Effect of joint design on ballistic performance of quenched and tempered steel welded joints

A study was carried out to evaluate the effect of joint design on ballistic performance of armour grade quenched and tempered steel welded joints. Equal double Vee and unequal double Vee joint configuration were considered in this study. Targets were fabricated using 4 mm thick tungsten carbide hardfaced middle layer; above and below which austenitic stainless steel layers were deposited on both sides of the hardfaced interlayer in both joint configurations. Shielded metal arc welding process was used to deposit for all layers. The fabricated targets were evaluated for its ballistic performance and the results were compared in terms of depth of penetration on weld metal. From the ballistic test results, it was observed that both the targets successfully stopped the bullet penetration at weld center line. Of the two targets, the target made with unequal double Vee joint configuration offered maximum resistance to the bullet penetration at weld metal location without any bulge at the rear side. The higher volume of austenitic stainless steel front layer and the presence of hardfaced interlayer after some depth of soft austenitic stainless steel front layer is the primary reason for the superior ballistic performance of this joint.     

Effect of welding wire and groove angle on mechanical properties of high strength steel welded joints

Mechanical properties of high strength steel welded joints strictly depend on the welding process, the filler material composition and the welding geometry. This study investigates the effects of using cored and solid welding wires and implementing various groove angles on the mechanical performance of weld joints which were fabricated employing the gas metal arc welding process. It was found that weld joints of low alloy, high strength steels using low alloy steel cored welding wires exhibited higher tensile strength than that of low alloy steel solid wire and chromium‐nickel steel bare welding wire when the method of gas metal arc welding is employed. The effect of groove angle on the strength and toughness of V‐groove and double V‐groove butt‐joints was investigated. V‐groove joints, with higher tensile strength than double V‐groove joints in the whole range of groove angles, were superior in toughness for small groove angles, but impact toughness values of both joints were comparable for large angles. The effect of heat input and cooling rate on the weld microstructure and weld strength was also investigated by performing thermal analysis employing the commercial software ANSYS. It was concluded that cooling rate and solidification growth rate determined the microstructure of the weld zone which had great consequences in regard to mechanical properties.     

Effect of back-stress on cyclic stress-strain behaviour of a quenched and tempered low alloy steel

Low cycle fatigue was considered in relation to back-stress hardening. Cyclic stress-strain behaviours under controlling strain and stress conditions were investigated for a quenched and tempered low alloy steel which contained cementite particles. The cyclic stress-strain states obtained by the two prescribed tests were uniquely described using a parameter which reflected the cumulative reversible plastic work associated with the back-stress hardening. It was suggested that the effect of back-stress hardening on cyclic deformation should appear directly on the cyclic stress-strain curves defined as the loci of the tips of stable hysteresis loops. The initial slopes of the cyclic stress-strain curves for several steels were demonstrated to coincide with the theoretical work-hardening rates calculated on the basis of back-stress hardening due to the included carbide particles. Finally, the Manson-Coffin law was explained from the view that the surface damage would progress in parallel with the structure change in the bulk according to the persistency of slips resulting from the reversible back-stress hardening.     

Microstructure based flow stress modeling for quenched and tempered low alloy steel

Quenching and tempering (Q&T) process is commonly applied in part making industries for improving mechanical properties of carbon low alloy steels. After Q&T, microstructure of the steel consists of temper martensite and carbide precipitations. In this work, material modeling for describing flow stress behavior of the SNCM439 alloy steel under different tempering conditions was introduced. Microstructure based models were developed on both macro- and micro-scale. The models were afterwards applied in FE simulations for predicting stress–strain responses of the tempered steels. For the macroscopic model, the Ludwik equation was used, in which precipitation strengthening depending on particle size was incorporated by the Ashby–Orowan relationship. For the microscopic model, representative volume elements (RVEs) were generated considering microstructure characteristics of the examined steels. Flow curves of the individual constituents were described based on dislocation theory and chemical compositions. The FE simulations of tensile tests and RVE simulations under uniaxial tension were performed using the introduced models. The influences of the carbide precipitations on mechanical behavior of the tempered steels were investigated. The resulted effective stress–strain curves were determined and compared with the experimental ones. Both macroscopic and microscopic approaches accurately predicted mechanical properties and strain hardening behaviors of the tempered steels.     

