Microstructure,hardness and petroleum corrosion evaluation of 316L/AWS E309MoL-16 weld metal

The current work presents some observations about the effect of welding heat input on the microstructure, hardness and corrosion resistance of AWS E309MoL-16 weld metal, diluted with AISI 316L austenitic stainless steel plates. Such welds are widely used during overlay of equipment in the petroleum and gas industries. Results show that the welds contained δ-ferrite varying between vermicular to lathy morphology, typically encountered in welds which solidify in ferrite–austenite mode (FA). Conversely, contents and morphology of δ-ferrite in the weld metals were altered, showing an increase of welding heat input. The corrosion rate of the weld metal indicated that when higher levels of welding heat input are used the corrosion rate is reduced. This may be attributed to metallurgical changes, especially variations in the proportion of δ-ferrite, caused by changes in cooling rate.     

Microstructure,mechanical and corrosion properties of friction stir welded high nitrogen nickel-free austenitic stainless steel

Friction stir welding (FSW) was applied to a 2.4 mm thick high nitrogen nickel-free austenitic stainless steel plate using tungsten–rhenium (W–Re) tool. The high-quality weld was successfully produced at a tool rotational speed of 400 rpm and a traveling speed of 100 mm/min. The microstructure, mechanical and corrosion properties of the weld were studied. The nitrogen content of the weld was almost identical to that of base metal (BM). FSW refined grains in the stir zone (SZ) through dynamic recrystallization and led to increase in hardness and tensile strength within the SZ, while the ductility was slightly decreased. The failure of tensile specimens occurred in the BM. TEM results revealed precipitates of Cr₂₃C₆ of size ~ 1 μm in the SZ, although their content was small. The precipitation of Cr₂₃C₆ and increase in δ-ferrite in the SZ led to small decrease in both pitting and intergranular corrosion resistance.     

The effect of repeated repair welding on mechanical and corrosion properties of stainless steel 316L

The purpose of this study is to evaluate changes in the mechanical, micro structural and the corrosion properties of stainless steel 316L under repeated repair welding. The welding and the repair welding were conducted by shielded metal arc welding (SMAW). The SMAW welding process was performed using E316L filler metals. Specimen of the base metal and different conditions of shielded metal arc welding repairs were studied by looking in the micro structural changes, the chemical composition of the phases, the grain size (in the heat affected zone) and the effect on the mechanical and corrosion properties. The microstructure was investigated using optical microscopy (OM) and scanning electron microscopy (SEM). The chemical composition of the phases was determined using energy dispersive spectrometry (EDS). The corrosion behavior in 1 M H₂SO₄ + 3.5% NaCl solution was evaluated using a potentiodynamic polarization method. Tensile tests, Charpy-V impact resistance and Brinell hardness tests were conducted. Hardness of the heat affected zone decreased as the number of repairs increased. Generally an increase in the yield strength (YS) and the ultimate tensile strength (UTS) occurred with welding. After the first repair, a gradual decrease in YS and UTS occurred but the values of YS and UTS were not less than values of the base metal. Significant reduction in Charpy-V impact resistance with the number of weld repairs were observed when the notch location was in the HAZ. The HAZ of welding repair specimen is more sensitive to pitting corrosion. The sensitivity of HAZ to pitting corrosion was increased by increasing the number of welding repair.     

Local mechanical properties of radial friction welded supermartensitic stainless steel pipes

In this work, supermartensitic stainless steel pipes were radial friction (RF) welded and their microstructures and local mechanical properties (hardness, fracture toughness and micro-tensile strength) were characterized in the as-welded condition. Defect-free RF welds were produced with a matching consumable ring (CR) under optimized welding conditions. The formation of a fine structure consisting of a mixture of virgin martensite and some stable austenite retained in the CR region was observed. On the other hand, the presence of virgin martensite plus δ-ferrite was found in the microstructures of the heat affected zone (HAZ) and thermo-mechanically affected zone (TMAZ). A ductile fracture was detected in the CR and weld interface regions at −40 °C. Moreover, both the CR and weld interface regions showed higher hardness and strength values than those of the base material (overmatching), without presenting significant losses in ductility and fracture toughness, which was attributed directly to the fine transformed microstructure of the weld region.     

