Use of phosphide phase additions to promote liquid phase sintering in 316L stainless steels

Phosphide powders in the form of Cu₃P and Fe₃P were used to enhance the sintering behaviour of 316L stainless steel by enabling the formation of a phosphide eutectic liquid phase at temperatures around 1050°C. Sintering in vacuum, with between 8 and 10% phosphide addition and at temperatures between 1200 and 1250°C, produced high density (>96% full density with <0·2% interconnected porosity). A 316L stainless steel sintered under similar conditions, but without phosphide addition, only succeeded in achieving approximately 80% full density with interconnected porosity as high as 17%. Decomposition of copper phosphide additions into a mixture of copper and an iron–chromium rich phosphide phase was responsible for densification via the formation of grain boundary eutectic liquid phase produced by complex eutectic reactions composed of austenite plus complex phosphide phases of either (Fe,Cr)₃P or (Fe,Cr)₂P. Liquid phases formed by the melting of a copper rich phase also provided a further means of densification but no such reaction took place in alloys containing Fe₃P owing to the fact that no copper rich phase was formed in these alloys. Reasonable agreement was obtained by comparing the observed microstructures and liquid phase reaction temperatures with those predicted by computer generated phase equilibrium of the alloy systems involved. Mechanical test data indicated useful levels of tensile strength and hardness in fully sintered alloys but their ductility was low owing to the presence of brittle grain boundary phosphide eutectic network.