As-welded Specimens Research Articles

Improvement of Extremely Low Cycle Fatigue Strength of Welded Joints by Toe Finishing

The improvement of extremely low cycle fatigue strength of welded joints by TIG dressing and burr grinding was investigated. The weld toes of T-shape welded joints were improved by TIG dressing or burr grinding and tested under displacement control conditions. The crack initiation life of finished specimens was at least threefold longer than that of as-welded specimens. There was no clear difference in improvement between TIG dressing and burr grinding. We also analysed the local strain around the cracking point by finite element analysis and performed local strain-based fatigue assessment. The results indicated that extremely low cycle fatigue strength of the joints improved by TIG dressing and burr grinding can be assessed by evaluating the local strain and referring to the extremely low cycle fatigue strength curve.

공압식 헤머피닝을 이용한 용접부 피로수명 연장기술 연구

Fatigue failures are often occurred at welded joints where stress concentrations are relatively high due to the joint geometry. Although employing good detail design practices by upgrading the welded detail class enables to improve the fatigue performance, in many cases, the modification of the detail may not be practicable. As an alternative, the fatigue life extension techniques that reduce the severity of the stress concentration at the weld toe region, remove imperfections and introduce local compressive welding residual stress, have been applied. These techniques are also used as definite measures to extend the fatigue life of critical welds that have failed prematurely and have been repaired. In this study, a hammer peening procedure for using commercial pneumatic chipping hammer was developed, and the effectiveness is quantitatively evaluated. The pneumatic hammer peening makes it possible to give the weld not only a favorable shape reducing the local stress concentration, but also a beneficial compressive residual stress into material surface. In the fatigue life calculation of non-load carrying cruciform specimen treated by the pneumatic hammer peening, the life was lengthened about ten times at a stress range of 240MPa, and fatigue limit increased over 65% for the as-welded specimen.

Investigation into Microstructures of Maraging Steel 250 Weldments and Effect of Post-Weld Heat Treatments

This study was undertaken to gain a better understanding of microstructures obtained by multipass gas tungsten arc welding in maraging steel grade 250. Metallography and microhardness measurements were carried out on sheet and welded joints in as-welded and post-weld aged conditions. It was found that there was a significant amount of reverted austenite formed on cell boundaries of weld metal after aging at 758-823 K for 3-5 h, and was stable at room temperature. Aging at higher temperatures led to an increase in the continuous network of patchy austenite along the cell boundaries. The reason for the above, in our opinion, is the concentrational heterogeneity which characterizes the microstructure of maraging steel welds. No reverted austenite was observed in as-welded specimens. Solution annealing at 1093 K for 1 h did not completely eliminate the chemical heterogeneity associated with weld structures. However, homogenizing at 1373 K produced homogenous structure that on subsequent aging produces austenite-free lath martensitic structure.

Microstructure and Notch Properties of Heat-Treated Ti-4.5Al-3V-2Mo-2Fe Laser Welds

The microstructure and transformation behavior of Ti-4.5Al-3V-2Mo-2Fe alloy and its laser welds after various heat treatments were investigated. Notch properties, such as impact toughness and notched tensile strength, were also measured on the welds to choose an appropriate post-weld heat treatment (PWHT) of the alloy. The temperature regimes in whichtransformed into � 0 , � 00 or retained asafter quenching were identified and discussed. The results indicated that � 00 could be obtained primarily by rapid quenching from 880� 840 � C solution temperatures and was identified as a base-centered orthorhombic (centered on the C face) with the lattice parameters of a ¼ 0:305 nm, b ¼ 0:489 nm, and c ¼ 0:457 nm. The as-welded specimen exhibited fine acicularin thematrix with hardness considerably higher than the mill-annealed base metal. For a PWHT temperature lower than 800 � C, the change in microstructure and hardness of the welds depended mainly on the temperature, not on the cooling rate. If the PWHT temperature was higher than 800 � C, both the temperature and the cooling rate were important in altering microstructure and hardness of the welds. The welds after a 704 � C/4 h treatment could prevent notch brittleness, reduce hardness variation in different regions of the weld, and obtain notch properties similar to the mill-annealed base metal. (doi:10.2320/matertrans.MRA2008370)

Effect of Cu and B addition on tempering behavior in the weld CGHAZ of high strength low alloy plate steel

