As-welded Specimens Research Articles

Fatigue life prediction in AlMgSi1 lap joint weldments

The fatigue behaviour of an AlMgSi1 alloy lap welded joint and the improvement in fatigue strength due to post-weld heat treatment were investigated. Two series of fatigue tests were performed: one in as-welded specimens and another in heat-treated specimens. The prediction of crack initiation life under a plane strain condition was made based on the local strain approach using Morrow's modified equation. The local inelastic stresses and strains at the weld toe were calculated for the heat-treated specimens using three methods: Neuber, Glinka and finite element analysis. The accuracy of the different methods were evaluated and discussed.

溶接止端部改良による疲労強度向上法

In this study, fatigue strength improvement methods of weld joints, such as TIG-dressing, hammer peening and burrgrinding are discussed with T-form fillet welded joint specimens. Concerning the crack initiation life, crack propagation life, and improvement mechanism of fatigue strength, fatigue testing, FEM analysis, measuring residual stresses, observation of crack surface and fracture mechanics approach are carried out.As a result of fatigue testiness, fatigue strengths of all types of improved speciments, especially burr-grinded specimens are higher than as-welded specimens. Effect of decreasing stress concentration by developing the configuration of welded toe is found only at 1 mm thick from treated surface, so that fatigue strength improvement effect of TIG-dressing and burr-grinding is observed only during fatigue crack length is small (Nc). On the other hand, by hammer peening high compressive residual stresses are induced about at 4 mm thick from treated surface so that improvement effect is obtained during crack propagation life (Np). However, hammer peening induces ditch which is about from 0.2 to 0.4 mm deep from the surface and the ditch work as source of stress concentration like under-cut, improvement effect of hammer peening is not found during crack is small (Nc).

Analytical Study on Fatigue Strength Reduction Factor of Small-Diameter Socket-Welded Pipe Joints

Four-point bending and rotating bending fatigue tests were conducted on socket-welded joints made of carbon, stainless, and Cr-Mo steels for clarification of the effects of diameter, welding pass sequence and post-weld heat treatment (PWHT) on fatigue strength. The results were evaluated quantitatively. Fatigue strength of socket-welded joints was found to strongly depend on weld pass sequences in fillet welds, this being possibly due to large change in residual stress distribution at roots and toes. The effects of residual stress were thus examined quantitatively by comparison of fatigue strength of PWHT stress-free specimens with that of as-welded specimens. By the modified Goodman’s method, the lowest S-N curve corresponding to maximum tensile residual stress and the highest S-N curve corresponding to maximum compression residual stress were obtained for different steels and diameters. Conventional S-N data of socket-welded joints were situated between these two limiting curves. Based on the lowest curve, fatigue strength reduction factors of socket-welded joints were proposed.

Hydrogen sulphide stress corrosion cracking of 2.25Cr-Mo steel weldments

Constant extension rate tensile (CERT) tests were performed to investigate the fracture characteristics of the A387 steel weldments in an H 2S charging environment. The heterogeneous (HG) welds using a stainless steel ER309-16 as a filler metal displayed inferior mechanical properties. In the as-welded condition, the heat-affected zone (HAZ) with coarse-grained structures initiated the crack. The propagation proceeded within the diluted region along the fusion boundary, owing to the formation of martensite in that region. Both the untempered HAZ and the diluted region revealed a high susceptibility to hydrogen embrittlement. The precipitation of Cr-carbides along the fusion boundary was found after tempering at 690 °C for 1 h, creating a more favorable path for the crack growth. The homogeneous (HM) welds, which were made by using the equivalent composition alloy as a filler metal (ER9016-B3), revealed that the coarse-grained HAZ of the as-welded specimen caused premature failure in H 2S. The tempered HM welds showed somehow improvement in strength and ductility in H 2S, but the crack still initiated at the coarse-grained HAZ of a weld.

Fractographic Studies on Charpy Impact Tested Ti6Al4V Weld Metals / Bruchflächenuntersuchungen an Charpy-Kerbschlagproben aus Ti6Al4V-Schweißgut

Fractographic features of the charpy impact tested weld metals of Ti6Al4V (ELI and commercial grades) made by autogenous welding of electron beam and gas tungsten arc welding processes have been studied and correlated with their impact toughness values. The instrumented impact testing was carried out on the as-welded and post weld heat treated specimens. GTA and two pass EB B-o-B weld metals showed less or no facets on the fractured surface compared to stringer bead EB weld metals. As-welded specimens of EBW (stringer bead) and samples water quenched from above β-transus temperature which showed lower CVN impact values showed intergranular fracture. Weld metals heat treated below β-transus which showed relatively better impact energy values exhibited transgranular type fracture.

