- Email: [email protected]
Failure analysis of a mine hoist rope
EngineeringFailureAnalysis,Vol. 4, No. 1, pp. 25~38, 1997 Publishedby ElsevierScienceLtd Printed in Great Britain 1350~307/97$17.00 + 0.00
~ ) Pergamon PII:S 1350-6307(96)00029-5
FAILURE
ANALYSIS
OF A MINE
HOIST
ROPE
K. SCHREMS Department of Energy, Albany Research Center, 1450 Queen Avenue, S.W., Albany, O R 97321-2198, U.S.A.
and D. MACLAREN Henderson Mine, Empire, CO 80438, U.S.A. (Received 10 October 1996) A k s t r a e t - - A n extensive investigation was carried out to determine the cause of the early retirement of an inservice hoist rope. The rope was retired earlier than expected because it met the criteria for removal based on the number and distribution of wire breaks. Its chemistry, strength and ductility compared favorably to standards for new ropes. Metallography revealed minor anomalies, but these appeared indiscriminately in both the good and the bad segments, and in both broken and unbroken wires. Only one item appeared to be related to the failure of wires, and this was the appearance of a construction anomaly. This anomaly, termed a "dive", disrupts the construction of the strand, and m a y cause excessive crown wear or unusual wear patterns. The wire breaks removed from the rope were all found in the vicinity of dives. The investigation suggests that the dives are responsible for the premature retirement of the hoist rope. Published by Elsevier Science Ltd.
1. INTRODUCTION Wire ropes transmit large axial loads, and exhibit extreme flexibility. In addition, a wire rope is designed so that it can withstand some wire breaks without a loss of integrity. These characteristics make wire rope a versatile component in many systems. Wire ropes are used in many industries, with applications that include mining, offshore oil production, and towing or mooring of ships. The Albany Research Center has been studying the degradation mechanisms of wire ropes with the goal of more accurately predicting when the end of the useful life of a rope has been reached. In partnership with Henderson Mine, Albany Research Center personnel investigated a hoist rope that was retired after an unexpectedly short service life. After 9 months of service, the rope was retired because it exceeded the allowable number of broken wires per lay length [1]. Two rope segments were selected for investigation. One segment was at or near the location that required retirement because of broken wires (hereafter referred to as the bad segment), and the other segment came from the dead wrap on the drum (hereafter referred to as the good segment). The two segments were analyzed for differences in construction, steel composition and processing, and mechanical properties. Since it was felt that this rope was retired prematurely, the analysis was designed to identify, and, if possible, quantify, any differences that might account for the wire breaks. Initial examination focussed on two broad questions: (1) what are the differences between the wires in the good and the bad rope segments, and (2) are there any physical or mechanical differences between broken and unbroken wires in the bad segment?
2. BACKGROUND Wire ropes are composed of wires wound into bundles called "strands", which are then wound into the final rope (Fig. 1). The centermost wire of a strand, known as the king wire, provides support for the wires wrapped around it. One or more layers may be wrapped around the king wire to form the strand. The last layer of wires forms the outside of the strand, and, hence, the wires are called outside wires. The number, size and arrangement of wires in a strand, and the number of strands in a rope, determine its construction. The wires within the strand and the strand within the 25
K. SCHREMS and D. MACLAREN
26
P ....
rand
fJ
t
\, Fig. 1. The three basic components of a wire rope are the wire, the strand, and the core. The wire is a single, continuous length of metal that is drawn from a rod. The strand is a symmetrically arranged and helically wound assembly of wires. The core is the central member of a wire rope, about which the strands are laid. It can be made of a fiber, a wire strand, or an independent wire rope. The assembly of these three components results in a wire rope. rope can be w o u n d in either a right or a left helix. Wire r o p e t e r m i n o l o g y refers to a right regular lay or left lang lay. T h e terms right a n d left refer to the helix o f the s t r a n d within the rope, while the lay refers to the relationship between the helix o f the wires in the s t r a n d a n d the helix o f the s t r a n d in the rope. A regular lay r o p e has the wires in the s t r a n d w o u n d in the o p p o s i t e direction to the s t r a n d s in the rope, whereas a lang lay r o p e has the wires in the s t r a n d a n d the strands in the rope w o u n d in the same direction. The rope core can be either a n o t h e r strand, a smaller r o p e [called the i n d e p e n d e n t wire r o p e core ( I W R C ) ] , or a fiber. A n o n - r o t a t i n g rope (also k n o w n as a r o t a t i o n resistant rope) is a specialty rope t h a t consists o f multiple layers o f strands where different types o f lays are a l t e r n a t e d to reduce the n a t u r a l r o t a t i o n o f the rope. The wear that occurs within ropes used in mine hoisting o p e r a t i o n s usually occurs as the result o f one o f three types o f contact: (1) c o n t a c t o f the o u t e r strands o f the r o p e with an external m e m b e r , such as a sheave, d r u m , or layer o f rope on the d r u m , a c o n t a c t that is often called c r o w n wear; (2) line c o n t a c t between wires within a single s t r a n d or between strands; a n d (3) p o i n t c o n t a c t between wires within a single s t r a n d or between strands. A c t u a l wear o f the wires results from the c o m b i n a t i o n o f stresses that develop at these c o n t a c t areas d u r i n g tensioning o f the rope, a n d d u r i n g localized m o v e m e n t as the r o p e is bent, l o a d e d and unloaded. C r o w n wear a p p e a r s as a reduced cross section on the outside wires o f the rope [Fig. 2(a)]. W e a r between s t r a n d s a p p e a r s as nicks, which are easily visible as o b l o n g wear scars [Fig. 2(b)]. A characteristic pattern, consisting o f one or m o r e nicks with a similar o r i e n t a t i o n a n d depth, forms on each wire at the multiple-wire c o n t a c t site between strands. This characteristic p a t t e r n o f nicks created by p o i n t a n d line c o n t a c t s between strands is k n o w n as "trellis" o r i n t e r s t r a n d nicking [Fig. 2(c)]. One e v a l u a t i o n o f wear in a wire r o p e has revealed that c r o w n wear results from a b r a s i o n [Fig. 3(a)], a n d i n t e r s t r a n d nicking results f r o m fretting [Fig. 3(b)], with differences in a p p e a r a n c e due to the severity o f the wear m e c h a n i s m [2].
3. E X P E R I M E N T A L
PROCEDURE
The two r o p e segments were a p p r o x i m a t e l y 10 ft in length. The b a d segment c o n t a i n e d n u m e r o u s breaks, a n d was in the vicinity o f the l o c a t i o n requiring retirement. T h e g o o d segment c o n t a i n e d
Failure analysis of a mine hoist rope
(a)
27
(b)
(c) Fig. 2. (a) Fatigue crack at crown wear site. Crown wear appears on the outside wires of a rope where it is in contact with sheaves, drums or other external members. (b) Fatigue crack at nick. Nicks result from contact between wires. Large, prominent nicks are found where a wire in one strand has contacted a wire in an adjacent strand. (c) Contact between adjacent strands usually involvesa number of wires, and produces a recognizable pattern of nicks called a trellis pattern. Nicks are indicated by white arrows. The three nicks indicated form one trellis pattern.
no breaks, and came from the dead wrap on the drum (Fig. 4). During service, the bad segment of the rope experienced cyclic bending stresses from both the head sheave and the drum, as well as varying tensile stress from the weight of the rope and the counterweight. The good segment of the rope experienced some tensile stress from the weight of the rope and the counterweight. All wires were disassembled and labeled by rope segment, layer (outside strands = layer 1 to core = layer 5), strand, and wire position, as shown in Fig. 5. Broken wires were only found in the bad rope segment. In this segment, all breaks were contained within two outside layer strands, and one strand from the third layer. These three strands also contained the construction anomaly referred to as a "dive". These were named dives because, while visually following outside wires around a strand, it was noticed that a wire would occasionally "dive" into the interior of the strand, and could no longer be followed visually. A different wire would come out of the interior of the strand, and take the position of the outside wire that disappeared. It was later determined that, beyond the axial location of the dive, the king wire from before the dive functioned as an outside wire, and the outside wire from before the dive functioned as a king wire. One such dive is shown in Fig. 6. Additionally, one strand from the good rope segment contained a dive, but no associated wire break. Although only the strands that contained dives contained wire breaks, not all dives had a corresponding wire break, nor were all wire breaks found at a dive. During a dive, a king wire and an outside wire physically change position within the strand. This presented a difficulty in labeling the wires and performing statistical comparisons. For labeling purposes, king wires were initially identified by strand position referenced to one end of the rope segment. However, m a n y of the planned evaluations were based on groupings of nominally identical wires. Therefore, for analyses, both king and outside wires were determined solely by diameter. Implicit in these analyses is that the conclusions pertain to a strand that does not contain dives. The chemistry of all groups of wires in the rope segments were examined. Since the alloying composition of a wire can have a large affect on the mechanical response, (1) the alloying composition of broken and unbroken wires in the bad segment were evaluated for significant differences, and (2) the overall alloying composition of each individual layer was evaluated. The alloying composition of the good segment was assumed to be identical to that of the bad segment: therefore, chemical analyses of the wires from the good segment were not performed. However, since king wires do not have the same diameter as the outside wires, there is no reason to expect that they are from the
28
K. SCHREMS and D. MACLAREN
(a)
(b) Fig. 3. (a) Abrasive wear is commonlyseen as the principal mode of damage in wires that are exposed to external surfaces. (b) Fretting results from the relative motion between wires, such as is seen at nicks. Fracture craters due to delamination are present. Some material has been extruded from the area of contact.
