Microstructure Of Weld Metal Research Articles

Solidification phenomena in creep resistant 9Cr weld metals and their implications on mechanical properties

Martensitic 9% Cr steels play an important role in the implementation of modern and efficiency-enhanced power generation technologies. In the presented study, metallurgical solidification phenomena in heat-resistant 9% Cr filler metals and their effects on the mechanical properties of the weld metal were analysed. The focus was on welded joints of steel grades P91 and CB2, which were welded with flux-cored wires. The investigation of welded joints and weld metals in the creep-damaged and unloaded condition provided detailed inspects into the formation and development of inhomogeneous areas. The microstructure of the individual weld metals was characterized in detail in the as-welded and in the heat-treated condition. It became apparent that inhomogeneities were formed in large areas of the weld metal. In particular, EDX measurements made it possible to explain these solidification phenomena and trace their development within the manufacturing process of the welded joints. It was found that even a slight uneven distribution of chromium and a diffusion of carbon caused extensive negative effects on the development of weld metal microstructures. In addition, the influence of these microstructural inhomogeneities on both the mechanical weld metal properties and the creep rupture strength is discussed. Finally, test welds were performed to optimize the microstructure of the flux-cored wire weld metal and possibilities to avoid microstructural inhomogeneities were derived. The results show that the short-term and long-term properties of the weld metal are affected by the inhomogeneous areas within the weld metal. In summary, it can be assumed that for steel grades P91 and CB2 the safety of plants in high-temperature operation is not endangered.

Characterization of Multi-layer Weld Metal and Creep–Rupture Behavior of Modified 10Cr–1Mo Welded Joint

The rupture behavior of the modified 10Cr–1Mo steel multi-layer welded joint is determined by the fine-grain zones of the weld metal adjacent to the fusion line during the long-term creep test at 620 °C. The microstructures of multi-layer weld metal before and after the creep tests were characterized in detail, and its role in creep behavior was systematically investigated. Most grain boundaries of subgrains represented the low-angle boundaries in the weld metal adjacent to the fusion line both before and after the creep test. The widths of grains in the fine-grain zones were about 0.5–1 μm. The fracture morphology appeared as “wave” structure due to the cracking initiating from multi-layer grain boundaries in the fine-grain zones. Some W elements that melted into weld metal adjacent to the fusion line altered the thermodynamic and kinetic conditions of the Laves phase formation during long-term creep exposure. Laves phase particles mainly distributed along the grain boundaries due to the faster diffusion and segregation of Mo, W, and Si elements. Moreover, higher-density grain boundaries in the fine-grain zones led to easier nucleation and growth of Laves phase particles. Compared with other areas in the welded joint, the size of Laves phase particles in the fine-grain zones of the weld metal adjacent to the fusion line was the largest ones. The interface between Laves phase particles and the matrix acted as the nucleation site of creep micro-cavities. The creep micro-cavities grew up at the expense of fine-grain boundaries and even grew across the grain boundary deeply into adjacent grains, and then developed to cracks in the fine-grain zones.

The Effects of TIG Welding Rod Compositions on Phase Distributions and Corrosion Properties of Dissimilar 304L and 420 Stainless Steel Welds

304L austenitic and 420 martensitic stainless steels are demanded in wide range of industries. 304L alloy exhibits good resistance to oxidizing medias up to 760°C and they also maintain superior impact properties at cryogenic temperatures while 420 alloy provides the strength values close to tool steels in with satisfactory corrosion properties in ambient atmospheres. In this work; 420 plate is TIG (Tungsten Inert Gas) welded with 304L plate with both thicknesses of 3 mm. Welding operation is applied by two passes under pure argon gas also with shielding the weld root. 3 types of TIG welding rods; ER312, ER316L and ER2209 are used in TIG welding for ensuring 3 different weld metal compositions. The effects of TIG welding rod type on weld metal phase ratios with microstructural and corrosion properties are investigated. Microstructural inspections and corrosion (weight loss) tests are applied to all joints after welding operations. The sample joined by ER312 TIG rod transformed the weld metal into dendritic microstructure and the sample joined by ER2209 TIG rod resulted in globular type of weld metal microstructure. The specimen that welded by ER316L type TIG welding rod resulted in the best corrosion test values among all welded samples.

