Austenitic Stainless Steel Weld Metal Research Articles

Effects of Aging at 550 °C on α′-Martensitic Transformation of High Si-Bearing Metastable Austenitic Stainless Steel Weld Metal

Effects of Aging at 550 °C on α′-Martensitic Transformation of High Si-Bearing Metastable Austenitic Stainless Steel Weld Metal

Understanding and Controlling the Weld Microstructure of Steels

In this article, selected studies are reviewed with a focus on the analysis of microstructure development in steel weld metals. In the study of austenitic stainless steel weld metals, microstructure development in the primary ferrite solidification mode (FA mode) was clarified and related to why FAmode welds are resistant to hot cracking. In studies of duplex stainless steel weld metals and high-Cr ferritic stainless steel weld metals, nitrogen-driven microstructure development and TiN-assisted grain refinement, respectively, were described, and discussions about the mechanism of equiaxed grain formation in the weld metals were added in the latter. In the study of low-alloy steel weld metals, the roles of titanium oxide and titanium nitride (TiN) inclusions on intragranular ferrite formation and the refinement of weld microstructure were described based on crystallographic analysis and the first principles calculation. At the end, the potential importance of the application of different multiscale, multiphysics simulations to welding research was pointed out.

Role of Ni content on microstructural and mechanical responses of Nb-stabilized metastable austenitic stainless steel weld metals

Three Nb-stabilized metastable austenitic stainless steel (15Cr–9Ni–Nb steel) weld metals with Ni contents of 8.4 wt%, 9.2 wt%, and 10.2 wt% were designed and prepared by gas tungsten arc welding. The effects of Ni content on the solidification behavior, microstructure, and the mechanism of deformation-induced martensitic transformation in the tensile process were investigated via scanning electron microscopy, transmission electron microscopy, scanning transmission electron microscopy, and high-resolution transmission electron microscopy, and their effects on mechanical properties were also discussed. As the Ni content increased, the solidification mode of weld metals changed from ferrite-austenite mode to austenite-ferrite mode, which made the segregation of Nb and Si at the interdendritic region more obvious and increased the total volume and the average size of primary NbC significantly. The large amount of coarse primary NbC in the AF weld metal damaged the impact toughness and reduction of area by inducing intergranular cracks and intergranular fractures. Moreover, both the three weld metals had poor mechanical stability and the deformation-induced martensitic transformation (DIMT) (γ→α′) occurred during the tensile process, which contributed to increased tensile strength. As the Ni content increased, the mechanical stability of austenite increased, and the strengthing effect of DIMT decreased, while the uniform elongation of weld metals increased.

Effect of ferrite morphology on pitting corrosion resistance of austenitic stainless steel weld metals

The effect of the ferrite morphology on the pitting corrosion resistance of austenitic stainless steel weld metals solidified with primary ferrite was investigated. Most of the pitting corrosion occurred at the interface between ferrite and austenite—particularly at the vermicular ferrite/austenite interface; less pitting corrosion occurred at the lacy ferrite/austenite interface. This was because a Kurdjumov–Sachs orientation relationship was established between lacy ferrite and austenite. Consequently, Cr carbide was less likely to precipitate owing to the low interfacial energy, and the lacy ferrite/austenite interface was less likely to be an initiation site of pitting corrosion.

Mechanism of δ-ferrite decomposition in high Si-bearing austenitic stainless steel weld metal during aging at 550°C