same steel heat. Therefore, t,he chemistry of the king wires was evaluated separately from the outside wires. The following elements were determined: carbon, sulfur, silicon, phosphorus, manganese, chromium and nickel. Carbon and sulfur were determined by gas analysis, and silicon, manganese, phosphorus and chromium by wet chemistry methods. In order to obtain a statistical representation of the alloying composition, multiple samples from each layer were analyzed. The results are reported in Table 1. Torsion tests were performed according to the American Petroleum Institute (API) Specification for Wire Rope [3]. Table 2 lists the requirements for the minimum number of torsions (i.e. the number of twists to failure) to be attained by wires made out of electric furnace steel after fabrication into wire rope. In addition to the API requirements for minimum torsions, Table 2 also lists the average number of experimentally determined torsions for wires in the good segment and the bad segment, and for broken wires in the bad segment (XGOOD,XBAD,XBROKEN,respectively). A column labeled 0.3J(GOODis included as a comparison to a practice by the Ontario Ministry of Labour. The Ontario Ministry of Labour tests ropes before they are put into service to determine initial reference values, and subsequently tests periodic cutoffs. They recommend caution when the number of
Failure analysis of a mine hoist rope
29
Headframe
Double drum hoist
/ E L 10425'
cage Location of "good" segment drum dead wraps
Location of "bad" section Production level EL 7700'
1000'<
Haulage level EL 7500'
(1
Counterweight Fig. 4. Schematic of hoisting operation. The rope in question came from the counterweight of a double drum hoist. The segment requiring removal was at the location marked "bad" section. The comparison sample ("good" segment) came from the dead wraps on the drum.
torsions d r o p s to 30% o f the initial reference value, a n d r e c o m m e n d t h a t ropes be retired w h e n the n u m b e r o f t o r s i o n s d r o p s b e l o w 15% [4]. Tensile tests were p e r f o r m e d a c c o r d i n g to the A P I Specification for W i r e R o p e [3]. T a b l e 3 lists the r e q u i r e m e n t s for the a v e r a g e m i n i m u m b r e a k i n g strength to be a t t a i n e d b y wires m a d e o u t o f electric furnace steel after f a b r i c a t i o n into wire rope. I n a d d i t i o n to the A P I r e q u i r e m e n t s for m i n i m u m tensile strength, T a b l e 3 also lists the e x p e r i m e n t a l l y d e t e r m i n e d average b r e a k i n g strength for wires in the g o o d segment, the b a d segment, a n d b r o k e n wires in the b a d segment ()?GOOD, XBAD, "~BROKEN, respectively). M e t a l l o g r a p h i c a n d f r a c t o g r a p h i c investigations were c a r r i e d o u t in o r d e r to identify the cause(s) o f failure o f i n d i v i d u a l wires. F o r the m e t a l l o g r a p h i c investigation, one o u t s i d e a n d one king wire f r o m each s t r a n d layer o f b o t h the g o o d a n d the b a d segments were e v a l u a t e d in the transverse d i r e c t i o n at sites o f general wear, c r o w n wear, a n d nicks b e t w e e n a d j a c e n t strands. These s a m p l e s
13
L~
Layer 3
L~
Strand 2
w~ i ~ jwir~ 2 wi~ 7 ~ )
~
Wire 3
Wire 6 /Wire 5
"Wire 4
Fig. 5. Construction ofretired hoist rope. Wires in the rope were examined as a function oflayer (outside=layer 1, core = layer 5) and strand position (wires 1~5 = outside wires, wire 7 = king wire). Strands in each layer were labeled in a clockwise relationship to strand 1, an arbitrarily chosen reference strand. For illustration purposes, the two strands from layer 1 that contained wire breaks (strands 2 and 13) are shown.
30
K. S C H R E M S and D. M A C L A R E N
Fig. 6. Appearance of a dive. A dive is a location where an outside wire and a king wire switch positions in the strand structure. White arrows indicate positions where an outside wire moves into the interior of the strand to assume the function of a king wire.
Table I. Steel composition of bad rope segment: results are the average of a random selection of wires from all strands within a given layer, with outside wires and king wires evaluated separately, and are reported in wt %
Layer 1
Wire position Outside Broken King Broken Outside King Outside King Broken Outside King Outside King
2 3
4 5
Carbon (%)
Sulfur (%)
Silicon (%)
Phosphorus (%)
Manganese (%)
Chromium (%)
Nickel (%)
0.746 0.753 0.738 0.738 0.743 0.738 0.742 0.735 0.730 0.723 0.703 0.704 0.754
0.0171 0.0171 0.0254 0.0259 0.0168 0.0251 0.0169 0.0258 0.0246 0.0196 0.0215 0.0219 0.0189
0.27 0.26 0.24 0.24 0.25 0.22 0.26 0.23 0.24 0.24 0.23 0.23 0.27
0.012 0.012 0.013 0.012 0.013 0.014 0.012 0.014 0.014 0.016 0.027 0.026 0.022
0.68 0.68 0.76 0.75 0.68 0.76 0.68 0.76 0.75 0.70 0.70 0.69 0.66
0.048 0.051 0.038 0.036 0.047 0.039 0.048 0.038 0.038 0.054 0.069 0.068 0.052
0.029 0.028 0.038 0.037 0.028 0.037 0.029 0.037 0.037 0.031 0.031 0.032 0.028
Table 2. Torsion results for wires removed from retired rope: APt requirements are for wires removed from a newly fabricated rope. Ontario Ministry of Labour recommendations (0.3J?GcXm) are for wires removed from a used rope Layer l 2 3 4 5
Wire position Outside King Outside King Outside King Outside King Outside King
API requirements 12 I1 12 11 12 1I 41 34 34 31
0.3.J(GOOD* 7.5 7.1 8.4 7.5 8.3 6.9 23.0 19.7 21.0 16.8
X(~OOI)
~BAD
"J(BROKEN
25.1 23.7 27.9 25.0 27.8 23.0 76.8 65.7 + 70.0 56.0
19.8 24.3 20.7 2t .3 22.9 23.3 40.9 58.3* 52.0 46.0
17.3 26.5 N/A? N/A N/A 21.00 N/A N/A N/A N/A
*Since an initial reference value is not available, it is assumed that the torsions in the good rope segment have not changed, and can be substituted for torsions from a newly fabricated rope. ?Not applicable. +Analysis includes results not acceptable by API specifications, e.g. those that failed too close to the grips.
Failure analysis of a mine hoist rope
31
Table 3. Average breaking strength for wires removed from retired rope: listed API requirements are for the average minimum breaking strength to be attained by wires produced from electric furnace steel after fabrication into wire rope Layer
Wire position
API requirements (kN)*
"(GOOD
"ITBAD
1
Outside King Outside King Outside King Outside King Outside King
11.216 12.153 11.216 12.153 11.216 12.153 1.920 2.527 2.527 2.999
10.92 11.84 10.95 11.97 10.62 11.86 1.530 2.341 2.336* 2.999
10.84 12.24 10.78 12.50 10.89 12.34++ 1.511 2.382++ 2.338 2.981 :~
2 3 4 5
-t~aROKEN 9.85 12.49 N/A•" N/A N/A 12.40++ N/A N/A N/A N/A
*Requirements listed are for the nominal wire diameter of each layer and position. Values are interpolated from tables in [3]. tNot applicable. ++Analysisincludes results not acceptable by API specifications, e.g. those that failed too close to the grips.
were e v a l u a t e d for d e c a r b u r i z a t i o n , cracks, martensite, a n d the a p p e a r a n c e o f the wear scar. In a d d i t i o n , wires involved with dives were also evaluated.
4. E X P E R I M E N T A L
RESULTS
AND DISCUSSION
4.1. Rope construction T h e c o n s t r u c t i o n a n o m a l i e s were n a m e d dives, a n d are l o c a t i o n s where a n o u t s i d e wire a n d a king wire switch p o s i t i o n in the s t r a n d structure. A l o n g the s t r a n d axis, the i n t e r c h a n g e o f the two wires will t a k e place over a length o f several centimeters, a n d results in a larger t h a n n o r m a l s t r a n d d i a m e t e r (Fig. 7). U n u s u a l a n d u n e x p e c t e d wear a n d / o r d e f o r m a t i o n will t a k e place between the wires within the strand. In s o m e cases, as also s h o w n in Fig. 7, this is o b s e r v e d as deep nicks (gouges). I n o t h e r cases, the result will be extensive flattening o f the wire surface a n d / o r excessive c r o w n w e a r (Fig. 8), with the a m o u n t o f c r o w n wear increasing as the p r o x i m i t y to the dive increases. In a d d i t i o n to the w e a r a n d d e f o r m a t i o n , the i n t e r c h a n g e o f wires, especially o f different diameters, will alter the l o a d d i s t r i b u t i o n in the strand. D u r i n g disassembly, 12 dives were identified in three different s t r a n d s in the b a d segment. A l l wire b r e a k s were f o u n d in these three different s t r a n d s (Figs 9-11), a n d were often l o c a t e d at o r between dives. In c o n t r a s t , o n l y one dive was f o u n d in the g o o d segment (Fig. 12), a n d there was no a s s o c i a t e d wire b r e a k . I n wire r o p e design, king wires t y p i c a l l y have a larger d i a m e t e r t h a n o u t s i d e wires. Overall, the king wires (wire 7) h a d a d i a m e t e r o f a p p r o x i m a t e l y 2.95 m m , in c o m p a r i s o n with
Fig. 7. Dive from layer 1, strand 2. The strand diameter at the location of the dive is larger than elsewhere, as is shown by the two white arrows. The gouge produced as a result of this expanded diameter is shown by the black arrow.
32
K. SCHREMS and D. MACLAREN
'
?'
crown wear Fig. 8. Dive from layer 1, strand 13. A total of four wire fractures are visible, and two are matching fractures. The location where the king wire switches position and becomes an outside wire is at the location marked "'dive." This wire shows extensive flattening and crown wear just prior to its fracture location. The two gray arrows point out differences in the severity of the crown wear. As the location of the dive is approached, the crown wear of the surrounding wires becomes more severe.
Wire 1
~+~L 2.82 2.82
Wire 2
+
2.84
, 2.84
Wire 3
+
,
2.95
, 2.82
Wire 4
2.82
.1.
7.82
2.82
T 2.82
2.82
Wire 5 2.82
Wire6 2.82
Wire 7 2.82
2.82
Layerl, Strand 2
....