Laser welding of TiNi shape memory alloy and stainless steel using Ni interlayer

Laser welding of TiNi shape memory alloy wire to stainless steel wire using Ni interlayer was investigated. The results indicated that the Ni interlayer thickness had great effects on the chemical composition, microstructure, gas-pore susceptibility and mechanical properties of laser-welded joints. With an increase of Ni interlayer thickness, the weld Ni content increased and the joint properties increased due to decreasing brittle intermetallic compounds (TiFe₂ and TiCr₂). The joint fracture occurred in the fusion zone with a brittle intermetallic compound layer. The tensile strength and elongation of the joints reached the maximum values (372 MPa and 4.4%) when weld Ni content was 47.25 wt.%. Further increasing weld Ni content resulted in decreasing the joint properties because of forming more TiNi₃ phase, gas-pores and shrinkage cavities in the weld metals. It is necessary to select suitable Ni interlayer thickness (weld composition) for improving the mechanical properties of laser-welded joints.     

Keyhole gas tungsten arc welding of AISI 316L stainless steel

Keyhole gas tungsten arc welding (K-TIG) was used to weld AISI 316L stainless steel of mid-thickness (thickness ranging 6–13 mm). 316L plates of 10-mm thickness were jointed using an I-groove in a single pass without filler metal. The effects of welding parameters on the fusion zone profile were investigated. The weld properties, including mechanical properties, microstructure, and corrosion resistance, were analyzed. The primary weld microstructures were austenite and δ-ferrite. The tensile strength and impact property of the weld were almost the same as those of the base metal, while the corrosion resistance of the weld was even better than that of the base metal. High-quality 316L stainless steel joints can be realized through K-TIG welding with high productivity and low processing cost. The practical application of K-TIG welding to join mid-thickness workpieces in industry is well demonstrated and an ideal process for welding AISI 316L of mid-thickness with high efficiency and low cost is presented.     

Effects of pulsed Nd:YAG laser welding parameters and subsequent post-weld heat treatment on microstructure and hardness of AISI 420 stainless steel

Martensitic stainless steels are often used in cases where high strength and medium corrosion resistance are required. In this study, pulsed Nd:YAG laser welding of AISI 420 martensitic stainless steel is considered. Welding of samples were carried out autogenously. The spacing between samples was set to almost zero. All samples were butt welded. The effect of welding parameters such as voltage, laser beam diameter, frequency, pulse duration, and welding speed on the weld dimensions were investigated and the optimum values were obtained for the 450 V voltage, 0.6 mm focal diameter, 6 Hz frequency, 5 ms pulse duration and 1.5 mm/s welding speed. Microstructure of weld pool and heat affected zone (HAZ) were investigated by optical microscopy (OM) and scanning electron microscopy (SEM). Micro-hardness studies were also carried out. The results showed the presence of some remaining delta-ferrite in the martensitic weld structure and coarsening of M₂₃C₆ carbides in HAZ. The magnitude of hardness in the HAZ was higher than that of the weld zone. To reduce the hardness of weld and HAZ and to increase the toughness in these regions, two types of post-weld heat treatments (PWHTs) were carried out. In type 1, samples tempered for 2 h. In type 2, samples austenitizied for 0.5 h at 1010 °C and then tempered for 2 h. In order to achieve high strength and toughness, optimum temper temperatures for type 1 and 2 heat treatments were obtained for 595 and 537 °C, respectively. The results showed higher toughness for type 2 than type 1.     

Effect of post weld heat treatment on the microstructure and mechanical properties of ITER-grade 316LN austenitic stainless steel weldments

The effect of postweld heat treatment (PWHT) on the microstructure and mechanical properties of ITER-grade 316LN austenitic stainless steel joints with ER316LMn filler material was investigated. PWHT aging was performed for 1 h at four different temperatures of 600 °C, 760 °C, 870 °C and 920 °C, respectively. The microstructure revealed the sigma phase precipitation occurred in the weld metals heat-treated at the temperature of 870 °C and 920 °C. The PWHT temperatures have the less effect on the tensile strength, and the maximum tensile strength of the joints is about 630 MPa, reaching the 95% of the base metal, whereas the elongation is enhanced with the rise of PWHT temperatures. Meanwhile, the sigma phase precipitation in the weld metals reduces the impact toughness.     

