Weld Metal Toughness Research Articles

Fracture toughness of electron beam welded Ti6Al4V

Abstract Two pass and three pass bead-over-bead (BoB) electron beam weld technique was attempted on Ti6Al4V to improve its Charpy impact and fracture toughness of weld metal portion. Hardness and microstructural studies were also done to support the findings. The hardness values of the weld metals were found to be higher than their base metal counterpart. The weld metal hardness came down when deposited with bead-over-bead technique. Conventional stringer bead welding caused reduction in Charpy impact toughness compared to its base metal counterpart. Bead-over-bead technique improved the notch toughness of weld metals. The fracture toughness values of the weld metals were found to be superior to that of base metal. This is in contrast to the Charpy impact toughness values where the impact toughness of base metal was better than the stringer and bead-over-bead weld metals. The better fracture toughness of weld metals was related to the basket-weave structure compared to the equi-axed structure of base metal. Bead-over-bead technique was found to help in improving the fracture toughness of weld metal further. Overall, the study showed that the bead-over-bead technique improved both notch toughness and fracture toughness of Ti6Al4V electron beam weld metals.

Effect of laser beam welding on fracture toughness of a Ti-6.5Al-2Zr-1Mo-1V alloy sheet

Investigation of fracture toughness on Ti-6.5Al-2Zr-1Mo-1V alloy thin sheet and its laser-welded joints has been carried out. In the test compact tension (CT) specimens and single specimen technology were used. In addition, hardness distribution and microstructure of the welded joints were examined. Fracture test indicates that brittle unstable fracture occurs after slow crack propagation for all the specimens, except that one heat affected zone (HAZ) specimen is brittle crack initiation. It is found that rolling directions have no obvious effect on fracture toughness of base metal. Moreover, fracture toughness of weld metal is obviously decreased in comparison with base metal whatever in as-welded condition or in stress relief condition. Post-weld heat treatment (PWHT) leads to fracture toughness of the welds further decreasing. Fractography observation shows that the fracture mode is predominantly dimpled in base metal. However, there exists intergranular fracture in the weld metal. Thus, the transition of fracture mode from both base metal and HAZ to weld metal may lead to dramatic decrease in fracture toughness. Microstructure examination reveals that the microstructure of weld metal consists of large grains with fine acicular structure. The formation of fine α acicular structure is due to rapid cooling during laser welding. After PWHT, the acicular structure is coarsened.

Tensile and Toughness Properties of Pipeline Girth Welds

The primary objective of this study was to develop a better understanding of all-weld metal tensile testing using both round and strip tensile specimens in order to establish the variation of weld metal strength with respect to test specimen through-thickness position as well as the location around the circumference of a given girth weld. Results from a series of high strength pipeline girth welds have shown that there can be considerable differences in measured engineering 0.2 % offset and 0.5 % extension yield strengths using round and strip tensile specimens. To determine whether or not the specimen type influenced the observed stress-strain behaviour a series of tests were conducted on high strength X70, X80 and X100 line pipe steels and two double joint welds produced in X70 line pipe using a double-submerged-arc welding process. These results confirmed that the same form of stress-strain curve is obtained with both round and strip tensile specimens, although with the narrowest strip specimen slightly higher strengths were observed for the X70 and X100 line pipe steels. For the double joint welds the discontinuous stress-strain curves were observed for both the round and modified strip specimens. Tests conducted on the rolled X100 mechanized girth welds established that the round bar tensile specimens exhibited higher strength than the strip specimens. In addition, the trends for the split-strip specimens, which consistently exhibit lower strength for the specimen towards the OD and higher for the mid-thickness positioned specimen has also been confirmed. This further substantiates the through-thickness strength variation that has been observed in other X100 narrow gap welds. A second objective of this study was to provide an evaluation of the weld metal toughness and to characterize the weld metal microstructure for the series of mechanized girth welds examined.

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.

Effects of Nitrogen on Weld Metal Microstructure and Toughness in Submerged Arc Welding

The effects of nitrogen content on weld metal impact toughness in submerged arc welding were investigated and interpreted in terms of microstructural changes and solid solution hardening. The weld metal impact toughness in as-welded condition decreased with increasing nitrogen content from 110 to 200 ppm. The weld metal microstructure changed with increasing nitrogen content; ferrite with second phase increased at the expense of tough acicular ferrite. In addition to microstructural changes, the microhardness of acicular ferrite increased gradually with the nitrogen content. Therefore, the loss of impact toughness can be attributed to a combination of the effects of microstructural changes and solid solution hardening.

