Post-weld Heat Treatment Procedures Research Articles

Effect of post-weld heat treatment on the microstructure, phase composition and mechanical properties of dissimilar Al-Mg-Li/Al-Cu-Li laser welded joints

Butt joints of the V-1461 and 1424 heat treated aluminum–lithium alloys of the third generation were welded with a CO2 laser and studied in detail for the first time. In particular, spatial distributions of the formed phases and their evolution upon post-weld heat treatment (PWHT), determining the mechanical properties of the joints, were investigated. The PWHT included quenching and subsequent artificial aging. The key factors, affecting the metallurgical processes in a weld pool, were the following: 1) the alloying elements and the thermal properties of the alloys, 2) the dynamics of the local non-equilibrium melting, the convective transfer of the molten metal in the weld pool and its subsequent solidification, 3) the cooling rate, and 4) the PWHT procedure. By using synchrotron radiation and scanning electron microscopy, new θ(Al2Cu), T1(Al2CuLi) and S1(Al2MgLi) phases were found in the weld metal that were absent in both base metals. This phenomenon reduced the mechanical properties of the joints to the levels, corresponding to about 50 % of the 1424 alloy as the weakest one. Quenching contributed to the formation of the δ′(Al3Li) strengthening phase in the weld metal. As a result, the ultimate tensile strength increased up to 70 % and elongation up to 95 % of the corresponding levels of the 1424 alloy. Artificial aging made it possible to form the T1(Al2CuLi) strengthening phase, to increase the content of the δ′(Al3Li) one, as well as to improve the mechanical properties of the joint. In this case, the ultimate tensile strength was 411 MPa, the yield point was 327 MPa and elongation was 1.7 %. These values were close to those for the 1424 alloy (about 80 %, 89 % and 23 %, respectively).

A Review on The Effect of Post-Weld Heat Treatment (PWHT) on Its Thermal Analysis and Mechanical Properties of Welded Metallic Pipe

Welding is a commonly used process in manufacturing and engineering, and it may create residual stress in the welded region that can cause problems during service. Thermal expansion of the post-weld heat treatment (PWHT) of steel carbon induces residual stress. To relieve the internal stresses in the weld zone, PWHT is used to soften the hardening zone, improve microstructures, and lower hydrogen content in the welded region. The pipe size, heating width, insulation conditions, heating rates, soaking temperature, and holding time are factors that influence PWHT procedures. This paper will explain clearly about the PWHT process and the effect of PWHT on the metallic metal.

Effects of post-weld heat treatment on the microstructure and properties of the matching SMAW filler metal for weld joints in MarBN steel

AbstractA detailed investigation has been carried out of the matching composition filler metal for welding MarBN steel to explore the influence of welding procedure and post-weld heat treatment (PWHT) on the microstructure and properties of the weld metal and a MarBN steel weld joint. The filler metal was in the form of a shielded metal arc welding (SMAW) electrode. With different interpass temperatures and PWHT procedures applied, the microstructure and mechanical properties, particularly toughness, of the all-weld metal deposits and welded MarBN alloy joints were examined. The differences in microstructure (grain structure, precipitates, etc.) were evaluated to provide a better understanding of how the PWHTs influence the starting conditions of the matching weld metal and welded MarBN steel joints before service. The current work found that for a 30-mm-thick all-weld metal coupon, reasonable impact toughness was achieved after PWHT of 735 °C × 8 h; longer heat treatment durations (16 h and 36 h) did not further increase the toughness. For thicker cast blocks, a PWHT of 735 °C × 16 h was needed for weld toughness comparable to the base metal, hence was considered more appropriate for actual engineering applications. Evenly distributed Laves phase precipitation was observed after 735 °C × 8 h; the precipitates gradually increased in size and area fraction as the duration increased, and appeared to be stable when approaching 36 h. Differences in the precipitation were observed between different weld regions. The weld cap region had a lower density of Laves precipitates compared to the lower weld area. From the current investigation, aspects for further investigation are also identified.

