Alison Mark

Design of weld fillers for mitigation of residual stresses in ferritic and austenitic steel welds

Abstract Residual stresses that arise as a result of welding can cause distortion, and also have significant implications for structural integrity. Martensitic filler metals with low transformation temperatures can efficiently reduce the residual stresses generated during welding, because the strains associated with the transformation compensate for thermal contraction strains during cooling. However, it is vital that a low weld transformation temperature is not obtained at the expense of other important material properties. This article outlines the alloy design process used to develop appropriate low transformation temperature filler materials for the mitigation of residual stresses in both low alloy ferritic and austenitic stainless steel welds. Residual stresses in single pass, 6 mm bead in groove welds, on 12 mm thick plates, have been measured and compared against those obtained with commercially available conventional austenitic and ferritic filler materials. The filler metals developed here exceeded requirements in terms of weld mechanical properties, while significantly reducing the maximum residual stress in the weld and heat affected zone.

Nonintrusive estimation of anisotropic stiffness maps of heterogeneous steel welds for the improvement of ultrasonic array inspection

It is challenging to inspect austenitic welds nondestructively using ultrasonic waves because the spatially varying elastic anisotropy of weld microstructures can lead to the deviation of ultrasound. Models have been developed to predict the propagation of ultrasound in such welds once the weld stiffness heterogeneity is known. Consequently, it is desirable to have a means of measuring the variation in elastic anisotropy experimentally so as to be able to correct for deviations in ultrasonic pathways for the improvement of weld inspection. This paper investigates the use of external nonintrusive ultrasonic array measurements to construct such weld stiffness maps, representing the orientation of the stiffness tensor according to location in the weld cross section. An inverse model based on a genetic algorithm has been developed to recover a small number of key parameters in an approximate model of the weld map, making use of ultrasonic array measurements. The approximate model of the weld map uses the Modeling of anIsotropy based on Notebook of Arcwelding (MINA) formulation, which is one of the representations that has been proposed by other researchers to provide a simple, yet physically based, description of the overall variations of orientations of the stiffness tensors over the weld cross section. The choice of sensitive ultrasonic modes as well as the best monitoring positions have been discussed to achieve a robust inversion. Experiments have been carried out on a 60-mm-thick multipass tungsten inert gas (TIG) weld to validate the findings of the modeling, showing very good agreement. This work shows that ultrasonic array measurements can be used on a single side of a butt-welded plate, such that there is no need to access the remote side, to construct an approximate but useful weld map of the spatial variations in anisotropic stiffness orientation that occur within the weld.

Residual stress control of multipass welds using low transformation temperature fillers

ABSTRACT Low transformation temperature (LTT) weld fillers can be used to replace tensile weld residual stresses with compressive ones and reduce the distortion of single-pass welds in austenitic plates. By contrast, weld fillers in multipass welds experience a number of thermal excursions, meaning that the benefit of the smart LTT fillers may not be realised. Here, neutron diffraction and the contour method are used to measure the residual stress in an eight pass groove weld of a 304 L stainless steel plate using the experimental LTT filler Camalloy 4. Our measurements show that the stress mitigating the effect of Camalloy 4 is indeed diminished during multipass welding. We propose a carefully selected elevated interpass hold temperature and demonstrate that this restores the LTT capability to successfully mitigate residual tensile stresses. This paper is part of a thematic issue on Nuclear Materials.

Finite Element Modelling and Measurements of Residual Stress and Phase Composition in Ferritic Welds

This paper describes finite element (FE) modelling and neutron diffraction (ND) measurements to investigate the development of residual stresses in two different geometries of ferritic weld. All specimens were produced using SA508 Grade 3 steel plates, depositing a low carbon SD3 weld filler by mechanised TIG welding. The FE analyses were carried out using Abaqus/VFT and the behaviour of the SA508 steel was modelled using a simplified (Leblond) phase transformation model with isotropic hardening using VFT’s UMAT-WELD subroutine, which includes the change in volume due to phase transformation. Single bead-on-plate specimens were manufactured using a range of mechanised TIG welding parameters. One pass and three pass groove welds were also produced, in order to investigate the cyclic hardening behaviour of the materials, as well as phase transformation effects in a multi-pass weld. FE analyses were then performed to determine how accurately these effects could be modelled. During manufacture, a number of thermocouples were attached to each of the specimens in order to calibrate the heat input to the FE models. The residual stresses in each of the bead on plate welds, as well as the groove weld after the first and the third passes, were then measured using ND at the middle of the plate. The ND measurements for the three pass weld showed no significant cyclic hardening behaviour although some was predicted by the FE analysis. Another key finding of the FE modelling that was seen in all of the models was that the phase transformation acts to reduce the stress levels in the deposited weld metal leaving the largest tensile stresses in a ring at the outer edge of the full heat affected zone (HAZ). There are plans to refine the FE studies using improved material properties when material testing of SA508 and SD3 are completed in the near future.Copyright

