Steel Weld Metal Research Articles

Effect of cerium on microstructure and microsegregation behavior of novel cryogenic high-Mn austenitic steel weld metal

The novel cryogenic high-Mn austenitic steel weld metal with four different cerium (Ce) contents were prepared under submerged arc welding by adding cerium dioxide and calcium silicon reducing agents into the flux to refine the solidification structure and alleviate the microsegregation. The effects of the Ce element on the microstructure, inclusion, and microsegregation behavior of weld metal were systematically investigated via optical, scanning, and transmission electron microscopy and using an electronic probe micro-analyzer. The results showed that the dendrite arm spacing of high-Mn austenitic steel weld metal was significantly reduced with an increase in the Ce content from 0.018 to 0.087 wt%. The typical inclusions in the weld metal were mainly Al2O3, CeAlO3, Ce2O3, and Ce2O2S for the Ce content ranging from 0 to 0.087 wt%. The number density and the size distribution of effective inclusions influenced the microstructure variation. Numerous small-sized effective inclusions (Ce2O3 and Ce2O2S) in the weld metal with 0.087 wt% Ce were distributed at the dendrite core, promoting the nucleation for austenite during the solidification and refining the dendrite structures. The co-segregation of Mn, C and Si occurred in the interdendritic regions. The segregation ratio of Mn, C and Si gradually decreased with the increase in Ce content, alleviating the element microsegregation.

Microstructure evolution and mechanical properties of 16Cr–25Ni superaustenitic stainless steel weld metal during 600 °C aging

The effect of high temperature aging on the microstructure evolution and mechanical properties of 16Cr–25Ni superaustenitic stainless steel weld metal was investigated at 600 °C for up to 5000 h. Interdendritic micro-size M6C carbide, nano-size M6C and M23C6 carbides were identified in the 200 h aged weld metal, which appeared in clusters and distributed dispersedly. With the aging time increased to 1000 h, the interdendritic region was filled with newly formed precipitates, which were identified as nano-size M12C and M23C6 carbides. Compared with M6C carbide, M12C carbide was preferred to precipitate for lower Gibbs free energy, and the pre-existed M6C carbide had transformed to M12C carbide through the release of C atoms to the matrix. As the aging time further increased to 5000 h, the M12C carbide kept stable with the M23C6 carbide. Due to the precipitation and coarsen of M12C and M23C6 carbides that consumed massive solute atoms from matrix, which weakened the precipitation strengthening effect and suppressed the dynamic strain aging, the tensile properties kept superior stability during aging. The evolution of impact toughness was also closely related to the interdendritic precipitates. As the interdendritic region was occupied by precipitates since the aging time increased from 200 h to 1000 h, the impact toughness rapidly decreased from 76 J to 39 J, and then slightly decreased to 33 J with the aging time increased to 5000 h.

Hydrogen effect on mechanical properties and cracking of creep-resistant 9% Cr P92 steel and P91 weld metal

Martensitic 9% Cr steels like P91 and P92 can show an increased susceptibility to delayed hydrogen-assisted cracking. The focus of this study was the microstructure and heat treatment effect on the mechanical properties of P92 base material and P91 multi-layer weld metal in both as-welded and post weld heat treated (PWHT) condition. Tensile tests with hydrogen-free reference samples and electrochemically hydrogen charged samples were carried out; the mechanical properties were assessed and supported by detailed fractographic analysis. Finally, a hydrogen and microstructure-dependent fracture criterion is established. All investigated microstructures showed a hydrogen-influenced degradation of the mechanical properties compared to the hydrogen-free reference samples. The as-welded martensitic P91 weld metal had the highest degree of degradation in the presence of hydrogen. The P91 PWHT weld metal and the P92 base material had comparable properties. From that point of view, a significantly increased risk for hydrogen-assisted cold cracking during welding fabrication of P91 weld joints must be considered before any heat treatment is conducted.

