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Impact-abrasion behavior of low alloy white cast irons
WEAR ELSEVIER
Wear209 (1997) 308-315
Impact-abrasion behavior of low alloy white cast irons Ma Qian a.., Wang Chaochang b Department of Mechanical Engineering, Tsinghua University; Beijing 100084. P.R. China b Department of Metallurgy (Foundry). University of Science and Technology. Beijing 100083. P.R. China Received 2l November 1995; accepted 6 September 1996
Abstract The impact-abrasionbehavior of a series of low alloy white cast irons with different moo-phologiesof eutectic carbide was investigated usinga repetitive impact-abrasiveweartester. By meansof a defined morphologicalparameter,the roundnessareafractionSF~,the morphology oftbe eutectic carbide in each alloy was quantitatively assessed.It was found that the mean weight loss oftbe experimental alloys increased with increasing volume fl'action of the network eutectic carbide. This is in general agreementwith the results reported for high chromium white irons. Through one-time impact-abrasion, static indentation and pure repetitive impact tests in addition to the usual wear surface scanning electron microscope (SEM) examination, various kinds of wear damage of low alloy white cast irons in the repeated impactabrasiontesting were revealed. In order to understandthe wear damage, the concept of true impact stress on the wear surfacewas emphasized and used to analyzethe mechanical interaction between the abrasive panicles and the wear surface. © 1997 Elsevier Science S.A. Keywords: Whilecast iron;Abrasivewear:Impactabrasion:Eutecticcarbide
1. Introduction The research and development of abrasion resistant low alloy white cast irons have been somewhat neglected due to their inferior toughness and wear resistance to those of high chromium cast irons. However, the high chromium content of the latter alloy inevitably makes the price of the high chromium iron castings relatively high, especially in countries such as China where the chromium resource is not abundam. Hence, to develop low-cost but impact-abrasion resistant low alloy white cast irons is always of practical importance. By using rare earth (RE) modification techniques and heat treatment to change the morphology and distribution of eutectic (Fe,M)3C [ 1-4], the authors had successfully developed a Mn--Cr--Cu-Mo low alloy white cast iron (Fe-(3.0--4.0)Mn-( 1.5-2.0)Cr-(0.8-1.5)Cu(0.5-4).8)Mo, where the composition here and throughout is given in weight per cent), and put it into practical use for manufacturing liner plates of dimensions 670 × 440 × 30 ~ 50 mm of ball mills ( diameter 2.87 m; length 4.70 m). The liner lasted three years of field tests in milling coal in electric power plants without failure, showing a life 2 ~ 3 times that of the anstenitic Mn steel liner [ 5 ]. Then, attempts * Correspondingauthor.Presentaddress:MaterialsDivision.Department of Mechanical and ProductionEngineering,NationalUniversityof Singapore.Singapore119260.E-mail:[email protected] 0043-1@48/97/$17.00 © 1997ElsevierScienceS.A.All rightsreserved Pll S0043-1648 ( 96 ) 07345-0
[6,7] were made to insert steel wires in the newly developed white cast iron to improve its toughness. As all these efforts were aimed at ensuring the safe use of low alloy white irons in impact-abrasion conditions, it is necessary to gain a good understanding of the impact-abrasion behavior of these alloys for optimizing their micmstructures. A review of the abrasive wear study of alloy white cast irons has recently been given by Sare et al. [8]. After having classified the reported wear tests that had involved impact loading into nine groups [ 8 ], they pointed out that few of the tests had combined impact loading with abrasion for white cast irons. Owing to the recent work by Sate et al. [8], Sare and Arnold [9] and Xu et ai. [ 10], the impact-abrasion behavior of high chromium white irons has been brought to light to some extent through both laboratory and field wear tests. Though Ref. [ 8 ] also presented some information about the impact-abrasion behavior of Ni-Cr I and Ni-Cr 2 white cast irons, such information remains very scarce with respect to low alloy white irons. In this work, we first defined a morphological parameter--the roundness area fraction SFf-to quantitatively describe the morphology of the eutectic carbide. Then a series of low alloy white cast irons with different morphologies of eutectic cementite was tested using a repetitive impact-abrasion tester. To understand the wear mechanisms, we designed one-time impact-abrasion, static indentation, and pure repetitive impact tests in addition to the
M. Qian, W. Chaochang/ Wear209 (1997)308-315 usual wear surface scanning electron microscope (SEM) examination. Finally, we estimated the value of the true impact stress on the wear surface and used it to judge the wear damage involved.
2. Experimental procedures 2.1. Materials and specimen preparatiop The Mn-Cr-Cu low alloy white cast iron used in this investigation had a nominal composition of Fe-(2.2-3.4)C! .20Si-4.0Mn-! .80Cr-1.50Cu-0.53S-0.065P similar to that of the Mn-Cr-Cu-Mo alloy. It was melted in an induction furnace with the superheating temperature of 1450+20 °C. Modified by RE, R E A l and Re-AI-N-Ti [ 3 ], the alloy melt was poured into green sand molds of round bars (diameter 8 ram; length 70 ram) and rectangular bars (dimensions I ! x ! I × 80 mm), respectively. All of the as-cast samples were tempered at 250 °C for 2.5 hours. The specimens prepared for the impact-abrasion test were precisely cut into bars of diameter 8 mm and length 25 mm, but for the one-time impact-abrasion test one of the two end surfaces of the specimen should be polished and etched. Specimens for the static indention test were prepared at the size of 10 × 10 × 15 ram, polished and etched. For the pure repetitive impact test, specimens of dimensions 10 × l0 x 70 mm were used with one of the rectangular surfaces (length 70 mm; width 10 mm) being polished and etched. 2.2. Quantimtive analysis of the eutectic carbide
309
buritic for SF<0.25, the irregular for SF= 0.25 ~0.50, and the blocky and nodular for SF=0.50~ !.00. Let At, A2 and A3 be the area fraction of each of these three types of eutectic carbides, SF~, SF2 and SF3 will then represent the fraction of the network-like, the irregular, and the blocky eutectic carbides, respectively. Having analyzed more than two hundred viewing fields using SF~, we have found that this parameter could reasonably describe the morphology of the eutectic carbides in white cast irons as shown in Fig. 2. For each alloy tested, 20 viewing fields were analyzed under a magnification of 200 times on a Cambridge Quantimet 900. In order to avoid the effects of secondary carbides, carbide particles below 5 p.m in size were neglected. 2.3. Impact-abrasion test 2.3.1. Test machine The wear tester used here is known as the MLD- I 0 impactabrasive wear test machine made in China. Fig. 3(a) shows a schematic diagram of the machine (see also [ 12] ). The impact work depe,ds upon the falling distance of the dropping hammer with an adjustable impact frequency. The counterweadng part, arrowed as the lower specimen in Fig. 3(a), is a ring of dimensions 4350X1330 X 20 mm made of quenched 40Cr steel having a uniform hardness distribution (HRC 50 ~ 51 ). A rotating speed of 200 rpm was chosen for the lower specimen in this test. 2.3.2. Abrasives Quartz particles (polyhedral) with 99.78% about 1.5 ~ 4 mm in size were utilized as abrasives. The average flow rate of the abrasive particles through the outlet (section size, 25 × 13 mm) was 2.6 kg rain- i.