Study on threshold fatigue crack growth characteristics of electron beam welding in AISI 4130 steel

Near-threshold fatigue crack growth of an electron beam weldment in AISI 4130 steel is studied. To increase the fatigue resistance ability, two different post-weld heat treatment processes are adopted to compare the near-threshold fatigue crack growth data with that of an as-welded specimen. The electron beam tempering in the vacuum chamber immediately after the welding and the traditional furnace tempering treatment are chosen for comparison. Strength of the welded joints is evaluated by a tensile test and near-threshold fatigue properties are investigated by K-decreasing method. The resistance to near-threshold fatigue crack growth is improved with post-weld heat treatment, due to the residual stress relief effect and existence of a toughened tempered structure. For electron beam post-weld heat treatment with 6mm dot pattern width, the threshold stress intensity factor range increases by 14% compared to that of an as-welded specimen and the data of furnace post-weld heat treatment at 550°C can be improved by as much as 26%.

Residual Stresses and Fatigue Precracking Techniques for Weldment Fracture Toughness Specimens

Abstract This article examines the ability of the local compression, stepwise high R-ratio, and reverse bend weldment fatigue precracking techniques to remove the residual stresses in the uncracked ligaments of as-welded fracture mechanics test specimens. The stepwise high R-ratio and reverse bend techniques appeared to have no significant influence on the residual stresses in the ligament. This was demonstrated by the occurrence of nonuniform fatigue cracks when these techniques were followed by the standard fatigue precracking procedures. Specimens given these treatments, and other specimens fatigue precracked using the standard procedures, showed evidence of crack closure and initial compliances that were lower than normal for these specimen geometries. In contrast, local compression reduced the residual stresses in the uncracked ligaments to low, uniform levels. This was indicated by acceptable fatigue precrack front-straightness and normal specimen compliance. Therefore, of the three precracking techniques investigated, only the local compression technique is recommended.

Effect of Carbon Content on the Characteristics of AS-Welded and Step-Cooled AWS E-8018B2 Type Weld Metal

The effect of carbon content on the microstructure and mechanical properties of weld metal deposited with an AWS E-8018B2 type electrode was studied in both as-welded and heat treated states. Carbon influences markedly the microstructure and carbide precipitation. In the present study, experimental coated electrodes were prepared in order to deposit weld metal with carbon in the range 0.04–0.16 wt%. The presence of grain boundary components was progressively reduced and a general refinement of the microstructure was observed as carbon content increased. Stress relief treatment resulted in an improvement in notch toughness and produced a weld metal with lower hardness than the corresponding as-welded specimens. After step cooling, a decrease in notch toughness was observed for carbon levels over 0.10% as a result of carbide coarsening and transformation.

Fatigue Strength of Electron-Beam Welded Joint of Injection-Moulded and Sintered Stainless Steel.

Mechanical properties of the electron-beam welded joints of the sintered SUS304L which are produced by the injection moulding method are investigated to clear the influence of welding on the tensile and the fatigue strength.The results obtained are summarized as follows;(1)Formation of welding defects is strongly affected by beam active parameter and welding speed.When they have appropriate values, welding defects aren't created, except that large voids are formed at or near the bond.(2)When the sintered stainless steel is welded under the optimum welding condition, the tensile strength of the as-welded joint decreases about 10% and the fatigue limits of the as-welded joint and the joint whose undercut is removed decrease about 20% and 5% respectively as compared with that of the un-welded one.(3)In as-welded specimen, the fatigue crack of the electron-beam welded joint initiates at the root of the undercut in welds, on the other hand, in the specimen whose surface irregularity is removed, it initiates at the intereface of delta phase and austenite phase near the bond.(4)The fatigue crack of the electron-beam welded joint of the sintered stainless steel propagates along the bond because large voids exist there.(5)It is confirmed that the electron-beam welded joints of the sintered stainless steels under the optimum welding condition can be put into practical use.