The 15Cr-9Ni-Nb austenitic stainless steel weld metal with 3.5 wt% Si content was designed and prepared via tungsten inert-gas arc welding and then stabilized heat treatment (SHT) at 900 °C for 3 h. The mechanism of δ-ferrite decomposition during the aging process at 550 °C was investigated for both the as-welded (AW) and the SHT weld metals via scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and high-resolution transmission electron microscopy (HRTEM), and its impact toughness effect was also discussed. The original δ-ferrite for both the AW and SHT weld metals decomposed to the σ-phase, A16Mn6Si7-type G-phase, and more stable δ-ferrite during aging. The coarse σ-phases were distributed at the δ/γ interface and the relatively fine G-phases (A16Mn6Si7) were at the δ/γ interface or inside the δ-ferrite. Si promoted the formation of the G-phases and σ-phases during aging, consumed a large amount of Cr, Ni, Si, and Mn from the δ-ferrite, and stabilized the remaining δ-ferrite. The impact toughness of the AW weld metal sharply dropped during aging due to the coarse σ-phases promoting the formation and propagation of intergranular cracks. The SHT reduced the δ-ferrite in the weld metal, thus avoiding the formation of a large number of σ-phases during aging and therefore improving the impact toughness.

Effect of surface machining on the environmentally-assisted cracking of Alloy 182 and 316L stainless steel in light water reactor environments: results of the collaborative project MEACTOS

Abstract The main objective of the EU-funded project mitigating environmentally-assisted cracking through optimisation of surface condition (MEACTOS) was to gain knowledge on the ability of different surface machining procedures to mitigate environmentally-assisted cracking (EAC) in typical light water reactor structural materials and environments. Surfaces of cold-worked (CW) type 316L austenitic stainless steel and nickel-based weld metal Alloy 182 flat tapered tensile specimens were machined using different processes. EAC initiation susceptibility of these specimens was evaluated using constant extension rate tensile (CERT) tests under simulated boiling water reactor (BWR) and pressurized water reactor (PWR) conditions and assessed using constant load experiments. More than a hundred tests were performed covering about 10 years of autoclave testing time. Only minor or no measurable improvements in EAC initiation susceptibility as a function of surface treatments (grinding or advanced machining) compared to the standard industrial face milling were demonstrated. In most cases, the stress thresholds for EAC initiation determined in constant load tests confirmed the trend obtained from CERT tests. This paper summarises the most important results and conclusions concerning the EAC initiation behaviour for the CW 316L and Alloy 182 under reducing PWR and oxidizing BWR conditions.

Effects of Stabilization Heat Treatment on Microstructure and Mechanical Properties of Si-Bearing 15Cr–9Ni–Nb Austenitic Stainless Steel Weld Metal

Effects of Stabilization Heat Treatment on Microstructure and Mechanical Properties of Si-Bearing 15Cr–9Ni–Nb Austenitic Stainless Steel Weld Metal

Effect of electrode flux composition on impact toughness of austenitic stainless-steel weld metal

The aim of this investigation was to determine whether the composition of a shielded-metal arc-welding electrode coating affected the low-temperature impact toughness of austenitic stainless-steel weld metal. It is generally accepted that increases in the 5-ferrite and nitrogen contents result in a decrease in toughness at low temperatures. Weld metal from electrodes with a basic coating also generally exhibit better toughness than those from rutile (acidic) electrodes. An increase in basicity was expected to decrease the number and size of inclusions, which in turn provides a tougher weld metal. Three commonly available potassium-rutile E308L electrodes were used, complying with the E308L-16 and E308L-17 specifications. Analysis of the electrode coatings showed very similar chemistry and basicity. Significant differences in the inclusion contents of the weld metals were observed: the E308L-17 weld metal had a lower inclusion content (1.4% by volume) than the E308L-16 weld metal (3.7%). The former had higher impact toughness at all temperatures, despite a slightly higher nitrogen content. Regression analysis confirmed that the inclusion content had a significant effect on the impact toughness at all temperatures.

Microstructural evolution and stress relaxation cracking mechanism for Super304H austenitic stainless steel weld metal

• Stress relaxation cracking in Super304H austenitic stainless steel weld metal. • The pre-compressed CT technique combined with FEM analysis was successfully used. • The mechanism responsible for stress relaxation cracking formation was illustrated after short-term aging and long-term aging. The pre-compressed CT technique was used to quantitatively investigate the formation of stress relaxation cracks under different tensile residual stresses and aging time in Super304H austenitic stainless steel weld metal. The statistical results revealed that intergranular cracks could occur within 2000 h under 650 °C when the residual stress was applied with greater than 18 KN pre-compression force. Detailed grain interior and boundary analyses showed that the growth of intragranular Cu-rich particles could induce a strong grain interior, and the intergranular Nb(C, N) carbides were one of the causes to crack under short-term aging time. For long-term aging time conditions, the intergranular M 23 C 6 carbides were more susceptible to crack than intergranular Nb(C, N) carbides. Finally, the mechanism responsible for stress relaxation cracking formation was carefully illustrated for the weld metals after short-term aging and long-term aging, respectively.