morphology As there is no general rule to describe the morphology of the eutectic carbides, few morphological parameters for eutectic carbides have been reported. Here a parameter defined as roundness area fraction (SFi) is utilized, where SFi =AJAo, SF denotes the roundness of a carbide particle (SF = 2vI( ~rA)/Lp [ I I ] ), A~is the area fraction of the carbide with certain roundness in the viewing fields, and Ao is the area fraction of all of the carbides. By the classification of the morphology of particles in terms of roundness (see Fig. I ) [ i i ], the shape of the eutectic carbide can be roughly classifted into three types. Namely, the network-like and lede-
~t ~O Fig. I. Classification of the molphology of particles according to the value
of roundness(SF) [ I I l.
Fig. 2. (a) A structurein whichthe networkeutectic carbide predominates (SF1:0.843. SF2:0.128, 5F~:0.029); (b) a structure in which the isolated eutectic carbide predominates (SFI=0.207. SF2=0.564. SF~=0.229).
M. Qian. W. Chaochang I Wear209 (1997)308-315
310
Co)
U Fig. 3. Schematicdiagramsof (a) the im tact-abresiontesterand (b) the purerepetitiveimpacttester. 2.3.3. One-time impact-abrasion test In order to identify the true impact stress on the wear surface and the impact patterns of the abrasive particles against the wear surface, the one-time impact-abrasion test was c&aried out for the etched specimens under an impact work of 0.98 J cm -2 (the falling distance of the dropping hammer was 5 ram). 2.3.4. Repetitive impact-abrasion test Prior to the actual test each specimen was subjected to a preliminary impact-abrasion test for 20 min to maximize its contact area with the lower specimen under an impact work of 0.49 J cm -2 and an impact frequency of 108 blows/rain. The worn specimen was ultrasonically cleaned and weighed using an analytical balance (resolution to ! ttg). Then the specimen underwent the actual impact-abrasion test with an impact work of 0.98 J cm -2 and the same impact frequency for 30 rain. After the test, the specimen was similarly cleaned and weighed. For each alloy, three specimens were tested for an average weight loss. The deviation of the weight loss for each specimen was within I1%. The worn surface of the specimen was examined using a scanning electron microscope. Z4. Static indentation test
As a kind of well-accepted approach to study the fracture behavior of brittle solids [ 13], the static indentation test has been shown to be a simple and effective method to investigate the microfracture mechanism of white cast irons [ 14]. In a three-body impact-abrasiou system, each abrasive particle can approximately be regarded as an individual indenter. Therefore, it is possible to qualitatively simulate the intrusion of an abrasive particle into the wear surface by means of an indentation test. Since the shapes of the abrasive particles are usually polyhedral or nearly round, a Vickers indenter and a Rockwell indenter were adopted to simulate the polyhedral and round abrasive particles, respectively. After an indent was created on the surface of the specimen using a standard hardness tester (Vickers or Rockwell), SEM examinations were carried out around the indent for microcracks.