Numerical and experimental study for electron beam welding process of Al6061-T6 material

Joining the aluminium 6061-T6 alloy (Al6061-T6) using Electron Beam Welding (EBW) wais important, especially in aerospace. Although the EBW method has been used for different materials in the literature, there are a limited number of studies for Al6061-T6 alloy. The study aimed at the weldability and welding quality of 6061-T6 aluminium alloy plates by experimentally and numerically using the EBW technique. A numerical model has been generated to obtain the appropriate voltage, beam current, and welding speed parameters to simulate the EBW process instead of the hit and trial method, which is costly. The numerical model is based on a user-defined function based on the heat source as a function of temperature, welding parameters and material properties. The validation of the model has been compared by obtained width from numerical model and experimental work. Three EBW parameter sets were employed with the numerical model. Three different experimental parameter sets were determined according to the most suitable processing parameter that emerged from the numerical model, and experimental studies were carried out. Experimental studies showed defects in the microstructure due to process defects by unsuitable parameters. While the mechanical strength was 70% of the tensile strength performance compared to the main material, the elongation performance was 75%. The microhardness of the heat affected zone was measured as 84% of the base material, and a hardness reduction of 40% was also observed in the fusion zone. In addition, it showed that using the proper parameter set can eliminate such defects. The application of post-weld heat treatment procedures that can effectively improve the mechanical properties could be investigated in future studies.

A primary plus secondary local PWHT method for mitigating weld residual stresses in pressure vessels

Local post-weld heat treatment (PWHT) is often used for mitigating welding-induced residual stresses in pressure vessels and piping systems. As large and ultra-large pressure vessels are increasingly used in petrochemical and power generation industries, traditional local PWHT procedures stipulated in Codes and Standards become difficult to implement for some of the ultra-large vessels due to their very large diameter and overall length. In this paper, a new method referred to as primary-secondary heating based local PWHT technique (PS-PWHT) is proposed. Both finite element residual stress modeling and experimental residual stress measurements were performed for both validating the effectiveness of the proposed technique and elucidating the underling mechanisms. The results show that the PS-PWHT method can reduce welding-induced residual stresses in a significant manner, even leading to some level of compressive residual stresses at the vessel weld region.

The State of the Art of Underwater Wet Welding Practice: Part 2

Developments in underwater wet welding (UWW) over the past four decades are reviewed, with an emphasis on the re-search that has been conducted in the last ten years. Shielded metal arc welding with rutile-based coated electrodes was established as the most applied process in the practice of wet welding of structural steels in shallow water. The advancements achieved in previous decades had already led to control of the chemical composition and microstructure of weld metals. Research and development in consumables formulation have led to control of the amount of hydrogen content and the level of weld porosity in the weld metal. The main focus of research and development in the last decade was on weldability of naval and offshore structural steels and acceptance of welding procedures for Class A weld classification according to American Welding Society D3.6, Underwater Welding Code. Applications of strictly controlled welding techniques, including new postweld heat treatment procedures, allowed for the welding of steels with carbon equivalent values greater than 0.40. Classification societies are meticulously scrutinizing wet welding procedures and wet weld properties in structural steels at depths smaller than 30 m prior to qualifying them as Class A capable. Alternate wet welding processes that have been tested in previous decades — such as friction stir welding, dry local habitat, and gas metal arc welding — have not achieved great success as originally claimed. Almost all of the new UWW process developments in the last decade have focused on the flux cored arc welding (FCAW) process. Part 1 of this paper covered developments in microstructural optimization and weld metal porosity control for UWW. Part 2 discusses the hydrogen pickup mechanism, weld cooling rate control, design, and qualification of consumables. It ends with a description of the advancements in FCAW applications for UWW.

