Evolving mechanism of eutectic carbide in as-cast AISI M2 high-speed steel at elevated temperature

Abstract

The evolution in type, size and shape of carbides in as-cast American Iron and Steel Institute (AISI) M2 high-speed steel before and after annealing were investigated. The micromechanism which was responsible for those changes was also analyzed and discussed. At the initial stage of reheating, metastable M2C-type carbide decomposed continuously. M6C-type carbide nucleated at the interface of M2C/γ firstly and grow from surface to center. Then MC-type carbide nucleated at both surface of M6C/M6C and inner of M6C. With the increasing decomposition of the metastable M2C-type carbide, the rod-shaped construction of eutectic carbide began necking, fracturing and spheroidizing gradually. Held enough time or reheated at higher temperature, particle-shaped product aggregated and grew up apparently, while secondary carbide precipitated in cell and grew up less significantly than the former. Based on the above microstructural observation, the thermodynamic mechanism for decomposition of M2C carbide, for spheroidization of products, and for the growth of particles were analyzed. The rate equations of carbides evolution were derived, too. It shows that the evolving rate is controlled by diffusion coefficients of alloy atoms, morphology of eutectic carbides and heating temperature.

Fabrication of Ultrafine Grained High Speed Steel with Satisfactory Carbide Dissolution by Electropulsing Treatment

“…18) M 2 C-type carbide was a metastable phase that can decompose into M 6 C and MC types of carbides from 897.2 to 1 221.5°C. 11,19) New precipitated carbides could nucleate on the pre-existed secondary carbide, 5) increasing the carbide size and thermodynamical stability. However, the supersaturated matrix will not decompose during rapid heating.…”

Effects of Austenitizing Conditions on the Microstructure of AISI M42 High-Speed Steel

“…However, when GBs are wetted by a second solid phase, the completely wetted GBs will decrease with the increase of temperature [17,18]. Moreover, Ms point will decrease and the stability of austenite will increase because the hightemperature austenite dissolves more carbon and alloying elements with the increase of austenitizing temperature [19,20]. When the austenitizing temperature reaches 1473 K (1200 °C), more carbides dissolve into the metallic matrix so that the quantity of carbides reduced rapidly as shown in Figure 5e.…”

Evolution of Carbides in H13 Steel in Heat Treatment Process

“…3) M 2 C is unstable and easily decomposed as M 6 C and MC 16) . The mass percent of M 6 C reached a maximum 0.11% at 862 C and decomposed rapidly with the decreasing of temperature, the precipitation of M 7 C 3 began to accelerate till the content of M 7 C 3 reach the maximum; 4) M 7 C 3 transformed to M 23 C 6 at 795 C and then reach the at peak.…”

“…There is a large difference in the content of each type of carbide, this is because a large amount of secondary M 7 C 3 precipitated, M 6 C dissolved and MC remained unchanged under the suf cient thermodynamic and kinetic conditions during annealing process. M 2 C is dif cult to generate during annealing, because the conditions of both thermodynamics and kinetics of M 6 C + MC → M 2 C are poor 16) . The carbides in quenched-tempered H13 steel are MC, M 23 C 6 and a small amount of M 7 C 3 and M 6 C. The evolution of carbides in H13 steel can be speculated according to the generating curves of three types of carbides in Fig.…”