Experimental study to investigate microstructure and continuous strain rate sensitivity of structural steel weld zone using nanoindentation

In this study, a series of experiments consisting of continuous stiffness measurement (CSM) nanoindentation experiments, optical microscope (OM), atomic force microscopy (AFM) examinations, and finite element (FE) analysis were performed to study the microstructures, the indentation size effects, and the rate-dependent behavior of mechanical properties in three phases of SM490 structural steel weld joint. The microstructures of base metal (BM), heat-affected zone (HAZ), and weld metal (WM) were observed using OM examinations. The size-dependent behaviors of mechanical properties in BM, HAZ, and WM were characterized and interpreted through the strain gradient theory. The CSM nanoindentation experiments were carried out in the strain rate range of 0.01–0.1 s−1 to investigate the rate-dependent behavior of indentation hardness and continuous strain rate sensitivity. The results indicated that indentation hardness depended on not only the indentation size but also the strain rate of the indentation level. The strain rate sensitivity (SRS) of BM, HAZ, and WM showed the depth-dependent behavior. The SRS of WM is quite high, over 0.4 at the indentation depth of 200 nm, quickly drops to 0.1, and finally is around 0.0298 at large indents. Similarly, the SRS behavior in the case of HAZ is the same, however, the SRS values at larger indents are higher than those obtained in the WM region. At larger depths, the SRS values of the WM region are lower than those of BM and HAZ, while BM has the highest SRS value compared with WM and HAZ. The continuous SRSs of WM, HAZ, and BM are attributed to the change in the dislocation behaviors during the indentation process. The results of the present study can be used to access and understand the rate- and the size-dependent behaviors in structural steel weld zone.

Effect of surface treatments on microstructure and stress corrosion cracking behavior of 308L weld metal in a primary pressurized water reactor environment

Effect of surface treatments on microstructure and stress corrosion cracking (SCC) of 308L weld metal in primary pressurized water reactor environment was evaluated by microstructure characterization and bent beam tests. The results showed that no SCC was observed on colloidal silica slurry polished surface while the change in composition and etched phase boundary promote SCC initiation after electropolishing. Further, milling and grinding produce a nanocrystalline layer and an underlying deformed layer on the surface. The nano-sized δ ferrite phase has a detrimental effect on SCC due to the synergistic effect of high concentration of dislocations and its nano size.

Relationship between Microstructure and Corrosion Behavior of Stainless Steel Weld Metals

Relationship between Microstructure and Corrosion Behavior of Stainless Steel Weld Metals

ANALYSIS OF THE PROPERTIES OF AW2099 ALUMINIUM-LITHIUM ALLOY WELDED BY LASER BEAM WITH AW5087 ALUMINIUM-MAGNESIUM FILLER MATERIAL

EN AW2099 aluminium lithium alloy, 2.0mm in thickness, was used as an experimental material. EN AW2099 belongs to the 3rd generation of aluminium lithium alloys. The third generation was developed to improve the disadvantages of the previous generation, such as anisotropy in mechanical properties, low fracture toughness, corrosion resistance and resistance to fatigue crack growth, as well. Aluminium magnesium 5087 filler wire with a diameter of 1.2mm was used for the welding. Crack free weld joints were produced after an optimization of welding parameters. The microstructure of weld metal and mechanical properties of weld joints were investigated. Equiaxed zone (EQZ) was observed at the fusion boundary. The character of grains changed in the direction towards the weld centre, from the columnar dendrite zone to equiaxed dendrite zone in the weld centre. The microstructure of the weld metal matrix consisted of -aluminium. Alloying elements enrichment was found at the inter-dendritic areas, namely copper and magnesium. The microhardness decrease in the weld metal due to a dissolution of strengthening precipitates was measured. The microhardness was slightly higher in comparison to a weld produced by a laser welding without a filler material. The tensile strength of the weld joint reached around 67% of the base material’s strength and the fracture occurred in the weld metal.

