Internal residual strain distribution in chromium-molybdenum steel after carburizing and quenching measured by neutron strain scanning

A hollow cylinder of a chromium-molybdenum steel composed of 0.20 mass% C was used as a model that simplified a motorcycle transmission gear. The cylinder was carburized in a carrier gas and quenched in an oil bath. After quenching, the internal residual strain and peak width distributions in the radial, hoop, and axial directions were nondestructively mapped by neutron strain scanning, and were compared to the results of an elastoplastic finite element analysis. The carbon content and hardness gradients were also measured to determine the case depth. The residual strain was evaluated from the lattice spacing change. The lattice spacing change was obtained from the peak shift from an unstressed peak position. In this study, the unstressed peak position was determined using coupons that were cut from the interior of the same quenched cylinder. As a result, the carbon content, hardness, and internal residual strain distributions almost accorded with the results of finite element analysis. Although the measured residual hoop strains near the hardened layer partially deviated from the analyzed weighted average strains, the validity of the internal strain field measured by neutron strain scanning was confirmed.