CYCLIC STRESSING OF GAS NITRO-CARBURIZED LOW-CARBON STEEL

Bending fatigue studies of low carbon steel after gaseous nitrocarburizing have shown that the fatigue strength is increased by about 140 pct relative to annealed material. X-ray diffraction and electron microscopy studies have demonstrated that fine scale precipitation occurs in the outermost 400 μ region of the diffusion zone and, beyond this, both nitrogen and carbon essentially remain in solid solution. Substantial cyclic hardening has been observed during the fatigue of nitrocarburized material. The higher the applied stress the greater the depth of hardening and at high applied stresses the depth of hardening extends beyond the diffusion zone into the core of the material. Fading of surface residual stresses during fatigue is observed, however subsurface compressive stresses are present throughout the life of the material. At low applied stresses subsurface plastic deformation occurs within the nitrocarburized material. Under long life fatigue conditions the amount of bulk plastic deformation required to cause fracture of the nitrocarburized material is significantly greater than for annealed material, but for short lives, the reverse is observed. © 1979 American Society for Metals and the Metallurgical Society of AIME.