What are the reasons for the failure of picks？
The picks are divided into shearer picks and roadheader picks. They are parts of mining and tunneling machines. They are used in conjunction with the shears and are suitable for coal mining. The main failure modes of the picks are cutter head falling off, chipping and cutter head , The cutter body is worn, and the cutter body is often broken under certain working conditions and the pick fails. Because the mechanical performance of the cutter body directly affects the service life of the pick, the material of the cutter body is selected reasonably And effective heat treatment methods have positive significance in reducing the wear and breaking of the pick cutter body, reducing the consumption of coal shearer picks, improving the operation rate of coal mining machinery, and increasing the comprehensive economic benefits of coal production. Among them, the roadheader Picks require higher hardness. Next, let's talk about the reasons for the failure of picks.
1. The quality of the cemented carbide tip. The carbide tip contains graphite impurities, the grain distribution is uneven, and there are cracks in some cemented carbide. Under the action of impact load, the pick bit is in a high stress state. When encountering hard coal and rock, embrittlement occurs when the high pressure stress exceeds the strength limit of the cemented carbide. This is the main reason for the cracking of the cemented carbide head. If the content of Co in the cemented carbide is insufficient, the toughness will be insufficient. Under the action of impact load, the carbide tip is also prone to brittle fracture, the alloy has a large upper and lower density difference, many pores, and low hardness, and it is also easy to cause chipping.
2. The brazing residual stress is large, and the picks are mostly copper-zinc solder, and the welding temperature reaches 920 degrees Celsius or more. The expansion coefficients of cemented carbide, brazing filler metal, and base metal are very different, and the three materials shrink differently during cooling. Among them, copper has the largest shrinkage, followed by base metal, and cemented carbide is the smallest. In this way, there is bound to be a great tensile stress between the cemented carbide and the brazing filler metal, and the brazing filler metal and the base metal. Because the coefficient of thermal expansion of the alloy cutter head and the tooth material is quite different, and the shrinkage difference during cooling increases with the increase of the brazing temperature. As a result, the strength of the weld is reduced, and the cutting is subjected to a strong impact load, resulting in brittle cracking of the cemented carbide head.
3. Abrasive wear. The content of some alloying elements in the alloy does not meet the requirements, the content of cobalt is high, the structure is incomplete, the density is low, the hardness is not high, and the structure is uneven, resulting in poor wear resistance of the pick alloy bit, which is caused by the cemented carbide The main reason for head wear: Due to the unstable quality and performance of the raw material of the tooth body, improper control of the heat treatment process causes the pick head to have cracks and low hardness, which affects its wear resistance.
4. Abrasion caused by low red hardness. Red hardness refers to the ability of the tool material to maintain high hardness at high temperatures. Theoretically, cemented carbide can maintain high hardness at a high temperature of 800-1000℃, but due to the production of cemented carbide There are some practical problems that make the red hardness of the picks low. When the picks are cutting coal and rock, the surface temperature of the cutter head can reach 600-800℃, and the hardness decreases by about 50%. The softening of the material accelerates the wear of the picks. Due to welding and shape design, the alloy head falls off.