Interface microstructure of diffusion bonded Ni3Al intermetallic alloy and austenitic stainless steel

The diffusion bonding of a Ni₃Al intermetallic alloy to an austenitic stainless steel has been carried out at temperatures 950, 1000 and 1050 °C. The influence of bonding temperature on the microstructural development and hardness across the joint region has been determined. The microvoids in the interface have been found to decrease with increasing bonding temperature. The intermetallic phase Al₃Ni has been detected at the Ni₃Al side of the diffusion couple. Diffusion of Cr and Fe from the stainless steel to the Ni₃Al alloy has been observed.     

Characterization of transient-liquid-phase-bonded joints in a duplex stainless steel with a Ni–Cr–B insert alloy

Transient liquid-phase bonding of a duplex stainless steel was performed with a Ni–Cr–B insert alloy. The microstructure of the joint region was investigated by cross-sectional and layer-by-layer characterization. According to the experimental studies, prior to completion of isothermal solidification, the bond microstructure can be expressed as γ-Fe + δ-Fe/γ-Fe + δ-Fe + BN/γ-Ni(Fe) + BN/γ-Ni + Cr-rich borides/γ-Ni + Ni₃B + Cr-rich borides (CrB, CrB₂, Cr₂B₃, Cr₃B₄, Cr₅B₃ and CrB₄), from the base metal side to the bonded-interlayer side. Complete isothermal solidification occurred at 1090 °C within 3600 s. Only the γ-Ni solid solution phase was present in the bonded interlayer, and BN precipitates were not removed after isothermal solidification. The formation of secondary-phase precipitates might be responsible for the presence of peak microindentation hardness in the bond region.     

Micro flash butt welding of super duplex stainless steel with Zr-based metallic glass insert

Micro flash butt welding of super duplex stainless steel with Zr-based metallic glass insert was carried out. Zr₅₅Cu₃₀Ni₅Al₁₀ of Zr-based metallic glass with thickness of 50 μm and Zr metal with thickness of 500 μm were used as the insert materials. After welding, Zr-based metallic glass insert became much thinner than that of Zr metal insert. The supercooled liquid of Zr-based metallic glass insert at the interface was protruded outside of the specimen during welding. The formation of the protrusion discharged the oxide films on the butting surfaces and contact surface, resulting in metallurgical bonding through the fresh surfaces. The Fe-Zr metallic compounds were observed at the bonding interface for the Zr metal insert, but the metallic compound for Zr-based metallic glass insert was hardly observed. The micro flash butt welding of stainless steel with Zr-based metallic glass insert was successfully welded.     

Microstructure and mechanical properties of low alloy steel T11–austenitic stainless steel 347H bimetallic tubes

Abstract

A low alloy steel (T11) has been bonded to an austenitic stainless steel 347H by hot coextrusion under industrial conditions. The final product was a seamless bimetallic tube with 347H cladding the exterior for corrosion resistance in severely corrosive environments at high temperatures. The microstructures of the coextruded bonding have been compared to those obtained in the laboratory, after diffusion bonding experiments, using hot isostatic pressing (hipping). In all cases both the interdiffusion of the different elements across the interface and the microstructure have been analysed by optical microscopy, SEM, and TEM. On the 347H side a profuse precipitation, mainly of NbC, was found in a region near the interface. Only in the hipped specimens, as result of nickel and chromium diffusion from the stainless steel to the T11 steel, a martensite band was observed parallel to the interface. The heat treatment performed on the bimetallic tubes, to obtain the optimum combination of mechanical properties and corrosion resistance, consisted of austenitisation between 1050–1100°C, water quenching, and a stabilisation treat ment at 850–900°C, followed by slow cooling.     

