Out-of-plane constraint effect on local fracture resistance of a dissimilar metal welded joint

An experimental investigation on effect and mechanism of out-of-plane constraint induced by specimen thickness on local fracture resistance of two cracks (A508 heat-affected-zone (HAZ) crack and A508/Alloy52Mb interface crack) located at the weakest region in an Alloy52M dissimilar metal welded joint (DMWJ) between A508 ferritic steel and 316L stainless steel in nuclear power plants has been carried out. The results show with increasing out-of-plane constraint (specimen thickness), the fracture mechanism of the two cracks changes from ductile fracture through mixed ductile and brittle fracture to brittle fracture, and the corresponding crack growth resistance decreases. The crack growth path in the specimens with different out-of-plane constraints deviates to low-strength material side, and is mainly controlled by local strength mismatch. For accurate and reliable safety design and failure assessment of the DMWJ structures, it needs to consider the constraint effect on local fracture resistance. The new safety design and failure assessment methods incorporating both in-plane and out-of-plane constraint effects need to be developed for the DMWJ structures.     

Correlation of material constraint with fracture toughness of interface regions in a dissimilar metal welded joint

In this work, the constraint parameter A_p based on crack‐tip equivalent plastic strain was calculated by finite element analyses for the cracks located at different locations in two interface regions in a dissimilar metal weld joint (DMWJ). The capabilities of the parameter A_p for characterizing material constraint and establishing correlation of material constraint with fracture toughness of the interface region cracks have been examined. The results show that the parameter A_p can characterize material constraint effect caused by material mismatch and initial crack positions in the interface regions. Based on the A_p, the correlation lines and formulae of material constraint with fracture toughness of the interface region cracks in the DMWJ can be established, and they may be used for obtaining material constraint‐dependent fracture toughness for the interface region cracks. The results in this work combining with those in the previous studies indicate that the parameter A_p may be a unified constraint parameter that can characterize both geometry constraint (including in‐plane and out‐of‐plane constraints) and material constraint, and it may be used in accurate fracture assessments of welded components with different geometry and material constraints.     

An experimental investigation of local fracture resistance and crack growth paths in a dissimilar metal welded joint

An experimental investigation on the local fracture resistance and crack growth behavior in a Alloy52M dissimilar metal welded joint (DMWJ) between A508 ferritic steel and 316L stainless steel has been carried out by using the single-edge notched bend (SENB) specimens. The local J-resistance curves and crack growth paths of 13 cracks located at various positions in the DMWJ were determined, and the effects of the local strength mismatch on local fracture resistance, crack growth paths and integrity assessment for the DMWJ were analyzed. The results show that the cracks always deviate to the materials with lower strength, and the crack path deviations are mainly controlled by the strength mismatch, rather than toughness mismatch. The J-resistance curve with larger crack path deviation only reflect the apparent fracture resistance along the crack growth region, rather than the intrinsic fracture resistance of the material at the initial crack-tip region. Without considering the local fracture resistance properties of heat affected zone (HAZ), interface and near interface zone, the use of the J-resistance curves of base metals or weld metals following present codes will unavoidably produce non-conservative (unsafe) or excessive conservative assessment results. In most cases, the assessment results will be potentially unsafe. Therefore, it is recommended to obtain and use local mechanical and fracture resistance properties of all regions of the DMWJ if the complex local mismatch situation is a concern. And new integrity assessment methods based on local damage and fracture models also need to be developed for the DMWJs.     

Role of butter layer in low-cycle fatigue behavior of modified 9Cr and CrMoV dissimilar rotor welded joint

The present work aims at studying the role of butter layer (BL) in low-cycle fatigue (LCF) behavior of modified 9Cr steel and CrMoV steel dissimilar welded joint. The significant difference of the chemical composition of base metals (BMs) makes it a challenge to achieve sound welded joint. Therefore, buttering was considered to obtain a transition layer between the dissimilar steels. The LCF tests of two kinds of specimens without and with butter layer were performed applying strain-controlled cyclic load with different axial strain amplitudes. The test results indicated that the number of cycles at higher strain amplitudes of welded joint without butter layer was greatly higher than that of the joint with butter layer, while the fatigue lifetime to crack initiation (2N_f) became closer to each other at low and middle strain amplitudes. The failure was in the tempered heat affected zone (HAZ) at the CrMoV side for specimens without BL, while the fracture occurred at the tempered HAZ in the BL for specimens with BL. The microstructure details of BM, BL, HAZ and weld metals (WMs) were revealed by optical microscopy (OM). It was found that the tempered martensite was major microstructure for welded joint and much more carbides were observed in tempered HAZ than other parts due to the repeated tempering. Microhardness test indicated a softest zone existing tempered HAZ of BL and also there was a softer zone in tempered HAZ at the CrMoV side due to repeated tempering during welding and post weld heat treatment (PWHT). And scanning electron microscopy (SEM) was applied to observe the fractography. It was indicated that the fatigue crack initiation occurred from the specimen surface and all specimens were ductile–brittle mixed fractures. It is deemed that the softening behavior in BL caused by twice tempering correspondingly decreased the LCF lifetime at higher strain amplitudes. So suitable welding parameters and heat treatment processes became a key measure to ensure LCF property without losing other properties for welded joint with BL.     

