Effect of interlayer addition on microstructure and mechanical properties of NiTi/stainless steel joint by electron beam welding

NiTi/Stainless Steel(SS) sheets have been welded via a vacuum electron beam welding process, with three methods(offsetting electron beam to SS side without interlayer, adding Ni interlayer and adding Fe Ni interlayer), to promote mechanical properties of the Ni Ti/SS joints. The joints with different interlayers are all fractured in the weld zone near the Ni Ti side, which is attributed to the enrichment of intermetallic compounds including Fe2 Ti and Ni3 Ti. The fracture mechanisms of different joints are strongly dependent on the types of interlayers, and the joints without interlayer, adding Ni interlayer and adding Fe Ni interlayer exhibit cleavage fracture, intergranular fracture and mixed fracture composed of cleavage and tearing ridge, respectively. Compared with the brittle laves phase Fe2 Ti, Ni3 Ti phase can exhibit certain plasticity, block the crack propagation and change the direction of crack propagation. The composite structure of Ni3 Ti and Fe2 Ti will be formed when the Fe Ni alloy is taken as the interlayer, which provides the joint excellent mechanical properties, with rupture strength of 343 MPa.     

Ultrasonic vibration assisted tungsten inert gas welding of dissimilar magnesium alloys

The effects of ultrasonic vibration assisted (UVA) treatment on the microstructures and mechanical properties of MB3/AZ31 dissimilar magnesium (Mg) alloy joints were studied by microstructural characterization, micro-hardness testing and tensile testing. Results indicate that the welding pores are eliminated and coarse ??-Mg grains of fusion zone are refined to 26 ??m, owing to the acoustic streaming effect and cavitation effect induced by the UVA treatment with an optimal ultrasonic power of 1.0 kW. In addition, Mg₁₇Al₁₂ precipitation phases are fine and uniformly distributed in the whole fusion zone of weldment. Micro-hardness of fusion zone of the Mg alloy joints increases to 53.5 HV after UVA process, and the maximum tensile strength with optimized UVA treatment increases to 263 MPa, which leads to fracture occurrence in the Mg alloy base plate. Eventually, it is experimentally demonstrated that robust MB3/AZ31 Mg alloy joints can be obtained by UVA process.     

Microstructures and mechanical properties of welded joints of novel 3Cr pipeline steel using an inhouse and two commercial welding wires

The welded joints of the novel 3Cr pipeline steel were fabricated via the gas tungsten arc welding (GTAW) technique using an inhouse welding wire labeled as R01 and two kinds of commercial wires (H08Cr3MoMnA and TGS-2CML). Microhardness, impact toughness and tensile properties of the joints were measured, and microstructure characteristics were observed by scanning electron microscopy (SEM). The results show that under selected welding procedure, the joints of R01 can achieve quite good mechanical properties without preheating and post weld heat treatment (PWHT). After thermal refining, elongation (15.2%) doubled and met the DNV-OS-F101 standard. For low carbon or super low carbon pipeline steels such as 3Cr steel, the revised formula with the carbon applicable coefficient (A(c)) was quite good for predicting the maximum hardness in heat affected zone (HAZ). Compared with these two selected commercial wires, the inhouse welding wire R01 can provide the highest cost-performance ratio.     

Pulsed MIG welding of AZ31B magnesium alloy

Abstract

Pulsed metal inert gas welding of AZ31B magnesium alloy is carried out, and continuous butt joints of high quality are obtained at optimised parameters. The effects of parameters on weld formation and welding stability are studied. The microstructure, mechanical properties and fracture of weld beads with different filler wires are investigated. The results show that it is a stable drop transition process with optimised parameters, which belongs to globular transfer. The precipitates in fusion zone and heat affected zone (HAZ) are uniform, dispersive and almost granular. The grain size in fusion zone is fine, and the grain size does not grow too large in HAZ compared with the base metal. The ultimate tensile strength of weld beads can be 94% of base metal, and the average elongation is 11%. Dimples and coarse tearing ridges can be observed on the fracture of the weld bead.     