A comparative analysis of metallurgical and mechanical properties of friction welded and post weld heat treated (oil quenched) duplex stainless steel joints

The effects of post weld heat treatment (PWHT) and oil quenching on the metallurgical and mechanical properties of the duplex (UNS S31803) welded joints were evaluated at three different temperatures namely 1080, 1150 and 1200 °C. The microstructural variation, austenite/ferrite phase changes, grain size measurements and microhardness aspects of the welded joint were observed. The fraction of ferrite and austenite phases was equivalent at 1150 °C. Nickel element was more efficient in controlling the twin phase balance. Finer grain structure was achieved at 1150 °C due to recrystallization effect. Twin phase presence and absence of precipitates were confirmed through XRD and TEM which followed Kurdjumov–Sachs relationship. At a heating pressure of 40 MPa, heating time of 4 s, an upsetting pressure of 80 MPa, and an upsetting time of 2 s during a PWHT at 1150 °C, a 50/50 balance between the duplex phases, fine grains, and increased microhardness were obtained.     

Effect of post-weld aging treatment on mechanical properties of Tungsten Inert Gas welded low thickness 7075 aluminium alloy joints

This paper reports the influence of post-weld aging treatment on the microstructure, tensile strength, hardness and Charpy impact energy of weld joints low thickness 7075 T6 aluminium alloy welded by Tungsten Inert Gas (TIG). Hot cracking occurs in aluminium welds when high levels of thermal stress and solidification shrinkage are present while the weld is undergoing various degrees of solidification. Weld fusion zones typically exhibit microstructure modifications because of the thermal conditions during weld metal solidification. This often results in low weld mechanical properties and low resistance to hot cracking. It has been observed that the mechanical properties are very sensitive to microstructure of weld metal. Simple post-weld aging treatment at 140 °C applied to the joints is found to be beneficial to enhance the mechanical properties of the welded joints. Correlations between microstructures and mechanical properties were discussed.     

Microstructure and mechanical properties of ultrasonic spot welded copper-to-magnesium alloy joints

High-power ultrasonic spot welding (USW) was used to join copper-to-AZ31B magnesium alloy at different welding energy levels, focusing on the interfacial microstructure and strength of the dissimilar joints. The enhanced diffusion during USW led to the presence of an interface diffusion layer mainly consisting of a eutectic structure of Mg and Mg₂Cu. The thickness of the interface diffusion layer increased with increasing welding energy or temperature at the joint interface. A unique diffusion pattern formed at high levels of welding energy of 2000 and 2500 J was attributed to the outburst of near-eutectic liquid at localized hot spots under internal pressure, which was explained in four stages. The tensile lap shear strength of the joints was observed to increase initially, reach a peak value, and then decrease with increasing welding energy. The failure of the joints made with the optimum welding parameters of 1500 J and 0.75 s occurred in the mode of cohesive failure in the eutectic structure of the interface diffusion interlayer.     

An investigation of mechanical and metallurgical properties of explosive welded aluminum-dual phase steel

The aim of this paper is investigation of microstructure and property relationship in aluminum-HSLA steel and aluminum-dual phase steel bimetals fabricated by explosive welding technique. Dual phase steel was produced by intercritical annealing and water quenching from 1.45Mn-0.2Si-0.186C HSLA steel. Hardness, tensile shear strength, tensile strength, toughness and microstructure of explosively welded aluminum-HSLA steel and aluminum-dual phase steel were evaluated. Both bimetals have a straight bonding interface. It was also seen that plastic deformation of dual phase steel was higher than HSLA steel near interfaces of bimetals. The hardness was increased near the bond interface of bimetals. Tensile and tensile shear strength tests showed that aluminum-dual phase steel is superior than aluminum-HSLA steel. Also, impact toughness of aluminum-dual phase steel was found significantly higher than that of aluminum-HSLA steel.     

