Prediction of residual stresses in welds of similar and dissimilar steel weldments

The fabrication of structural member using dissimilar steels renders steel structures lighter and more economical. However, it always involves welding process and produces different residual stresses in welds as compared with welding of similar steels. This paper presents the characteristics of residual stresses in welds of similar and dissimilar steel weldments by carrying out three-dimensional (3-D) thermal elastic-plastic finite element (FE) analysis. The materials used in this investigation were SM400, SM490, SM520 and SM570, widely used structural steels in welded structure. Results show that the maximum longitudinal residual stresses in welds of the similar steel weldments increase with increasing yield stress of the steel welded (SM400 < SM490 < SM520 < SM570). In case of the dissimilar steel weldments, the difference between the longitudinal residual stresses in welds increases with increasing yield stress of the steel welded together with SM400 (SM490 < SM520 < SM570).     

Characterisation of dissimilar P91 and P92 steel welds joint

In the present study, dissimilar weld joint was prepared using the P91 and P92 steel plate of 8-mm thickness, using the multi-pass gas tungsten arc (GTA) welding with filler (weld 1) and autogenous tungsten inert gas welding (A-TIG) process (weld 2). Evolution of δ-ferrite patches was studied in weld zone and heat affected zone (HAZ) for both weld 1 and weld 2. Effect of varying post weld heat treatment (PWHT) duration was also studied on δ-ferrite patches and mechanical properties of the dissimilar weld joint. PWHT was carried out at 760°C. For weld 2, weld zone showed poor impact toughness and higher peak hardness as compared to weld 1. After the PWHT, a considerable reduction in hardness was obtained for both weld 1 and weld 2,while impact toughness of weld zone showed a continuous increment with PWHT duration. For weldments characterisation, optical microscope, scanning electron microscope (SEM) and microhardness tester were utilised.     

Weldability in dissimilar welds between Type 310 austenitic stainless steel and Alloy 657

Microstructural evolution and solidification cracking susceptibility of dissimilar metal welds between Type 310 austenitic stainless steel and Inconel 657, a nickel-based alloy, were studied using a combination of electron microscopy analysis and Varestraint testing techniques. In addition, the effect of filler metal chemistry on the fusion zone composition, microstructure, and resultant weldability was investigated. The good cracking resistance of welds prepared with Inconel A was due to a small amount of secondary phase (NbC) and narrow solidification temperature range. The relatively poor cracking resistance of welds prepared with Inconel 82 and Type 310 stainless steel (310 SS) was a result of a wide solidification temperature range and an increase in the amount of secondary phases. Consequently, it is concluded that for the joint between Inconel 657 and 310 SS, filler material of Inconel A offers the best weldability.     

Microstructure and residual stress distribution of similar and dissimilar electron beam welds – Maraging steel to medium alloy medium carbon steel

The influence of parent metal heat treatment condition on the residual stress distribution in dissimilar metal welds of maraging steel to quenched and tempered medium alloy medium carbon steel has been investigated. It has been observed that the residual stress distribution would be more compressive if the maraging steel is in soft condition. This is attributed to stress absorbing nature of highly yielding soft maraging steel.     

Microstructure characterization and properties of carbon steel to stainless steel dissimilar metal joint made by friction welding

Dissimilar metals of 1045 carbon steel and 304 stainless steel are joined successfully by friction welding. The microstructure variation and mechanical properties are studied in detail. The weld interface can be clearly identified in central zone, while the two metals interlock with each other by the mechanical mixing in peripheral zone. On carbon steel side, a thin proeutectoid ferrite layer forms along weld interface. On stainless steel side, austenite grains are refined to submicron scale. The δ-ferrite existing in stainless steel decreases from base metal to weld interface and disappears near the weld interface. Severe plastic deformation plays a predominant role in rapid dissolution of δ-ferrite compared with the high temperature. Carbide layer consisting of CrC and Cr₂₃C₆ forms at weld interface because of element diffusion. Metastable phase CrC is retained at room temperature due to the highly non-equilibrium process and high cooling rate in friction welding. The fracture appearance shows dimple fracture mode in central zone and quasi-cleavage fracture mode in peripheral zone. Further analysis indicates that welding parameters govern tensile properties of the joint through influencing the thickness of carbide layer at weld interface and heterogeneous microstructure in thermo-mechanically affected zone on carbon steel side.     