To understand the effect of Cu and B on mechanical properties of the weld coarse-grained heat-affected zone (CGHAZ) of high strength low alloy plate steels, six alloys including different Cu and B content were prepared. The CGHAZ and tempered conditions were simulated by a Gleeble simulator and mechanical properties were measured by Vickers hardness test and charpy impact test. Toughness of as-welded specimens was deteriorated with comparing to base steels and decreased with increasing Cu + B content. Meanwhile, Vickers hardness showed an opposite tendency, which is due to formation of martensite in the weld CGHAZ. For as-tempered conditions, Vickers hardness at first decreased with increasing tempering temperature and then was reinforced at around 550 °C with an unexpected decrease of toughness, which was mainly due to the precipitation of Cu particles and carbides, such as (Ti, Nb)C and (Mo, Mn) 2C.

Effects of residual stress and heat treatment on fatigue strength of weldments

Effects of residual stress and heat treatment on fatigue strength of weldments were investigated with lateral link specimens and sway bar bracket specimens. Residual stresses in the thickness direction at a weld toe location were measured by the X-ray diffraction (XRD) method and the hole drilling method for as-welded and post-heat-treated specimens. Compressive residual stresses on the surface were obtained from the as-welded specimens, while zero or very low tensile residual stresses were obtained from the heat-treated specimens. Microstructural differences between as-welded and heat treated specimens were also investigated in this study. Heat-treated specimens showed fine grains at the weld zone compared to that of as-welded specimens. Fatigue strength of as-welded specimens was found higher than that of heat-treated specimens due to compressive residual stresses induced at the weld toe areas. This study also predicted the fatigue life of as-welded specimens with commercial fatigue software and the life predictions were compared with experimental results.

Fatigue Life Improvement Using Mechanical Post Treatment for Weldment

Mechanical post treatments for welded structures have been applied in various industrial fields and, in most cases, have been found to cause substantial increase in their fatigue strength. These methods, generally, consist of the modification of weld toe geometry and the introduction of compressive residual stresses. In hammer peening, the weld profile is modified due to removal or reduction of minute crack-like flaws; compressive residual stresses are also induced by repeated hammering of the weld toe region with blunt-nosed chisel. In this study, a hammer peening procedure, using commercial pneumatic chipping hammer, was developed; a quantitative measure of fatigue strength improvement was performed. The fatigue life of hammer-peened specimen was prolonged by approximately 10 times in S=240MPa, and was doubled for the as-welded specimen.

Effect of multiple repairs in girth welds of pipelines on the mechanical properties

This work presents the results of multiple weld repairs in the same area in seamless API X-52 microalloyed steel pipe. Four conditions of shielded metal arc welding repairs and one as-welded specimen of the girth weld were characterized to determine changes in the microstructure, grain size in the heat affected zone, and to evaluate their effect on the mechanical properties of the weld joints. The mechanical properties by means of tension tests, Charpy-V impact resistance and Vickers hardness of the welds were analyzed. The results indicate that significant changes are not generated in the microstructural constituents of the heat affected zone. Grain growth in the heat affected zone at the specimen mid-thickness with the number of repairs was observed. Tensile strength of the weld joints meets the requirement of the API 1104 standard even after the fourth weld repair. Significant reduction in Charpy-V impact resistance with the number of weld repairs was found when the notch location was in the intersection of the fusion line with the specimen mid-thickness. A significant increase in the Vickers hardness of the heat affected zone occurred after the first repair and a gradual decrease in the Vickers hardness occurred as the number of repairs increases.

Effect of Post-Weld Straining on Temper-Rolled Austenitic Stainless Steel Welds

Temper-rolling increases the mechanical strength in metastable austenitic stainless steels of 1.4310 (AISI 301) type. However, these grades are subject to local softening in the weld region after welding. This loss has been characterised for various temper-rolled conditions after GMAW, GTAW and Nd:YAG laser welding. Different levels of post-weld straining were applied across the welds in an attempt to restore the strength by a combination of normal work hardening and hardening due to martensite formation. The martensite transformation in this type of steel can be used for improved formability, so called transformation induced plasticity (TRIP). The effects of post-weld straining of the weldment were evaluated for each welding method and material by means of hardness profiles, tensile tests and optical strain measurements. The extent of strength loss and compensation found, with and without weld bead reinforcement has been assessed. The results are discussed in terms of differences between welding methods, the behaviour of individual regions in the weldments and practical applications. Laser welds gave the highest strength in the as-welded condition while GTA welded joints were the most ductile and showed the greatest potential for strength restoration by post-weld straining. For GMAW the filler metal determined the strength level of the joint. Fatigue test results of as-welded GMA specimens are reported.

