Residual stress measurement in steel plates and welds using critically refracted longitudinal (LCR) waves

The application of acoustoelasticity using critically refracted longitudinal (L_CR) waves is described for measuring residual stress in welded steel plates. Residual stresses are self-equilibrating and may exist in a material that has been deformed in a nonhomogeneous manner. When unknown residual stress is present in a structure, the true stress may become significantly greater than the working stress. In a corrosive environment, highly stressed areas that have not been properly stress relieved are prone to stress corrosion cracking. Areas near welds are particularly susceptible to stress corrosion cracking.Two welded plates were investigated for the present work: one hot-rolled and the other cold-rolled. Residual stresses are usually greatest after welding. Further, longitudinal stress (i.e., stress parallel to the weld bead) is typically greater than the component transverse to the weld bead. Since the acoustoelastic behavior of the L_CR wave is largest when propagated parallel to a uniaxial stress, the L_CR wave traveling parallel to the weld bead was used to investigate the stress changes after stress relieving of the welded plates. Both 1 MHz and 2.25 MHz probe frequencies were used in this study. The stress changes in the welds and in the cold-rolled plate were clearly indicated by the L_CR data.Two verification methods were used: hole drilling (HD) and neutron diffraction (ND). The stress relief was verified by the hole-drilling technique. While the HD technique showed about the same stress magnitude as found by the L_CR results, the orientation was reversed. The stress orientation was probably caused by the grinding process used to flatten the weld bead. Texture was also investigated using a neutron diffraction (ND) technique on the (001)[110] texture. The through-the-thickness technique yields an average of the orientation distribution of the (110) planes. At locations in the parent metal and in the weld, the distribution was found to be very similar, indicating uniform texture throughout the weld and parent metal zones.     

Effect of residual stress around cold worked holes on fracture under superimposed mechanical load

Cold working is one method used to enhance the fatigue life of holes in aerospace structures. The method introduces a compressive stress field in the material around the hole and this reduces the tendency for fatigue cracks to initiate and grow under superimposed cyclic mechanical load. To include the benefit of cold working in design the stress intensity factors must be evaluated for cracks growing from the hole edge. Two-dimensional (2D) finite element analyses have been carried out to quantify the residual stresses surrounding the cold worked hole. These residual stresses have been used in a finite element calculation of the effective stress intensity factor for cracks emanating from the hole edge normal to the loading direction. The results of the 2D analysis have been compared with those derived using a weight function method. The weight function results have been shown always to underestimate the stress intensity factor. A three-dimensional (3D) FEA has been carried out using the same technique for stress intensity factor evaluation to investigate the effect of through thickness variation of residual stress. Stress intensity factors calculated with the 3D analysis are generally higher than those calculated using the 2D analysis.     

Residual Stress Measurement in Steel Plates and Welds Using Critically Refracted Longitudinal (LCR) Waves

Abstract

The application of acoustoelasticity using critically refracted longitudinal (L_CR) waves is described for measuring residual stress in welded steel plates. Residual stresses are self-equilibrating and may exist in a material that has been deformed in a nonhomogeneous manner. When unknown residual stress is present in a structure, the true stress may become significantly greater than the working stress. In a corrosive environment, highly stressed areas that have not been properly stress relieved are prone to stress corrosion cracking. Areas near welds are particularly susceptible to stress corrosion cracking.

Two welded plates were investigated for the present work: one hot-rolled and the other cold-rolled. Residual stresses are usually greatest after welding. Further, longitudinal stress (i.e., stress parallel to the weld bead) is typically greater than the component transverse to the weld bead. Since the acoustoelastic behavior of the L_CR wave is largest when propagated parallel to a uniaxial stress, the L_CR wave traveling parallel to the weld bead was used to investigate the stress changes after stress relieving of the welded plates. Both 1 MHz and 2.25 MHz probe frequencies were used in this study. The stress changes in the welds and in the cold-rolled plate were clearly indicated by the L_CR data.

Two verification methods were used: hole drilling (HD) and neutron diffraction (ND). The stress relief was verified by the hole-drilling technique. While the HD technique showed about the same stress magnitude as found by the L_CR results, the orientation was reversed. The stress orientation was probably caused by the grinding process used to flatten the weld bead. Texture was also investigated using a neutron diffraction (ND) technique on the (001)[110] texture. The through-the-thickness technique yields an average of the orientation distribution of the (110) planes. At locations in the parent metal and in the weld, the distribution was found to be very similar, indicating uniform texture throughout the weld and parent metal zones.     