Microstructural evolutions in dissimilar welds between AISI 310 austenitic stainless steel and Inconel 657

This investigation has been performed to characterize dissimilar metal welds between type 310 austenitic stainless steel (SS) and Inconel 657 superalloy. The welds were produced using four types of filler materials: Inconel 82, Inconel A, Inconel 617, and type 310 SS. The weldments were characterized in detail using optical metallography and scanning electron microscopy. It can be concluded that Inconel A weld metal does not promote severe hot cracking. Continuous NbC precipitates in the Inconel 82 weld metal can sensitize the weld metal to solidification cracking. The presence of high amounts of Mo in Inconel 617 weld metal led to the formation of brittle phases. In addition, continuous precipitates were observed in the 310 SS weld metal, which can lead to poor resistance of the weld metal to hot cracking. In the aged condition, Inconel 82 and Inconel A exhibited good thermal stability, whereas Inconel 617 and type 310 SS exhibited poor thermal stability. Also, after subjecting the heat-affected zone and interface between Inconel weld metal and base metals to aging treatment, unmixed zone of Inconel 657 base metal side has disappeared. Elimination of this region can be attributed to high-temperature interdiffusion of alloying elements. Finally, it is found that Inconel A and Inconel 82 weld metals are the best choices for the dissimilar welds performed here, respectively.     

Sulphur segregation and precipitation in HAZ of boiler steel thick section welds

Abstract

Stress relief cracking can occur in weld heat affected zones (HAZ) after post-weld heat treatment (PWHT) and periods of service at elevated temperatures. Stress relief cracking is generally believed to occur by sulphur induced decohesion ahead of a growing sharp crack. The impurity segregation behaviour in a microalloyed steel, typical of that used in the construction of a power station boiler where intermittent cracks were observed along the weld fusion boundaries, has been assessed. In particular the type and amount of segregation in the coarse grained HAZ (CGHAZ) before and after PWHT has been determined. It was found that significant sulphur segregation occurred during the CGHAZ thermal cycle resulting in elemental sulphur on the prior austenite grain boundaries. Following PWHT some desegregation of sulphur, coupled with the formation of sulphides and carbides on the prior austenite grain boundaries, was observed; in addition, significant phosphorus segregation to the prior austenite grain boundaries and grain boundary precipitate/matrix interfaces was seen.     

Interfacial bonding mechanism in Al/coated steel dissimilar refill friction stir spot welds

Defect-free dissimilar Al/zinc coated steel and Al/AlSi coated steel welds were successfully fabricated by refill friction stir spot welding. However, Al alloy and uncoated steel could not be welded under the same welding condition. Al-Zn eutectic layer formed at the Al/zinc coated steel interface showed non-uniformity in thickness and nanoscale intermetallic (IMC) produced was discontinuous. The bonding formation between the Al-Zn layer and the surrounding materials was attributed to a liquid/solid reaction mechanism. Bonding formation at Al alloy and AlSi coated steel interface was attributed to a solid/solid reaction mechanism, as the joining process did not involve with melting of base metals or AlSi coating materials. Kissing bond formed at the weld boundary acted as a crack initiation and propagation site, and the present study showed that weld strength of Al 5754/AlSi coated steel was greatly influenced by properties of original IMC layer.     

Phase formation in Ti/Ni dissimilar welds

We explore phase formation in Ti/Ni dissimilar welds using a combination of microscopy and composition analysis (TEM, SEM and EDS). Main microstructural features are NiTi dendrites and Ti₂Ni grains in the inter-dendritic space. The high temperature B2 phase of NiTi is found to transform to trigonal ‘R’-phase, B19^′martensite, and rhombohedral Ni₄Ti₃ phase; these different transformation products highlight the composition inhomogeneity in the parent B2 phase and probable non-equilibrium solidification events during rapid cooling of the weld. Solidification sequence of NiTi and Ti₂Ni was found to vary depending on local conditions in the weld. Formation of impurity nitride phases of titanium is observed which signify incomplete shielding during welding.     

Influence of fusion zone size and failure mode on mechanical performance of dissimilar resistance spot welds of AISI 1008 low carbon steel and DP600 advanced high strength steel

The work here addresses the investigation of the effect of the welding parameters (welding time, welding current and electrode force) on the overload failure mode and mechanical performance of dissimilar resistance spot welds between drawing quality special killed AISI 1008 low carbon steel and DP600 dual phase steel. Mechanical properties of spot welds are described in terms of failure mode, peak load and energy absorption during the quasi-static tensile-shear test. Three distinct failure modes were observed during the tensile-shear test: interfacial, pullout and partial thickness–partial pullout failure modes. Correlations among failure mode, welding parameters, weld physical attributes and weld mechanical performance are analyzed. Effect of expulsion on mechanical performance of welds is also investigated.     

