Origin and Subsequent Behaviour of Inclusions in Water-Atomized Low-Alloy Steel Powder

Abstract

The origin of inclusions in a water-atomized 3·5%Ni steel powder and their subsequent behaviour during sintering have been investigated. Four samples of the powder were produced under different melting and atomizing conditions. For example, both acid SiO₂ and basic MgO linings were used in the melting furnace. The volume fraction of inclusions was measured for the powders in the as-delivered and sintered conditions. Similarly, the chemical composition of individual inclusions was determined using electron microprobe techniques. The analysis showed the inclusions to be mainly complex oxides. Those oxides formed from elements less stable than SiO₂ were reduced by the sintering process. These oxides originated mainly from the alloying elements present in the steel powder. The residual stable complex oxides containing silicon, aluminium, magnesium, calcium, sodium, and sulphur were shown to originate from the refractories and atomizing medium used. A much lower volume fraction of inclusions was measured for the powder produced with fewer silicon-containing refractories in the atomizing system. Other measures to help minimize the inclusion levels in water- atomized low-alloy powders during their manufacture are highlighted. PM/0175