Study On Secondary Hardening And Strength And Toughness Of Cr12 And Cr12MoV Die Steel

The effects and mechanism of conventional hardening and secondary hardening heat treatment on strength and toughness of Cr12 and Cr12MoV tool steel were investigated by means of Metallographic examination, Scanning electron microscopy method, X-ray diffraction, Energy spectrum analysis, Impact test. The initial microstructure, austenite grain size, characteristics of carbides, decomposition of martensite and residual austenite were taken into account. This article aims at improving the manchanical properties of Cr12 and Cr12MoV steel and offering reference to optimum heat treatment.The research on initial microstructure and the effect of quenching temperature on austenite grain size of Cr12 and Cr12MoV steel indicates that, the unequally distributed cardides and alloying elements will do harm to the homogeneity of austenite grains.The research on the effect of conventional heat treatment which consists of quenching and low temperature tempering on the microstructure and properties of Cr12 and Cr12MoV shows that, fine austenite grains, large quantitives of undissolved carbides, fewer amount of retained austenite and higher quenching hardness can be obtained by quenching at low temperature. With the rising of quenching temperature, high alloying content austenite and martensite can be gained, which means stronger resistence of martensite to tempering softening and more retained austenite. When the 1000℃ quenching process is applied to Cr12 and Cr12MoV steel, the size of austenite grains will be kept in fine level and the two kinds of material will suit for some operating conditions which require high toughness and red hardening more than conventional reinforced work piece. Along with the increasing amount of retained austenite, the toughenss of both materials gets higher. Science the austenite grains of Cr12MoV grow faster than that of Cr12 in the same quenching condition, the amount of retained austenite plays leading roles in the toughening of Cr12 and Cr12MoV steel.The research on the microstructure and properties of Cr12 and Cr12MoV steel in secondary hardening treatments indicate that, it is more beneficial to take high quenching temperature in secondary hardening treatments of Cr12 and Cr12MoV steel to obtained high alloying content austenite which improves the resistance of martensite to tempering softening and ability of precipitation hardening. When quenching at 1050℃, the hardness peak temperature of Cr12 steel in between 520℃ and 550℃ while that of Cr12MoV steel is around 550℃. The Cr12MoV steel shows favorable strength and toughness when treated in 1050℃ quenching and 550℃ tempering twice process.To improve the strength and toughness of steel like Cr12 and Cr12Mo V, the initial microstructure, pretreatment process and heat treatment should be taken into account. Optimum properties can be obtained by taking steps of applying large forging ratio to smash the eutectic carbides, using spheroidizing annealing as pretreatment and taking proper heating rate and quenching temperature to get fine grains and appropriate alloying content of austenite.