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Abstract
Water‐atomized iron and steel powder is commonly used as the base material for powder metallurgy (PM) of ferrous components. The powder surface chemistry is characterized by a thin surface oxide layer and more thermodynamically stable oxide particulates whose extent, distribution, and composition change during the sintering cycle due to a complex set of oxidation–reduction reactions. In this study, the surface chemistry of iron and steel powder was investigated by combined surface and thermal analysis. The progressive reduction of oxides was studied using model sintering cycles in hydrogen atmospheres in a thermogravimetric (TG) setup, with experiments ended at intermediate steps (500–1300°C) of the heating stage. The surface chemistry of the samples was then investigated by means of X‐ray photoelectron spectroscopy (XPS) to reveal changes that occurred during heating. The results show that reduction of the surface oxide layer occurs at relatively lower temperature for the steel powder, attributed to an influence of chromium, which is supported by a strong increase in Cr content immediately after oxide layer reduction. The reduction of the stable oxide particulates was shifted to higher temperatures, reflecting their higher thermodynamic stability. A complementary vacuum annealing treatment at 800°C was performed in a furnace directly connected to the XPS instrument allowing for sample transfer in vacuum. The results showed that Fe oxides were completely reduced, with segregation and growth of Cr and Mn oxides on the particle surfaces. This underlines the sequential reduction of oxides during sintering that reflects the thermodynamic stability and availability of oxide‐forming elements.
Highlights
The surface chemistry of iron and steel powder used in powder metallurgy (PM) has been widely studied over the last few decades,[1,2,3] where special attention has been given to the characterization of oxide particulates that are distributed across the powder surfaces.[3,4,5]
A comparable result has been shown in another study where reduction of iron oxide in hydrogen was found to be promoted by doping with chromium.[11]
The progressive change of surface chemistry during sintering was analyzed for water-atomized iron powder and steel powder pre-alloyed with chromium
Summary
Sintering is more complicated due to oxide transformation events during the heating stage of sintering, wherein oxygen from the less stable iron-rich oxides can be transferred to react with elements that form more stable oxides.[5,7,8] The ability to reduce oxides and subsequently remove the reaction products play a crucial role in the successful sintering of PM steels.[5] The aim of this study is to further characterize the trajectory of oxide transformation and reduction during sintering of water-atomized iron and low-alloyed steel powder commonly used in the PM industry For this purpose, interrupted sintering trials were conducted in a thermogravimetric analyzer (TGA) emulating a sintering furnace with well-controlled temperature and atmosphere conditions. The chemical changes revealed were correlated to the recorded mass losses obtained by TGA to detail the progression of oxide reduction
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