Fatigue behaviour of friction welded medium carbon steel and austenitic stainless steel dissimilar joints

This paper reports the fatigue behaviour of friction welded medium carbon steel–austenitic stainless steel (MCS–ASS) dissimilar joints. Commercial grade medium carbon steel rods of 12 mm diameter and AISI 304 grade austenitic stainless steel rods of 12 mm diameter were used to fabricate the joints. A constant speed, continuous drive friction welding machine was used to fabricate the joints. Fatigue life of the joints was evaluated conducting the experiments using rotary bending fatigue testing machine (R = −1). Applied stress vs. number of cycles to failure (S–N) curve was plotted for unnotched and notched specimens. Basquin constants, fatigue strength, fatigue notch factor and notch sensitivity factor were evaluated for the dissimilar joints. Fatigue strength of the joints is correlated with microstructure, microhardness and tensile properties of the joints.     

Numerical analysis of residual stresses in welds of similar or dissimilar steel weldments under superimposed tensile loads

This paper presented the characteristics of residual stresses in welds of similar or dissimilar steel weldments by carrying out three-dimensional (3-D) thermal elastic–plastic finite element (FE) analyses. Moreover, residual stress behavior in welds of the weldments under superimposed tensile loads was further investigated. The materials used in this investigation were SM400, SM490, SM520 and SM570, widely used structural steels in welded structure. Results show that the maximum longitudinal residual stresses in welds of the similar steel weldments increase with increasing yield stress of the steel welded (SM400 < SM490 < SM520 < SM570). When tensile loads are superimposed to edges of the weldments, the maximum longitudinal residual stresses are increased to a higher value. In case of the dissimilar steel weldments, the difference between the longitudinal residual stresses in welds increases with increasing yield stress of the steel welded together with SM400 (SM490 < SM520 < SM570). When tensile loads are superimposed to edges of the weldments, same pattern is noted for the longitudinal stresses in welds and the difference is almost the same as that between the longitudinal residual stresses in welds.     

Microstructure and failure behavior of dissimilar resistance spot welds between low carbon galvanized and austenitic stainless steels

Resistance spot welding was used to join austenitic stainless steel and galvanized low carbon steel. The relationship between failure mode and weld fusion zone characteristics (size and microstructure) was studied. It was found that spot weld strength in the pullout failure mode is controlled by the strength and fusion zone size of the galvanized steel side. The hardness of the fusion zone which is governed by the dilution between two base metals, and fusion zone size of galvanized carbon steel side are dominant factors in determining the failure mode.     

Microstructure,hardness and residual stress distribution of dissimilar metal electron beam welds: maraging steel and high strength low alloy steel

Abstract

The present investigation reports on a study that has been taken up to develop an understanding of the electron beam welding characteristics of similar and dissimilar combination of maraging steel and high strength low alloy steel, which are in the hardened condition, i.e. maraging steel, in a solution that was in treated and aged condition, whereas high strength low alloy steel in a quenched and tempered condition before welding. The joint characterisation studies include microstructural examination, microhardness survey across the weldment and measurement of residual stresses. Maraging steel weld metal is under compressive stress rather than tensile stress as observed in low alloy steel welds because the martensite transformation occurs at a relatively low temperature. It has been observed that, in dissimilar metal welds, tensile stress is observed at the fusion boundary of low alloy steel and weld metal, whereas compressive stress is obtained at the location between weld and maraging steel fusion boundary. Dissimilar weldment contains a soft region beside the interface on maraging steel side because of the diffusion of manganese from low alloy steel towards maraging steel. The observed residual stresses, hardness distribution across the similar and dissimilar metal welds are correlated with the observed microstructures.     