Experimental and computational analyses on fatigue fracture and microstructure in dissimilar metal weldments with circular sharp stress raiser

Fatigue strengths of dissimilar metal weldments with a circular sharp stress raiser, which were the inner peripheral side welded joints of flange pipes with tungsten inert gas (TIG) or metal inert gas (MIG) welding process, were defined through bending fatigue experiments. Fatigue strengths decreased according to stress concentration factors in some cases but not in samples with a different welding process, and stress intensity factors at the sharp weld notch were applied for analyses using the energy release rate method. The crack states corresponded to the fact that the fatigue cracks propagated at nearly right angles to the maximum stress direction, and it is possible to predict their propagation direction. The local fatigue resistance of the martensite-ferrite structure with the TIG welding was higher than that of the austenite-ferrite structure with the MIG welding which has a largely different penetration rate. In the samples which have similar weld metal microstructure, the nominal fatigue strength declines according to the increase in the stress concentration factor. The TIG samples and the MIG samples reach their fatigue failure in the same level of stress intensity factors, except for a MIG sample which has a large excess metal shape and a different fracture mode. The stress intensity factors can be used as the fatigue fracture threshold criterion in the weldments with a circular sharp stress raiser, where have the complex stress effects.

Effect of Trace Element on Microstructure and Fracture Toughness of Weld Metal

The microstructure and fracture toughness of weld metal under Ti-free and Ti-containing different fluxes were investigated in this study. It was found that the trace element Ti of flux in submerged arc welding produced significant influence on the fracture toughness. The addition of 60 ppm Ti induced the sharp increase in J0.2 value from 232.78 to 364.08 kJ/m2. Microstructure characterization revealed that a large number of oxide inclusions prompted the nucleation of acicular ferrites and refined grains, which improved the fracture toughness of Ti-containing weld metal greatly. Moreover, the crack propagation path was more tortuous and bifurcated due to the small amount of carbide precipitations along grain boundaries and blocky martensite–austenite islands for Ti-containing weld metal. Meanwhile, the large-angle grain boundaries caused crack deflection and increased the resistance of crack propagation compared to Ti-free weld metal.

Coupling effect of microstructure and hydrogen absorbed during service on pitting corrosion of 321 austenitic stainless steel weld joints

Pitting corrosion of austenitic stainless steel weld joints served in a hydrocracking unit for 6 years was examined. The microstructure of weld metal (WM) and fusion line (FL) was austenite and ferrite. After the immersion test, many pits were observed on the FL, whereas few pits were observed on the heat-affected zone (HAZ). Pits were microscopically initiated at ferrite/austenite boundaries. Hydrogen concentrations at FL and WM were higher than those of HAZ, and hydrogen segregation occurred at ferrite/austenite boundaries, which was consistent with the pit position. Hydrogen enrichment conditions varied under different microstructures, which induced the difference in pitting susceptibility.

Weld Metal Toughness of Autogenous Laser-Welded Joints of High-Strength Steel Domex 960

Abstract The results of investigations on autogenous laser welding of 5.0-mm-thick high-strength steel Domex 960 are presented in this article. The experimental plates delivered directly from the steel manufacturer were used for butt joint welding. The disk laser with maximum output power (maximum capacity of the laser generator) of 3.3 kW, emitting at 1.03 μm, and with the beam spot diameter of 200 μm was used for the trails of welding. Initially, the bead-on-plate welding tests were carried out, and then the test butt joints were laser welded. The influences of basic technological welding parameters, especially the energy input of laser welding on the shape of the fusion zone, the microstructure of weld metal and heat affected zone, and the impact toughness were analyzed. Laser welding trials were conducted in a wide range of energy input from 100 to 400 J/mm. Despite the low energy input of the laser welding process and also the short cooling times t8/5, tendency to cold crack was found neither in the weld metal nor in the heat affected zone. The carbon equivalent (CET), determined by the chemical analysis of the experimental melt, was just 0.341, indicating moderate tendency to increase hardness after welding because of martensitic transformation. It was found that the energy input has a clear influence on the microstructure and the impact toughness of the weld metal. The weld metal of the test butt joint welded at the energy input of 198 J/mm showed the average impact toughness at approximately 80 % of the base metal, whereas the weld metal of the test butt joint welded at a lower energy input of 132 J/mm showed the average impact toughness at the level of just 60 % of the base metal of Domex 960 steel.

