Microstructure, Corrosion and Mechanical Behavior of 15-5 PH Stainless Steel Processed by Direct Metal Laser Sintering

Abstract

The present research deals with producing complicated-shaped 15-5 precipitation-hardened (PH) stainless steel (SS) alloy structure using direct-metal-laser-sintering (DMLS) technique. Systematic microstructural characterization throughout the different planes and sections of the as-build (AB) structure reveals occurrence of non-uniform microstructures with nominal porosity distribution. The size and morphology of the microstructural features are noted to vary along the top and bottom sections of build direction. Such microstructural inhomogeneity is related to non-uniform heating and cooling rates associated during the manufacturing. Presence of trace amount of austenite phase is also noted in the as-build alloy. Post-build heat treatments, including solutionizing and two standard aging or precipitation treatments at 900 and 925 °F, are pursued next, termed as ST, H900 and H925, respectively. ST and H900 alloys reveal existence of homogenized microstructure and absence of austenite phase. This phase is noted to form again in H925 alloy due to diffusion of austenitic stabilizing elements, Cu and Ni, from martensite during the aging process. Presence of Nb-rich precipitates in the aged alloys is also confirmed. Potentiodynamic polarization tests reveal that H900 specimens exhibit highest corrosion resistance in comparison with the AB, ST and H925 conditions. It is due to the stress relaxation of the martensite and auto-passivation of the copper precipitates, formed during aging. Detailed mechanical characterization of as-build, solutionized and aged specimens exhibits interesting results. Hardness is noted to vary along the top and bottom sections of asbuild specimen, former being higher owing to finer grain size. Nevertheless, aging the alloy appears beneficial in enhancing hardness and strengths further, particularly owing to the presence of precipitates. It is apparent from the series of systematic investigations that optimal aging condition (H900) homogenizes microstructure, stabilizes the phase, and thereby leads to optimum combination of improved corrosion resistance, higher hardness, strength, ductility and toughness of DMLS-manufactured 15-5 PH SS alloy.

Effect of Microstructure Modifications on Stress Corrosion Endurance of 15-5 PH Stainless Steel Formed by Wire Laser Additive Manufacturing (WLAM)

“…The corrosion degradation may also be attributed to the relatively massive formation of an austenitic phase that creates micro-galvanic corrosion at the interface with the martensitic matrix. A similar electrochemical performance in terms of reduced potential corrosion and increased corrosion rate were obtained by Avula et al [50] in a 15-5 PH alloy, post-H925 heat treatment, that was produced through direct metal laser sintering.…”

“…Apart from the martensitic phase, the WLAM alloy, after H-1150M treatment, contains 54% austenitic phase compared to only 11% in the counterpart alloy. It is believed that the large amount of austenitic phase in the WLAM alloy can be partly attributed to the interaction between this alloy and the protective gas atmosphere (N 2 ) during the printing process [50,51]. This assumption was mainly supported by the presence of a secondary Nb(CN) phase in the form of Nb 0.3 C 0.3 N 0.7 .…”

Recent Advances on Composition-Microstructure-Properties Relationships of Precipitation Hardening Stainless Steel

“…PH17-4 steel manufactured by SLM consists of 72% metastable austenite and 28% martensite phase, and the distinguished mechanical properties result from the strain-induced transformation of austenite and microstructural features of dual phase [ 164 , 165 ]. The difference of corrosion behavior with microstructural inhomogeneity was systematically studied [ 32 , 113 ]. The size and morphology of the microstructure are observed to vary from the top and side perspectives of building directions employing the direct-metal-laser-sintering (DMLS) method [ 166 ].…”

“…Meanwhile, the quantity density and size of Cu-rich particles and Cr-rich precipitates were found to increase with the increasing service time, indicating the hardening effect of PH17-4 steel under the nuclear power plant environment [ 112 ]. The Cu precipitation promotes the formation of reverted austenite via the diffusion of elements [ 113 ]. The preferential corrosion nucleation regions with the weak passive film are prone to form at the interface between the Ni 3 Ti precipitates and the matrix in Custom 465 steel [ 64 ].…”