Properties of Cross-Rolled Low Alloy White Cast Iron Grinding Ball

Available online at www.sciencedirect.com

ScienceDirect JOURNAL OF IRON

AND STEEL RESEARCH, INTERNATIONAL. 2007, 14(5): 47-51

Properties of Cross-Rolled Low Alloy White Cast Iron Grinding Ball CHANG Li-min'

,

LIU L i d ,

LIU Jian-hua3

(1. Analysis and Measure Center, Jilin Normal University, Siping 136000, Jilin, China; 2. Department of Basic Medical, Tangshan Vocational Technology College, Tangshan 063000, Hebei, China; 3. State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, Hebei, China)

Abstract: The low-energy , multi-impact fracture resistance and the abrasiveness of the cross-rolled low alloy white cast iron grinding balls were studied after heat treatments a t residual rolling temperature. Moreover, the means by which they are damaged and characters of the wear surface were analyzed. The results show that high resistance to impact fracture and high abrasiveness can be achieved after appropriate heat treatment at residual rolling temperature. This kind of heat treatment technology has several advantages under low impact and hard abrasive. These results are very useful for determining the optimized heat treatment technology at residual rolling temperatures. Key words: low alloy white cast iron; grinding ball; cross rolling; impact fracture resistance; abrasiveness

Low alloy white cast iron has high hardness and wear-resistance; however, the material has low ductility, because the carbide in the structure is distributed in the form of continuous network, which limits its application. Therefore, the method to improve the toughness of this material has attracted the interests of the researchers worldwide. The experimental results showed that when white cast iron is hot deformed, the network structure of carbide breaks and becomes independent blocks, which results in the improvement of the ductility of the white cast iron. Because of the improved ductility of white cast i r ~ n [ l - ~ 'it, shows extensive prospects for application. The appearance of forging white cast iron grinding ball is of utmost Integration of white cast iron under hot working conditions with cross rolling grinding balls to make balls is a novel and valuable method for the improvement of the properties, productivity, and operating conditions of the grinding ball. But there are only few reports that explain how to roll the material that has permanent brittleness into balls. Although experiments have shown that it is possible to make low alloy white cast grinding ball using cross rolling, its property has not been elucidated. In this study, the mechanical property of the ball, especially the resistance

to impact fracture and abrasion are discussed.

1

Experimental

Experimental material The experimental material is low alloy white cast iron. Its composition (mass percent, %) is: C 1. 95, Mn 0. 65, Si 0.85, Cr 2.75, P 0. 034, and S 0.032. T h e materials were prepared by melting in a 250 kg intermediate frequency induction furnace, using pig iron, waste steel, and chromium iron as raw materials. Aluminum was added as deoxidant when the temperature of the molten metal reached 1 450 "C. T h e molten materials were then modified using RE Si-Fe alloy (0.20%) and were then poured into a ladle. Finally, it was cast into an ingot with the size of $50 mm X 80 mm in metal moulds. 1. 1

Experimental procedure T h e ingot was heated to 1 000 "C in an electric resistance furnace KJX-8-13 and was held at this temperature for 40 min. It was then cross rolled into $50 mm grinding ball ( t h e deformation amount measured is about 35% in the course of shaping of the grinding ball, which is controlled by the hole model). Following the cross rolling, the heat treatment was carried out with residual rolling tempera1.2

Foundation Item: Item Sponsored by Guiding Program of Science and Technology Research of Jilin Province of China (20000513) 9iography:CHANG Likmin(1966-), Male, Doctor, Professor; E-mail: aaaa2139B.163. cam; Revised Date: February 27, 2006

J o u r n a l of I r o n a n d Steel R e s e a r c h , International

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ture according to the technology listed in Table 1, and its hardness was then measured. Wind cooling : Wind cooling involves winding with air blower; concentration of the SSTlOl cooling media is 1. 2 % and the cooling capacity is between that of water and No. 20 oil. The low-energy impact fracture resistance of the grinding ball treated under different conditions was tested using self-made drop ball test machine. The procedure is as follows: 25 balls as a group, were dropped from a height of 3. 5 m and 4. 2 m respectively, and these balls were allowed to fall on another ball until the ball was crushed or surface shelled off. ( T h e mass of the shelled lump was greater than 100 mg. ) T h u s , N is the number of times that the ball is dropped, which is the average of three grinding ball crushings. Abrasion test was carTable 1

