| After nitrogen permeation heat treating the Al and Cu alloyed 13wt%Cr stainless steels in the nitrogen gas pressure of 1㎏/㎟ mainly at 1200℃, the surface nitrogen content,, phase changes, hardness variations and corrosion properties were investigated. The nitrogen dissolution and exhaustion of Al owing to the nitride precipitation cause to $quot;change the microstructure of the nitrogen permeated surface layer from α+γ(martensite) phase to γ single phase. The surface austenite phase contains round type aluminum nitrides and needle type carbonitrides. The nitrogen content of the surface layer increases after showing low content at the outmost surface due to the hindrance effect of carbon diffusing from interiors to surface. The Al and Cu move from γ phase to precipitate and Cr moves from precipitate to γ phase during nitrogen permeation heat treatment. At the nitrogen permeation temperature, the specimen representing γ single phase shows gradual decrease in the hardness with increasing depth below surface. while the specimen representing α+γ phase shows abrupt decrease in hardness due to the obvious nitrogen permeation layer boundary. The corrosion resistance of nitrogen permeated surface layer increases and remarkablely decreases in the 5%HCl solution and the 5%H₂SO₄ solution, respectively. compared to that of matrix. |
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