Effect of heat treatment on the microstructure and mechanical property of GX4CrNi13-4 manufactured by laser cladding

Abstract

BackgroundLaser cladding, recognized for its cost-effectiveness and high efficiency, has become a focal point in the field of laser remanufacturing. GX4CrNi13-4 martensitic stainless steel produced by laser cladding is a widely used structural material in nuclear power plants.PurposeThis study aims to enhance the mechanical properties of GX4CrNi13-4 martensitic stainless steel, fabricated using laser cladding technology, through different heat treatments that cause microstructure modification.MethodsThe GX4CrNi13-4 stainless steel sample was prepared using laser cladding technology, and its heat treatment microstructure was studied in details. Firstly, thermal expansion experiments identified the onset temperature of austenitic phase transformation of sample at 620 °C, serving as a pivotal reference for developing heat treatment schemes. Two distinct heat treatment processes, i.e., solution treatment plus aging (STPA) at 1 050 °C for 1 h followed by a similar treatment at 550 °C for 4 h and single aging (SA) at 620 °C for 2 h, were applied to experiments. The effects of these treatments on the microstructure and mechanical performance of the cladding were comparatively analyzed by using X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were employed to characterize the post-treatment microstructure and phase distribution. Tensile tests at room temperature were performed on samples before and after heat treatment.ResultsExperimental results indicate that the as-cladded GX4CrNi13-4 stainless steel exhibits a dual-phase microstructure primarily comprising martensite and ferrite, with continuous network-like ferrite precipitated along martensitic boundaries, accompanied by a minor presence of residual austenite. Post STPA, the matrix still predominantly comprises martensite and ferrite, but the continuous network-like ferrite decomposes, and numerous micrometer-scale transgranular precipitates within the martensite are observed. This led to a slight improvement in plasticity but a significant decrease in strength. The SA treatment of the cladded samples, performed at the critical temperature for austenitic phase transformation, induces the formation of the reversed austenitic phase. This phase, during tensile deformation, triggers the transformation induced plasticity (TRIP) effect. Furthermore, the network-like ferrite precipitated along the martensite decomposes into a dispersed distribution post-SA. The combined effect of TRIP and ferrite decomposition notably enhances the plasticity of the laser-cladded GX4CrNi13-4 stainless steel while effectively maintaining its strength.ConclusionsThe use of austenitic phase transition temperature for aging in this study, coupled with the synergistic effect of reversed austenite TRIP and ferrite decomposition, successfully achieves a balanced strength-plasticity performance in laser-cladded GX4CrNi13-4 stainless steel. Appropriate heat treatment and microstructural control emerge as effective strategies to improve the comprehensive mechanical properties of materials.

Keywords

• martensitic stainless steel
• laser cladding
• reversed austenite
• toughening heat treatment
• coefficient of thermal expansion
• austenite transformation temperature