Effect of Triple Tempering Temperature on Microstructure, Mechanical, and Wear Properties of K340 Cold Work Tool Steel

The effect of triple tempering temperature (500-580 °C) on microstructure and properties of K340 steel, a new generation of the high Cr cold work tool steel, was investigated. Applied heat treatment consisted of preheating at 400, 600, and 800 °C for 20 min and consequently austenitizing at 1050 °C for 30 min, quenching in a salt bath at 250 °C, three times tempering at 500, 520, 540, 560, and 580 °C for 2 h, and cooling in the air. The microstructure comprised tempered martensite, irregular M6C, and spherical secondary carbides. Results showed that coarsening of M6C and reduction of the finest carbides' density have occurred. Variations of carbide features correlated with the mechanical properties of tempered samples. Impact toughness increased by 100% with an increase in tempering temperature from 500 to 580 °C. Moreover, the hardness value increased to 63.4 HRC with a rise in temperature up to 520 °C and then decreased by 13% in the highest tempering temperature condition. The fracture mode changed from intergranular cleavage to transgranular dimples. The highest weight loss and coefficient friction were obtained for the tempering at 580 °C, which decreased by 72 and 38%, respectively, for 520 °C. The dominant wear mechanism was abrasive in all tempered specimens.