Luis Daniel Cozzolino

Effect of high pressure rolling on weld-induced residual stresses

Abstract The formation of large residual stresses continues to be a problematic side effect of all common welding processes. In this work, localised high pressure rolling of gas metal arc welds to relieve these residual stresses has been investigated using strain gauging and neutron diffraction. Rolling was found to remove undesirable tensile stresses and even induce large compressive ones, though only when applied after rather than during welding. Strain measurements taken during combined welding and rolling operations show that this is because material at the weld line continues to yield as it cools. This erases any beneficial effect on the stress distribution of rolling at high temperature. A method of rolling using an oscillating force is also presented and found to be just as effective as the equivalent static force process.

High pressure rolling of low carbon steel weld seams: Part 2 – Roller geometry and residual stress

Abstract Large residual stresses are an undesirable but inevitable side effect of fusion welding operations, and localised high pressure rolling of the weld seam is a proposed method for eliminating them. In this study, neutron diffraction has been used to map the residual stresses within low carbon steel weld seams treated with high pressure rolling. The effect on the residual stress distribution of using different roller types was determined, along with the influence of these different rollers on final weld seam geometry. Rolling was found to completely change the residual stress state in the weld, creating large compressive longitudinal residual stresses. It was effective for this purpose regardless of whether it was applied directly to the weld seam or to regions either side of it. The fatigue life of welded specimens was shown to be reduced by rolling; however, it is suggested that this is due to geometric and metallurgical effects.

Residual strain measurement for arc welding and localised high-pressure rolling using resistance strain gauges and neutron diffraction

Neutron diffraction and foil resistance strain gauges have been used to study the state of residual stress introduced by localised high-pressure rolling of structural steel plates, and compare it to that caused by gas metal arc welding. Rolling creates a region in which the residual stress state is highly compressive in the rolling direction. Furthermore, this region is sharply defined, making it potentially very suitable for cancelling out the tensile residual stresses caused by welding. It is also demonstrated that non-destructive strain measurements made during the welding and rolling processes can be used to indicate residual elastic strain and stress, and that this method shows good agreement with conventional neutron diffraction measurements. Determination of residual stresses in this way requires consideration of the effect of curvature on the values of strain measured at the surface of the object.

High pressure rolling of low carbon steel weld seams: Part 1 – Effects on mechanical properties and microstructure

Abstract One technique for reducing residual stress in welds is high pressure rolling of the weld seam. In this study, a variety of experimental techniques, including microhardness measurements and cross-weld tensile tests with digital image correlation, have been used to characterise the effects of rolling on the mechanical properties and microstructure of the weld material in welded structural steel specimens. It is shown that rolling applied at high temperature, as welding is carried out, promotes the formation of acicular ferrite in the weld metal. This produces a weld material with a greater yield strength and hardness, but slightly reduced impact toughness compared to unrolled welds. Rolling of the weld metal once it has cooled instead causes work-hardening. These effects are discussed as they relate to the use of rolling for weld residual stress reduction.

The Effect of Pre-Weld Rolling on Distortion and Residual Stress in Fusion Welded Steel Plate

Local rolling and other mechanical tensioning techniques can be highly effective at reducing residual stress and distortion in thin plate welds prone to buckling. However, the issues of high capital cost and low scalability currently prevent wider adoption of such processes. Pre-weld rolling aims to address these issues and can be applied easily to each component prior to fabrication. The results of an initial trial are presented, and indicate that post-weld distortion can be reduced by an average of 38% when correct rolling parameters are used. Finally, the mechanism by which pre-rolling acts to modify the state of residual stress around a weld line is discussed.