POST WELD HEAT TREATMENT EFFECTS ON FATIGUE CRACK PROPAGATION IN AN ELECTRON BEAM WELDMENT

This paper studies the effect of two post-weld heat treatment processes on the fatigue behaviour of an electron beam weldment in 9 mm AISI 4130 steel. Electron beam tempering, in a vacuum chamber, immediately after welding and a traditional furnace tempering treatment were compared. Fatigue crack propagation resistance was assessed by a linear elastic fracture mechanics analysis. The resistance to fatigue crack growth was improved with post weld heat treatment due to residual stress relief and the existence of a toughened tempered microstructure. The specimens with an electron beam post-weld heat treatment showed better fatigue properties than those of furnace-treated specimens. An electron beam post-weld heat treatment causes the fatigue crack growth rate to decrease with increasing energy input and decreasing micro-dot-pattern width. For a furnace post-weld heat treatment, the fatigue crack growth rate decreases with increasing tempering temperature.     

Identification of microstructural zones and thermal cycles in a weldment of modified 9Cr-1Mo steel

This paper presents the results of a study on the microstructural and microchemical variations in a multipass Gas Tungsten Arc weld (GTAW) of modified 9Cr-1Mo steel. The changes brought about in the steel due to the heating and cooling cycles during welding and the subsequent effects due to reheating effects during multipass welding are described. Detailed analytical transmission electron microscopy has been carried to study the type and composition of the primary and secondary phases in this steel. The systematic changes in microstructural parameters such as Prior Austenite Grain Size, martensite lath size, number density, size and microchemistry of carbides, have been understood based on the different transformations that the steel undergoes during the heating and cooling process. Based on the observed microstructure, an attempt has been made to identify distinct microstructural zones and possible thermal cycles experienced by different regions of the weldment.     

Optimization of GTAW Al 3003 weld using fabricated nanocomposite filler metal

Novel Al–3 wt.% TiO₂ (fillers) nanocomposite has been successfully fabricated by accumulative roll bonding method to join Al 3003 alloy with the parameters of gas tungsten arc welding process such as the shielding gas flow rate (G), the welding speed (S), and the current (I) identified by response surface methodology. After preparation of weldment, studies involving X-ray diffraction and advanced electron microscopy techniques (scanning electron microscope and transmission electron microscope) have been carried out. FESEM has revealed the presence of second-phase hard ceramic particles which are embedded homogeneously in the matrix material. Energy dispersive spectroscopy analysis indicates the absence of foreign particles other than Al, Ti, and oxides. Further, the results show that the increase in mechanical properties (hardness and ultimate tensile strength) of the weldment is due to grain refinement and uniform distribution of TiO₂ nanoparticles. The analysis of variance test identified the welding speed as a significant parameter on the homogeneous distribution of TiO₂ particles followed by current and gas flow rate in sequential order.     

Microstructure,hardness and residual stress distribution of dissimilar metal electron beam welds: maraging steel and high strength low alloy steel

Abstract

The present investigation reports on a study that has been taken up to develop an understanding of the electron beam welding characteristics of similar and dissimilar combination of maraging steel and high strength low alloy steel, which are in the hardened condition, i.e. maraging steel, in a solution that was in treated and aged condition, whereas high strength low alloy steel in a quenched and tempered condition before welding. The joint characterisation studies include microstructural examination, microhardness survey across the weldment and measurement of residual stresses. Maraging steel weld metal is under compressive stress rather than tensile stress as observed in low alloy steel welds because the martensite transformation occurs at a relatively low temperature. It has been observed that, in dissimilar metal welds, tensile stress is observed at the fusion boundary of low alloy steel and weld metal, whereas compressive stress is obtained at the location between weld and maraging steel fusion boundary. Dissimilar weldment contains a soft region beside the interface on maraging steel side because of the diffusion of manganese from low alloy steel towards maraging steel. The observed residual stresses, hardness distribution across the similar and dissimilar metal welds are correlated with the observed microstructures.     

