Temperature distribution and residual stresses due to multipass welding in type 304 stainless steel and low carbon steel weld pads

Welding is a reliable and efficient metal-joining process widely used in industry. Due to the intense concentration of heat in the heat source of welding, the regions near the weld line undergo severe thermal cycles, thereby generating inhomogeneous plastic deformation and residual stresses in the weldment. Plates of different thickness are used in industry and these plates are normally joined by multipass welding. In a multipass welding operation, the residual stress pattern developed in the material changes with each weld pass. In the present experimental work, thermal cycles and transverse residual stresses due to each pass of welding have been measured in the weld pads of AISI type 304 stainless steel and low carbon steel with 6, 8 and 12 mm thickness. X-ray diffraction method was used for residual stress measurements. The welding process used was the Manual Metal Arc Welding (MMAW) process. In this paper, the peak temperatures attained at different points during deposition of weld beads in stainless steel and low carbon steel weld pads are compared. The residual stress patterns developed, the change in the peak tensile stress with the deposition of weld beads, and the relation between the peak temperatures and the residual stresses in the weld pads are discussed.     

Predictions and measurements of residual stress in repair welds in plates

This paper presents the work, from the European Union FP-5 project ELIXIR, on a series of rectangular repair welds in P275 and S690 steels to validate the numerical modelling techniques used in the determination of the residual stresses generated during the repair process. The plates were 1,000 mm by 800 mm with thicknesses of 50 and 100 mm. The repair welds were 50%, 75% and 100% through the plate thickness. The repair welds were modelled using the finite element method to make predictions of the as-welded residual stress distributions. These predictions were compared with surface-strain measurements made on the parent plates during welding and found to be in good agreement. Through-thickness residual stress measurements were obtained from the test plates through, and local to, the weld repairs using the deep hole drilling technique. Comparisons between the measurements and the finite element predictions generally showed good agreement, thus providing confidence in the method.     

Direct measurement of the residual stresses near a ‘boat-shaped’ repair in a 20 mm thick stainless steel tube butt weld

Neutron diffraction measurements have been performed to quantify directly the three-dimensional residual stress field in a stainless steel pipe girth weld containing a part-circumference weld repair. Adjacent to the repair, the measured through-wall axial stress profile was found to be membrane in character and of tensile magnitude equal to about 60% of the base metal 1% proof stress. The measurements show that the repair procedure substantially increased axial and hydrostatic components of residual stress in the neighbouring heat-affected zone.     

Through thickness measurement of residual stresses in a stainless steel cylinder containing shallow and deep weld repairs

A programme of residual stress measurements was undertaken using the deep hole method. The measurements obtained the through-thickness residual stress profiles before and after introducing deep and shallow part-circumferential weld repairs into a 37 mm thick stainless steel cylinder. Measured through-thickness distributions were obtained at mid-width and in the heat affected zone of both the original weld and repairs. The results show that the membrane and bending components of the in-plane residual stresses are generally increased when weld repairs are introduced.     

NeT bead-on-plate round robin: Comparison of residual stress predictions and measurements

The members of the European network NeT have undertaken parallel round robin activities measuring and simulating the residual stresses generated by laying a single Tungsten Inert Gas (TIG) weld bead on an AISI Type 316L austenitic stainless steel flat plate. This is a strongly three-dimensional configuration with many of the characteristics of a repair weld. The round robin finite element predictions of weld residual stresses are compared with each other in order to identify the effects on the predicted residual stresses of material hardening model, global heat input, mechanical and thermal boundary conditions, and the handling of high temperature inelastic strains. Comparison with the residual stress measurements then leads to the optimum choices for these variables.     

Spatial variation of residual stresses in a welded pipe for high temperature applications

Measurements of residual macro-stresses have been undertaken in a feature multipass circumferential single V butt-weld made from a P91 ferritic steel pipe over different spatial depths: (i) ≤10 μm by X-ray diffraction, (ii) ≤1 mm by incremental centre-hole drilling and (iii) through wall section using deep-hole drilling. The ability to make near-surface X-ray residual stress measurements on as-oxidised surfaces has been demonstrated and the implications for use in the evaluation of overall integrity are discussed. Each of the three measurement techniques provides complementary and consistent measurement of induced residual stresses for weld metal, heat affected zone and parent metal for the as-welded and the post-weld heat-treated conditions over the complete spatial range. The results are discussed with respect to the importance of the weld capping run in introducing near-surface compressive residual stresses, the through-wall profiles of the residual stresses measured at the weld metal position in hoop and axial directions and the presence of existing surface oxide.     

