Stainless Steel Joints Research Articles

Underwater wet laser welding of duplex stainless steel under various water depths

Reports on experiments of underwater wet laser welding (UWLW) are scarce because of the poor forming quality of weldments. To address the key manufacturing challenges such as failure of welding with water depth exceeding 7 mm and the generation of pores and cracks which caused by the rapid water cooling, a specific self-designed flux assisted welding at different water depths (0–30 m) was innovatively proposed in this study. The microstructure, phase distribution, and properties of underwater wet laser welded duplex stainless steel joints were examined. The welding process successfully produced joints free from pores or cracks, even when performed within a 15 m water depth. In conjunction with thermodynamic calculation, the evolution process of welds and the effect of nickel from the flux on phases structure were analyzed. As the water depth increases and the welding speed decreases, the number of pores inside welds increase significantly, and no cracks are found. The penetration of all welds made in deep water is obviously smaller than that of shallow water and increased heat input does not seem to contribute to the increase of weld penetration. With the increase of water depth, a large amount of coarsened austenite consisted of columnar grains and cellular grains appear in weld zone. Texture and dislocation of the weld made in shallow water were investigated. Many dislocations including edge dislocations and dislocation entanglements arranged in austenite phase, while seldom dislocations appear in ferrite phase.

Study on the Microstructure and Mechanical Properties of Ultrasonic-Assisted TIG Welded 441 Ferritic Stainless Steel Joints

A self-designed ultrasonic-assisted welding platform was built to improve the poor microstructure and properties of conventional TIG welded ferritic stainless steel. The ultrasonic vibration was transmitted to the weld pool through the base metal in the manner of point–surface contact in the optimal position after calculation. The results show that the coarse columnar grains in the welded joint can be transformed into very fine equiaxed grains under ultrasonic vibration, especially the coarse columnar grains near the fusion line where cliff-like refinement occurs. The maximum grain size in the weld seam is reduced from 420 μm to 260 μm, and the average size is reduced by 60%. At the same time, the grain orientation tends to be harmonized. The microhardness of the welded joint is greatly improved on the whole, and the softening of the heat-affected zone caused by grain coarsening is effectively inhibited. Compared with the welded joints without ultrasonic assistance, the tensile strength and yield strength can be increased by 61 MPa and 47 MPa, respectively, under 130 W ultrasonic vibration. By strengthening the weak part of the welded joint, the weldability and toughness reserve of 441 ferritic stainless steel can be significantly improved.

Influence of iron on the structure and strength of the Kovar to stainless steel joints by using Cu–Mn–Co–Fe brazing filler metal

Influence of iron on the structure and strength of the Kovar to stainless steel joints by using Cu–Mn–Co–Fe brazing filler metal

Improving Plasticity of Ferritic Stainless Steel Welded Joints via Laser Spot Control.

The plasticity of welded 441 ferritic stainless steel joints was controlled by varying the laser beam spot diameter during laser welding. A stainless steel plate thickness of 1.2 mm was used. The microstructures of the welded joints were analyzed for various spot diameters. The elongation of breaks and the reduced area of tensile specimens were analyzed to study the effects of spot diameter on plasticity of the welded joints. The results showed that the weld melt width and weld column crystal size increased with the spot diameter, but isometric crystals in the center of the weld were gradually reduced. Increasing spot diameter resulted in decreased tensile strength, elongation after breaks, and area reduction. So the parameter must be controlled during the laser welding.

Unveiling the effect of Ni interlayer on diffusion bonded reduced activation Cr0.8FeMnNi medium entropy alloy/316 stainless steel joint: Interfacial microstructure, diffusion behavior and mechanical properties

This study explored the diffusion bonding of Cr0.8FeMnNi medium entropy alloy to 316SS with and without a Ni interlayer using spark plasma sintering. Direct bonding led to the formation of Cr2O3 and Mn3O4 oxides at the interface, resulting in stress concentration and reduced tensile strength. However, the introduction of a Ni interlayer effectively reduced oxide size and improved mechanical properties by accelerating diffusion and relieving stress concentrations. Optimal joints were achieved at 820 °C / 60 min / 25 MPa, producing joints with tensile strength and elongation comparable to 316SS. Furthermore, diffusion behavior, growth kinetics of diffusion layers, and activation energy of diffusion in various atoms and layers were analyzed. The activation energy of each element in the joints with Ni interlayer was decreased compared to direct diffusion bonding. This research highlights the significance of interlayer in enhancing diffusion bonding outcomes for nuclear power applications.

