This paper illustrates the importance of residual stress characterization in welds and post weld processes. The failure to characterize residual stresses created during welding and/or post weld processes can lead to unexpected occurrences of stress corrosion cracking, distortion, fatigue cracking as well as instances of over design or over processing. The development of automated residual stress mapping and the availability of portable and fast equipment have now made the characterization of residual stresses using xray diffraction (XRD) practical for process control and optimization. The paper presents examples where xray diffraction residual stress characterization techniques were applied on various kinds of welds including arc welds, TIG welds, resistance welds, laser welds and electron beam welds. The nondestructive nature of the x-ray diffraction technique has made the residual stress characterization of welds a useful tool for process optimization and failure analysis, particularly since components can be measured before and after welding and post welding processes. Some examples presented show the residual stresses before and after the application of post weld processes such as shot peening, grinding and heat treatment.
INTRODUCTIONThe advantages of XRD and three areas of concem regarding residual stress and weld quality, stress corrosion cracking, fatigue, and stress concentrations, will be briefly discussed.
Fatigue and stress concentrationsTensile residual stress fields created during the welding process often contribute to decreases in the fatigue life ofwelded components, especially when they exist in the HAZ. The residual stress state existing in certain weld toes and undercuts can also be critical when stress concentration geometries exist which can magnify the effects ofapplied loads. When issues offatigue cracking are considered, potentially harmful tensile residual stresses alone or in combination with stress concentrations can lead to fatigue crack initiation and propagation. This means that accurate residual stress characterization must be performed in key areas such as the toe and the HAZ of welds to understand fatigue failures, help in the experimental verification of stress concentration factors predicted by finite element models and to reduce instances of over-design and unneeded increases in weight.
Environmentally assisted weld crackingAlso known as Stress Corrosion Cracking or 5CC, environmentally assisted cracking is a major source ofpotential failures in the process industries, in pulp mills, in storage vessels, and even in aircraft. Most often, 5CC occurs in the heat affected zone (I-IAZ) immediately adjacent to a weld simply because the HAZ is left in a state of very high residual tensile stress as a result ofthe shrinkage and differential cooling occurring in most welds. Tensile stresses (residual or applied) are the main component ofthe stress corrosion cracking triangle: the other two are a susceptible metal and an environment that often needs to be only slightly corrosive to that...
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