Working conditions, failure forms and property requirements of cold working die steels were presented. This review attempted to introduce the development of cold working die steels, the ways to improve its quality, heat treatment and surface treatment technology at home and abroad. Meanwhile, the characteristic and application prospects of spray-formed cold working die steels were analyzed.
The T15 high speed steel (HSS) spray formed (SF) billets were manufactured using a twin-atomizer spray forming facility at Beijing Institute of Aeronautical Materials (BIAM) AVIC, China. The billets were then hot isostatically pressed (HIP) following hot forge (HF) and heat treatment (HT). The as-spray formed billet is integrally dense and has a high relative density of >98%, and the macrostructure is free of macrosegregation. The microstructure consists of polygonal grains of 20µm with tempered martensite inside the grains and refined carbides uniformly distributed along the grain boundaries. Hardness and three-point bending properties were also measured. The spray formed materials was comparable to those made by powder metallurgy in terms of microstructure uniformity, hardness and bending property.
Spray forming has attracted considerable attention for the production of high speed steels due to its potential and priority in the microstructure refining and cost saving. In this study, high-quality large billets of 2060 high speed steel were successfully produced by spray forming process using a twin-atomizer facility. As-deposited billet was subsequently processed by hot forging, quenching in oil at 1180 °C and a triple tempering in the temperature range of 500-580 °C. The microstructures and hardness of the deposit and their subsequent development resulting from hot forging and heat treatment were investigated. This paper was designed to provide insight and have a better understanding of such a system for the steel. The results showed that the as-deposited microstructure was composed of the fine equiaxed grains with V-rich MC and W-Mo-rich M2C carbides non-uniformly distributed along the grain boundaries and inside the grains. M2C presented rod-like or unconnected net-shaped morphologies in the as-deposited microstructure. Following hot forging, metastable M2C carbides were completely decomposed into refined MC and M6C nearly spherical carbides uniformly distributed throughout the microstructure. A hardness value of 31HRC was attained for the spray deposited and hot forged samples. With increasing the tempering temperature, hardness was increased firstly and then decreased. Secondary hardening peak appeared at 540 °C for spray formed 2060 steel austenitized at 1180 °C, and the corresponding peak hardness reached 71HRC.
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