Cracking by elevated temperature embrittlement in the HAZ of alloy 800H

Several cases of cracking failures in the HAZ of alloy 800H pipe have been experienced in equipment operating in the temperature range of 550-700°C. The influence of several heat treatments on the cracking sensitivity by cylindrical type restraint cracking test was investigated. And the mechanism of this crack formation was considered.Main results obtained in this research were as follows.(1) Intergranular oixidation and cracking were detected at the bottom of notched HAZ area of as-welded specimens in the temperature range of 600-650°C. The area of cracking coincided with the area of γ' phase precipitation.(2) By PWHT of solution heat treatment, HAZ cracking was not detected in the specimens aged for 3000 hours at 600°C, while only intergranular oxidation was detected. The sensitivity of this cracking can be reduced remarkably by the use of the PWHT of solution heat treatment that reduces the initial weld residual stress.(3) When the cohesive force between grain boundaries is severely reduced by intergranular oxidation and weld tensile residual stress acts upon grain boundaries, the intergranular cracking occurs.

Ａｌｌｏｙ ８００ Ｈ溶接熱影響部の高温経年劣化割れ

Several cases of cracking failures in the HAZ of alloy 800H pipe have been experienced in equipment operating in the temperature range of 550-700°C. The influence of several heat treatments on the cracking sensitivity by cylindrical type restraint cracking test was investigated. And the mechanism of this crack formation was considered.Main results obtained in this research were as follows.(1) Intergranular oixidation and cracking were detected at the bottom of notched HAZ area of as-welded specimens in the temperature range of 600-650°C. The area of cracking coincided with the area of γ' phase precipitation.(2) By PWHT of solution heat treatment, HAZ cracking was not detected in the specimens aged for 3000 hours at 600°C, while only intergranular oxidation was detected. The sensitivity of this cracking can be reduced remarkably by the use of the PWHT of solution heat treatment that reduces the initial weld residual stress.(3) When the cohesive force between grain boundaries is severely reduced by intergranular oxidation and weld tensile residual stress acts upon grain boundaries, the intergranular cracking occurs.

Effect of heat treatment on mechanical behavior of electron beam welded sintered molybdenum

The effect of heat treatment on the weldability of sintered molybdenum was examined by tensile properties at room temperature. Fracture surfaces were characterized by scanning electron microscopy and scanning Auger electron spectroscopy. In as-welded specimens, tensile strength exhibited a considerable increase by stress relief annealing and a rapid decrease by the annealing before welding. An increase of the annealing temperature before welding served to reduce ductility. Post-weld heat treatment reduced the strength in the specimens which were annealed up to 1173 K before welding. But much improvement in strength was gained by increasing post-weld annealing temperature in the annealed welds before welding. Ductility was also improved considerably with an increase of post-weld annealing temperature. Fracture initiated predominantly at grain boundaries and propagated in a mixed manner in weld metal. Nitrogen and oxygen segregation seemed to promote decohesion of grain boundaries. Carbon segregation and precipitation by heat treatment, especially during post-weld annealing, strengthened the cohesion of grain boundaries and resulted in improving the ductility.

Environmental strength evaluation of welded steel joint in seawater

A study on corrosion fatigue was experimentally conducted for the as-welded and PWHT specimens of the steels, HT80 and SM53B in 3.5% NaCl solution. Submerged arc welding was done. PWHT was carried out at comparatively high temperature of 650° C. Besides, in order to simulate the residual stress in weld HAZ, the stress of 98MPa was applied during PWHT. Corrosion fatigue crack growth was dependent upon the materials and PWHT conditions. In the case of HT80, crack growth in corrosion environment was faster than that in air. However, the crack growth of the main crack for SM53B in 3.5% NaCl solution was decreased in comparison with that in air, unlike HT80. The sensitivity to corrosion environment was reduced due to PWHT. The applied stress in HAZ during PWHT acted to enhance the crack growth compared with that of the PWHT specimen without stress.