Evolution of inclusions in novel cryogenic high-Mn austenitic steel weld metal with cerium addition

Cerium (Ce) was effectively transitioned into the high-Mn austenitic steel weld metal under submerged arc welding by adding cerium dioxide and calcium silicon reducing agents into flux. The evolution behaviours of the composition, number density and size of inclusions in weld metal with different Ce contents were systematically investigated by thermodynamic calculation and experiments. The results show that the number density of inclusions increased and average size decreased with the increase of Ce content from 0 to 0.087 wt-%. Meanwhile, and the percentage of inclusions number with size less than 0.80 μm increased from 79.28% to 91.43%. The evolutionary process of inclusions was Al2O3 → Al11O18Ce → CeAlO3 → Ce2O3 → Ce2O2S with the increase of Ce content in the present weld metal. In particular, Ce-O-S-Al inclusion was extracted by in-situ sample preparation method, which proved its internal structure and evolution process. The complete formation model of Ce-Al-O-S system was also established.

Effect of boron treatment on the microstructure and toughness of Ti-containing steel weld metals

Ti-containing steel weld metals with boron addition contents of 0-85 ppm were prepared, and their microstructural characteristics as well as the impact toughness were investigated. The results show that in these microstructures, compared to the weld metal without boron, the addition of 22-39 ppm boron results in a remarkable increase in the amount of acicular ferrite at the expense of grain boundary ferrite, idiomorphic ferrite and side-plate ferrite. However, with a further increase in the boron content up to 61-85 ppm, the bainitic ferrite is formed, accompanied with a drop in the amount of acicular ferrite. In the acicular ferrite, the size of martensite-austenite (M/A) islands is much smaller, and the amount is much lower than those found in the bainitic ferrite. In the case of the weld metals primarily composed of acicular ferrite, during the fracture of the impact specimens, the crack propagation path is more bent in comparison with the weld metals with large amounts of grain boundary ferrite, idiomorphic ferrite, side-plate ferrite or bainitic ferrite, which that the presence of acicular ferrite improves the toughness of the weld metals. The coarse martensite-austenite islands readily induce micro-cracks at the interface between martensite-austenite islands and ferrite matrix, deteriorating the toughness. The weld metals with B contents of 22-39 ppm exhibit outstanding impact toughness because of high amount of acicular ferrite, accompanied with fine martensite-austenite islands.

Effect of preheating and preplaced filler wire on microstructure and toughness in laser-arc hybrid welding of thick steel

Acicular ferrite (AF) is the most important microstructural constituent to achieve high toughness at low temperatures in weld metal of steels. This is due to the relatively small grain size and large misorientation angles. AF is known to form at non-metallic inclusions (NMIs), but under high cooling rates, as in deep and narrow laser-arc hybrid welding (LAHW), this phenomenon is scarcely studied. In deep and narrow LAHW, insufficient transportation of filler wire to the root results in low amount of NMIs, thus bainite-martensite mainly forms due to fast cooling. In this work, a 45 mm thick high strength low alloy steel was welded by double-sided LAHW using different groove preparations. The effect of different cooling times on the microstructure in the weld metal and the heat-affected zone was studied. A low fraction of AF and high hardness were achieved in the root of weld metal when using standard LAHW. This was related to a rapid cooling time (Δt8/5 < 1.0 s) and a low population of NMIs due to lack of filler wire. Micro X-ray computed tomography revealed centerline solidification cracks and porosity. Two external methods were conducted to assist the formation of AF and to reduce hardness, base plate preheating and preplaced metal-cored filler wire into the weld groove prior to welding. Preheating reduced hardness and increased the AF fraction due to increased cooling time. In addition, preheating showed to mitigate porosity. Preplaced filler wire provided an enhanced population of NMIs in the root thus significantly increased the fraction of AF. It was found that the diameter and chemical composition of NMIs has a strong correlation with the cooling time Δt8/5. High impact toughness (> 35 J) was achieved at −50 °C by combining preheating and preplaced filler wire, and up to 45 % fraction content of AF was reached. However, many NMIs were still inactive due to a small diameter (< 200 nm) and unfavorable chemical composition related to the high cooling rate. The external methods had no influence on the occurrence of weld centerline cracks in the root, which will require further attention to secure mechanical properties and integrity.

