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This paper describes a high-current Gas Metal Arc Welding (GMAW) process using a constant voltage power source developed for high-efficiency welding of thick stainless steel plates. A short arc len...

The formation mechanism of microstructure of duplex stainless steel weld metals with different N content and heat input and the effects of microstructural factors on pitting corrosion resistance of...

Solidification cracking has been a serious problem in austenitic stainless steels. Since solidification cracking often occurs at the solidification grain boundary, the grain boundary character distribution (GBCD) in the weld metal may affect the solidification cracking susceptibility. On the other hand, the columnar grains of the weld metal are formed by the epitaxial growth from the grains in the base metal. Therefore, it is expected that microstructure and cracking susceptibility of the weld metal can be reduced by controlling the microstructure of the base metal. In this study, the effect of microstructure of the base metal on the microstructure and solidification cracking susceptibility of the weld metal was examined. The GBCDs of the base metal of the type 310S stainless steel were changed by the thermomechanical process. The solidification cracking susceptibility was evaluated by the Trans-Varestraint test. The thermomechanically processed specimen had larger ratio of coincidence site lattice (CSL) boundaries compared with those of the as-received specimen. Moreover, the specimen was found less susceptible to solidification cracking than was the as-received material. The GBCD analysis indicated that the complex shape and distribution of the grain boundaries in the thermomechanically processed specimen affect the initiation and propagation of solidification cracks.

The relationship between grain boundary segregation behaviour of sulphur and intergranular fracture in 10% nickel steel weld metal solidified with austenite single phase was investigated. In the wel...

The 355 aluminium alloy is known to have excellent thermodynamic characteristics that render it suitable as a raw material for rheocasting and thixoforming. However, besides the controllable transition from solid to liquid phase, the refined microstructure required in the semisolid range is one of the key factors with a strong influence on the rheology of the material. This paper intends to analyse the in situ behaviour of the microstructure, in terms of morphological change, using high-temperature laser scanning confocal microscopy. The 355 alloy was prepared via conventional casting, and refined with a 30-s exposition via ultrasonic melt treatment (UST - 20 Hz, 2 kW). The material was reheated up to the thixoforming target temperature of 595 °C at which it was maintained for 0, 30, 60, 90, and 120 s, after which all the samples were cooled in water. The samples subjected to UST prior to the heat treatment were more refined in terms of microstructural evolution; they exhibited reduction in grain size (~107 ± 16 μm), smallest primary phase particle size (~81 ± 7 μm), and high circularity shape factor (~0.59 ± 0.19 μm). In situ observation methods were employed to analyse evolution mechanisms such as Ostwald ripening and coalescence.

The nanoscale structure of ferrite in a 22% Cr duplex stainless steel weld metal (fabricated with grade 2209 filler wire) aged at 400 °C was characterized via scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy through the Super-XTM detector; the results were compared with those of a corresponding grade base metal (2205). The weld metal-ferrite exhibited a more pronounced spinodal decomposition than the base metal-ferrite because of its higher Ni content. Moreover, the presence of Ni-rich solute clusters with a body-centered-cubic structure was notably evident in the weld metal-ferrite. The investigation of the precipitation hardening mechanism using the Ardell model confirmed that, besides the extended spinodal decomposition, Ni-rich solute clusters significantly contributed to the improved age hardening performance of the weld metal-ferrite in the 22% Cr duplex stainless steel.

The present study has demonstrated the differences in the 475 °C embrittlement behavior of the base and weld metals of a 22 mass% Cr-duplex stainless steel. A grade 2205 solution-annealed plate was used as the base metal. The corresponding Ni-rich weld metal, which comprised multiple layers, was fabricated using a grade 2209 filler wire. The base and weld metals were subjected to isothermal aging for various durations (up to 1000 h) at 400 °C. Microstructural characterization, Charpy impact toughness testing, and Vickers microhardness measurements were conducted before and after aging. Furthermore, the microstructure of the ferrite phase, after aging, was evaluated using transmission electron microscopy (TEM). There was a substantial decrease in the impact toughness in both the base and weld metals with increasing aging time, although the decrease in the impact toughness was more pronounced for the weld metal. The hardening of ferrite was more prominent in each layer of the weld metal than in the base metal. The TEM observations indicated that the ferrite phase in both the base and weld metals underwent spinodal decomposition, which was accelerated in the weld metal. Regarding the major alloying elements (i.e., Cr, Mo and Ni) contained in the ferrite phase, the content of each, Cr and Mo, was lower in each layer of the weld metal than in the base meal, while the Ni content was, conversely, higher in each layer of the weld metal than in the base metal. The higher Ni content promoted the decomposition of ferrite in each layer of the weld metal during aging. This resulted in an acceleration of the 475 °C embrittlement in the multilayered weld metal of the 22 mass% Cr duplex stainless steel compared to the base metal.

