0.3～0.4%窒素を含有する高Cr-高Niオーステナイト鋼におけるCr₂Nからπ相への窒化物反応

抄録

The precipitation procosses occurring during high temperature aging in high chromium-high nickel austenitic steels containing 0.3-0.4% nitrogen were followed by optical microscopy and X-ray powder difrraction analysis. Steels, mainly with a composition of 25% Cr-26% Ni-2% Mo-0.36% N, were aged at 800°C for periods of up to 10000h. X-ray powder diffraction photographs of extracted residues indicated that the precipitation of dichromium nitride, Cr₂N, at the earlier stage was followed by the formation of σ phase and π phase (a nitride with the structure of β-manganese type) and that the relative amount of Cr₂N in the extracted residues decreased with time of aging after 100 h. Microscopic observation of the electrolytically etched microstructure revealed that Cr₂N precipitates covering entire grain boundary area at the earlier stage completely redissolved and transformed to π and σ phases after 10000h aging. The resultc onfirmed a reaction of Cr₂N→π phase. It was suggested that π phase precipitation took place in steels bearing lower concentration of nitrogen without accompanying the preceding Cr₂N precipitation It should be emphasized that the solubility of nitrogen in high chromium-high nickel austenltic steels was determined not by Cr₂N but by π phase.