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 ionmicroscopy (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 concentrationin 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 occursafter long-term aging.
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 copperin 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 materialshave to serve for a very long period of time during the life span of the plants, understanding the embrittlement mechanism at slightly above the service temperature is very important. The precipitation sequence in 17-4 PH stainless steel begins with formation of coherent copper precipitates, which occurs during the tempering treatment before service. These coherent particles were reported totransform to incoherent fcc-Cu particles after long term aging at temperatures around 400°C . In addition, since the Cr concentration in 17-4 PH is within the spinodal line, phase decomposition of the martensite into the Ferich α and the Cr-enriched α’ is expected on aging below 450°C. Much work has shown that stainless steels are embrittled when α’ phase precipitates by spinodal decomposition .Such α’ embrittlement is anticipated in the 17-4 PH stainless steel as well.
Several studies on the effect of aging 17-4 PH stainless steel were carried out [3-6]. Early work by Anthony  proposed mechanical properties of 17-4 PH are influenced by precipitation of α’ phase, but no direct evidence for α’ precipitation was presented. Later, Jack and Kalish  observed copper precipitation onaging and correlated it to mechanical property changes; however, there was no mention of phase decomposition in the martensite phase. More recently, Yrieix and Guttmann  reported that 17-4 PH stainless steel exhibits high susceptibility to aging embrittlement at 400°C, and they concluded that it was essentially due to α’ precipitation. In their study, however, no microstructural observationresults were shown. Employing atom probe field ion microscopy (APFIM) and transmission electron microscopy (TEM), Miller and Burke  showed direct evidence for a’ precipitation after aging at 482°C. They also reported that significant amounts of iron, nickel and manganese were contained in the ε-Cu precipitates even in the overaged condition. However, their aging temperature is rather high...