Fatigue properties of arc-welded lap joints with weld start and end points

Fatigue properties of arc‐welded lap joints with weld start and end points were investigated through experiments with 2.3‐mm and 3.2‐mm thick 440 MPa‐class steel sheets. Macroscopic fatigue crack‐initiation sites depended on the length of the weld bead to the specimen width. In joints with shorter weld beads, cracks mainly initiated at the toe of the weld start points, while joints with longer beads had initial cracks at the toe of the bead centre. Crack‐propagation analyses, taking stress distribution around the weld toe and residual stress into account, suggested that residual stress distribution could move crack‐initiation sites from the weld start point to the bead centre, although the applied stress at the toe of the weld start point remains the highest.     

On the analysis of the causes of cracking in a wind tower

This paper presents an analysis of the cracking causes in a wind tower of a wind farm. The cracks were detected in several towers of the farm, in the welded joint between the lower ring of the towers and the flange connecting the towers to their corresponding foundation. In the extreme case, here analysed, the crack was a through thickness crack.In order to clarify the cracking causes, the component was inspected in situ, non-destructive tests were performed on the base material, the weld bead and the Heat Affected Zone (HAZ), and a Finite Elements (FE) simulation was carried out to determine the stress state in the welded joints and to develop the corresponding fatigue analysis following the Fatigue Module (Chapter 7) of the FITNET FFS Procedure.The analysis has demonstrated that the main cause of the cracking process is an inadequate design of the joint, with high stress concentrations and an insufficient resistant section on the flange.Finally, two tentative solutions (grinding and grinding plus soft transition) have been analysed but none of them have provided satisfactory results.     

Simplified FE welding simulation of fillet welds – 3D effects on the formation residual stresses

In this study two- and three-dimensional finite element welding simulations have been carried out. The welded component studied is a T-type fillet weld which is frequently used in the heavy vehicle machine industry with plate thicknesses of eight and 20 mm, respectively. The software’s used for the welding simulations is MSC.Marc and ANSYS. The objective is to study the formation of the residual stresses due to 3D effect of the welding process. Moreover, welding simulations using solid models and contact models in the un-fused weld roots were carried out in order to investigate the possible effect with respect to the residual stresses. Residual stress measurements were carried out using X-ray diffraction technique on the manufactured T-welded structure. The 2D residual stress predictions shows good agreement with measurements, hence the 2D welding simulation procedure is suitable for residual stress predictions for incorporation in further fatigue crack growth analysis from weld defects emanating from the weld toe and the un-fused root.     

船体外板装配焊接裂纹分析

某船体尾部外板装配焊接后出现一条贯穿于焊缝和母材的裂纹,通过对船板原材料进行理化性能检验和裂纹断面进行宏、微观检查,对船体外板裂纹产生的原因进行了分析。结果表明:船体外板开裂主要是由于焊接顺序及焊接措施不当,导致船板强制装配拘束应力大,焊缝中存在较多焊接缺陷,从而使得裂纹在两条焊缝交接处的高应力区萌生并延伸扩展至母材区。     

Simulation and Validation of Welding Residual Stresses Based on Non‐Linear Mixed Hardening Model

Abstract: This study contributes to Phase 2 of the Task Group 1 round robin in the NeT European Network. To obtain better prediction results, in the thermal analysis, two significant changes are used. The welding efficiency, η, is fixed at 75%, and the weld bead fusion boundary profiles are based upon macrographs taken from welded specimens, which have been destructively examined. In the subsequent mechanical simulation, a non‐linear kinematic or mixed isotropic–kinematic hardening model should be employed, and a progressive annealing scheme or explicit consideration of visco‐plastic or creep effects should be implemented to handle high‐temperature inelastic strains and reduce stress discontinuities. In this study, an uncoupled 3D thermal and mechanical analysis was carried out using the software code SYSWELD. In the thermal simulation, a two‐offset‐double‐ellipsoid heat source model was developed, and the parameters were fitted using the heat source fitting tool. Power intensity was applied to simulate 1‐s dwelling time at the weld start end. Offset distances between two double ellipsoids were adjusted to obtain the weld bead transverse fusion boundary profiles at different positions. Predicted temperatures were compared with the measured data by thermocouples on the test pieces. In the mechanical analysis, a new material constitutive model, non‐linear mixed hardening model, was developed. Tension–compression cyclic tests were simulated at different temperatures using three different material hardening models (isotropic hardening model, kinematic hardening model and non‐linear mixed isotropic–kinematic hardening model), and the predicted cyclic stress–strain curves were compared with the measured data. Effects of three different hardening models on the welding residual stresses were studied. Compared with the measured data, the optimum material hardening model was confirmed.     