Microstructure and the fatigue crack propagation in the dissimilar low alloy/stainless steel GMAW welded joints

Joining dissimilar metals is a frequent task for engineers in the power industry. The resulting dissimilar joints are then critical parts of the constructions and have to be able to withstand various loads during the operation of power devices. Due to the metals dissimilarity, the interface between ferritic side and the austenitic side of the joint is considered to be a critical area regarding the joint resistance to failure under static or cyclic loading. Proper understanding of the microstructures formed at the interface is key for production of solid and reliable dissimilar joints. In the present study, two dissimilar joints were prepared by gas metal arc welding (GMAW) using different low alloy steels. The microstructure of both joints is extensively studied using electron microscopy methods, with the emphasis taken on the interface between low alloy steel and austenitic weld metal. It was shown, that the interface between low alloy steel and weld metal is not a preferential crack path in case of non-environmental fatigue crack propagation, and the cracks are rather propagating through to the heat-affected zones of the low alloy steel side of the dissimilar joint. There were recorded significant differences in the threshold stress intensity factor between two tested welded joints (5.8 MPa × m1/2 vs. 8.7 MPa × m1/2. Therefore, the choice of the low alloy steel has a significant effect on the resistance to the fatigue crack propagation, and even small differences in the microstructure of low alloy steel may affect the resistance to the fatigue crack propagation at low stress ratios of the whole dissimilar joint.

Deformation and cracking behaviors of proton-irradiated 308L stainless steel weld metal strained in simulated PWR primary water

The proton-irradiated 308L stainless steel weld metal was strained by using constant extension rate tensile testing in simulated PWR primary water, and its deformation microstructures and irradiation assisted stress corrosion cracking (IASCC) behavior were investigated. The results suggest that the irradiation significantly increases the SCC susceptibility of 308L weld metal and causes various deformation microstructures including lathy faulted planes, dislocation channels and deformation twins in austenite and atomic plane rotation in δ-ferrite. The propagation of intergranular IASCC cracks is closely related to the location of the crack tip. For the crack tip in the specimen matrix interior, localized deformation is likely the key factor responsible for the crack growth. For the crack tip close to the specimen surface, however, localized corrosion along the grain boundary rather than the localized deformation appears to dominate the crack propagation. Unlike the intergranular cracks, the IASCC cracks along the δ-ferrite/austenite phase boundary can initiate either by crack initiation at the phase boundary or by crack propagation from the grain boundary. In both cases, the cracked phase boundaries contain a large number of carbides and are severely corroded, but no deformation microstructures are observed, which implies that the localized corrosion may play an important role in the IASCC along the phase boundary. In addition, δ-ferrite can retard the IASCC crack propagation along the grain boundary, which is probably related to the reduction of localized deformation by δ-ferrite.

Effect of titanium content on solidification structure of ferritic stainless steel gas-tungsten and gas-metal arc welds

Ferritic stainless steel is utilized to fabricate automotive exhaust systems using a ferritic weld metal. Ductility of the weld metal is higher if its microstructure contains a significant proportion of equiaxed grains. The formation of equiaxed (rather than columnar) grains is favoured by a higher titanium weld metal content. In this study, the Ti content of ferritic stainless steel weld metal was changed by using Ti-free (Type 436) and Ti-containing (441) ferritic stainless steel as base metals. The metal-cored welding consumable contained 0.4% Ti. Gas-tungsten arc welding and gas-metal arc welding processes were compared. The weld metal Ti content ranged from zero to 0.5% Ti, as determined from scanning electron microscopy supplemented by inductively coupled plasma optical emission spectroscopy. Cross-sections of the weld beads were subjected to point counting (to estimate the fraction of equiaxed grains) and image analysis (to estimate the average grain size). Point counting proved to be more reliable. The fraction of equiaxed grains was sensitive to the Ti content, but not to the welding process. Below 0.4% Ti, the fraction of equiaxed grains gradually increased with an increase in the weld metal Ti content; above 0.4% Ti, the fraction of equiaxed grains rapidly increased with increasing Ti content. The transition in behaviour at 0.4% Ti corresponded to a Ti content at which Ti-rich precipitates became stable at the estimated liquidus temperature of the weld metal.

