Application of new MQ-P-T technology in white cast iron balls (1)

Abstract: Fe-2.4C-12Cr high chrome white cast iron balls with a diameter of 80mm was quenched and tempered with water and air by MQ-P-T method, analysis of mechanical properties and microstructure, weaving, alignment, comparison. The strengthening and toughening mechanism of MQ-P-T treated high chromium cast iron was investigated using micro-and microstructure characterization. The results show that the average radial hardness of high chromium cast ball treated by MQ-P-T is about 60HRC, which is about 17HRC higher than that of air-cooled cast ball and about 5HRC higher than that of oil quenched samples. After MQ-P-T treatment, the average impact toughness reached 12.6 J/CM2, about 4 times of air-cooled impact toughness, about 2 times of oil-quenched impact toughness. After treated by MQ-P-T, the cast ball has high hardness and hardness uniformity, and its impact property is also greatly improved. MQ-P-T treated cast iron’s excellent mechanical properties are mainly attributed to the martensite matrix, a considerable amount of secondary carbides, and stable, residual, retained, austenite structure.

Keywords: multi-cycle quenching-distribution-tempering (MQ-P-T) process; mechanical properties; martensite; carbide; retained austenite; toughening mechanism

China is a big country of iron and steel, but not a strong iron and steel; high-strength steel, wear-resistant steel, and other special steel products have not yet reached the advanced international level. Moreover, the technology level is low; the energy consumption is too big, the pollution is severe. The backwardness of heat treatment technology in China has become a bottleneck restricting the national economy’s development. For nearly one hundred years, high chromium cast iron has been treated by air cooling or oil quenching. The mechanical properties of high chromium cast iron can be improved by water quenching treatment, but the high chromium cast iron is easy to appear quenching cracks when the cast iron containing casting defects is directly water quenched, white cast iron” and other national standards and production process are strictly prohibited water quenching.” Whether we can further improve the mechanical properties and wear resistance of high chromium cast iron, without changing the composition, through the improvement of the process and technology, it is a challenging subject and a research direction that the domestic and foreign counterparts have been paying attention to.

In the early 2000s, Speer and others quenched and partitioned Si. MN steels, and treated them with Q & P processes to obtain martensite steels with higher strength than dual-phase (DP) steels and TRIP steels. Because of the CCE theory of the Q & P process, the application of precipitation strengthening is excluded. To this end, academician Xu Zuyao proposed in 2007 the Q-P-T process, which aims to further enhance the strength of steel by adding alloy elements formed by carbide. The results show that Q-P-T martensite steel’s strength and strength are much higher than that of Q & P martensite steel. Although the Q-P-T process can improve the mechanical properties of steel in an all-round way, it is obviously affected by the workpiece’s size. The former Q-P-T reports are all the cold-rolled sheet results made of Si — mn steel with thickness, not more than 3mm. The current Q-P-T process is carried out. In the laboratory using two different temperatures, a salt bath or a salt bath and a hot water tank, only the treatment, small sample size, can not be extended to the engineering practice.

In this article Fe-2.4C-12Cr high chromium grinding ball with a diameter of 80 mm was quenched and tempered by multi-cycle quenching, and a one-distribution tempering (MQ-P-T) process with water and air as quenching medium, the mechanical properties, and microstructure of chromium cast ball were compared and analyzed. The strengthening and toughening mechanism of the MQ-P-T treating steel ball was discussed by means of material and microstructure characterization.

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