Influence of Thermal Aging on the Microstructure and Mechanical Behavior of Dual-Phase, Precipitation-Hardened, Powder Metallurgy Stainless Steels

Abstract

i ABSTRACT Increasing demand for high strength powder metallurgy (PM) steels has resulted in the development of dual phase PM steels. In this work, the effects of thermal aging on the microstructure and mechanical behavior of dual phase precipitation hardened powder metallurgy (PM) stainless steels of varying ferritemartensite content were examined. Quantitative analyses of the inherent porosity and phase fractions were conducted on the steels and no significant differences were noted with respect to aging temperature. Tensile strength, yield strength, and elongation to fracture all increased with increasing aging temperature reaching maxima at 538 o C in most cases. Increased strength and decreased ductility were observed in steels of higher martensite content. Nanoindentation of the individual microconstituents was employed to obtain a fundamental understanding of the strengthening contributions. Both the ferrite and martensite hardness values increased with aging temperature and exhibited similar maxima to the bulk tensile properties.Due to the complex non-uniform stresses and strains associated with conventional nanoindentation, micropillar compression has become an attractive method to probe local mechanical behavior while limiting strain gradients and contributions from surrounding features. In this study, micropillars of ferrite and martensite were fabricated by focused ion beam (FIB) milling of dual phase precipitation hardened powder metallurgy (PM) stainless steels. Compression testing was conducted using a nanoindenter equipped with a flat punch indenter.The stress-strain curves of the individual microconstituents were calculated from ii the load-displacement curves less the extraneous displacements of the system.

Mechanical Properties of a Thermally-aged Cast Duplex Stainless Steel by in Situ Tensile Test at the Service Temperature

“…It has been reported that the thermal aging embrittlement in CDSS was caused by the precipitation that formed in the ferrite phase after long-term thermal aging, e.g., the G-phase precipitates and the spinodal decomposition during which the ferrite phase decomposes into a Fe-rich α-phase and a Cr-rich α'-phase [10,11,[13][14][15]. Many researchers have found that the mechanical properties of CDSS are strongly dependent on the strength of each individual phase [16,17] and their microstructures, e.g., the volume fraction and the morphology of the ferrite phase, have meaningful influence on the fracture toughness, thermal aging embrittlement, high-temperature ductility, impact properties and corrosion resistance of duplex stainless steel [18][19][20][21][22][23][24].…”

Survey of Modeling and Simulation Techniques for Advanced Manufacturing Technologies Volume II – Predicting Material Performance from Material Microstructure

“…Also, researchers found that at some point over-aging occurs at higher temperatures because the carbide precipitates coarsen and causes a decrease in strength. The ductility was also observed to increase slightly with aging due to tempering of the martensite [46].…”

“…During this process, the carbon diffuses out of the martensite, meaning that the distortion of the martensite phase is reduced and thus lowers the residual stresses and strength of the material. If precipitation hardening elements such as copper or aluminum are introduced into dual-phase steels, the opposite occurs and the material is actually strengthened with increasing thermal exposure [46].…”

Three-Dimensional (3D) Microstructure-Based Modeling of a Thermally-Aged Cast Duplex Stainless Steel Based on X-ray Microtomography, Nanoindentation and Micropillar Compression

“…For the unaged duplex stainless steel, a higher content of ferrite can enhance the mechanical properties, while the impact energies of duplex stainless steel are found to decrease with the increasing ferrite content for the aged condition. Besides the effect of the microstructure, as a multi-phase material, the bulk mechanical properties of CDSS are also highly dependent on the mechanical properties of each individual phase [22,23]. Stewart [22] applied a conventional rule of mixtures approach to the stress-strain data obtained from microconstituents in a dual phase precipitation hardened (DPPH) steel to predict the bulk behavior.…”