The effect of non-metallic inclusions in refining microstructure and improving cryogenic toughness of novel high-Mn steel weld metals for LNG tanks

The effect of non-metallic inclusions in refining microstructure and improving cryogenic toughness of novel high-Mn steel weld metals for LNG tanks

Mechanism for enhanced age hardening of 22 % Cr duplex stainless steel weld metal fabricated with grade 2209 filler material

In industrial applications, 22 % Cr duplex stainless steel is extensively used to manufacture welded components. However, the steel becomes brittle on thermal exposure at 250–500 °C (known as 475 °C embrittlement). Most existing studies have focused on the solution-treated material, leaving the weld metal largely unexplored. In this study, we characterized a nanoscale structure of 22 % Cr duplex stainless steel weld metal, fabricated using a grade 2209 filler wire and aged at 400 °C for 1,000 h. The presence of Ni-Mn rich solute clusters within BCC-structured ferrite phase was evident, along with spinodal decomposition in the weld metal. A simulation conducted using Thermo-Calc indicated that the clusters were the precursors of the G-phase. The residual stress and the higher Ni content are considered to accelerate the spinodal decomposition of the weld metal. The solute clusters and pronounced spinodal decomposition contributed to an enhanced age hardening of the ferrite in the weld metal.

Role of Si content on microstructural and mechanical responses of 16Cr–25Ni superaustenitic stainless steel weld metals

In this work, three 16Cr–25Ni superaustenitic stainless steel weld metals with the Si content of 0.03 wt%, 1.22 wt% and 2.01 wt%, were prepared to study the microstructural evolution and its effect on mechanical properties. The results showed that with the increase of Si content, the size and number density of Mo-rich M6C carbide were increased in the weld metals. In particular, massive long strip shape M6C carbides were observed in the weld metal with the Si content of 2.01 wt%. For tensile tests, good match of strength and ductility was obtained in the weld metals with the Si content of 0.03 wt% and 1.22 wt%. However, the ductility of the weld metal was sharply decreased when the Si content further increased to 2.01 wt%, owing to the long strip shape M6C carbide that weakened the coordinated deformation with the matrix. The impact toughness was monotonously decreased from 185 J to 12 J with the increase of Si content, for the M6C carbide generated easier local stress concentration and fracture that the local stress concentration in the matrix was increasingly difficult to favor the twinning deformation. The results in this work are meaningful for the design of the weld metal with optimized microstructure and high mechanical performance for liquid lead-bismuth eutectic environment.

Effect of tempering heat treatment on the microstructure and impact toughness of a Ni–Cr–Mo–V steel weld metal

Microstructure evolution and impact toughness of Ni–Cr–Mo–V weld metal in as-welded (AW) and different post-weld heat treatment (PWHT) conditions were investigated using Transmission Electron Microscopy (TEM), Transmission Kikuchi Diffraction (TKD), and Charpy impact test. The enrichment of Mn and Ni in inter-dendritic (ID) regions caused by welding solidification segregation lowered the Ac1 temperature, making partial untempered martensite transform into reversed austenite during PWHT at 580 °C, which was confirmed directly by Laser Scanning Confocal Microscope (LSCM). The transformation kinetics and element distribution during austenite reversion were revealed by DICTRA simulation, suggesting the enrichment of C, Mn and Ni in reversed austenite. The microstructure at room temperature of ID regions depended on the tempering cooling rates. When weld metal was cooled by water after tempering, the microstructure in ID regions consisted of finer fresh martensite and over tempered martensite, which was a major contributor to the higher impact toughness. While weld metal was cooled at a rate of 5 °C/h, the reversed austenite in ID regions transformed into fresh ferrite and cementite, which was difficult to resist crack propagation, and resultantly the impact toughness was deteriorated. The impact absorbed energy of weld metal increased with the increase of tempering cooling rates, which was attributed to the microstructure change in ID regions from ferrite to martensite. The inherited substructures of martensite in ID regions had a hindering effect on crack propagation, and resultantly more energy was consumed.