铝/镀锌钢搅拌摩擦铆焊接头组织与力学性能

为实现铝钢之间的优质连接,采用搅拌摩擦铆焊新方法对6061铝合金和DP600镀锌钢进行搭接点焊,利用扫描电子显微镜、能谱仪及拉伸试验对接头的微观组织及力学性能进行了研究.结果表明:接头成形平整美观,中心没有匙孔;接头包含铆接区和扩散区,其中在铆接区铝合金以铝柱的形式嵌入到钢板的圆孔中,形成了一个"铝铆钉",底部有富铝的α固溶体偏聚,圆孔四周形成扩散区,铝和钢形成了冶金结合,依靠金属间化合物Fe Al3连接在一起;接头有3种断裂形式,在最佳工艺参数下接头的抗剪力达到8.2 k N;铝柱上断口的微观形貌是被拉长的韧窝,扩散区的断口由灰色基体和白色颗粒组成.     

1.0 GPa low carbon medium Mn heavy steel plate with excellent ductility

ABSTRACT

A vanadium-bearing medium Mn heavy steel plate was designed, and the yield strength reaches around 1.0?GPa and the total elongation reaches around 18% after the annealing treatments. Meanwhile, the steel also possesses relatively good cryogenic impact toughness. Although the dislocation density of ～2.65?×?10¹⁴?m^?2 in the hot-rolled steel decreases to ～1.13?×?10¹⁴?m^?2 in both annealed steels and the volume fractions of retained austenite reach ～7.9% and ～13.5% for the steels annealed at 620°C for 1 and 2?h, there is no or small decrease in yield strength owing to numerous nanometer-sized carbides precipitated in martensite matrix.

This paper is part of a Thematic Issue on Medium Manganese Steels.     

Laser,tungsten inert gas,and metal active gas welding of DP780 steel: Comparison of hardness,tensile properties and fatigue resistance

The microstructural characteristics, tensile properties and low-cycle fatigue properties of a dual-phase steel (DP780) were investigated following its joining by three methods: laser welding, tungsten inert gas (TIG) welding, and metal active gas (MAG) welding. Through this, it was found that the size of the welded zone increases with greater heat input (MAG > TIG > laser), whereas the hardness of the weld metal (WM) and heat-affected zone (HAZ) increases with cooling rate (laser > TIG > MAG). Consequently, laser- and TIG-welded steels exhibit higher yield strength than the base metal due to a substantially harder WM. In contrast, the strength of MAG-welded steel is reduced by a broad and soft WM and HAZ. The fatigue life of laser-and TIG-welded steel was similar, with both being greater than that of MAG-welded steel; however, the fatigue resistance of all welds was inferior to that of the non-welded base metal. Finally, crack initiation sites were found to differ depending on the microstructural characteristics of the welded zone, as well as the tensile and cyclic loading.     

High-strength medium Mn quenching and partitioning steel with low yield ratio

ABSTRACT

A new microstructural design is proposed to develop a strong and ductile quenching and partitioning (Q&P) steel with low yield ratio. This steel has a heterogeneous dual phase microstructure which is developed by varying austenite thermal stability through Mn segregation. The heterogeneous microstructure contains large austenite grains which contribute to the low yield strength. The ultra-high tensile strength and good ductility are ascribed to the enhanced strain hardening behaviour resulted from the continuous transformation-induced plasticity (TRIP) effect. The present microstructural design enables a conventional medium Mn steel with high tensile strength, good ductility and low yield ratio, which promises easy forming and potential applications in automotive industries.

This paper is part of a Thematic Issue on Medium Manganese Steels.     

Flash butt welding application on 16MnCr5 chain steel and investigations of mechanical properties

In this study, 16MnCr5 chain steels were welded by flash butt welding and welded samples were annealed. Their microstructure and mechanical properties have been investigated. Upsetting current time was chosen as 1.6 s and build up pressure was chosen as 2, 2.5, and 3 bar. It is pointed out that welding parameters and annealing affected the mechanical properties. For all samples, the highest hardness was measured from weld metal. Non-annealed samples gave the highest tensile strength with the parameter of highest build up pressure. It is also found that annealing reduces the tensile strength of all samples.     

Microstructure and mechanical properties of friction stir welded 18Cr–2Mo ferritic stainless steel thick plate

In this study, microstructure and mechanical properties of a friction stir welded 18Cr–2Mo ferritic stainless steel thick plate were investigated. The 5.4 mm thick plates with excellent properties were welded at a constant rotational speed and a changeable welding speed using a composite tool featuring a chosen volume fraction of cubic boron nitride (cBN) in a W–Re matrix. The high-quality welds were successfully produced with optimised welding parameters, and studied by means of optical microscopy (OM), scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD) and standard hardness and impact toughness testing. The results show that microstructure and mechanical properties of the joints are affected greatly, which is mainly related to the remarkably fine-grained microstructure of equiaxed ferrite that is observed in the friction stir welded joint. Meanwhile, the ratios of low-angle grain boundary in the stir zone regions significantly increase, and the texture turns strong. Compared with the base material, mechanical properties of the joint are maintained in a comparatively high level.     