Numerical investigation of the influence of friction stir welding parameters on the microstructure of AISI 410S ferritic stainless steel joints

Stainless steels are widely used in industry due to their corrosion resistance properties. Ferritic stainless steels are cheaper than austenitic steels because the former do not contain nickel, an expensive alloying element. However, when ferritic stainless steels are used in manufacturing industries, they undergo microstructure transformations that can result in corrosion. Furthermore, the conventional welding processes that are most commonly used can also cause microstructural changes, such as high grain growth, due to the high thermal inputs applied, and precipitation of deleterious phases. On the other hand, Friction Stir Welding (FSW) avoids such problems due to its intrinsic features and has, therefore, become a viable option for the welding of ferritic stainless steels. This technique imposes high deformation rates, enough heat to soften the material and provides the necessary conditions to promote dynamic recrystallisation and, consequently, prevent the grain growth that occurs in conventional welding. The current investigation is clearly justified due to the vast potential of ferritic stainless steels in the industry using FSW. The main goal of this work is to simulate the FSW process, analyse the possible structural changes in the material and provide the factors that most influence the quality of welds when using this process, such as temperature, strain rate, and changes in the viscosity of the material. The results were able to predict the regions with the smallest grain sizes due to dynamic recrystallisation. Furthermore, the calculated Zener-Hollomon values could be directly linked to the grain size: when the Zener-Hollomon parameter increases the grain size decreases.

Experimental investigation of microstructural, mechanical and corrosion properties of 316L and 202 austenitic stainless steel joints using cold metal transfer welding

The objective of the current experimental study is to investigate influence of dissimilar welding between 316 L Austenitic and 202 Austenitic Stainless Steel. This is achieved through the utilization of Cold Metal Transfer (CMT) Welding in conjunction with the GTAW mode. Various welding approaches are employed, including the use of ER316L and ER309L fillers, as well as a no-filler (autogenous) welding technique. Microstructural observation shows martensitic formation in the interface region. The tensile test was conducted by using Universal Testing Machine for the weldment and found that ER316L filler (721 MPa) has higher strength than ER309L (672 MPa) and autogenous weld (528 MPa). Charpy impact test was used to determine toughness of the weldment and found that ER309L Filler weld has toughness (87 J) which is higher than ER316L filler weld (5 4 J) and autogenous weld (23 J). Vicker's hardness test shows that autogenous weld hardness (avg. 193.34 HV) which is highest than ER309L filler (avg.188 HV) and ER316L filler weld (avg. 184.67 HV). The intergranular corrosion test by double loop electrochemical potentiokinetic reactivation (DLEPR) test shows sensitization at highest degree for autogenous weld (18.25 %) than ER316L filler weld (9.22 %) and ER309L filler weld (7.28 %).

Microstructure and mechanical properties of AISI 410S ferritic stainless steel narrow gap weld using flux strip constraining arc welding

The achievement of high comprehensive mechanical properties in welded joints of ferritic stainless steels has attracted considerable attention in the manufacturing industry. Here, a low heat input flux strip constrained arc welding (FSCAW) was performed on a hot-rolled AISI 410S sheet. Defect free joints with a small HAZ width were obtained by one side welding both sides formation. The microstructure of HAZ consists of fine ferrite grains and a small amount of M23C6-decorated martensite due to short high-temperature residence time and rapid cooling rate, resulting in high toughness and hardness. Influenced by the stirring effect of the constraining arc and the promotion of heterogeneous grain nucleation by Ti-rich MX particles originating from the transition of alloying elements in the flux strip, the fusion zone (FZ) exhibits a unique precipitates-strengthened martensite+austensite uniform microstructure with weak martensitic texture. This translates into a simultaneous increase in impact toughness and hardness of FSCAW FZ compared to the MAG joint with a non-uniform mixed microstructure, strong martensitic texture, and large grain size. This work establishes the potential for using FSCAW welding to join ferrite stainless steels.