Through thickness residual stresses in large rolls and sleeves for metal working industry

Abstract

There is little experimental knowledge about the initial state of through thickness residual stresses in rolls and sleeves for the steel rolling industry. This is surprising bearing in mind the impact that residual stress has on the performance of the roll and sleeve materials in the highly aggressive loading environments of the metal working industry. Previous work has been confined to measurement of very near surface residual stresses and numerical predictions of residual stress distributions. In the present paper through thickness residual stress measurements were carried out using a deep hole drilling technique on a series of rolls and sleeves representative of those used in the rolling industry. Different features of the manufacturing processes used in their production are shown to influence the magnitude and distribution of the residual stresses. It is also shown that the measurements can be used, together with a finite element analysis, to determine the volumetric distribution of the residual stresses.     

An improved back computation procedure for the parting-out step of a destructive method for measuring residual stresses in pipes

One method for measuring through the thickness residual stress distributions in welded pipes involves a destructive, three step laboratory procedure consisting of parting out, splitting, and layer removal operations. In the parting out step, a coupon of material is removed from the wall of the pipe while strain gauges on the inner and outer surfaces are monitored. In back computing the through the thickness residual stresses from the parting-out strain gauge data, it is generally assumed that stress changes through the thickness of the coupon vary as a straight line. If the circumferential dimension of the coupon is large compared to the thickness, this assumption is valid. However, in the case of a pipe, the circumferential dimension is usually limited to reduce curvature effects during the subsequent splitting and layer removal steps. If this dimension is too small, the assumption of straight line stress changes through the thickness can lead to serious inaccuracies. In this paper, the validity of the straight line assumption is examined over a range of conditions. To handle cases where the straight line assumption is not valid, a modified back computation procedure for the analysis of parting-out strain gauge data is developed and presented with numerical results.     

Failure Investigation of Boiler Water Wall Tubes of a Thermal Power Station

Failure investigation was carried out on boiler water wall tubes of a thermal power plant through visual inspection, chemical analysis, and metallurgical analysis. Failure was in the form of thin/micro cracks along the length of the tubes which were located at the girth welding joint of tubes. Experimental results revealed that the cracking was from inward to outward of the tube thickness. Discontinuities/cavities were observed in the welded region which might have occurred due to lack of fusion of base metal and the weld metal. Cracks were initiated from the sharp corner/crack tip of the cavities/discontinuities present at the welded region under the action of hoop/thermal stress existed during the operation. Nature of the crack propagation indicates the case of typical hydrogen-induced cracking. Moreover, the presence of the cavities/discontinuities reduced the cross-sectional area of tubes resulting increased stress intensity. Increased stress beyond the flow stress of the material assisted by hydrogen-induced effect resulted the cracking of the tubes. In order to mitigate the problem, proper welding of tubes joints should be carried out followed by proper inspection after weld. Secondly, hydrogen dissolution during welding should be prevented and treatment for its removal after welding should be carried out.     

On crack propagation in the welded polyolefin pipes with and without the presence of weld beads

The main aim of the paper is to study the influence of both material inhomogeneity and weld bead geometry on crack propagation in welded polyolefin pipes. Axially and circumferentially oriented cracks are studied and the stress intensity factors are compared considering different positions of the cracks. Two cases of the welded pipe system are compared, one considering the optimal weld bead geometry and the other one considering the geometry after removing the weld bead once the welding process is finished. In both cases the inhomogeneous distribution of material properties inside the welded region is considered. The results show that the weld might have a negative effect on the lifetime especially when the weld bead is removed. Though the weld bead increases the stress concentration near the notches due to which a circumferential crack may appear the resulting lifetime is still comparable to that of an axial crack propagating in a homogeneous pipe.     

