Residual stress relaxation in shot peened high strength low alloy steel and its implications for hydrogen assisted cracking

Abstract

The relaxation of compressive residual stress in shot peened and laser peened 300M high strength low alloy steel in response to applied loads has been investigated and for the shot peened specimens the implication for resistance to hydrogen assisted cracking assessed. The near surface residual stress was measured predominantly by X-ray diffraction (first 10 μm) but supplemented by measurements using high resolution, fine increment hole drilling to yield depth profiling. Relaxation of residual stress was most significant under compressive loading (single load cycle) with a loss of ∼70% of the peak residual stress when the applied stress was at the compressive 0·2% proof stress. Constant load tests in 3·5%NaCl were conducted on cadmium or SermeTel coated specimens at open circuit and with uncoated specimens at an applied potential corresponding to the minimum value of the protection potential associated with these coatings. For all peened specimens, stress relaxation was induced by a prior applied stress, usually close to the compressive 0·2% proof stress. Most tests were conducted with notched specimens with a target notch depth about 50–60 m m, close to the peak residual stress on shot peened specimens. Despite the comparative severity of the tests, with applied tensile stresses up to 95% of the tensile 0·2% proof stress and the maximum expected residual stress relaxation, none of the shot peened specimens failed. This is reassuring from an engineering perspective. However, notched specimens did fail below the tensile 0·2% proof stress under slow strain rate conditions and the performance under corrosion fatigue conditions remains to be established.