Effects of annealing treatment on microstructure and hardness in the 28 wt% Cr cast iron with Mo/W addition

Abstract

In this study, the effects of annealing on the hardness and microstructure of 28 wt% Cr-2.6 wt% C iron with 1.4 wt% Mo/1 wt% W addition have been investigated. The as-cast samples were heated to 800℃ and held for 4 h followed by slow cooled with a cooling rate of 20℃h-1 to 500℃. Microstructures were characterized by X-ray diffractometry, optical microscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Vickers macro-hardness and micro-hardness were measured. It was found that the as-cast microstructure in the hypoeutectic 28 wt% Cr iron without Mo or W addition consisted of primary austenite dendrite, eutectic M7C3 carbide and martensite. In the iron with 1.4 wt% Mo addition, multiple eutectic carbides of M7C3, M23C6 and M6C were observed. In contrast the addition of 1 wt% W changed the structure to hypereutectic containing primary M7C3, eutectic M7C3 and martensite. After the annealing heat treatment, ferrite +secondary carbides and some pearlite were present in the irons, due to decomposition of austenite during annealing. The macro-hardness in the as-cast condition of the iron without alloying and the irons with 1.4 wt% Mo/ 1 wt% W addition were 506, 529, and 576 HV30, respectively. Annealing heat treatment reduced the macro-hardness to about 390, 463, and 428 HV30, respectively.