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

Microstructure and mechanical properties of Al-1050 during incremental ECAP

Incremental ECAP is a new method of ECAP process were the severe shear deformation is incrementally applied on the sample resulting in grain refining and new texture developing. The fundamental objective of the present work is an observation of effect of different passes of I-ECAP on microstructure and mechanical properties of AA1050 billet. To that end, 8 pass of I-ECAP have been carried out using Bc route and microstructure evolution and mechanical properties of the I-ECAPed samples have been studied. The EBSD and TEM analyses indicates that I-ECAP is as capable as conventional ECAP to grain refinements and a UFG structure is resulted after I-ECAP cycles. Tensile testing and hardness measurements indicates that mechanical properties of the Al-1050 billets increases dramatically by increasing the I-ECAP passes

Modelling and experimentation of the evolution of texture in an Al-Mg alloy during earing cupping test

Earing and thinning are often the major manufacturing problems occur during deep drawing processes. Thinning occurs when a section of a part undergoes localised deformation, and earing is the formation of wavy edges at the open end of a drawn part that must be trimmed at final stage leading to higher manufacturing costs. The anisotropic mechanical behavior of the initial sheet metal is the predominant source of thinning and earing problems. This work aims to establish a relationship between the properties of a sheet blank and thinning and earing issues during deep drawing by studying the evolution of crystallographic texture throughout the sheet forming process using crystal plasticity simulation modelling and experimental measurements. Firstly, to understand the impact of individual texture components on the mechanical properties of the material, Lankford coefficients for FCC crystal structure during uni-axial tensile loading were analysed using Visco-Plastic Self Consistent (VPSC) model. Subsequently, Finite Element (FE) analyses were carried out to study the effect of initial state of the material on earing and thinning issues occurred during deep drawing. It was observed that the existing Cube and Goss texture components evolved during annealing heat treatments were responsible for the generation of troughs along 45° to the rolling direction (RD) and peaks along the transverse direction (TD), respectively. Optical 3D scanning of a manufactured part confirmed that earing is less prominent in the case of as-rolled and shear-formed condition due to weakening of Cube and Goss texture components. Furthermore, a combination of FE simulation and the VPSC model has been used to simulate texture evolution during a standard earing cupping test at various points of interest. The results of texture evolution simulations were compared to those measured experimentally by electron backscatter diffraction (EBSD), and a good qualitative agreement is achieved

Development of process induced residual stress during flow forming of tubular 15-5 martensitic stainless steel

Flow forming is a near net shape process for manufacturing of dimensionally accurate hollow components such as shaft in gas turbines, that is currently at its development stage for aerospace industry. The process has several advantages such as reducing material wastage, extremely fast manufacturing time, and eliminating extra manufacturing processes such as machining. Due to the nature of this complicated cold deformation process, significant magnitude of residual stress is introduced into the component. Understanding the magnitude and distribution of residual stress is essential to tailor the flow forming process to achieve parts within dimensional tolerances and desired mechanical properties. The present research is aiming to explore the generation and evolution of residual stress at various stages of flow forming process in a tubular component made from martensitic 15Cr-5Ni stainless steel, using different techniques of neutron scattering, x-ray diffraction (XRD) and hole-drilling based on electronic speckle pattern interferometry (ESPI). Residual stress measurements were carried out in pre-formed and flow formed components at surface, near-surface and in the bulk of components using XRD, ESPI based hole-drilling and neutron diffraction techniques. These measurements were conducted at different levels of reduction in the thickness of the original part (i.e. after 20% and 40%), by applying identical forming parameters for all samples. The XRD results show significant change in hoop and axial residual stress levels with a reduction in the wall thickness. This is more pronounced for the axial component where the average stress switches from relatively high tensile (~ 450MPa) in the original part to significant compressive stress (~ -600MPa) in the formed part, after 20% of reduction. The bulk residual stress components measured in the middle of thickness of the parts, using neutron scattering, show a general increase in the magnitude of residual stress by higher level of deformation (i.e. reduction in the wall thickness). The measured bulk stress components through the thickness were tuned to tensile after reducing the wall thickness by 40%. The results of XRD and neutron diffraction stress measurements suggest that the residual stress along the length of the samples (i.e. axial direction) is consistent with ±800 MPa and ±400 MPa after 20% and 40% reduction by forming process, respectively. The results of ESPI based hole-drilling show tensile hoop residual stress (≈600 MPa) and an abrupt fluctuation (i.e. tension-compressive-tension) in the axial residual stress near the surface of the part following flow forming. The stresses measured by ESPI based hole-drilling are complementary to the results of the XRD on surface and neutron diffraction in the bulk to reconstruct the residual stress profile form the surface through to the bulk

A complete reassessment of standard residual stress uncertainty analyses using neutron diffraction emphasizing the influence of grain size

The determination of residual stress in engineering materials with large grains is a challenge when it comes to using diffraction techniques. Not only are the accuracies of the residual stresses themselves important but also the accurate evaluation of their uncertainties. An austenitic steel three-pass slot weld (NeT- TG4) with varying grain size high-lights the potential problems with the data evaluation when grain size is not taken into account whilst measuring strain. Neutron diffraction results are compared with each other (with combinations of slit and radial oscillating collimator beam defining optics) and with high energy synchrotron radiation results with a spiral slit gauge volume defining system. The impact of the grain size on the determination of residual stresses and their associated uncertainties when using diffraction techniques in engineering components is emphasized and discussed. A simple model to estimate the extra random uncertainty contribution due to the so-called grain size statistics is applied and verified. The benefit of continuous or stepwise oscillation to increase the number of detected grains on the detector is discussed together with how to optimize the time of a measurement. From the data obtained, best practice guidelines will be suggested on dealing with large grains when determining strain and stress with neutron diffraction.JRC.G.I.4-Nuclear Reactor Safety and Emergency Preparednes