--
-
J
-
Fig. 9. Wire diameters and location of wire breaks for layer 1, strand 2, bad rope segment. The shaded portion of the wire has a significantly larger diameter. The bottom illustration shows the relationship of dives and wire breaks in the assembled strand. (Dimensions are in mm.)
2.82 m m for the o u t s i d e wires. In a t t e m p t i n g to g r o u p like wires/sizes, it w a s f o u n d that a c o n t i n u o u s k i n g wire c o u l d n o t be identified in strands 2 a n d 13 f r o m layer 1 as illustrated in Figs 9 a n d 10. This, in a d d i t i o n to the difficulty u n w i n d i n g strands c o n t a i n i n g dives, suggests that the a n o m a l y w a s n o t created d u r i n g service.
33
Failure analysis of a mine hoist rope
t
Wire 1
t
ii
2.84
t
ii
2.82
t t
Wire 2
2.82
t ,
ii
2.84
t
,j_., t
t
-f2.82
2.84
,
Wire 4
t
+
,' 2.82
Wire 6
,
,
÷
t
I
2.95
strand13
'
2.79
+, 2,82
i
t
,+ ,' + , + t 2.82
2.92
2.84
2.82
2.84
Wire 7
|
2.82
Wire 3
Wire 5
i
2.82
2.82
2.82
2.82
t t
i
2.$4
n
o
t
¢
Fig. 10. Wirc diameters and location of wire breaks for layer 1, strand 13, bad rope segment. The shaded portion of the wire has a significantly larger diameter. The bottom illustration shows the relationship of dives and wire breaks in the assembled strand. (Dimensions are in mm.)
Layer 3, Strand 3
, ~, t
Fig. 11. Location of dive and break in layer 3, strand 3, bad rope segment.
Layer 1, Strand 7 Good Rope Segment Fig. 12. Location of dive in layer I, strand 7, comparison segment. This was the only dive found in thc comparison scgmcnt. There were no associated wire breaks. -
4.2. Chemical analysis Wire ropes used in the United States are not required to meet alloying standards. However, the API does require that the wire be produced from: (1) acid or basic open-hearth, (2) basic oxygen, or (3) electric furnace steelmaking processes; and that the wire so produced meets certain mechanical property specifications, e.g. breaking strength and torsional requirements, dependent upon the steelmaking process used. Breaking strength and torsional requirements are highly dependent on alloying composition, and API-acceptable results have been developed for each of the three different steelmaking processes. Therefore, the type of steelmaking process needs to be determined for later comparisons with API specifications. The chemistries of the wires in Table 1 are typical of an electric furnace steel [5]. Residual alloying elements (manganese, chromium and nickel) and impurities (sulfur and phosphorus) are generally higher in electric furnace steel than in open-hearth or basic oxygen steel. In general, higher levels of
34
K. SCHREMS and D. MACLAREN
alloying elements result in lower ductility and higher strength. This is reflected in the API specifications, where the electric furnace steel has the highest requirement for tensile strength and the lowest for torsion. The steel used in this rope was considered to be produced in an electric furnace. This was later verified by the rope manufacturer. A multivariate analysis of the chemical analysis data was performed to determine if differences in chemical composition exist between the different layers of wires. The analysis revealed that there is a significant difference in the chemistries between the first three layers and the two layers that comprise the independent wire rope core. The results can be summarized as follows: (1) Layers 1 3 contain wires with very similar composition, and are probably obtained from the same heat of steel. Furthermore, the outside wires from layers I-3 were obtained from one heat of steel, and the king wires from the same layers were obtained from another heat. It should be noted that layer 2 appears to be produced from the same steel heats as layers 1 and 3, yet contains no wire breaks of either king or outside wires. (2) Layers 4 and 5 have significantly different composition from the first three layers, and are probably not obtained from the same heat as layers 1 3. (3) Layers 4 and 5 differ significantly from each other, and probably do not come from the same heat. Again, the king wires appear to be from a different heat than the outside wires. In all, it appears that there are six distinct heats of steel represented in this rope. Independent of the type of steelmaking process used, one of the primary questions to be addressed is whether the wire material itself is responsible for premature failure of the wires. As can be seen in Table 1, the chemical analyses of broken and unbroken wires in the bad rope segment are very similar. A multivariate analysis of variance shows no significant difference between the chemistries, with the possible exception of the nickel content of the layer 1 king wires. Although the statistical analysis identifies the nickel content as being significantly different, from a practical standpoint the difference is not great enough to affect the behavior of the material. It appears that the steel used for this wire rope came from six distinct heats from an electric furnace. All of the broken wires were found in layers 1 and 3, which would comprise only two of six distinct steelmaking heats identified. No significant difference was found between the broken and unbroken wires. It can, therefore, be concluded that it is highly unlikely that the overall chemistry of the wires was responsible for the wire breaks.
4.3. Torsion results For the rope segments examined, the number of torsions may reasonably be expected to be lower than those in the API specifications, due to fatigue and wear degradation during service. However, as can be seen from Table 2, the torsions generally met or exceeded the AP! specifications. It should be noted that the API specification evaluates the individual torsions, not the averages, whereas Table 2 lists the averages. When individual torsion results are evaluated, more than 90% of outside wires from layer 1 (both segments) and 50% of the outside wires from layer 4 (bad segment) meet this criteria. Since torsion results are highly dependent on surface imperfections such as crown wear and trellis patterns (wear produced as a result of contact between the outside wires in adjacent strands), lower torsion results for these two wire groups are not unusual. As can be seen from Table 2, there does not appear to be a difference in torsions between broken and unbroken wires. The torsion results of the outside wires of the two segments differ significantly, whereas the torsion results of the king wires do not. It should be noted that all of the layers in the bad segment contained readily identified nicks, whereas the good segment showed little or none. On the other hand, the king wires only showed uniform line contact. These findings agree well with the observed torsion results. The king wires experienced little or no detrimental wear, and, for these wires, differences between the segments are not observed. Layer 1 of both segments experienced crown wear, and these wires show a reduction in the number of torsions. The effects of wear on the torsion results can be summarized as follows: (1) The outside wires of a strand experience crown wear and/or trellis contact, a cumulative process that affects the surface quality of the wire and lowers the overall torsions.
Failure analysis of a mine hoist rope
35
(2) King wires do not experience crown wear or trellis contact, and no appreciable difference in the torsion values was seen. The torsion test is geared towards testing ductility. The API specifications provide for a minimum ductility to be present in a newly fabricated rope. On the other hand, the Ontario Ministry of Labour recommendations provide guidelines for removing a rope based on a decrease in ductility. The initial ductility is primarily a function of the steel chemistry and the wire drawing process. After a rope is put into service, the ductility will change as a function of fatigue and wear. As can be seen, the ductility is similar to that required for a newly fabricated rope, and far exceeds the Ontario Ministry of Labour guidelines for removal. Therefore, it appears that the ductility of the wires in both the good and the bad segments is sufficient. All the torsion results met or exceeded the API requirements for wires removed from newly fabricated wire rope, even though the wires were removed from a used rope. The number of torsions of wires removed from the good rope segment was significantly greater than for those removed from the bad rope segment, but was limited to the outside wires of a strand. These differences are most likely a result of crown wear and/or trellis patterns. There was virtually no difference in the number of torsions between broken and unbroken wires. Therefore, it is highly unlikely that the wires failed due to poor ductility.