Hardness–fracture toughness–creep rupture strength relationship for welds in P91 main steam pipe

An experimental study on the impact toughness, hardness, fracture toughness and creep rupture strength of P91 steel weld metals under two conditions (R and D) has been carried out. The results show that weld metal having higher hardness, lower impact toughness and lower fracture toughness (sample R) has higher creep rupture strength. The creep rupture strength is closely related to creep life of component. Thus, assessment of the weld hardness of P91 steel is of value in assessing the creep life of pipes in service. The lower toughness of sample R is attributed mainly to the higher welding current and input line energy used. The lower hardness and strength of sample D, which received a second post-weld heat treatment, partially result from too high a secondary tempering temperature being employed.

Effect of laser beam welding on tear toughness of a 1420 aluminum alloy thin sheet

A tear test has been conducted for a 1420 Al–Li alloy thin sheet and its laser welded joint. The test was based on the ASTM B 871-01, standard test method for tear testing of aluminum alloy products. Moreover, the hardness distribution and microstructure of the alloy and welded joints were investigated. The test results indicate that there exists slight orientation dependence of tear energies for the L–T and T–L base metal. In addition initiation energies are higher than the propagation energies for all the tear test specimens, and the tear energies of base metal are higher than those of weld metal and HAZ. Thus, it is considered that the tear toughness of the laser-welded joints is obviously decreased for the 1420 Al–Li alloy thin sheet. Fracture surface shows strong delamination structure for the base metal. Moreover, there exists a feature of fracture along grains or subgrains for the weld metal. Thus, the obvious decrease in tear toughness of weld metal is due to the transition of fracture mode from transgranular to intergranular.

Significance of Fracture Toughness Test Results of Beam Welds in Evaluation of Brittle Fracture Performance of Girth Welded Pipe Joints

Abstract High power beam welds, such as electron beam welds or laser welds, sometimes provide fracture path deviation (FPD) in standardized Charpy V-notch fracture toughness testing due to narrow bead profile together with higher overmatching in strength between weld metal and base metal. Moreover, it should be noted that these typical features of beam welds might result in a plastic constraint loss around both the notch and crack tip in fracture toughness test specimens. Even in the temperature range where FPD would not occur, the fracture toughness test results could not necessarily be an intrinsic value of such beam welds. These fracture properties make it difficult to evaluate fracture performance of girth welded pipe joints. In this paper, the estimation method of intrinsic fracture toughness of beam weld metal itself using standard toughness test specimens is proposed on the basis of “Weibull stress criterion.” The predicted intrinsic fracture toughness was found to be lower than the test results both in standard Charpy specimen and in three-point bend specimen with fatigue precrack. The assessment of brittle fracture performance of girth welded pipeline was conducted from the estimated intrinsic fracture toughness of girth welds by means of Weibull stress criterion. It was demonstrated that the low intrinsic fracture toughness of beam welds could not directly lead to the low fracture performance of a pipe joint under tensile loading. This is because of a lower plastic constraint compared to a three-point bend specimen due to difference in loading mode together with constraint loss in pipe joints and shielding effect of straining in weld metal due to highly overmatched narrow welds.

Micromechanisms of cleavage fracture initiation from inclusions in ferritic welds: Part II. Quantification of local fracture behaviour observed in fatigue pre-cracked testpieces

The quantification of the local fracture behaviour and the fracture toughness of high strength low alloy weld metals have been studied using fatigue pre-cracked CTOD testpieces. In CTOD testpieces, when brittle fracture occurs, with or without prior stable crack growth, cleavage usually starts from an inclusion located at the initiation site within allotriomorphic ferrite for weld metals with a classical microstructure, or within bainite or martensite packets in weld metals with a lath-like microstructure. The mean value of fracture stress, σ F( X 0), when evaluated using CTOD testpieces, has proved to be higher than when it is evaluated using SENB testpieces. This is deduced to be as a direct consequence of the reduction of the inclusion size found at initiation sites in CTOD testpieces. The effective surface energy here was evaluated to be approximately 8.6 J m −2, for the weld metal with a classical microstructure and 14 J m −2, for the weld metal with a lath-like microstructure. These values are closely similar to the values of the effective surface energy evaluated using SENB testpieces.

Size distribution of oxides and toughness of steel weld metals

Experiments have been conducted to modify the size distribution of oxide particles in steel weld metals which have predominantly martensitic microstructures. It is found that the toughness improves significantly when the particles are refined, even when the total concentration of oxygen is unchanged in the process.

Influence of Zirconium on Microstructure and Toughness of Low-Alloy Steel Weld Metals

The influence of zirconium on microstructure and toughness of low-alloy steel weld metal was studied. Weld metals with different zirconium contents were obtained adding iron-zirconium alloy in the welding flux formulation. Weld metal chemical composition proved that zirconium was able to be transferred from the flux to the weld metal. The addition of zirconium refined the weld metal microstructure, increasing the acicular ferrite content. Weld metal toughness, determined by means of impact Charpy-V tests, showed that the zirconium addition is beneficial up to a content of 0.005 wt.%. Above this level, zirconium was not able to produce further microstructure refinement, although the toughness was reduced, possibly due to the formation of microconstituent such as the martensite-austenite constituent (M-A), which is considered to be deleterious to the weld metal toughness.