2.5. Pure repetitive impact test
The purpose of this test is to reveal some of the fracture behavior of low alloy white cast irons under pure repetitive impact loading. The test unit (see Fig. 3(b)) is a modified form of the same impact-abrasion test machine. The etched surface of the horizontal specimen lies downwards. The impact work was 1.96 J c m - 2. After the horizontal specimen was fractured under the repeated impact loading ( 108 blows/ rain), examinations were conducted on the etched surface for any possible microcracks using an SEM. 3. Results and discussion 3.1. Impact-abrasion test results
The results of weight loss for the alloys tested together with their microstructural parameters are summarized in Table l. The matrix structure for each alloy was practically identical, that is, a mixture of martensite, m(ained anstenite, small amount of pearlite, and some secondary carbides. By means of linear regression analys:% a meaningful trend was found between the mean weight loss A W and the volume fraction of the network eutectic carbide SFj Vo i.e. A W=0.1398+ 1.6389× iO-2×SFjVr
( I)
with F=16.64>Fo.ofll,n-2)=12.2 ( n ~ 9 ) , and R = 0.839>Ro.ot(n-2) =0.798. This reveals that weight loss increases with increasing amount of the network eutectic carbide over the range investigated. Note that most of the eutectic carbides in the alloys tested in Ref. [8] were network-like: the present impact-abrasion test results thus coincide well with those of Ref. [ 8 ], which showed that the mean weight loss of alloy white cast irons in the repeated impactabrasion testing increased with increasing volume fraction of the eutectic carbides [8]. Also, in both studies, the intercarbide spacing exerted no linear effects upon A W. 3.2. Wear surface characteristics
The wear damage that appeared on the worn surface of the low alloy white cast iron in this test is similar to that reported
M. Qian. W. Chaochang / Wear 209 f l997) 308-315
311
Table 1 Microstructural parameters and wear test results of the alloy~ tested Alloys
I 2 3 4 5 6 7 8 9 IO II 12
lntercarbide spacing (tim)
Eutectic carbide volume (V~%)
Fraction of network-like eu~ctic carbides ( SFt % )
Fraction of irregular eutectic carbides (SF,%)
Fraction of blocky eutectic carbides (SF3%)
Bulk hardness tHRC)
Mean weight loss (AWing)
122.39 I 15.91 124.71 69.04 120.53 IO7.97 60.74 93.'/9 68.06 59.45 55.32 52.78
5.53 7.00 6.83 I 1.84 8.33 9.41 14.63 I 1.16 16.37 20.27 23.68 28.82
22.63 36.69 36.27 36.49 46.59 61. I0 6~.81 80.72 85.17 84.36 89.92 88.38
44.20 47.10 43.11 39.73 41.24 27.57 21.80 13.95 I 1.80 9.23 7.00 8.97
33.17 19.30 20.62 23.78 12.1 "; I 1.33 8.39 5.33 3.03 6.41 2.08 2.65
50.5 51.7 50.8 53.1 52.5 53.4 54.2 53.6 54.9 55.3 54.7 55.0
0.13785 0.12278 0.13109 0.24620 0.25345 0.28317 0.31295 0.32371 0.31315 - a - • -"
a Meaningless It, measure A W due to local macrofractare at the wear surface.
Fig. 4. SEM micrographs of the wear damage on the worn surface: (a) gouges and plastic deformation; (b) spalliog craters; (c) microcut grooves and microcracks; (d) local macroscopic fracture.
for high chromium white cast irons [ 8,10,12], such as local plastic deformation, gouges ( Fig. 4(a ) ), fatigue spalling craters ( Fig. 4(b ) ), microcut grooves ( Fig. 4(c) ), and microcracks (Fig. 4( c ) ). All these phenomena have been observed on the worn surface of a specimen. This implies that the wear conditions at different areas of the wear surface may vary significantly. Fig. 4(d) refers to the area at which local macroscopic fracture occurred on a wear surface that contains a relatively large amount of network eutectic carbide. This has less frequently been reported with respect to high chromium white cast irons. Fig. 5 shows the impact patterns of the abrasive particles against the wear surface. Fig. 5(a) and (b) were taken from the one-time impact-abrasion test. As shown in Fig. 5(a), abrasive particles could directly impact against the eutectic
carbide network structure on the wear surface, it is presumed that accumulation of such damage from repeated impactabrasion would readily cause the brittle network structure to fracture and spall (Fig. 5(c) ). Fig. 5(b) shows that the tips of the abrasive particles could also intrude themselves into the metal matrix when the tip size is less than the interearbide spacing or the net diameter. More detailed examinations under high magnifications revealed that the intrusion of abrasive particles into the wear surface could cause microcracks to form around the inlrusion periphery (Fig. 5(d)) due to the surrounding tensile stress. The intrusion of abrasive particles and local plastic deformation are two of the most common features observed on the worn surface. As the former is also accompanied by local plastic deformation, it is necessary to make clear how local
312
M. Qian, W. Chaochang I Wear 209 (1997) 308-315
Fig. 5. Impact patterns of abrasive particles against the weac surface: (a) against the carbide netwoflcstructure; (b) intrusion of abrasive particles; ( c ) network eutectic carbide on the worn surface: (d) microcracks formed at the intrusion periphery.
plastic deformation affects the wear process other than the apparent work hardening effect. 3.3. Microfracture behavior caused by local plastic deformation Fig. 6(a) and (b) show respectively the Vickers and the Rockwell indents created on the surface of the specimen. Local plastic deformation occurred both inside and outside the indents. For the series of alloys tested, it is noted that when the load applied to the Vickers indenter reached 49 N, microcracks appear to form around the indent (Fig. 6(c)). For the Rockwell indenter, the critical load causing microcracking is about 306.25 N, but few microcracks were found inside the indent, which is to be expected because of the compressive stress state there. The microcracks in Fig. 6(c) and (d) appeared to resemble those detected in Fig. 5(d) in nature. Besides, it was shown by the static indentation tests that the network eutectic carbide in low alloy white cast irons cracked easily (Fig. 6(e) ) in contrast to the blocky entectic carbides (Fig. 6(f) ), and the initiated microcracks tended to propagate along the eutectic carbide network structure [ 14]. Fig. 6 indirectly demonstrates that the local plastic deformation on the wear surface, occasioned by the intrusion or impact of abrasive particles, would potentially cause the microcmcks to initiate and propagate in the wear surface layer so long as the local plastic deformation or the fatigue damage exceeds a certain limit. The pure repetitive impact test also provides an insight into the microfracture behavior of the experimental alloys. The network eutectic carbide first underwent severe fragmentation due to the repeated impact loading (Fig. 7(a)). Then further development of this phenomenon caused the fragmented eutectic carbide to spall (Fig. 7(b)), thus naturally