ried out using M L D 1 0 dynamic wear test machine to test its resistance to impact wear. All samples were found at the same site of each grinding ball, and their sizes were 10 mmX 10 mmX30 mm. The impact energy is 3.5 J. The abrasive in the test was hard abrasive (No. 80 Al,O,) and soft abrasive (No. 60 cement wrought material). During the test, the time taken by hard and soft abrasive are 10 and 30 min, respectively. T h e metallographic microstructure, crack initiation and propagation of the spalling lump, fracture surface, and characteristics of abrasion surface are observed under NEOPH21 metallographic microscope and KYKY lOOOB scanning electron microscope. T h e results are compared with those of the cast ball, which, with the same compositions, are cast in metal model and tempered at 200 "C for 2 h.

Heat treatment with residual temperature of cross rolling ~~

Number

2 2.1

Heat treatment with residual temperature

HRC

1 2

Castflow-temperature tempering (200 C ' for 2 h)

3

Air cooling after cross rollingfhigh-temperature tempering (500 'c for 2 h)

4

Wind cooling after cross rolling+low-temperature tempering (200 'c for 2 h)

Air cooling after cross rolling+ low-temperature tempering (200 'C for 2 h)

42-44 41 -43 41-43 48-50

5

Wind cooling after cross rollingfhigh-temperature tempering (500 'C for 2 h)

6

SSTlOl cooling after cross rolling+ low-temperature tempering (200 C ' for 2 h)

48-50 54-56

7

SSTlOl cooling after cross rolling+high-temperature tempering (500 'c for 2 h)

50-51

Results and Analysis Impact fracture resistances under low energy and repeated impacting

Various heat-treatment conditions of the crossrolling grinding ball result in different drop ball experimental numbers and different fracture forms (Table 2). The drop ball experiment numbers of the grinding ball tempered at low temperature is Table 2

smaller than those tempered at high temperature. Fractures appeared on the whole body of the ball tempered at low temperature whereas the ball tempered at high temperature is shelled off only on the surface. Besides, the cooling conditions after the cross rolling influences the result of the ball-on-ball impact tests. T h e quicker the cooling rate, the fewer the test numbers of the drop ball experiment,. and the bigger the fracture trend. As shown by the

Result of ball-on-ball impact tests B

N Number

Characteristics of fracture surface

3. 5 m

4. 2 m

1

28 147

25 105

2

32 468

28 170

3

38 895

4

32 115

33 286 26 954

5 6

38 602 28 342

7

32 717

3. 5 m

4. 2 m

Form of fracture

1

1

Central crack

Plain fracture surface, the structure is rayed

0. 89

0.89

Central crack

Plain fracture surface, the structure is compact

0.72

0.75

Small spalling

T h e thickness of spalling is I. 5-4

0.88

0. 93

Central crack

Plain fracture surface, the structure is compact

0. 73 0. 99

0. 77 1

Small spalling

T h e thickness of spalling is 1. 5-4

25 185

Central crack

Plain fracture surface, the structure is compact

27 247

0.86

0.92

Small spalling

Plain fracture surface, the structure is compact, the thickness of spalling is 1. 5-4 mm

32 528

Note: Relative fatigue fracture capacity ( B ) is cast ball number/cross rolled ball number.

mm mm

No. 5

49

Properties of Cross-Rolled Low Alloy White Cast Iron Grinding Ball -

relative fatigue fracture capacity ( B ) , the higher the height of drop ball, the larger the impact energy, and the smaller the difference of the resistance to repeated impacting fatigue fracture between crossrolled ball and cast ball. T h e final result is that the dropping number of cross-rolled ball is greater than that of the cast ball. As shown in macro-observation studies, the microstructure of fracture surface of the cross-rolled ball is obviously more compact than that of the cast ball, and there is no evidence of cast defect in the cross-rolled ball. SEM observation of the spalling from the grinding ball fractured in ball-on-ball impact tests shows that the cracks are centralized over the range of 100 pm on the surface of the grinding ball. The cracks are initiated on the carbide or on the interface between the carbide and matrix, and then propagated and converged (Fig. 1). T h e fractured surface of the cast ball showed the classical cleavage fracture whereas the partial region of the cross-rolled ball showed the meta-cleavage fracture. The cross-rolled ball has fine grain size. Tear ridge, and clear and tough nest appear in some small regions (Fig. 2 1 , which indicates that more energy is required for the formation of fracture, i. e. , the toughness of the cross-rolled ball is better than that of the cast ball. Stress under impact load is caused because of the brittle nature of the carbide in the grinding ball. Cracks are initiated inside the carbide when the stress is beyond its limit intensity. Moreover, the plasticity of the carbide and matrix are different, so are their resistances t o plastic deformation. T h u s , it easily causes stress concentration at the interface and results in the formation of cracksCg1.After repeated impacts, the cracks that have been initiated propagate continually and join together, which results in lump spalling. The carbide in cast ball mostly exists

