フェライト系ステンレス鋼圧粉体の窒素吸収に伴う組織変化

抄録

Sintering treatment in 1atm-N₂ gas atmosphere was applied for 23mass% Cr ferritic stainless steel powder compacts, and structural changes of the powder compacts by nitrogen absorption were investigated by means of optical microscopy, chemical analysis and X-ray diffraction. Mechanical properties of the sintered materials were also examined in relation to nitrogen content and microstructures. The results obtained are as follows :
(1) In the sintering of powder compacts in N₂ gas atmosphere, nitrogen absorption into powder particles causes phase transformations from ferrite to other phases. The phases formed are dependent on the temperature and nitrogen content
(2) The amount of nitrogen absorbed into steel powder particles is determined by the surface equilibrium between N₂ gas and nitrogen content of steels, so that the saturation nitrogen content increases with a fall in the sintering temperature. At temperatures below 1473K, nitrogen is concentrated enough to form nitrides such as CrN and Cr₂ N.
(3) At around 1473K, the steel powder can absorb about 1mass% of nitrogen, and this causes a structural change of the matrix from ferrite to austenite which dissolves all of nitrogen. The austenitic structure obtained is so stable at room temperature that the sintered steel does not undergo martensitic transformation during tensile deformation.
(4) A sintered steel with the chemical composition of 23mass% Cr-1mass% N has about three times large 0.2% proof stress in comparison with a sintered SUS304L steel with little nitrogen, and also has a good ductility in spite of containing about 12vol.% of retained pores.
(5) Sintering in N₂ gas atmosphere makes the use of a large amount of nitrogen possible for stainless steels, therefore, stable austenitic structure is easily obtained without adding expensive alloying elements such as nickel.