4.4. Tensile results The breaking strengths listed in Table 3 generally do not meet the minimum requirements for wires produced from electric furnace steel. Breaking strength requirements should more accurately be called breaking load requirements because the requirements list the minimum load-carrying ability of a given diameter wire. Minimum breaking loads will depend on the cross-sectional area as well as the material property known as the ultimate tensile strength. When crown wear and trellis patterns are present, the cross-sectional area is reduced. The result is that the wire will generally break at these sites of reduced cross section. However, the API specifications are for wires removed from newly fabricated ropes, presumably with uniform cross sections. It would be reasonable to find lower strengths in wires from a rope removed from service, e.g. a rope with crown wear and trellis contact. This effect is probably responsible for the drop in strength seen in the broken outside wires from layer 1. Dives create "high" wires (analogous to high strands) which experience more material removal from crown wear and deeper nicks from trellis contact. The differences in breaking load between the king wires in the good and bad segments for the first three layers was found to be highly significant. An effect similar to this has been seen by the Bureau of Mines Pittsburgh Research Center when testing rope segments [6]. They have noted that the rope breaking strength may initially increase, and then drop off significantly as the rope approaches the end of its useful life. For ropes, the initial increase in breaking strength is generally attributed to "break-in" and the flattening of contact sites between wires. Although there is a significant difference between the breaking loads for the wires in the good and bad segments, there is not a significant difference between the breaking loads for the broken and unbroken wires in the bad segment. Although interesting, the increase in strength of the wires does not appear to be related to wire failure. The tensile test is used to evaluate the minimum strength of the wires. Wires that are not of the minimum strength run the risk of being overloaded during normal use, and will also have shorter fatigue lives. Signs of overloading, such as ductile cup-cone failures, were not seen. The average strength of the wires is generally acceptable when compared with the strengths listed for other steelmaking processes. With the exception of results for the broken wires in the outside layer (which appears to be related to a loss of metallic area), there is no significant difference in the broken and unbroken wires in the bad segment. The average breaking strength of the wires removed from both the good and bad segments failed to meet the minimum API specifications for wires made out of electric furnace steel. With a few exceptions, the average breaking strengths exceeded those required for basic oxygen steel. The loss of strength of broken outside wires from layer 1 is attributed to the presence of crown wear and trellis patterns. King wires showed an unexplained increase in strength between the good segment and the bad segment, but there was no significant difference in strength between the broken and
36
K. SCHREMS and D. MACLAREN
unbroken king wires. It seems unlikely that inadequate tensile strength is responsible for the wire breaks. 4.5. Metallography Decarburization was observed on wires from layers 1-3. The full depth of decarburization was approximately 15-20/tm, or a little over 1% of the diameter of the wire. Although decarburization detrimentally affects fatigue, the effect is much smaller in magnitude than that of a surface blemish such as crown wear or trellis contact. The amount measured should not have a noticeable effect on the fatigue life of the rope [7]. Two types of cracks were observed in the metallographic samples. The first type of crack was radial, less than 50 pm, and generally emanated from a surface pit. The pits and cracks appear to follow the incursion of decarburization into the wire. This is not unexpected since the ferrite resulting from the decarburization will pit preferentially during the pickling process. Since the cracks appear in almost all of the samples from layers 1 3, regardless of location, it is highly unlikely that the radial cracks account for the differences in fracture behavior between the good and bad segments. The second type of crack propagated parallel to the surface of the wire, and was located at both crown wear and nick sites. The size and appearance of this type of crack took a variety of forms. At crown wear scars, various degrees of abrasion were found, many samples having wedge formation and heavy plastic deformation. Some cracks were seen separating the wedge from the main body of the wire. Smaller cracks often appeared in the middle of the crown wear site, and separated the lip of material left after abrasion from the base metal itself. Unlike the abrasion found at the crown wear sites, the wear mechanism present at the nick sites tended to be sliding wear or fretting. The sliding wear produced considerable deformation in the pearlitic structure, while fretting tended to remove material by spalling. In all cases, the cracks appeared roughly parallel to the surface, and propagation of the cracks produced a metallic flake as opposed to a transverse fatigue crack. These mechanisms, which are what would be expected given the nature of the interactions within the rope and with its environment, are the same as found at crown wear and nick sites in other ropes [2]. It is highly unlikely that these mechanisms contributed to premature removal of the rope. Martensite, a suspected nucleation site for fatigue cracks, was not readily identified. Using a nital etchant, only small, thin areas were identified as martensite. It should be noted that there is some question as to whether the "white-etching layer" observed in wear of ferrous materials is actually martensite. A thin white layer, less than 15/tm, was observed at some nick and crown wear sites in both segments. At some locations, surface cracks that turned and followed the boundary of the white layer were visible. It is felt that the cracks associated with the white layer led to spalling, and are not responsible for the premature removal of the rope. A wide disparity of inclusions was noticed on initial examination. In relation to each other, these inclusions ranged from minimal in size and number to large in size. Since inclusions can reduce the load-carrying capability of a wire as well as decrease its fatigue performance, the number and size of inclusions was investigated. The Making, Shapin9 and Treatin9 of Steel mentions a cleanliness rating of 0.1~).3 vol % inclusions in the range of 10-50 ~m in diameter for continuous casting [8]. Of 69 transverse samples, only 12 had inclusions that were in that range, and these were typically ! 025 #m. In the longitudinal direction, the cleanliness rating ranged from < 0.1 vol % to approximately 0.75%. (It should be noted that all inclusions are included in this value, not just those greater than 10/~m. In addition, only one field was used, leading to a large statistical error.) These observations suggest that, although large inclusions are present, they are within the range expected in this type of steel. Large inclusions and/or clusters of inclusions were found in both the good and the bad segments, as well as at general wear sites, crown wear, and trellis sites. Therefore, it does not appear that there is a correlation between inclusion content and/or size and rope segment, layer, wire, or wear type. Extensive metallographic analyses were performed on wires removed from the two rope segments. Decarburization was found on all outside wires from the first three layers, but was uniform between the segment in question and the comparison. Two types of cracks were observed in the samples. The first type was a radial crack that appeared in relation to the decarburization, and was observed the length of the wire. The second type of crack occurred at wear sites on both the good and bad segments, and on both the broken and unbroken wires, and was observed to propagate parallel to
Failure analysis of a mine hoist rope
37
the wire surface. This type of crack most likely contributed to spalling and material removal at the wear site as opposed to initiating a fatigue crack in and of itself. An extensive search was undertaken to locate the appearance o f martensite at wear surfaces. The search revealed small locations that could possibly be martensite, but the depth is much less than has been reported in the literature. All anomalies noticed during the metallographic analysis appeared in both segments without regard to broken or unbroken wires. Therefore, it is highly unlikely that microstructural anomalies were responsible for the wire breaks. 4.6.
Fracto#raphy
A fractographic study of 12 fracture locations including five matching fractures was performed to identify characteristics of: (1) fracture location, and (2) crack initiation. O f the 12 fracture locations, nine were identified as occurring at crown wear sites, two as occurring at trellis sites, and one was too obliterated to determine its location. The relationship of the wire failures to dives was also sought. The movement of wires within a strand creates faint wear patterns between wires that are in contact, allowing the placement of adjacent wires to be identified. Adjacent wires within a strand follow a helical path around the king wire, and, although the wires twist, the patterns around the wires are evenly spaced, and continuous between crown wear patterns. The disruption of these wear patterns was considered as evidence of a dive (a location where two wires switch position in the strand). As such, nine of the fractures were positively identified as being in the location of a dive. Most of the wire fractures had two identifiable crack initiation sites. For the failures occurring at crown wear sites, initiation often occurred at the actual crown wear site, located either at the center or at a corner. There was usually another failure site located opposite the crown wear site at the location of the faint wear patterns. Often this wear pattern was of the wire associated with the dive. For the two failures that occurred at trellis sites, the wear associated with the dives was closely associated with one of the nicks. The amount of deformation present as well as the remaining material was evaluated. Some of the crown wear fractures had one-third to one-half of the wire cross section removed by abrasion. Others experienced considerable flattening in addition to the loss of cross-sectional area. The trellis nicks associated with the wire fractures were also (qualitatively) larger than the nicks seen elsewhere on the ropes. Therefore, it is felt that the effect of the wire dives on the wire fractures was twofold. First, the dives disrupted the close-packed structure of the strand, and produced locations where certain wires were "high". If the high wires were on the outside of the rope, the result was increased wear and/or compression of the wire. If the high wires were in the interior of the rope, the result was deeper than normal nicks on adjacent strands. Second, the wear characteristics and stress distribution were altered in the presence of a dive. In m a n y cases, this contributed to either primary or secondary crack initiation. In summary, fractography was performed on wire breaks removed from the rope. O f the fractures that were not obliterated, most could be identified as being in the location of a dive by wear patterns. The process by which the wires failed varied. Clearly, some of the wires had excessive crown wear, and failed in relation to a loss of metallic area. Fractures at crown wear sites associated with dives had as much as one-third to one-half of the cross-sectional area removed. The interchange of wires at a dive produced unusual wear patterns with adjacent wires, and these wear patterns often appeared as an initiation site of the fatigue crack. Fractures at trellis sites experienced deeper nicks than were seen elsewhere in the rope. In one case, a large gouge caused by an adjacent wire (larger than that shown in Fig. 7) was identified as initiating the wire break. Although a variety of failure mechanisms were present, the c o m m o n factor was the proximity of a dive. The dive either accentuated a normally occurring mechanism (crown wear) or was the cause of an unforeseen mechanism (gouging).
5. C O N C L U S I O N S A series of experiments was designed: (1) to compare the broken wires in the bad segment with the unbroken wires in the bad segment, and (2) to compare the wires in the bad segment with those
38
K. SCHREMS and D. MACLAREN
in the comparison segment. Chemical analyses, tensile tests, torsion tests, metallography, and fracture analysis have all been performed. It was determined that, although six different steelmaking heats were identified, there was no correlation with broken and unbroken wires. A significant difference was found in the torsions between the good and the bad segments (attributed to in-service surface wear), but not between the broken and unbroken wires. The results all met or exceeded the API requirements for wires removed from a new rope, so it is highly unlikely that the wires failed due to poor ductility. The tensile results exceeded those for new wires made from basic oxygen steel. Again, a significant difference between the broken wires and the unbroken wires was not observed, so it seems unlikely that inadequate tensile strength is responsible for the wire breaks. Inclusions, decarburization, cracks and martensite were all evaluated. All anomalies noticed during the metallographic analysis appeared in both segments without regard to broken or unbroken wires. It is highly doubtful that the metallurgical structure was responsible for the wire breaks. Differences in the chemical analyses, tensile tests, torsion tests, and metallography do not correlate with the presence or absence of broken wires. The construction anomaly called a "dive" was found to be related to the presence of broken wires. The three strands in the bad segment that had wire breaks were also the three strands that contained dives. One strand in the good segment contained a single dive, but no wire break. Dives produce a larger than normal strand diameter in the local vicinity, which results in unusual and unexpected wear and/or deformation. Fractography of the wire breaks showed the breaks to be associated with gouges, flattening, or severe crown wear, all of which can be attributed to the disruption of the strand structure produced by a dive. It appears that the wear and/or deformation caused by the presence of the dive accounts for the wire breaks. The number and distribution of wire breaks were, in turn, responsible for the rope being retired prematurely.
REFERENCES 1. U.S. Code of Federal Regulations. Title 30 Mineral Resources; Chapter l--Mine Safety and Health Administration, Department of Labor; Subchapter N--Metal and Nonmetallic Mine Safety;Part 56, Subpart R, and Part 57, Subpart R: Subchapter O~Coal Mine Safety and Health; Part 75, Subpart O, and Part 77, Subpart O; 1 July 1989. 2. Sehrems, K. K., Do,an, C. P. and Hawk, J. A., Journal o["Materials Engineerin 9 and PerJormance, 1995,4, 136. 3. AmericanPetroleum Institute, Spec!fication/br Wire Rope. APISpec(fication 9A (Spec 9A), 23rd edn. Washington, DC, 1984. 4. Djivre, M., Mine ShaJ~ Ropes: Ontario Destructit~e Wire Testin,q Program. Ontario Ministry of Labour, Sudbury, Ontario, 16 January 1991, p. 5. 5. Dove, A. B., Ferrous Wire. Vol. I: The Manu/~tcture ~fFerrous Wire. The Wire Association International, Guilford, CT, 1989. 6. Miscoe,A. J., Private communication, 1990. 7. Mayer, M., Private communication, 1995. 8. Lankford, W. T., Jr., Samways,N. L.. Craven, R. F. and McGannon, H. E., eds, The Making, Shaping, and Treating o[ Steel, 10th edn. Associationof Iron and Steel Engineers, Pittsburgh, PA, 1985.
