Characterisation of ASTM A747-grade CB7Cu-1 precipitation hardening cast stainless steel

Abstract

This paper reports on a microstructural characterisation of standard ASTM A747/A747M grade CB7Cu-1 precipitation hardening cast stainless steel with a martensitic matrix, a material widely applied today in rotary and static components subject to wear, although the majority of publications about this family of stainless steels focus on wrought and rolled stainless steels. An optical microscopy analysis was performed to clearly identify the constituents of the microstructure in the as cast state and in the solution annealed condition at 1140°C for 2 h, followed by water quenching. In the as cast state, the microstructure was composed of splinter shaped martensitic dendrites constituting the matrix and delta ferrite precipitated in the interdendritic regions. The same phases were found in the solution annealed state, but the dendritic structure was ‘broken’ by the heat treatment. Quantitative metallography using image analysis software showed that the volumetric concentrations of delta ferrite in the as cast and solution annealed states were 15·22 and 12·44% respectively. The results of the energy dispersive spectroscopy microanalysis revealed higher concentrations of chromium and silicon in the delta ferrite and of nickel and copper in the martensite, which is a product of transformation of the previous austenite. A comparison between two different chemical analysis methods: optical emission spectroscopy and microanalysis indicated a slight difference (<2%) in the elements chromium, nickel and silicon, while the difference for the element copper was close to 12%. Aging heat treatments at various temperatures showed that the maximum hardness was obtained in the range of 400–500°C, with 2 h for soaking time.