ROLE OF SURFACE OXIDES IN SINTERING OF CHROMIUM-ALLOYED STEEL POWDER
During water-atomisation and handling of steel powder the surface inevitably oxidises. The surface bound oxide is an important factor in the processing of powder metallurgical (PM) steels, especially in case the powder contains elements forming stable oxides, such as Cr, Mn, and Si. The surface products formed on the metallic particles may obstruct the sintering due to inadequate metallic contact between the particles. Hence, insufficient sintering may lead to limited bonding between metallic particles as well as entrapment of residual oxides in the final component. Consequently the mechanical properties of the final component are lowered. In order to understand and control the surface reactions during sintering, it is important to know the composition, morphology and amount of oxides that are present on the powder surface. Furthermore, it is vital to understand the parameters influencing the surface reactions during pressing and sintering.
In the sintering stage, at least partial surface oxide reduction has to be onset in order to facilitate sintering and neck growth. An overall aim of research in this field is to achieve high density (close to theoretical density of steel) in order to compete with traditionally fabricated materials. When density is increased, the surface reactions is even more crucial, since the sintering gas into the component, as well as the reduction products out of the component may be restricted due to the low degree of open porosity in the compact.
This work deal with surface product formation on Cr-alloyed water-atomised PM steel. By applying surface sensitive analytical tools, such as AES and XPS, in combination with microscopy it was found that the surface oxide on powder particles were consisting of a thin layer of Fe-rich oxide with particulates of thermodynamically stable oxides (e.g. Cr-, Mn-, and Si-oxides). This was actually found to be beneficial from a sintering point of view, since sintering starts at the Fe-oxide, which is easily reduced compared to the Cr-, Mn- and Si-oxides. Sintered materials (1120ºC-1250ºC) were analysed by fractographic analyses (fracture in UHV) using AES and SEM. It was shown that the particulate surface oxide grow during the sintering stage (from about 0.1 µm to 1 µm), whereas the thin oxide is reduced. It was also found that there is a transformation from more Cr-rich particulates to more Si-rich ones during sintering. Hence, Si, which is a trace element in the material, act as a deoxidiser of the metal particles during sintering. This was also evident by model experiments where the initial stage of sintering by annealing powder in vacuum and reducing atmosphere (H2/N2 - 10/90) at 800ºC -1000ºC,. By these experiments it was observed that the relatively stable oxides grow on expense of the less stable ones at early stage of sintering.