Optimization of deep penetration laser welding of thick stainless steel with a 10 kW fiber laser

Deep penetration laser welding of 12 mm thick stainless steel plates was conducted using a 10 kW high-power fiber laser. The effect of the processing parameters on the weld bead geometry was examined, and the microstructure and mechanical properties of the optimal joint were investigated. The results show that the focal position is a key parameter in high-power fiber laser welding of thick plates. There is a critical range of welding speed for achieving good full penetration joint. The type of top shielding gas influences the weld depth. The application of a bottom shielding gas improves the stability of the entire welding process and yields good weld appearances at both the top and bottom surfaces. The maximum tensile stress of the joint is 809 MPa. The joint fails at the base metal far from the weld seam with a typical cup–cone-shaped fracture surface. The excellent welding appearance and mechanical properties indicate that high-power fiber laser welding of a 304 stainless steel thick plate is feasible.     

The effect of thermal ageing on low cycle fatigue behaviour of 316 stainless steel welds

It is well known that welds are the weak links in any structure. Therefore, it is of out most importance to characterize the mechanical properties of welds. Moreover, the changes in the microstructure that occur in welds on exposure to high temperatures affect the mechanical properties and must be studied by ageing the welds at high temperature. In this paper the low cycle fatigue behaviour of thermally aged 316 stainless steel weld metal is presented. Weld pads with single V configuration were prepared by the shielded metal arc welding process using 316 electrodes. Thermal ageing was done for 10,000 h at 823 and 873 K. Total strain controlled low cycle fatigue tests were conducted at a constant strain rate of 3 × 10^?3 s^?1 with strain amplitudes in the range ±0.25% to ±0.6% at 823 and 873 K. Weld metal exhibited initial hardening followed by cyclic softening prior to failure. The aged samples exhibited higher stress response as compared to the unaged samples. At both the temperatures and all strain amplitudes fatigue life was inferior to that of unaged samples. The metallography of the aged and tested material was studied through optical, scanning and transmission electron microscopy. The effect of transformation of δ-ferrite to sigma phase and carbides in the weld metal on low cycle fatigue behaviour was evaluated.     

Experimental investigation on dissimilar pulsed Nd:YAG laser welding of AISI 420 stainless steel to kovar alloy

This paper presents the results of an investigation on autogeneous laser welding of AISI 420 stainless steel to kovar alloy using a 100 W pulsed Nd:YAG laser. The joints had a circular geometry and butt welded. The joints were examined by optical microscope for cracks, pores and for determining the weld geometry. The microstructure of the weld and the heat affected zones were investigatedby scanning electron microscope. The austenitic microstructure was achieved in the weld. The morphology of weld zone solidification was basically cellural, being influenced by the temperature gradient. It was found that the start of solidification in the kovar side of weld zone occurred by means of epitaxial growth. When the temperature gradient was high, the columnar grains were created in the fusion boundary of 420 stainless steel side toward weld zone. Measurements taken by X-ray spectrometry for dispersion of the energy in the weld zone indicated a significantly heterogeneous distribution of chromium element. The variations in chemical compositions and grains morphologies significantly alter the Vickers microhardness values in the weld zone.     

Investigation on microstructure and properties of dissimilar joint between SA553 and SUS304 made by laser welding with filler wire

The dissimilar joints between SA553 and SUS304 were produced by CO₂ laser welding with the ERNiMo-8 and ER308L filler wire. After welding parameters were optimized, qualified weld formations were made. Investigation on the microstructure showed that there were dual phases (martensite and austenite) in the ER308L weld, but only austenite in the ERNiMo-8 weld. For both joints, not only the microstructure gradient, but also the element gradient was observed near interfaces between weld metals and base metals. The Charpy impact and tensile test at room (25 °C) and low temperature (− 196 °C) was implemented. The cryogenic impact energy of the ER308L weldment was 51 J, lower than the value (84 J) of the ERNiMo-8 weldment. The corresponding cryogenic tensile strength of the two weldments was 1070 MPa and 960 MPa, respectively. The cryogenic tensile properties of both weldments were rather higher than requirements in the relevant standards. The ERNiMo-8 weldment showed relatively better comprehensive performance when the cryogenic toughness was considered.     