Critical assessment 42: acicular ferrite formation and its influence on weld metal and heat-affected zone properties of steels

Toughness and fatigue resistance of welds and associated heat-affected zones (HAZs) in high-strength steels are important design parameters in structural applications. Acicular ferrite microstructures, which can be achieved through control of weld process and nucleant (inclusions or precipitates) characteristics, are often regarded as key to enhancing weld metal and HAZ properties of welded steels. This paper addresses the transformation characteristics of acicular ferrite related to alloying and weld processing as well as the specific role of acicular ferrite in weld performance, along with future research needs to identify weld processes suitable to specific alloying strategies.

Microstructure, non-metallic inclusions and impact toughness of high-Mn cryogenic steel weld metal

Microstructure, inclusions and cryogenic impact toughness of Fe-22.03Mn-0.45C austenitic weld metal were investigated. The results show that microstructure of weld metal consisted of straight columnar dendrites with different orientations. Microsegregation of Mn was observed in the interdendritic regions and Mn depletion thus occurred in the dendrite core, which is suggested to lead to the different morphologies of MnS oxides (Mn–Al–Si–O) depending on their location. MnS patch-type oxides were found at dendrite boundary and MnS shell-type oxides were found at dendrite core. Inclusions as crack sources were distributed in the dimple of cryogenic impact fractography, it was found that MnS patch-type oxides were more ready to act as a crack source and cause the formation of dimples than MnS shell-type oxides.

Metallurgical Design Rules for High-Strength Steel Weld Metals

A review of a dilatometric analysis of selected Fe-C-Mn high-strength steel shielded metal arc weld metals showed that balanced Ti, B, Al, O, and N additions reduced the austenite-to-ferrite transformation-start (TS) temperature. These microalloy additions must match the following aim levels for composition control: Ti at 400 ppm (0.04 wt-%), B at 40 ppm (0.004 wt-%), Al at 200 ppm (0.020 wt-%), O at 400 ppm (0.04 wt-%), and N preferably below 80 ppm (0.008 wt-%) to ensure effective deoxidation, form complex inclusions, and distribute them to enable development of highly fracture-resistant refined weld metal microstructures. It may be wiser to avoid the rich and lean ends for these microalloy additions, except N, which should be held at the lean end, preferably much below 80 ppm (0.008 wt-%). The balanced Ti, B, Al, O, and N additions offered nearly a 100°C shift in lowering the Charpy V-notch (CVN) test temperature for either 28 or 100 J absorbed energy. Dilatometric evaluations of reheated weld metals showed that 1) the balanced Ti, B, Al, O, and N additions lowered the actual TS temperature by about 60°C compared to the calculated austenite-to-ferrite transformation temperature obtained from the constitutional equation; 2) N with more than 100 ppm (0.010 wt-%) effectively nullified the beneficial effects of Ti, B, and Al additions in lowering the transformation temperature; and 3) at N content much below 80 ppm (0.008 wt-%), both a lower TS temperature and a narrow start-to-finish (TS–Tf) temperature range helped in achieving exceptional weld metal CVN impact toughness.

Influence of precipitate evolution on the impact fracture behavior in Nb-containing 16Cr–25Ni superaustenitic stainless steel weld metal

In this work, the precipitate evolution and its influence on the impact fracture behavior of Nb-containing 16Cr–25Ni superaustenitic stainless steel weld metal were systematically studied via scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Results show completely transgranular ductile fracture mode of as-welded condition, of which the M6C carbides and Nb(C, N) carbonitrides contributed to the crack initiation, but the interface between M6C carbide and matrix was more prone to crack under impact load. In contrast, due to the semi-continuous distributed M23C6 carbides on the grain boundary, an intergranular/transgranular mixed mode was observed in the post-weld heat treatment (PWHT) sample. Additionally, the crack sources in grain interior were not limited to primary precipitation phases, whereas secondary M2(C, N) carbonitrides and M23C6 carbides also acted as preferential sites for crack initiation. The massive M2(C, N)/matrix and M23C6/matrix interfaces led to easier crack propagation path simultaneously. Moreover, the deformation mechanism were governed by dislocation slip and twinning in the impact fracture of as-welded sample, but the mechanical twins disappeared in the fractured sample at PWHT condition, which was attributed to the local stress concentration in the matrix could not exceed the critical twinning stress. Therefore, the combination of secondary precipitates induced embrittlement and weakened plastic deformation capacity of matrix, leading to the impact toughness declined from 91.0 ± 3.6 J to 34.3 ± 4.6 J after PWHT.