Performance Evaluation of Underwater (UW) Welding System with Friction Stir Welding (FSW) Method on 316L Stainless Steel Exposed to Sea Water

Friction Stir Welding (FSW) is a solid-state welding technique that can produce high-quality joints without defects that commonly occur in liquid welding. However, FSW has limitations in underwater applications due to the friction between the tool and the material that causes a decrease in heat efficiency and shear force. To overcome this problem, this study proposes an underwater welding (UW) system with the FSW method on 316L stainless steel exposed to seawater. This UW-FSW system uses a high-speed rotating tool surrounded by an inert gas shroud to prevent direct contact between the tool and seawater. The aim of this study is to evaluate the performance of this UW-FSW system in terms of mechanical properties, microstructure, and corrosion resistance of 316L stainless steel joints. The results of this study show that the UW-FSW system can produce joints with better tensile strength, hardness, and corrosion resistance than conventional FSW. This is due to the rapid cooling, reduction of oxidation, and formation of a thicker passive layer on the joint surface. This study provides a contribution to the development of environmentally friendly and efficient underwater welding technology.

A 2D and 3D segmentation-based microstructure study on the role of brittle phases in diffusion brazed AISI 304L/NiCrSiFeMoB joints

Nickel-based filler metals are frequently used in high temperature vacuum diffusion brazing for austenitic stainless-steel joints when components are subjected to high static or dynamic loads, corrosive environments and elevated temperatures. Due to melting point depressing metalloids such as silicon and boron, hard and brittle intermetallic phases are formed during the brazing process depending on the diffusion mechanisms. These brittle phases significantly affect mechanical and corrosive properties of the compounds. To quantify the influences of their amount, morphology and distribution, deep learning image segmentation was applied to segment these phases of the athermal solidification zone and the diffusion zone. Subsequently, characteristic microstructure parameters were calculated from these. The parameters of six different brazed joint variations were compared with their experimental characterization of mechanical and corrosive properties so that several correlations could be identified. Finally, a layer-by-layer removal of a brazed joint was performed using a focused ion beam, and a 3D model was reconstructed from the generated images to gain a mechanism-based understanding beyond the previous 2D investigations.

Hardness joint distribution as a function of the bevel angle in stainless steel joints

Hardness joint distribution as a function of the bevel angle in stainless steel joints

Distortion in laser micro welding of stainless steel SS304 foils

Stainless Steel 304 is a fine material selection for industrial usage in laser micro welding for e-mobility applications wherein battery components are fabricated. Laser micro welding whilst very precise can produce good quality welds for thick sheets, and the risk of distortion is minimal. However, when the thickness of the sheets was reduced to a few hundred micrometres, this is where the challenge of joining emerged. Hence, this study focuses on identifying the defects that were formed by laser micro welding parameters on the geometry of the joint of Stainless Steel 304 of 100 µm thickness. Wobbling diameter and distance varied between 0.2 mm and 0.5 mm. Sample No. 5 has a higher angle distortion which was about 25° on the right side, while 11° on the left side. Whereas Sample No. 6 and No. 9 manifested no angle distortion at all. The Laser marking loop can significantly influence angle distortion on the sample that is being welded.