forming a microcrack along the eutectic carbide network (Fig. 7(b)). Finally, the microcracks initiated at different eutectic carbides linked up with each other (Fig. 7(c)). Figs. 5-7 have presented a general explanation of the impact-abrasion test results from the mierostmctural aspects. The following section will concentrate on the mechanical aspects. 3.4. True impact stress on the wear surface For a given abrasive wear system, the mechanical interaction between the abrasive particles and the wear surface determines the wear process. Consider a moving body impacting against another, between them abrasive particles exist just similar to all three-body impact-abrasion conditions. Clearly, contact can only occur over local areas a~, a2, etc., which gives the true area of contact. A,=a~ +a2+a3+...a,
(2)
where a~denotes the contact area with the ith abrasive particle. Suppose that the impact load W or the weight of the moving body is allowed to fall freely onto the impacted body, the equivalent static load (WEO) can be written as WEQ = KdW
(3)
in which Kd, the impact factor, is given by [ 15,16] Kd-----l + ( l + 2h/~5~t)I/2
(4)
where h is the falling distance of the impact load and 8~t is the static deflection of the impacted body under a gradually applied load W. The true impact stress 8, is then formulated
by 8, = KdW/A,
(5)
M. Qian, W. Chaochang / Wear 209 (1997) 308-315
313
Fig. 6. (a) A Vickersindent: (b) a Rockwellindent: (¢) microcracksaround(a): (d) microcracksaround(b): (e) crackedcarbide network;(f) cracked blockycarbides. We now use Eq. (5) to calculate the true impact stress in this test. The impact to which the upper specimen was subjected in Fig. 3(a) is equivalent to applying an impact load of IO kgfto it with the same falling distance (h = 5 mm). For 1=25 ram, Ao=50.25 mm2, W=98 N, and E--- 165 Gpa [ 17], we have 8~,=Wl/EA=2.956× IO-4 ram. Then substituting for 8~, and h in Eq. (4) results in gj-~ 183.93. By measuring the impacted area, i.e. the white area in Fig. 8 for a total of six specimens, the true impacted area A, was determined in the range 5.3 -- ! 5.2%Ao. Thus Eq. (5) finally gives 6t -- 2360 ~ 6769 N mm -2, a value which is significantly greater than the nominal static stress (W/Ao) and the nominal impact stress (KdW/Ao). Of course, such a magnitude of stress may rarely be attained in practice, because the quartz abrasive particles will quickly get fragmented before that. But it helps us to qualitatively understand the intrusion of abrasive particles into the wear surface and the impact damage imposed on the network eutectic carbide and the matrix structure by comparing the value of o', with the mechanical properties of these micro-constituents. The difference in the true impact stress at different areas on the wear surface con-
tributed to various kinds of wear damage observed on the worn surface. Owing to the impact fatigue caused by the local high repetitive impact stress, the alloys with a large amount of network eutectic carbide exhibited poor structural integrity (Table 1). The role that the network eutectic carbide possibly plays in this process has been demonstrated in Figs. 5-7. it should be noted that even for high chromium white cast irons, which possess relatively good structural integrity, their resistance to impact-abrasion decreases as the amount of the eutectic carbide increases [ 8]. Hence, by this work, we concluded that the favorable volume fraction of the eutectic carbide in low alloy white cast irons for impact-abrasion use should be around 10%. The less the amount of the network eutectic carbide, the better the structural integrity and the impactabrasion resistance would be. 4. Conclusions I. A series of low alloy white cast irons with different morphologies of eutectic carbides has been tested using a
314
M. Oian, W. Chaochang / Wear 209 (1997) 308-315
Fig. 7. Fracture behavior of white cast ire~nsunder repeated impact loading: (a) fragmented carbide network; (b) spalling of fragmented carbides; (c) wopagation of mir,rocracks.
ment, and gradually turn into microcracks under repeated impact loading. 5. The true impact stress on local areas o f the w e a r surface is probably much greater than the strength o f the microconstituents o f the w e a r surface. To understand the w e a r damage, the true impact stress should always be considered.
Fig. 8. Impacted area (white) on the worn surface after one-time impactabrasion test.
Acknowledgements Helpful c o m m e n t s from the reviewer are appreciated.
repetitive impact-abrasion tester. The mean weight loss o f the experimental alloys increased with increasing volume fraction o f the network eutectic carbide. The favorable volume fraction o f the eutectic carbide in low alloy white cast irons for impact-abrasion use should be around 10% according to this work. 2. lntrnsion o f abrasive particles into the w e a r surface could directly cause microcracks to form around the intrusion periphery. The eutectic carbide network structure on the w e a r surface .was subjected to strong repetitive impact from the abrasive panicles. 3. It w a s revealed b y the static indentation test that the local plastic deformation on the w e a r surface occasioned by the intrusion or impact o f the abrasive particles could energetically cause the surrounding eutectic carbide to crack. 4. It was shown by the repetitive impact test that the network eutectic carbide in low alloy white cast irons would frag-
References [ I ] Q. Ma, B. Liu and C. Wang. Breakup of eutectic carbide network of white cast irons at high temperatures.J. Mater. Sei., 30 (1995) 33833386. [2] C. Wang. H.T. Hsu and Q. Ma. Formation of spheroidal carbide in vanadium white cast iron by RE modification. Mater. Sci. Tech.. 6 (1990) 905-910. [ 3 ] Q. Ma, C. Wang and S. Harada, Modification of hypoeutectic low alloy white cast irons../. Mater. ScL.31 (1996) 1865-1871. [4l Q. Ma and C. Wang. Effect of rare earth modification on the
agglomerationof eutectic carbides in heat treatmentof whitecast irons, Iron andSteel, 25 (1990) 1I, 52-57 (in Chinese). [ 51 Shanxi Electric Power Bureau, Appraisal report on KmTBMn3Cr2Mo wear resistant ball n',ill liner plates in electric power industry (in Chinese ). No. 89-2-008. Oct. 6. 1989 (Shanxi Electric Power Bureau. Shanxi Province. P.R. China). [6l Q. Ma. S. Harada, Y. Li and D. Man. On fabrication of steel wire reinforced white cast irons, Mater. Sei. Eng.. A206 (1996) 104-109.