Microstructure evolution and mechanical properties of linear friction welded S31042 heat-resistant steel

S31042 heat-resistant steel was joined by linear friction welding(LFW) in this study. The microstructure and the mechanical properties of the LFWed joint were investigated by optical microscopy, scanning electronic microscopy, transmission electron microscopy, hardness test and tensile test. A defect-free joint was achieved by using LFW under reasonable welding parameters. The dynamic recrystallization of austenitic grains and the dispersed precipitation of NbCrN particles resulting from the high stress and high temperature in welding, would lead to a improvement of mechanical property of the welded joint.With increasing the distance from the weld zone to the parent metal, the austenitic grain size gradually increases from ~1μm to ~150μm, and the microhardness decreases from 301 HV to 225 HV. The tensile strength(about 731 MPa) of the welded joint is comparable to that of the S31042 in the solution-treated state.     

Enhanced strength-ductility of medium Mn steel by quenching,partitioning and tempering

ABSTRACT

A quench and partition (Q&P) process was combined with tempering in a medium Mn steel. The partitioning treatment enriched the austenite in carbon, and stabilised the austenite against transformation during cooling. The ductility of Q&P steel is significantly improved by tempering, with negligible loss in strength. The ductility was found to be determined by the martensite in the structure, rather than the austenite in the present case. The reason for the significant improvement in the ductility with tempering was suggested to be the reduction in the dislocation density after extended tempering treatment. The energy absorption of this alloy was increased to 28.5?GPa·% together with an ultrahigh tensile strength ～1400?MPa, which is one of the largest observations in this system.     

Microstructural evolution and mechanical properties of friction stir welded joint of Fe–Cr–Mn–Mo–N austenite stainless steel

2 mm thick Fe–18.4Cr–15.8Mn–2.1Mo–0.66N high nitrogen austenite stainless steel plate was successfully joined by friction stir welding (FSW) at 800 rpm and 100 mm/min. FSW did not result in the loss of nitrogen in the nugget zone. The arc-shaped band structure, consisting of a small amount of discontinuous ferrite aligning in the bands and fine austenite grains, was a prominent microstructure feature in the nugget zone. The discontinuous ferrite resulted from newly formed ferrite during welding and the remained ferrite, whereas the fine austenite grains were formed due to dynamic recrystallization of the initial austenite during FSW. The fine dynamically recrystallized grains in the nugget zone significantly increased the hardness compared to that of the base material. The strength of the joint was similar to that of the base material, with the joint failing in the base material zone.     

Time-evolution of microstructure and mechanical behaviour of double annealed medium Mn steel

Cold rolled 0.1C-4.7Mn (wt-%) steel was submitted to double annealing. The holding time of second intercritical annealing at 650°C was varied between 3 min and 30?h. Tensile behavior after each treatment was measured and analysed. Microstructure characterisation was performed using field emission gun scanning electron microscope, transmission electron microscope and saturation magnetisation method. Moreover, interrupted tensile tests were done to obtain the kinetics of austenite destabilisation during straining. An important effect of soaking time on the microstructure and associate mechanical properties was revealed and analysed. Considering thermal and mechanical stability of retained austenite, the optimum combination of phases, providing the best strength-ductility balance, was found after 2?h holding.

This paper is part of a Thematic Issue on Medium Manganese Steels.     

Effects of tool rotational and welding speed on microstructure and mechanical properties of bobbin-tool friction-stir welded Mg AZ31

The effects of rotational and welding speeds on the microstructure and mechanical properties of bobbin-tool friction stir welded (BT-FSW) Mg AZ31 were investigated. The results indicated that the thermo-mechanically affected zone (TMAZ) consisted of equiaxed grains, which were inconsistent with the deformed, rotated and elongated grains found in the TMAZs of bobbin-tool friction stir welded Al alloys and friction stir welded Al and Mg alloys. The average grain size increased as the ratio of the rotational speed to welding speed increased. Excellent welds with no degradation in hardness were produced using a low heat input. Mechanical tests revealed that the ultimate tensile strengths gradually increased with increasing welding speed while keeping the rotational speed constant. The rotational and welding speeds had only slight influences on the yield stress and fracture elongation.     