Effect of nickel and molybdenum additions on weld metal toughness in a submerged arc welded HSLA line-pipe steel

The effects of alloy additions of nickel (Ni), molybdenum (Mo), and Ni and Mo together on the impact toughness of an API HSLA-70 steel by submerged arc welding in the laboratory were investigated and micro-structural factors which affect the impact toughness were discussed. Ni additions resulted in a low impact toughness and an increased fracture appearance transition temperature (FATT) in weld metal (WM). The above influences of Ni should be attributed to the formation of acicular ferrite (AF) suppressed by increasing the Ni content. Conversely, the combined presence of Ni and Mo in the WM decreased the volume fractions of grain-boundary ferrite (GBF) and promoted formation of high toughness of AF. The increase of Mo content created an acicular ferrite-predominant weld metal microstructure with impressively improved toughness. Mo addition of 0.881 wt.% in the WM gave the optimal impact toughness at −45 °C with a microstructure of 77% AF and 20% granular bainite (GB).

The influence of the oxygen equivalent in a gas-mixture on the structure and toughness of microalloyed steel weldments

Testing were carried out on two steels. The first was microalloyed with Nb and second with Ti, Nb and V. The impact toughness of weld metals of these steels was evaluated using an instrumented Charpy pendulum. Five different gas mixtures (Ar, CO2, O2) were used to determine the optimal gas shielded metal arc process for both steels. The oxygen equivalent was used as a representative parameter of a mixture to follow, in particularly, its effect on the microstructure, toughness and crack propagation energy of the weld metal. For these investigated steels, the optimum gas mixture was established (5%CO2, 0.91%O2, balance Ar), which provided the maximum crack propagation energy, due to the microstructure which consisted dominantly of acicular ferrite.

Assessment of Properties and Microstructure of X100 Pipeline Girth Welds

Over the last few years, interest in the development and construction of large diameter, high pressure pipelines from the Mackenzie Delta in northern Canada and Alaska’s North Slope has steadily increased as a result of the projected decline in recoverable natural gas from existing reserves in western Canada. The success of such enormous pipeline projects, which will require utilisation of a strain-based design approach, is largely based on successful application of high strength line pipe steels, such as X80 and X100. To fully realise the economic benefits of high strength steels, welding processes and procedures must be developed to ensure that the requirements for high strength and good low temperature toughness are confidently obtainable in field girth welds used in the construction of such pipelines. This study aims to provide a better understanding of the factors that control weld metal strength and toughness for a series of mechanised X100 girth welds produced using a range of narrow-gap gas-metal arc welding procedures. The attainment of high yield strength (≥ 810 MPa) is considered to be an important target to ensure overmatching strength relative to the longitudinal property distribution of X100 line pipe steel. Some factors that have been considered in this evaluation include influences of gas-metal arc welding process variants (single and multi-wire) and changes in welding procedure specifications, including joint preparation. Key components of this work relate to the reliable measurement of weld metal strength variation through-thickness and as a function of position around the circumference of a given girth weld.

An Evaluation Method for Laser Weld Metal Toughness by Side-Notched Charpy Test

Ultra-fine grained (UFG) steel has been developed on a laboratory scale, which has both high strength and superior toughness. When welded, however, grain growth takes place at the heat-affected zone (HAZ), and consequently HAZ softening occurs. To avoid the detrimental effect of HAZ softening, laser welding is one of the promising methods. The hardness distribution of laser welded joints is so steep in a narrow region that fracture path deviation (FPD) is often observed in the standard Charpy V-notch (STD-Cv) test in and above the transition temperature region. Side-notch Charpy (SN-Cv) test is widely used to prevent FPD. The SN-Cv test gives a higher transition temperature and lower absorbed energy as compared with the STD-Cv test. Therefore, it is necessary to evaluate quantitatively the difference between the STD-Cv and the SN-Cv test. In this paper, STD-Cv and SN-Cv tests were carried out on laser weld metal for UFG steel and on base plates of conventional steels from mild steel to 800 MPa high strength steel. The fracture surface of Cv test was categorized into three types, that is, fibrous fracture at the notch tip and at the edge of the specimen, brittle fracture, and slant fracture at both sides. The area of each fracture surface was measured for all specimens tested and each unit fracture energy was determined by using the regression analysis method. Then, the difference in the formation of each fracture surface between STD-Cv and SN-Cv tests was discussed, and finally a method to estimate STD-Cv energy from the SN-Cv test results was proposed.