Capacitor discharge welded bars of Inconel 718 and TiAl6V4 superalloys under fatigue

This work concerns the mechanical behaviour analysis of Capacitor Discharge Welded joints made with Inconel 718 and TiAl6V4 superalloys in the form of cylindrical bars. The research activities have been conducted in the frame of AWFORS project, financed by the European Community. The Capacitor Discharge Welding (CDW) technique is of particular interest for repair processes; it allows the substitution of the damaged part after its removal and the replacement of the new component without consistent additional machining. Good mechanical properties and geometrical integrity are to be achieved; consequently, the multipoint contact profile for the CDW process has been conceived to enhance the microstructural weld characteristics and improve the fatigue and creep behaviour at high temperatures. Fatigue tests have been performed both at room and working temperature, comparing the results between base material and as-welded specimens with the same heat treatment. The microstructure has been deeply analyzed through optic and scanning electron microscope (SEM) observations; this way the microstructural modifications and the Heat Affected Zones have been studied in the welded section as well as the interaction between the weld layers material and the parent metal.

A Study on Fatigue Strength Improvement of Aluminum Alloy Resistant Spot Welds by Cold Working

Resistance spot welds (RSW) have been widely used in the sheet metal joining process due to high productivity, low cost and convenience. The application of aluminum alloys in automobile manufacturing has resulted in the reduction of weight in vehicle, fuel economy and improvement of drivability and driving performance. Therefore, the resistance spot weld (RSW) process has attracted attention to fabrication of aluminum structures. However, applications of Al RSW are limited due to the unreliable durability of the weld-bonded joints. This study presents an experimental investigation on the use of a post-weld cold working process to improve the fatigue strength of Al RSW. The post-weld cold working process includes a pair of uniquely shaped indenters that are pressed or driven into the structure in order to induce the compressive residual stresses around the RSW. The mechanical properties of the post-weld cold worked Al RSW were investigated, including the experimental results of fatigue. The effect of the post-weld cold working process parameters on the fatigue strength of the Al RSW was also investigated. Comparisons of the mechanical properties and qualitative results between the as-welded RSW specimens and the post-weld cold worked RSW specimens of each post-cold working parameter have been carried out. Also, discussions on the experimental results using the finite element method (FEM) for the post-weld cold working parameters are performed.

The influence of aging treatments on sulfide stress corrosion cracking of PH 13-8 Mo steel welds

Slow displacement rate tensile tests were carried out to study sulfide stress corrosion cracking (SSCC) of PH 13-8 Mo stainless steel welds in a saturated H 2S solution. The welds aged in the temperature range of 482–593 °C were susceptible to SSCC; the fracture surfaces revealed mainly quasi-cleavage fractures after notched tensile tests. However, the SSCC susceptibility in terms of the percentage loss of the notched tensile strength (NTS) of the welds was dependent on the aging treatment. The SSCC resistance and the austenite content of the welds increased with the aging temperatures. The presence of greater amounts of austenite, mainly reverted austenite, in the W1100 specimen (the weld aged at 1100 °F or 593 °C) than that in other aged specimens could account for its lower hardness and better SSCC resistance. On the other hand, the AW (as-welded) specimen containing a small amount of retained austenite films in a soft matrix exhibited a slightly improved SSCC resistance than that in the W1100 specimen. The lower hardness of the AW specimen was owing to the absence of fine coherent precipitates, leading to a reduced local stress and an enlarged plastic zone located in front of the notch in the test. With lower hardness, the local stress would also be lower and less likely to exceed some critical stress for failure in the saturated H 2S solution. For the aged specimens, the hardness/strength level and the amount of reverted austenite were the important factors that affect SSCC susceptibility.