M. Qian. W. Chaoehang / Wear 209 (1997) 308-3!5 [71 A. Zhao and C. Wang. Strengthening, toughening and fracture behavior of composite white cast irons, J. Univ. ScL Tech. Beijing. ~'3 { i'~)i J 123-129 (in Chinese). [ 8 ] I.R. Sare, B.K. Arnold. G.A. Dunlop and P.G. Lloyd. Repeated impactabrasion testing of alloy white cast irons, Wear. 162-164 { 1993) 790801. [91 l.R. Sate and B,K. Arnold. The effect of heat treatment on the gouging abrasion resistance of alloy white cast irons, ?detail. Trans.. 26A (1995) 357-370. 10] L. Xu, C. Vose and D. St. John. Abrasive wear study of selected white cast irons as liner materials for the mining industry, Wear, 162-164 ( 1993 ) 820-832. I I ] S.A. Sahykov. Stereometrische Metallographie, VEB Deut. Veil. f. Grundstoffindustrie, Leipzig. 1974) p. 334 (cited in E.E. Underwood and J.T. Berry. AFS Trans., 89 (1981) 755).
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[ 12] L Tong, Y. Zhuu. T. ,qhe- and H. Dt..Z~. The influence of retained auslenite in high chromium ca.st iron on impact-abrasive wear. Wear, 135 { 1990) 217-226. [13i C.B. Pomon and R.D. Rawlings, Viekers indentation fracture toughness test, Part I, Mater. Set. Tech,, 5 (1989) 865-872. [14l Q. Ma. C. Wang and S. Harada. Initiation and propagation of microcracks in white cast irons under static indentation test, AFS Trans., 104 (1996) (in press). 15] F.E, Miller and H.A. Doedngsfeld, Mechanics of Materials, Imemational Textbook Company. Semmon. Penn., USA. 1955, pp. 417-419. 16[ A. Jensen and H.H. Chenoweth. Applied Strength of Materials. McGraw-HilL New York, USA. 2nd edn.. 1967. pp. 323-325. 17I ASM Handbook Cammitte¢. Metals Handbaok. Properties and Selectians: Irons and Steels. Vol. I. American Society for Metals. Metals Park. OHI0, USA. 9th edn.. 1987, pp. 86--87.
Wear209 (1997) 308-315
Impact-abrasion behavior of low alloy white cast irons Ma Qian a.., Wang Chaochang b Department of Mechanical Engineering, Tsinghua University; Beijing 100084. P.R. China b Department of Metallurgy (Foundry). University of Science and Technology. Beijing 100083. P.R. China Received 2l November 1995; accepted 6 September 1996
Abstract The impact-abrasionbehavior of a series of low alloy white cast irons with different moo-phologiesof eutectic carbide was investigated usinga repetitive impact-abrasiveweartester. By meansof a defined morphologicalparameter,the roundnessareafractionSF~,the morphology oftbe eutectic carbide in each alloy was quantitatively assessed.It was found that the mean weight loss oftbe experimental alloys increased with increasing volume fl'action of the network eutectic carbide. This is in general agreementwith the results reported for high chromium white irons. Through one-time impact-abrasion, static indentation and pure repetitive impact tests in addition to the usual wear surface scanning electron microscope (SEM) examination, various kinds of wear damage of low alloy white cast irons in the repeated impactabrasiontesting were revealed. In order to understandthe wear damage, the concept of true impact stress on the wear surfacewas emphasized and used to analyzethe mechanical interaction between the abrasive panicles and the wear surface. © 1997 Elsevier Science S.A. Keywords: Whilecast iron;Abrasivewear:Impactabrasion:Eutecticcarbide
1. Introduction The research and development of abrasion resistant low alloy white cast irons have been somewhat neglected due to their inferior toughness and wear resistance to those of high chromium cast irons. However, the high chromium content of the latter alloy inevitably makes the price of the high chromium iron castings relatively high, especially in countries such as China where the chromium resource is not abundam. Hence, to develop low-cost but impact-abrasion resistant low alloy white cast irons is always of practical importance. By using rare earth (RE) modification techniques and heat treatment to change the morphology and distribution of eutectic (Fe,M)3C [ 1-4], the authors had successfully developed a Mn--Cr--Cu-Mo low alloy white cast iron (Fe-(3.0--4.0)Mn-( 1.5-2.0)Cr-(0.8-1.5)Cu(0.5-4).8)Mo, where the composition here and throughout is given in weight per cent), and put it into practical use for manufacturing liner plates of dimensions 670 × 440 × 30 ~ 50 mm of ball mills ( diameter 2.87 m; length 4.70 m). The liner lasted three years of field tests in milling coal in electric power plants without failure, showing a life 2 ~ 3 times that of the anstenitic Mn steel liner [ 5 ]. Then, attempts * Correspondingauthor.Presentaddress:MaterialsDivision.Department of Mechanical and ProductionEngineering,NationalUniversityof Singapore.Singapore119260.E-mail:[email protected] 0043-1@48/97/$17.00 © 1997ElsevierScienceS.A.All rightsreserved Pll S0043-1648 ( 96 ) 07345-0
[6,7] were made to insert steel wires in the newly developed white cast iron to improve its toughness. As all these efforts were aimed at ensuring the safe use of low alloy white irons in impact-abrasion conditions, it is necessary to gain a good understanding of the impact-abrasion behavior of these alloys for optimizing their micmstructures. A review of the abrasive wear study of alloy white cast irons has recently been given by Sare et al. [8]. After having classified the reported wear tests that had involved impact loading into nine groups [ 8 ], they pointed out that few of the tests had combined impact loading with abrasion for white cast irons. Owing to the recent work by Sate et al. [8], Sare and Arnold [9] and Xu et ai. [ 10], the impact-abrasion behavior of high chromium white irons has been brought to light to some extent through both laboratory and field wear tests. Though Ref. [ 8 ] also presented some information about the impact-abrasion behavior of Ni-Cr I and Ni-Cr 2 white cast irons, such information remains very scarce with respect to low alloy white irons. In this work, we first defined a morphological parameter--the roundness area fraction SFf-to quantitatively describe the morphology of the eutectic carbide. Then a series of low alloy white cast irons with different morphologies of eutectic cementite was tested using a repetitive impact-abrasion tester. To understand the wear mechanisms, we designed one-time impact-abrasion, static indentation, and pure repetitive impact tests in addition to the
M. Qian, W. Chaochang/ Wear209 (1997)308-315 usual wear surface scanning electron microscope (SEM) examination. Finally, we estimated the value of the true impact stress on the wear surface and used it to judge the wear damage involved.