Effect of heat treatment on mechanical properties of low alloy steel foams

Effect of heat treatment on compressive properties of low alloy steel foams (Fe–1.75 Ni–1.5 Cu–0.5 Mo–0.6 C) having porosities in the range of 47.4–71.5% with irregular pore shape, produced by the space holder-water leaching technique in powder metallurgy, was investigated. Low alloy steel powders were mixed with different amounts of space holder (carbamide), and then compacted at 200 MPa. Carbamide in the green compacts was removed by water leaching at room temperature. The green specimens were sintered at 1200 °C for 60 min in hydrogen atmosphere. Sintered compacts were heat treated by austenitizing at 850 °C for 30 min and then quenched at 70 °C in oil and tempered at 210 °C for 60 min. In this porosity range, compressive yield strengths of as-sintered and heat treated specimens were 28–122 MPa and 18–168 MPa, respectively. The resultant Young’s moduli of the as-sintered and heat treated specimens were 0.68–3.12 GPa and 0.47–3.47 GPa, respectively. The heat treatment enhanced the Young’s modulus and compressive yield strength of the foams having porosities in the range of 47.4–62.3%, as a consequence of matrix strengthening. However, the compressive yield stress and Young’s modulus of the heat treated foam having 71.5% porosity were lower than that of the as-sintered foam’s, as a result of cracks in the structure. The results were discussed in light of the structural findings.     

Effect of heat input on microstructure and mechanical properties of dissimilar joints between super duplex stainless steel and high strength low alloy steel

In the present study, microstructure and mechanical properties of UNS S32750 super duplex stainless steel (SDSS)/API X-65 high strength low alloy steel (HSLA) dissimilar joint were investigated. For this purpose, gas tungsten arc welding (GTAW) was used in two different heat inputs: 0.506 and 0.86 kJ/mm. The microstructures investigation with optical microscope, scanning electron microscope and X-ray diffraction showed that an increase in heat input led to a decrease in ferrite percentage, and that detrimental phases were not present. It also indicated that in heat affected zone of HSLA base metal in low heat input, bainite and ferrite phases were created; but in high heat input, perlite and ferrite phases were created. The results of impact tests revealed that the specimen with low heat input exhibited brittle fracture and that with high heat input had a higher strength than the base metals.     

微缺陷对在役焊接接头力学性能的影响

为了从原子层次揭示裂纹、空位等微缺陷对在役焊接接头力学性能的影响,从而得到焊接接头中最易失效的区域,本文采用焊接模拟软件SYSWELD对在役焊接过程进行模拟,计算熔池周围的温度场和应力场分布,然后利用LAMMPS软件建立含微缺陷的结构模型,以在役焊接过程中熔池下方区域的瞬时温度、应力状态作为计算条件,研究其在服役环境下的力学性能及微缺陷积累、扩展的动态演化机理.结果表明:在役焊接烧穿是一个宏观、微观结合的多层次过程;焊接电弧经过时,烧穿失稳更易发生在熔深最大处后方;熔池下方的各微区中,熔合线附近力学性能较差,在焊接热和应力的作用下最先失效;管壁材料烧穿时经历了微缺陷的形核、积累及扩展,整个失效过程伴随着空位、微裂纹的形成和扩展;空位、裂纹等微缺陷显著降低了结构的强度,在应力集中的作用下沿缺陷处起裂,严重破坏了结构的稳定性,大大加速了失效进程.     

Effect of microstructure on mechanical properties in weld-repaired high strength low alloy steel

To understand the effect of microstructure on mechanical properties of weld-repaired high strength low alloy (HSLA), as-received and weld-repaired HSLA with and without buffer layers (BLs) were prepared. Microstructure analysis was carried out using optical microscope and SEM, and mechanical properties were measured by Vickers hardness test and fatigue test.The fatigue resistance of weld-repaired HSLA without BL was deteriorated with comparing to parent metal (PM). Meanwhile, Vickers hardness (VH) showed an obviously reduction in the melted parent metal (MPM), which was due to formation of predominately block ferrite. For the weld-repaired HSLA with BL, the VH and fatigue resistance increased with the incorporation of 4 mm BL, which was mainly due to formation of lath ferrite and fine-grained pearlite and bainite. When BL thickness increased to 10 mm, the VH and fatigue resistance decreased, which was because the thick BL diluted the MPM. VH number from low temperature (below melting point) heat affected zone (HAZ) fluctuated, but had a little scatter. However, the fatigue crack growth rate from HAZ was not obviously affected by the welding as comparison with the PM.     

