Determination of X-Ray Elastic Constants for Q1N Submarine Pressure Hull Steel

The Canadian Navy has a requirement to conduct on-site accurate residual stress investigations to improve the efficacy of risk assessment and damage tolerance analysis of critical structures. This paper describes the techniques for qualification of the portable miniature X-ray diffractometer (mXRD) for residual stress analysis on Canadian VICTORIA Class submarine (VCS) pressure hulls. The effect that thermomechanical processing has on the X-ray elastic constant (XREC) and resultant stress analysis is discussed. The approach for calibration of the mXRD equipment and calculation of residual stress values from strain measurements is explained. A new methodology for determination of the XREC, based on the 'multiple angle method', is presented discussed and applied to describe the stress-strain relation for tempered Q1N steel. This method, which expands on the ASTM standard, examines the change in lattice strain at multiple psi angles (partial derivative d(phi psi)/partial derivative sin(2)psi) and provides improved statistical error over the ASTM standard. An XREC of 195 +/- 6 GPa was experimentally determined for Q1N steel using the {211} crystallographic planes of bcc Fe (using Cr K alpha radiation) using the multiple angle method.