Transition and fracture shift behavior in LCF test of dissimilar welded joint at elevated temperature

This work focused on the low-cycle fatigue(LCF) behavior of modified 9Cr/CrMoV dissimilar welded joint at elevated temperature. Narrow gap submerged arc welding(NG-SAW) process via multi-pass and multi-layer techniques was employed to fabricate the welded joint. LCF tests at different strain amplitude range from 0.22% to 0.75% were performed at strain ratio R =-1. The two-slope behavior based on fracture location shift was presented both on the cyclic stress-strain(CSS) curve and Manson-Coffin(M-C) curve, which could be applied to predict the fatigue life more precisely especially at relatively low strain amplitude. The results indicated that the joint failed in CrMoV-base metal(BM) at relatively low strain amplitude below 0.4% while failure shifted to CrMoV-over tempered zone(OTZ) at higher strain amplitude above 0.4%. Fatigue failure occurred in CrMoV-BM at low strain amplitude could be attributed to temperature softening effect in CrMoV-BM combined with cyclic strengthening in CrMoV-OTZ. While CrMoV-OTZ with a comparable number of grain boundaries and much lower hardness than that of CrMoV-BM was deemed to be the weakest zone across the welded joint at higher strain amplitude.EBSD investigations also revealed that CrMoV-BM experienced more fatigue damage at relatively low strain amplitude, while CrMoV-OTZ accumulated more plastic strain at higher strain amplitude.     

Formation of the anomalous microstructure in the weld metal of Co-based alloy/AISI 410 stainless steel dissimilar welded joint

The Co-based alloy/AISI 410 stainless steel dissimilar welded joint was fabricated by the electron beam welding (EBW) technique. The anomalous microstructure containing the element transition zone (ETZ) and/or core of tail-like zone (CTLZ) is in the weld metal (WM) adjacent to the fusion line. The melting temperature difference between the WM and AISI 410 steel, melt stirring effect and element diffusion can trigger the formation of such anomalous microstructure. In particular, the larger distance of the region in WM away from the fusion line, the smaller CTLZ and larger ETZ occurred. Compared with the fine and ellipsoidal precipitates in the as-welded CTLZ, a large number of chain-type clustered precipitates were detected in the CTLZ and ETZ interface after the aging treatment at 566 °C for 1 000 h. The element diffusion under elevated temperature in WM is regarded as the crucial factor for such anomalous microstructure evolution during the aging treatment.The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-022-00396-z     

Experimental investigation of local tensile and fracture resistance behaviour of dissimilar metal weld joint: SA508 Gr.3 Cl.1 and SA312 Type 304LN

The aim of the paper is to evaluate the local tensile and fracture toughness properties of the dissimilar metal weld joints between SA508Gr.3 Cl.1 and SA312 Type 304LN pipe. Weld joints have been prepared by manual gas tungsten arc welding (GTAW) process with conventional V‐groove and automatic hot wire gas tungsten arc welding with narrow gap using different filler wires/electrode such as Inconel 82/Inconel 182 and ER309L/ER308L. The tensile and fracture toughness test specimens have been machined from different regions of dissimilar metal weld such as heat affected zones, fusion lines, buttering layer, weld metal and both base metals. Tensile and fracture toughness tests have been carried out as per the ASTM standard E8 and E1820 respectively. Tensile and fracture toughness results of all the regions of dissimilar metal weld joints have been discussed in this paper. Metallurgical and fracture surface examinations have also been reported to substantiate the tensile and fracture toughness results. Need for the local properties for integrity assessment of the dissimilar metal weld joints has also been brought out.     