Studies on intragranular ferrite with ultra-low O content are scarce, and establishing a technology for the refinement of the microstructure of a weld metal with ultra-low O content through the formation of intragranular ferrite is challenging. In this report, the effect of addition of Ti and Al on the formation of intragranular ferrite in ultra-low-oxygen weld metals was studied. The formation process of the oxide was discussed based on the detailed observation of the oxides and thermodynamic calculations. In the weld metal of Ti added steel with the highest intragranular ferrite area fraction, it was observed that the significant Mn depleted zone (MDZ) formed around a complex oxide containing Ti, Mn and Si. The formation of the MDZ increases the transformation temperature around the oxide and promotes the formation of intragranular ferrite with the oxide as a nucleus. Since the increase in Mn content at equilibrium due to the change in the stable phase during cooling is larger for the inclusion in Ti added steel, it was considered that more Mn diffused from the matrix to the inclusion in Ti added steel and the MDZ formed at the interface between matrix and the inclusion. The promotion of intragranular ferrite formation in an ultra-low-oxygen weld metal is enhanced by oxides with a large increase in the equilibrium Mn content during cooling.

The pyrolytic reactive granulation process, yielding ceramic spherical porous granules, is simple, consisting of typical ceramic processing methods, viz. , wet-ball milling of powders, vacuum drying, granulation via sieving through a screen mesh, and one-step heat treatment for local reactive sintering within each granule. Here, the microstructural development of spherical porous granules was successfully visualized by in situ high-temperature confocal laser-scanning microscopy during the heating up to 1400 °C in air. Based on the result of the in situ observation, a simple but powerful size-controlling process of spherical porous granules, viz. , multiscreen sieving after the heating was demonstrated. Nearly monodispersed spherical porous granules composed of pseudobrookite-type MgFeNbO 5 were easily obtained.

A novel strategy with shot peening (SP) pretreatment was proposed to prevent heat-affected zone liquation cracking during laser cladding of K447A nickel-based superalloy powder over its non-weldable cast structure. High dislocation density accumulated by the SP pretreatment drove the high-temperature zone of the heat-affected zone to recrystallize during the laser cladding process, as a result, the coarse columnar grain with an average dendrite stem and secondary dendrite arm width of 102.67 ± 4.24 μm was transformed into a fine equiaxed grain with an average size of 10 μm. SP could effectively inhibit liquation cracking. With the increase of SP duration, the tendency of the liquation cracking reduced. Compared with no SP BM, the total length of the liquation cracking in the longitudinal section of the laser cladding sample reduced from 1649 μm to 248.8 μm. In-situ experimental observation and process simulation were opted to study the evolution behavior of recrystallization. Recrystallization firstly developed in the region of the intergranular zone at ~1214 °C and gradually the base metal recrystallized between 1232 and 1242 °C except for the region with γ/γ′ eutectic structure. As the temperature rose further, a fine liquid film network developed in the range of 1260–1267 °C. Finally, the stress and strain-based criteria were exercised to evaluate the crack susceptibility of the liquid film. The fine liquid film network adequately constrained the liquation cracking by reducing the driving stress applied to the liquid film as well as by lowering the pressure drop of the liquid film caused by driving strain.

The solidification mechanism of austenitic stainless steels solidified with primary ferrite is clarified in terms of crystallography. The formation of austenite was expected to take place by the peritectic/eutectic reaction in the interdendritic region of the primary ferrite, but no unique orientation relationship was confirmed between the primary ferrite and the austenite. The austenite was found to keep growing along its direction, independently of the primary ferrite, even after the primary ferrite changed its growth direction. This was confirmed by solidification experiments with changing heat flow directions, and was supported by the observation that the austenite forms crystallographic domains different from those of the primary ferrite in the final microstructure. Thus, it is concluded that the austenite is not crystallographically restricted by the primary ferrite, and that the growth of the primary ferrite and that of the austenite are crystallographically independent of each other.

