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Near threshold fatigue behaviour of flake graphite cast irons microstructures
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Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
Near threshold fatigue behaviour of flake graphite cast irons microstructures J.H. Bulloch * Electricity Supply Board, Head Off'we, Dublin 2, Ireland
Abstract The present paper attempts to describe a study concerning the influence of mean stress or R-ratio and mierostruetural variation, percentage polygonal ferrite, on the near threshold fatigue crack extension characteristics of a flake graphite cast iron. Basically significant effects of R-ratio and microstructure on the fatigue threshold stress intensity range, AKth , were observed. Also at intermediate AK levels the flake cast iron microstrueture exh~ited a much higher slope in the two-parameter fatigue crack growth relationship than those observed in ductile low alloy steels. It was also observed that at low R-ratio the AKth values of the flake cast irons were significantly larger than those recorded for iron-based and low alloy steel materials. The high levels of AKth were explained in terms of roughness induced crack closure and crack deflection processes which resulted from the very rough topography of the fatigue fracture surfaces. Finally the role played by the graphite constituent in the fatigue fracture processes prevalent in cast iron microstructures was discussed.
1. Introduction For many years the transport industry, and in particular the railways, have b e e n foremost in utilising many engineering c o m p o n e n t s fabricated from cast irons. A significant n u m b e r of these components are used under cyclic environmental service conditions which can involve mechanical loading, thermal cycling or a combination of both. Cast irons can be classified in terms of the general graphite morphology, viz., flake, compacted flake and spheroidal.
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For over a half a century it has been generally accepted that the mechanisms of failure in flake cast irons, both in the as-cast and annealed condition, are dictated primarily by the brittle characteristics of the graphite flakes. As a result of its heterogeneous microstructure flake cast irons exhibit non-linear behaviour and as such the conventional variables which are usually used for assessing the tensile characteristics of homogeneous materials do not apply. Indeed, it was suggested [1] that microcracking occurs under low tensile stresses which causes a progressive alteration of the elastic modulus and, as such, no tangible elastic limit can be measured. U n d e r tensile loading conditions failure occurs in a discontinuous fashion which is controlled by numer-
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66
J.H. Bulloch / Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
ous microstructural factors and hence the elongation or tensile strength cannot be taken as real measurements of the flake cast iron's tensile properties. Upon initial inspection it would appear that the fatigue characteristics of flake graphite cast iron are similar to that portrayed by low alloy steels inasmuch that definite fatigue limits were observed in both materials. Indeed the endurance ratio, which is the ratio of the fatigue limit to tensile or ultimate tensile strength, of a number of steels is around 0.4 to 0.5 which agrees well with that value recorded for pearlitic flake cast irons. However, as a result of the heterogeneous nature of flake cast irons, the tensile strength is not a measure of the stress required for post-yield instability, but rather the brittle failure stress with is controlled by the fracture toughness, and the dimensions of the graphite flakes which act as crack initiation locations [2,3]. It has been reported [4] that the tensile strength was typically 30% of the yield strength measured in compression. As a direct consequence, if the endurance ratio is considered in terms of the yield strength, it was typically 30% of the yield strength measured in compression. As a direct consequence, if the endurance ratio is considered in terms of the yield strength, flake graphite cast irons exhibit inordinately low fatigue resistance. Another important difference between steels and flake cast irons is the fact that, while significant strength increased in the latter can be attained by quench and tempering, little effect on the fatigue strength of flake cast irons was observed [4]. The present study was aimed at assessing the influence of microstructure and specifically the amount of ferrite present in the microstructure on the mechanical and threshold fatigue properties of flake graphite cast irons.
2. Experimental techniques The average bulk chemical composition of the graphite flake cast iron can be found in Table 1. The graphite flake iron in the as-cast condition exhibited a microstructure which was 100%
Table 1 Chemical composition of graphite flake cast iron %C
%Si
%Mn
%S
%P
3.28
2.14
0.45
0.049
0.038
pearlite. It has been demonstrated [6] that the heat treatment of flake cast irons resulted in the breakdown of the pearlite and deposition of graphite on existing graphite flakes. In an effort to vary the amounts of pearlite and polygonal ferrite phases present in the flake cast irons, segments of the casting were subjected to prolonged heat treatment schedules either below or above the critical temperature. In the present study segments of the casting were held at 800°C for various times (in the range 5 to 20 min) and then furnace cooled after holding at 690°C for 1 hour. These treatments produced mixed ferritepearlite microstructures. The fully ferritic graphite flake cast iron microstructure was obtained by a normal or standard commercial heat treatment, viz., (i) a 4 hour hold at 900°C (ii) a furnace cool from 900 to 690°C, (iii) a 8 hour hold at 690°C and (iv) a furnace cool from 690°C to ambient temperatures. These particular heat treatments resulted in the polygonal ferrite phase being preferentiaUy nucleated adjacent to graphite flakes. The mechanical property test data and details of the metallographic parameters are listed in Table 2. Fatigue crack propagation tests were conducted using precracked standard Compact Tension test specimens with thickness 20 mm and width 40 mm. Threshold stress intensity range, AKth, measurements were carried out on an Amster vibraphone fatigue machine at frequencies in the range of 90 to 120 Hz. Three separate R-ratio values of 0.05 0.3 and 0.7 were studied, and all tests were conducted in air under ambient conditions. Note that R = min. load/max, load. A conventional load shedding technique was adopted for all fatigue threshold assessment tests. Fatigue crack growth was first measured in the 10 -5 m m / c growth rate region and subsequently the loads were reduced. At each specific load level the crack growth rate was measured. At each specific load level the crack tip was allowed
J.H. Bulloch /Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
67
Table 2 Mechanical properties of graphite flake iron Microstructure
Ferrite 73F-P 16F-P Pearlite
Mechanical properties
MetaUographic details
Yield stress (MPa)
Tensile stress (MPa)
148 154 183 243
144 165 198 258
to grow a distance of four monotonic plastic zone sizes before subsequent fatigue crack extension wad deemed to be unaffected by prior plasticity effects. The fatigue crack length was monitored both optically and continuously using a DC electric potential method, capable of measurement to within 0.1/mm of absolute crack length, and of detecting changes in crack length of the order of 10 p,m. Fatigue crack growth rates were calculated from the crack length-number of fatigue cycles data using an incremental polynominal method which involved fitting a second order polynominal (parabola) to sets of seven successive data points. In this particular study the threshold stress intensity range, A K t h , w a s experimentally defined as the AK value at which a crack extension rate of 10 - 7 m m / c was recorded. Scanning electron microscopy was used in an effort to assess the fracture topography in selected test specimens. A quantitative assessment of the various fracture modes prevalent on the fatigue surfaces was performed by measuring several fractographs at various A K levels. Crack profde samples were fractographs at various AK levels. Crack profile samples were also taken in an effort to asses the crack path tortuosity or crack deflection ratio. Metallographic sections were taken and the average pearlite interlamellar spacing, and the average graphite flake size was established.
Hardness (UPN)
% ferrite
% pearlite
110
100 73 16 -
27 84 100
123
154 216
-
fraction pearlite and details of the wholly pearlitic microstructure are given in Fig. 1. The lamellar pearlite spacing of the pearlite matrix was fairly coarse, and the average pearlite spacing was assessed at 2 Ixm. Also from Fig. 1 it is evident that the graphite morphology was Type A and that the average size (length) of the graphite flakes was estimated at around 230 p,m; the relative frequency distribution of the graphite flakes is given in Fig. 2. The area fraction of the graphite flakes in all microstructures was assessed at around 13%.
3. 2. Mechanical properties The mechanical properties of the various flake graphite microstructures are given in Table 2. It is evident from this that these particular microstruetures exhibited little signs of ductility in that the yield and tensile strength levels were very close to each other. Generally increased
3. Experimental results
3.1. Metallographic details As listed in Table 2, it can be seen that four microstructures ranging from zero to unit volume
Fig. I. Details of the fully pearlitic microstructure.
J.H. Bulloch / Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
68
0.3 AVERAGE FLAKE LENGHT = 230pro
0.2 RELATIVE FREQUENCY
0.1
0
10 0
20 0
30 0
50 0
40 0
GRAPHITE FLAKE LENGHT {urn) Fig. 2. Relative frequency distribution of graphite flake size.