~ ) Pergamon PII:S 1350-6307(96)00029-5
FAILURE
ANALYSIS
OF A MINE
HOIST
ROPE
K. SCHREMS Department of Energy, Albany Research Center, 1450 Queen Avenue, S.W., Albany, O R 97321-2198, U.S.A.
and D. MACLAREN Henderson Mine, Empire, CO 80438, U.S.A. (Received 10 October 1996) A k s t r a e t - - A n extensive investigation was carried out to determine the cause of the early retirement of an inservice hoist rope. The rope was retired earlier than expected because it met the criteria for removal based on the number and distribution of wire breaks. Its chemistry, strength and ductility compared favorably to standards for new ropes. Metallography revealed minor anomalies, but these appeared indiscriminately in both the good and the bad segments, and in both broken and unbroken wires. Only one item appeared to be related to the failure of wires, and this was the appearance of a construction anomaly. This anomaly, termed a "dive", disrupts the construction of the strand, and m a y cause excessive crown wear or unusual wear patterns. The wire breaks removed from the rope were all found in the vicinity of dives. The investigation suggests that the dives are responsible for the premature retirement of the hoist rope. Published by Elsevier Science Ltd.
1. INTRODUCTION Wire ropes transmit large axial loads, and exhibit extreme flexibility. In addition, a wire rope is designed so that it can withstand some wire breaks without a loss of integrity. These characteristics make wire rope a versatile component in many systems. Wire ropes are used in many industries, with applications that include mining, offshore oil production, and towing or mooring of ships. The Albany Research Center has been studying the degradation mechanisms of wire ropes with the goal of more accurately predicting when the end of the useful life of a rope has been reached. In partnership with Henderson Mine, Albany Research Center personnel investigated a hoist rope that was retired after an unexpectedly short service life. After 9 months of service, the rope was retired because it exceeded the allowable number of broken wires per lay length [1]. Two rope segments were selected for investigation. One segment was at or near the location that required retirement because of broken wires (hereafter referred to as the bad segment), and the other segment came from the dead wrap on the drum (hereafter referred to as the good segment). The two segments were analyzed for differences in construction, steel composition and processing, and mechanical properties. Since it was felt that this rope was retired prematurely, the analysis was designed to identify, and, if possible, quantify, any differences that might account for the wire breaks. Initial examination focussed on two broad questions: (1) what are the differences between the wires in the good and the bad rope segments, and (2) are there any physical or mechanical differences between broken and unbroken wires in the bad segment?
2. BACKGROUND Wire ropes are composed of wires wound into bundles called "strands", which are then wound into the final rope (Fig. 1). The centermost wire of a strand, known as the king wire, provides support for the wires wrapped around it. One or more layers may be wrapped around the king wire to form the strand. The last layer of wires forms the outside of the strand, and, hence, the wires are called outside wires. The number, size and arrangement of wires in a strand, and the number of strands in a rope, determine its construction. The wires within the strand and the strand within the 25
K. SCHREMS and D. MACLAREN
26
P ....
rand
fJ
t
\, Fig. 1. The three basic components of a wire rope are the wire, the strand, and the core. The wire is a single, continuous length of metal that is drawn from a rod. The strand is a symmetrically arranged and helically wound assembly of wires. The core is the central member of a wire rope, about which the strands are laid. It can be made of a fiber, a wire strand, or an independent wire rope. The assembly of these three components results in a wire rope. rope can be w o u n d in either a right or a left helix. Wire r o p e t e r m i n o l o g y refers to a right regular lay or left lang lay. T h e terms right a n d left refer to the helix o f the s t r a n d within the rope, while the lay refers to the relationship between the helix o f the wires in the s t r a n d a n d the helix o f the s t r a n d in the rope. A regular lay r o p e has the wires in the s t r a n d w o u n d in the o p p o s i t e direction to the s t r a n d s in the rope, whereas a lang lay r o p e has the wires in the s t r a n d a n d the strands in the rope w o u n d in the same direction. The rope core can be either a n o t h e r strand, a smaller r o p e [called the i n d e p e n d e n t wire r o p e core ( I W R C ) ] , or a fiber. A n o n - r o t a t i n g rope (also k n o w n as a r o t a t i o n resistant rope) is a specialty rope t h a t consists o f multiple layers o f strands where different types o f lays are a l t e r n a t e d to reduce the n a t u r a l r o t a t i o n o f the rope. The wear that occurs within ropes used in mine hoisting o p e r a t i o n s usually occurs as the result o f one o f three types o f contact: (1) c o n t a c t o f the o u t e r strands o f the r o p e with an external m e m b e r , such as a sheave, d r u m , or layer o f rope on the d r u m , a c o n t a c t that is often called c r o w n wear; (2) line c o n t a c t between wires within a single s t r a n d or between strands; a n d (3) p o i n t c o n t a c t between wires within a single s t r a n d or between strands. A c t u a l wear o f the wires results from the c o m b i n a t i o n o f stresses that develop at these c o n t a c t areas d u r i n g tensioning o f the rope, a n d d u r i n g localized m o v e m e n t as the r o p e is bent, l o a d e d and unloaded. C r o w n wear a p p e a r s as a reduced cross section on the outside wires o f the rope [Fig. 2(a)]. W e a r between s t r a n d s a p p e a r s as nicks, which are easily visible as o b l o n g wear scars [Fig. 2(b)]. A characteristic pattern, consisting o f one or m o r e nicks with a similar o r i e n t a t i o n a n d depth, forms on each wire at the multiple-wire c o n t a c t site between strands. This characteristic p a t t e r n o f nicks created by p o i n t a n d line c o n t a c t s between strands is k n o w n as "trellis" o r i n t e r s t r a n d nicking [Fig. 2(c)]. One e v a l u a t i o n o f wear in a wire r o p e has revealed that c r o w n wear results from a b r a s i o n [Fig. 3(a)], a n d i n t e r s t r a n d nicking results f r o m fretting [Fig. 3(b)], with differences in a p p e a r a n c e due to the severity o f the wear m e c h a n i s m [2].
3. E X P E R I M E N T A L
PROCEDURE
The two r o p e segments were a p p r o x i m a t e l y 10 ft in length. The b a d segment c o n t a i n e d n u m e r o u s breaks, a n d was in the vicinity o f the l o c a t i o n requiring retirement. T h e g o o d segment c o n t a i n e d
Failure analysis of a mine hoist rope
(a)
27
(b)
(c) Fig. 2. (a) Fatigue crack at crown wear site. Crown wear appears on the outside wires of a rope where it is in contact with sheaves, drums or other external members. (b) Fatigue crack at nick. Nicks result from contact between wires. Large, prominent nicks are found where a wire in one strand has contacted a wire in an adjacent strand. (c) Contact between adjacent strands usually involvesa number of wires, and produces a recognizable pattern of nicks called a trellis pattern. Nicks are indicated by white arrows. The three nicks indicated form one trellis pattern.
no breaks, and came from the dead wrap on the drum (Fig. 4). During service, the bad segment of the rope experienced cyclic bending stresses from both the head sheave and the drum, as well as varying tensile stress from the weight of the rope and the counterweight. The good segment of the rope experienced some tensile stress from the weight of the rope and the counterweight. All wires were disassembled and labeled by rope segment, layer (outside strands = layer 1 to core = layer 5), strand, and wire position, as shown in Fig. 5. Broken wires were only found in the bad rope segment. In this segment, all breaks were contained within two outside layer strands, and one strand from the third layer. These three strands also contained the construction anomaly referred to as a "dive". These were named dives because, while visually following outside wires around a strand, it was noticed that a wire would occasionally "dive" into the interior of the strand, and could no longer be followed visually. A different wire would come out of the interior of the strand, and take the position of the outside wire that disappeared. It was later determined that, beyond the axial location of the dive, the king wire from before the dive functioned as an outside wire, and the outside wire from before the dive functioned as a king wire. One such dive is shown in Fig. 6. Additionally, one strand from the good rope segment contained a dive, but no associated wire break. Although only the strands that contained dives contained wire breaks, not all dives had a corresponding wire break, nor were all wire breaks found at a dive. During a dive, a king wire and an outside wire physically change position within the strand. This presented a difficulty in labeling the wires and performing statistical comparisons. For labeling purposes, king wires were initially identified by strand position referenced to one end of the rope segment. However, m a n y of the planned evaluations were based on groupings of nominally identical wires. Therefore, for analyses, both king and outside wires were determined solely by diameter. Implicit in these analyses is that the conclusions pertain to a strand that does not contain dives. The chemistry of all groups of wires in the rope segments were examined. Since the alloying composition of a wire can have a large affect on the mechanical response, (1) the alloying composition of broken and unbroken wires in the bad segment were evaluated for significant differences, and (2) the overall alloying composition of each individual layer was evaluated. The alloying composition of the good segment was assumed to be identical to that of the bad segment: therefore, chemical analyses of the wires from the good segment were not performed. However, since king wires do not have the same diameter as the outside wires, there is no reason to expect that they are from the
28
K. SCHREMS and D. MACLAREN
(a)
(b) Fig. 3. (a) Abrasive wear is commonlyseen as the principal mode of damage in wires that are exposed to external surfaces. (b) Fretting results from the relative motion between wires, such as is seen at nicks. Fracture craters due to delamination are present. Some material has been extruded from the area of contact.