Laser cladding with a paraxial powder-feeding device was used to repair as-cast K447A nickel-based superalloy, a kind of non-weldable alloy with a high Hf content and working as gas-turbine hot-section components. In the present study, the metallurgical evolution behavior of liquid film and liquation cracking mechanisms are studied. The results showed that a kind of uncommon Ni21Hf8 phase was found in the incipient melting regions (IMR) of the intergranular region. In the thermal cycle, γ/Ni21Hf8 eutectic liquefaction, M5B3 dissolved, and γ/γ’ eutectic liquefaction successively occurred in the heating process in IMR, and liquid → γ + γ/γ’ + M5B3 + γ/Ni21Hf8 → γ + γ’(3) + γ/γ’ + γ/Ni21Hf8 + M5B3 occurred in the cooling process. The inhomogeneity of the intergranular structure leads to the liquid film with the characteristics of discontinuous distribution and uneven width. The cracking criterion of the liquid film was established based on pressure drop. When the closed liquid film meets the condition of 2 γ sl 1 / K * ( ρ s / ρ l − 1 * R * dt + ∆ h e ) , the liquid film with a thickness that is greater than critical thickness will be induced into a liquation cracking.

To investigate the effect of alloying elements on the microstructure and mechanical properties of medium-carbon steel weldments, 0.4% carbon steels with different chromium and titanium contents were fabricated by melting and hot-rolling for friction stir welding (FSW). The microstructures in the stir zones of friction stir welded (FSWed) medium-carbon steels without chromium, with 1% chromium and 4% chromium are ferrite and pearlite, bainite, and fully lath martensite, respectively. The prior austenite grain size was refined by the addition of titanium. High densities of ultrafine-twins are observed in the martensite of FSWed 0.4%C–4%Cr and 0.4%C–4%Cr-0.2%Ti steels, while dense auto-tempered iron carbides with less ultrafine-twins are observed in the lath martensite of FSWed 0.4%C–4%Cr-0.4%Ti steel. The strength of FSWed 0.4%C–4%Cr steels is improved by the martensitic structure, while the ductility of 0.4%C–4%Cr steels decreases significantly compared to that of 0.4%C steels with low chromium content. A small amount of 0.4% titanium addition significantly increases the ductility of 0.4%C–4%Cr-0.4Ti steel, leading to a good combination of high strength of 2270 MPa and high elongation of 18.3%. The substructure changes of the lath martensitic structure caused by the addition of titanium contribute to the brittle to ductile transition of FSWed medium-carbon steel with 4% chromium.

The purpose of this study is to evaluate the corrosion resistance of weld metal by electrochemical methods and discuss the relationship between microstructure and corrosion resistance. Intergranular and pitting corrosion resistances were measured using electrochemical potentiokinetic reactivation (EPR) test and pitting potential measurement respectively. The reactivation ratio and pitting potential corresponded to its chemical composition. The specimens containing more Cr and Mo showed higher resistance. In the EPR test, the dendrite core with a relatively low Cr content was corroded. In the pitting corrosion test, Nb carbide became the initiation site of pitting corrosion which propagated along the cell structure.

The effect of secondary phase formation such as MC carbide during welding on solidification cracking susceptibility of fully austenitic stainless steels with various amounts of alloying elements and carbon was investigated. The solidification cracking susceptibility of Fe-24mass%Cr-26mass%Ni stainless steels with various amounts of niobium, titanium, zirconium, and carbon was evaluated by the Trans-Varestraint test. The addition of alloying elements such as niobium, titanium, and zirconium increased the brittle temperature range (BTR). The BTRs depend on the type and combination of the alloying elements and carbon content. Titanium was the most influential element on the increase in BTR compared to the other elements. The increase in carbon content tended to decrease the BTR, especially for the specimen containing titanium. The alloying elements formed phases such as the film-like MC-type carbide (MC) phase and granular Laves phase in the specimens containing 0.05% C, and a lot of the acicular MC phases in the specimens containing 0.20% C. The start temperature of the secondary phase formation differed depending on the type of the alloying element, and the temperature tended to increase with increasing carbon content. When the additions of niobium and titanium were compared, the amount of titanium segregation during solidification was smaller due to the formation of MC phases. Solidification simulation revealed that a higher amount and faster formation of the MC phase during solidification in 2Ti–20C contributed to reducing the BTR by increasing the carbon content. Thus, it is considered that the weld solidification cracking susceptibility depended on the type and amount of formation morphology, as well as start temperature during solidification corresponding to the composition of alloying elements and carbon.