Microstructural transformations in austenitic-ferritic transition joints

The relatively complex microstructures developed at the interface between ferritic steel and weld metal on austenitic-ferritic transition joints have been examined by metallographic observation and by hardness tests in the as-welded condition and in the as-welded-and-tempered condition. Both austenitic stainless steel and nickel-based filler metals were used in welds. On as-welded specimens a sharp change of hardness in low-alloy steel has been measured, with increasing distance from weld metal; the hardness values have been related to the observed metallographic constituents. On post-weld heat treated specimens, the behaviour is different according to the composition of filler material, either austenitic steel or nickel-based alloy. In the case of austenitic filler material, a dark-etching narrow diffusion region of carbon toward weld metal is formed, with an adjacent markedly decarburized zone, exhibiting the minimum microhardness values in a narrow band of about 60 micrometres. Since this sharp structural variation is recorded just in the zone where often failures occur, the final post-weld heat treatment appears to be proposed with due caution. In the case of nickel-based filler material, carbon diffusion is inhibited by the precipitation of alloy carbides at the weld interface. This determines a more homogeneous heat affected zone (HAZ) in the ferritic steel and a reduced decarburization near the fusion line after a post-weld heat treatment, confirming the reasons of the preference recognized to this filler material, especially when service temperature is elevated and submitted to frequent changes, or whenever a post-welded heat treatment is required.

オーステナイトステンレス鋼溶接金属の低温じん性 ＩＩＩ ＳＵＳ３１６Ｌ溶接金属の低温じん性に及ぼす再熱の影響

Type 316L weld metals contained various delta ferrites ranging from 0% to 9.5%, which were deposited by one pass submerged arc welding, were heat treated at 923 K to 1523 K for 2 s to 86.4 Ks (24 h). From the results of absorbed energy at 77 K and analysis of precipitates by electron and X-ray diffraction, the influence of reheating on low temperature toughness was investigated.In the weld metal of eutectic delta ferrite (delta=2.7%) which had minimum absorbed energy, the heat treatment for 2 s at the temperature above 1123 K enhanced its absorbed energy considerably as compared to as-welded specimen and in particular the tendency was remarkable at the temperatures of 1423 K and 1523 K.The absorbed energies of weld metals which contained some delta ferrites decreased with the increase in the heat treating time and this decreasing rate was pronounced at more delta ferrite content and high heat treating temperature. However, in the case for short heat treating time within 120 s, the absorbed energy had a tendency to increase, compared with as-welded one, while in the fully austenitic weld metal, the decrease in absorbed energy could be scarcely detected independent of the heat treating time.Retained delta ferrite contents after the heat treatment decreased faster at eutectic delta ferrite than at primary delta ferrite. This was considered to be due to the precipitation of M23C6 and sigma phase facilitated in the case of eutectic delta ferrite because of poor coherency of delta/gamma interface.

熱処理によって残留応力を軽減したＨＴ８０鋼溶接継手の疲れき裂伝ぱ特性

The fatigue crack propagation rate and the fatigue threshold were measured on transverse butt welded joints. The center cracked type specimens were made from as-welded joints and heat treated joints. The heat treatment were performed at 630°C and 720°C. The residual stresses were tensile around the middle of specimen and compressive near the edges. The tensile residual stresses were about 200 MPa, 20 MPa and 5 MPa for as-welded and heat treated specimens at 630°C and 720°C, respectively. The fatigue crack propagation properties obtained at various stress ratios on as-welded and heat treated specimens at 630°C were similar to each other and inferior to those obtained at the stress ratio of 0 on the base metal specimens. The fatigue threshold for as-welded specimens was about a quarter of that for base metal specimens. These properties obtained at the stress ratio of 0 on the heat treated specimen at 720°C were similar to those for the base metal specimens. While, the properties obtained avoiding crack closure at the very high stress ratio on the heat treated specimen at 720°C coincided with those for as-welded specimens. The coincidence of these properties means that the critical stress ratio conditions at which crack tips are fully open during all range of loading are made around crack tips by welding tensile residual stresses of 200 and 20 MPa.