The State of the Art of Underwater Wet Welding Practice: Part 1

Developments in underwater wet welding (UWW) over the past four decades are reviewed, with an emphasis on the research that has been conducted in the last ten years. Shielded metal arc welding with rutile-based coated electrodes was established as the most applied process in the practice of wet welding of structural steels in shallow water. The advancements achieved in previous decades had already led to control of the chemical com-position and microstructure of weld metals. Research and development in consumables formulation have led to control of the amount of hydrogen content and the level of weld porosity in the weld metal. The main focus of research and development in the last decade was on weldability of naval and offshore structural steels and acceptance of welding procedures for Class A weld classification according to American Welding Society D3.6, Under-water Welding Code. Applications of strictly controlled welding techniques, including new postweld heat treatment procedures, allowed for the welding of steels with carbon equivalent values greater than 0.40. Classification societies are meticulously scrutinizing wet welding procedures and wet weld properties in structural steels at depths smaller than 30 m prior to qualifying them as Class A capable. Alternate wet welding processes that have been tested in previous decades — such as friction stir welding, dry local habitat, and gas metal arc welding —have not achieved great success as originally claimed. Al-most all of the new UWW process developments in the last decade have focused on the flux cored arc welding (FCAW) process. Part 1 of this paper covers developments in microstructural optimization and weld metal porosity control for UWW. Part 2 discusses the hydrogen pickup mechanism, weld cooling rate control, design, and qualification of consumables. It ends with a description of the advancements in FCAW applications for UWW.

Influence of local post-weld heat treatment and its thermal analysis on thick wall carbon steel pipe

Local post-heating was a common process during the field fabrication and repair of engineering components. Post Weld Heat Treatment (PWHT) attempts to relax residual stress when service requirement dictates the need for PWHT. The additional objectives such as hardness reduction stress relaxation aimed to be below a specific threshold level. Many factors have influenced on PWHT procedures, such as the size of the pipe, heating width, insulation conditions, heating rates, soaking temperature and holding time, material composition, etc. However, up to now, the influences of these factors on PWHT were not clearly understood and different criteria for sizing the parameters. It can be found in different codes. PWHT was to attempt and duplicate the outcome of furnace heating (i.e heating the whole weldment) within a localized region, referred to the soak band surrounding the weld. The width of heating coil from 500 to 600 mm significantly reduced the temperature gradient in between OD and ID of the carbon steel pipe. The increase in thermal energy moderately enhanced the temperature gradient on the carbon steel pipe.

Influence of Metal Transfer Modes and Postweld Heat Treatment on Tensile Properties of Gas Metal Arc–Welded AA6061-T6 Aluminum Alloy Joints

Abstract The issues of dissolution or overaging of precipitates in the weld metal or heat-affected zone of aluminum alloy joints welded by conventional or constant-current gas metal arc welding (CC-GMAW) process are the key factors responsible for the formation of softening region, which leads to the degradation of the mechanical properties of the joint. In order to prevent the precipitate dissolution, a low heat input welding process is required. Further, to improve the mechanical properties of the welded joints, reprecipitation is necessary, and this can be achieved by postweld heat treatment (PWHT). The main aim of this investigation is to study the combined effect of metal transfer modes (i.e., CC-GMAW, pulsed-current gas metal arc welding [PC-GMAW], and cold metal transfer gas metal arc welding [CMT-GMAW]) and heat treatment on the tensile properties of gas metal arc–welded AA6061 aluminum alloy joints. This investigation effectively implements the low heat input GMAW variants like PC-GMAW and CMT-GMAW to control the dissolution of precipitates. Moreover, PWHT procedure (solutionizing at 530°C for 1 h, followed by quenching in water and then artificial aging at 170°C for 19 h, and finally cooling in air at an ambient temperature) was imposed on the specimens in order to recover the mechanical properties of the welded joints. It is observed from the tensile properties that PWHT joints exhibited improved mechanical properties compared with the as-welded joints. But the joint welded by CC-GMAW process has shown no improvement in the tensile properties because of the segregation of alloying elements that are observed along the grain boundary. The PWHT CMT-GMAW joint showed huge improvement in tensile properties (i.e., 13 % higher than the CC-GMAW joint and 9 % higher than the PC-GMAW joint) because of the controlled segregation of alloying elements with nucleation of β″ precipitates.

