Measuring Residual Stresses in Stainless Steel - Recent Experiences within a Vamas Exercise

Residual stresses impact on a wide variety of industrial sectors including the automotive, power generation, industrial plant, construction, aerospace, railway and transport industries, and a range of materials manufacturers and processing companies. The X-ray diffraction (XRD) technique is one of the most popular methods for measuring residual stress [1], used routinely in quality control and materials characterisation for validating models and design. The VAMAS TWA20 Project 3 activity on the “Measurement of Residual Stresses by Xray Diffraction” was initiated by NPL in 2005 to examine various aspects of the XRD test procedure in support of work aimed at developing an international standard in this area. The purpose of this project was to examine and reduce some of the sources of scatter and uncertainty in the measurement of residual stress by X-ray diffraction on metallic materials, through an international inter-comparison and validation exercise. One of the major issues the inter-comparison highlighted was the problem associated with measuring residual stresses in austenitic stainless steel. The following paper describes this intercomparison, reviews the results of the exercise and details additional work looking at developing best practice for measuring residual stresses in austenitic stainless steel, for which X-ray measurements are somewhat unreliable and problematic. INTRODUCTION The main objective of the VAMAS TWA20 Project 3 was to examine various aspects of the measurement process associated with X-ray residual stress measurement, and contribute to an internationally agreed standard that has been drafted through the CEN/TC138/WG10 committee and the ASTM E28.13 subcommittee on Residual Stress Measurement. The project complimented the activity of CEN group by carrying out additional testing and characterisation in support of the proposed standard, through an inter-comparison exercise. For the inter-comparison a simple component was required that would have a reasonably high stress with good repeatability whilst providing a significant measurement challenge and yet could be produced cost effectively in the numbers required for the first inter-comparison exercise. From previous experience [2, 3] a shot peened stainless steel plate was chosen to meet all these criteria. The specimens were manufactured from a 316 stainless steel plate, which was cut into 16 large sections and shot peened on one face by Metal Improvement Company, using MI 230 hard media, with an intensity of 0.010 “A and a 200% coverage, where A refers to the Almen measurement, which is a measure of the deflection on a Almen test strip exposed to the peening process. 312 Copyright ©JCPDS-International Centre for Diffraction Data 2009 ISSN 1097-0002