in the form of network, which cuts apart the matrix. Besides, .the three-dimensional space network structure makes the free revolving, which lightens the stress concentration when impacted, difficult. As impact proceeds, the stress is concentrated at the sharp place of carbide, which results in the initiation of cracks. But the grinding ball after high-temperature rolling not only reduces cast defect (loose), but also crushes the network carbide and forms isolated lump (Fig. 3 ) , which protects the continuity of the matrix and alleviates the stress concentration and the initiation of cracks. Meanwhile, fine carbide is precipitated in the matrix, which hinders the propagation of the cracks and improves its toughness. The main reason for this is that the impact fracture resistances under lower energy and the repeated impacting of cross-rolled ball are better than those of cast ball. T h e quicker the cooling rate after rolling and the lower the tempering temperature, the stronger is the integral fracture tendency of the grinding ball, which shows as the decrease of the number of the ball-onball experiment (Table 1). As shown from the test results, the integral fracture of the grinding ball is formed before the lump spalling. Although the grinding balls cooled by SSTlOl are tempered at 500 " C , the integral fracture phenomenon still exists. It indicates that the residual stress in the ball is the main reason for the initiation of fracture. From the test result, it can be observed that the larger the impact energy, the smaller is the difference

( a ) Cast ball;

Fig. 2

(2)

Fig. 1

Morphologies of crack

(b) Cross-rolled ball (No. 5)

Morphologies of balls after impact fracture

Cast ball:

Fig. 3

f b ) Cross-tolled hall (No. 5 )

Morphologies of different balls

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50

Journal of Iron and Steel Research, International

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in the repeated impact fracture resistances between the cross rolled ball and the cast ball. It shows that the modification of the appearance and the distribution of the carbide can improve its toughness, but the effect under large impact energy is not very obvious as that under low impact. This may be mainly attributed to the brittleness of material. I t can be predicted that the cross-rolled low alloy white cast iron grinding ball has greater advantage over cast ball under low impact energy.

2. 2

Abrasiveness It can be observed from Table 3 that the abra-

siveness of the cross-rolled ball is better than that of the cast ball, regardless of whether it is under the hard abrasive or the soft abrasive test. Heat treatments with residual temperature considerably affect the abrasiveness of the cross-rolled ball. T h e abrasiveness is better when the cooling rate is faster and the tempering temperature is higher. It shows that the cross-rolled ball has better advantages under hard abrasive than under soft abrasive. Because the abrasiveness of low alloy white cast iron matrix is not so good as that of carbide, the carbide after wearing becomes convex when compared with the matrix. T h u s , the convex carbide will be impacted by the abrasive. T h e carbide has no resist; therefore, ance to the impact of A1203 abrasiveC9] the fracture of carbide results in lump spalling under repeated impact of abrasives. T h e carbide in cast balls exists in the form of continuous network, and its distribution is not homogeneous; therefore, the abrasive can be easily thrust into the matrix where the distance of carbide is larger. T h u s , these sites wear out first and result in the formation of convex carbide. T h e continuous network of carbide limits the Table 3

