Comparative study of evolution of residual stress state by local mechanical tensioning and laser processing of ferritic and austenitic structural steel welds.

Complex thermal stresses generated in welded structures are undesirable but inevitable in fusion welding. The presence of residual stresses can be detrimental to the integrity of a welded joint. In this research, redistribution of residual stress magnitude and profile was studied and compared in two multi-pass welded structural alloys (API X100 and 304L stainless steel) after cold rolling and laser processing. The residual stress field was studied by neutron diffraction using the SALSA strain scanner at their reactor neutron source at ILL, Grenoble. In addition to a complex distribution of residual stress state, multi-pass welds also forms dendritic grain structure, which are repeatedly heated, resulting in segregation of alloying elements. Dendritic grain structure is weaker and segregation of alloying elements may result in formation of corrosion microcells as well as reduction in overall corrosion prevention due to depletion of alloying elements in certain areas. The modification of as-welded residual stress state was done by cold rolling which was followed by laser processing to create a recrystallized microstructure to minimise segregation. The main objective of this study is to understand the suitability of this novel manufacturing technique to create a stress free weldment with recrystallised grain structure. Hardness evolution in the welded structures was scanned following welding, post weld cold rolling and cold rolling followed by laser processing. Hardness distribution in both the structural alloys showed a significant evidence of plastic deformation near the cap pass of the weld metal. Residual stress redistribution was observed up to 4 mm from the capping pass for ferritic steel, while in austenitic steel weld, post weld cold rolling was effective in modifying the residual stress redistribution throughout the entire thickness. Laser processing in both cases reinstated the as-welded residual stress distribution and resulted in softening of the strained area

Analysis of Residual Stresses in Laser-Shock-Peened and Shot-Peened Marine Steel Welds

Laser peening is now the preferred method of surface treatment in many applications. The magnitude and depth of the compressive residual stress induced by laser peening can be influenced strongly by the number of peen layers (the number of laser hits at each point) and by processing conditions including the use of a protective ablative layer. In this study, residual stresses have been characterized in laser and shot-peened marine butt welds with a particular focus at the fatigue crack initiation location at the weld toe. X-ray diffraction, synchrotron X-ray diffraction, incremental center-hole drilling, and the contour method were used for determination of residual stress. Results showed that the use of ablative tape increased the surface compressive stress, and the depth of compressive stress increased with an increase in number of peening layers. A key result is that variation of residual stress profile across laser peen spots was seen, and the residual stress magnitude varies between the center and edges of the spots

Through-Thickness Residual Stress Profiles in Austenitic Stainless Steel Welds: A Combined Experimental and Prediction Study

Economic and safe management of nuclear plant components relies on accurate prediction of welding-induced residual stresses. In this study, the distribution of residual stress through the thickness of austenitic stainless steel welds has been measured using neutron diffraction and the contour method. The measured data are used to validate residual stress profiles predicted by an artificial neural network approach (ANN) as a function of welding heat input and geometry. Maximum tensile stresses with magnitude close to the yield strength of the material were observed near the weld cap in both axial and hoop direction of the welds. Significant scatter of more than 200 MPa was found within the residual stress measurements at the weld center line and are associated with the geometry and welding conditions of individual weld passes. The ANN prediction is developed in an attempt to effectively quantify this phenomenon of ‘innate scatter’ and to learn the non-linear patterns in the weld residual stress profiles. Furthermore, the efficacy of the ANN method for defining through-thickness residual stress profiles in welds for application in structural integrity assessments is evaluated