Fatigue life evaluation for ring-welded lap joints of stainless steel based on fracture mechanics approach

This study aims to evaluate the fatigue life of ring-welded lap joints of stainless steel with various geometries. It involves welding morphologies observations, static and fatigue tests under tensile-shear loading, and fatigue life evaluation based on fracture mechanics approach. Test results show that the interfacial failure occurs for each specimen under the static loading, and the static strength increases with an increase in weld area, as well as the degree of bending deformation during the test. In addition, three failure modes were identified under fatigue loadings, with crack propagation from the weld root to the weld metal surface accounting for over 90% of the fatigue life for most specimens. Taking into account the fatigue failure modes, a fracture mechanics-based method was used to unify the Fa-Nf curves of ring-welded lap joints with different geometries. This unified curve considers the effects of specimen geometries, crack length, loading ratio, and bending deformation. The analysis demonstrates that the fatigue life of ring-welded lap joints increases with an increase in the thickness of the sheet with a hole and the hole diameter, while decreases with the increasing loading ratio and bending deformation. By utilizing this method, the predicted fatigue life of various ring-welded lap joints can be controlled within a range that is four times of the measured values.

Effect of welding current on the microstructural and corrosion behavior of GTA welded austenitic stainless steel joints

In the present study, effect of welding current was analysed on the microstructural and corrosion properties of 316L austenitic stainless steel (ASS) joints welded by gas tungsten arc welding (GTAW). Three welding current were chosen from the GTAW process's operational range and welded joints fabricate using these currents were exposed to microstructural, microhardness as well as corrosion evaluations. Microstructural examination showed the lathy and vermicular morphology of δ-ferrite in the weld metal microstructures while different degrees of grain coarsening in the HAZ microstructures were seen. DLEPR method was used to assess the welds' resistance to corrosion, which depicted that high welding current specimen revealed relatively higher value of DOS (indicate higher amount of corrosion rate) as compared to low welding current specimen. This was due the fact that vermicular morphology of δ-ferrite was present in the high welding current specimens, which promoted the corrosion rate. However, lathy morphology of δ-ferrite was found in the low welding current specimens, which demotes the corrosion rate.

Mechanical characteristics and stretch-bend failure analysis on ultra high frequency pulsed gas tungsten arc welded thin FSS 409/430 dissimilar joints

The Mechanical and Stretch-Bend Failure studies on Ultra High Frequency Pulsed Gas Tungsten Arc Welded dissimilar joints of AISI409-AISI430 Ferritic Stainless Steels were conducted. Welding was conducted with 5 ultra high frequencies (50 Hz, 150 Hz, 250 Hz, 350 Hz, 450 Hz). Mechanical characteristics evaluation on the joints included tensile strength, microhardness variations across the welds and creep. Microstructural and metallurgical investigations included weld cross section evaluation, comparing grain variations in high, medium and low thermal heat affected zones, weld zones and base material region. Stretch bend failure studies included studies on angular distortion, fracture limit strain, and coefficient of friction. Tests revealed that joints welded at 350 Hz was better, compared to other joints. Dissimilar AISI409-AISI430 joint fabricated at 350 Hz exhibited 267 ± 3 MPa as yield and 409 ± 6 MPa and as ultimate tensile strength. Its creep fracture duration was 72.7 min (highest among the joints). Microstructural studies revealed grain growth, partially coarse and partially fine grains in heat affected zones. Depending on the difference in grain sizes, on both sides of the welds, heat affected regions were identified as three distinct zones. In AISI430 side; high temperature austenitic, martensitic, delta ferrites and in AISI409 side; needle like martensitic structures, mixture of ferritic-austenitic, δ-ferrite with carbide precipitation were found in high, medium and low thermal heat affected zones, respectively. On increasing the ultra high frequency pulses, angular distortion increased, fractures changed from tensile/shear type to mixed type. In shear bend tests, on increasing the ratio of radius: thickness, fracture limit strain on outer surface, across sheet thickness, due to stretching increased.

Effect of post-welding sensitisation on the degree of sensitisation of the welding zones of AISI 304 resistance spot welding joints studied by using an electrochemical minicell

ABSTRACT In resistance spot welding (RSW) joints of austenitic stainless steel (ASS), a small-scale electrochemical cell (minicell) was used for assessing individually, on each of the three welding zones, of size less than 1000 µm (fusion zone (FZ), heat affected zone (HAZ) and base metal (BM)), the combined effect of a RSW process and post-welding sensitisation on the degree of sensitisation (DOS). The results show that the three welding zones have different microstructures that make each of them respond differently to post-welding sensitisation. The DOS varies with post-welding sensitisation time in all three welding zones, but it varies at a different rate in each welding zone (the highest rate in the FZ). This variation is due to the fact that when the DOS reaches a certain level, which is observed when plotting the reactivation charge (Qr) versus the post-welding sensitisation time, a microstructural regeneration occurs.

