To obtain an alloy material free from the generation of pitting and having a long service life by diffusing nitrogen having a specified conc. over the surface of an alloy material.
Over the surface of an alloy material such as a low alloy steel, nitrogen is diffused to so as to regulate the concn. of nitrogen to 0.1 to 1.0%. The depth of the diffusion of nitrogen from the surface of the alloy material is made deeper than a surface abnormal layer formed on the alloy material. In this way, its hardenability improves to suppress the formation of the surface abnormal layer and also to form carbon nitrides in the alloy material by which the bearing strength of the material can be improved. The alloy material preferably contains, by compositional ratio, ≤0.15wt.% Si, ≤0.55wt.% Mn, ≤0.70wt.% Cr, ≥0.50wt.% Ni and ≥0.40wt.% Mo. By reducing the alloy components oxidized in the process of carburizing treatment, intergranular oxidation causing fracture cracking can be suppressed. The reduction of this hardenability caused by the reduction of the alloy components can be suppressed by the addition of Mo and Ni.
UCHINO RYUICHI
HIROMURA TOSHIKI
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