Effect of restraint stress on martensite transformation in low transformation temperature weld metal

In this study, martensite transformation behavior of low transformation temperature (LTT) weld metals under unrestrained and restrained conditions was analyzed by thermal simulation technology. It is found that martensite transformation start (Ms) temperature and martensite transformation temperature range of LTT weld metal under restrained condition increased by 85 K and 50 K, respectively, compared with that of under unrestrained condition, which is attributed to the significant increase Gibbs free energy produced by restraint stress for martensite transformation. The increasement of Ms due to the effect of restraint stress is quantitatively described by Fe–X–C ternary regular solution-elastic energy model. Based on the transformation kinetics curve, the effect of restraint stress on martensite transformation of LTT weld metal can be divided into three stages, including stress-induced nucleation, transition and without effect stages. In initial stage, the martensite nucleates at a relatively high temperature with a low phase transformation rate due to the directional effect of restraint stress; in second stage with martensite transformation fraction range of 0.2–0.6, the elastic energy caused by restraint stress decreases significantly, and the chemical free energy of martensite transformation under restrained condition is lower than that under unrestrained condition, which results in the martensitic transformation rate of LTT weld metal under restraint condition being lower than that under unrestrained condition; in final stage, the small value of restraint stress has little effect on martensite transformation of LTT weld metal, and the transformation rate of martensite mainly depends on the content of austenite. This study provides a theoretical base for better understanding phase transformation and microstructure of LTT weld metal under restrained condition.

Investigations on Mechanical Strength and Microstructure of Multi-Pass Welded S690QL HSLA Steel Using MAG and FCAW

Abstract High strength low alloy (HSLA) steels are quite crucial in weight reduction in manufacturing of vehicles and constructions with high steel thickness. Quenched and tempered S690QL HSLA steel with 700 MPa yield strength was multi-pass welded with 7 passes using MAG (Metal Active Gas) and FCAW (Flux Cored Arc Welding) methods. Heat inputs for both methods were tried to keep close to each other as much as the process parameters allowed. The effects of the welding wires and the process parameters for both methods on weld metals and heat affected zones (HAZ) were investigated. Weld metals, which were deposited by solid and cored wire electrodes, were subjected to mechanical tests and their toughness values were measured with Charphy V-notch impact test and analyzed using optical and SEM micrographs. Inclusions were present for both weld metals and HAZs. Inclusions were spherical oxides of Si, Mn and Ti for the FCAW specimen. The main microstructures of weld metals for both processes were acicular ferrite accompanied by grain boundary ferrite (GBF) and ferrite side plate (FSP) for MAG and GBF for FCAW. In coarse grain heat affected zone (CGHAZ) FCAWed specimens formed lath type martensite, which contributed to higher impact toughness compared with MAG specimens.

On the Dissimilar Metal Welding of 1.4742 Ferritic to 310S Austenitic Stainless Steels Utilizing Different Filler Metals

In this work, the microstructural, mechanical, and physical properties of dissimilar welding between the 1.4742 ferritic and 310S austenitic stainless steels have been investigated. The welding was performed by the manual gas tungsten arc welding process utilizing ER310, ERNiCr-3, and ER446 filler metals. The weld metal microstructure of the ER310 and ERNiCr-3 filler metals was fully austenitic. The ER446 weld metal exhibited a ferritic-austenitic dual-phase microstructure. Carbide precipitates enriched with Cr, Fe, and or Nb were detected in all of the three weld metals. The unmixed zone was observed on both sides of the ERNiCr-3 weld metal, while for the ER310 and ER446 weld metals it was formed solely on one side. The ER446 weld metal displayed the maximum hardness values, 221.9 ± 5 HV, with relatively large fluctuations. The highest and lowest impact energy was achieved in the ERNiCr-3 weld metal, 40 ± 3 J, and ER446 weld metal, 25 ± 3 J, respectively. The ERNiCr-3 weld metal had a linear coefficient of thermal expansion (CTE) value about the mean value of the linear CTE of the two base metals. At last, for the dissimilar welding of these two base metals, it was concluded that the ERNiCr-3 filler metal is a better selection.