Influence of high-temperature exposure on the microstructure and mechanical properties of dissimilar metal welds between modified 9Cr-1Mo steel and alloy 800

Transition joints between ferritic steel and austenitic stainless steel are commonly encountered in high-temperature components of power plants. Service failures in these are known to occur as a result, mainly, of thermal stresses due to expansion coefficient differentials. In order to mitigate the problem, a trimetallic configuration involving an intermediate piece of a material such as Alloy 800 between the ferritic and austenitic steels has been suggested. In our work, modified 9Cr-1Mo steel and 316LN stainless steel are used as the ferritic and austenitic components and the thermal behavior of the joints between modified 9Cr-1Mo steel and Alloy 800 is described in this article. The joints, made using the nickel-base filler material INCONEL 82/182 (INCONEL 82 for the root pass by gas-tungsten arc welding and INCONEL 182 for the filler passes by shielded-metal arc welding), were aged at 625 °C for periods up to 5000 hours. The microstructural changes occurring in the weld metal as well as at the interfaces with the two parent materials are characterized in detail. Results of across-the-weld hardness surveys and cross-weld tension tests and weld metal Charpy impact tests are correlated with the structural changes observed. Principally, the results show that (1) the tendency for carbon to diffuse from the ferritic steel into the weld metal is much less pronounced than when 2.25Cr-1Mo steel is used as the ferritic part; and (2) intermetallic precipitation occurs in the weld metal for aging durations longer than 2000 hours, but the weld metal toughness still remains adequate in terms of the relevant specification.

Effect of vibration on weld metal hardness and toughness

Vibration of welded parts is usually applied to achieve effects similar to thermal stress relief. With vibration, it is not only residual stresses that are affected. Using two different welding processes, the influence of vibration on hardness and toughness of the weld was measured. For each welding process, two series of Charpy specimens were made over the temperature range from -60 to +20°C. The only difference between the two series was in performing welding with or without vibration. Slight differences in weld metal hardness were observed. Toughness measurements show an increase in impact toughness and a significant increase in fracture toughness in samples which were vibrated during welding.

High power laser welding of Nb microalloyed steel plates

Laser welding studies on 12 mm thick Nb microalloyed steels were done using a 25 kW CO2 laser at welding speeds of up to 3 m min−1 with the aim of identifying the influences of Nb and carbon on laser weldability and weld properties. Welds were examined for solidification flaws, penetration characteristics, microstructure and mechanical properties. Solidification cracking did not appear to be influenced by carbon or Nb in the ranges studied when welding at power levels between 22 and 25 kW. The area fraction of martensite was shown to increase with decreasing energy input and increasing carbon content. Weld metal toughness was improved by reducing carbon content in the range 0·08–0·05%C but was little influenced by Nb. Weld metal and heat affected zone hardness levels were reduced, as expected, by reducing carbon content. The results show that increasing Nb content leads to higher strength material without significant loss of toughness, while lowering carbon content can improve toughness without loss of strength. Thus low carbon Nb microalloyed steels with about 0·05%C can achieve a good combination of parent plate and laser weld properties and appear to be suitable for high power laser welding situations.

Effect of controlled atmosphere on the mig-mag arc weldment properties

Due to their higher welding speed, automation and weld pool protection against to the atmosphere gases, gas metal arc welding (GMAW) process is widely used in industry. Due to the less stable arc associated with the use of consumable electrodes, GMAW process is not clean as good as gas tungsten arc welding process. Furthermore, the greater arc length in GMAW process also reduces the protective effect of the shielding gas. Due to electrochemical and thermochemical reactions between weld pool and arc atmosphere, it is quite important, especially weld metal toughness and joining of reactive materials to entirely create inert atmosphere for GMAW process. Therefore, a controlled atmosphere cabinet was developed for GMAW process. Low carbon steel combinations were welded with classical GMAW process in argon atmosphere as well as controlled atmosphere cabinet by using similar welding parameters. The mechanical and metallurgical properties of both weldments were evaluated. Result shows that toughness of the weld metal that was obtained in the controlled atmosphere cabinet much higher than that of classical GMAW process. The metallographic examination also clarified that there was not any gas porosity and inclusion in the weld metal compared with classical process.

Microstructures and properties of laser/arc hybrid welds and autogenous laser welds in pipeline steels

Lasers are capable of producing welds with deep penetration, low distortion and faster travel speeds, compared to arc welding. More recently, laser/arc hybrid welding processes have also been generating interest for industrial fabrication. In this paper, six carbon–manganese, mainly pipeline, steels were welded using both autogenous Nd:YAG laser welding, and Nd:YAG laser/MAG hybrid welding. The improvements in weld microstructures and weld metal toughness that are possible when using the hybrid process are described and illustrated. Laser/arc hybrid welding is shown to be a process that can generate good quality welds in commercially available pipeline steels. It also has the potential to complete girth welds in these steels with significantly fewer welding passes than are currently required for arc welded pipelines, reducing the joint completion time.