Post-Weld Heat Treatment Stress Relaxation in Zircaloy 4 Plasma Welds

We have studied the effect of a post-weld heat treatment on plasma arc welds on Zircaloy 4 plates. The samples consist of two 100 mm long, 50 mm wide, and 6.25 mm thick plates, welded along the rolling (longitudinal) direction. The heat-treatment consisted of a steady increase in temperature from room temperature to 450oC over a period of 4.5 hours; followed by cooling with an equivalent cooling rate. Residual strains and stresses along the longitudinal, transverse and normal directions on an as-welded and a heat-treated specimen were measured by neutron diffraction on the ENGIN-X beamline at the Isis Facility, Rutherford Laboratory, UK. Peak tensile stresses of (105±25) MPa were found in the as-welded specimen, which were reduced to (70±20) MPa after the heat-treatment. Thermal compressive stresses of (-80±20) MPa were found along the normal direction, which were not affected by the heat treatment. The use of a full-pattern Rietveld refinement for the determination of bulk strains in Zircaloy specimens is also discussed.

二相ステンレス鋼溶接金属における水素とシグマ相の相互作用

The effect of sigma phase on hydrogen embrittlement in 329J3L duplex stainless weld metal was studied. In addition, in order to clarify the mechanism, hydrogen behavior was also investigated by means of internal friction and fractography technique. The internal friction measurements were conducted over the temperature range from 77K to 373K by using an inverted torsion pendulum for a frequency of about 1.5Hz. After hydrogen charging, almost hydrogen can be diffused easily into the austenite through the ferrite and ferrite/austenite interfaces. Consequently, a sharp significant peak due to hydrogen in austenite was detected at 245K in the as-welded specimen. In the case of as-welded specimen a little reduction of strength was recognized, while the significant loss of strength was observed in the weld metal containing significant sigma phase after hydrogen charging.The sigma phase clearly accelerated hydrogen embrittlement. In the hydrogen-charged specimen containing significant sigma phase, the internal friction analysis revealed that hydrogen entered in the sigma phase lattice and hydrogen was also trapped sigma/austenite phase boundaries. Furthermore, on the fracture surface, the many secondary cracks associated with interaction between sigma phase and hydrogen were observed. Therefore, it could be concluded that sigma phase itself and sigma/widmanstatten austenite phase boundaries were preferential hydrogen cracking sites. It was found that the reduction of notch tensile strength was mainly attributed to many secondary cracks associated with hydrogen and sigma phase.

Electrochemical study of hydrogen embrittlement and optimum cathodic protection potential of welded high strength Steel

Electrochemical properties and mechanical properties of welded high strength steel were investigated by the slow strain rate test (SSRT) method with an applied constant cathodic potential. There was no correlation between maximum tensile strength, yield strength, stress at failure, and hydrogen embrittlement. However, the elongation, time-to-fracture, and strain-to-failure ratio decreased with shifting potential of the negative direction. Elongation, time-to-fracture, and strain-to-failure ratio for the various welding methods displayed the highest values when the potential was −770 mV regardless of post-weld heat treatment (PWHT) conditions. The elongation, time-to-fracture, and amount of dimples by PWHT were increased. The optimum potential region for cathodic protection without hydrogen embrittlement was observed between −770 and (above) −850 mV (SCE) in the post-weld specimens heat treated at 550°C and the as-welded specimens.

スーパー二相ステンレス鋼溶接金属における水素ぜい性の破壊解析

The influence of sigma phase on the hydrogen embrittlement in the 329J4L super duplex stainless weld metal was investigated by 3-dimensional fracture analysis. In the case of ferrite-austenite as-welded specimen, a little reduction of notch tensile strength was recognized since a little hydrogen cause the hydrogen embrittlement of ferrite. On the basis of the microstructure examination, hydrogen cracking preferentially occurred in the ferrite, while widmanstatten austenite showed ductile fracture. On the one hand, when hydrogen charging was carried out for weld metal precipitated 40% sigma phase (sigma phase-austenite microstructure), the notch tensile strength drastically dropped compared to hydrogenated as-welded specimen. This strength loss had a dependency of the amount of sigma phase. The susceptibility to hydrogen embrittlement of sigma phase was very high and it was clearly found that sigma phase strongly enhanced hydrogen embrittlement.The fracture surface was corresponded to typical widmanstatten microstructure. The area of sigma phase was always lower than other ductile area on the 3-dimensional fracture surface since sigma phase strongly promoted hydrogen embrittlement. From the viewpoint of microstructure, the ferrite almost decomposed and 40% sigma phase was present. Consequently, it could be concluded that sigma phase itself and sigma/widmanstatten austenite phase boundaries were preferential hydrogen cracking sites in the 329J4L weld metal.