2. Experimental procedures 2.1. Materials and specimen preparatiop The Mn-Cr-Cu low alloy white cast iron used in this investigation had a nominal composition of Fe-(2.2-3.4)C! .20Si-4.0Mn-! .80Cr-1.50Cu-0.53S-0.065P similar to that of the Mn-Cr-Cu-Mo alloy. It was melted in an induction furnace with the superheating temperature of 1450+20 °C. Modified by RE, R E A l and Re-AI-N-Ti [ 3 ], the alloy melt was poured into green sand molds of round bars (diameter 8 ram; length 70 ram) and rectangular bars (dimensions I ! x ! I × 80 mm), respectively. All of the as-cast samples were tempered at 250 °C for 2.5 hours. The specimens prepared for the impact-abrasion test were precisely cut into bars of diameter 8 mm and length 25 mm, but for the one-time impact-abrasion test one of the two end surfaces of the specimen should be polished and etched. Specimens for the static indention test were prepared at the size of 10 × 10 × 15 ram, polished and etched. For the pure repetitive impact test, specimens of dimensions 10 × l0 x 70 mm were used with one of the rectangular surfaces (length 70 mm; width 10 mm) being polished and etched. 2.2. Quantimtive analysis of the eutectic carbide
309
buritic for SF<0.25, the irregular for SF= 0.25 ~0.50, and the blocky and nodular for SF=0.50~ !.00. Let At, A2 and A3 be the area fraction of each of these three types of eutectic carbides, SF~, SF2 and SF3 will then represent the fraction of the network-like, the irregular, and the blocky eutectic carbides, respectively. Having analyzed more than two hundred viewing fields using SF~, we have found that this parameter could reasonably describe the morphology of the eutectic carbides in white cast irons as shown in Fig. 2. For each alloy tested, 20 viewing fields were analyzed under a magnification of 200 times on a Cambridge Quantimet 900. In order to avoid the effects of secondary carbides, carbide particles below 5 p.m in size were neglected. 2.3. Impact-abrasion test 2.3.1. Test machine The wear tester used here is known as the MLD- I 0 impactabrasive wear test machine made in China. Fig. 3(a) shows a schematic diagram of the machine (see also [ 12] ). The impact work depe,ds upon the falling distance of the dropping hammer with an adjustable impact frequency. The counterweadng part, arrowed as the lower specimen in Fig. 3(a), is a ring of dimensions 4350X1330 X 20 mm made of quenched 40Cr steel having a uniform hardness distribution (HRC 50 ~ 51 ). A rotating speed of 200 rpm was chosen for the lower specimen in this test. 2.3.2. Abrasives Quartz particles (polyhedral) with 99.78% about 1.5 ~ 4 mm in size were utilized as abrasives. The average flow rate of the abrasive particles through the outlet (section size, 25 × 13 mm) was 2.6 kg rain- i.
morphology As there is no general rule to describe the morphology of the eutectic carbides, few morphological parameters for eutectic carbides have been reported. Here a parameter defined as roundness area fraction (SFi) is utilized, where SFi =AJAo, SF denotes the roundness of a carbide particle (SF = 2vI( ~rA)/Lp [ I I ] ), A~is the area fraction of the carbide with certain roundness in the viewing fields, and Ao is the area fraction of all of the carbides. By the classification of the morphology of particles in terms of roundness (see Fig. I ) [ i i ], the shape of the eutectic carbide can be roughly classifted into three types. Namely, the network-like and lede-
~t ~O Fig. I. Classification of the molphology of particles according to the value
of roundness(SF) [ I I l.
Fig. 2. (a) A structurein whichthe networkeutectic carbide predominates (SF1:0.843. SF2:0.128, 5F~:0.029); (b) a structure in which the isolated eutectic carbide predominates (SFI=0.207. SF2=0.564. SF~=0.229).
M. Qian. W. Chaochang I Wear209 (1997)308-315
310
Co)
U Fig. 3. Schematicdiagramsof (a) the im tact-abresiontesterand (b) the purerepetitiveimpacttester. 2.3.3. One-time impact-abrasion test In order to identify the true impact stress on the wear surface and the impact patterns of the abrasive particles against the wear surface, the one-time impact-abrasion test was c&aried out for the etched specimens under an impact work of 0.98 J cm -2 (the falling distance of the dropping hammer was 5 ram). 2.3.4. Repetitive impact-abrasion test Prior to the actual test each specimen was subjected to a preliminary impact-abrasion test for 20 min to maximize its contact area with the lower specimen under an impact work of 0.49 J cm -2 and an impact frequency of 108 blows/rain. The worn specimen was ultrasonically cleaned and weighed using an analytical balance (resolution to ! ttg). Then the specimen underwent the actual impact-abrasion test with an impact work of 0.98 J cm -2 and the same impact frequency for 30 rain. After the test, the specimen was similarly cleaned and weighed. For each alloy, three specimens were tested for an average weight loss. The deviation of the weight loss for each specimen was within I1%. The worn surface of the specimen was examined using a scanning electron microscope. Z4. Static indentation test
As a kind of well-accepted approach to study the fracture behavior of brittle solids [ 13], the static indentation test has been shown to be a simple and effective method to investigate the microfracture mechanism of white cast irons [ 14]. In a three-body impact-abrasiou system, each abrasive particle can approximately be regarded as an individual indenter. Therefore, it is possible to qualitatively simulate the intrusion of an abrasive particle into the wear surface by means of an indentation test. Since the shapes of the abrasive particles are usually polyhedral or nearly round, a Vickers indenter and a Rockwell indenter were adopted to simulate the polyhedral and round abrasive particles, respectively. After an indent was created on the surface of the specimen using a standard hardness tester (Vickers or Rockwell), SEM examinations were carried out around the indent for microcracks.
