Segregants at Prior Particle Boundaries in Powder Metallurgical Martensitic Stainless Steel

Abstract

The segregant behaviour in powder metallurgical (PM) martensitic stainless steel produced by hot isostatic pressing of gas atomised powder has been investigated by characterisation of the surface products on atomised powder and of the precipitate pattern on the prior particle boundaries (PPBs) in the consolidated material. Special attention was paid to the distribution of minor alloy constituents and impurities. Attempts were made to correlate the PPBs with microstructural features such as former austenite grain boundaries (FAGBs). The powder surfaces were analysed using electron spectroscopy for chemical analysis, Auger electron spectroscopy, and secondary ion mass spectroscopy (SIMS). The particles were covered primarily by Fe, Cr, and Mn oxides. Compared with the alloy composition Mn and Cr were preferentially oxidised. Oxides of the minor constituents W, Nb, and V were also found in the surface layer. In B alloyed material this element was enriched within the surface oxide. The impurities Sn, Cu, Zn, and S were enriched on the powder surfaces during atomisation, primarily as a result of the lowering of the surface tension of the liquid metal droplet, and Ca was also observed on the powder surfaces. Higher amounts of Mn and Cr oxides, as well as enrichment of C, N, and S, were found on smaller than on coarser particles. These differences are a result of the larger undercooling of the smaller particles. Mechanisms are suggested for how this factor alters the diffusion in and evaporation from liquid droplets as well as the solidification structure. The PPBs were localised by means of imaging SIMS of segregants contained in precipitates and correlated with the microstructure via intensity contrast from micro hardness indents. The established method has proved useful for PM materials with PPB decoration that can not be visualised using optical microscopy. About 10% of the FAGBs were contiguous with the PPBs in compacts of 10% Cr steel, with and without added B. Evidence for the retention of segregants on the PPBs is shown. These boundaries were decorated with oxide compounds that contain the alloying elements Mn, Si, V, Cr, and B as well as the impurities Ca, Na, Al, and Ti. All these elements were mostly present in mixed compounds or coprecipitates that were coarser and more dispersed in the B alloyed material than in the material without added B. PM/0728