Extension to the blind hole drilling technique for residual stress determination with airabrasive hole forming

For an austenitic clad ferritic steel plate it was necessary to determine the residual stress state between the austenitic surface and a region below the austenitic/ferritic interface.
The thickness of claddings was 6 mm and to determine the stress variation between the surface and 8 mm an incremental analysis is required. Any material removal en masse in conjunction with small hole residual stress measurements would have resulted in a stress redistribution and hence a distortion in the stress field being measured. In conjunction with strain change measurements on the other face the sensitivity would not have been sufficient. These two options assume that a thin layer of material over relatively large areas can be removed by a non-stress inducing technique.
A blind hole residual stress measurement using a 13 mm diameter hole 13 mm deep will facilitate accurate incremental analysis to a depth of 8 mm.
The end mill method of hole forming cannot be used with stainless steels because of the random damage effect which is induced in the readings. The airabrasive method which induces no damage effect was used.
The standard airabrasive method of hole forming produces a 2 mm diameter and a 2 mm deep hole.
The modification of the standard airabrasive system is discussed which enables 13 mm diameter and 13 mm deep holes to be formed.
A deviation from the standard hole/gauge layout geometry has been investigated due to non-availability of suitable standard rosettes, and a significant increase in sensitivity has been obtained.
The incremental stress results for the austenitic layer and the austenitic/ferritic interface are discussed using the results from 1.6 mm, 3.2 mm and 13 mm diameter/deep holes.     

Modellierungskonzept für die Versagensbewertung von Laserschweißverbindungen

Failure assessment of laser weldments based on numerical modelling Classical fracture mechanics based assessments are no more sufficient to provide realistic predictions of the deformation and failure behaviour of welded structures. This situation can be improved by numerical modelling based on damage mechanics. A new concept will be provided, which is based on a cohesive model for crack growth simulation. The determination of the relevant material parameters is also considered where testing is combined with numerical simulation. For a laser weld joint, the gradient of the material properties has to be properly characterized. With miniature sized specimens, the material properties can be discretized by homogeneous layers. A new method, based on the digital image technique, has been introduced to determine the stress‐strain curves also in the large strain region due to necking. Test results on small bend bars containing a thin laser weld and a crack like defect in the centre show different crack path developments resulting from a competitive fracture situation. Mainly shear fracture mode occurs, in some cases also a pure normal fracture mode or a combination of both were observed. The concept presented is able to consider the crack development, if all occuring fracture modes are included in the analysis. However, a complete simulation of an extensive crack extension through a heterogeneous structure has not yet been verified.     

Application guidelines for the parting out step in a through thickness residual stress measurement procedure

The first step in an often used destructive procedure for determining through thickness residual stresses in pipes and plates is a parting out step in which a coupon is removed from the parent material while strain gauges monitor the coupon's deformations. The changes in stress that occur at the gauged surfaces when the coupon is parted out, together with information about the stresses in the parted out coupon are used in a back computation method to estimate the stress distributions in the coupon before it was parted out. Either of two back computation methods can be used. One method assumes that the through thickness stress distribution changes that occur when the coupon is parted out lie on a straight line. This method is computationally quick and easy to use, but can be inaccurate if the lateral dimensions of the coupon are too small. The other method uses influence coefficients and greatly extends the range of coupon dimensions for which accurate results can be obtained; but, it is more elaborate than the straight line model and involves two rather time consuming steps. This paper presents guidelines for determining when the more elaborate influence coefficient method must be used in place of the straight line model in terms of the error that the simpler model would produce in the estimated residual stress distribution. This paper also simplifies the application of the influence coefficient method by tabulating the results of the two time consuming steps over the range of coupon dimensions for which this method is typically required. Example problems using the two back computation methods are presented.     

Microstructure and texture evolution in continuously cooled AA 3004 hot rolled sheet

Abstract

A quantitative study of variations in microstructure and texture evolution in the through thickness direction of industrially hot rolled AA 3004 aluminium alloy has been carried out. The microstructural features of the specimens were examined with the aid of the electron channelling contrast technique in conjunction with an image analysis system. The number of recrystallised grains and the size distributions of coarse and fine intermetallics were measured to evaluate the variation between surface and centre. Significant differences in the number of recrystallised grains and the average size of coarse intermetallics in the through thickness direction of the hotband were observed. After isothermal annealing of the hotband for various times the fine intermetallic area fraction increased and was higher at the centre than at the surface. Quantitative texture analysis was carried out on the specimens and various texture components estimated to characterise the through thickness texture evolution. The proportion of cube texture component was higher near the surface than at the centre and this difference increased after isothermal annealing. Since negligible change occurred in the cube content of the centre specimen, growth of cube grains was deduced to have taken place primarily near the surface region. These observations illustrate that mechanisms of cube texture formation, heterogeneous nucleation of precipitates causing retarded recrystallisation, and Zener drag are evidently applicable even to complex commercial alloys.

