Temperature dependent tensile behavior of Ni–Fe based weld metal by gas tungsten arc welding (GTAW) was evaluated in the range of 350–820 ℃ at a strain rate of 0.035/min. Intergranular fracture and intermediate temperature brittleness (ITB) took place at around 750 ℃. The microstructure and the fracture surface morphology observation by optical microscopy, scanning and transmission electron microscopy showed that the ITB to a large extent depended on the grain boundary sliding (GBS) and the gamma prime (γ′) precipitation during the elevated temperature tensile deformation. X-ray diffraction (XRD) confirmed that the primary phases formed during the last stage of solidification were mainly TiN nitrides, MC carbides, and Laves phases in the form of Laves/γ eutectics. Quantitative statistics of the Laves/γ eutectics and the MX(MC, TiN) phases were processed. These primary phases had a pinning effect on the migration of grain boundaries and accordingly made the grain boundaries tortuous. The tortuous grain boundaries were expected to inhibit the GBS, relieving ITB. Transmission electron microscopy confirmed that fine Laves particles precipitated along the grain boundary in the weld metal with higher Mo content during the deformation process, and these Laves particles were also supposed to be one factor for inhibiting the GBS.

•Effects of Nb and solution treatment temperature on pitting corrosion were studied.•Nb in solid solution state hindered metastable pit initiation and growth.•Niobium additions more than a critical amount ennobled the pitting potential (Epit).•Increasing solution treatment temperature ennobled Epit of Nb-bearing samples.•Solution treatment temperature had little influence on Epit of Nb-free samples.Effects of niobium and solution treatment temperature on pitting corrosion of Super304H austenitic stainless steels were investigated by potentiodynamic and potentiostatic polarization tests and surface analysis. Results indicated that niobium in solid solution stabilised passive films against pit initiation and formed insoluble niobium-rich corrosion products partially covering pit walls inhibiting pit growth, whereas niobium as carbonitride accelerated metastable pit growth by reducing nitrogen in austenite matrix. Niobium additions more than a critical amount ennobled the pitting potential of Super304H. Moreover, increasing solution treatment temperature increased pitting potential for niobium-bearing Super304H samples but had little influence on that for niobium-free ones.

A series of Ni–Cr–Fe welding wires with different Nb and Mo contents were designed to investigate the effect of Nb and Mo on the microstructure, mechanical properties and the ductility-dip cracking susceptibility of the weld metals by optical microscopy (OM), scanning electron microscopy, X-ray diffraction as well as the tensile and impact tests. Results showed that large Laves phases formed and distributed along the interdendritic regions with high Nb or Mo addition. The Cr-carbide (M23C6) was suppressed to precipitate at the grain boundaries with high Nb addition. Tensile testing indicates that the ultimate strength of weld metals increases with Nb or Mo addition. However, the voids formed easily around the large Laves phases in the interdendritic area during tensile testing for the weld metal with high Mo content. It is found that the tensile fractographs of high Mo weld metals show a typical feature of interdendritic fracture. The high Nb or Mo addition, which leads to the formation of large Laves phases, exposes a great weakening effect on the impact toughness of weld metals. In addition, the ductility-dip cracking was not found by OM in the selected cross sections of weld metals with different Nb additions. High Nb addition can eliminate the ductility-dip cracking from the Ni–Cr–Fe weld metals effectively.

The distribution of boron and the microstructure of grain boundary (GB) precipitates (M23(C, B)6 and M2B) have been analyzed with their effects on the susceptibility of ductility-dip-cracking (DDC) and tensile properties for NiCrFe-7 weld metal, using optical microscopy (OM), secondary ion mass spectroscopy (SIMS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results show that boron segregates at GBs in NiCrFe-7 weld metal during the welding process. The segregation of boron at GBs promotes the formation of continuous M23(C, B)6 carbide chains and M2B borides along GBs. The addition of boron aggravates GB embrittlement and causes more DDC in the weld metal, by its segregation at GBs presenting as an impurity, and promoting the formation of larger and continuous M23(C, B)6 carbides, and M2B borides along GBs. DDC in the weld metal deteriorates the ductility and tensile strength of the weld metal simultaneously.

