Received 11 December 2009; accepted 22 March 2010 *Corresponding author. Tel: 86 516 83591916 E-mail address: sulyflying@cumt.edu.cn doi: 10.1016/S1674-5264(09)60283-0

Preparation and properties of a new cutting pick of coal shearers

LUO Yong1,*, ZHANG Dekun1, WANG Qingliang1, LIU Hongtao1, CHENG Gang2, GE Shirong2

1School of Material Science and Engineering, China University of Mining & Technology, Xuzhou 221116, China 2School of Mechanical and Electrical Engineering, China University of Mining & Technology, Xuzhou 221116, China

Abstract: Reliability and wear resistance of cutting picks play a significant role in coal mine exploitation with coal shearers. Tool bit separation, blade breaking, severe erosion of the cutting body and fatigue fractures are the main reasons for failure of cutting picks. We carried out carburization on a 30CrMnMo alloy to synthesize a new cutting pick material with improved mechanical properties and high wear resistance. The results indicated that carburization can effectively strengthen the surface of the 30CrMnMo alloy by forming a thick carburized layer and thus significantly improve the surface hardness and wear resistance. In addition, the excellent toughness of 30CrMnMo alloy as a substrate of cutting picks can prevent brittle ruptures and fatigue frac-tures under high impact stress conditions. The significant decrease in both frictional coefficient and rate of erosion of this carbur-ized 30CrMnMo alloy suggests that this alloy is a potential material for cutting picks of coal shearers after rational carburization. Keywords: cutting picks; coal shearer; carburization; wear

1 Introduction Based on a structural energy survey, both coal and

gas accounted each for 24.1% of primary energy consumption and oil for 34.8% in the world in 2008. However, China is still one of the leading consumers of coal in the world, accounting for 69% of primary energy consumption in 2008. Therefore, coal industry still plays an important role in economic development and is expected to maintain its irreplaceable position for a long time to come. Thus, coal exploitation is very important for which we expect high efficiency with a low cost coal shearer.

Cutting picks of coal shearers are most vulnerable to damage due to high impact stress in the process of coal exploitation shown as Fig. 1[1-8]. The SEM pho-tograph of damaged cutting picks is shown in Fig. 2, indicating that the wear and cracks could significantly accelerate the failure of cutting picks of coal shearers. Therefore, the failure of cutting picks could not only decrease the efficiency of coal exploitation consid-erably, if the worn and corrupted cutting picks were not timely replaced, but would also increase the cost of exploitation. For the material of cutting picks, one generally chooses a mild carbon alloy steel such as

35CrMnSi, 30CrMnSi, 40Cr and 45, where the ce-mented carbides are welded on the mild carbon alloy steel substrate to improve wear resistance[9-13]. It has been pointed out that tool bit separation, blade break-ing, severe wear of the cutting body and fatigue frac-tures are the main reasons for failure of cutting picks of coal shearers during mine exploitation[14-15]. Thus, it is necessary to synthesize cutting picks with high toughness and good wear resistance to decrease the cost.

Fig. 1 Cracks on blade of cutting picks of coal shearer

Fig. 2 Damage and broken cutting picks of coal shearer

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LUO Yong et al Preparation and properties of a new cutting pick of coal shearers 795

In the present study, we chose a 30CrMnMo alloy as the substrate of cutting picks due to its high toughness. Then we performed carburization on this alloy to strengthen the surface and improve its wear resistance. After that, surface topography and me-chanical properties of carburized 30CrMnMo alloy were investigated. Finally, the friction and wear of this carburized alloy was studied to evaluate the po-tential application in cutting picks of coal shearers.

2 Experimental

A 30CrMnMo alloy was selected as the material for cutting picks of our coal shearer. The composition of this alloy is shown in Table 1.

Table 1 Composition of 30CrMnMo alloy (%)

Composition C Si Mn Cr Mo Fe

Content 0.3 0.4 1.3 1.0 0.7

Five 30CrMnMo alloy specimens were machined into discs with a diameter of 36, 5 mm thick. Before carburization, the specimens were polished with Ra=0.2 m and then ultrasonically cleaned in acetone for 15 min. Then, carburization was performed in a drip-style carburizing furnace. The temperature wasincreased to 925 °C over a period of one hour and then kerosene and methanol was dripped into the furnace at speeds of 240 and 120 drip/min. After 4 hours carburization, the specimens were kept at 880 °C, quenched in #20 oil and then tempered at 220 °Cfor 2.5 hours.