Effect of Welding Processes on the Microstructure and Mechanical Properties of Duplex Stainless Steel Weld Joints

Abstract The objective of the present investigation involves studying the microstructure and mechanical properties of duplex stainless steel (DSS) alloy 2205 and super DSS (SDSS) alloy 2507 weld joints fabricated by tungsten inert gas (TIG) welding and its variant, activated TIG (A-TIG) welding. Square butt weld joints were fabricated from 10-mm-thick plates of DSS and SDSS using single pass A-TIG and multi-pass TIG welding processes. The filler wire ER2209 was used for multi-pass TIG welding of DSS 2205, and ER2594 filler wire was used for multi-pass TIG welding of SDSS 2507. The microstructure in the A-TIG weld metals was found to be coarser, which was caused by high peak temperatures during A-TIG welding. The microstructures exhibited different forms of austenite, including grain boundary allotriomorphs, Widmanstätten side plates, and intragranular precipitates in the weld metal. In the A-TIG weld metals, the microstructure exhibited a balanced ferrite-to-austenite ratio. The hardness values of multi-pass TIG weld joints were higher compared to A-TIG weld joints. The tensile and yield strengths of multi-pass TIG weld joints were higher compared to A-TIG weld joints, which was mainly due to higher delta ferrite content and finer microstructure. The toughness values obtained by Charpy impact testing were higher in A-TIG weld joints compared with multi-pass TIG weld joints at room temperature, which was mainly due to the balanced ferrite-to-austenite ratio in the weld microstructure. However, at subzero temperatures, the A-TIG weld joints exhibit lower impact toughness because of their coarser weld metal structure.

Metallurgical effects of nitrogen on the microstructure and hot corrosion behavior of Alloy 718 weldment

The effects of nitrogen added by autogenous gas tungsten arc (GTA) welding on weld microstructure and hot corrosion resistance of Alloy 718 exposed to Na2SO4–25%wt NaCl salt compound at 650 °C have been studied. The weld metal microstructure was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that nitrogen has low solubility in Alloy 718, leading to precipitation of (Ti,Nb)CN and (Ti,Nb)N precipitates inside the weld. The optimum nitrogen content for hot corrosion resistance was 150 ppm due to formation of compact Cr2O3 and adhesive Nb2O5 layers. Higher concentration of nitrogen aggravates hot corrosion resistance owing to severe microsegregation of Nb and Mo solutes as well as increased NbC and Laves phase in interdenderitic regions.

Weld metal composition and aging influence on metallurgical, corrosion and fatigue crack growth behavior of austenitic stainless steel welds

AISI 316 austenitic stainless steel joints were fabricated using gas tungsten arc welding (GTAW) process selecting two filler compositions viz. AISI 316L (conventional) and AISI 347 (Nb-based). Both the welds were subjected to an aging treatment of 750 °C/24 h. AISI 316L aged weld metal microstructure exhibited interdendritically branched δ-ferrite morphology which facilitated a strong network for the formation of carbides, whereas in 347 weld similar ferrite morphology occurred but branching effect was not observed. EPMA (Electron probe microanalyzer) results show that carbide precipitation in the dendritic regions of both the welds in the aged condition accounted for their respective corrosion that occurred interdendritically. Corrosion performance evaluation of the welds carried out by measuring their degree of sensitization using DLEPR (double loop electrochemical potentiokinetic reactivation) technique showed niobium contained 347 weld performed better than a 316L weld. Fatigue crack growth resistance of both the welds in the aged condition increased due to dispersion strengthening effect that occurred due to carbides formation. Further, 316L weld exhibited better fatigue performance than 347 weld, both in the as-welded as well as in the aged condition. This study establishes that if Nb addition to the weld metal can improve the sensitization performance of AISI 316 welds, it can at the same time; decrease their fatigue crack growth resistance also.

