Harry Edward Coules

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.

Contemporary approaches to reducing weld induced residual stress

Abstract Self-equilibrating residual stresses may occur in materials in the absence of external loading due to internal strain inhomogeneity. While favourable distributions of residual stress can bestow an object with the appearance of superior material properties, most welding processes leave behind residual stresses in particularly unfavourable patterns, causing a greater susceptibility to fracture based failure mechanisms and unintended deformation. Currently, heat treatment is the primary means of removing these stresses, but since the formation of residual stress is dependent upon many material and process factors, there are several other viable mechanisms (using thermal, mechanical or phase transformation effects) by which it may be modified. It is only now, using relevant advances in numerical and experimental methods, that these techniques are being fully explored. This article gives a brief introduction to weld induced residual stresses and reviews the current state of the art with regard to their reduction. Emphasis is placed on the recent development of unconventional techniques, and the mechanisms by which they act.

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.

Residual stresses in clad nuclear reactor pressure vessel steels; prediction, measurement and reconstruction

Most residual stress measurement methods are limited in terms of their stress and spatial resolution, number of stress tensor components measured and measurement uncertainty. In contrast, finite element simulations of welding processes provide full field distributions of residual stresses, with results dependent on the quality of the input conditions. Measurements and predictions are often not the same, and the true residual stress state is difficult to determine. In this paper both measurements and predictions of residual stresses, created in clad nuclear reactor pressure vessel steels, are made. The measurements are then used as input to a residual stress mapping technique provided within a finite element analysis. The technique is applied iteratively to converge to a balanced solution which is not necessarily unique. However, the technique aids the identification of locations for additional measurements. This is illustrated in the paper. The outcomes from the additional measurements permit more realistic and reliable estimates of the true residual state to be made. The outcomes are compared with the finite element simulations of the welding process and used to determine whether there is a need for additional input to the simulations.Copyright

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.

Multiscale Measurements of Residual Stress in a Low-Alloy Carbon Steel Weld Clad with IN625 Superalloy

Fatigue fracture is one of the major degradation mechanisms in the low-alloy 4330 carbon steel pumps that are utilized in the hydraulic fracturing process operating under cyclic loading conditions. A weld cladding technology has been developed to improve the ability of these components to resist fatigue crack initiation by cladding them with a secondary material. This process introduces a residual stress profile into the component that can be potentially detrimental for fatigue performance. The cladding technology under examination is a low-alloy 4330 carbon steel substrate weld that is clad with the nickel-chromium–based superalloy IN625 and is investigated herein using several experimental residual stress measurement techniques. Understanding the magnitude and distribution of residual stress in weld clad components is of the utmost importance to accurately assess the performance of the component in service. This study summarizes the results of residual stress measurements that were determined using X-ray diffraction, i.e., hole drilling based on electronic speckle pattern interferometry, deep-hole drilling, and the contour method, to obtain the residual stress distributions from the surface of the weld clad, through the clad layer, and into the substrate material. The results of deep-hole drilling and the contour method show large-scale tensile residual stress in the clad layer and compressive residual stress in the majority of the substrate. However, the X-ray diffraction and hole drilling methods indicate the presence of short-scale compressive residual stress on the surface and near the surface of the clad layer. It was shown that these measurement techniques are complementary in assessing the residual stress profile throughout the entire component.

Assessment of the effect of residual stresses in elastic-plastic fracture of dissimilar welded components

Abstract Residual stresses in welds pose a significant threat to the structural integrity of a component, especially in the presence of defects and are required to be accounted for in assessing component safety. Although the R6 assessment procedure suggests various approximate methods for incorporating these effects in defect assessment, most of them are overly conservative and not very cost-effective. A more reliable approach is to characterise the weld residual stresses around a defect and study how they interact with primary load. The current paper analyses the effects of weld residual stresses on the fracture of a dissimilar weld in the presence of defect. The weld is made between modified 9Cr–1Mo steel and 316LN stainless steel using autogenous electron beam welding. A C(T) specimen was extracted from the centre of the weld and a crack introduced in the fusion zone using electro-discharge machining. The residual stresses around the crack were measured on a grid of measurement points at mid-thickness of the C(T) specimen using neutron diffraction on the strain diffractometer SALSA at ILL, Grenoble. The measured residual stresses around the crack-tip were incorporated into a finite element model and the interaction of these with applied load was predicted under fracture.