Residual stress measurement of a 316l stainless steel bead-on-plate specimen utilising the contour method

This paper describes the mapping of transverse residual stresses within a single bead-on-plate round robin test specimen. The purpose of these measurements was to quantify the magnitude and shape of the residual stress field arising from a single weld bead laid down on an austenitic stainless steel plate. Measurements were made through the thickness of the specimen using the contour method. The contour method is a new destructive, stress relaxation method allowing the full field residual stress to be measured. Results from these measurements show transverse tensile residual stresses over 150 MPa below the plate surface along the length of the weld bead with peak stresses of up to 210 MPa close to the weld stop position. Finally, as these measurements are insensitive to local microstructure variations within the specimen (i.e. texture or variations in lattice parameter), they are useful in helping to validate diffraction based residual stress measurements made within this round robin measurement program.     

Effect of weld metal properties on fatigue crack growth behaviour of gas tungsten arc welded AISI 409M grade ferritic stainless steel joints

The effect of filler metals such as austenitic stainless steel, ferritic stainless steel and duplex stainless steel on fatigue crack growth behaviour of the gas tungsten arc welded ferritic stainless steel joints was investigated. Rolled plates of 4 mm thickness were used as the base material for preparing single ‘V’ butt welded joints. Centre cracked tensile (CCT) specimens were prepared to evaluate fatigue crack growth behaviour. Servo hydraulic controlled fatigue testing machine was used to evaluate the fatigue crack growth behaviour of the welded joints. From this investigation, it was found that the joints fabricated by duplex stainless steel filler metal showed superior fatigue crack growth resistance compared to the joints fabricated by austenitic and ferritic stainless steel filler metals. Higher yield strength, hardness and relatively higher toughness may be the reasons for superior fatigue performance of the joints fabricated by duplex stainless steel filler metal.     

Characterization of gradients in mechanical properties of SA-533B steel welds using ball indentation

Gradients in mechanical and fracture properties of SA-533B steel welds were studied using ball indentation technique. The local stress–strain behaviors of different microstructural zones of the weld were determined at various temperatures. Gradients in the strength of the base metal, weld metal and the different positions in the heat affected zone were observed to be consistent with the changes in the microstructure. The maximum in yield and the corresponding minimum in indentation energy to fracture occurred at around 1 mm from the fusion line.     

Temperature distribution during multipass welding of plates

Welding is one of the most important material-joining processes widely used in industry. Low carbon steel plates and AISI type 304 stainless steel plates with 6, 8 and 12 mm thicknesses are widely used in the fabrication of pressure vessels and other components. These plates are mostly joined together by multipass welding methods. The temperature distribution that occurs during multipass welding affects the material microstructure, hardness, mechanical properties and the residual stresses that will be present in the welded material. Very limited experimental data regarding temperature distribution during multipass welding of plates is available in the literature. Experimental work was carried out to find out the temperature distribution during multipass welding of the above plates. The temperature distribution curves obtained during the experiments are presented. Average maximum temperature rise during each pass of welding is calculated and plotted against the distance from the weld pad centre line. From these plots, the maximum temperature rise expected in the base plate region during any pass of welding operation can be estimated.     

Residual stresses in austenitic stainless steel primary coolant pipes and welds of pressurized water reactors

Surface and through thickness residual stress measurements were performed on an aged cast austenitic-ferritic stainless steel pipe and on an orbital TIG weld representative of those of primary coolant pipes in pressurized water reactors. An abrasive-jet hole drilling method and a block removal and layering method were used. Surface stresses and through thickness stress profiles are strongly dependent upon heat treatments, machining and welding operations. In the aged cast stainless steel pipe, stresses ranged between −250 and +175 MPa. On and near the orbital TIG weld, the outside surface of the weld was in tension both in the axial and hoop directions, with maximum values reaching 420 MPa in the weld. On the inside surface, the hoop stresses were compressive, reaching −300 MPa. However, the stresses in the axial direction at the root of the weld were tensile within 4 mm depth from the inside surface, locally reaching 280 MPa.     