Effect of Nb on the microstructural evolution and microhardness of Super304H austenitic stainless steel weld metals during long-term aging

Abstract The microstructural evolution and mechanical properties of Super304H austenitic stainless steel weld metals with Nb-free and 1.0 % Nb-additive were investigated during 650 °C long-term aging. Massive nanoscale M 23C6 particles distributed in the interdendritic regions of the Nb-free weld metal while most of the M 23C6 formed and coarsened on the phase interface between the primary Nb(C, N) phase and matrix in the Nb-bearing weld metal. Nano-sized Cu-rich particles dispersed inside the grains in both weld metals. All of these nano-sized particles gradually coarsened over time. The Cu-rich particles with high distribution density and nanometer size affect the microhardness of both weld metals most during long-term aging. The addition of Nb could increase the microhardness in the relatively short aging time, while the effect of Nb-addition on the microhardness in long-term aging is insignificant.

Intergranular precipitation behavior and its influence on the stress relaxation cracking susceptibility of Super304H austenitic stainless steel weld metal during long-term aging

Stress relaxation cracking failure is widely reported in the materials used for thermal power plants, which seriously affected the safety of thermal power units under long-term service. In our study, the influence of intergranular precipitation behavior on the stress relaxation cracking susceptibility of Super304H austenitic stainless steel weld metal was investigated based on the pre-compressed CT technique. The 45° misorientation grain boundary has the lowest critical interfacial nucleation energy, followed by the grain boundaries away from the 45° misorientation. As a result, the coarse intergranular M23C6 carbides were more prone to precipitate at 45° misorientation grain boundary, which led to the lowest resistance to stress relaxation cracking susceptibility at 35 to 45° grain boundary. Further analysis indicated that intergranular cavities were easily initiated at the non-coherent M23C6/γ interface. Since the non-coherent interface with a low Cr concentration zone and high specific interfacial energy had a lower strength, the lattice distortion at the non-coherent interface could lead to higher strain concentration.

Investigation on effect of thermal aging embrittlement on residual stresses of austenitic stainless steel repair welds in nuclear components subject to seismic loads

This paper presents investigation results on effect of thermal aging embrittlement on residual stresses of austenitic stainless steel repair welds in nuclear components subject to seismic loads. First, the welds on austenitic stainless steel 316L plates were manufactured consisting of the austenitic stainless steel weld metal with the δ—ferrite content sensitive to the thermal aging embrittlement. One of the welds was embrittled via conducting thermal aging acceleration test. Second, validity of finite element weld residual stress analysis was verified by performing finite element residual stress analysis and holedrilling method for the unrepaired welds. Third, effect of the thermal aging embrittlement on the residual stresses of the repair welds was evaluated by comparing the residual stresses according to whether the original welds were thermally embrittled. Last, effect of seismic loads was reviewed for the case where the repair welding was performed on the original welds being thermally embrittled. As a result, it is found that the effect of the thermal aging embrittlement on the residual stresses of the unrepaired welds is insignificant whereas tensile residual stresses of the repair welds increase in proportion to the increase in yield strength at room temperature due to the thermal aging embrittlement. In addition, the residual stresses are significantly mitigated after the seismic loads.

Proton irradiation effects on the electron work function, corrosion and hardness of austenitic stainless steel phases

The effects of proton irradiation on the electron work function of the 308 L austenitic stainless steel weld metal were investigated by scanning Kelvin probe force microscopy. The electron work function of ferrite increased, while austenite decreased after proton irradiation on the irradiated surface. The electron work function of the two phases decreased with irradiation depth. Softening of austenite and hardening of ferrite by irradiation with an obvious boundary between the irradiated and unirradiated regions on the polished cross-section surface were measured by nanoindentation. The corrosion morphology was consistent with the electron work function measured in the cross-section.

