ABSTRACT: The heat treatment process of high chromium grinding ball produced by sand coated iron mouldcasting process was researched. Shrinkage and porosity defects have not been discovered after cutting along the diameter of grinding ball. The grinding ball in cast state has poor mechanical properties. Mechanical properties of grinding ball significantly increase after heat treatment After heat treatment of 950 °C * 2 h oil quenching and 350 °C *2 h tempering, grinding ball gets the best comprehensive properties. Compared with cast state, impact toughness and Rockwell hardness are increased by 55% and 26% respectively. After determination of metal matrix microhardness and microstructure observation, it can be judged that metal matrix in cast state is mainly austenite. During quenching, most of the austenite transforms into martensite and micro-hardness increases obviously*Ehning tempering, martensite transforms to tempering martensite or tempering troostite.
The quenching stress is relieved, impact toughness increases and microhardness decreases.
KEYWORDS： sand coated iron mould; microhardness; tempering martensite; tempering troostite
High Chromium wear-resistant cast iron is widely used because of its excellent wear resistance. High Chromium cast iron grinding balls are widely used in cement, mining, thermal power generation, etc. To improve the production and heat treatment process of grinding balls and actively explore the relevant laws in the production process has a good guiding role for the actual production. Meanwhile, the production process can be simplified, the product quality can be improved, and the production cost can be reduced.
1.CHEMICAL COMPOSITION
High Chromium wear-resistant white cast iron contains more chromium elements, the cast will produce more high hardness of chromium carbon compounds. When the CR / C ratio in high chromium white cast iron is over 3.5, the main carbides are MTC3 and its micro-hardness reaches 1200 something 1800HVra. The relationship between the content of carbides and the content of elements in castings is shown in formula (1). MARTENSITE was selected to improve the wear resistance of castings.
Formula (1) Carbides (vol%%)12.33 XC + 0.55 xcr-15.2.
2.CASTING PROCESS SELECTION
The traditional production technology of wear-resistant grinding ball is sand mold casting and metal mold casting.The too slow cooling speed of sand casting can easily lead to shrinkage cavity and porosity, at the same time, the coarse grain, the impact toughness and the service life of grinding ball will be decreased sharply. Metal Mold casting has the high cooling rate to refine grains, but the too fast cooling rate will cause great casting stress. The sand-coating process with iron mold combines the advantages of sand mold and metal mold casting. This casting process cooling speed moderate, mold use many times will not appear mold cavity depression, the production of the high-chromium grinding ball smooth surface. Mold with the exhaust hole, exhaust effect is good, the production of high-chromium grinding ball stable performance. Is now widely used in the production of wear-resistant grinding balls.
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