Accelerated emergence of CoNi-based medium-entropy alloys with emphasis on their mechanical properties

The concept of alloying has evolved over the centuries and in the past decade and a half, the emergence of the high entropy alloying concept has completely changed our perception of alloy design. This alloying strategy has been found to exhibit exciting properties such as high strength, excellent corrosion resistance, high cryogenic fracture toughness, thermal stability, and irradiation resistance. While the fcc-structured equiatomic CrMnFeCoNi has been very popular over the years, the discovery of the superior properties by a ternary CoNiCr alloy, kick-started a new era for medium-entropy alloy-focused research in the last 5–10 years due to the realization that “medium is better”. Here we review the recent progress made in the development of medium entropy alloys from a binary CoNi building block (CoNi-M, where M is Fe, Cr, or V), which are prototype systems of medium-entropy alloys. We discuss the relationship between their microstructure and properties (mainly mechanical ones), and how the stacking fault energy, and/or short-range order (SRO) determines the corresponding deformation mechanism. The influence of minor-alloying on their crystal structure and variations in deformation modes are critically discussed. Lastly, some insights and challenges are outlined.     

Revealing heterogeneous C partitioning in a medium Mn steel by nanoindentation

The present work employs nano-indentation technique to investigate the C partitioning in a medium Mn steel subjected to quenching and tempering process. It is found that the C partitioning between martensite and austenite is inhomogeneous. Particularly, the large lenticular martensite has negligible C partitioning into the austenitic matrix as it maintains an ultra-high nanohardness comparable to the one without tempering. Strategies to suppress the formation of large martensite are discussed.     

Carbide-free bainite in medium carbon steel

Heat-treatment processes to obtain carbide-free upper bainite, low bainite and low-temperature bainite in the 34MnSiCrAlNiMo medium-carbon steel were explored. Results show that in the steel bainite transformation mainly goes through three stages: short incubation, explosive nucleation and slow growth. When transformation temperature, T > Ms + 75 °C, upper bainite consisted of catenary bainitic ferrite and blocky retained austenite is obtained in the steel. When Ms + 10 °C < T < Ms + 75 °C, lower bainite is the main morphology composed of lath-like bainitic ferrite and flake-like retained austenite. When T < Ms + 10 °C, the lower bainite, also known as low-temperature bainite, is obtained, which contains much thinner lath-like bainitic ferrite and film-like retained austenite. Mechanical testing results show that the lower the transformation temperature is, the better comprehensive performance is. The low-temperature bainite has the very high tensile strength and impact toughness simultaneously. The lower bainite has lower tensile strength and higher impact toughness. The upper bainite has higher tensile strength and lower impact toughness. The big difference of the mechanical performance between these kinds of bainite is mainly caused by interface morphology, size, and phase interface structure of the bainitic ferrite and the retained austenite. Additionally, when the bainite transformation temperature is decreased, the high-angle misorientation fraction in packets of bainite ferrite plates is increased. High-angle misorientation between phase interfaces can prevent crack propagation, and thus improves impact toughness.     

Investigation on microstructure,mechanical properties and corrosion behavior of AISI 316L stainless steel to ASTM A335-P11 low alloy steel dissimilar welding joints

In the present study, the microstructure, mechanical properties and corrosion resistance of AISI 316L austenitic stainless steel to ASTM A335-P11 low alloy steel dissimilar joints, which are widely employed in the oil and gas industries especially for manufacturing of heat exchangers over 600°C, were investigated. For this purpose, two filler metals of ER309L and ERNiCrMo-3 were selected to be used with GTAW process. The results of microstructural evaluation revealed that the ERNiCrMo-3 weld metal contains dendritic and interdendritic zones, and the ER309L weld metal microstructure includes skeletal ferrites in an austenitic matrix. The maximum impact fracture energy and microhardness values were obtained for the ERNiCrMo-3 weld metal specimens; however, no significant difference was observed between the tension properties. The corrosion test results showed that the ERNiCrMo-3 has a higher corrosion resistance than ER309L. Finally, it was concluded that ERNiCrMo-3 would be a suitable filler metal for joining AISI 316L to A335-P11 for a variety of applications.