PSI - Issue 14

Vijay Sai et al. / Procedia Structural Integrity 14 (2019) 491–498 Ch. Vijay Sai/ Structural Integrity Procedia 00 (2018) 000–000

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material adjacent to it. Thus greater the decrease in compressive residual stress, greater will be the tensile residual stress. This explains the approximately equal difference between peak compressive and tensile residual stress in all joints with P91. Owing to the open structure of plate vis-à-vis the closure of pipe, compressive stress does not decrease in circumferential weld joint. This explains the lower tensile residual stress in circumferential weld joints vis-à-vis plate weld joints with P91 as a base metal. Also owing to difference in circumference across the thickness, eigen strains vary across the thickness of pipes, with a higher magnitude occurring in the smallest circumference i.e. inside of pipe. In the dissimilar weld, SS304 encounters steeper temperature profile and higher restraint from P91 side due to its lower coefficient of expansion, in comparison to its conventional joint. Hence residual stress of SS304 should rise in the dissimilar weld joint. This is distinctly observed in the pipe weld joint, where this increase and locking of compressive stress, results in tensile residual stress being higher in SS304. In the plate joint, the release of compressive stress accompanied by expansion of adjacent material is similar to the force applied in mechanical stress proofing, releasing the eigen strain in SS304 and reducing its residual stress. Magnitude of eigen strain being higher, in SS304 as observed by Javadi et al. (2017), and SS304 being weaker than P91, stress is relieved in SS304 side of the joint. P91 in the dissimilar weld encounters lower restraint due to higher thermal expansion of SS304 and decrease in steepness of temperature, in comparison to its conventional joint. This reduces the residual stress of P91 in dissimilar weld in comparison to P91 similar base metal joint. This is evidenced by higher compressive stress in martensitic zones in the dissimilar welds, which are indicative of smaller tensile stress prior to martensite formation. In the pipe joints, the difference in magnitude of peak compressive stress is approximately 200 MPa

Fig. 8. Hoop residual stress in (a) SS304 circumferential weld joint; (b) P91 circumferential weld joint; (c) Dissimilar circumferential weld joint