Investigation of Transformation Induced Plasticity and Residual Stress Analysis in Stainless Steel Welds

The aim of this paper is to investigate the implications for weld residual stresses of martensitic transformation induced plasticity (TRIP) in stainless steel filler metal. The TRIP strains occurring during cooling under different uniaxial load levels have been obtained using digital image correlation (DIC) for a residual stress relieving low transformation temperature weld filler known to show little variant selection on cooling as a function of stress. In order to investigate the efficacy of current FE transformation plasticity models of different levels of sophistication in simulating TRIP strains, a finite element model, incorporating the so-called Greenwood-Johnson effect was used to simulate these constrained dilatometry measurements. To assess the implications of the different approaches to modelling TRIP for weld residual stresses, the TRIP coefficients determined from the above experiments were incorporated into an FE model simulating the residual stresses that are generated when a single weld bead is deposited on to a stainless steel base plate. It was found that including TRIP had a significant influence on the weld stresses, while the differences between the models were much smaller.

On the Stress Development in SA508 Autogenous Weld

Considering the significant role that residual stresses play in determining the lifetime-service of materials, it is mandatory to have a good understanding of and a means of predicting those that develop during welding processes. For this purpose, a User MATerial subroutine (UMAT) is developed to study the effects of various parameters that influence solid state phase transformations and residual stress evolution during welding of SA508 ferritic steel. The temperature dependent elastic and kinematic hardening parameters for each of the individual phases that can potentially develop during cooling from elevated temperatures are measured and are used for calculating stress development during low (75 mm/min) and high (300 mm/min) speed gas-tungsten arc welding (GTAW) on SA508 grade 3. These two speeds are selected to cover a wide range of cooling rates in the heat affected zone so that different phase proportions would be present. The results of the numerical simulations for residual stresses are compared against those measured by neutron diffraction. It is shown here that a low speed weld results in bainite formation whereas a high speed weld results in bainitic as well as subsequent martensitic phase transformations where each welding rate results in different residual stress development.

Comparison of grain to grain orientation and stiffness mapping by spatially resolved acoustic spectroscopy and EBSD

Our aim was to establish the capability of spatially resolved acoustic spectroscopy (SRAS) to map grain orientations and the anisotropy in stiffness at the sub‐mm to micron scale by comparing the method with electron backscatter diffraction (EBSD) undertaken within a scanning electron microscope. In the former the grain orientations are deduced by measuring the spatial variation in elastic modulus; conversely, in EBSD the elastic anisotropy is deduced from direct measurements of the crystal orientations. The two test‐cases comprise mapping the fusion zones for large TIG and MMA welds in thick power plant austenitic and ferritic steels, respectively; these are technologically important because, among other things, elastic anisotropy can cause ultrasonic weld inspection methods to become inaccurate because it causes bending in the paths of sound waves. The spatial resolution of SRAS is not as good as that for EBSD (∼100 μ m vs. ∼a few nm), nor is the angular resolution (∼1.5° vs. ∼0.5°). However the method can be applied to much larger areas (currently on the order of 300 mm square), is much faster (∼5 times), is cheaper and easier to perform, and it could be undertaken on the manufacturing floor. Given these advantages, particularly to industrial users, and the on‐going improvements to the method, SRAS has the potential to become a standard method for orientation mapping, particularly in cases where the elastic anisotropy is important over macroscopic/component length scales.

