Specimens of 16Cr -5Ni -1Mo stainless steel were solution treated at 1050°C for 1 h followed by heating in the temperature range 400 -750°C for different holding times (1 -16 h). After heat treatment, optical microscopy, scanning (SEM) and transmission (TEM) electron microscopy, and X-ray diffraction examinations were conducted. The microstructure of all aged specimens was found to consist of martensite with variable fractions of d ferrite and reversed austenite. Very ne precipitates of Mo carbides were revealed in the specimens aged at 475°C. The specimens aged at 625°C showed a decrease in the dislocation density and a high volume fraction of austenite and precipitation of Fe 2 Mo Laves phase was detected by X-ray analysis. Above 625°C, Cr 23 C 6 and TiC became the predominate carbides heterogeneously precipitated in the martensitic matrix. Partial transformation of reversed austenite to unaged martensite was observed at temperatures above 625°C.MST/5714

Mechanical property characterisation has been carried out on specimens of 16Cr -5Ni -1Mo stainless steel, subjected to various aging cycles. The heat treatment cycles involved solution treatment at 1050°C for 1 h followed by heating in the temperature range 400 -750°C for different holding times (1 -16 h). After heat treatment, tensile, hardness, impact, and creep tests were conducted. Specimens aged at 475°C exhibited maximum values of tensile strength and hardness with minimum values of ductility and impact toughness, while specimens aged at 625°C had maximum values of impact toughness and ductility. The results were correlated with the microstructural data presented in Part 1 of this study. Softening of the martensitic matrix at 625°C occurs as a result of the elimination of internal stresses, the decrease in the dislocation density, and the high volume fraction of austenite which lead to the drop in values of tensile strength and hardness. The results of the study reveal that aging at 550°C for 4 h gives the optimum combination of strength, hardness, ductility and toughness for this steel.MST/5715

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