•The areas of CGHAZ and unaltered reheated CGHAZ have severe local brittle effect.•Grain coarsening and retained delta ferrite deteriorate the toughness.•Fine grains formed by phase transformation recrystallization improve the toughness.•Welding input should be low to reduce areas of CGHAZ and unaltered reheated CGHAZ.Samples of enlarged heat affected zones (HAZs) submitted to single and double welding thermal cycles were fabricated by a thermal-mechanical physical simulator (Gleeble 1500). Typical welding thermal cycle curves were extracted based on the measurement of the non-equilibrium phase transformation of 9Cr2WVTa steel, and the simulated and experimental thermal cycles were compared. Observation of the microstructure revealed that the blocky δ-ferrite that existed in the coarse grained heat affected zone (CGHAZ) was the main cause of the decrease in impact toughness. Each of the areas retained the δ-ferrite with different sizes and shapes after the CGHAZ underwent the second welding thermal cycle. Hardness tests, tensile tests, and instrumented impact tests were carried out to investigate the corresponding mechanical properties. When the peak temperature of the second welding thermal cycle was 1315 °C, grain and structure coarsening was the main causes of the decrease in impact toughness. When the peak temperature of the second welding thermal cycle was decreased to 1100 °C, the toughness increased the most due to the fine grains formed by phase transformation recrystallization.

EP-823 steel is one of the candidate materials for accelerator-driven systems/lead-cooled fast reactors (ADS/LFR). Its weldability was investigated by mechanical property tests and microstructure analysis on the enlarged heat-affected zones (HAZs) made by numerical and physical simulation. The finite element numerical simulation could simulate the welding thermal cycle of the characteristic regions in HAZs with extremely high accuracy. The physical simulation performed on a Gleeble simulator could enlarge the characteristic regions to easily investigate the relationship between the microstructure evolution and the mechanical properties of the HAZs. The results showed that the simulated partially normalized zone comprising tempered martensite, newly formed martensite and more tiny carbides has the highest impact energy. The fully normalized zone exhibits the highest hardness because of the quenched martensite and large carbides. The ductile property of the overheated zone is poor for the residual delta-ferrite phases and the quenched martensite.

A new welding method named double shielded tungsten inert gas (TIG) has been developed to improve the TIG weld penetration. The main principles to increase the weld depth have been discussed. Results show that the critical oxygen content in the weld pool is around 100 × 10−6 as the temperature coefficient of surface tension changes from negative to positive. The tracer test using pure silver shows that the direction of Marangoni convection changes as the oxygen content increases in the weld pool. The effect of arc constriction on the weld depth has been evaluated on a water-cooled copper plate, and the result indicates that the torch of double shielded can give a more powerful arc. Heavy oxide on the pool surface has undesirable impacts on the increasing of weld depth as the oxygen excessively accumulates in weld pool. It is possible to form chromium oxide in the weld process, while the iron oxide may form as the weld surface exposes to the air after the shielded gas moving away.

The effects of double shielded TIG (tungsten inert gas) torch's structural parameters, including the flow rate ratio between the inner and outer layers of gas and the extended length of the electrode (abbreviated as ELE in this work), on the fusion zone profile have been investigated for 0Cr13Ni5Mo martensitic stainless steel. Results show that the double shielded TIG process yields relatively high penetration of the weld pool in a broad range of the structural parameters. ELE over 3 mm is too large and causes adverse reactions on the protection of electrode. The outer gas with relatively high flow rate or the outer layer with high oxygen content is conducive to the oxygen dissolved into the arc, which results in the oxidation of the weld pool surface and the electrode tip. The double shielded TIG welded metal was tested and presented good impact property.

The welding heat source models and the plastic tension zone sizes of a typical weld joint involved in the double floor structure of high speed train under different welding parameters were calculated by a thermal-elastic-plastic FEM analysis based on SYSWELD code. Then, the welding distortion of floor structure was predicted using a linear elastic FEM and shrinkage method based on Weld Planner software. The effects of welding sequence, clamping configuration and reverse deformation on welding distortion of floor structure were examined numerically. The results indicate that the established elastic FEM model for floor structure is reliable for predicting the distribution of welding distortion in view of the good agreement between the calculated results and the measured distortion for real double floor structure. Compared with the welding sequence, the clamping configuration and the reverse deformation have a significant influence on the welding distortion of floor structure. In the case of 30 mm reverse deformation, the maximum deformation can be reduced about 70% in comparison to an actual welding process.