Pulsed laser spot welding of intersection points for Zircaloy-4 spacer grid assembly

A pulsed laser spot welding technology has been developed for joining intersection points of 0.457 mm-thick Zircaloy-4 straps. Weld beads size, mechanical properties and microstructure characterization of the weld beads were investigated. The results indicate that peak power of the pulsed laser has significant influence on the penetration depth of weld beads. Pulse width and number of shots should be taken into consideration mainly to control the weld width. The average value of ultimate tensile loads of the intersection points are continuously increasing as the penetration depth and weld width of weld beads increase. Fracture was located at the fusion line and the fracture surface could be characterized as a mixture of ductile and cleavage feature. Column grains and equiaxed grains were observed in fusion zone and heat affected zone, respectively. The fusion zone consists of a mixture of α-Zr and β-Zr phases among which lamellar Widmanstatten structure α-Zr + Zr₃Fe was distributed. The Zr(Fe, Cr)₂ second phase particles were precipitated inter and intragranular of α-Zr grains.     

Creep of austenitic stainless steel welds

Austenitic stainless steels (SS) find extensive application in power, petrochemical and nuclear industries in view of their excellent elevated temperature mechanical properties, corrosion resistance, formability and weldability. However, they are susceptible to hot cracking during fusion welding. To avoid this problem, chemical composition of the welding consumable is generally adjusted to promote primary ferrite mode of solidification and retain about 3 to 10%δ-ferrite in the as-welded condition. The duplex microstructure of the weld metal undergoes transformation to carbides and a variety of intermetallic phases during elevated temperature service and causes deterioration in the mechanical properties. This paper presents a comprehensive review of the current understanding of the solidification microstructures, ageing processes and their influence on the creep behaviour of types 308 and 316 SS weld metals. The effects of varying chemical composition,δ-ferrite content, electrode coating and welding processes on creep strength and ductility are examined. Current trends in the design of welded components for creep application are also discussed.     

Quenching microstructure and properties of 300M ultra-high strength steel electron beam welded joints

The 300M steel was welded by electron beam welding (EBW) with optimized welding parameters in the annealed state. As-welded, for comparison, and as-quenched (oil quenching at 870 °C × 1 h and tempering at 315 °C × 2 h) welded joints were investigated in this paper. The microstructure and fracture morphology were analyzed using scanning electron microscopy (SEM) and optical microscope. X-ray energy spectrum analysis was used to determine chemical composition of phases formed at the joint. The microhardness and tensile strength were evaluated. Results indicate that the weld metal microstructures of the as-welded joint are lower bainite, retained austenite and pro-eutectoid ferrite; the heat affected zone microstructure is sorbite with undissolved particles. The microstructure of as-quenched joint is tempered martensite. The tensile strength of the joints after quenching reached 1900 MPa.     

Feasibility study of friction spot welding of dissimilar single-lap joint between poly(methyl methacrylate) and poly(methyl methacrylate)-SiO2 nanocomposite

In this work, the feasibility of friction spot welding (FSpW) of a commercial poly(methyl methacrylate) (PMMA) GS grade and a PMMA 6 N/2 wt% silica (SiO₂) nanocomposite was investigated. Single-lap joints welded at rotational speeds of 1000, 2000 and 3000 rpm were produced. The analysis of the joint microstructure and material flow pattern indicated that joints could be produced using all of the tested welding conditions. However, the joint produced at 1000 rpm displayed sharp weld lines (weak links), indicating insufficient heat input, while the welds produced at 3000 rpm displayed excessive plastic deformation (bulging of the bottom plate), volumetric defects and a lack of material mixing in the welded area, associated with higher heat input. The weld produced at a rotational speed of 2000 rpm resulted in improved material mixing, which was indicated by the absence of weld lines and volumetric defects due to the more correct heat input. This welding condition was selected for further mechanical testing. Lap shear testing of PMMA GS/PMMA 6 N/2 wt% SiO₂ nanocomposite single lap joints welded at 2000 rpm resulted in an average ultimate lap shear strength of 3.9 ± 0.05 MPa. These weld strength values are equal to or better than those obtained using state-of-the-art welding techniques for PMMA materials, thereby demonstrating the potential of friction spot welding for thermoplastic nanocomposites.     