2.5. Pure repetitive impact test
The purpose of this test is to reveal some of the fracture behavior of low alloy white cast irons under pure repetitive impact loading. The test unit (see Fig. 3(b)) is a modified form of the same impact-abrasion test machine. The etched surface of the horizontal specimen lies downwards. The impact work was 1.96 J c m - 2. After the horizontal specimen was fractured under the repeated impact loading ( 108 blows/ rain), examinations were conducted on the etched surface for any possible microcracks using an SEM. 3. Results and discussion 3.1. Impact-abrasion test results
The results of weight loss for the alloys tested together with their microstructural parameters are summarized in Table l. The matrix structure for each alloy was practically identical, that is, a mixture of martensite, m(ained anstenite, small amount of pearlite, and some secondary carbides. By means of linear regression analys:% a meaningful trend was found between the mean weight loss A W and the volume fraction of the network eutectic carbide SFj Vo i.e. A W=0.1398+ 1.6389× iO-2×SFjVr
( I)
with F=16.64>Fo.ofll,n-2)=12.2 ( n ~ 9 ) , and R = 0.839>Ro.ot(n-2) =0.798. This reveals that weight loss increases with increasing amount of the network eutectic carbide over the range investigated. Note that most of the eutectic carbides in the alloys tested in Ref. [8] were network-like: the present impact-abrasion test results thus coincide well with those of Ref. [ 8 ], which showed that the mean weight loss of alloy white cast irons in the repeated impactabrasion testing increased with increasing volume fraction of the eutectic carbides [8]. Also, in both studies, the intercarbide spacing exerted no linear effects upon A W. 3.2. Wear surface characteristics
The wear damage that appeared on the worn surface of the low alloy white cast iron in this test is similar to that reported
M. Qian. W. Chaochang / Wear 209 f l997) 308-315
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Table 1 Microstructural parameters and wear test results of the alloy~ tested Alloys
I 2 3 4 5 6 7 8 9 IO II 12
lntercarbide spacing (tim)
Eutectic carbide volume (V~%)
Fraction of network-like eu~ctic carbides ( SFt % )
Fraction of irregular eutectic carbides (SF,%)
Fraction of blocky eutectic carbides (SF3%)
Bulk hardness tHRC)
Mean weight loss (AWing)
122.39 I 15.91 124.71 69.04 120.53 IO7.97 60.74 93.'/9 68.06 59.45 55.32 52.78
5.53 7.00 6.83 I 1.84 8.33 9.41 14.63 I 1.16 16.37 20.27 23.68 28.82
22.63 36.69 36.27 36.49 46.59 61. I0 6~.81 80.72 85.17 84.36 89.92 88.38
44.20 47.10 43.11 39.73 41.24 27.57 21.80 13.95 I 1.80 9.23 7.00 8.97
33.17 19.30 20.62 23.78 12.1 "; I 1.33 8.39 5.33 3.03 6.41 2.08 2.65
50.5 51.7 50.8 53.1 52.5 53.4 54.2 53.6 54.9 55.3 54.7 55.0
0.13785 0.12278 0.13109 0.24620 0.25345 0.28317 0.31295 0.32371 0.31315 - a - • -"
a Meaningless It, measure A W due to local macrofractare at the wear surface.
Fig. 4. SEM micrographs of the wear damage on the worn surface: (a) gouges and plastic deformation; (b) spalliog craters; (c) microcut grooves and microcracks; (d) local macroscopic fracture.
for high chromium white cast irons [ 8,10,12], such as local plastic deformation, gouges ( Fig. 4(a ) ), fatigue spalling craters ( Fig. 4(b ) ), microcut grooves ( Fig. 4(c) ), and microcracks (Fig. 4( c ) ). All these phenomena have been observed on the worn surface of a specimen. This implies that the wear conditions at different areas of the wear surface may vary significantly. Fig. 4(d) refers to the area at which local macroscopic fracture occurred on a wear surface that contains a relatively large amount of network eutectic carbide. This has less frequently been reported with respect to high chromium white cast irons. Fig. 5 shows the impact patterns of the abrasive particles against the wear surface. Fig. 5(a) and (b) were taken from the one-time impact-abrasion test. As shown in Fig. 5(a), abrasive particles could directly impact against the eutectic
carbide network structure on the wear surface, it is presumed that accumulation of such damage from repeated impactabrasion would readily cause the brittle network structure to fracture and spall (Fig. 5(c) ). Fig. 5(b) shows that the tips of the abrasive particles could also intrude themselves into the metal matrix when the tip size is less than the interearbide spacing or the net diameter. More detailed examinations under high magnifications revealed that the intrusion of abrasive particles into the wear surface could cause microcracks to form around the inlrusion periphery (Fig. 5(d)) due to the surrounding tensile stress. The intrusion of abrasive particles and local plastic deformation are two of the most common features observed on the worn surface. As the former is also accompanied by local plastic deformation, it is necessary to make clear how local
312
M. Qian, W. Chaochang I Wear 209 (1997) 308-315
Fig. 5. Impact patterns of abrasive particles against the weac surface: (a) against the carbide netwoflcstructure; (b) intrusion of abrasive particles; ( c ) network eutectic carbide on the worn surface: (d) microcracks formed at the intrusion periphery.