Effect of Cr and Ni on diffusion bonding of Fe3Al with steel

Microstructure at the diffusion bonding interface between Fe₃Al and steel including Q235 low carbon steel and Cr18-Ni8 stainless steel was analysed and compared by means of scanning electron microscopy and transmission electron microscopy. The effect of Cr and Ni on microstructure at the Fe₃Al/steel diffusion bonding interface was discussed. The experimental results indicate that it is favourable for the diffusion of Cr and Ni at the interface to accelerate combination of Fe₃Al and steel during bonding. Therefore, the width of Fe₃Al/Cr18-Ni8 interface transition zone is more than that of Fe₃Al/Q235. And Fe₃Al dislocation couples with different distances, even dislocation net occurs at the Fe₃Al/Cr18-Ni8 interface because of the dispersive distribution of Cr and Ni in Fe₃Al phase.     

Yield stress of duplex stainless steel specimens estimated using a compound Hall–Petch equation

In this study, the 0.2% yield stress of duplex stainless steel was evaluated using a compound Hall–Petch equation. The compound Hall–Petch equation was derived from four types of duplex stainless steel, which contained 0.2–64.4 wt% δ-ferrite phase, had different chemical compositions and were annealed at different temperatures. Intragranular yield stress was measured with an ultra-microhardness tester and evaluated with the yield stress model proposed by Dao et al. Grain size, volume fraction and texture were monitored by electron backscattering diffraction measurement. The k_γ constant in the compound equation for duplex stainless steel agrees well with that for γ-phase SUS316L steel in the temperature range of 1323–1473 K. The derived compound Hall–Petch equation predicts that the yield stress will be in good agreement with the experimental results for the Cr, Mn, Si, Ni and N solid-solution states. We find that the intragranular yield stress of the δ-phase of duplex stainless steel is rather sensitive to the chemical composition and annealing conditions, which is attributed to the size misfit parameter.     

Tribological properties of oxidised boride coatings grown on AISI 4140 steel

In this study, we investigated the wear behaviour of borided and borided + short-duration oxidized AISI 4140 steel. Boronizing was carried out in a slurry salt bath consisting of borax, boric acid and ferro silicon. Also, short-duration oxidizing treatment was applied to borided steel to produce glass-like boron oxide layer. The short-duration oxidizing was performed at 750 °C for 3 min. Optical and scanning electron microscope (SEM) cross-sectional examinations of borided layer revealed a needle-shaped morphology. The presence of non-oxide boride type ceramics FeB and Fe₂B formed on the surface of steel substrate was confirmed by classical metallographic technique and X-ray diffraction (XRD) analysis. The hardness of borides formed on the surface of steel substrate and unborided steel substrate were 1446–1690 HV_0.1 and 280 HV_0.1, respectively. The wear behaviour of borided steel were characterised by using a pin-on-disc technique. The borided and short-duration oxidized steels, in the form of pins were allowed to slide against a hard AISI 440C stainless steel disc (63 HRc). The sliding velocity of 1 m s^− 1 for borided and short-duration oxidized steel and the nominal load on the pin was 20 N. The highest wear rates were observed on disc slide against the base steel, whilst the lowest wear rates occurred during sliding against the borided and short-duration oxidized steel surfaces. It was observed that the friction coefficient of unborided (hardened + tempered) and borided steels ranged from 0.50 to 0.60, but after short-duration oxidizing, the friction coefficient of borided steel was dropped to 0.12.     

Microstructure characterization and properties of carbon steel to stainless steel dissimilar metal joint made by friction welding

Dissimilar metals of 1045 carbon steel and 304 stainless steel are joined successfully by friction welding. The microstructure variation and mechanical properties are studied in detail. The weld interface can be clearly identified in central zone, while the two metals interlock with each other by the mechanical mixing in peripheral zone. On carbon steel side, a thin proeutectoid ferrite layer forms along weld interface. On stainless steel side, austenite grains are refined to submicron scale. The δ-ferrite existing in stainless steel decreases from base metal to weld interface and disappears near the weld interface. Severe plastic deformation plays a predominant role in rapid dissolution of δ-ferrite compared with the high temperature. Carbide layer consisting of CrC and Cr₂₃C₆ forms at weld interface because of element diffusion. Metastable phase CrC is retained at room temperature due to the highly non-equilibrium process and high cooling rate in friction welding. The fracture appearance shows dimple fracture mode in central zone and quasi-cleavage fracture mode in peripheral zone. Further analysis indicates that welding parameters govern tensile properties of the joint through influencing the thickness of carbide layer at weld interface and heterogeneous microstructure in thermo-mechanically affected zone on carbon steel side.     

