a b s t r a c t AISI Type 304L austenitic stainless steels are extensively used in industries, and welding is an indispensable tool used for joining these materials. In a multi-pass weld, the development of residual stress to a large extent depends on the response of the weld metal, heat affected zone and parent material to complex thermo-mechanical cycles during welding. Earlier researchers on this area used either mechanical tensioning or heat treatment to modify the residual stress distribution in and around the weld. In this research, microstructural refinement with modification of residual stress state was attempted by using high pressure cold rolling followed by laser processing in 12 mm thick 304L austenitic stainless steels which is a novel technique. The hardening of the weld metal was evaluated after welding, post weld cold rolling, and post weld cold rolling followed by laser processing. The residual stress was determined nondestructively by using neutron diffraction. Residual stress analysis show that post weld cold rolling was effective in modifying the longitudinal residual stress distribution throughout the entire thickness. Post weld cold rolling followed by laser processing performed in this research was to induce recrystallization of the cold rolled grains. However, post weld cold rolling followed by laser processing showed minor grain refinement but was not effective as it reinstated the stress state.

Multi-pass fusion welding by a filler material (wire) is normally carried out to join thick steel sections used in most engineering applications. Multiple thermal cycles from a multi-pass weld resulted in a variable distribution of residual stress field across the weld and through the thickness. Presence of tensile residual stresses can be detrimental to the integrity and the service behaviour of the welded joint. In addition to a complex distribution of residual stress state, multipass welds also form dendritic grain structure, which are repeatedly heated, resulting in segregation of alloying elements. In this research, microstructural refinement with modification of residual stress state was attempted by applying post-weld cold rolling followed by laser processing and then cold rolling. The residual stress was determined non-destructively by using neutron diffraction. Post-weld cold rolling followed by laser processing was carried out to induce recrystallization of the cold rolled grains. Microstructural characterisation indicates a significant grain refinement near the capping pass. However, post-weld cold rolling followed by laser processing reinstates the lock-in stress. In this study, it was demonstrated that a complete recrystallized microstructure with compressive state of stress can be formed when a further cold rolling is applied on the laser processed, recrystallized microstructure.

In multi-pass welding, the weld metal and the associated heat-affected zone are subjected to repeated thermal cycling from successive deposition of filler metals. The thermal straining results into multi-mode deformation of the weld metal which causes a variably distributed residual stress field through the thickness and across the weld of a multi-pass weldment. In addition to this, the as-welded fusion zone microstructure shows dendritic formation of grains and segregation of alloying element. This may result in formation of micro-corrosion cells and the problem would aggravate in case of highly alloyed materials. Local mechanical tensioning is an effective way of elimination of the weld tensile residual stress. It has been shown that application of cold rolling is capable not only of removing the residual stress, but depending on its magnitude it may also form beneficial compressive stress state. Multi-pass structural steel welds used as structural alloy in general engineering and structural applications. Such alloys are subjected to severe inservice degradation mechanisms e.g., corrosion and stress corrosion cracking. Welds and the locked-in residual stress in the welded area often initiate the defect which finally results in failure. In the present study, a multi-pass structural steel weld metal was first subjected to post-weld cold rolling which was followed by controlled heating by a fiber laser. Cold straining resulted in redistribution of the internal stress through the thickness and controlled laser processing helps in reforming of the grain structure. However, even with controlled laser, processing the residual stress is reinstated. Therefore, a strategy has been adopted to roll the metal post-laser processing so as to obtain a complete stress-free and recrystallized microstructure.

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