The Effect of the Microstructure and Heat Treatments on the Ductility of 50%Cr-Ni Castings

Abstract

A 50%Cr-Ni cast alloy has excellent corrosion resistance at high temperatures, but thicker castings show poorer ductility at room temperature. To find out the cause of this degradation the complex microstructure in castings with various thickness were analyzed qualitatively and quantitatively. And solution treatments to improve the ductility of castings and preconditionings to prevent them from being brittle in service conditions were respectively tried. Results are as follows:
(1) The solidification of this alloy begins at lower temperatures than the eutectic line for Cr-Ni binary system and ends around 1295°C. The mean cooling rates, v, depend upon the casting thickness, d: v∝d^-1.79.
(2) The eutectic reaction in this alloy is retarded extremely by “halo effect. ” Dominant phases in cast structures are primary α₁ (Cr-rich solid solution), which contains Widmanstätten-type γ₂ (Ni-rich solid solution), and γ₁, in which grain boundary reaction (GBR) advances during cooling down. In thicker castings with lower cooling rates, larger portion of γ₁ is occupied by the colony of cellular GBR precipitates, α+γ.
(3) The GBR colony acts as the site of initiation and propagation of cracks, so thicker castings with more colonies are less ductile.
(4) Solution treatments at higher temperatures make α in GBR (α+γ) discontinuous and/or dispersed and improve the ductility of castings. But the application of this method to big ones is impractical.
(5) The GBR progresses in service at high temperatures and impairs severely the ductility of castings. Preconditionings of the microstructure to leave discontinuous and separate α are effective to suppress fresh GBR and then to keep castings ductile.