MST/1849     

Application of different concepts for fatigue design of welded joints in rotating components in mechanical engineering

Several concepts are used for the fatigue design of welded joints. In this paper investigations are presented, which were carried out in a joint project between five research institutes [1]. The aim is to investigate currently applied fatigue concepts with respect to their limitations, compatibility and reliability, in order to improve the accuracy of lifetime estimation and to simplify the choice of the optimum fatigue concept. Here, the results of the investigation of welded joints in rotating universal joint shafts are shown [2]. In the critical weld, a structural steel and a quenched and tempered steel are joined. In practice, stresses result from rotating bending, torsion and also residual stresses are sometimes present. Several welding techniques, MAG, TIG and laser welding, and two seam geometries were investigated with regard to their influence on fatigue strength. Experiments were conducted with welded tube specimens representative of the actual component application and with derived flat specimens as detail specimens. The welded sheet thickness was 5.5 mm. Fatigue strength was investigated from 10⁴ to 10⁷ numbers of cycles. In numerical analyses, nominal stress, structural hot spot stress and elastic notch stress with reference radii of 0.3 mm and 0.05 mm were calculated. In the comparison of the concepts, their respective advantages and disadvantages have been demonstrated. A comparison of the results with the IIW recommendation for fatigue design of welded joints and components [3] has been carried out and improvements have been suggested.     

Acoustic Horn Design and Effects of Process Parameters on Properties of Dissimilar Ultrasonic Welding Aluminum to Brass

Ultrasonic welding technology is a new and emerging concept, which can be applied to many industrial applications. The vital part of this technology is the horn, which acts as a tool, upon which the pressure and high-frequency vibration is applied to create a solid-state weld. In this study, a special type of horn is designed and its length is determined analytically. Dimensions obtained by finite-element method (FEM) are employed for the horn used in the present experiments to weld brass with aluminum sheet of thickness 0.1 mm. The dynamic analysis is also performed to find out the stress and amplitude distribution in the horn under loading conditions. Welding has been carried out using various parameter combinations in order to improvise the weld strength. Maximum weld strength of 4.05 MPa is obtained under the optimum welding conditions of 0.21 s weld time, 0.26 MPa weld pressure, and 60 µm vibration amplitude. The microhardness test also has been done on the optimized results to show the plastic flow at the weld zone. It confirms that the hardness of both materials up to 20 µm distance around the weld zone is increased with respect to the parent materials.     

Accurate measurement of residual stress on any steel using the centre hole method

The centre hole method has been shown to be a simple and reasonably accurate method of measuring residual stress. With a 1.6 mm diameter hole, the method can be applied on operational plant and the equipment required is sufficiently portable for use on site. Its application has, however, been limited to use on low-strength steels, due to problems associated with forming the hole.
This paper describes the development of an air-abrasive technique for forming the hole, which allows the centre hole method to be used on any steel with a measurement accuracy of better than±8%.     

Influence of Co,Cu and W on microstructure of 9%Cr steel weld metals

Abstract

There is an increasing interest in the use of the 9-12%Cr grades of ferritic steel containing Mo, V, Nb, N and W with further possible Co additions, for high temperature applications. There is a parallel need for weld metal development to match these materials. A series of weld metals, based on 9CrMoV but with Cu and Co additions and W replacement of the Mo, have been examined and the effects on delta ferrite distributions and contents identified. The results show that the compositional differences have a direct effect on the delta ferrite content. The delta ferrite form and content are different for the last weld bead, or the as deposited structure, and the centre of the weld pad, where the structure has been reheated by subsequent weld beads. In general, a blocky form of ferrite is associated with the as deposited weld bead and an intergranular form for the reheated weld metal positions but these are not exclusive. A simplified mechanism is proposed for explaining the ferrite distribution. Finally, the measured ferrite contents are considered with respect to the existing empirical relationships from the literature, based on equivalent Cr and Ni levels, for estimation of the ferrite levels.