Effect of Electrode Coating on Austenitic Stainless Steel Weld Metal Properties

The effect of electrode coating on austenitic stainless steel weld metal properties was studied. Manual metal arc welding method was used to produce the joints with the tungsten inert gas welding serving as the control. Metallographic and chemical analyses of the fusion zones of the joints were conducted. Results indicate that the weldment produced from E 308-16/12 lime-titania electrode has a higher ductility and strength of about 36% in terms of percentage elongation and 517 N/mm2respectively, compared to 26% and 18% and 475 N/mm2and 425 N/mm2respectively, obtained from weldments produced from E 308-16/10 rutile and E 308-16/12 rutile electrodes respectively. The presence of lime which is a slag former in E 308-16/12 lime-titania electrode was relevant in slowing down the cooling rate of both the weld pool and the just solidified weld metal resulting in the overall improvement of the resultant weld metal properties. It was found that the values of the strain hardening exponent were 0.379 for E 308-16 gauge 10, rutile electrode, 0.406 for E 308-16 gauge 12 rutile electrode, 0.382 for TIG welding, 0.353 for E 308–16 gauge 12, lime-titania electrode, 0.435 for E 310-16 gauge 10, rutile electrode. E 310 – 16 gauge 10, rutile electrode had the greatest strength and strain hardening coefficients of 1180 N/mm2and 0.435 respectively, and will be more amenable to cold working. Keywords: Austenitic stainless steel, microstructure, electrode coating, welding, joints.

Assessment of Thermal Aging of Austenitic Stainless Steel Weld Metal by Using the Double Loop Electrochemical Potentiokinetic Reactivation Technique

Double loop electrochemical potentiokinetic reactivation (DL-EPR) was applied to evaluate thermal aging of 308L stainless steel weld metal. It was found that the activation and reactivation peaks of DL-EPR curve were induced by dissolution of austenite and δ-ferrite, respectively. Before saturation of hardness, the linear relationship between reactivation ratio and hardness could be used for assessing the thermal aging-induced hardening. In the following thermal aging process, the reactivation ratio is applicable to assess the occurrence of the saturation of thermal aging-induced hardening. The results demonstrated that the DL-EPR test is applicable to assess the evolution of thermal aging.

Irradiation-induced precipitation and inverse coarsening of G-phase in austenitic stainless steel weld metal

The irradiation-induced precipitation and inverse coarsening of G-phase in austenitic stainless steel weld metal were studied by using transmission electron microscopy and atom probe tomography. Following 0.5-dpa (displacement per atom) irradiation, Ni/Si/Mn/P-enriched G-phase was observed in delta-ferrite. By increasing the irradiation dose from 0.5 to 3 dpa, coarsening of G-phase occurred at first, followed by an obvious inverse coarsening. The segregation of Ni and Si atoms at α/α′ interface due to spinodal decomposition was directly observed, which contributed to the G-phase precipitation. The inverse coarsening of G-phase was caused by irradiation recoil and could be interpreted by the cell model.

An assessment of impact toughness of the pressure vessel steel A533-B and its 308 austenitic stainless steel weld metal

The steel A533-B is a low alloy steel used as a structural material for the construction of light water nuclear reactor pressure vessels. It is normally cladded with the 308 austenitic stainless steel weld metal overlay for protection against extension of surface flaws generating during operation. In this investigation the impact toughness of the A533-B steel and its 308 weld metal were evaluated. The microstructure of the A533-B steel consisted of bainitic structure while that of the 308 weld metal comprised austenite with ferrite phase of about 5%. The impact toughness was assessed using instrumented impact testing machine at temperatures between -196 and 300 C. The determined impact toughness parameters involved the total impact fracture energy, the energy expended in the fracture crack initiation and crack propagation, the ductile to brittle transition temperature as well as the dynamic yield strength. The A533-B steel exhibited distinct ductile to brittle transition behavior with superior resistance to ductile fracture at high test temperatures. In contrast, the 308 weld metal did not display such transition. However, it showed low energy ductile fracture performance with evident resistance to brittle fracture even at low test temperatures. The findings were substantiated with the load-time traces derived from the instrumented impact tests as well as the fracture surface morphology. The results were discussed in relation to the difference in microstructure and flow properties of the examined steels.

