Microestructural evalution 17-4ph

Published in Metall. Mater. Trans. A. Vol. 30A, pp. 345-353. 1999

Microstructural Evolution in a 17-4 PH Stainless Steel after Aging at 400°C
M. MURAYAMA, Y. KATAYAMA and K. HONO
The microstructure of 17-4 PH stainless steel at various stages of heat treatment, i.e. after solution heattreatment, tempering at 580°C and long term aging at 400°C, have been studied by atom probe field ion microscopy (APFIM) and transmission electron microscopy (TEM). The solution treated specimen consists largely of martensite with a small fraction of δ-ferrite. No precipitates are present in the martensite phase, while spherical fcc-Cu particles are present in the d-ferrite. After tempering for 4 h at 580°C, coherent Cu particles precipitate in the martensite phase. At this stage, the Cr concentration in the martensite phase is still uniform. After 5000 h aging at 400°C, the martensite spinodaly decomposes into Fe-rich α and Cr-enriched α’. In addition, fine particles of the G-phase (structure type D8a, space group Fm3m ) enriched in Si, Ni and Mn have been found in intimate contact with the Cu precipitates. Following spinodal decomposition of the martensite phase, G-phase precipitation occurs after long-term aging.

I. INTRODUCTION

P RECIPITATION-hardened stainless steels are widely used as structural materials for chemical and power plants because of their balanced combination of good mechanical properties and adequate corrosion resistance. 17-4 PH stainless steel is a martensitic stainless steel containing approximately 3 wt pct Cu and is strengthened by precipitation of copper in the martensite matrix [1-8]. After a solution heat-treatment, this alloy is precipitation hardened by tempering at about 580°C for about 4 hours. Typical service temperatures in power plant applications are below 300°C, but increases in hardness and tensile strength accompanied by embrittlement was reported at temperatures ranging from 300 to 400°C after long term aging. Since these materials