Low heat input gas metal arc welding for dissimilar metal weld overlays part III: hydrogen-assisted cracking susceptibility

Dissimilar metal weld overlays of nickel-base alloys on low-alloy steel components are commonly used in the oil and gas, petrochemical, and power generation industries to provide corrosion and oxidation resistance in a wide range of service environments and temperatures. Traditionally, dissimilar weld overlays are produced using cold or hot wire gas tungsten arc welding. This study aims to identify and evaluate potential advantages of low heat input gas metal arc welding processes over the conventional gas tungsten arc welding in the production of such overlays. Parts I and II of this publication series described characteristics of the heat-affected zone and the transition zone region of alloy 625 on grade 22 steel overlays. These results indicate a good resistance against hydrogen-assisted cracking, which is being verified within this third part of the publication series. To determine the hydrogen-assisted cracking susceptibility, welded samples are tested using the delayed hydrogen-assisted cracking test. Fractography is performed using scanning electron microscopy along with energy dispersive spectroscopy. The results confirm the suitability and efficiency of low heat input gas metal arc welding for dissimilar weld overlays. Variation of the postweld heat treatment procedure bears potential for improvement in this respect.

Effect of post-weld heat treatment on joint properties of dissimilar friction stir welded 2024-T4 and 7075-T6 aluminum alloys

Abstract The effect of post-weld heat treatment on dissimilar friction stir welded AA7075 and AA2024 joints was studied. After welding in constant parameters, solution heat treatment and various aging treatments were given to the welded joints. Microstructural and phase characterizations were done using optical microscope, SEM, FE-SEM, XRD and EDS techniques. Finally, mechanical properties of post-weld heat treated joints were evaluated and compared with as-welded joints. Results show that both 2024-T6 and 7075-T6 post-weld heat treatment procedures considerably improve the mechanical strength of the welded joint, with higher strength obtained for the 7075-T6 procedure, in comparison with the as-welded joint. This is explained by the formation of fine precipitates during the aging process, despite the abnormal grain growth. Fracture occurs at the interface between thermo-mechanical affected zone (TMAZ) and heat affected zone (HAZ) on the retreating side (AA7075) of as-welded joint, while by applying post-weld heat treatment fracture location shifts towards the stir zone (SZ) of the welded joint. Also, for post-weld heat treated samples, fracture surface is predominantly inter-granular, while in as-weld joint, fracture surface is mostly trans-granular. This is explained by dissolution and coarsening of precipitates within grains in post-weld heat treated joints.

Effect of post-weld heat treatment on fusion boundary microstructure in dissimilar metal welds for subsea service

Abstract Microstructural features at the fusion boundary in dissimilar metal welds significantly influence the susceptibility to local hydrogen embrittlement under subsea service conditions. Two subsea dissimilar metal welds using nickel-based filler metal on low alloy steel substrate were studied in this work in order to investigate the nature and microstructural evolution of the dissimilar interface during post-weld heat treatment (PWHT), and to determine resistance to hydrogen embrittlement. The delayed hydrogen cracking test (DHCT) was utilized to determine hydrogen-assisted cracking (HAC) susceptibility. Thermokinetic modeling was conducted to study phase formation and carbon diffusion across the dissimilar interface. Diffusion calculations incorporated initial compositional gradients after welding and the nonmoving phase boundary during PWHT. Resistance to HAC was in good agreement with previously obtained DHCT results for the tested dissimilar metal welds. Tempered martensite in the heat-affected zone of the steel substrate was observed as a function of PWHT temperature. Fresh martensite with high local hardness formed during cooling in highly diluted weld metal regions. The cell model incorporated in DICTRA effectively predicted differences in carbon concentration profiles across the dissimilar interface and type of carbide precipitation as a function of PWHT procedure and steel substrate.