Microstructure and intergranular stress corrosion cracking susceptibility of a SA508-52M-316L dissimilar metal weld joint in primary water

Correlation of microstructure and intergranular stress corrosion cracking (IGSCC) susceptibility for the SA508-52M-316L dissimilar metal weld joint in primary water was investigated by the interrupted slow strain rate tension test following a microstructure characterization. The susceptibility to IGSCC in various regions of the dissimilar metal weld joint was observed to follow the order of Alloy 52 Mb> the heat affected zone of 316L> the dilution zone of Alloy 52 Mw> Alloy 52 Mw weld metal. The chromium-depletion at the grain boundary is the dominant factor causing the high IGSCC susceptibility of Alloy 52 Mb. However, IGSCC initiation in the heat affected zone of 316L is attributed to the increase of residual strain adjacent to the grain boundary. In addition, the decrease of chromium content and increase of residual strain adjacent to the grain boundary increase the IGSCC susceptibility of the dilution zone of Alloy 52 Mw.     

An experimental investigation of the static and fatigue fracture behaviour of hybrid composite/metal joints for a tilting car body

The hybrid composite joint structures considered in this work, for application in a tilting railroad car body, are subjected to shear and bending loads. Two types of the joint specimens were fabricated and tested under both static and fatigue conditions: a hybrid bolted-joint specimen subjected to a shear loads, and a hybrid beam-joint specimen for the bending tests. The fracture behaviours of these specimens under static loads were different from those under cyclic loads. For the hybrid bolted-joint specimens, static shear loads caused a pure shear fracture in the bolt pin body itself. However, cyclic fatigue shear loads brought about an opening-mode fracture at the local site of the bolt which was the valley of the screwed region of the bolt pin and/or the perpendicularly angled region between the bolt head and the pin body. On the other hand, for the hybrid beam-joint specimens, static bending loads caused shear deformation and fracture in the honeycomb core region, while fatigue cyclic bend loading caused delamination along the interface between the composite skin and the honeycomb core, and/or caused a fracture in the welded part jointed with the steel under-frame. These fracture behaviours could arise in other industrial hybrid joints with similar sub-structures, and were used in developing a design parameter to improve a hybrid joint structure.     

Combined effect of strength & sheet thickness on fatigue behaviour of resistance spot welded joint

Fatigue performance of spot welded lap shear joint is primarily dependent on weld nugget size, sheet thickness and corresponding joint stiffness. Two automotive steel sheets having higher strength lower thickness and lower strength higher thickness are resistance spot welded with established optimum welding condition. The tensile‐shear strength and fatigue strength of lap shear joint of the two automotive steel sheets are determined and compared. Experimental fatigue life of spot welded lap shear joint of each steel are compared with predicted fatigue lives using different stress intensity factor solutions for kinked crack and spot weld available in literature. Micrographs of fatigue fractured surfaces are examined to understand fracture micro‐mechanisms.     

Structure‐property investigations on a laser beam welded dissimilar joint of aluminium AA6056 and titanium Ti6Al4V for aeronautical applications. Part II: Resistance to fatigue crack propagation and fracture

Investigations were continued on the dissimilar laser beam welds of AA6056 and Ti6Al4V, fabricated by inserting Ti‐sheet into the profiled Al‐sheet and melting AA6056 alone. By using microstructure, hardness and strength as the criteria, sites exhibiting non‐uniform microstructure and localized plastic deformation due to strength mismatch were investigated in two orientations: ? crack parallel to the weld and ? crack perpendicular to the weld for fatigue crack propagation and fracture toughness at room temperature. Effect of temper of AA6056 on these properties was studied for two conditions; welding in T4 followed by post weld heat treatment T6, and welding in T6 and naturally aged for a defined period. The orientation “crack parallel to the weld” was investigated in 3 locations on the side of AA6056: the interface and the two changeovers on the Al‐side. Firstly, between the fusion zone and the heat affected zone (3 mm from the interface) and secondly, between (primary) heat affected zone and towards the base material (7 mm from the interface). Although brittle intermetallic TiAl₃ had been formed at the interface, uncontrolled separation or debonding at the interface was not observed. Insofar the bond quality of the weld was good. However, the ranking of interface was the lowest since fatigue crack propagation was relatively faster than that in the fusion zone and heat affected zone, and fracture toughness was low. Therefore, unstable fatigue crack propagation is observed when the crack propagates perpendicular to the weld from AA6056 towards Ti6Al4V. The results have shown that the dissimilar joints exhibit improved performance when laser beam welded in the T6 condition.