Residual stress distribution in nitrided En41B steel as function of case depth

Abstract

Specimens of a ‘Nitralloy’ type of En 41B steel were subjected to glow-discharge nitriding at 525°C for between 0.5 and 24h in order to produce specimens with differing nitride case depths. Microhardness tests were used to determine the specimen case depth and to indicate the reproducibility between batches of nitrided specimens. The residual stress distribution was obtained by the Sachs boring technique. ‘Change-in-strain’ at the specimen surface was measured by four precision strain gauges connected in two separate Wheatstone bridge circuits. Strain functions were calculated from the strain data and a computer program was produced which fitted robust, weighted least-squares polynomials to these strain functions. The triaxial residual stress condition was obtained on differentiation of the fitted polynomials. The residual stresses introduced into the specimen by drilling were minimized to a consistent level for which corrections were made in the calculation of residual stress. Conditions of high biaxial compression exist in the case of nitrided specimens, while the core is subjected to triaxial tension. The greatest core residual stress is in the longitudinal direction, but both the tangential and radial residual stresses have magnitudes of 60% of that of the longitudinal core residual stress.