Effect of heat input and weld chemistry on mechanical and microstructural aspects of double side welded austenitic stainless steel 321 grade using tungsten inert gas arc welding process

Stainless steel 321 is a stabilized austenitic grade that prevents the formation of chromium carbides at the grain boundaries and subsequently reduces the risk of corrosion attack at the weld surface by forming titanium carbide. It is primarily used in industries such as pressure vessels, boilers, nuclear reactors, carburetors and car exhaust systems. In order to assess the effect of gas tungsten arc welding process parameters on weld penetration, the proposed Taguchi L₉ orthogonal matrix has been selected with two factors and three levels for welding austenitic stainless steel 321 by adjusting the welding current and welding speed. Bead-on-plate experiments were performed on base metal of 6 mm thick plate by changing the process parameters, and corresponding weld bead measurement and macrostructure images are examined. Maximum depth of penetration −3.3017 mm is achieved with a heat input −1.4058 kJ/mm, i. e., welding current-220 A and welding speed-120 mm/min. Double-side arc welding technique is used to obtain full penetration on 6 mm thick plate. The quality of the weldment was assessed using non-destructive radiography inspection. Mechanical integrity and microstructural characteristics of the weldments were studied using tensile (transverse and longitudinal), bend, impact, microhardness, optical microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction analysis, ferrite number measurement and scanning electron microscope. The results reveal that the double side-tungsten inert gas weldment have better mechanical properties. It is corroborated from the weld metal microstructure that it contains γ-austenite, δ-ferrite and titanium carbides (intermetallic compounds). X-ray diffraction analysis and energy dispersive x-ray spectroscopy plots confirm the increase in the ferrite phase in weld metal. The ferrite measurement results show that the ferrite volume in the base metal and weld metal is 1.2 percent and 6.1 percent respectively. In addition, the higher δ-ferrite volume in the weldment helps in attaining superior mechanical integrity. Fractography shows that the failure mode of the weld metal and the base metal is ductile.     

Characterization of microstructure,mechanical properties and corrosion resistance of dissimilar welded joint between 2205 duplex stainless steel and 16MnR

The joint of dissimilar metals between 2205 duplex stainless steel and 16MnR low alloy high strength steel are welded by tungsten inert gas arc welding (GTAW) and shielded metal arc welding (SMAW) respectively. The microstructures of welded joints are investigated using scanning electron microscope, optical microscope and transmission electron microscopy respectively. The relationship between mechanical properties, corrosion resistance and microstructure of welded joints is evaluated. Results indicate that there are a decarburized layer and an unmixed zone close to the fusion line. It is also indicated that, austenite and acicular ferrite structures distribute uniformly in the weld metal, which is advantageous for better toughness and ductility of joints. Mechanical properties of joints welded by the two kinds of welding technology are satisfied. However, the corrosion resistance of the weldment produced by GTAW is superior to that by SMAW in chloride solution. Based on the present work, it is concluded that GTAW is the suitable welding procedure for joining dissimilar metals between 2205 duplex stainless steel and 16MnR.     

低焊接裂纹敏感性钢回火组织及力学性能研究

为优高强度低焊接裂纹敏感性钢的力学性能,对其热轧态钢板进行了不同温度的回火实验.通过光学显微镜、扫描电镜和透射电镜观察了回火显微组织的演变特征,并结合相应的力学性能检测手段分析了不同回火温度下显微组织与力学性能的关系.结果表明,550℃回火后屈服强度和抗拉强度较热轧态强度分别提高了115和30 MPa,平均冲击功提高了...     