same steel heat. Therefore, t,he chemistry of the king wires was evaluated separately from the outside wires. The following elements were determined: carbon, sulfur, silicon, phosphorus, manganese, chromium and nickel. Carbon and sulfur were determined by gas analysis, and silicon, manganese, phosphorus and chromium by wet chemistry methods. In order to obtain a statistical representation of the alloying composition, multiple samples from each layer were analyzed. The results are reported in Table 1. Torsion tests were performed according to the American Petroleum Institute (API) Specification for Wire Rope [3]. Table 2 lists the requirements for the minimum number of torsions (i.e. the number of twists to failure) to be attained by wires made out of electric furnace steel after fabrication into wire rope. In addition to the API requirements for minimum torsions, Table 2 also lists the average number of experimentally determined torsions for wires in the good segment and the bad segment, and for broken wires in the bad segment (XGOOD,XBAD,XBROKEN,respectively). A column labeled 0.3J(GOODis included as a comparison to a practice by the Ontario Ministry of Labour. The Ontario Ministry of Labour tests ropes before they are put into service to determine initial reference values, and subsequently tests periodic cutoffs. They recommend caution when the number of
Failure analysis of a mine hoist rope
29
Headframe
Double drum hoist
/ E L 10425'
cage Location of "good" segment drum dead wraps
Location of "bad" section Production level EL 7700'
1000'<
Haulage level EL 7500'
(1
Counterweight Fig. 4. Schematic of hoisting operation. The rope in question came from the counterweight of a double drum hoist. The segment requiring removal was at the location marked "bad" section. The comparison sample ("good" segment) came from the dead wraps on the drum.
torsions d r o p s to 30% o f the initial reference value, a n d r e c o m m e n d t h a t ropes be retired w h e n the n u m b e r o f t o r s i o n s d r o p s b e l o w 15% [4]. Tensile tests were p e r f o r m e d a c c o r d i n g to the A P I Specification for W i r e R o p e [3]. T a b l e 3 lists the r e q u i r e m e n t s for the a v e r a g e m i n i m u m b r e a k i n g strength to be a t t a i n e d b y wires m a d e o u t o f electric furnace steel after f a b r i c a t i o n into wire rope. I n a d d i t i o n to the A P I r e q u i r e m e n t s for m i n i m u m tensile strength, T a b l e 3 also lists the e x p e r i m e n t a l l y d e t e r m i n e d average b r e a k i n g strength for wires in the g o o d segment, the b a d segment, a n d b r o k e n wires in the b a d segment ()?GOOD, XBAD, "~BROKEN, respectively). M e t a l l o g r a p h i c a n d f r a c t o g r a p h i c investigations were c a r r i e d o u t in o r d e r to identify the cause(s) o f failure o f i n d i v i d u a l wires. F o r the m e t a l l o g r a p h i c investigation, one o u t s i d e a n d one king wire f r o m each s t r a n d layer o f b o t h the g o o d a n d the b a d segments were e v a l u a t e d in the transverse d i r e c t i o n at sites o f general wear, c r o w n wear, a n d nicks b e t w e e n a d j a c e n t strands. These s a m p l e s
13
L~
Layer 3
L~
Strand 2
w~ i ~ jwir~ 2 wi~ 7 ~ )
~
Wire 3
Wire 6 /Wire 5
"Wire 4
Fig. 5. Construction ofretired hoist rope. Wires in the rope were examined as a function oflayer (outside=layer 1, core = layer 5) and strand position (wires 1~5 = outside wires, wire 7 = king wire). Strands in each layer were labeled in a clockwise relationship to strand 1, an arbitrarily chosen reference strand. For illustration purposes, the two strands from layer 1 that contained wire breaks (strands 2 and 13) are shown.
30
K. S C H R E M S and D. M A C L A R E N
Fig. 6. Appearance of a dive. A dive is a location where an outside wire and a king wire switch positions in the strand structure. White arrows indicate positions where an outside wire moves into the interior of the strand to assume the function of a king wire.
Table I. Steel composition of bad rope segment: results are the average of a random selection of wires from all strands within a given layer, with outside wires and king wires evaluated separately, and are reported in wt %
Layer 1
Wire position Outside Broken King Broken Outside King Outside King Broken Outside King Outside King
2 3
4 5
Carbon (%)
Sulfur (%)
Silicon (%)
Phosphorus (%)
Manganese (%)
Chromium (%)
Nickel (%)
0.746 0.753 0.738 0.738 0.743 0.738 0.742 0.735 0.730 0.723 0.703 0.704 0.754
0.0171 0.0171 0.0254 0.0259 0.0168 0.0251 0.0169 0.0258 0.0246 0.0196 0.0215 0.0219 0.0189
0.27 0.26 0.24 0.24 0.25 0.22 0.26 0.23 0.24 0.24 0.23 0.23 0.27
0.012 0.012 0.013 0.012 0.013 0.014 0.012 0.014 0.014 0.016 0.027 0.026 0.022
0.68 0.68 0.76 0.75 0.68 0.76 0.68 0.76 0.75 0.70 0.70 0.69 0.66
0.048 0.051 0.038 0.036 0.047 0.039 0.048 0.038 0.038 0.054 0.069 0.068 0.052
0.029 0.028 0.038 0.037 0.028 0.037 0.029 0.037 0.037 0.031 0.031 0.032 0.028
Table 2. Torsion results for wires removed from retired rope: APt requirements are for wires removed from a newly fabricated rope. Ontario Ministry of Labour recommendations (0.3J?GcXm) are for wires removed from a used rope Layer l 2 3 4 5
Wire position Outside King Outside King Outside King Outside King Outside King
API requirements 12 I1 12 11 12 1I 41 34 34 31
0.3.J(GOOD* 7.5 7.1 8.4 7.5 8.3 6.9 23.0 19.7 21.0 16.8
X(~OOI)
~BAD
"J(BROKEN
25.1 23.7 27.9 25.0 27.8 23.0 76.8 65.7 + 70.0 56.0
19.8 24.3 20.7 2t .3 22.9 23.3 40.9 58.3* 52.0 46.0
17.3 26.5 N/A? N/A N/A 21.00 N/A N/A N/A N/A
*Since an initial reference value is not available, it is assumed that the torsions in the good rope segment have not changed, and can be substituted for torsions from a newly fabricated rope. ?Not applicable. +Analysis includes results not acceptable by API specifications, e.g. those that failed too close to the grips.
Failure analysis of a mine hoist rope
31
Table 3. Average breaking strength for wires removed from retired rope: listed API requirements are for the average minimum breaking strength to be attained by wires produced from electric furnace steel after fabrication into wire rope Layer
Wire position
API requirements (kN)*
"(GOOD
"ITBAD
1
Outside King Outside King Outside King Outside King Outside King
11.216 12.153 11.216 12.153 11.216 12.153 1.920 2.527 2.527 2.999
10.92 11.84 10.95 11.97 10.62 11.86 1.530 2.341 2.336* 2.999
10.84 12.24 10.78 12.50 10.89 12.34++ 1.511 2.382++ 2.338 2.981 :~
2 3 4 5
-t~aROKEN 9.85 12.49 N/A•" N/A N/A 12.40++ N/A N/A N/A N/A
*Requirements listed are for the nominal wire diameter of each layer and position. Values are interpolated from tables in [3]. tNot applicable. ++Analysisincludes results not acceptable by API specifications, e.g. those that failed too close to the grips.
were e v a l u a t e d for d e c a r b u r i z a t i o n , cracks, martensite, a n d the a p p e a r a n c e o f the wear scar. In a d d i t i o n , wires involved with dives were also evaluated.
4. E X P E R I M E N T A L
RESULTS
AND DISCUSSION
4.1. Rope construction T h e c o n s t r u c t i o n a n o m a l i e s were n a m e d dives, a n d are l o c a t i o n s where a n o u t s i d e wire a n d a king wire switch p o s i t i o n in the s t r a n d structure. A l o n g the s t r a n d axis, the i n t e r c h a n g e o f the two wires will t a k e place over a length o f several centimeters, a n d results in a larger t h a n n o r m a l s t r a n d d i a m e t e r (Fig. 7). U n u s u a l a n d u n e x p e c t e d wear a n d / o r d e f o r m a t i o n will t a k e place between the wires within the strand. In s o m e cases, as also s h o w n in Fig. 7, this is o b s e r v e d as deep nicks (gouges). I n o t h e r cases, the result will be extensive flattening o f the wire surface a n d / o r excessive c r o w n w e a r (Fig. 8), with the a m o u n t o f c r o w n wear increasing as the p r o x i m i t y to the dive increases. In a d d i t i o n to the w e a r a n d d e f o r m a t i o n , the i n t e r c h a n g e o f wires, especially o f different diameters, will alter the l o a d d i s t r i b u t i o n in the strand. D u r i n g disassembly, 12 dives were identified in three different s t r a n d s in the b a d segment. A l l wire b r e a k s were f o u n d in these three different s t r a n d s (Figs 9-11), a n d were often l o c a t e d at o r between dives. In c o n t r a s t , o n l y one dive was f o u n d in the g o o d segment (Fig. 12), a n d there was no a s s o c i a t e d wire b r e a k . I n wire r o p e design, king wires t y p i c a l l y have a larger d i a m e t e r t h a n o u t s i d e wires. Overall, the king wires (wire 7) h a d a d i a m e t e r o f a p p r o x i m a t e l y 2.95 m m , in c o m p a r i s o n with
Fig. 7. Dive from layer 1, strand 2. The strand diameter at the location of the dive is larger than elsewhere, as is shown by the two white arrows. The gouge produced as a result of this expanded diameter is shown by the black arrow.
32
K. SCHREMS and D. MACLAREN
'
?'
crown wear Fig. 8. Dive from layer 1, strand 13. A total of four wire fractures are visible, and two are matching fractures. The location where the king wire switches position and becomes an outside wire is at the location marked "'dive." This wire shows extensive flattening and crown wear just prior to its fracture location. The two gray arrows point out differences in the severity of the crown wear. As the location of the dive is approached, the crown wear of the surrounding wires becomes more severe.
Wire 1
~+~L 2.82 2.82
Wire 2
+
2.84
, 2.84
Wire 3
+
,
2.95
, 2.82
Wire 4
2.82
.1.
7.82
2.82
T 2.82
2.82
Wire 5 2.82
Wire6 2.82
Wire 7 2.82
2.82
Layerl, Strand 2
....
--
-
J
-
Fig. 9. Wire diameters and location of wire breaks for layer 1, strand 2, bad rope segment. The shaded portion of the wire has a significantly larger diameter. The bottom illustration shows the relationship of dives and wire breaks in the assembled strand. (Dimensions are in mm.)