plastic deformation affects the wear process other than the apparent work hardening effect. 3.3. Microfracture behavior caused by local plastic deformation Fig. 6(a) and (b) show respectively the Vickers and the Rockwell indents created on the surface of the specimen. Local plastic deformation occurred both inside and outside the indents. For the series of alloys tested, it is noted that when the load applied to the Vickers indenter reached 49 N, microcracks appear to form around the indent (Fig. 6(c)). For the Rockwell indenter, the critical load causing microcracking is about 306.25 N, but few microcracks were found inside the indent, which is to be expected because of the compressive stress state there. The microcracks in Fig. 6(c) and (d) appeared to resemble those detected in Fig. 5(d) in nature. Besides, it was shown by the static indentation tests that the network eutectic carbide in low alloy white cast irons cracked easily (Fig. 6(e) ) in contrast to the blocky entectic carbides (Fig. 6(f) ), and the initiated microcracks tended to propagate along the eutectic carbide network structure [ 14]. Fig. 6 indirectly demonstrates that the local plastic deformation on the wear surface, occasioned by the intrusion or impact of abrasive particles, would potentially cause the microcmcks to initiate and propagate in the wear surface layer so long as the local plastic deformation or the fatigue damage exceeds a certain limit. The pure repetitive impact test also provides an insight into the microfracture behavior of the experimental alloys. The network eutectic carbide first underwent severe fragmentation due to the repeated impact loading (Fig. 7(a)). Then further development of this phenomenon caused the fragmented eutectic carbide to spall (Fig. 7(b)), thus naturally
forming a microcrack along the eutectic carbide network (Fig. 7(b)). Finally, the microcracks initiated at different eutectic carbides linked up with each other (Fig. 7(c)). Figs. 5-7 have presented a general explanation of the impact-abrasion test results from the mierostmctural aspects. The following section will concentrate on the mechanical aspects. 3.4. True impact stress on the wear surface For a given abrasive wear system, the mechanical interaction between the abrasive particles and the wear surface determines the wear process. Consider a moving body impacting against another, between them abrasive particles exist just similar to all three-body impact-abrasion conditions. Clearly, contact can only occur over local areas a~, a2, etc., which gives the true area of contact. A,=a~ +a2+a3+...a,
(2)
where a~denotes the contact area with the ith abrasive particle. Suppose that the impact load W or the weight of the moving body is allowed to fall freely onto the impacted body, the equivalent static load (WEO) can be written as WEQ = KdW
(3)
in which Kd, the impact factor, is given by [ 15,16] Kd-----l + ( l + 2h/~5~t)I/2
(4)
where h is the falling distance of the impact load and 8~t is the static deflection of the impacted body under a gradually applied load W. The true impact stress 8, is then formulated
by 8, = KdW/A,
(5)
M. Qian, W. Chaochang / Wear 209 (1997) 308-315
313
Fig. 6. (a) A Vickersindent: (b) a Rockwellindent: (¢) microcracksaround(a): (d) microcracksaround(b): (e) crackedcarbide network;(f) cracked blockycarbides. We now use Eq. (5) to calculate the true impact stress in this test. The impact to which the upper specimen was subjected in Fig. 3(a) is equivalent to applying an impact load of IO kgfto it with the same falling distance (h = 5 mm). For 1=25 ram, Ao=50.25 mm2, W=98 N, and E--- 165 Gpa [ 17], we have 8~,=Wl/EA=2.956× IO-4 ram. Then substituting for 8~, and h in Eq. (4) results in gj-~ 183.93. By measuring the impacted area, i.e. the white area in Fig. 8 for a total of six specimens, the true impacted area A, was determined in the range 5.3 -- ! 5.2%Ao. Thus Eq. (5) finally gives 6t -- 2360 ~ 6769 N mm -2, a value which is significantly greater than the nominal static stress (W/Ao) and the nominal impact stress (KdW/Ao). Of course, such a magnitude of stress may rarely be attained in practice, because the quartz abrasive particles will quickly get fragmented before that. But it helps us to qualitatively understand the intrusion of abrasive particles into the wear surface and the impact damage imposed on the network eutectic carbide and the matrix structure by comparing the value of o', with the mechanical properties of these micro-constituents. The difference in the true impact stress at different areas on the wear surface con-
tributed to various kinds of wear damage observed on the worn surface. Owing to the impact fatigue caused by the local high repetitive impact stress, the alloys with a large amount of network eutectic carbide exhibited poor structural integrity (Table 1). The role that the network eutectic carbide possibly plays in this process has been demonstrated in Figs. 5-7. it should be noted that even for high chromium white cast irons, which possess relatively good structural integrity, their resistance to impact-abrasion decreases as the amount of the eutectic carbide increases [ 8]. Hence, by this work, we concluded that the favorable volume fraction of the eutectic carbide in low alloy white cast irons for impact-abrasion use should be around 10%. The less the amount of the network eutectic carbide, the better the structural integrity and the impactabrasion resistance would be. 4. Conclusions I. A series of low alloy white cast irons with different morphologies of eutectic carbides has been tested using a
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M. Oian, W. Chaochang / Wear 209 (1997) 308-315
Fig. 7. Fracture behavior of white cast ire~nsunder repeated impact loading: (a) fragmented carbide network; (b) spalling of fragmented carbides; (c) wopagation of mir,rocracks.
ment, and gradually turn into microcracks under repeated impact loading. 5. The true impact stress on local areas o f the w e a r surface is probably much greater than the strength o f the microconstituents o f the w e a r surface. To understand the w e a r damage, the true impact stress should always be considered.
Fig. 8. Impacted area (white) on the worn surface after one-time impactabrasion test.
Acknowledgements Helpful c o m m e n t s from the reviewer are appreciated.
repetitive impact-abrasion tester. The mean weight loss o f the experimental alloys increased with increasing volume fraction o f the network eutectic carbide. The favorable volume fraction o f the eutectic carbide in low alloy white cast irons for impact-abrasion use should be around 10% according to this work. 2. lntrnsion o f abrasive particles into the w e a r surface could directly cause microcracks to form around the intrusion periphery. The eutectic carbide network structure on the w e a r surface .was subjected to strong repetitive impact from the abrasive panicles. 3. It w a s revealed b y the static indentation test that the local plastic deformation on the w e a r surface occasioned by the intrusion or impact o f the abrasive particles could energetically cause the surrounding eutectic carbide to crack. 4. It was shown by the repetitive impact test that the network eutectic carbide in low alloy white cast irons would frag-
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