热处理对特种设备用高强钢激光–电弧复合焊接头氢脆断裂行为的影响研究

目的 提高800 MPa级特种设备用低碳贝氏体高强钢激光–电弧复合焊接头的抗氢脆性能。方法 采用预充氢后慢应变速率拉伸试样的方法,定量评估焊态、焊后直接高温回火和焊后调质3种状态下800MPa级低碳贝氏体高强钢激光–电弧复合焊接头的氢脆敏感性,结合扫描电镜下的初始微观组织和断裂特征,讨论抗氢脆性能的改善机理。结果 焊后调质处理有效消除了焊接热循环形成的马氏体组织,使接头各区域的微观组织趋于一致,接头的抗氢脆性能较焊态和直接焊后高温回火态的显著提高,断裂特征也从沿晶和穿晶的混合断裂转变为穿晶解理断裂。结论 焊后调质处理可以有效提高800MPa级低碳贝氏体高强钢激光–电弧复合焊接头的抗氢脆性能。     

Hot Corrosion Behavior of Friction Welded AISI 4140 and AISI 304 in K2SO-60% NaCl Mixture

Understanding the behavior of weldment at elevated temperatures and especially the corrosion behavior has become an object of scientific investigation recently.Investigation has been carried out on friction welded AISI 4140 and AISI 304 in molten salt of K 2 SO 4-60% NaCl environment at 550,600 and 650 C under cyclic condition.The resulted oxide scales in the weldment have been characterized systematically by surface analytical techniques.From the results of the experiments,it is observed that the scale thickness on low alloy steel side was higher than that on stainless steel side.Furthermore,weld interface has been found to be more susceptible to degradation than base metals due to inter diffusion of element across the interface and the formation of intermetallic compound.The influences of welding parameters and the temperature of exposure on the hot corrosion behavior of the weldment were discussed in this paper.     

Fracture responses of microstructures of electron beam-welded D6AC

In this study, D6AC steel was produced from flow forming (wall thickness reduction: 67%) and then it was subjected to electron beam welding and tempering treated. We characterized the specimens in terms of their microstructures and nanotribological properties. After tempering treatment at 315 °C, the yield strength increased from 381 to 1334 MPa and the percentage elongation increased remarkably from 0.5 to 7.8%. We performed these mechanical studies for the samples subjected to each set of experimental conditions, and determined the tribological characteristics from the heat-affected zone and the fusion zone. Lower degrees of adhesion reflected the presence of interlinks and rearrangements of indenter tip at lateral force that have been responsible for fluctuations in the coefficients of friction.     

Microstructural characterization of shielded metal arc weldments of a copper-bearing HSLA stee

Abstract

A systematic microstructural characterization in the heat-affected zone (HAZ) of two ASTM A710 grade A steel weldments (one preheated and the other pre–cooled), employing identical shielded metal arc welding conditions, has been performed. The microstructure in both the HAZ and the weld metal of both welds has been characterized by optical, scanning, and transmission electron microscopy in conjunction with microhardness traverses. No difference in microstructure was observed in the HAZ on comparing the preheated and non-preheated weldments. The only significant difference observed in the two weldments was the width of the HAZ, which is about 1 mm wider for a preheated weldment. Examination by transmission electron microscopy revealed the following microconstituents in the HAZ of both the weldments: polygonal ferrite, acicular ferrite, ferrite–carbide aggregates, ε-copper and fine cementite precipitates, martensite, tempered martensite, retained austenite, and transformation-twinned martensite. The microhardness traverse revealed almost identical hardness gradients in the two welds. The microstructural and microhardness data are discussed with regard to the preheating requirements for this alloy.