The effects of residual stress on elastic-plastic fracture: two diffraction studies

Brittle fracture in structural materials is strongly influenced by the presence of residual stresses. During more ductile fractures the influence of residual stress is reduced by plastic deformation prior to the initiation of fracture. In structural integrity assessments, it is therefore important to account for the interaction between applied loads and residual stresses. This interaction was studied in two experiments. In the first, residual stresses in aluminium alloy 7075-T6 three-point bend specimens were measured prior to loading using neutron diffraction. In the second, the stress field in compact tension specimens of ferritic pressure vessel steel was mapped using energydispersive synchrotron X-ray diffraction as the specimens were loaded. In both cases, finite element modelling of the elastic-plastic fracture was performed using residual stresses either reconstructed from, or validated using, experimental results. This method of analysis allowed us to compare the effect of residual stress on two different fracture mechanisms. The aluminum specimens fractured in brittle manner, and analysis of experimental load-displacement results showed that residual stresses were superimposed almost linearly with externally-applied stresses at the point of fracture initiation. In the steel specimens, fracture initiated by ductile tearing and it was shown using finite element analysis that residual stress had only a very small effect on the crack-driving force at higher levels of applied load. In fact, at the point of tearing initiation prior strain-hardening caused by the indentation process used to introduce residual stresses had a greater effect on the crack-driving force than the residual stress state itself. Introduction Residual stresses can strongly affect the initiation of fracture in brittle materials [1]. However, in more ductile fractures the effect is diminished by partial relaxation of the residual stress, which is enabled by plasticity prior to fracture initiation. As a consequence, some structural integrity assessment procedures such as the R6 procedure [2] and the British Standard BS7910:2013 [3] include methods to account for this effect. This is achieved using approximate methods. For example, in the Failure Assessment Diagram (FAD) approach used in R6, a plasticity correction factor V is introduced to represent the reduction in the effect of “secondary” stresses. Secondary stresses are defined as those stresses that can influence fracture initiation but do not contribute to plastic collapse when this is the dominant failure mechanism. Typically, residual and thermal stresses are classed as secondary. By contrast “primary” loads are those which affect both failure mechanisms and include, for example, pressurisation and gravity loads. Although it is relatively simple to apply, the plasticity correction method can give overly-conservative results for some geometries. In certain situations, disproportionately severe plastic deformation can occur close to a defect as a result of high residual stresses remote from it – a situation referred to as elastic follow-up. In cases where there is significant elastic follow-up, the V factor approach has the potential to give a non-conservative assessment result unless the elastic follow-up effect is considered [4]. Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 353-358 doi: http://dx.doi.org/10.21741/9781945291173-60 354 In the past, the interaction between primary and secondary stresses during fracture has been widely studied using finite element analysis, often for the purpose of developing improved techniques for structural integrity assessment. However, there has been little experimental work on this topic due to the difficulty involved in measuring the complex stress fields which occur close to a crack tip. We performed two experiments to observe these stress fields and, using this data in conjunction with finite element analysis, investigated the effect of residual stress on the mechanism of elastic-plastic fracture [5], [6]. Method Brittle fracture. Single Edge Notched Bend (SEN(B)) specimens of 7075-T6 aluminium alloy with dimensions 120 x 30 x 15 mm were prepared as shown on Fig 1. Half of the 16 SEN(B) specimens were compressed in the out-of-plane direction using a pair of circular indenters, before the “crack” (actually an Electrical Discharge Machined notch) was cut into them. The other half were notched in the as-received condition. The residual elastic strain field on the mid-thickness plane of the specimen in the region of the notch (see Fig 1) was measured using neutron diffraction. The measurements were performed using the SALSA diffractometer at the Institut Laue-Langevin (Grenoble, France). Measurements were taken at 2.5 mm intervals over most of the measurement plane and at 1 mm intervals in a small region surrounding the crack tip. Finally, both the indented and non-indented sets of specimens were loaded to failure in three-point bending and their apparent plane-strain fracture toughness was calculated. Elastic-plastic finite element analysis was performed to estimate the elastic-plastic strain energy release rate at the point of fracture for both sets of specimens. This analysis used a residual stress field for the indented specimens that was reconstructed from the measured neutron diffraction data using an iterative technique [7]. Loading of the residually-stressed specimen was then simulated and the energy release rate was determined by calculating the J-integral as a function of throughthickness position on the crack front. To account for the effect of residual stress, the modified Jintegral formulation due to Lei was used [8], [9]. Figure 1: Indented Single Edge Notched (SEN(B)) and Compact Tension (C(T)) fracture specimens used in the experiments. Ductile tearing. This experiment used modified Compact Tension (C(T)) specimens of a ferritic pressure vessel steel (Fig 1). As in the previous experiment, a residual stress field was introduced in some of the specimens by local compression ahead of the notch tip. However, the steel specimens exhibited a much more ductile fracture process and were loaded until the initiation of ductile tearing. Loading of the specimens was carried out on the I12 beamline at Diamond Light Source (Oxfordshire, UK). Energy-Dispersive X-ray Diffraction (EDXD) was used to measure the stress field surrounding the crack at incremental loading steps at a spatial resolution of 1 mm over most of the specimen, and 0.25 mm in a region surrounding the crack tip. X-ray radiography was used to detect the initiation of ductile tearing at the notch tip (Fig 2c). Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 353-358 doi: http://dx.doi.org/10.21741/9781945291173-60