Research on fatigue behavior of electron beam welding joint of 06Cr19Ni10 austenitic stainless steel sheet

06Cr19Ni10 austenitic stainless steel sheet square butt joints without filler metal addition were fabricated using EBW (electron beam welding) processes. Fatigue properties, tensile properties and microstructure of the welded joints were studied. It was found that the yield strength, tensile strength, elongation and Vickers hardness of EBW joint can reach the level of base material performance. Fusion zones consist of coarse columnar dendritic grains, which are perpendicular to the weld pool boundary. The hardness of heat affected zone is lower than weld centreline. The reason is that the grain of heat affected zone under the action of electron beam heat source happened recovery and recrystallization. The present work suggests that S–N curve slope of welding joint should be revised to m = 10 under the condition of high stress levels, and S–N curve slope is still m = 3.0 under the condition of low stress levels. Fatigue cracks did not extend along the minimum thickness of the section. On the contrary, fatigue cracks extended along the maximum height of the section.     

Microstructure evolution in heat affected zone of T4003 ferritic stainless steel

In this work, microstructural characteristics and development within the heat affected zone (HAZ) of T4003 ferritic stainless steel (FSS) welded joint were investigated combining experimental measurement with finite element simulation of welding temperature field. The results indicate that the HAZ was characterized with heterogeneous microstructure due to the extensive peak temperature range which could be divided into three sub-zones named as HAZ1, HAZ2 and HAZ3. The HAZ1 (the region next to weld zone boundary) experienced peak temperatures of 1300–1500 °C during welding process. This region presented almost fully δ ferrite microstructure with irregular grain, which was attributed to the high element diffusion rate and the absence of elevated-temperature austenite. The HAZ2 (center region of HAZ) suffered the peak temperatures of 1150–1300 °C. It presented martensite + δ ferrite dual microstructure with limited grain growth due to the formation of γ phase at grain boundaries. The HAZ3 (the region closed to the base metal) was undergone the peak temperatures of 830–1150 °C and was characterized with both martensite and ferrite structure.     

Microstructure and mechanical behaviors of electron beam welded NiTi shape memory alloys

The vacuum electron beam welding (EBW) technique was employed to weld Ni_50.8Ti_49.2 shape memory alloy sheets, and the microstructure, transformation behaviors and mechanical behaviors of the welding joints were investigated systematically. The microstructure observation showed that the weld seam was composed of coarse columnar crystals at the center and relatively fine columnar crystals near the fusion line. The abnormal high intensity of B2_2 0 0 peak in XRD patterns and preferred orientation in EBSD indicated that the grains in the weld seam have grown preferentially along the 〈1 0 0〉 crystal orientation. Differential scanning calorimetry (DSC) curves exhibited an increase of the martensite start temperature (M_s) of the weld seam, which led to the mixed microstructure of martensite and austenite at room temperature. As a result, the ultimate tensile strength of the welding joint was 85% as high as that of the base metal at room temperature, while it could reach 93% at 223 K when both the weld seam and the base metal were in pure martensitic state.     

Microstructure and interface characterization of dissimilar friction stir welded lap joints between Ti–6Al–4V and AISI 304

The feasibility of dissimilar friction stir welding (FSW) in overlap configuration between Ti–6Al–4V alloy (Ti64) and AISI 304 austenitic stainless steels (304SS) was investigated. Sound joints were achieved when placing titanium as the upper workpiece. Joints were successfully produced by employing a welding speed of 1 mm/s and rotational speeds of 300 and 500 rpm. A lamellar microstructure was formed in the stir zone of Ti64, where grain size was found to increase with increasing rotational speed, and austenitic equiaxed grains were obtained near the interface of 304SS coupon. Energy dispersive X-ray spectroscopy (SEM-EDS) of the interface revealed a thin intermixed region and suggested intermetallic compound formation. Microhardness data in the titanium weld zone for both rotational speeds exhibited slightly lower values than the base material, with the lowest values in the heat affected zone, whereas the microhardness values in the stainless steel side around the weld center were found to be higher than those obtained for the base material.