MST/118     

Study of welding characteristics of 0.3C–CrMoV(ESR) ultrahigh strength steel

A new ultrahigh strength low alloy steel 0.3C–CrMoV(ESR), having an ultimate tensile strength and 0.2% proof strength of above 1,700 and 1,500 MPa, respectively, in quenched and tempered condition, was developed primarily as a cost effective material for space launch vehicle applications. Welding is a major step in the fabrication of most of the pressure vessels, structures and equipments. Steels with carbon equivalent in excess of 0.40 wt% show a tendency to form martensite on welding, and therefore are considered difficult to weld. 0.3C–CrMoV(ESR) steel has a carbon equivalent value of nearly 1.0 that classifies it as a ‘very difficult to weld’ steel. In addition it has a niobium content of about 0.10% and a vanadium content of 0.25%. It is known that niobium content of more than 0.02 wt% has a deleterious effect on the toughness properties of low carbon welds. It has also been reported that the effect of niobium on weld metal toughness is more deleterious in the presence of vanadium. Hence, in the present study, the properties of the weldment of this new steel under different heat treatment conditions (HT-1 and HT-2) have been studied. In HT-1 condition, the plates were welded in hardened and tempered condition and no further heat treatment was given after welding, while in HT-2 condition, the annealed plates were subjected to welding followed by hardening and tempering heat treatments. For HT-1 condition, only tensile properties were evaluated. The welded plates under HT-2 condition were evaluated for tensile properties, fracture toughness, residual strength and microstructure features.     

Creep properties of 12% Cr and improved 9% Cr weldments

Martensitic Cr-alloyed high-temperature materials offer interesting opportunities for design and construction of advanced power plants. An extensive research programme has been carried out at the Research Centre of the Belgian Welding Institute and Laborelec on martensitic 12% Cr steel for gaining a better understanding of the failure mode and the deformation mechanism of welded joints under uniaxial and multiaxial loads. A large number of pipe girth welds were realized by three Belgian manufacturers (Cockerill Mechanical Industries, Fabricom and Mannesmann-Carnoy). Different filler metals were used and the influence of the welding regime (austenitic and martensitic), the post-weld heat treatment (PWHT) (single or double) and the base metal wall thickness on the high-temperature properties of the different weldments was evaluated. It was found that the creep properties of a 12% Cr weldment are not influenced by the welding regime and the base metal wall thickness. As would be expected, the creep strengths of the original 12% Cr base metal as well as the temperature of the PWHT have some effect. The existence of a typical failure in the intercritical zone (type IV region) is demonstrated and explained. The consequences for the design of welded 12% Cr components are indicated. More recently the research was extended towards improved 9% Cr steel (T91). A rather small preliminary programme for the orientation of further research showed a similar failure location as for 12% Cr steel, although the observed loss in strength of the weldment compared to the base metal tended to be considerably lower. The so-called ‘half-tempering’ treatment was tried out and the effect on the creep strength of the weldment is shown. A more fundamental national research programme on P91 steel has been established and is actually running.     

Process Parameters Optimization of Laser Beam Welded Joints by Neural Network

Laser beam welding of C-Mn steel plates with Ni powder filler metal has been performed. Metallography samples of the welded cross-section have been observed by scanning electron microscopy (SEM) and submitted to energy dispersive spectroscopy to obtain Ni concentration profiles. On the basis of the experimental results, neural networks have been carried out. These networks were first validated and then utilized to foresee Ni concentration along the welded thickness. The objective of obtaining the best Ni penetration and minimizing powder loss was reached optimizing, by numerical simulation, process parameters, such as powder rate and joint geometry.     

Microstructure evolution of hot-work tool steels during tempering and definition of a kinetic law based on hardness measurements

Microstructural evolutions of the 55NiCrMoV7 steel during tempering were investigated by transmission electron microscopy, scanning electron microscopy and X-ray diffraction in order to describe the main mechanisms of softening. The softening resistance is strongly associated with evolution of obstacles to the movement of dislocations (prior austenitic grain boundary, lath boundary, secondary carbides, etc.). Only the average size of carbides was found to be influenced by tempering conditions. Moreover, a strong correlation observed between the hardness measured after tempering and the average size of carbides showing that this easy test could in this case partially characterize the state of the microstructure after tempering. Performing hardness measurements at the as-quenched, tempered and annealed states, a kinetic law of tempering based on the work of Johnson, Mehl and Avrami has been proposed. This law was validated in the case of complex tempering and for other steels and can well describe the evolution of hardness during tempering.     