Crack branching behavior and amorphous film formation mechanism during SCC expanding test for multi-layers weld metal of NiCrMoV steels

Multi-layer weld metal of NiCrMoV steels used in nuclear rotor was adopted to perform stress corrosion cracking (SCC) test at 180 ℃, which indicated the crack growth rate in 3.5% NaCl solution was lower than that in pure water. The results obtained from high resolution transmission electron microscopy (HRTEM) analysis revealed that the combined effect of crack branching behavior and amorphous film made contributions to the slower crack growth rate. The aggressive Cl− ions invaded grain in multi-directions that was promoted by dislocation motion, facilitating the main crack to bifurcate. Besides, nanocrystals also enhanced branching behavior by deflecting the crack growth path. An amorphous film forming in crack evolved from the disorder of atom arrangement produced by dislocation accumulation and grew stably with the supplement of carbon atom generated in selective corrosion. The complete amorphous film was absent connected path for migration of aggressive anions, thus weakened the driving force for crack propagation and improved resistance to localized corrosion attack.

Model description of hydrogen-induced damage evolution in high-strength steel weld metal based on results of a novel test method

In this paper, the development of a novel method for testing welded tensile specimens under two-stage external tensile loading is presented. The test method is intended to investigate the susceptibility of submerged arc welded high-strength steel specimens of type S690QL-S3Ni2.5CrMo to hydrogen-induced damage. Multiple cracking events within one specimen prove that the locally prevailing boundary conditions decisively determine the hydrogen-induced damage sequences.By means of SEM and EBSD analyses on specimens after test completion, interrelationships about the microstructural damage behavior are being examined. The crack tip environments are of particular importance here, since complex stress and strain conditions tend to cause hydrogen accumulation and further propagation of inter- and transgranular cracks here. In the vicinity of crack tips, retained austenite is of special importance. This metastable phase might undergo a deformation-induced transformation into crack-sensitive martensite and simultaneously release high local hydrogen contents as a former hydrogen trap. Finally, a model description including various sections of the stress- and hydrogen-induced damage evolution is derived from the empirical-analytical results.

Effect of the strength of steel sheets and wires on residual stress of thin steel sheet lap-fillet GMA weld joint

Weld residual stress of the lap fillet GMA welding joints which was performed using two strength grades of steel sheets and two strength grades of welding wires were measured. In addition, to consider the effects of steel sheet strength and weld metal strength on welding residual stress, thermal elastoplastic analysis using FEM was carried out for the experiments. As a result of the analysis using the temperature history of the heat-affected zone as a fitting parameter, the residual stress distributions near the toe were in agreement with the measurement in the FE analysis. Comparing the residual stress distributions in the direction perpendicular to the weld line on the weld metal surface near the toe, which is the occurring point of fatigue cracks in the FE analysis results, with the same steel sheet, the higher the weld metal strength the smaller the tensile residual stress. And comparing with the same wire, the higher the steel sheet strength the higher the tensile residual stress. The reason for the former is that when using high-strength wire, cooling transformation occurs at a lower temperature than using low-strength wire. As a result, it is considered that the tensile residual stress was effectively reduced by causing transformation expansion while the surrounding strength was high. The reason for the latter is considered to be that each stress value changes based on the yield stress of the steel sheet and weld metal because the plastic strain continues to occur in each direction until transformation occurs after welding. It was considered that the larger the ratio of the hardness of the weld metal to the base metal, the smaller the ratio of the residual stress of the weld metal near the toe to the yield stress.