Thermal aging of 16Cr-5Ni-1Mo stainless steel - Part 1 - Microstructural analysis

Specimens of 16Cr - 5Ni - 1Mo stainless steel were solution treated at 1050degreesC for 1 h followed by heating in the temperature range 400 - 750degreesC 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 6 ferrite and reversed austenite. Very fine precipitates of Mo carbides were revealed in the specimens aged at 475degreesC. The specimens aged at 625degreesC showed a decrease in the dislocation density and a high volume fraction of austenite and precipitation of Fe2Mo Laves phase was detected by X-ray analysis. Above 625degreesC, Cr23C6 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 625degreesC.