10 .3 --
LE GE ND • R= 0.05 - R = 0.3 • R = 0.7
10"4
FATIGUE
GROWTH (ram/c) 10 -5
&A
--
FERRITIC S T E E L REF. 7 10 .6
10.7
I 2
I 3
I 4
l 6
8
I 10
I
I
20
30
AK (MPaVm)
Fig. 3. Fatigue crack growth characteristics of the fully fcrritic microstructure at the fully ferritic microstructure at various R-ratio levels.
J.H. Bulloch /Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
69
LEGEND 1() 3,
• • •
R = 0.05 R=0.3 R=0.7
-4 10
FATIGUE CRACK 10.5 GROWTH (mrrVc)
lo-S -
10 `7 I
1
I
I
I
I
I
I
2
3
4
6
8
10
20
30
AK(MPaVm) Fig. 4. Fatigue crack growth characteristics of the 73% ferrite-pearlitic microstructure at various R-ratio levels.
LEGEND -3 10
• • •
R = 0,05 R=0,3 R=0,7
/
-4 10
FATIGUE CRACK GROWTH (ram/c)
10 "5
;_
10"6-
-7 10 -
I 2
i 3
I 4
I 6
I 8
I
I
I
10
20
30
AK (MPaVm)
Fig. 5. Fatigue crack growth characteristics of the 16% ferrite-oearlitic microstructure at various R-ratio levels.
70
J.H. Bulloch/Theoretical and Appfied FractureMechanics 24 (1995) 65-78
amounts of pearlite increased both the strength and hardness values. Indeed the introduction of pearlite caused the yield and tensile strengths to increase by around 60% and 80% respectively, while the hardness was almost doubled. 3.3. Fatigue crack extension characteristics
The fatigue crack growth behaviour of the various microstructures examined in the present investigation are exhibited in Figs. 3 to 6. In the case of the fully ferritic microstructure (Fig. 3), it can be seen that the influence of the R-ratio is significant in that decreasing the R-ratio from 0.7 to 0.3 and 0.05 increased the value of the threshold stress intensity range, AKth, by a factor of about 4 and 7 respectively. Also intermediate crack growth rates are very sensitive to the R-ratio where the exponent m of the two-parameter fatigue crack growth relation da/dN=
C(AK)"
(1)
was constant at a value around 6. The threshold fatigue characteristics of the duplex ferrite pearlite flake graphite microstructures are shown in Figs. 4 and 5. From these figures it was again evident that significant effects of the R-ratio were recorded in these microstructures with the value of AKth increasing by three times and four to six times at intermediate and low R-ratio levels respectively. Again all intermediate fatigue crack growth data, which exhibited a linear relationship between log AK and log d a / d N , exhibited a slope or exponent value of around 6. Similar marked beneficial effects of a decreasing R-ratio, in inhibiting the crack growth of long macrocracks, were evident in the case of the fully pearlitic microstructure (see Fig. 6), where A Kth levels were increased by a factor of 5. The fatigue crack extension results of all the tests conducted in this study are illustrated in Fig. 7. From this figure it can be seen that the data
LEGEND
.3 r 10
• R = 0.05 • R=0.3 A R=0.7
-4
10
FATIGUE CRACK GROWTH
10-5
(mrn/c)
PERARLITE STEEL REF. 7
~o-6 -7 10
-
I
1
1
1
I
I
I
f
2
3
4
6
8
10
20
30
AK (MPaVm) Fig. 6. Fatigue crack growth characteristics of the pearlitic microstructure at various R-ratio values.
J.H. Bulloch /Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
-3 10
--
LEGEND n FULLYFERRITIC •
16%FERRITE
A•
~z
,o-,_ FATIGUE CRACK GROWTH 10"5 (ram/c)
71
lie&
o.,x~:,,oM.s/.:X
_
10"6 _
'tr o.--o,
,o
o.
r~.
,o.,
•
z
•
°'. ! I " I I
I
I
I
2
3
4
6
I
I
8
1
l
10
30
20
&K (MPaqm) Fig. 7. Fatigue crack growth d a t a f o r various R - r a t i o a n d microstructure types.
can be separated into sets of different R-ratios with the high R-ratio results exhibiting the least amount of scatter at fatigue crack growth rates
above 10 - 6 mm/c, and approaching 10 -5 m m / c the different microstructural results tended to merge together and the intermediate fatigue crack
LEGEND • 100% FERR K~(0) = 14.5 MPaV'm o 73PAFERR Kill(0) = 12.7 MPaV'm •~ 16% FERR K~(0) = 9.7 MPaqm • 0% FERR Kth(0) = 11.3 MPaY'm
16
• Q~ , , , , , , .~
12
&
"'-,
THRESHOLD STRESS INTENSITY RANGE (MPaVm)
i 0
l
I 0.2
[ 0.4
I 0.6
I 0.8
R-RATIO Fig. 8. Influence o f R - r a t i o o n the threshold stress intensity range f o r the various microstructures.
J.H. Bulloch/ Theoreticaland Appfied FractureMechanics24 (1995) 65-78
72
propagation behaviour could be described by the following expressions;
da/dN
~,-
(mm/c)
= 6 . 1 2 x 1 0 - 1 3 ( A K ) 6"7 for
da/dN
R=0.05,
(2)
R = 0.30
(3)
(mm/c)
= 1.35 x 1 0 - t 1 ( A K ) 6"2 for and
da/dN
CRACK GROWTH DIRECTION
Ii,
(mm/c)
= 2.59 x 1 0 - 9 ( A K ) 65
I
for
R =0.70.
(4)
The average value of m in Eq. (1) was around 6.5 and this value was significantly higher than those reported for steels, typically 2 to 4, Indeed, from Figs 3 and 6, where the average data scatter for mild ferritic steels and pearlitic steels [6] are shown respectively, it is evident that for both microstructures (i) the graphite flake cast irons exhibited much higher values of AKth, (ii) the steels showed slower fatigue crack extension characteristics at intermediate AK levels and (iii) the steel results exhibited a much lower exponent value, between 2 to 4, which was typical of materials undergoing ductile fatigue type failure. The influence of the R-ratio on the threshold fatigue stress range, Agth , for the various microstructures is given in Fig. 8. From this figure it is evident that the relationship between AKth and the R-ratio for the graphite flake cast iron microstructures was not linear. However extrapolation back to R = 0 yields the values of A gth(0), these are listed in Fig. 8. Also it can be seen that the effects of R-ratio increase with decreasing R-ratio value is the greatest and least in the ferritic and pearlite microstructures respectively.
s
Fig. 9. Schematicillustration of crack deflectioneffectsi
where S is the horizontal segment and D is the deflected portion at an angle 0 of the fatigue crack profile (see Fig. 9). By careful measurement of a series of different crack profiles and assessment of the extent of crack deflection during the fatigue process in the ferritic graphite, flake cast iron microstructure could be made. In the R = 0.05 test sample the value of D / ( D + S) was measured at 0.3 while deflection angles of as much as 70° were recorded. In the case of the pearlite dominated microstructures it was difficult to identify or resolve fatigue striations; such features were commonly found however in ferritic microstructures (see Fig. 10). In numerous instances it was observed that the regions of ductile striated fatigue crack
3.4. Fractographic details The fracture topography of the fatigue failure surfaces varied from rough to very rough, especially in the fully ferritic microstructures the crack path tortuosity showed large crack deflections. The crack path tortuosity was characterised by the crack deflection ratio [7] which is given by crack defection ratio = D / ( D + S),
(5)
Fig. 10. Details of fatigue striations present in the fullyferritic microstructure.
J.H. Bulloch / Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
73
Fig. 11. Illustration of crack deflection effects. Cracks cut across a pearlite colony.