Corrosion failures of AISI type 304 stainless steel in a fertiliser plant

A urea plant, operating on ammonia and carbon dioxide (CO₂) gases, had to be shutdown due to corrosion in the intercooler and aftercooler of its CO₂ gas cleaning circuit. Extensive general corrosion of AISI type 304 stainless steel parts, such as sealing strips, fins, demisters and the shell, of these two components which were in contact with the duplex stainless steel tubes, caused the shutdown of the fertiliser plant within 6 months. Investigations of the corrosion products by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) techniques showed the presence of carbon and ammonia based compounds, thus suggesting the role of ammonia and CO₂ gases, or the product of their reactions, in the corrosion of type 304 stainless steel. Electrochemical polarisation studies showed that duplex stainless steel possessed a more positive open circuit potential and a nobler critical pitting potential than type 304 stainless steel thus confirming that the corrosion of type 304 stainless steel was caused by the galvanic action with the duplex stainless steel heat transfer tubes. Hence, it was recommended that (i) the same material (type 304 stainless steel) be used for all parts of the intercooler and aftercooler to avoid galvanic corrosion, (ii) condense water carried over by CO₂ gas by cooling it to low temperatures immediately after it comes out from the scrubber, (iii) slight modification of the process to add up to 0.8% oxygen in the CO₂ gas before entry into the intercooler, which will help in retaining/formation of an effective passive film on type 304 stainless steel.     

Strengthening of AISI 2205 duplex stainless steel by strain ageing

In this study, static strain ageing behavior of commercially available and solution heat treated duplex stainless steel was investigated and the effect of static strain ageing on the mechanical properties was also determined in detail. Some of as-received duplex stainless steel test specimens were pre-strained in tension by 5% and then aged at 100 °C, 200 °C, 300 °C, 400 °C, 500 °C and 600 °C for 30 min in furnace. Some of duplex stainless steel test specimens were solution heat treated at 1050 °C for 30 min, water quenched and then pre-strained for 5% in tension shortly after the solution heat treatment.In order to identify the effect of static strain ageing on the mechanical properties, the tensile strength, the change in the strength due to ageing (ΔY), elongation fracture and hardness were determined. The test results showed that the mechanical properties were affected by static strain ageing mechanism which was applied at different temperatures for same time interval.     

Effect of annealing temperature on the pitting corrosion resistance of super duplex stainless steel UNS S32750

The pitting corrosion resistance of commercial super duplex stainless steels SAF2507 (UNS S32750) annealed at seven different temperatures ranging from 1030 °C to 1200 °C for 2 h has been investigated by means of potentiostatic critical pitting temperature. The microstructural evolution and pit morphologies of the specimens were studied through optical/scanning electron microscope.Increasing annealing temperature from 1030 °C to 1080 °C elevates the critical pitting temperature, whereas continuing to increase the annealing temperature to 1200 °C decreases the critical pitting temperature. The specimens annealed at 1080 °C for 2 h exhibit the best pitting corrosion resistance with the highest critical pitting temperature. The pit morphologies show that the pit initiation sites transfer from austenite phase to ferrite phase as the annealing temperature increases. The aforementioned results can be explained by the variation of pitting resistance equivalent number of ferrite and austenite phase as the annealing temperature changes.     

Deformation mechanisms of a modified 316L austenitic steel subjected to high pressure torsion

The present work is assigned to the microstructural evolution of a modified 316L stainless steel during high pressure torsion (HPT) in a temperature range between −196 °C and 720 °C. The aspect of microstructural evolution is similar to that of materials with low stacking fault energy: at high deformation temperatures (T_def > 450 °C) the dominant deformation mechanism is dislocation glide whereas for medium temperatures (450 °C > T_def > 20 °C) mechanical twinning is observed. At very low deformation temperatures (20 °C > T_def > −196 °C) mechanical twinning is replaced by the deformation induced martensite transformation γ(fcc) → ?(hcp). Based on the present results, the formation mechanisms of nanocrystalline austenite are discussed.     