Visualization of the microstructure formation process in SUS410 was considered combining the confocal scanning laser microscope (CSLM) and the multi-phase field method (MPFM). According to the in situ observation using CSLM, γ-phase precipitation temperature falls greatly with an increase in cooling rate. Moreover, solidification and solid-state transformation behaviour was analysed by MPFM. Calculated tensile strength depending on the phase state corresponds to experimental results. It is thought that tensile strength and density in the high-temperature region are affected by the cooling rate due to the delay of γ-phase formation.

Prevention of weld cracking is necessary for ensuring the reliability of high strength steel structures. Tensile residual stress in the weld metal is one of the major factors causing the weld cracking, therefore, it is important to clarify the residual stress distribution in the weld metal. Conventional stress measurement, the stress relief method using strain gauges and the X-ray diffraction technique, can only provide the stress information in the surface region of the steel weld. The neutron diffraction is the only non-destructive method that can measure the residual stress distribution inside the steel weld [1-3]. The neutron stress measurement was applied for the 980MPa class high strength steel weld and it was revealed that high level of tensile residual stress can affect the weld cracking to a significant degree [4-5]. Recently, it was reported that Ni-Cr type steel weld exhibit higher resistance to the weld cracking compared with conventional low alloy type weld. Increase of tensile residual stress is prevented by lower transformation temperature of the Ni-Cr type weld metal and retained austenite phase is dispersed in the martensite microstructure. It is considered that lower level of tensile residual stress and the existence of retained austenite may prevent hydrogen accumulation in the weld metal [6]. However, retained austenite and the residual stress conditions in the Ni-Cr type high strength steel weld is not well understood. In this study, neutron diffraction analysis was conducted on the Ni-Cr type steel weld joint with the tensile strength level of 980MPa in order to investigate the effect of the retained austenite and the residual stress distribution on the weld cracking.

We have investigated the martensite transformation phenomena of a Cr-Ni type weld metal, especially the effects of stress and plastic strain that exist before the martensite transformation start. Tensile specimens machined from the weld metal were firstly heated up to 800 °C to austenitize the specimens and then cooled down. During the cooling, we applied stress and plastic strain before the martensite transformation starts. The results show that the stress increases the M s temperature, while the plastic strain decreases the M s temperature. In addition, the transformation-induced plasticity was also investigated. Finite element method (FEM) analysis was conducted considering these effects, and the results show that these effects on the residual stress and the weld distortion are considerably large. The FEM analysis shows that the compressive residual stress in the weld metal is likely to be decreased by the transformation-induced plasticity. We also conducted hydrogen diffusion analysis for the case that the retained austenite exists in the weld metal by varying the M s temperature in order to consider the appropriate time period to keep a cold cracking test specimen before the crack observation.

The carbon and chromium contents significantly affected the microstructure evolution in the stir zones of medium-carbon steels during the friction stir welding (FSW). The stir zone of the 0.2%C–4%Cr steel has a high strength over 1.5 GPa with a good elongation of 30.3%, while the stir zones of the 0.4%C–2%Cr and 0.4%C–4%Cr steels had ultrahigh tensile strengths over 2 GPa with low elongation. Auto-tempered martensite is the main reason for the excellent balance of strength and ductility in the stir zone of 0.2%C–4%Cr steel.