Fig. 13. General view of fatigue fracture surface showing extensive formations of graphite flakes.
growth radiated out from certain discrete graphite flakes which suggested that the flakes aided fatigue crack growth processes. In pearlitic microstructure two types of failure were recognised viz., (a) an etched type of fracture feature (see Fig. 11) occurred when the growing crack cut across a pearlite colony and (b) a cleavage type failure event, that happened when an extending crack grew between the cementiteferrite lamellar layers. When ferrite was present in the microstructures the extent of intergranular failure increased; indeed in the fully ferritic microstructure
this was the dominant failure mode (see Fig. 12). A quantitative assessment of the extent of intergranular failure as a function of the percent ferrite phase in the microstructure was attempted but no accurate measurements could be made because of the extremely rough nature of the fracture surface. However, qualitatively, the extent of intergranular failure increases with increasing amount of ferrite in the flake graphite cast irons. It was generally observed that a surprising amount of graphite flakes were revealed on the fatigue fracture surfaces. Indeed at one particular location (see Fig. 13) the area percentage of graphite flakes was assessed at about some 29% which was over twice that recorded for the metallographic specimen. The graphite flakes observed on the fatigue fracture surfaces exhibited, in almost every instance, cleavage failure; indeed the graphite flakes cleaved along the (0001) basal planes which looked like a series of distorted hexagons. It was evident that the graphite flakes played an important part in the fatigue fracture processes of these particular microstructures.
4. Discussion
Fig. 12. Details of intergranular failure facets prevalent in the predominantly ferrite microstructure.
From the microstructures tested in the present investigation it was evident that the differences in
74
J.H. Bulloch /Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
strength and hardness properties were the result of different amounts of pearlite in the various microstructures. A limited literature survey was conducted to examine the influence of pearlite, in basically ferrite-pearlite flake cast irons, on the yield strength [8-13]. Where composition and grain size assessments were given the results were corrected to account for these differences. The data was corrected to a constant substitutional solid solution hardening element composition of 3% Si and 0.5% Mn using the strength vectors [14], and a constant matrix grain size of 40 p,m (d -1/2 5 mm -1/2) utilising a ky value of 16.5 reported in [15] for the Hall-Petch equation. The corrected yield strength levels are plotted against volume or area fraction pearlite as illustrated in Fig. 14. Also shown in this figure are the data reported for spheroidal graphite cast irons [16]. From this figure it can be seen that the yield strength of the flake graphite cast irons increase with increasing amounts of pearlite. Taking a linear dependence, and allowing for the considerable scatter in the data, the influence of pearlite on the yield strength of flake cast irons can be simply described as
Also from Fig. 14 it can be seen that the spheroidal graphite cast iron results (i) exhibited markedly higher yield strength levels than corresponding flake graphite cast irons and (ii) showed a strengthening effect from the pearlite microstructure constituent. In this instance the strengthening effects were greater than those recorded for the flake graphite cast irons and could be described by the following expression;
-
% = 435 + 2.5(% pearlite).
The value of the pearlite strengthening vectors observed for both types of cast irons show good agreement with the values of 1.5 to 2.0 reported for plain carbon steels [16]. The linear relationship between try and percentage of pearlite phase does not really exhibit the real trends in ferrite-pearlite microstructures as Gladman and Pickering have demonstrated that the yield strength varies in a non-linear fashion with pearlite content [14]; viz.:
=
-
% = 110 + 1.6(% pearlite).
O'y =f•trferrite + (1 - fn)trpearlite ,
(6)
LEGEND ~PRESENTDATA • REF11 V REF.18 o REF.10 II .
~
~
~.~---.~\ \ \ \ \ \ \ \
60 C
DATASCATTERSPHEROIDAL GRAPHITECASTIRONS
40 C--
YIELD STRENGTH (MPa) 20
: 0
I 20
(8)
which represents a modified law of mixtures approach where f is percent ferrite, trferrite and O'pearlite a r e the yield strength values of ferrite and pearlite respectively and 77 is an index which
80 C
o'/
(7)
I 40
I 60
I 80
J 10 0
%PEARLITE Fig. 14. Influence of ¢2 ~ pearlite phase on the yield strength on ferrite-pearlite cast iron microstructures.
J.H. Bulloch /Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
14
75
SPHEROIDALGRAPHITE
12 &Kth (o) o
THRESHOLD 10 STRESS INTENSITY RANGE AT 8 R=0
\
P%'E'DATA
."
FLAKE GRAPHITE CAST IRONS
C] REF.10 • REF.20 • REF.21
I
i
I
I
I
0
20
40
60
80
I 10 0
% PEARLITE
Fig. 15. Influence of pearlite on the threshold stress intensity range at R = 0.
ful influence on the A Kth levels of the various microstructures, (ii) microstructural characteristics (the proportions of ferrite and pearlite phases) significantly affect AKth especially the value at R = 0 i.e., Agth(0) , and (iii) they exhibited higher m exponent values than those shown by ductile low alloy steels. The influence that pearlite levels have on the AKth(0) values is illustrated in Fig. 15 together with other A Kth(0) values for flake
allows for the non-linear variation of % with pearlite level. However when n values of 1 / 2 and 2 were used (see Fig. 14) it was evident that the linear expression best suited the trends in the flake graphite cast iron results. This was also the case for the spheroidal graphite cast iron microstructures from the foregoing section; it has been clearly demonstrated in flake graphite cast irons that (i) the R-ratio value exhibited a power-
• o Z~ • • •
16 •
&Kth (0) THRESHOLD STRESS INTENSITY AT R=0 (MPaVm)
•
12
~
LEGENp PRESENTDATA REF. 19 REF. 10 REF.21 REF. 17 REF,20
&
B
4
E
REF'23~
~/
I 0
10 0
~
MIJTCH&FI~O/AKRISHIWlN LOW ALLOYSTEEL, REF. 22
. . . . . . . . . . . . .
L 20 0
I 30 0
I 40 0
oY, YIELD STRENGTH (MPa) Fig. |6. Relationship
between yield stress and A Kth(0) for iron based alloys,
I 50 0
J.H. Bulloch /Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
76
and sphroidal cast irons reported in the literature. From this figure it is clear that little data is available from the literature. An increase in pearlite content decreased the AKth(0) levels and the flake graphite cast irons exhibited values which were higher than correspondence spheroidal graphite irons. The relationship between the threshold stress intensity range at R = 0 , Agth(0), and yield strength has been well reported in low alloy steels [17]. Thus for direct comparison purposes the results, in terms of yield strength, from the present study and other reported data for other materials [16,18-20] are portrayed in Fig. 16. From this figure it is clear that (i) the low alloy steel [17,21] and binary ferrite date (22) except for Ishii et al.'s data for binary ferrites [23] within the range 100 to 200 MPa, and the Armco iron results [24] exhibited good agreement and indicated AKth values between 6 to 9 MPaCm- over the yield strength range 130 to 350 MPa, (ii) no distinct differences exist between the A Kth values for the flake and spheroidal cast irons although the flake results resided in the upper section of the data scatter, and (iii) the A Kth(0)
values portrayed by the cast iron microstructure were 30% to 60% higher than those recorded for low alloy steels at similar yield strength values. The fractographic and metallographic studies indicated that the flake graphite microstructures, and in particular the fully ferritic microstructure, exhibited a very rough fatigue fracture topography and showed a significant extent of crack deflection. Indeed large crack deflection ratios and significant deflection angles were measured respectively. It has been reported [25] that in certain dual phase microstructures inordinately high values of A Kth(0) were attributed primarily to the very rough crack path topography which caused deflection and roughness induced crack closure processes. Fig. 17 illustrated the threshold results for pure iron [22] and the present fully ferritic microstructure, which are essentially similar, and the large difference in AKth, of some 55%, is explained in terms of roughness induced crack closure and crack deflection. Consider a somewhat simplified schematic of a deflected crack (see Fig. 9), where 0 is the angle of deflection, D is the distance over which the
165 - -
PURE IRON DATA GEBERICH & ESAKAUL REF. 23 FATIGUE CRACK GROWTH (ram/c)
2~' Y ~" / ,.." "'r'
0=0.75
10-6 - -
PRESENT DATA 100% FERRITE FLAKE GRAPHITE CAST IRON
10.7 - -
I 8
/ f. I
10
1S
I 20
~K {MPa4m)
Fig. 17. N e a r threshold fatigue crack growth characteristics of pure iron and the p r e s e n t ferritic microstructure at R = 0.