Number

Result of wear tests

Wear weightlessness amount/mg Hard abrasive

Soft abrasive

F

Hard abrasive

Soft abrasive

1

123

46

1. 00

1. 00

2

94

41

1. 3 1

1. 12

3

90

40

1. 37

1. 1 5

4

78

32

1. 58

1.44

5

75

30

1. 64

1. 53

6

63

24

1. 95

1. 92

7

70

27

1. 76

1. 70

Vol. 14

deformation of subsurface and causes the stress centralization at the interface between the bottom of the convex carbide and the matrix, which initiates cracks and causes lump spalling and low abrasiveness. T h e carbide in the cross-rolled balls exists in the form of isolated lump, and its distribution is relatively homogeneous. Meanwhile, the hard granulated carbide precipitates from the matrix, which hinders the stress concentration and the initiation and propagation of cracks. Therefore, the trend of spalling is reduced and its abrasiveness is improved. T h e lower the tempering temperature, the bigger the inner remains stress. Thus, it is easy to initiate cracks, which can be peeled during impact wear, which is the reason of its poor abrasiveness. It can be observed from the appearance of the wear that the spalling blocks of the cross-rolled ball are smaller than that of the cast ball and its amount is fewer. It implies that the wear of grinding ball concludes not only a kind of microcutting of the abrasive onto the surface of sample, but also the fatigue spalling. Therefore, the hardness of the cast ball is close t o that of the crossrolled ball, but its abrasiveness is lower. T h e carbide has the ability of resisting the impact of soft abrasives, such as the cement wrought material. Even after repeated impact of cement wrought material, the carbide is not fractured. Its trend of spalling is relatively smaller than that of the hard abrasive. Its wear mechanism is mainly cutting. Therefore, the advantages of the cross-rolled balls, in the case of the cement wrought material, is not so good as that of the hard abrasive. With the increase in the cooling rate after rolling, the hardness of the matrix increases and the wear weightlessness of the matrix under the same wear condition decreases. Meanwhile, it matches well with the hardness of carbide, which is favorite to support the carbide. Thus, the tendency to lump spalling is reduced. The grinding ball cooled in SSTlOl has the highest hardness and abrasiveness, which has been supported by the wear appearances (Fig. 4). In general, the resistance to impact fracture and the abrasiveness of the cross-rolled ball is superior to that of the cast ball made from the same materials. Heat treatments with residual temperature can be a means that can be used to modify its properties to meet the needs of different working conditions.

3

Conclusions

Note: Relative abrasiveness ( E ) =wear weightlessness amount of

cast ball/wear weightlessness amount of cross-rolled ball

(1) T h e cross-rolled bail under low-energy re-

No. 5

P r o p e r t i e s of Cross-Rolled Low Alloy White Cast I r o n G r i n d i n g Ball

( a ) Hard abrasive (No. 51;

Fig. 4

(b) Soft abrasive (No. 5);

(c) Soft abrasive (No. 7)

Morphologies of wear surface of cross-rolled ball Structure in White Cast Irons [J]. Material in Engineering Application, 1979, 7(1): 143.146. Chakrabarti A K. Hot Forging of White Cast Irons [J]. Trans Indian Institute of Metals, 1980, 33(6): 467-473. Yuichiro Sato. On the Properties and Application of Forged White Cast Iron [J]. Trans of Japan Metals Inst, 1981, 23

peated impact fractures in the form of small pieces spalling and integral fracture were studied. The lower the tempering temperature, the faster the cooling rate after rolling, and the larger the residual stress inside the ball, the stronger the integral fracture trend. ( 2 ) Reducing the residual stress inside the grinding balls, completely crushing the carbide and homogenously distributing it are effective ways to improve the complex mechanical properties of the grinding balls. (3) T h e resistance to the low-energy repeated impact fracture and the abrasiveness of the grinding balls could be improved by heat treatment with residual rolling temperature, especially when the impact energy is not very large and it is under hard abrasive. ( 4 ) According to the test results, combined with the actual conditions, the best residual heat treatment conditions of the cross rolling are: wind cooling followed by tempering at 500 "C for 2 h.

(8) : 702-704.

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1994, 29(3): 53-55. XING Shu-ming, YANG Sheng-li, S H E N Huan-zhu. Study on Fracture Resistance of Forged White Iron Balls Under Repeated Impacts [J]. Iron and Steel, 1997, 32(5): 64-68. LI Wo-guang, C H E N Xian-chao, LIU Ke-ru. Experiment on Forged Manganese White Cast Irons Milling Balls [J]. Hot Working Technology, 1995, 117(5): 38-40. MA Qian, WANG Zhao-chang. Study of Initiation and Propagation of Microcracks in White Cast Iron by Indentation Method [J]. Physical Testing and Chemical Analysis, 1991, 12(6):

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