Lattice Plane Spacing Change in Hardened Layer of Chrome Molybdenum Steel after Carburizing and Quenching

Abstract

Block specimens of chrome molybdenum steel with 0.20 mass% C, SCM420, were carburized in carrier gas and quenched in oil bath. The hardness and carbon content gradients in the hardened layer were measured experimentally. The carburized surface of one block specimen was gradually removed by electrolytic polishing. Ten thin plates were cut from the total case depth of the other block specimen. An experimental method to determine the stress-free lattice plane spacing of the hardened layer was examined using x-ray and neutron radiations. As a result, the stress-free lattice plane spacing change in the hardened layer could be determined successfully by measuring neutron diffraction peaks from Fe-211 of the thin plates during rotating ±90° around the specimen axis. Using x-ray, the stress-free lattice plane spacing at the carburized surface could be also determined by measuring the 2θ-sin2ψ diagrams of either removed surface of block specimen or thin plate. However, under the carburized surface, the Kα2 diffraction from the heat-treated eutectoid phase was superimposed on x-ray diffraction peak because the subsurface microstructure was composed of martensitic and heat-treated eutectoid phases. The stress-free lattice plane spacing under the carburized surface could not be determined using x-ray. Furthermore, the stress-free lattice plane spacing of Fe-211 was found to decrease with increasing the distance from the carburized surface and be expressed by the cubic function of the carbon content in the hardened layer.