Effects of peening on mechanical properties in friction stir welded 2195 aluminum alloy joints

The effects of surface treatment techniques like laser and shot peening on the mechanical properties were investigated for friction stir welded 2195 aluminum alloy joints. The loading in the tensile specimens was applied in a direction perpendicular to the weld direction. The peening effects on the local mechanical properties through the different regions of the weld were characterized using a digital image correlation technique assuming an iso-stress condition. This assumption implies that the stress is uniform over the cross-section and is equal to the average stress. The surface strain and average stress were used giving an average stress–strain curve over the region of interest. The extension of the iso-stress assumption to calculate local stress–strain curves in surface treated regions is a novel approach and will help to understand and improve the local behavior at various regions across the weld resulting in a sound welding process. The surface and through-thickness residual stresses were also assessed using the X-ray diffraction and the contour methods. The laser peened samples displayed approximately 60% increase in the yield strength of the material. In contrast, shot peening exhibited only modest improvement to the tensile properties when compared to the unpeened FSW specimens. The result that laser peening is superior to shot peening because of the depth of penetration is original since this superiority has not been presented before regarding mechanical properties performance.     

Role of pulsed current on metallurgical and mechanical properties of dissimilar metal gas tungsten arc welding of maraging steel to low alloy steel

This research work encompasses the investigations carried out on the mechanical and metallurgical properties of maraging steel and AISI 4340 aeronautical steel weldments. The materials were joined by continuous current gas tungsten arc welding (CCGTA) and pulse current (PCGTA) gas tungsten arc welding processes using ErNiCrMo-3 filler wire. Cross sectional macrostructures confirmed proper deposition of the fillers and lack of discontinuities. Optical microscopy studies revealed that at the maraging steel–weld interface, martensite in distorted and block forms prevailed in CCGTA and PCGTA weldments whereas tempered martensite was predominant at the low alloy–weld interfaces of both the welds. Scanning electron microscopy (SEM) with energy dispersive analysis of X-rays (EDAX) analysis apparently showed less elemental migration in PCGTA weldments as compared to the other. Results of X-ray diffraction analysis recorded possible phase formations in various zones of the weldments. Microhardness profiles in either weld zones followed a constant trend whereas it showed a downtrend in the heat affected zones (HAZ) of the maraging steel and very high hardness profiles were observed in the low alloy steel side. Tensile studies on various factors and impact testing showed that PCGTA weldments outperformed the continuous ones in terms of strength, ductility and toughness. Fractograph analysis depicted the nature of failures of tensile and impact tested specimens. Comparison analyses involving influence and nature of pulsed current welds over continuous ones were done to determine the possibility of implementing these joining processes in aerospace applications. Weldments fabricated using PCGTA technique proved to be superior to the other, resulting in exceptional mechanical properties.     

Effect of welding parameters on microstructure and mechanical properties of friction stir welded joints of AA7039 aluminum alloy

A high strength Al–Zn–Mg alloy AA7039 was friction stir welded by varying welding and rotary speed of the tool in order to investigate the effect of varying welding parameters on microstructure and mechanical properties. The friction stir welding (FSW) process parameters have great influence on heat input per unit length of weld, hence on temperature profile which in turn governs the microstructure and mechanical properties of welded joints. There exits an optimum combination of welding and rotary speed to produce a sound and defect free joint with microstructure that yields maximum mechanical properties. The mechanical properties increase with decreasing welding speed/ increasing rotary speed i.e. with increasing heat input per unit length of welded joint. The high heat input joints fractured from heat affected zone (HAZ) adjacent to thermo-mechanically affected zone (TMAZ) on advancing side while low heat input joints fractured from weld nugget along zigzag line on advancing side.     