The influence of niobium on the plastic deformation behaviors of 310s austenitic stainless steel weld metals at different temperatures

The deformation behaviors of 310s stainless steel weld metals (WMs) with different Nb contents are investigated under uniaxial tension at temperatures ranging from ambient temperature to 1000 °C. The results show that Nb effectively improves the strength of the WMs in the whole test temperatures range. The addition of Nb causes a change in the fractures of the WMs from intergranular brittle fractures to intragranular ductile fractures at elevated temperature. Hence, elemental Nb enhances the elevated temperature plasticity of the WMs. However, as the eutectic Nb(C, N) promotes the nucleation and propagation of the cracks, Nb decreases the elongation of the WMs at temperature below 700 °C. With the increase in the deformation temperature, the strength of the WMs decreases monotonously. However, the elongation of the WMs shows a nonmonotonic relationship with the temperatures. The deformation twins can occur at room temperature and improve the strength and elongation of the WMs. Meanwhile, the cracks nucleate around the eutectic Nb(C, N) at 1000 °C, leading to the minimum elongation of the Nb-bearing WMs in the whole test temperature range. The differential scanning calorimetry result suggests that the melting of the eutectic Nb(C, N) may be the dominant reason for the nucleation of the cracks and result in the reduction in the elongation of the Nb-bearing WM at 1000 °C.

Effect of aging treatment on the microstructures and mechanical properties evolution of 25Cr-20Ni austenitic stainless steel weldments with different Nb contents

The microstructure evolutions and the mechanical properties of the 25Cr-20Ni austenitic stainless steel weld metals with different Nb contents were investigated during the long term aging treatment at 700 °C. M23C6, Nb(C, N), α-Cr phase and Nb-nitride phase (Z phase) were observed in the microstructures of the aged weld metals. The results showed that the α-Cr phase precipitated in the interdendritic regions of the weld metals after being exposed to 700 °C for 500 h and the element Nb accelerated the precipitation of the α-Cr phase significantly. The density of the α-Cr phase decreased with the increase of the distance away from the primary Nb(C, N). Additionally, the α-Cr phase showed a crystallographic relationship with the austenitic matrix, [11¯1¯]α-Cr // [11¯0]γ and (011¯)α-Cr // (1¯1¯1)γ. It was observed that the Z phase precipitated in the periphery of the Nb(C, N) and may replace the Nb(C, N) after long term exposure to high temperature. The transformation of the Nb(C, N) into Z phase suggested that the Z phase had a higher stability than the Nb(C, N) particles at 700 °C for long term aging. The tensile strength of the Nb-bearing weld metal showed a continuous decrease at the initial stage of the aging treatment and then went up slightly with the prolonged aging time. However, the elongations and the impact energies of the weld metals decreased monotonously with the increase of the aging time.

On Characterizing a Composite Microstructure in 316LN Stainless Steel Weld Metal and a New Damage Micromechanism During Creep

The austenitic stainless steel weld metal fabricated by multipass welding exhibits a composite microstructure. Microstructural characterization of the weld metal revealed that there are two distinct regions on either side of the weld-pass interface. The variations in dislocation substructure and delta ferrite morphology are the two microstructural attributes which delineate these regions. The generation of subsequent thermal cycles during the fabrication of multipass weld joint is the paramount factor influencing the formation of the composite microstructure. During creep exposure, the extent of creep cavitation and propagation varies substantially in these two regions due to differences in their microstructures. This results in preferential damage during creep exposure of austenitic stainless steel weld metal.