2.82 m m for the o u t s i d e wires. In a t t e m p t i n g to g r o u p like wires/sizes, it w a s f o u n d that a c o n t i n u o u s k i n g wire c o u l d n o t be identified in strands 2 a n d 13 f r o m layer 1 as illustrated in Figs 9 a n d 10. This, in a d d i t i o n to the difficulty u n w i n d i n g strands c o n t a i n i n g dives, suggests that the a n o m a l y w a s n o t created d u r i n g service.
33
Failure analysis of a mine hoist rope
t
Wire 1
t
ii
2.84
t
ii
2.82
t t
Wire 2
2.82
t ,
ii
2.84
t
,j_., t
t
-f2.82
2.84
,
Wire 4
t
+
,' 2.82
Wire 6
,
,
÷
t
I
2.95
strand13
'
2.79
+, 2,82
i
t
,+ ,' + , + t 2.82
2.92
2.84
2.82
2.84
Wire 7
|
2.82
Wire 3
Wire 5
i
2.82
2.82
2.82
2.82
t t
i
2.$4
n
o
t
¢
Fig. 10. Wirc diameters and location of wire breaks for layer 1, strand 13, bad rope segment. The shaded portion of the wire has a significantly larger diameter. The bottom illustration shows the relationship of dives and wire breaks in the assembled strand. (Dimensions are in mm.)
Layer 3, Strand 3
, ~, t
Fig. 11. Location of dive and break in layer 3, strand 3, bad rope segment.
Layer 1, Strand 7 Good Rope Segment Fig. 12. Location of dive in layer I, strand 7, comparison segment. This was the only dive found in thc comparison scgmcnt. There were no associated wire breaks. -
4.2. Chemical analysis Wire ropes used in the United States are not required to meet alloying standards. However, the API does require that the wire be produced from: (1) acid or basic open-hearth, (2) basic oxygen, or (3) electric furnace steelmaking processes; and that the wire so produced meets certain mechanical property specifications, e.g. breaking strength and torsional requirements, dependent upon the steelmaking process used. Breaking strength and torsional requirements are highly dependent on alloying composition, and API-acceptable results have been developed for each of the three different steelmaking processes. Therefore, the type of steelmaking process needs to be determined for later comparisons with API specifications. The chemistries of the wires in Table 1 are typical of an electric furnace steel [5]. Residual alloying elements (manganese, chromium and nickel) and impurities (sulfur and phosphorus) are generally higher in electric furnace steel than in open-hearth or basic oxygen steel. In general, higher levels of
34
K. SCHREMS and D. MACLAREN
alloying elements result in lower ductility and higher strength. This is reflected in the API specifications, where the electric furnace steel has the highest requirement for tensile strength and the lowest for torsion. The steel used in this rope was considered to be produced in an electric furnace. This was later verified by the rope manufacturer. A multivariate analysis of the chemical analysis data was performed to determine if differences in chemical composition exist between the different layers of wires. The analysis revealed that there is a significant difference in the chemistries between the first three layers and the two layers that comprise the independent wire rope core. The results can be summarized as follows: (1) Layers 1 3 contain wires with very similar composition, and are probably obtained from the same heat of steel. Furthermore, the outside wires from layers I-3 were obtained from one heat of steel, and the king wires from the same layers were obtained from another heat. It should be noted that layer 2 appears to be produced from the same steel heats as layers 1 and 3, yet contains no wire breaks of either king or outside wires. (2) Layers 4 and 5 have significantly different composition from the first three layers, and are probably not obtained from the same heat as layers 1 3. (3) Layers 4 and 5 differ significantly from each other, and probably do not come from the same heat. Again, the king wires appear to be from a different heat than the outside wires. In all, it appears that there are six distinct heats of steel represented in this rope. Independent of the type of steelmaking process used, one of the primary questions to be addressed is whether the wire material itself is responsible for premature failure of the wires. As can be seen in Table 1, the chemical analyses of broken and unbroken wires in the bad rope segment are very similar. A multivariate analysis of variance shows no significant difference between the chemistries, with the possible exception of the nickel content of the layer 1 king wires. Although the statistical analysis identifies the nickel content as being significantly different, from a practical standpoint the difference is not great enough to affect the behavior of the material. It appears that the steel used for this wire rope came from six distinct heats from an electric furnace. All of the broken wires were found in layers 1 and 3, which would comprise only two of six distinct steelmaking heats identified. No significant difference was found between the broken and unbroken wires. It can, therefore, be concluded that it is highly unlikely that the overall chemistry of the wires was responsible for the wire breaks.
4.3. Torsion results For the rope segments examined, the number of torsions may reasonably be expected to be lower than those in the API specifications, due to fatigue and wear degradation during service. However, as can be seen from Table 2, the torsions generally met or exceeded the AP! specifications. It should be noted that the API specification evaluates the individual torsions, not the averages, whereas Table 2 lists the averages. When individual torsion results are evaluated, more than 90% of outside wires from layer 1 (both segments) and 50% of the outside wires from layer 4 (bad segment) meet this criteria. Since torsion results are highly dependent on surface imperfections such as crown wear and trellis patterns (wear produced as a result of contact between the outside wires in adjacent strands), lower torsion results for these two wire groups are not unusual. As can be seen from Table 2, there does not appear to be a difference in torsions between broken and unbroken wires. The torsion results of the outside wires of the two segments differ significantly, whereas the torsion results of the king wires do not. It should be noted that all of the layers in the bad segment contained readily identified nicks, whereas the good segment showed little or none. On the other hand, the king wires only showed uniform line contact. These findings agree well with the observed torsion results. The king wires experienced little or no detrimental wear, and, for these wires, differences between the segments are not observed. Layer 1 of both segments experienced crown wear, and these wires show a reduction in the number of torsions. The effects of wear on the torsion results can be summarized as follows: (1) The outside wires of a strand experience crown wear and/or trellis contact, a cumulative process that affects the surface quality of the wire and lowers the overall torsions.
Failure analysis of a mine hoist rope
35
(2) King wires do not experience crown wear or trellis contact, and no appreciable difference in the torsion values was seen. The torsion test is geared towards testing ductility. The API specifications provide for a minimum ductility to be present in a newly fabricated rope. On the other hand, the Ontario Ministry of Labour recommendations provide guidelines for removing a rope based on a decrease in ductility. The initial ductility is primarily a function of the steel chemistry and the wire drawing process. After a rope is put into service, the ductility will change as a function of fatigue and wear. As can be seen, the ductility is similar to that required for a newly fabricated rope, and far exceeds the Ontario Ministry of Labour guidelines for removal. Therefore, it appears that the ductility of the wires in both the good and the bad segments is sufficient. All the torsion results met or exceeded the API requirements for wires removed from newly fabricated wire rope, even though the wires were removed from a used rope. The number of torsions of wires removed from the good rope segment was significantly greater than for those removed from the bad rope segment, but was limited to the outside wires of a strand. These differences are most likely a result of crown wear and/or trellis patterns. There was virtually no difference in the number of torsions between broken and unbroken wires. Therefore, it is highly unlikely that the wires failed due to poor ductility.