Measurement of the residual stresses in a stainless steel pipe girth weld containing long and short repairs

A series of residual measurements were made to obtain the through-thickness residual stress profiles in an as-welded and repair welded stainless steel pipe. Long and short length repairs were manufactured after initial measurements in the original girth weld. Measurements were made using neutron diffraction, deep hole and surface hole techniques. The various measurement methods were found to complement each other well. All the measurements revealed a characteristic profile for the through-thickness distribution of the residual stresses in the heat-affected zone. The residual stresses at mid-length of the heat affected zone of the short repair were found to be higher than in the long repair.     

Weld repair of grade 91 steel without post-weld heat treatment

Abstract

A weld repair technique to be used without post-weld heat treatment has been developed for use on grade 91 steel. The approach makes use of standard (non- modified) 9CrMo weld metal. The work has extended an approach used successfully on the low alloy steel ½CrMoV, which utilised a low strength 2CrMoL weld metal, to the more advanced steel grade 91, using an equivalently lower strength weld metal, standard (non-modified) 9CrMo. This has considerably lower creep strength than matching modified 9CrMo weld metal. Two variants of standard 9CrMo weld metal were chosen for investigation: a specially commissioned low carbon 9CrMoL version, with carbon below the normal minimum for this grade of weld metal (0·05 wt-%), and a conventional batch of 9CrMo weld metal, but selected to have carbon in the bottom half of the normal range. Comparison between the 9CrMoL weld metal and the standard 9CrMo weld metal, on the basis of residual stress level and creep and fracture toughness properties, has shown the latter to be the better option. The most likely repair scenario envisaged was to a retrofit grade 91 header on a UK coal fired power station. The goal was to achieve a lifetime for such a repair greater than the 4 year period between major overhauls for a typical power station of this type, corresponding to >20 kh operating hours.     

Residual stress simulation in thin and thick-walled stainless steel pipe welds including pipe diameter effects

In this paper, residual stresses in welded components are discussed and a brief review of weld simulation is presented. The general methodology of the FE analysis methods used for welded sections of steel pipes is explained. FE analyses are performed for two axisymmetric butt welds in stainless steel pipes having a 4-pass or a 36-pass weld in a pipe with a wall thickness of 7.1 or 40.0 mm, respectively. In addition, more FE models with inside radius to wall thickness ratio ranging from 1 to 100 have been analysed to investigate the effect of pipe diameter on residual stresses. Residual axial and hoop stresses are plotted for the considered range of pipe diameters for the two simulated pipe wall thicknesses and the differences are discussed.     

Thermomechanical modelling of weld microstructure and residual stresses in P91 steel pipe

Abstract

A multipass circumferentially butt welded P91 steel pipe, typically used for high temperature applications in power plants, has been numerically analysed to determine residual stresses, induced by the process of welding, as well as microstructural regions in the weld, caused by thermal cycles. The finite element (FE) method has been applied to simulate residual stresses generated in the weld region and heat affected zone (HAZ), which are then validated by published experimental data. The axisymmetric FE simulation incorporates solid state phase transformation by allowing for volumetric changes and associated changes in yield stress and hardening behaviour due to austenitic and martensitic transformations. The thermal cycles during welding cause different microstructural regions to emerge within the weld metal and HAZ. Columnar and equiaxed microstructural zones have been numerically modelled in the weld region of the pipe. The predicted FE microstructural regions have been corroborated by columnar and equiaxed zones that have been mapped out on a cross-sectional macroimage of the weld.     