Spinodal decomposition mechanism study on the duplex stainless steel UNS S31803 using ultrasonic speed measurements

This work, focuses on the spinodal decomposition mechanism study on the duplex stainless steel duplex UNS S31803, composed by austenite (γ) and ferrite (α) phases, at 425 °C and 475 °C temperatures by ultrasonic speed measurements. This temperature range is responsible for the transformation mechanism of α_initial phase to α phases (poor in chromium) and α′ (rich in chromium) by spinodal decomposition. The techniques to accomplish this analysis are based mainly on X-ray diffraction measures and ultrasonic speed. The obtained results show that it is possible to conclude that the use of ultrasonic speed measurements indicates a promising technique for following-up the phase transformation and spinodal decomposition on the steel studied.     

不锈钢—碳钢单、双面复合板的爆炸焊接及性能研究

研究了不锈钢－碳钢单面、双面复合板爆炸焊接质量,结合界面的微观结构,剪切强度及耐蚀性能,结果表明,单、双面不锈钢－低碳爆炸焊接复合板的结合界面均为波状结构,结合面两侧存在一定组织变形,近界面处为角结晶组织,稍远处为拉伸变形后的维状组织,结合界面碳钢－侧过渡区存在增碳区,不锈钢一侧有一个脱碳层,双面复合界面的结合过渡区的单面为１．５倍宽,脱碳区也接近单面的１．５倍,采用切割爆炸焊接法有利于改善不锈钢－低碳钢复合板的边缘焊合,在同一基板上进行了双面不锈钢复合时,第一面复合界面的剪切强度比第二面复合界面的差,还是双面复合板,其界面结面强度均由起爆端的末端逐渐降低,结合界面的脱碳层对复合的耐蚀性能无明显影响。     

Hot working behavior of 2205 austenite-ferrite duplex stainless steel characterized by constitutive equations and processing maps

The hot deformation characteristics of the 2205 duplex stainless steel were analyzed using constitutive equations and processing maps. The hot compression tests were performed at temperature range of 950-1200 °C and strain rate of 0.001-1 s⁻¹. Flow stress was modeled by the constitutive equation of hyperbolic sine function. However, the stress exponent and strain rate sensitivity were different at low and high deformation temperatures where austenite and ferrite are dominant, respectively. It was recognized that strain at the peak point of flow curve increases with the Zener-Hollomon parameter, Z, at low temperature deformation while at high temperature deformation it actually decreases with Z. The power dissipation map, instability map and processing map were developed for the typical strain of 0.3. It was realized that dynamic restoration mechanisms could efficiently hinder the occurrence of flow instability at low and medium strain rates. Otherwise, the increase in strain rate at low and high temperatures could increase the risk of flow instability.     

Vacuum hot roll bonding of titanium alloy and stainless steel using nickel interlayer

Abstract

Vacuum hot roll bonding of titanium alloy and stainless steel using a nickel interlayer was investigated. No obvious reaction or diffusion layer occurs at the interface between stainless steel and nickel. The interface between titanium alloy and nickel consists of an occludent layer and diffusion layers, and there are the intermetallic compounds (TiNi₃, TiNi, Ti₂Ni and their mixtures) in the layers. The total thickness of intermetallic layers at the interface between titanium alloy and nickel increases with the bonding temperature, and the tensile strength of roll bonded joints decreases with the bonding temperature. The maximum tensile strength of 440·1 MPa was obtained at the bonding temperature of 760°C, the reduction of 20% and the rolling speed of 38 mm s^–1.     