The surface hardness of the carburized alloy was measured on a Vickers hardness tester with a normalload of 1.96 N. In addition, the phase and structure of this carburized alloy was analyzed with the help of metallographic photographs. In the end, we investi-gated the friction and wear of the carburized alloy on a CETR universal multifunction tester against a GCr15 pin under dry friction.

3 Results and discussion

The surface hardness of our carburized 30CrMnMo alloy is shown in Fig. 3. We found the surface hard-ness of the carburized 30CrMnMo alloy to be 544.27 HV, a significant increase of 305.8% compared to the hardness of the 30CrMnMo alloy. This indicates that the carburization effectively strengthened the surface of the 30CrMnMo alloy. The carburizing layer was 1.5 mm thick, which indicated that the carburizing layer could provide effective support to the substrate of the 30CrMnMo alloy. Furthermore, the very toughness of the 30CrMnMo alloy and outstanding bonding between the carburizing layer and the sub-strate makes carburized 30CrMnMo alloy a potential candidate to be manufactured as the cutting pick of coal shearers.

Fig. 3 Surface hardness of carburized 30CrMnMo alloy

A metallurgical photograph of a carburized 30CrMnMo alloy is shown in Fig. 4. It is to be noted that a large amount of fine martensity phase was formed after the quenching process. Since the car-burization provides for a high concentration of carbon atoms, it is easy to understand the formation of mart-ensity after the quenching process. Therefore, the surface of 30CrMnMo alloy has strengthened owing to this hard martensity, which provides for a complete understanding of the increase in hardness (see Fig. 3).

The coefficient of friction of carburized 30CrMnMo alloy is shown in Fig. 5. It is too noted that the coefficient of friction of 30CrMnMo alloy quickly increases to 0.65, under dry friction condi-tions and fluctuates over time. Since the 30CrMnMo alloy is soft, the surface of alloy is vulnerable to plas-tic deformation, adhesion and scratch, resulting in a high coefficient of friction and caused damage to the alloy. It is also noted that the coefficient of friction of the carburized 30CrMnMo alloy decreased to 0.4 and remained stable. The results indicated that the car-burization has effectively strengthened the surface of the 30CrMnMo alloy and improved resistance to de-formation, leading to reduced damage.

Fig. 4 Microstructure of carburized 30CrMnMo alloy

Fig. 5 Coefficient of friction of carburized 30CrMnMo alloy

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The rate of erosion of carburized 30CrMnMo alloy is shown in Fig. 6. It is demonstrated that this rate of 30CrMnMo alloy decreased from 3.2×10–3 to 0.5×10–3 g/m after carburization, indicating that its erosion resistance has significantly improved due to the formation of a thick and hard carburizing layer. These results suggest that carburized 30CrMnMo alloy is a potential candidate for cutting picks of coal shearers, thanks to the combination of high toughness and excellent surface wear resistance.

Fig. 6 Rate erosion of carburized 30CrMnMo alloy

4 Conclusions

Carburization was performed on a 30CrMnMo al-loy to synthesize a new cutting pick material with improved mechanical properties and high wear resis-tance. The results indicated that carburization can effectively strengthen the surface of 30CrMnMo alloy by forming a thick carburized layer and thus signifi-cantly improve the surface hardness and wear resis-tance. In addition, the excellent toughness of this al-loy as a substrate of cutting picks could prevent brit-tle rupture and fatigue fractures under high impact stress conditions. As well, the significant decrease in both the frictional coefficient and rate of erosion of carburized 30CrMnMo alloy suggest that this alloy is a potential material for the cutting pick of coal shear-ers after rational carburization.

Acknowledgements

Financial support for this study, provided by the Youth Foundation of China University of Mining & Technology (No.2009A056), the Tribology Science Fund from State Key Laboratory of Tribology at Tsinghua University (No.SKLTKF08A01), the Na-tional Natural Science Foundation of China (Nos. 50905180 and 51005234) and the National Science and Technology Pillar Program in the Eleventh

Five-Year Plan Period (No.2008BAB36B02) are gratefully acknowledged.

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