A double-shielded TIG welding process using pure He gas as the inner shielding layer and He and CO2 mixed gas as the outer shielding layer was proposed for the welding of Cr13Ni5Mo martensitic stainless steels. This proposed welding process can successfully address the problem of electrode oxidation with mixed-gas TIG welding and the issue of low weld depth and welding efficiency of traditional TIG welding. A change in the direction of the surface tension convection mode was the primary mechanism that affected the fusion zone profile. When the oxygen content in the weld pool was in the range of 80–120 ppm, the surface tension convection direction changed from outward to inward, resulting in both a larger weld depth and a larger weld depth/width ratio. This process not only allows for a high welding efficiency comparing with traditional TIG welding but also produces better weld impact properties than those of MAG welding (metal active gas welding).

The effects of filler metal (FM) composition on inclusions and inclusion defects for ER NiCrFe-7 weldments have been investigated and analyzed. Results show that as Al, Ti content in FM increases from 0.14 wt% Al, 0.30 wt% Ti to 0.42 wt% Al, 0.92 wt% Ti, the Al, Ti reduction will increase during welding. Inclusion defects (point-like defects named by welding workers) are prone to form in the high Al, Ti content weldments. Inclusion defects with Mg, Ca, Al, and Ti as major metallic elements have been found on the surface and interior of the weldments, as Al, Ti content in FM is over 0.29 wt% Al, 0.62 wt% Ti. Less Ti content in FM cannot prevent ductility-dip-cracking (DDC) through producing enough intragranular precipitates and lessening intergranular M23C6 precipitates. Nb can be used to replace Ti to reduce the sensitivity of the DDC in the NiCrFe-7 alloy weldments.

The effects of the minor elements Ti and Nb on the microstructure and mechanical properties for multi-pass weldments from the alloy ER NiCrFe-7 were studied using an optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), electron probe micro analysis (EPMA), as well as tensile and bend tests. The results show that grain size does not change significantly by increasing the Ti content from 0.28 wt% to 0.87 wt% in the weld metal (WM), whereas the grain boundaries become tortuous. The intragranular precipitate in the WM with Ti are AlO and Ti(C, N), whereas the intragranular precipitate in the WM with Nb are AlO and MX (M=Nb, Ti, X=C, N). As the Ti and Nb content increased in the WM, more MX was produced. Furthermore, the majority of C was fixed in the grain, not segregated to the grain boundaries; and less M23C6 (M=Cr, Fe) formed at the grain boundaries. Fewer ductility-dip-cracking (DDC) was observed for WM with higher levels of Ti and Nb. The tensile strength and elongation simultaneously increased with an increase in Ti and Nb in WM. The number and length of the cracks in the bend specimens decreased upon adding Ti and Nb.

A double-shielded TIG method was proposed to improve weld penetration and has been compared with the traditional TIG welding method under different welding parameters (i.e., speed, arc length and current). The strength of the Marangoni convection was calculated to estimate the influence of the welding parameters on the variations in weld pool shapes. The results show that the changes in the welding parameters directly impact the oxygen concentration in the weld pool and the temperature distribution on the pool surface. The oxygen content and heat distribution on the weld pool surface are determinants of the pattern and strength of the Marangoni convection. For a negative temperature coefficient of surface tension (∂σ/∂T < 0), an outward Marangoni convection leads to a wide and shallow weld pool shape. The narrow and deep weld pool shape occurs when the Marangoni convection flows along an inward direction (∂σ/∂T > 0). The oxide layer that may appear with the relatively high oxygen content in the weld pool is harmful for the heat flow along the pool surface so as to reduce the welding efficiency especially in the double shielded TIG process.

