Technical Ways to Improve the Performance of Picks

时间:2019-09-30 作者: baidu baike 阅读量:
  Analysis of damage forms of pick
  Through the investigation of the damage forms of the pick in China and the analysis and impact test of some domestic and foreign picks, the main reasons for the failure of the pick are as follows:
  Low impact resistance
  Through a large number of observation of damaged pick, it is found that the damage of most pick is not normal wear failure, especially in hard rock cutting. The reasons are as follows.
  (1) The performance of alloy tool head is poor. There are graphite impurities in domestic alloys, uneven grain distribution and cracks in some alloys, which are the main causes of cemented carbide head cracking. Secondly, the pressing technology of domestic alloys is relatively backward, the density difference between upper and lower alloys is large, the porosity is large, and the hardness is low.
  (2) The residual stress of alloy head brazing is large. Because the thermal expansion coefficient of alloy cutter head and tooth material is quite different, and the shrinkage difference increases with the increase of brazing temperature. Copper-zinc brazing filler metal is mostly used in domestic pick. The welding temperature is above 950 C, and the welding residual stress is large.
  (3) The cracking and eccentric wear of the head of the pick make the alloy head lose its protective support prematurely. The main reason is that the performance of domestic raw materials is unstable and some raw materials are loosened seriously, which causes potential cracks inside the pick body. Some pick can not rotate while working, which often causes eccentric wear of the tooth body.
  Poor wear resistance
  Wear resistance refers to the wear resistance of alloy tool head and tooth head.  Domestic pick alloys generally have high cobalt content, incomplete structure, low density and low hardness, resulting in poor wear resistance of the pick alloy tool head. The pick head is due to the unstable quality of raw materials and improper heat treatment process, resulting in cracks in the pick head, low hardness, affecting its wear resistance.
  Drop alloy head
  Cutting off alloy head is one of the main failure modes of cutting teeth. The phenomenon of alloy head dropping of pick can be divided into early, middle and late stages. Among them, the most serious hazard is caused by the early drop of the alloy head. In a very short working time, the cemented carbide tool head brazed on the pick body falls off the pick body, leading to the early scrap of the pick. The main reasons for the alloy head dropping in the early and middle stages of the pick are poor quality of the weld, low strength and lax control of the processing dimension of the pick body, resulting in too large or too small gap between the weld seams, resulting in difficult flow of solder, resulting in "extrusion" or "lack of welding" of the alloy head. For this reason, China's coal industry standard MT246-1996 "Coal Mine Picks" requires that the shear strength of the brazing seam be greater than 180 MPa, the shear strength of the brazing seam of the high-quality pick be greater than 230 MPa, and the filling degree of the solder in the welding seam be not less than 80%. The other reasons for cutting teeth to drop the alloy head are improper brazing process and poor pre-welding treatment of the alloy head. The reason for the late alloy head drop of the pick is that the excessive wear of the front end of the pick body makes the carbide tool head protrude too much and the weld area decreases, which leads to the insufficient strength of the weld and causes the alloy head drop of the pick. Therefore, in order to solve the problem of alloy head dropping at the later stage of pick, besides improving the brazing quality and weld strength, the more important thing is to improve the wear resistance of the front end of pick body.
  Technical Ways to Improve the Performance of Picks
  Development of impact-resistant, wear-resistant and crack-free cemented carbide tool head
  In the field of cemented carbide research, the cemented carbide was studied in cooperation with Zhuzhou Cemented Carbide Factory. Through the anatomical analysis of cemented carbide tool heads at home and abroad, it is concluded that the three properties (magnetic force, specific gravity and hardness) of foreign and domestic alloys are basically similar. It is also difficult to see that there is a big gap between the two in the microstructure of the alloy. However, the impact resistance and wear resistance of foreign tool heads are obviously better than that of domestic alloy, mainly due to the low purity of raw materials and the backward powder forming process. Therefore, the new material and technology of cemented carbide were studied. High quality raw materials are selected for the developed pick head, and three technical measures, including improving the forming process and alloy preparation process (high purity tungsten powder, high temperature reduction, high temperature carbonization), are adopted to improve the service life of the pick head. The alloy has high toughness, wear resistance and impact resistance.
  Surfacing of Wear Resistant Layer
  In order to solve the problem of rapid wear and tear failure of pick in coal mining process, a circular strip with a width of 20-30 mm and a thickness of 2-3 mm was surfaced along the circumferential direction at the part below the cone top of pick (cemented carbide cutter head) by surfacing welding (or other metallurgical methods). The annular strip is called wear-resistant surfacing layer. Commonly used wear-resistant surfacing materials are martensitic surfacing alloys (D317, D327, DG7, etc.), high chromium Fe-Cr-C alloys (D688, D638, etc.), tungsten carbide, niobium carbide (Beijing Guben brand KB515), etc. In terms of mechanism, the function of wear-resistant surfacing layer of pick is as follows: 1. To compensate or change the performance deterioration caused by brazing process of cemented carbide bit around the tooth body (refers to the pick produced by heat treatment followed by brazing process). 2. Further optimize or strengthen the performance of the tool head. Of course, the latter plays a more prominent role in the manufacturing process of brazing the cutter head first and then heat treating the pick as a whole. However, the actual situation is not simple, the emergence and use of the wear-resistant surfacing layer of the pick will change the friction and impact between the cutter head and the cut coal and rock into or part of the friction and impact between the surfacing layer and the cut coal and rock. Friction, wear and impact damage between them not only depend on the characteristics of the two contact materials and their matching behavior, but also are affected by many factors in the mining process.
  Study on the Material of Cutter Body
  At present, most of the materials used in the cutter body of roadheader are alloy structural steel 35CrMnSiA, which is quenched, tempered at low temperature or quenched at isothermal temperature.



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