Microstructure, post thermal treatment response, and tribological properties of 3D printed 17-4 PH stainless steel

Abstract

The intention of the present study is to investigate the effect of laser energy density on the microstructure, post-heat treatment response, and wear properties of additively manufactured 17-4 P H stainless steel. Microstructural characterisation techniques such as Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Electro Back Scatter Diffraction (EBSD) and thermal analysis methods such as Differential Scanning Calorimetry (DSC) and Dilatometry (DIL) were employed to attain the quantitative information about the microstructural evolution and phase transformation before and after conventional heat treatment (solution + aging). It was shown that applying higher energy density resulted in a slightly more retained austenite perhaps due to the lower cooling rate. The response of the 3D printed material to post-thermal treatment was affected by the initial energy density enabling us to control the final volume fraction of austenite. Significant grain refinement was observed in the heat-treated specimens. Solution treatment at 1040 °C led to the formation of retained austenite and precipitation of M23C6/M7C3 carbides (M = Cr, Mo) and NbC carbides. The increase of solution time from 1hr to 4 hrs increased the volume fraction of carbides (NbC, M23C6 and/or M7C3) precipitated during solution treatment and decreased the volume fraction of retained austenite due to the localised change in the chemical composition of the matrix around carbides and rise of Ms temperature. Subsequent aging at 472 °C for 1hr led to the precipitation of further carbides (and Cu-rich clusters) in the matrix forming Ni-rich regions promoting the formation of reverse austenite in the matrix. The amount of the precipitated carbides and reverse austenite was more severe in those samples solutionised at 1040 °C for 1hr rather than 4 hrs regardless of the initial energy density. The indentation results showed that the sample subjected to the lower energy density did not respond well to the conventional heat treatment on 17-4 PH stainless steels as expected; however, the increase of energy density improved the post-thermal treatment response. The reciprocating dry sliding wear resistance of the as-printed sample subjected to the energy density of 53.12 J/mm3 was considerably better than that of 73.40 J/mm3. We showed that the initial energy density of the additively manufactured samples could have a significant influence on the wear resistance of post-thermally treated samples and results in different wear behaviour. The main wear mechanism was oxidation and delamination of the iron oxide layer from the surface.