J.H. Bulloch/ Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
tilted crack advances along the kink, and S the distance over which linear (Mode 1) crack growth occurred. The segment shown in Fig. 9 is then repeated. Suresh [26] has demonstrated, using linear elastic fracture mechanics at various angles or degrees of crack deflections, that the nominal driving force, AKNL , for each deflection segment could be expressed as, AKNL =
D cos 2 ( 0 / 2 ) + 5 D + S
AgE,
(9)
where AK L is the corresponding driving force for a linear crack of similar size in the mode I direction also the measured or apparent growth rate, (da/dN)NL for this simplified deflection crack will always be less than the actual growth rate, (da/dN)h, if crack deflections are not considered, i.e., NL
O + S
L"
(10)
From these equations it is possible to estimate the individual contribution of crack deflection effects and from Fig. 17 it can be seen that crack deflection effects can account for about 30% of the AKth(0) difference between the pure iron and the ferritic flake graphite cast iron. The cause of the other 70% increase can be attributed to surface roughness-induced crack Closure which arises from the premature contact of the fracture surface asperities due to shear displacements which are not fully reversible because of inelastic crack tip deformation and surface oxidation effects. Essentially in this instance the presence of coarse graphite flakes are responsible for the separate crack deflection and roughness inducted crack closure process which cause high values of A Kth in flake graphite cast irons fatigued at low R-ratio levels. Amongst the various fractographic features was the observation of the preponderance of fractured graphite flakes on the fatigue fracture surfaces. Indeed in certain instances the fatigue surfaces contained over double the amount of flakes that would be expected from the metallographic data; such an observation, in this instance graphite spheroids rather than flakes, has been recorded
77
for spheroidal or nodular cast irons. However, the important difference between the two types of cast irons is the role that the graphite constituent plays in the failure process i.e., (a) in the spheroidal form the crack extension consists of a series of repeatable progressive processes involving (i) crack approaching spheroid, (ii) crack deflection, (iii) decohesion at spheroid-matrix interface (iv) re-initiation and continued crack growth through the matrix while (b) in the flake form the cracks or defects are initiated into the surrounding matrix by the graphite flake itself being cleaved.
5. Concluding remarks It has been established that the threshold fatigue crack extension behaviour of a range of flake graphite cast irons was (i) markedly affected by mean stress or R-ratio and (ii) sensitive to the microstructural proportions of the ferrite and pearlite phases. At intermediate fatigue crack growth rates, typically above 5 x 10 -5 ram/c, the slope exhibited by the flake cast irons was significantly higher than those recorded for ductile steels. It was observed that as the percentage pearlite and yield strength increased the threshold stress intensity range, AKth, at low R-ratio, designated AKth , at low R-ratio, designated Agth(0), generally decreased. However at a yield strength within the range 150 to 300 MPa the AKth(0) values of the flake (and indeed spheroidal) cast irons were between 30% to 60% higher than those recorded for iron based alloys and low alloy steels. The high values of A gth(0) were shown to results from the very roughness nature of the fatigue surfaces which resulted in contributions from roughness induced crack closure and crack deflection processes. Finally, it was shown that in cast irons, the graphite constituent plays an important part in the crack extension process, tough its exact role depends on its morphology, viz., in spheroidal cast irons growing cracks preferentially encounter the spheroid-matrix interface causing decohesion while in the case of flake cast irons the graphite
78
J.H. Bulloch /Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
flake itself takes places in the fracture process by cleaving and producing matrix mierocraeks or defleets which provide energetically easy sites for subsequent crack extension. References [1] L. Haenny and G. Zambelli, The stiffness and modulus of elasticity of grey cast irons, J. Mater. Sci. Lett. 2, 239-242 (1983). [2] W.R. Clough and M.E. Shank, The deformation and rupture of grey cast iron, Trans. A S M 49, 241-262 (1957). [3] G.N.J. Gilbert, The components of strain due to deformation of the matrix and due to volume changes in a flake graphite cast iron under uniaxiai stress, BCIRA J. 11(14), 512-524 (1963). [4] M.R. Mitchell, Effects of graphite morphology, matrix hardness and structure on the fatigue resistance of grey cast iron, AFS Inter-Cast Metals J. 2, p 64-74 (1977). [5] A.A. Timmings, The decomposition of pearlite in grey cast iron, JISI 142, 123-140 (1940). [6] J.M.R. Ibabe and J.G. Sevillano, Fatigue fracture path in medium-high carbon ferrite-pearlite structures, 6th Int. Conf. on Fracture, ICF6, New Dehli, India, Dec. 1984, pp. 2073-2079. [7] Metals Handbook, 9th ed., ASM Int., Metals Park, Ohio, USA, Vol. 12, Fractography, p. 206. [8] A.G. Glover and G. Pollard, The brittle fracture of grey cast irons, JIM 120, 138-141 (1971). [9] R.N. Castillo and T.J. Baker, A fracture mechanics approach to fatigue in flake graphite cast iron, 6th Int. Conf. on Fracture, ICF6, New Dehli, India, Dec. 1984, pp. 2057-2064. [10] A. Tiziani, E. Ramous and A. Molinari, Fracture morphology of compacted graphite cast iron, 6th Int. Conf. on Fracture, ICF6, New Dehli, India, Dec. 1984, pp. 1490-1496. [11] P.A. Blakemore and K. Morton, Structure-property relationships in graphite cast irons, Int. J. Fatigue 149-155 (July 1982). [12] S. Ueda, Plastic fatigue strength of cast iron under rotating bending, 4th Int. Conf. on Fracture, ICF4, Waterloo, Canada, 1977, pp. 111-123.
[13] J. Pits, Fracture toughness of cast iron, 6th Int. Conf. on Fracture, ICF6, New Dehli, India, Dec. 1984, pp. 12551261. [14] T. Gladman and F.B. Pickering, The effect of grain size on the mechanical propeties of ferrous materials, Yield, Flow and Fracture in Polycrystals (Ed. T.N. Baker) (Elsevier Applied Science Publisher, London, 1983) p. 141. [15] B. Mintz, importance of Kv (Hall-Petch slope) in determining strength of steels, Metals Tech. 11,265-271 (1984). [16] D.J. Bulloch and J.H. Bulloch, the influence of R-ratio and micrstructures on the threshold fatigue crack growth characteristics of spheroidal graphite cast irons, Int. J. Pres. Ves. Piping 36, 289-314 (1989). [17] J.H. Bulloch and R. Kennedy, The influence of stress ratio and volume fraction martensite on the threshold fatigue crack growth behaviour of granular bainites, Res. Mechanica 15, 259-271 (1985). [18] A.J. Padkin, M.F. Brereton and W.J. Plumbridge, Fatigue crack growth in two phase alloys, Mat. Sci. TechnoL 3, 217-223 (1987). [19] F.D. Griswold and R.I. Stephens, Comparison of fatigue properties of nodular cast iron production and Y-block casting, Int. J. Fatigue 9, 3-10 (1980). [20] P. Clement, J.P. Angeli and A. Pineau, Short crack behaviour in nodular cast iron, Fatigue Eng. Mater. Struct. 7, 251-264 (1984). [21] Y. Mutoh and V.M. Radhakrishnen, "Effect of yield stress and grain size on threshold and fatigue, Trans. ASME 108, 174-178 (1986). [22] W.W. Gerberich, W. Yu and K. Esaklul, Fatigue threshold studies in Fe, Fe-Si, and HSLA steels: Part 1. Effect of strength and surface asperities on closure, Metall. Trans. 15A, 875-888 (1984). [23] H. Ishii, T. Kawarazaki and Y. Fujimura, Fatigue in binary alloys of bcc iron? Metall. Trans. 15A, 679-691 (1984). [24] M. Srinivas and G. Malakondaiah, Effect of grain size on threshold stress intensity for fatigue crack growth in ARMCO iron, Scripta Metall. 20, 689-692 (1986). [25] V.B. Dutta, S. Suresh and R.O. Ritchie, Fatigue crack propagation in dual phase steels: effects of ferritemartensite microstructures on crack path morphology, MetaU. Trans. 15A, 1193-1207 (1984). [26] S. Suresh, Crack defection characteristics during fatigue, MetaU. Trans. 14A, 2375-2381 (1983).