Effects of post-weld heat treatment cycles on microstructure and mechanical properties of electric resistance welded pipe welds

Two post-weld heat treatment cycles of one-step normalizing and two-step quenching and tempering have been performed by Gleeble, a thermo-mechanical simulator, to improve the toughness of fine-grained electric resistance welded pipe welds. Comparison was made to API X65 grade steel, which is widely used for pipeline parts. Microstructural evolution was investigated by optical microscopy and transmission electron microscopy. Vickers hardness and Charpy V-notch impact toughness tests were used to evaluate the mechanical properties. While the mechanical properties of one-step normalizing heat treatment satisfied the API specification, the two-step quenching and tempering heat treatments were conditional upon tempering temperature for X65 grade and fine-grained steels. As a result, a one-step normalizing heat treatment was more effective for both steel pipes.     

Microstructure of copper–AISI type 304L electron beam welded alloy

Abstract

Heterogeneous butt welding of copper and AISI type 304L stainless steel was carried out using the electron beam process. Examination by scanning and transmission electron microscopy has indicated the possibility of obtaining joints free of cracks and porosity. Energy dispersive microanalysis of the weld bead cross-section has demonstrated the presence of non-equilibrium phases. The results show that the binary Cu-Fe equilibrium diagram is unable to predict the weld microstructure even at the moderate cooling and solidification rates expected under the present welding conditions. The feasibility of the Cu-304L electron beam welding process is therefore hindered by the problem of microstructural stability of the joint because of possible phase transitions during the service life of welded components.     

Relational analysis between parameters and defects for electron beam welding of AZ-series magnesium alloys

In the last half century, lightweight magnesium alloy has gradually shifted from military applications to civil applications. More noteworthy is that its low melting point, high thermal conductivity, and superior fluidity are good for weld pool flow and welding parameter research. This paper presents a novel approach to these characteristics, which analyzes the influences of electron beam welding parameters on weldment strength and defect formation by linking Taguchi's method with the grey relational analysis. Not only are the parameter contribution and the defect weight individually quantified, but also the relationship between welding parameters and defect dimensions can also be obtained this way.     

On the microstructure and the origin of intermetallic phase seams in magnetic pulse welding of aluminum and steel

The microstructure of aluminum‐steel compounds fabricated by magnetic pulse welding are investigated. Electron backscatter diffraction, energy dispersive x‐ray spectroscopy and scanning electron microscopy revealed the existence of several different phases along the weld seam. While one layer could be identified as aluminum solid solution, a very thin layer close to the steel side of the compound eluded detailed characterization by scanning electron microscopy techniques. Transmission electron microscopy and selected area electron diffraction investigations of this layer revealed a mix of nanocrystalline and amorphous parts. When ambient pressure was reduced to 1 mbar during welding, no interlayers were observed in the samples. This suggests that the interlayers are precipitates of the jet that is formed during conventional impact welding.     

EFFECT OF ELECTRON BEAM WELDING ON FATIGUE CRACK GROWTH RATE IN AISI 4130 STEEL

Abstract— The purpose of this study is to investigate the effect of electron beam welding parameters on fatigue crack growth rate in AISI 4130 steel. The welding method was carried out by using nonoscillation and oscillation electron beam patterns. In nonoscillation welding, the welding speed, was varied; in oscillation welding just the oscillation pattern and the oscillation frequency were varied. After welding, X-ray inspection, metallography, fatigue crack growth tests and scanning electron microscopy were employed to characterize these specimens. From the results, the weld centerline solidification structure transforms from a longitudinal raft structure to an interlocked structure with increasing welding speed, and the fatigue crack growth rate increases slightly. The transverse oscillation pattern provides a lower fatigue crack growth rate than the longitudinal oscillation pattern. The higher oscillation frequency gives a lower fatigue crack growth resistence for the longitudinal oscillation pattern, but the opposite is the case for the transverse oscillation pattern.