Study on the microstructure and mechanical properties of hybrid laser + MIG welded joints of 316LN stainless steel

Study on the microstructure and mechanical properties of hybrid laser + MIG welded joints of 316LN stainless steel

Exploring the Impact of Cooling Rate on Microstructural Features, Mechanical Properties, and Corrosion Resistance of a Novel Nb-Stabilized Super Duplex Stainless Steel in Shielded Metal Arc Welding

The corrosion and mechanical response produced by quenching in the welded joint of a new Nb-doped stainless steel designed by the CALPHAD method and produced by open-atmosphere casting with recycled materials were investigated to contribute to the circular economy and to establish disruptive manufacturing criteria based on metallurgical principles. The steel was initially subjected to solubilization heat treatment and partial solubilization treatment at 1090 °C to obtain an appropriate α/γ balance and carbide solubilization. It was then welded by the SMAW process, quenched, and tempered at three different cooling rates. As a result, a good fit between the phases predicted by the CALPHAD method and those observed by X-ray diffraction and scanning electron microscopy were obtained, with minor differences attributable to the precipitation and diffusion kinetics required for dissolution or nucleation and growth of the phases in the system. The forced air quenching mechanism was identified as providing an α/γ phase equilibrium equivalent to 62/38 as the most effective quenching method for achieving the optimum mechanical and corrosion response, even with the post-weld σ phase and showing superior results to those of the base metal. The outstanding mechanical and corrosion responses resulted from a proper balance of the primary phases in the duplex steel with a precipitation-strengthening mechanism. The damage tolerance obtained by forced air quenching was superior to that obtained by water and air quenching, with a PSE of 24.71 GPa% post-welding.

Microstructure and Pitting Corrosion Characteristics of Tig Welded Joints of Super 304HCu Austenitic Stainless Steel

Abstract Super 304HCu is an advanced ultra-super critical (A-USC) boiler grade austenitic stainless steel with the distinct addition of 3 wt.-% of Copper. A-USC power plants intended to operate in chloride rich environments (sea shore, feed water residues, etc.) are susceptible to chloride assisted corrosion failures. In this study, the pitting corrosion behaviour of the Super 304HCu parent material and tungsten inert gas weld joints was studied using a potentiodynamic cyclic polarization test in 3.5 % NaCl solution at three different pH levels (pH = 3, pH = 7, and pH = 11). The Epit values of the parent material is found to be much nobler than that of the weld joints. The micrographs of the pitted weld joints and the oxalic acid etched structure of Super 304HCu joints are presented. From the micrographs it is revealed that the heat affected zone is the most susceptible region to pitting corrosion.

Effect of Brazing Fillet on the Microstructure and Mechanical Properties of Vacuum Brazing Stainless Steel Joints

The enhancement mechanism of the fillet on brazing joints is of great significance for vacuum brazing technology. Although a lot of research on the enhancement mechanism of the fillet has been carried out, some key components of a comprehensive systematic enhancement mechanism for brazing fillets have yet to be established. In this paper, the enhancement mechanism for brazing fillets of SS304/pure copper brazing joints was studied by both experimental and numerical simulations. The SEM and tensile experiments were used to characterize the microstructure and shear strength of the brazing joints. The results show that the brazing joints, using 60 µm thick pure copper filler metal, exhibit a good microstructure in the brazing seam; however, its mechanical properties are lower than those found in specimens with a thickness of 90 µm. The fracture behaviors of brazing joints were also investigated, the fracture of the brazing seam was a fracture of mode II due to shear stress, while the fracture of the brazing fillet was caused by a combination of tensile stress and shear stress (mode I and mode II).