•
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a ELSEVIER
~
Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
Near threshold fatigue behaviour of flake graphite cast irons microstructures J.H. Bulloch * Electricity Supply Board, Head Off'we, Dublin 2, Ireland
Abstract The present paper attempts to describe a study concerning the influence of mean stress or R-ratio and mierostruetural variation, percentage polygonal ferrite, on the near threshold fatigue crack extension characteristics of a flake graphite cast iron. Basically significant effects of R-ratio and microstructure on the fatigue threshold stress intensity range, AKth , were observed. Also at intermediate AK levels the flake cast iron microstrueture exh~ited a much higher slope in the two-parameter fatigue crack growth relationship than those observed in ductile low alloy steels. It was also observed that at low R-ratio the AKth values of the flake cast irons were significantly larger than those recorded for iron-based and low alloy steel materials. The high levels of AKth were explained in terms of roughness induced crack closure and crack deflection processes which resulted from the very rough topography of the fatigue fracture surfaces. Finally the role played by the graphite constituent in the fatigue fracture processes prevalent in cast iron microstructures was discussed.
1. Introduction For many years the transport industry, and in particular the railways, have b e e n foremost in utilising many engineering c o m p o n e n t s fabricated from cast irons. A significant n u m b e r of these components are used under cyclic environmental service conditions which can involve mechanical loading, thermal cycling or a combination of both. Cast irons can be classified in terms of the general graphite morphology, viz., flake, compacted flake and spheroidal.
" Tel. +353 1 676 5831, Fax +353 1 676 2043.
For over a half a century it has been generally accepted that the mechanisms of failure in flake cast irons, both in the as-cast and annealed condition, are dictated primarily by the brittle characteristics of the graphite flakes. As a result of its heterogeneous microstructure flake cast irons exhibit non-linear behaviour and as such the conventional variables which are usually used for assessing the tensile characteristics of homogeneous materials do not apply. Indeed, it was suggested [1] that microcracking occurs under low tensile stresses which causes a progressive alteration of the elastic modulus and, as such, no tangible elastic limit can be measured. U n d e r tensile loading conditions failure occurs in a discontinuous fashion which is controlled by numer-
0167-8442/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0167-8442(95 )00032- l
66
J.H. Bulloch / Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
ous microstructural factors and hence the elongation or tensile strength cannot be taken as real measurements of the flake cast iron's tensile properties. Upon initial inspection it would appear that the fatigue characteristics of flake graphite cast iron are similar to that portrayed by low alloy steels inasmuch that definite fatigue limits were observed in both materials. Indeed the endurance ratio, which is the ratio of the fatigue limit to tensile or ultimate tensile strength, of a number of steels is around 0.4 to 0.5 which agrees well with that value recorded for pearlitic flake cast irons. However, as a result of the heterogeneous nature of flake cast irons, the tensile strength is not a measure of the stress required for post-yield instability, but rather the brittle failure stress with is controlled by the fracture toughness, and the dimensions of the graphite flakes which act as crack initiation locations [2,3]. It has been reported [4] that the tensile strength was typically 30% of the yield strength measured in compression. As a direct consequence, if the endurance ratio is considered in terms of the yield strength, it was typically 30% of the yield strength measured in compression. As a direct consequence, if the endurance ratio is considered in terms of the yield strength, flake graphite cast irons exhibit inordinately low fatigue resistance. Another important difference between steels and flake cast irons is the fact that, while significant strength increased in the latter can be attained by quench and tempering, little effect on the fatigue strength of flake cast irons was observed [4]. The present study was aimed at assessing the influence of microstructure and specifically the amount of ferrite present in the microstructure on the mechanical and threshold fatigue properties of flake graphite cast irons.
2. Experimental techniques The average bulk chemical composition of the graphite flake cast iron can be found in Table 1. The graphite flake iron in the as-cast condition exhibited a microstructure which was 100%
Table 1 Chemical composition of graphite flake cast iron %C
%Si
%Mn
%S
%P
3.28
2.14
0.45
0.049
0.038
pearlite. It has been demonstrated [6] that the heat treatment of flake cast irons resulted in the breakdown of the pearlite and deposition of graphite on existing graphite flakes. In an effort to vary the amounts of pearlite and polygonal ferrite phases present in the flake cast irons, segments of the casting were subjected to prolonged heat treatment schedules either below or above the critical temperature. In the present study segments of the casting were held at 800°C for various times (in the range 5 to 20 min) and then furnace cooled after holding at 690°C for 1 hour. These treatments produced mixed ferritepearlite microstructures. The fully ferritic graphite flake cast iron microstructure was obtained by a normal or standard commercial heat treatment, viz., (i) a 4 hour hold at 900°C (ii) a furnace cool from 900 to 690°C, (iii) a 8 hour hold at 690°C and (iv) a furnace cool from 690°C to ambient temperatures. These particular heat treatments resulted in the polygonal ferrite phase being preferentiaUy nucleated adjacent to graphite flakes. The mechanical property test data and details of the metallographic parameters are listed in Table 2. Fatigue crack propagation tests were conducted using precracked standard Compact Tension test specimens with thickness 20 mm and width 40 mm. Threshold stress intensity range, AKth, measurements were carried out on an Amster vibraphone fatigue machine at frequencies in the range of 90 to 120 Hz. Three separate R-ratio values of 0.05 0.3 and 0.7 were studied, and all tests were conducted in air under ambient conditions. Note that R = min. load/max, load. A conventional load shedding technique was adopted for all fatigue threshold assessment tests. Fatigue crack growth was first measured in the 10 -5 m m / c growth rate region and subsequently the loads were reduced. At each specific load level the crack growth rate was measured. At each specific load level the crack tip was allowed
J.H. Bulloch /Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
67
Table 2 Mechanical properties of graphite flake iron Microstructure
Ferrite 73F-P 16F-P Pearlite
Mechanical properties
MetaUographic details
Yield stress (MPa)
Tensile stress (MPa)
148 154 183 243
144 165 198 258
to grow a distance of four monotonic plastic zone sizes before subsequent fatigue crack extension wad deemed to be unaffected by prior plasticity effects. The fatigue crack length was monitored both optically and continuously using a DC electric potential method, capable of measurement to within 0.1/mm of absolute crack length, and of detecting changes in crack length of the order of 10 p,m. Fatigue crack growth rates were calculated from the crack length-number of fatigue cycles data using an incremental polynominal method which involved fitting a second order polynominal (parabola) to sets of seven successive data points. In this particular study the threshold stress intensity range, A K t h , w a s experimentally defined as the AK value at which a crack extension rate of 10 - 7 m m / c was recorded. Scanning electron microscopy was used in an effort to assess the fracture topography in selected test specimens. A quantitative assessment of the various fracture modes prevalent on the fatigue surfaces was performed by measuring several fractographs at various A K levels. Crack profde samples were fractographs at various AK levels. Crack profile samples were also taken in an effort to asses the crack path tortuosity or crack deflection ratio. Metallographic sections were taken and the average pearlite interlamellar spacing, and the average graphite flake size was established.
Hardness (UPN)
% ferrite
% pearlite
110
100 73 16 -
27 84 100
123
154 216
-
fraction pearlite and details of the wholly pearlitic microstructure are given in Fig. 1. The lamellar pearlite spacing of the pearlite matrix was fairly coarse, and the average pearlite spacing was assessed at 2 Ixm. Also from Fig. 1 it is evident that the graphite morphology was Type A and that the average size (length) of the graphite flakes was estimated at around 230 p,m; the relative frequency distribution of the graphite flakes is given in Fig. 2. The area fraction of the graphite flakes in all microstructures was assessed at around 13%.
3. 2. Mechanical properties The mechanical properties of the various flake graphite microstructures are given in Table 2. It is evident from this that these particular microstruetures exhibited little signs of ductility in that the yield and tensile strength levels were very close to each other. Generally increased
3. Experimental results
3.1. Metallographic details As listed in Table 2, it can be seen that four microstructures ranging from zero to unit volume
Fig. I. Details of the fully pearlitic microstructure.
J.H. Bulloch / Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
68
0.3 AVERAGE FLAKE LENGHT = 230pro
0.2 RELATIVE FREQUENCY
0.1
0
10 0
20 0
30 0
50 0
40 0
GRAPHITE FLAKE LENGHT {urn) Fig. 2. Relative frequency distribution of graphite flake size.