The aim of the present study was to investigate the effect of chromium and carbon in medium carbon steels on the microstructure evolution during FSW process and their mechanical property. The materials are dynamically recrystallized in austenite during FSW followed by phase transformation from austenite to ferrite, pearlite, bainite, and martensite. In the case of 0.2%C, the microstructure of the stir zone transited from pearlite and ferrite to bainite and martensite with the increase in chromium content. On the other hand, in the case of 0.4%C, the microstructure transited from pearlite to martensite with the increase in chromium content. The specimens of 0.2%C-4%Cr, 0.4%C-2%Cr and 0.4%C-4%Cr showed high tensile strength of over 1.5 GPa. In addition, even though the microstructures in the stir zones of the specimens are martensite, the elongation of 0.2%C-4%Cr was 31%while those of the others were less than 5%. TEM observation revealed that many tiny carbides precipitated in martensite of the stir zone of 0.2%C-4%Cr and micro twin formed in martensite in those of 0.4%C-2%Cr and 0.4%C-4%Cr. Thus, it was considered that auto-tempering occurred in the stir zone of 0.2%C-4%Cr during FSW process. The carbide precipitation in martensite is more effective to improve the tensile elongation compared with micro twin formation.

The hydrogen-releasing properties from weld metals that mainly consist of martensite and contain some amount of retained austenite were experimentally investigated in order to determine the physical properties such as the activation energies for the simulation of the hydrogen distribution in welded joints and to consider the time period to age a cold cracking test specimen before crack observation. The numerical calculations for single-pass welding show that in the case of the 2.5 % and 4.2 % retained austenite, the hydrogen contents in the martensite become sufficiently low in 2 days. In the case of 16 % retained austenite, about 40 % of the initial hydrogen content still exists in the retained austenite 7 days after the completion of the welding. But the hydrogen content in the martensite becomes less than 1 % of the initial content in 7 days. Hence, the 7-day-ageing time before crack observation is considered to be adequate. The numerical calculations for multi-pass welded joints show that the 7-day ageing is also considered to be appropriate.

Hydrogen evolutions for martensitic-austenitic weld metals were measured at 45 °C. The specimen without the austenite released hydrogen in about two days, and this result agrees with the 72 h holding time required by JIS Z3118. In the case of the specimens that contain the retained austenite, hydrogen release continued for about 100 days or more. We derived the analytic solution of the McNabb & Foster diffusion equation by regarding the austenitic microstructures as trapping sites to compare the experimental and calculation results, and showed that both of them are in good agreement. We also showed that the initial ratio of free and trapped hydrogen atoms is not necessarily equal to the thermal equilibrium condition at the measurement temperature, which can be analysed using the present solution.

The effect of dilution on microstructure and toughness of dissimilar weld metal of super-austenitic stainless steel using nickel base alloy filler wire was investigated. As the dilution increased, the kind of inclusion was changed from Nb(C,N) to Laves phase and the amount of Laves phase increased because of higher phase stability at dendrite boundaries by solidification segregation. Consequently, the toughness of dissimilar weld metal was lower than that of deposited metal.

The effect of dilution on pitting corrosion resistance of dissimilar weld metal of super-austenitic stainless steel using nickel base alloy filler wire was investigated. As the dilution increased, Cr and Mo concentration at dendrite cores became lower because of solidification segregation and the amount of inclusions at dendrite boundaries increased. Consequently, the pitting corrosion resistance of large diluted dissimilar weld metal was deteriorated.

Electron backscattering diffraction was employed to study grain structure development and texture evolution during friction stir welding (FSW) of a low stacking fault energy material, S31254 superaustenitic stainless steel. Formation of the final stir zone (SZ) microstructure was deduced to be primarily governed by discontinuous recrystallization occurring during the FSW cooling cycle. The textural pattern formed in the SZ was interpreted in the terms of {1 1 1}〈 u v w 〉 and { h k l }〈1 1 0〉 partial simple shear fiber textures.

The numerical simulation by multi-phase field method (MPFM) has been performed in association with calculation of phase diagram. Microstructure formations during solidification, cooling and heat treatment in SUS304 are simulated in conditions of multi-component and multi-phase alloy. It is confirmed that dendrite growth and δ/γ transformation can be predicted and reasonable agreements with dendrite growth theory and previous works are obtained. Moreover, dissolution behavior of δ-ferrite during heat treatment can be also evaluated by MPFM. It will be a useful tool in analyzing the microstructure formation in welding process.