A numerical model of the welding arc is coupled to a model for the heat transfer and fluid flow in the weld pool of a SUS304 stainless steel during a moving GTA welding process. The described model avoids the use of the assumption of the empirical Gaussian boundary conditions, and at the same time, provides reliable boundary conditions to analyze the weld pool. Based on the two-dimensional axisymmetric numerical modeling of the argon arc, the heat flux to workpiece, the input current density, and the plasma drag stress are obtained. The arc temperature contours, the distributions of heat flux, and current density at the anode are in fair agreement with the reported experimental results. Numerical simulation and experimental studies to the weld pool development are carried out for a moving GTA welding on SUS304 stainless steel with different oxygen content from 30 to 220 ppm. The calculated result show that the oxygen can change the Marangoni convection from outward to inward direction on the liquid pool surface and make the wide shallow weld shape become narrow deep one. The calculated result for the weld shape and weld D/W ratio agrees well with the experimental one.

The mechanical properties and microstructure were evaluated and analyzed by optical microscopy (OM) and transmission electron microscopy (TEM) for micro-alloy carbon steel weld metal with and without Nb addition, respectively, under different heat treatment processes including stress relief annealing, normalizing, and no treatment after welding. The strength and elongation of the weld metal without treatment after welding were improved with the addition of Nb element, and the impact toughness was not affected obviously with the Nb addition. After stress relief annealing, the strength decreased for the Nb-free weld metal, while the elongation and impact toughness increased. However, for the Nb-bearing weld metal, stress relief annealing improved the strength of the weld metal significantly, and deteriorated the elongation and impact toughness. In the case of normalizing treatment to the weld metal, it was shown that with the increase of the holding time at the normalizing temperature of 920 °C, for both the weld metals with and without Nb addition, the microstructure of the columnar grain zone (CGZ) was transformed from one of columnar grain into one of equiaxed grain. The grain size of the equiaxed grain zone (EGZ) increased initially, then remained almost unchanged with the prolonging of the holding time. The mechanical properties of the weld metal with and without Nb addition showed no obvious change with the increasing holding time. With the increase of the normalizing temperature, the strength of the Nb-bearing weld metal increased, while the elongation and impact toughness decreased significantly. OM and TEM analysis found that the fine NbC particles were precipitated at the normalizing temperature of 920 °C, which refined the grains of the weld metal and increased the impact toughness. With the increase of the normalizing temperature, the content of widmanstatten ferrite (WF) in the Nb-bearing weld metal increased, whereas the quantity of the NbC particles decreased, which improved the strength and lowered the impact toughness.

The influences of argon and oxygen in helium base shielded GTA welding on the arc ignitability, bead protection and weld penetration are systematically investigated by bead-on-plate welding on SUS304 stainless steel. Experimental results show that the critical electrode tip work distance for arc ignition is increased from 1 mm under pure He shielding to 5 mm under He–50%Ar shielding. Small addition of oxygen content to the He–Ar mixed shielding can significantly change the weld shape from a wide shallow type to a narrow deep one, and the weld depth/width ratio can be doubled due to the change in the Marangoni convection from an outward to an inward direction.

•The competitive growth of microstructures in the weld is simulated by the CA model.•The thermal conditions and the weld metal characteristic are considered.•The effect of the S/L interface morphology on the competitive growth is discussed.•The formation mechanisms for axial structure and curved columnar grain are studied.•We obtain the grain structure morphologies for various welding speeds.The competitive growth of microstructures in the entire weld pool for both the Al–Cu alloy and the pure aluminum was simulated by the cellular automata method to comparatively investigate the micro-mechanisms for the morphological evolution of the axial structure and the curved columnar grain in the weld. The competitive mechanism of grains during the epitaxial growth and the morphological evolution of the grain structure in the weld with various welding speeds were studied. The results indicate that both the thermal conditions and the solidification characteristic of the weld metal exert an important influence on the grain competition and the resulting structure in the weld. For the Al–Cu alloy, the dendritic structure with a large S/L interface curvature appears during the epitaxial growth. The preferential orientation affects the competition result obviously. Owing to the anisotropic growth kinetics, the straight axial structure forms at low welding speeds. With the increase of the welding speed, the width of the axial region decreases and eventually disappears. For the pure aluminum, the S/L interface during the epitaxial growth is planar, and the grain competition is controlled by the thermal conditions completely. The columnar grains curve gradually to follow the highest temperature gradient direction at low welding speeds and become straight at high welding speeds.