10 .3 --
LE GE ND • R= 0.05 - R = 0.3 • R = 0.7
10"4
FATIGUE
GROWTH (ram/c) 10 -5
&A
--
FERRITIC S T E E L REF. 7 10 .6
10.7
I 2
I 3
I 4
l 6
8
I 10
I
I
20
30
AK (MPaVm)
Fig. 3. Fatigue crack growth characteristics of the fully fcrritic microstructure at the fully ferritic microstructure at various R-ratio levels.
J.H. Bulloch /Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
69
LEGEND 1() 3,
• • •
R = 0.05 R=0.3 R=0.7
-4 10
FATIGUE CRACK 10.5 GROWTH (mrrVc)
lo-S -
10 `7 I
1
I
I
I
I
I
I
2
3
4
6
8
10
20
30
AK(MPaVm) Fig. 4. Fatigue crack growth characteristics of the 73% ferrite-pearlitic microstructure at various R-ratio levels.
LEGEND -3 10
• • •
R = 0,05 R=0,3 R=0,7
/
-4 10
FATIGUE CRACK GROWTH (ram/c)
10 "5
;_
10"6-
-7 10 -
I 2
i 3
I 4
I 6
I 8
I
I
I
10
20
30
AK (MPaVm)
Fig. 5. Fatigue crack growth characteristics of the 16% ferrite-oearlitic microstructure at various R-ratio levels.
70
J.H. Bulloch/Theoretical and Appfied FractureMechanics 24 (1995) 65-78
amounts of pearlite increased both the strength and hardness values. Indeed the introduction of pearlite caused the yield and tensile strengths to increase by around 60% and 80% respectively, while the hardness was almost doubled. 3.3. Fatigue crack extension characteristics
The fatigue crack growth behaviour of the various microstructures examined in the present investigation are exhibited in Figs. 3 to 6. In the case of the fully ferritic microstructure (Fig. 3), it can be seen that the influence of the R-ratio is significant in that decreasing the R-ratio from 0.7 to 0.3 and 0.05 increased the value of the threshold stress intensity range, AKth, by a factor of about 4 and 7 respectively. Also intermediate crack growth rates are very sensitive to the R-ratio where the exponent m of the two-parameter fatigue crack growth relation da/dN=
C(AK)"
(1)
was constant at a value around 6. The threshold fatigue characteristics of the duplex ferrite pearlite flake graphite microstructures are shown in Figs. 4 and 5. From these figures it was again evident that significant effects of the R-ratio were recorded in these microstructures with the value of AKth increasing by three times and four to six times at intermediate and low R-ratio levels respectively. Again all intermediate fatigue crack growth data, which exhibited a linear relationship between log AK and log d a / d N , exhibited a slope or exponent value of around 6. Similar marked beneficial effects of a decreasing R-ratio, in inhibiting the crack growth of long macrocracks, were evident in the case of the fully pearlitic microstructure (see Fig. 6), where A Kth levels were increased by a factor of 5. The fatigue crack extension results of all the tests conducted in this study are illustrated in Fig. 7. From this figure it can be seen that the data
LEGEND
.3 r 10
• R = 0.05 • R=0.3 A R=0.7
-4
10
FATIGUE CRACK GROWTH
10-5
(mrn/c)
PERARLITE STEEL REF. 7
~o-6 -7 10
-
I
1
1
1
I
I
I
f
2
3
4
6
8
10
20
30
AK (MPaVm) Fig. 6. Fatigue crack growth characteristics of the pearlitic microstructure at various R-ratio values.
J.H. Bulloch /Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
-3 10
--
LEGEND n FULLYFERRITIC •
16%FERRITE
A•
~z
,o-,_ FATIGUE CRACK GROWTH 10"5 (ram/c)
71
lie&
o.,x~:,,oM.s/.:X
_
10"6 _
'tr o.--o,
,o
o.
r~.
,o.,
•
z
•
°'. ! I " I I
I
I
I
2
3
4
6
I
I
8
1
l
10
30
20
&K (MPaqm) Fig. 7. Fatigue crack growth d a t a f o r various R - r a t i o a n d microstructure types.
can be separated into sets of different R-ratios with the high R-ratio results exhibiting the least amount of scatter at fatigue crack growth rates
above 10 - 6 mm/c, and approaching 10 -5 m m / c the different microstructural results tended to merge together and the intermediate fatigue crack
LEGEND • 100% FERR K~(0) = 14.5 MPaV'm o 73PAFERR Kill(0) = 12.7 MPaV'm •~ 16% FERR K~(0) = 9.7 MPaqm • 0% FERR Kth(0) = 11.3 MPaY'm
16
• Q~ , , , , , , .~
12
&
"'-,
THRESHOLD STRESS INTENSITY RANGE (MPaVm)
i 0
l
I 0.2
[ 0.4
I 0.6
I 0.8
R-RATIO Fig. 8. Influence o f R - r a t i o o n the threshold stress intensity range f o r the various microstructures.
J.H. Bulloch/ Theoreticaland Appfied FractureMechanics24 (1995) 65-78
72
propagation behaviour could be described by the following expressions;
da/dN
~,-
(mm/c)
= 6 . 1 2 x 1 0 - 1 3 ( A K ) 6"7 for
da/dN
R=0.05,
(2)
R = 0.30
(3)
(mm/c)
= 1.35 x 1 0 - t 1 ( A K ) 6"2 for and
da/dN
CRACK GROWTH DIRECTION
Ii,
(mm/c)
= 2.59 x 1 0 - 9 ( A K ) 65
I
for
R =0.70.
(4)
The average value of m in Eq. (1) was around 6.5 and this value was significantly higher than those reported for steels, typically 2 to 4, Indeed, from Figs 3 and 6, where the average data scatter for mild ferritic steels and pearlitic steels [6] are shown respectively, it is evident that for both microstructures (i) the graphite flake cast irons exhibited much higher values of AKth, (ii) the steels showed slower fatigue crack extension characteristics at intermediate AK levels and (iii) the steel results exhibited a much lower exponent value, between 2 to 4, which was typical of materials undergoing ductile fatigue type failure. The influence of the R-ratio on the threshold fatigue stress range, Agth , for the various microstructures is given in Fig. 8. From this figure it is evident that the relationship between AKth and the R-ratio for the graphite flake cast iron microstructures was not linear. However extrapolation back to R = 0 yields the values of A gth(0), these are listed in Fig. 8. Also it can be seen that the effects of R-ratio increase with decreasing R-ratio value is the greatest and least in the ferritic and pearlite microstructures respectively.
s
Fig. 9. Schematicillustration of crack deflectioneffectsi
where S is the horizontal segment and D is the deflected portion at an angle 0 of the fatigue crack profile (see Fig. 9). By careful measurement of a series of different crack profiles and assessment of the extent of crack deflection during the fatigue process in the ferritic graphite, flake cast iron microstructure could be made. In the R = 0.05 test sample the value of D / ( D + S) was measured at 0.3 while deflection angles of as much as 70° were recorded. In the case of the pearlite dominated microstructures it was difficult to identify or resolve fatigue striations; such features were commonly found however in ferritic microstructures (see Fig. 10). In numerous instances it was observed that the regions of ductile striated fatigue crack
3.4. Fractographic details The fracture topography of the fatigue failure surfaces varied from rough to very rough, especially in the fully ferritic microstructures the crack path tortuosity showed large crack deflections. The crack path tortuosity was characterised by the crack deflection ratio [7] which is given by crack defection ratio = D / ( D + S),
(5)
Fig. 10. Details of fatigue striations present in the fullyferritic microstructure.
J.H. Bulloch / Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
73
Fig. 11. Illustration of crack deflection effects. Cracks cut across a pearlite colony.
