Hot workability—microstructure relationship in a CrNi steel

Abstract

Precipitation-hardening stainless steels are used in a variety of industrial applications and their successful fabrication by secondary hot-working operations is known to be sensitive to minor variations in processing parameters. In an effort to develop a further understanding of this problem, the hot-ductility characteristics of a CrNi steel have been studied with a view to determining the detailed role of chemistry, microstructure and temperature on the workability of the steel. Hot-ductility tests were conducted on two heats of ASTM A564 Type 630 steel which exhibited widely different hot-working characteristics. The results indicated that a ductility trough exists in the 1095–1290°C range, and is influenced by certain minor compositional variations in the steel. The “chromium equivalent” ( Cr eqv) designates the effective ferrite forming capability of the steel and is a convenient method for understanding the influence of minor variations in chemical composition on the microstructure of such steels. As the Cr eqv increases, the severity of the ductility trough has been observed to increase. Thus, the sensitivity of ductility to the hot-working temperature increases. This suggests that when working a high Cr eqv ASTM A564 Type 630 stock, extreme precautions need to be taken regarding temperature control. Further, the ductility of ASTM A564 Type 630 steel at the hot-working temperatures was related to the volume fraction and aspect ratio of delta ferrite present in the microstructure. The higher the volume fraction of delta ferrite, and the more acicular the delta ferrite, the lower is the ductility. The crack initiation mechanisms during hot working were also identified. Cavitation damage was found to be the mechanism in the 1095–1230°C range with cracks initiating at the austenite—delta ferrite interface. Wedge cracking, on the other hand, was found to be the cracking mechanism in the 1230–1290°C range.