4.4. Tensile results The breaking strengths listed in Table 3 generally do not meet the minimum requirements for wires produced from electric furnace steel. Breaking strength requirements should more accurately be called breaking load requirements because the requirements list the minimum load-carrying ability of a given diameter wire. Minimum breaking loads will depend on the cross-sectional area as well as the material property known as the ultimate tensile strength. When crown wear and trellis patterns are present, the cross-sectional area is reduced. The result is that the wire will generally break at these sites of reduced cross section. However, the API specifications are for wires removed from newly fabricated ropes, presumably with uniform cross sections. It would be reasonable to find lower strengths in wires from a rope removed from service, e.g. a rope with crown wear and trellis contact. This effect is probably responsible for the drop in strength seen in the broken outside wires from layer 1. Dives create "high" wires (analogous to high strands) which experience more material removal from crown wear and deeper nicks from trellis contact. The differences in breaking load between the king wires in the good and bad segments for the first three layers was found to be highly significant. An effect similar to this has been seen by the Bureau of Mines Pittsburgh Research Center when testing rope segments [6]. They have noted that the rope breaking strength may initially increase, and then drop off significantly as the rope approaches the end of its useful life. For ropes, the initial increase in breaking strength is generally attributed to "break-in" and the flattening of contact sites between wires. Although there is a significant difference between the breaking loads for the wires in the good and bad segments, there is not a significant difference between the breaking loads for the broken and unbroken wires in the bad segment. Although interesting, the increase in strength of the wires does not appear to be related to wire failure. The tensile test is used to evaluate the minimum strength of the wires. Wires that are not of the minimum strength run the risk of being overloaded during normal use, and will also have shorter fatigue lives. Signs of overloading, such as ductile cup-cone failures, were not seen. The average strength of the wires is generally acceptable when compared with the strengths listed for other steelmaking processes. With the exception of results for the broken wires in the outside layer (which appears to be related to a loss of metallic area), there is no significant difference in the broken and unbroken wires in the bad segment. The average breaking strength of the wires removed from both the good and bad segments failed to meet the minimum API specifications for wires made out of electric furnace steel. With a few exceptions, the average breaking strengths exceeded those required for basic oxygen steel. The loss of strength of broken outside wires from layer 1 is attributed to the presence of crown wear and trellis patterns. King wires showed an unexplained increase in strength between the good segment and the bad segment, but there was no significant difference in strength between the broken and
36
K. SCHREMS and D. MACLAREN
unbroken king wires. It seems unlikely that inadequate tensile strength is responsible for the wire breaks. 4.5. Metallography Decarburization was observed on wires from layers 1-3. The full depth of decarburization was approximately 15-20/tm, or a little over 1% of the diameter of the wire. Although decarburization detrimentally affects fatigue, the effect is much smaller in magnitude than that of a surface blemish such as crown wear or trellis contact. The amount measured should not have a noticeable effect on the fatigue life of the rope [7]. Two types of cracks were observed in the metallographic samples. The first type of crack was radial, less than 50 pm, and generally emanated from a surface pit. The pits and cracks appear to follow the incursion of decarburization into the wire. This is not unexpected since the ferrite resulting from the decarburization will pit preferentially during the pickling process. Since the cracks appear in almost all of the samples from layers 1 3, regardless of location, it is highly unlikely that the radial cracks account for the differences in fracture behavior between the good and bad segments. The second type of crack propagated parallel to the surface of the wire, and was located at both crown wear and nick sites. The size and appearance of this type of crack took a variety of forms. At crown wear scars, various degrees of abrasion were found, many samples having wedge formation and heavy plastic deformation. Some cracks were seen separating the wedge from the main body of the wire. Smaller cracks often appeared in the middle of the crown wear site, and separated the lip of material left after abrasion from the base metal itself. Unlike the abrasion found at the crown wear sites, the wear mechanism present at the nick sites tended to be sliding wear or fretting. The sliding wear produced considerable deformation in the pearlitic structure, while fretting tended to remove material by spalling. In all cases, the cracks appeared roughly parallel to the surface, and propagation of the cracks produced a metallic flake as opposed to a transverse fatigue crack. These mechanisms, which are what would be expected given the nature of the interactions within the rope and with its environment, are the same as found at crown wear and nick sites in other ropes [2]. It is highly unlikely that these mechanisms contributed to premature removal of the rope. Martensite, a suspected nucleation site for fatigue cracks, was not readily identified. Using a nital etchant, only small, thin areas were identified as martensite. It should be noted that there is some question as to whether the "white-etching layer" observed in wear of ferrous materials is actually martensite. A thin white layer, less than 15/tm, was observed at some nick and crown wear sites in both segments. At some locations, surface cracks that turned and followed the boundary of the white layer were visible. It is felt that the cracks associated with the white layer led to spalling, and are not responsible for the premature removal of the rope. A wide disparity of inclusions was noticed on initial examination. In relation to each other, these inclusions ranged from minimal in size and number to large in size. Since inclusions can reduce the load-carrying capability of a wire as well as decrease its fatigue performance, the number and size of inclusions was investigated. The Making, Shapin9 and Treatin9 of Steel mentions a cleanliness rating of 0.1~).3 vol % inclusions in the range of 10-50 ~m in diameter for continuous casting [8]. Of 69 transverse samples, only 12 had inclusions that were in that range, and these were typically ! 025 #m. In the longitudinal direction, the cleanliness rating ranged from < 0.1 vol % to approximately 0.75%. (It should be noted that all inclusions are included in this value, not just those greater than 10/~m. In addition, only one field was used, leading to a large statistical error.) These observations suggest that, although large inclusions are present, they are within the range expected in this type of steel. Large inclusions and/or clusters of inclusions were found in both the good and the bad segments, as well as at general wear sites, crown wear, and trellis sites. Therefore, it does not appear that there is a correlation between inclusion content and/or size and rope segment, layer, wire, or wear type. Extensive metallographic analyses were performed on wires removed from the two rope segments. Decarburization was found on all outside wires from the first three layers, but was uniform between the segment in question and the comparison. Two types of cracks were observed in the samples. The first type was a radial crack that appeared in relation to the decarburization, and was observed the length of the wire. The second type of crack occurred at wear sites on both the good and bad segments, and on both the broken and unbroken wires, and was observed to propagate parallel to
Failure analysis of a mine hoist rope
37
the wire surface. This type of crack most likely contributed to spalling and material removal at the wear site as opposed to initiating a fatigue crack in and of itself. An extensive search was undertaken to locate the appearance o f martensite at wear surfaces. The search revealed small locations that could possibly be martensite, but the depth is much less than has been reported in the literature. All anomalies noticed during the metallographic analysis appeared in both segments without regard to broken or unbroken wires. Therefore, it is highly unlikely that microstructural anomalies were responsible for the wire breaks. 4.6.
Fracto#raphy
A fractographic study of 12 fracture locations including five matching fractures was performed to identify characteristics of: (1) fracture location, and (2) crack initiation. O f the 12 fracture locations, nine were identified as occurring at crown wear sites, two as occurring at trellis sites, and one was too obliterated to determine its location. The relationship of the wire failures to dives was also sought. The movement of wires within a strand creates faint wear patterns between wires that are in contact, allowing the placement of adjacent wires to be identified. Adjacent wires within a strand follow a helical path around the king wire, and, although the wires twist, the patterns around the wires are evenly spaced, and continuous between crown wear patterns. The disruption of these wear patterns was considered as evidence of a dive (a location where two wires switch position in the strand). As such, nine of the fractures were positively identified as being in the location of a dive. Most of the wire fractures had two identifiable crack initiation sites. For the failures occurring at crown wear sites, initiation often occurred at the actual crown wear site, located either at the center or at a corner. There was usually another failure site located opposite the crown wear site at the location of the faint wear patterns. Often this wear pattern was of the wire associated with the dive. For the two failures that occurred at trellis sites, the wear associated with the dives was closely associated with one of the nicks. The amount of deformation present as well as the remaining material was evaluated. Some of the crown wear fractures had one-third to one-half of the wire cross section removed by abrasion. Others experienced considerable flattening in addition to the loss of cross-sectional area. The trellis nicks associated with the wire fractures were also (qualitatively) larger than the nicks seen elsewhere on the ropes. Therefore, it is felt that the effect of the wire dives on the wire fractures was twofold. First, the dives disrupted the close-packed structure of the strand, and produced locations where certain wires were "high". If the high wires were on the outside of the rope, the result was increased wear and/or compression of the wire. If the high wires were in the interior of the rope, the result was deeper than normal nicks on adjacent strands. Second, the wear characteristics and stress distribution were altered in the presence of a dive. In m a n y cases, this contributed to either primary or secondary crack initiation. In summary, fractography was performed on wire breaks removed from the rope. O f the fractures that were not obliterated, most could be identified as being in the location of a dive by wear patterns. The process by which the wires failed varied. Clearly, some of the wires had excessive crown wear, and failed in relation to a loss of metallic area. Fractures at crown wear sites associated with dives had as much as one-third to one-half of the cross-sectional area removed. The interchange of wires at a dive produced unusual wear patterns with adjacent wires, and these wear patterns often appeared as an initiation site of the fatigue crack. Fractures at trellis sites experienced deeper nicks than were seen elsewhere in the rope. In one case, a large gouge caused by an adjacent wire (larger than that shown in Fig. 7) was identified as initiating the wire break. Although a variety of failure mechanisms were present, the c o m m o n factor was the proximity of a dive. The dive either accentuated a normally occurring mechanism (crown wear) or was the cause of an unforeseen mechanism (gouging).
5. C O N C L U S I O N S A series of experiments was designed: (1) to compare the broken wires in the bad segment with the unbroken wires in the bad segment, and (2) to compare the wires in the bad segment with those
38
K. SCHREMS and D. MACLAREN
in the comparison segment. Chemical analyses, tensile tests, torsion tests, metallography, and fracture analysis have all been performed. It was determined that, although six different steelmaking heats were identified, there was no correlation with broken and unbroken wires. A significant difference was found in the torsions between the good and the bad segments (attributed to in-service surface wear), but not between the broken and unbroken wires. The results all met or exceeded the API requirements for wires removed from a new rope, so it is highly unlikely that the wires failed due to poor ductility. The tensile results exceeded those for new wires made from basic oxygen steel. Again, a significant difference between the broken wires and the unbroken wires was not observed, so it seems unlikely that inadequate tensile strength is responsible for the wire breaks. Inclusions, decarburization, cracks and martensite were all evaluated. All anomalies noticed during the metallographic analysis appeared in both segments without regard to broken or unbroken wires. It is highly doubtful that the metallurgical structure was responsible for the wire breaks. Differences in the chemical analyses, tensile tests, torsion tests, and metallography do not correlate with the presence or absence of broken wires. The construction anomaly called a "dive" was found to be related to the presence of broken wires. The three strands in the bad segment that had wire breaks were also the three strands that contained dives. One strand in the good segment contained a single dive, but no wire break. Dives produce a larger than normal strand diameter in the local vicinity, which results in unusual and unexpected wear and/or deformation. Fractography of the wire breaks showed the breaks to be associated with gouges, flattening, or severe crown wear, all of which can be attributed to the disruption of the strand structure produced by a dive. It appears that the wear and/or deformation caused by the presence of the dive accounts for the wire breaks. The number and distribution of wire breaks were, in turn, responsible for the rope being retired prematurely.
REFERENCES 1. U.S. Code of Federal Regulations. Title 30 Mineral Resources; Chapter l--Mine Safety and Health Administration, Department of Labor; Subchapter N--Metal and Nonmetallic Mine Safety;Part 56, Subpart R, and Part 57, Subpart R: Subchapter O~Coal Mine Safety and Health; Part 75, Subpart O, and Part 77, Subpart O; 1 July 1989. 2. Sehrems, K. K., Do,an, C. P. and Hawk, J. A., Journal o["Materials Engineerin 9 and PerJormance, 1995,4, 136. 3. AmericanPetroleum Institute, Spec!fication/br Wire Rope. APISpec(fication 9A (Spec 9A), 23rd edn. Washington, DC, 1984. 4. Djivre, M., Mine ShaJ~ Ropes: Ontario Destructit~e Wire Testin,q Program. Ontario Ministry of Labour, Sudbury, Ontario, 16 January 1991, p. 5. 5. Dove, A. B., Ferrous Wire. Vol. I: The Manu/~tcture ~fFerrous Wire. The Wire Association International, Guilford, CT, 1989. 6. Miscoe,A. J., Private communication, 1990. 7. Mayer, M., Private communication, 1995. 8. Lankford, W. T., Jr., Samways,N. L.. Craven, R. F. and McGannon, H. E., eds, The Making, Shaping, and Treating o[ Steel, 10th edn. Associationof Iron and Steel Engineers, Pittsburgh, PA, 1985.