잔류응력을 고려한 용접 이음부의 피로수명 평가 모델

According to our fatigue tests carried out at 20 Hz, R=0.1 on transversely butt-welded joints, fatigue strengths of as-welded specimens, that is, specimens having residual stress are higher than those of annealed specimens in short life range, but vice verse in long life range. This behavior seems to be concerned mainly with residual stress relaxation by applied loading. After analyzing the welding process, we conducted finite element analysis to quantify the degree of residual stress relaxation. By taking into account residual stress relaxation, modified Goodman diagram, and nominal stress, we evaluated the fatigue life of the welded joint from the S-N curve for the parent material. The estimated results are in a good agreement with the experimental results.

Fatigue crack growth of AISI 304 stainless steel welds in air and hydrogen

Fatigue crack growth behavior of AISI 304 stainless steel plate and its plasma weld was evaluated in laboratory air and gaseous hydrogen. Observed variation in crack growth characteristics are explained based on the martensitic transformations near the crack tip due to mechanical loading and environmental action. The microstructures of fusion zone consisted of lacy ferrites in the columnar grain and subgrain boundaries within the austenite matrix, versus equiaxial austenite grains with some twins inside the parent metal. The threshold Δ K of the as-welded (AW) specimen required to initiate crack growth was clearly higher than that of the other specimens, regardless of testing environments. The considerable retardation of crack growth vanished for welds subjected to stress-relief treatment. In addition, all specimens tested in gaseous hydrogen exhibited enhanced crack growth. The fatigue-fractured surface of welds was found to be very rough suggesting a zig-zag crack path, unlike the straight path in the base metal. The lack of twin boundary failures in addition to the irregular solidified microstructure could be the causes for the improved resistance to fatigue crack growth in case of the stress-relieved weld. Moreover, the thin martensite layer formed on the cracked surface was correlated with the hydrogen-enhanced crack growth, resulting in inducing extensive quasi-cleavage (QC) fracture for the specimens tested in hydrogen.

Inertia welding nickel-based superalloy: Part II. Residual stress characterization

The next generation of Ni-based alloys for aeroengines are richer in γ′ than existing alloys and are more difficult to weld by conventional means. Inertia welding is currently being developed as a joining technique for these alloys. Steep microstructural gradients have been observed in nickel-based superalloy RR1000 tube structures welded by inertia friction welding,[1] and in this article, the concomitant residual stresses are mapped at depth using neutron diffraction. One tube in the aswelded and two in the postweld heat-treated (PWHT) condition have been investigated. In the case of the as-welded specimen, it was necessary to establish the variation of the stress-free lattice parameter, a0, across the weld line to infer elastic strain from lattice spacing changes. A biaxial sin2ψ measurement on thin slices was used to determine a0 as a function of the axial position from the weld line. This was in excellent agreement with the variation inferred by imposing a stress balance on the axial measurements. The change of a0 across the weld line can be rationalized in terms of the observed variation in the element partitioning effect between the matrix (γ) and the precipitates (γ′). It was found that the residual stresses in the weld and heat-affected zone generated by the welding process are large, especially close to the inner diameter of the welded ring. The experimental results have shown that, in order to relax the residual stresses sufficiently, the heat-treatment temperature must be increased by 50 °C over the conventional heat-treatment temperature. This is due to the high γ′ content of RR1000.

The Influence of Vibratory Treatment on the Fatigue Life of Welds: A Comparison with Thermal Stress Relief

Abstract: A comparison has been made between the fatigue lives of welded specimens a) in the as welded condition b) after heat treatment, and c) after post‐weld vibration. By comparison with the as‐welded specimens, the fatigue lives of the thermally relieved specimens were found to decrease by 43%, while the vibration treated specimens showed an increase of between 17% and 30%. While these findings are interesting in that they offer a way of extending the fatigue lives of welded joints, they confirm the view that the mechanism of residual stress relieving in the vibratory stress relief (VSR) method, and its relationship with fatigue life is poorly understood.