Residual Stress Distribution in Hardened Case Layer of Cr-Mo Steel after Carburizing and Quenching

Abstract

The hollow circular cylinder specimen of Cr-Mo steel with 0.20 mass% C was carburized in carrier gas and quenched in oil bath. After quenching, the surface residual stress distributions in the radial, axial and hoop directions of the specimen were measured experimentally by x-ray as a function of the distance from the carburized surface. The case depth of the quenched specimen was about 0.8 mm. Diffraction from Fe-211 by Cr-Kα radiation was used to minimize the effect of carbon content gradient on the nonlinearity of 2θ-sin2ψ, because effective x-ray penetration depth was about 5.8 μm at sin2ψ=0. The hardened case layer was gradually removed by electrolytic polishing. X-ray stress measurement was repeated on the polished surface from the carburized surface to the interior. The ψ-splitting was not observed on the carburized surface. The 2θ-sin2ψ diagrams were found to shift from low to high angles in inverse proportion to carbon content. The residual stresses in the hardened case layer were compressive. The maximum compressive residual stresses on the hollow circle and periphery surfaces were about −559 and −544 MPa at the depth of 0.2 and 0.3 mm, respectively. On the other hand, the tensile residual stress was not observed. The full widths at half-maximum intensity of Fe-211 diffraction peaks in the hardened case layer were wider than that of the interior of the specimen. Martensitic transformation of the carbon-rich layer leaded to the broadening of diffraction peaks. Therefore the interior of carburized specimen were deformed elastically to balance the existence of the surface compressive residual stresses.