Microstructure and mechanical properties of resistance upset butt welded 304 austenitic stainless steel joints

Resistance upset welding (UW) is a widely used process for joining metal parts. In this process, current, time and upset pressure are three parameters that affect the quality of welded products. In the present research, resistance upset butt welding of 304 austenitic stainless steel and effect of welding power and upset pressure on microstructure, tensile strength and fatigue life of the joint were investigated. Microstructure of welds were studied using scanning electron microscopy (SEM). X-ray diffraction (XRD) analysis was used to distinguish the phase(s) that formed at the joint interface and in heat affected zone (HAZ). Energy dispersive spectroscopy (EDS) linked to the SEM was used to determine chemical composition of phases formed at the joint interface. Fatigue tests were performed using a pull–push fatigue test machine and the fatigue properties were analyzed drawing stress-number of cycles to failure (S–N) curves. Also tensile strength tests were performed. Finally tensile and fatigue fracture surfaces were studied by SEM. Results showed that there were three different microstructural zones at different distances from the joint interface and delta ferrite phase has formed in these regions. There was no precipitation of chromium carbide at the joint interface and in the HAZ. Tensile and fatigue strengths of the joint decreased with welding power. Increasing of upset pressure has also considerable influence on tensile strength of the joint. Fractography of fractured samples showed that formation of hot spots at high welding powers is the most important factor in decreasing tensile and fatigue strengths.     

Analysis of friction welding parameters on the mechanical metallurgical and chemical properties of AISI 1035 steel joints

In this present work, the analysis of process parameters on mechanical, metallurgical and chemical properties of American Iron and Steel Institute (AISI) 1035 steel rods of 12 mm diameter joints produced by friction welding is analyzed. The joints made with various process parameter combinations are subjected to tensile tests, hardness test and cyclic potentiodynamic polarization tests. The properties such as tensile strength, notch tensile strength, yield strength, % elongation, and vickers’s hardness, fully deformed zone (FDZ), flash formation and pitting corrosion has been analyzed for high level and low level process parameters. The optimized process variable is obtained by using Response Surface Methodology (RSM). The integrity of the welds has been investigated using high magnification optical microscopy. The fracture surface of the tensile test specimen is analyzed by using Scanning Electron Microscope (SEM) and Energy-dispersive X-ray spectroscopy (EDAX).     

Effect of friction time on mechanical and metallurgical properties of continuous drive friction welded Ti6Al4V/SUS321 joints

Dissimilar joint of Ti6Al4V titanium alloy and SUS321 stainless steel was fabricated by continuous drive friction welding. The effect of friction time on the mechanical properties was evaluated by hardness measurement and tensile test, while the interfacial microstructure and fracture morphologies were analyzed by scanning electron microscope, energy dispersive spectroscope and X-ray Diffraction. The results show that the tensile strength increases with friction time under the experimental conditions. And the maximum average strength 560 MPa, which is 90.3% of the SUS321 base metal, is achieved at a friction time of 4 s. For all samples, studied fracture occurred along the joint interface, where intermetallic compounds like FeTi, Fe₂Ti, Ni₃(Al, Ti) and Fe₃Ti₃O and many other phases were formed among elements from the two base metals. The width of intermetallic compounds zone increases with friction time up to 3 μm, below which it is beneficial to make a strong metallurgical bond. However, the longer friction time leads to oversized flash on the Ti6Al4V side and overgrown intermetallic compounds. Finally the optimized friction time was discussed to be in the range of 2–4 s, under which the sound joint with good reproducibility can be expected.     

Effect of backplate diffusivity on microstructure and mechanical properties of friction stir welded joints

Series of welds were made by friction stir welding (FSW) with various backplates made out of materials ranging from low diffusivity granite to high diffusivity copper in order to reveal the effect of backplate diffusivity on the joint microstructure and properties. The temperature, microstructure, microhardness and tensile properties of joints were compared and discussed. Results show that the backplate with high diffusivity effectively decreases the heat input to the workpiece during FSW. With decreasing the backplate diffusivity the sizes of equiaxed recrystallized grains in the nugget zone increase obviously, while the hardness of the nugget zone also increases a little. The interface between the thermo-mechanically affected zone and nugget zone at the retreating side disappears under the granite backplate. Moreover, the ductility of the joint is more excellent under the copper backplate, but under the granite backplate the failure has mixed fracture characteristics of quasi-cleavage and dimples.