Mechanical and microstructural behavior of C-Mn steel weld deposits with varying titanium contents

The effect of micro-alloying on the microstructure of C-Mn steel weld metals has been extensively studied in the past and the effect of titanium, in particular, has motivated several investigations, due to its essential contribution to the occurrence of acicular ferrite. However, there are cases where a substantial amount of acicular ferrite present in the last pass is not directly related to higher impact energy, since other parameters such as the proportion of the transformed phases by reheating, occurrence of micro-phases and non-metallic inclusions also influence the toughness of weld metal. Based on this scenario, the current work complements previous investigation and proposes an alternative methodology for evaluation of the behaviour of microstructure and impact toughness of C-Mn steel weld metals containing varying titanium contents, in the range of 6 and 255 ppm, based on comparison with the microstructural characteristics observed at the Charpy-V notch, where impact toughness was measured.The weld metal microstructure was characterized at the top bead and at the mid-thickness by Optical (OM), Electron Microscopy (SEM) and electron backscattered diffraction (EBSD). Vickers microhardness tests were performed at the same positions where metallographic examination was done. Charpy-V impact tests employing test samples removed from the center line and at mid-thickness of the weld deposits were carried out in order to obtain test temperatures corresponding to impact energy of 100 joules.The results evidenced that the increase of titanium content promoted a substantial amount of acicular ferrite and the refinement of microstructure at the top bead. At the Charpy-V notch location, the same behavior was observed, the difference being relative to the lower amount of acicular ferrite. Optimum impact toughness was attained for 28 ppm Ti, although the metallographic examination revealed that the higher amount of refined acicular ferrite presenting a lower effective grain size and increased frequency of high angle boundaries was observed for 255 ppm Ti. From these findings, a methodology involving OM, SEM and EBSD for an adequate characterization of the microstructure at the Charpy-V notch location is applied, in order to evaluate the influence of reheating, microstructure, effective grain size, frequency of high angle boundaries, presence of MA constituents and inclusions. The analysis revealed that the behavior of impact toughness is associated with the presence of the M-A constituents occurring at the Charpy-V notch for 255 ppm Ti.The obtained results indicate the need for a more stringent procedure for an explanation of the behaviour of the impact properties of C-Mn steel weld metals, due to the role of the thermal cycle build-up of multi-layer weldments.

Phase-field investigation of dendrite growth in the molten pool with the deflection of solid/liquid interface

The solidification process in the molten pool significantly determines the microstructures of weld metal. The motion of heat source makes the solidification parameters, temperature gradient G and growth rate R, changing continuously across the pool, as well as the solidification modes. It is meaningful to investigate the solidification evolution in the molten pool, with dynamic solidification parameters. The previous researches ignored the deflection of solid/liquid (S/L) interface with time, regarding the normal direction of interface being constant all the time. In this paper, we modified the macroscopic model for G and R without S/L interface deflection to the model with the interface deflection. The solidification parameters were calculated out through the two models and compared with each other. Then the phase-field simulations of dendrite growth in the molten pool were carried out, including single crystal and bi-crystals. The dendrite growth of single crystal and bi-crystals, with the diverging and converging directions, were compared and discussed. The simulations demonstrate that the model considering the interface deflection could represent the dendrite growth during welding more realistically. The investigations lay the foundation for revealing solidification behavior in the molten pool more precisely.

Solidification cracking susceptibility of ferritic stainless steels using Modified Varestraint Transvarestraint (MVT) method

The Modified Varestraint Transvarestraint (MVT) test was used to investigate the solidification cracking susceptibility of an unstabilised, a Nb-stabilised and two (Ti + Nb) dual-stabilised ferritic stainless steels. Two different welding speeds of 6 and 3 mm/s using autogenous gas tungsten arc welding were employed. At the welding speed of 6 mm/s, the high-content (Ti + Nb) steel was resistant and the Nb-stabilised steel was marginally susceptible to solidification cracking. At the welding speed of 3 mm/s, the Nb and the high (Ti + Nb) steels were found to be marginally susceptible to solidification cracking while the unstabilised and low-content (Ti + Nb) grades were resistant to solidification cracking. The weld metal microstructures transverse to the welding direction revealed columnar grains in all the samples for both welding speeds. The ferritic stainless steels were generally resistant to solidification cracking, except the Nb-stabilised steel, which was marginally susceptible to solidification cracking.