Diffusion bonding between Ti -6Al -4V alloy and microduplex stainless steel with copper interlayer

Abstract

In the present study, diffusion bonding was used to join Ti -6Al- 4V alloy to a microduplex stainless steel using a pure copper interlayer. The effects of heating rate and holding time on microstructural developments across the joint region were investigated. After bonding, microstructural analysis including metallographic examination and energy dispersive spectroscopy (EDS), microhardness measurements, and shear strength tests were carried out. From the results, it was seen that heating rate and holding time directly affect microstructural development at the joint, especially with respect to the formation of TiFe intermetallic compounds, and this in turn affects the shear strength of the bonds. A sound bond was obtained with a heating rate of 100 K min ^-1 and holding time of 5 min, and this was related to the small amount of TiFe intermetallics formed close to the duplex stainless steel side at this bonding condition. Although Ti₂Cu and TiFe intermetallics were formed in all specimens, it was seen that the most deleterious intermetallic was TiFe. As the heating rate was decreased and holding time increased the amount of TiFe intermetallics increased, and consequently shear strength decreased. As a result, from the microstructural observations, EDS analysis, microhardness measurements, and shear strength tests, it was concluded that a high heating rate and a short holding time must be used in the diffusion bonding of Ti-6Al- 4V to a microduplex stainless steel when pure copper interlayers are used.     

Investigations on the mechanical properties and microstructure of dissimilar cp-titanium and AISI 316L austenitic stainless steel continuous friction welds

Friction welding process is a solid state joining process that produces a weld under the compressive force contact of one rotating and one stationary work piece. In this study, the friction welding of dissimilar joints of AISI 316L stainless steel and cp-titanium is considered. The optical, scanning electron microscopy studies of the weld were carried out. Moreover, the X-ray diffraction analysis was performed. The integrity of welds was achieved by the micro hardness and tensile tests. The fracture surface was examined by the scanning electron microscopy. The study showed that the magnitude of tensile strength of the dissimilar welded specimen was below that of the titanium base material if preheating was not applied at the interface. The high weld tensile strength was achieved by preheating the 316L stainless steel material to 700 °C, smoothing and cleaning of the contact surfaces. Results illustrated that in dissimilar joints, different phases and intermetallic compounds such as FeTi, Fe₂Ti, Fe₂Ti₄O, Cr₂Ti and sigma titanium phase were produced at the interface. The presence of brittle intermetallic compounds at the interface resulted in degradation of mechanical strength which in turn led to premature failure of joint interface in the service condition. Preheating caused to produce oxide layer at the interface which was harmful for bonding. The oxide layer could be eliminated by applying pressure and smoothing the surface. Results of hardness tests illustrated that the high hardness was occurred in the titanium side adjacent to the joint interface. Moreover, the optimum operational parameters were obtained in order to achieve the weld tensile strength greater than the weak titanium material.     

Microstructure and mechanical properties of diffusion bonded SiC/steel joint using W/Ni interlayer

This paper describes the design and examination of W/Ni double interlayer to produce a joint between SiC and ferritic stainless steel. Diffusion bonding was performed by a two steps solid state diffusion bonding process. Microstructural examination and mechanical properties evaluation of the joints show that bonding of SiC to steel was successful. EDS and XRD analysis revealed that W₅Si₃ and WC were formed at SiC/W interface. The diffusion products at W/Ni interface, Ni-rich solid solution Ni(W) or intermetallic compound Ni₄W, was found to be dependent on the second step joining temperature. Neither intermediate phases nor reaction products was observed at Ni/steel interface for the joints bonded at the temperature studied. The average tensile strength of 55 MPa which is insensitive to the second step process was measured for as-bonded SiC/steel joint and the failure occurred at SiC/W interface. The hardness near the various bonded interfaces was also evaluated.     

Reactive brazing of ceria to an ODS ferritic stainless steel

This research study shows that a ceria ceramic can be bonded to an ODS ferritic stainless steel (MA956) by reactive brazing using a Ag₆₈-Cu_27.5-Ti_4.5 interlayer. The ability to join these materials provides an alternative to the current ceramic interconnects used in the development of solid oxide fuel cells. Initial results show that the ceramic-metal bonds survived the bonding process irrespective of the degree of porosity within the ceria ceramic. Metallographic analyses indicate that a reaction zone formed along the ceria/braze interface, which was not only titanium rich, but also consisted of a mixture of copper oxides. When the ceramic-metal bonds were exposed to high bonding temperatures or when subjected to thermal cycling at 700°C, this reaction layer increased in thickness and had a detrimental affect on the mechanical strength of the final joints.