オーステナイトステンレス鋼溶接金属の低温じん性 ＩＩ ＳＵＳ３１６Ｌ溶接金属の透過電子顕微鏡観察及びミクロ分析

An authors' previous paper which reported on the influence of delta-ferrite on low temperature toughness of Type 316L austenitic stainless steel weld metal, revealed that the toughness of as-welded specimen was dependent not only on the delta-ferrite content but also on the solidification mode.In this paper, various factors being affected on the toughness such as concentration difference, coherency, etc., were investigated in detail by using a transmission electron microscopy (TEM), an analytical electron microscopy (STEM/EDX) and a scanning electron microscopy equipped with an energy dispersive X-ray spectrometer (SEM/EDX).From an electron diffraction analysis, a crystallographic orientation of primary delta-ferrite was coherent with that of its adjacent austenite matrix but in the eutectic delta-ferrite, the relation was incoherent.The amount of molybdenum contained in the eutectic delta-ferrite was 1.8 times as that in the primary delta-ferrite and moreover, a cell or a cellular dendrite boundary adjacent to the eutectic delta-ferrite, was enriched in molybdenum slightly. Probably, in these molybdenum-rich sites, a lattice strain was considered to be higher than that in the primary delta-ferrite.From the SEM/EDX analysis on the fractured surface of the Charpy tested eutectic delta-ferrite specimen, the chromium and molybdenum rich zones were measured, where these compositions were equivalent to that of delta-ferrite or cellular dendrite. However, in the primary delta-ferrite specimen, such a zone could not be detected.From results above, when the eutectic delta-ferrite specimen was Charpy-tested, a crack initiation occurred at the delta-ferrite/austenite boundary or within the delta-ferrite and subsequently, the crack propagated preferentially along the cell or the cellular dendrite boundary or the delta-ferrite.In the specimen heat-treated at 923 K for 100 hrs, the concentration of the principal elements such as chromium, molybdenum and nickel within the prior delta-ferrite fluctuated widely, independent of its morphology, in which furthermore, the presence of carbide M23C6 and σ phase were confirmed.

電子ビーム溶接された球状黒鉛鋳鉄の継手組織と強度

Structures and strength of welded joints of ductile irons have been experimentally investigated which were electron-beam welded using Ni insert plates of 0.2 to 0.5 mm in thickness and then annealed for 1h at various temperatures from 823 to 1 123K. Acicular martensitic structures and high carbon martensitic structures were observed in the bond and around the spheroidal graphites in the heat affected zone, respectively. After annealing at higher temperature than 973 K, these martensites were completely disappeared and fine cementite and graphite particles were precipitated in both bead and bond zones. The hardness in the bond decreased to similar values in the bead and base metal zones by annealing at higher temperature than 923 K. The tensile strength of welded joints in which Ni plates of 0.30 to 0.35 mm thick had been inserted were almost recovered to the strength of base metal by annealing. Fine blowholes which were observed in the bead did not affect the tensile strength of annealed specimens. Charpy impact values of as-welded specimens were less than 20 J/cm2, while the values of annealed ones were kept at more than 60 J/cm2.

ＳＵＳ３１６Ｌオーステナイトステンレス鋼溶接金属の低温じん性に及ぼすδフェライトの影響 オーステナイトステンレス鋼溶接金属の低温じん性 １

The influence of δ-ferrite content on notch toughness and elastic-plastic fracture toughness of Type 316L austenitic stainless steel weld metal was investigated. The weld metals were prepared by deposition of single pass weld using submerged arc welding. The amount of δ-ferrite was changed in the range of 0.3 to 11.1 % by addition of nickel and chromium.In notch toughness test of as-welded specimens at 77 and 103 K, there was no simple correlation between the notch toughness and δ-ferrite content, differing from the results of other previous studies relative to this research. In more than about 3%δ-ferrite, the notch toughness increases with δ-ferrite content and reaches a maximum values at about 7%δ-ferrite.The solidification structure indicated that in the 3%δ-ferrite specimen of minimum notch toughness, an eutectic δ-ferrite appeared and furthermore with increasing δ-ferrite content, the morphology alters to vermicular and then lathy ferrite. From these results, it was evident that the δ-ferrite morphology considerably influenced to the notch toughness.On the other hand, the measurement of elastic-plastic fracture toughness Jrc, critical J and dJ/da revealed that these values related reasonably to the notch toughness.In heat treating the specimens at 923 K for 100 h, the absorbed energy decreased dramatically with increasing prior δ-ferrite content. This was considered because δ-ferrite changed to σ phase and carbide by heat treating.

EFFECT OF STRESS RELIEF ON FATIGUE STRENGTH OF HIGH STRENGTH STEEL CORNER JOINT WITH BLOWHOLE

The effect of low-temperature stress relieving was examined, using a partially penetrated groove weld with a blowhole. Box section specimens measuring 200mm (W)×250mm (H), made of 15mm thick 600MPa class steel were tested.The result proved that stress relief could reliably improve fatigue strength. Compared with the as-welded specimen, the 2000 000-cycle strength of the treated weld increased by about 20MPa.