Metallurgical characterization of arc-conditioned carbon coating on copper electrodes

During arc discharge between two parallel electrodes, the electrode surface is eroded by the arc. Carbon coating of the electrode surface often results in a decreased erosion. The surface structure of the arc-conditioned carbon-coated copper electrode was metallurgically characterized by the use of (1) scanning electron microscopy and (2) Auger in-depth profiling. Two different types of a.c. conditioning with varying pulse characteristics were studied.It was found that the peak current density, the pulse duration and the total number of pulses were the most important parameters in determining the metallurgical structure of the conditioned electrode surface. The vapor arcs which were produced by pulses with a high current density and short pulse duration were most effective in compacting the carbon coating and promoting good adhesion between the carbon and the copper electrode. The compaction and the adhesion are effected by a high ionic beam pressure on the cathode reaching up to 1000 atm.A lower degree of carbon penetration was effected when surface melting was produced by thermionic arcs which have a lower current density and longer pulse duration. In this case, carbon adhesion is due to the mechanical entrapment of carbon powder in the thin molten copper layer produced on the electrode surface by the arcs.     

Effect of welding parameters on residual stresses in dissimilar joint of stainless steel to carbon steel

In this study, a thermo-mechanical model was utilized to investigate the effects of welding parameters on the distribution of residual stresses in dissimilar TIG welds of low carbon and ferritic stainless steels. To solve the governing thermal and mechanical problems, a finite element program, ANSYS, was employed while the different aspects such as welding sequence and dilution were considered in the numerical solution. To validate the predictions, the model results were compared with the residual stresses measured by X-ray diffraction technique and a reasonable agreement was found. The results show that the magnitude of tensile residual stresses decrease as the welding current increases while lower residual stresses are produced in the longer samples. In addition, the magnitudes of residual stresses significantly decrease when a symmetric welding sequence is employed especially for the carbon steel part with the higher yield strength.     

Microstructure and fatigue properties of Mg-to-steel dissimilar resistance spot welds

The structural application of lightweight magnesium alloys in the automotive industry inevitably involves dissimilar welding with steels and the related durability issues. This study was aimed at evaluating the microstructural change and fatigue resistance of Mg/steel resistance spot welds, in comparison with Mg/Mg welds. The microstructure of Mg/Mg spot welds can be divided into: base metal, heat affected zone and fusion zone (nugget). However, the microstructure of Mg/steel dissimilar spot welds had three different regions along the joined interface: weld brazing, solid-state joining and soldering. The horizontal and vertical Mg hardness profiles of Mg/steel and Mg/Mg welds were similar. Both Mg/steel and Mg/Mg welds were observed to have an equivalent fatigue resistance due to similar crack propagation characteristics and failure mode. Both Mg/steel and Mg/Mg welds failed through thickness in the magnesium sheet under stress-controlled cyclic loading, but fatigue crack initiation of the two types of welds was different. The crack initiation of Mg/Mg welds was occurred due to a combined effect of stress concentration, grain growth in the heat affected zone (HAZ), and the presence of Al-rich phases at HAZ grain boundaries, while the penetration of small amounts of Zn coating into the Mg base metal stemming from the liquid metal induced embrittlement led to crack initiation in the Mg/steel welds.     

Softened zone formation and joint strength properties in dissimilar friction welds

The mechanical properties of dissimilar MMC/AISI 304 stainless steel friction welds with and without silver interlayers were examined. The notch tensile strengths of MMC/AISI 304 stainless steel and MMC/Ag/AISI 304 stainless steel friction welds increased when high friction pressures were applied during the joining operation. The higher notch tensile strengths of dissimilar MMC/AISI and MMC/Ag/AISI 304 stainless steel friction welds resulted from the formation of narrow softened zones in MMC material immediately adjacent to the bondline. The influence of softened zone width and hardness (yield strength) on the notch tensile strengths of dissimilar welds was analysed using finite element modelling (FEM). FEM in combination with the assumption of a ductile failure criterion was used to calculate the notch tensile strengths of dissimilar joints. The key assumption in this work is that dissimilar weld failure wholly depended on the characteristics (mechanical properties and dimensions) of the softened zone formed in MMC material immediately adjacent to the bondline. The modelling results produced based on this assumption closely correspond with the actual notch tensile strengths of dissimilar MMC/Ag/AISI 304 stainless steel and MMC/Ag/AISI 304 stainless steel friction welds.     

Metallurgical after-effect in steel

Results are reported of studies of the effect of the purity of steel, as determined by the application of various metallurgical processing treatments, on the physicomechanical and electrochemical properties of structural steels. Since the differences between steels of various degrees of purity persist even after thermal, mechanical, and other treatments, it is suggested that a term metallurgical after-effect be used to describe this phenomenon.