Development of Material Model Parameters Suitable for the Finite Element Simulation of Ferritic Welds

The prediction of residual stresses in ferritic welds using finite element techniques requires materials properties to describe the thermal, tensile, cyclic and phase transformation behaviour that the material undergoes during welding and also during creep as the effect of post weld heat treatment is also of interest. Ferritic steels will transform at a temperature above about 850°C to austenite. As the steel is cooled, a further phase transformation in the structure occurs. The precise structure formed depends on the detailed chemical make-up of the steel and on the rate at which it is cooled. On slow cooling from above 850°C a pearlite-ferrite microstructure is formed. On more rapid cooling, other microstructures, particularly bainite at intermediate cooling rates and martensite at the highest cooling rates are formed. Predicting the phase on cooling requires a Continuous Cooling Transformation diagram that is suitable for welding thermal cycles and reflects the time spent above the austenitisation threshold which influences the austenitic grain size formed and subsequently the phase of material on cooling. Material properties for a SA508 Grade 3 steel and a low carbon SD3 filler metal have been generated and fitted to constitutive models that are available in the finite element codes ABAQUS and SYSWELD. The choice of hardening model and its associated parameters have been evaluated on the basis of the observed cyclic behaviour in materials testing. Validation of these models has then been carried out by finite element simulations of welded mock-ups which have been measured using neutron diffraction. These include an autogenous weld beam and groove weld specimens containing up to eight weld passes. The rationale for using these simple specimens has been to:• Validate the capability of the model to predict the correct phase transformation behaviour and resulting stresses.• Account for the different behaviour of the parent and filler material.• To develop the capability for representing the material cyclic behaviour.On the basis of these simulations recommendations have been made on the material models (and their parameters) that may be used for the finite element simulation of the welding process.Copyright

A Comparison of Residual Stresses in Single-Pass and Multipass SA508 Steel Welds

The safe operation of a power plant is contingent on the integrity of its welded joints. In turn, the performance of these welded joints is strongly influenced by the residual stresses that remain after fabrication and, where applicable, post-weld heat treatment operations. It is thus critical that careful consideration is given to the influence of weld residual stresses on plant integrity at the design stage, as well as during subsequent service. Since it is generally difficult to measure weld residual stresses in thick-walled components, numerical models are employed to make predictions. Such models, however, will be limited by our understanding of material behaviour, particularly in the case of multipass welds, which involve several complex thermo-mechanical cycles. Here, we report on neutron diffraction measurements, which enable the residual stresses for a single weld bead deposited on to a 20 mm thick SA508 steel plate to be compared with those in an 8-pass groove weld. The same low-carbon (SD3) filler metal was used in the manufacture of both types of specimen. Interestingly, the residual stress distributions for each type of specimen were found to be very similar. This suggests that cyclic hardening effects did not play a decisive role in determining the final residual stress distribution within the multipass weld.Copyright

Annealing models in welding simulation: Conservative and non-conservative residual stress distributions

The influence of residual stress on degradation mechanisms must be taken into account when performing structural integrity assessments. Conservative hypothetical distributions are often assumed, in which the maximum stress is equal to yield. Finite element simulation is now being used in an attempt to derive more realistic residual stress profiles for incorporation into assessment procedures. It is now possible to include self-annealing effects in finite element simulations of the welding process, whereby stresses in existing material in the immediate vicinity of a weld pass are relaxed locally on a very short time scale. Annealing models used in practice vary in sophistication from a simple erasure of strain hardening history at a particular temperature, through certain non-physical assumptions about temperature dependence, to phenomenological models based on the assumed kinetics of underlying microstructural processes. Even more sophisticated models based on thermodynamical principles have been proposed. The original incentive for developing such models was the hope that peak residual stress predictions within the weld and heat affected zone would be reduced, removing the need for over-conservative assumptions when performing structural integrity assessments. Preliminary 2-D results using a model developed by one of the authors, however, suggest that peak stresses predicted with annealing may actually be higher, the hypothesis being that it is compressive stresses that are relaxed during the welding cycle, and that the final tensile residual stresses are increased via the Bauschinger effect. This paper considers results from a further 3-D welding simulation and looks a little more closely at different annealing models. A number of different modelling approaches to annealing are described, together with the basic modelling parameters used to simulate a weld that has been the subject of various European round-robins, both theoretical and experimental. A brief overview of how a kinetics based model can be implemented in a finite element code is also presented. Predicted residual stress distributions are compared with each other and with neutron diffraction measurements and conclusions drawn.Copyright