Characterization of residual stress and plastic strain in austenitic stainless steel 316L(N) weldments

Fusion welding processes commonly involve the localized input of intense heat, melting of dissimilar materials and the deposition of molten filler metal. The surrounding material undergoes complex thermo-mechanical cycles involving elastic and plastic deformation. This processing history creates large residual stress in and around the weld bead, which can be particularly detrimental in reducing the lifetime of fabricated structures, increasing their susceptibility to stress corrosion, fatigue and creep crack growth as well as reducing the fracture load. It is very important to have a proper knowledge of the residual stress distribution in and around the weld region of structured components because knowing this allows their fitness to be assessed and the service life of critical components to be predicted. Characterizing weld residual stress fields either by measurement or finite element simulation is not straightforward because of the strain field complexity, inhomogeneity of the microstructure and the complex geometry of structural weldments. The residual stress distribution in a slot weld benchmark sample made from AISI 316L(N) austenitic stainless steel was analysed using the neutron diffraction at pulsed source. The presence of crevices and hydrogen containing super glue in the stress-free cuboids are some of the main issues effecting the neutron residual stress measurements. A residual stress of 400-450MPa was observed in first pass weld metal and in the HAZ of a three pass welded plate. The strain hardening behaviour of AISI 316L(N) steel around the slot weld was studied taking account of the asymmetric cyclic deformation and the typical strain rates experienced; inferences are drawn regarding how such effects Should be modelled in finite element weld residual stress computations. The solution annealed material was tested under symmetric and asymmetric cyclic loading at both room and 550°C. During asymmetric cyclic loading, the 316L(N) material at room and high temperature was less strain hardened than in the same number of cycles of symmetric cyclic loading. At room temperature; the 316L(N) material deformed at fast strain rate showed higher strain hardening than at the slow strain rate. However, at high temperature (550°C); the 316L(N) material deformed at slow strain rate showed higher strain hardening than at the fast strain rate due to dynamic strain ageing. A mixed hardening model was to predict the strain hardening of the 316L(N) material at room and high temperature (550°C). However, the published mixed hardening parameters were unsuccessful in predicting the strain hardening of the symmetric cyclic deformation at high temperature. Finally, the accumulated cyclic plastic strain resulting from the addition of each weld bead was studied using Electron Backscatter Diffraction (EBSD) and hardness measurements. The EBSD metrics showed a gradual increase of plastic strain and equivalent yield stress from the parent zone (approximately 0.02) to the fusion boundary (approximately 0.05-0.09). Although, in strain controlled cyclic loading, none of the EBSD metrics used were capable of assessing the plastic strain, below 58% cumulative plastic strain path. The quantified plastic strain (from the EBSD) and hardness analysis of the parent material indicates that the material deformed plastically. The EBSD derived plastic strain and equivalent yield stress correlate well with hardness, finite element prediction and von Mises equivalent residual stress.