不锈钢中厚板激光全熔透焊的有限元模拟

运用有限元计算软件ABAQUS对16mm厚不锈钢板的激光全熔透焊的温度场和应力场进行了模拟.采用一体两面的复合焊接热源模型来刻画激光全熔透焊过程中的热输入特征,以柱状体热源代表焊接小孔传热模式,以2个超高斯面热源代表等离子体/金属蒸气云对熔池的辐射传热模式.结果表明:温度场模拟结果得到了与实验结果相一致的"沙漏状"焊缝;钢板内纵向残余应力最大,横向应力次之,板厚方向横向应力最小;纵向拉应力主要分布在焊缝两侧约25mm的区域内,最大值已超过材料的屈服强度;经测算,钢板焊后的角变形量仅为0.35°,这是由于激光焊接能量输入高且集中、可不用开坡口而一次性将钢板焊透.     

Residual stresses in girth butt welded pipes and treatments to modify these

A number of Type 304 stainless steel pipes are used in the primary cooling systems of nuclear plants. Intergranular stress corrosion cracks (IGSCC) were found at some welded joints in these piping systems due to very high tensile residual stress, sensitization of the material due to welding, and corrosive environment, all occurring simultaneously. Investigations have shown that at least one of the above factors must be eliminated to prevent IGSCC.

This report describes experimental results on the temperature variations during pipe welding by conventional techniques and by the heat sink welding (HSW) technique. The mechanism of residual stress generation due to welding is also discussed. The pipe used in these experiments was 4B Sch80 Type 304 stainless steel. It was found that the temperature distribution through the thickness of the pipes was almost uniform for the conventional welding technique, but had a very sharp gradient for HSW. In the pipe axial direction, the temperatures varied sharply for both welding techniques. This implies that the sensitization of metal due to HSW is lighter than that of conventional welding and that the residual stress on the inside surface of the heat sink welded pipe is compressive.

The induction heating stress improvement (IHSI) method has been investigated analytically and experimentally. In the IHSI method, a pipe is heated with an induction coil while cold water is pumped through it. This causes a temperature gradient throughout the pipe wall which generates high thermal stresses. This, in turn, generates compressive stresses on the inner surface of the pipe. This method is designed to eliminate tensile residual stresses near the weld heat affected zone on the inner surface.

Temperature analysis and subsequent thermoelastic-plastic analysis show that tensile weld residual stresses at a joint were changed into compressive stresses on the inner surface of a pipe. It was confirmed experimentally that these stresses suppressed fatigue crack propagation in the heat affected zone (HAZ) of a welded pipe. Therefore, the IHSI method is effective not only in preventing crack initiation but also in suppressing crack propagation.

As for the relaxation of residual stresses, no significant relaxation was measured when external loads were applied at as much as 80% of the yield strength in the experiments.     

Measurement and prediction of residual stress in a bead-on-plate weld benchmark specimen

This paper presents measurements and predictions of the residual stresses generated by laying a single weld bead on a flat, austenitic stainless steel plate. The residual stress field that is created is strongly three-dimensional and is considered representative of that found in a repair weld. Through-thickness measurements are made using the deep hole drilling technique, and near-surface measurements are made using incremental centre hole drilling. Measurements are compared to predictions at the same locations made using finite element analysis incorporating an advanced, non-linear kinematic hardening model. The work was conducted as part of an European round robin exercise, coordinated as part of the NeT network. Overall, there was broad agreement between measurements and predictions, but there were notable differences.     

Residual stress measurement in a repair welded header in the as-welded condition and after advanced post weld treatment

Measurements of the residual stresses in a type 316H stainless steel steam header were made using the deep hole drilling (DHD) technique to characterise the through thickness residual stress distribution in a circumferentially repaired weld in the as-welded condition and after an advanced post weld treatment (APWT) had been applied. The measured residual stress field was compared with finite element (FE) predictions. Overall there was a good correlation between the DHD measurement results and the FE results in the as-welded condition. The results highlighted the generation of high residual stresses due to repair welding. After APWT, there were noticeable differences between the measurements and the FE predictions. The study also demonstrated the feasibility of performing an APWT procedure on components and its significant effect on the redistribution of residual stresses. In particular, it was demonstrated that APWT promoted a finite volume of compressive stresses near the critical area of the header, thus providing increased confidence in the component integrity.