Fig. 13. General view of fatigue fracture surface showing extensive formations of graphite flakes.
growth radiated out from certain discrete graphite flakes which suggested that the flakes aided fatigue crack growth processes. In pearlitic microstructure two types of failure were recognised viz., (a) an etched type of fracture feature (see Fig. 11) occurred when the growing crack cut across a pearlite colony and (b) a cleavage type failure event, that happened when an extending crack grew between the cementiteferrite lamellar layers. When ferrite was present in the microstructures the extent of intergranular failure increased; indeed in the fully ferritic microstructure
this was the dominant failure mode (see Fig. 12). A quantitative assessment of the extent of intergranular failure as a function of the percent ferrite phase in the microstructure was attempted but no accurate measurements could be made because of the extremely rough nature of the fracture surface. However, qualitatively, the extent of intergranular failure increases with increasing amount of ferrite in the flake graphite cast irons. It was generally observed that a surprising amount of graphite flakes were revealed on the fatigue fracture surfaces. Indeed at one particular location (see Fig. 13) the area percentage of graphite flakes was assessed at about some 29% which was over twice that recorded for the metallographic specimen. The graphite flakes observed on the fatigue fracture surfaces exhibited, in almost every instance, cleavage failure; indeed the graphite flakes cleaved along the (0001) basal planes which looked like a series of distorted hexagons. It was evident that the graphite flakes played an important part in the fatigue fracture processes of these particular microstructures.
4. Discussion
Fig. 12. Details of intergranular failure facets prevalent in the predominantly ferrite microstructure.
From the microstructures tested in the present investigation it was evident that the differences in
74
J.H. Bulloch /Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
strength and hardness properties were the result of different amounts of pearlite in the various microstructures. A limited literature survey was conducted to examine the influence of pearlite, in basically ferrite-pearlite flake cast irons, on the yield strength [8-13]. Where composition and grain size assessments were given the results were corrected to account for these differences. The data was corrected to a constant substitutional solid solution hardening element composition of 3% Si and 0.5% Mn using the strength vectors [14], and a constant matrix grain size of 40 p,m (d -1/2 5 mm -1/2) utilising a ky value of 16.5 reported in [15] for the Hall-Petch equation. The corrected yield strength levels are plotted against volume or area fraction pearlite as illustrated in Fig. 14. Also shown in this figure are the data reported for spheroidal graphite cast irons [16]. From this figure it can be seen that the yield strength of the flake graphite cast irons increase with increasing amounts of pearlite. Taking a linear dependence, and allowing for the considerable scatter in the data, the influence of pearlite on the yield strength of flake cast irons can be simply described as
Also from Fig. 14 it can be seen that the spheroidal graphite cast iron results (i) exhibited markedly higher yield strength levels than corresponding flake graphite cast irons and (ii) showed a strengthening effect from the pearlite microstructure constituent. In this instance the strengthening effects were greater than those recorded for the flake graphite cast irons and could be described by the following expression;
-
% = 435 + 2.5(% pearlite).
The value of the pearlite strengthening vectors observed for both types of cast irons show good agreement with the values of 1.5 to 2.0 reported for plain carbon steels [16]. The linear relationship between try and percentage of pearlite phase does not really exhibit the real trends in ferrite-pearlite microstructures as Gladman and Pickering have demonstrated that the yield strength varies in a non-linear fashion with pearlite content [14]; viz.:
=
-
% = 110 + 1.6(% pearlite).
O'y =f•trferrite + (1 - fn)trpearlite ,
(6)
LEGEND ~PRESENTDATA • REF11 V REF.18 o REF.10 II .
~
~
~.~---.~\ \ \ \ \ \ \ \
60 C
DATASCATTERSPHEROIDAL GRAPHITECASTIRONS
40 C--
YIELD STRENGTH (MPa) 20
: 0
I 20
(8)
which represents a modified law of mixtures approach where f is percent ferrite, trferrite and O'pearlite a r e the yield strength values of ferrite and pearlite respectively and 77 is an index which
80 C
o'/
(7)
I 40
I 60
I 80
J 10 0
%PEARLITE Fig. 14. Influence of ¢2 ~ pearlite phase on the yield strength on ferrite-pearlite cast iron microstructures.
J.H. Bulloch /Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
14
75
SPHEROIDALGRAPHITE
12 &Kth (o) o
THRESHOLD 10 STRESS INTENSITY RANGE AT 8 R=0
\
P%'E'DATA
."
FLAKE GRAPHITE CAST IRONS
C] REF.10 • REF.20 • REF.21
I
i
I
I
I
0
20
40
60
80
I 10 0
% PEARLITE
Fig. 15. Influence of pearlite on the threshold stress intensity range at R = 0.
ful influence on the A Kth levels of the various microstructures, (ii) microstructural characteristics (the proportions of ferrite and pearlite phases) significantly affect AKth especially the value at R = 0 i.e., Agth(0) , and (iii) they exhibited higher m exponent values than those shown by ductile low alloy steels. The influence that pearlite levels have on the AKth(0) values is illustrated in Fig. 15 together with other A Kth(0) values for flake
allows for the non-linear variation of % with pearlite level. However when n values of 1 / 2 and 2 were used (see Fig. 14) it was evident that the linear expression best suited the trends in the flake graphite cast iron results. This was also the case for the spheroidal graphite cast iron microstructures from the foregoing section; it has been clearly demonstrated in flake graphite cast irons that (i) the R-ratio value exhibited a power-
• o Z~ • • •
16 •
&Kth (0) THRESHOLD STRESS INTENSITY AT R=0 (MPaVm)
•
12
~
LEGENp PRESENTDATA REF. 19 REF. 10 REF.21 REF. 17 REF,20
&
B
4
E
REF'23~
~/
I 0
10 0
~
MIJTCH&FI~O/AKRISHIWlN LOW ALLOYSTEEL, REF. 22
. . . . . . . . . . . . .
L 20 0
I 30 0
I 40 0
oY, YIELD STRENGTH (MPa) Fig. |6. Relationship
between yield stress and A Kth(0) for iron based alloys,
I 50 0
J.H. Bulloch /Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
76
and sphroidal cast irons reported in the literature. From this figure it is clear that little data is available from the literature. An increase in pearlite content decreased the AKth(0) levels and the flake graphite cast irons exhibited values which were higher than correspondence spheroidal graphite irons. The relationship between the threshold stress intensity range at R = 0 , Agth(0), and yield strength has been well reported in low alloy steels [17]. Thus for direct comparison purposes the results, in terms of yield strength, from the present study and other reported data for other materials [16,18-20] are portrayed in Fig. 16. From this figure it is clear that (i) the low alloy steel [17,21] and binary ferrite date (22) except for Ishii et al.'s data for binary ferrites [23] within the range 100 to 200 MPa, and the Armco iron results [24] exhibited good agreement and indicated AKth values between 6 to 9 MPaCm- over the yield strength range 130 to 350 MPa, (ii) no distinct differences exist between the A Kth values for the flake and spheroidal cast irons although the flake results resided in the upper section of the data scatter, and (iii) the A Kth(0)
values portrayed by the cast iron microstructure were 30% to 60% higher than those recorded for low alloy steels at similar yield strength values. The fractographic and metallographic studies indicated that the flake graphite microstructures, and in particular the fully ferritic microstructure, exhibited a very rough fatigue fracture topography and showed a significant extent of crack deflection. Indeed large crack deflection ratios and significant deflection angles were measured respectively. It has been reported [25] that in certain dual phase microstructures inordinately high values of A Kth(0) were attributed primarily to the very rough crack path topography which caused deflection and roughness induced crack closure processes. Fig. 17 illustrated the threshold results for pure iron [22] and the present fully ferritic microstructure, which are essentially similar, and the large difference in AKth, of some 55%, is explained in terms of roughness induced crack closure and crack deflection. Consider a somewhat simplified schematic of a deflected crack (see Fig. 9), where 0 is the angle of deflection, D is the distance over which the
165 - -
PURE IRON DATA GEBERICH & ESAKAUL REF. 23 FATIGUE CRACK GROWTH (ram/c)
2~' Y ~" / ,.." "'r'
0=0.75
10-6 - -
PRESENT DATA 100% FERRITE FLAKE GRAPHITE CAST IRON
10.7 - -
I 8
/ f. I
10
1S
I 20
~K {MPa4m)
Fig. 17. N e a r threshold fatigue crack growth characteristics of pure iron and the p r e s e n t ferritic microstructure at R = 0.
J.H. Bulloch/ Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
tilted crack advances along the kink, and S the distance over which linear (Mode 1) crack growth occurred. The segment shown in Fig. 9 is then repeated. Suresh [26] has demonstrated, using linear elastic fracture mechanics at various angles or degrees of crack deflections, that the nominal driving force, AKNL , for each deflection segment could be expressed as, AKNL =
D cos 2 ( 0 / 2 ) + 5 D + S
AgE,
(9)
where AK L is the corresponding driving force for a linear crack of similar size in the mode I direction also the measured or apparent growth rate, (da/dN)NL for this simplified deflection crack will always be less than the actual growth rate, (da/dN)h, if crack deflections are not considered, i.e., NL
O + S
L"
(10)
From these equations it is possible to estimate the individual contribution of crack deflection effects and from Fig. 17 it can be seen that crack deflection effects can account for about 30% of the AKth(0) difference between the pure iron and the ferritic flake graphite cast iron. The cause of the other 70% increase can be attributed to surface roughness-induced crack Closure which arises from the premature contact of the fracture surface asperities due to shear displacements which are not fully reversible because of inelastic crack tip deformation and surface oxidation effects. Essentially in this instance the presence of coarse graphite flakes are responsible for the separate crack deflection and roughness inducted crack closure process which cause high values of A Kth in flake graphite cast irons fatigued at low R-ratio levels. Amongst the various fractographic features was the observation of the preponderance of fractured graphite flakes on the fatigue fracture surfaces. Indeed in certain instances the fatigue surfaces contained over double the amount of flakes that would be expected from the metallographic data; such an observation, in this instance graphite spheroids rather than flakes, has been recorded
77
for spheroidal or nodular cast irons. However, the important difference between the two types of cast irons is the role that the graphite constituent plays in the failure process i.e., (a) in the spheroidal form the crack extension consists of a series of repeatable progressive processes involving (i) crack approaching spheroid, (ii) crack deflection, (iii) decohesion at spheroid-matrix interface (iv) re-initiation and continued crack growth through the matrix while (b) in the flake form the cracks or defects are initiated into the surrounding matrix by the graphite flake itself being cleaved.
5. Concluding remarks It has been established that the threshold fatigue crack extension behaviour of a range of flake graphite cast irons was (i) markedly affected by mean stress or R-ratio and (ii) sensitive to the microstructural proportions of the ferrite and pearlite phases. At intermediate fatigue crack growth rates, typically above 5 x 10 -5 ram/c, the slope exhibited by the flake cast irons was significantly higher than those recorded for ductile steels. It was observed that as the percentage pearlite and yield strength increased the threshold stress intensity range, AKth, at low R-ratio, designated AKth , at low R-ratio, designated Agth(0), generally decreased. However at a yield strength within the range 150 to 300 MPa the AKth(0) values of the flake (and indeed spheroidal) cast irons were between 30% to 60% higher than those recorded for iron based alloys and low alloy steels. The high values of A gth(0) were shown to results from the very roughness nature of the fatigue surfaces which resulted in contributions from roughness induced crack closure and crack deflection processes. Finally, it was shown that in cast irons, the graphite constituent plays an important part in the crack extension process, tough its exact role depends on its morphology, viz., in spheroidal cast irons growing cracks preferentially encounter the spheroid-matrix interface causing decohesion while in the case of flake cast irons the graphite
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J.H. Bulloch /Theoretical and Applied Fracture Mechanics 24 (1995) 65-78
flake itself takes places in the fracture process by cleaving and producing matrix mierocraeks or defleets which provide energetically easy sites for subsequent crack extension. References [1] L. Haenny and G. Zambelli, The stiffness and modulus of elasticity of grey cast irons, J. Mater. Sci. Lett. 2, 239-242 (1983). [2] W.R. Clough and M.E. Shank, The deformation and rupture of grey cast iron, Trans. A S M 49, 241-262 (1957). [3] G.N.J. Gilbert, The components of strain due to deformation of the matrix and due to volume changes in a flake graphite cast iron under uniaxiai stress, BCIRA J. 11(14), 512-524 (1963). [4] M.R. Mitchell, Effects of graphite morphology, matrix hardness and structure on the fatigue resistance of grey cast iron, AFS Inter-Cast Metals J. 2, p 64-74 (1977). [5] A.A. Timmings, The decomposition of pearlite in grey cast iron, JISI 142, 123-140 (1940). [6] J.M.R. Ibabe and J.G. Sevillano, Fatigue fracture path in medium-high carbon ferrite-pearlite structures, 6th Int. Conf. on Fracture, ICF6, New Dehli, India, Dec. 1984, pp. 2073-2079. [7] Metals Handbook, 9th ed., ASM Int., Metals Park, Ohio, USA, Vol. 12, Fractography, p. 206. [8] A.G. Glover and G. Pollard, The brittle fracture of grey cast irons, JIM 120, 138-141 (1971). [9] R.N. Castillo and T.J. Baker, A fracture mechanics approach to fatigue in flake graphite cast iron, 6th Int. Conf. on Fracture, ICF6, New Dehli, India, Dec. 1984, pp. 2057-2064. [10] A. Tiziani, E. Ramous and A. Molinari, Fracture morphology of compacted graphite cast iron, 6th Int. Conf. on Fracture, ICF6, New Dehli, India, Dec. 1984, pp. 1490-1496. [11] P.A. Blakemore and K. Morton, Structure-property relationships in graphite cast irons, Int. J. Fatigue 149-155 (July 1982). [12] S. Ueda, Plastic fatigue strength of cast iron under rotating bending, 4th Int. Conf. on Fracture, ICF4, Waterloo, Canada, 1977, pp. 111-123.
[13] J. Pits, Fracture toughness of cast iron, 6th Int. Conf. on Fracture, ICF6, New Dehli, India, Dec. 1984, pp. 12551261. [14] T. Gladman and F.B. Pickering, The effect of grain size on the mechanical propeties of ferrous materials, Yield, Flow and Fracture in Polycrystals (Ed. T.N. Baker) (Elsevier Applied Science Publisher, London, 1983) p. 141. [15] B. Mintz, importance of Kv (Hall-Petch slope) in determining strength of steels, Metals Tech. 11,265-271 (1984). [16] D.J. Bulloch and J.H. Bulloch, the influence of R-ratio and micrstructures on the threshold fatigue crack growth characteristics of spheroidal graphite cast irons, Int. J. Pres. Ves. Piping 36, 289-314 (1989). [17] J.H. Bulloch and R. Kennedy, The influence of stress ratio and volume fraction martensite on the threshold fatigue crack growth behaviour of granular bainites, Res. Mechanica 15, 259-271 (1985). [18] A.J. Padkin, M.F. Brereton and W.J. Plumbridge, Fatigue crack growth in two phase alloys, Mat. Sci. TechnoL 3, 217-223 (1987). [19] F.D. Griswold and R.I. Stephens, Comparison of fatigue properties of nodular cast iron production and Y-block casting, Int. J. Fatigue 9, 3-10 (1980). [20] P. Clement, J.P. Angeli and A. Pineau, Short crack behaviour in nodular cast iron, Fatigue Eng. Mater. Struct. 7, 251-264 (1984). [21] Y. Mutoh and V.M. Radhakrishnen, "Effect of yield stress and grain size on threshold and fatigue, Trans. ASME 108, 174-178 (1986). [22] W.W. Gerberich, W. Yu and K. Esaklul, Fatigue threshold studies in Fe, Fe-Si, and HSLA steels: Part 1. Effect of strength and surface asperities on closure, Metall. Trans. 15A, 875-888 (1984). [23] H. Ishii, T. Kawarazaki and Y. Fujimura, Fatigue in binary alloys of bcc iron? Metall. Trans. 15A, 679-691 (1984). [24] M. Srinivas and G. Malakondaiah, Effect of grain size on threshold stress intensity for fatigue crack growth in ARMCO iron, Scripta Metall. 20, 689-692 (1986). [25] V.B. Dutta, S. Suresh and R.O. Ritchie, Fatigue crack propagation in dual phase steels: effects of ferritemartensite microstructures on crack path morphology, MetaU. Trans. 15A, 1193-1207 (1984). [26] S. Suresh, Crack defection characteristics during fatigue, MetaU. Trans. 14A, 2375-2381 (1983).