Creep rates of heat-affected zone of grade 91 pipe welds as determined by stress-relaxation test

Cross-weld creep testing conducted at 650 °C under 69 MPa stress has reproduced either the Type I or Type IV failure mode in Grade 91 welds, depending on the post-weld heat treatment procedures. Welds post-weld heat treated below the AC1 temperature have ruptured in the Type VI failure mode, while welds heat-treated above the AC1 temperature of the alloy have ruptured in the Type I failure mode. Heat-treatments at lower temperatures and shorter durations have produced a reduced creep rate. The accelerated short term stress-relaxation test has been conducted to obtain the input creep rates for a finite element model for the cross-weld creep testing. The model predicted secondary creep rates are in good agreement with the results from the conventional cross-weld creep tests. From the finite element model, the creep damage by cavitation is believed to start at regions where the first principal stress and stress triaxiality concentrate.

On the Role of Modified Post Weld Heat Treatment in Eliminating Incipient Melting of Non-equilibrium Product Resulting During Solid-State Welding of Astroloy

The formation of non-equilibrium phases as a result of segregation during solidification of castings or in the welds of nickel-based superalloys has been a major concern limiting the enhancement of the optimal heat treatment procedure that could have improved the performance of the superalloys to their full potential strength in service. Much consideration has not been given to the dissolution of the non-equilibrium products resulting from welding of nickel-based superalloys. The present work on astroloy shows how non-equilibrium welding products could be completely eliminated by a modified post weld heat treatment procedure.

Residual stress measurements in a P92 steel-In625 superalloy metal weldment in the as-welded and after post weld heat treated conditions

Residual stress measurements have been undertaken in a multi-pass circumferential double J butt-weld made from a P92 martensitic steel pipe using an In625 superalloy weld metal. Stresses were evaluated in the as welded condition and after post weld heat treatment. The deep hole drilling and neutron diffraction techniques were used to provide through thickness measurements for the parent, weld metal and heat affected region for the as-welded and the post weld heat treated conditions. The use of multiple residual stress measurement techniques provides an understanding of the stress distributions as well as increasing confidence in the reliability of the measurements. The results presented are discussed with respect to the residual stress magnitude and location as well as with respect to the effectiveness of the post weld heat treatment procedure.

Correlation between microstructure and creep performance of martensitic/austenitic transition weldment in dependence of its post-weld heat treatment

This paper deals with the influence of post-weld heat treatment (PWHT) of T92/TP316H martensitic/austenitic transition weldment on the resulting microstructure and creep characteristics. Experimental weldments were fabricated by gas tungsten arc welding using a nickel-based weld metal (Ni WM). After the welding, two individual series of produced weldments were heat-treated according to two different PWHT procedures. The first “conventional PWHT” was carried out via subcritical tempering (i.e. bellow Ac1 temperature of T92 steel), whereas the other one, the so-called “full PWHT” consisted of a complete reaustenitization of the weldments followed by water-quenching and final tempering. The use of “conventional PWHT” preserved microstructural gradient of T92 steel heat-affected zone (HAZ), consisting of its typical coarse-grained and fine-grained subregions with tempered martensitic and recrystallized ferritic–carbidic microstructures respectively. In contrast, the “full PWHT” led to the complete elimination of the original HAZ via transformation processes involved, i.e. the reaustenitization and back on-cooling martensite formation. The observed microstructural changes depending on the initial PWHT conditions were further manifested by corresponding differences in the weldments’ creep performance and their failure mode. The weldments in “conventional PWHT” state ruptured after long-term creep tests by premature “type IV failure” within their recrystallized intercritical HAZs. On the contrary, the long-term creep behavior of the weldments processed by “full PWHT” was characterized by their remarkable creep life extension but also by the occurrence of unfavorable “decohesion failure” along T92/Ni WM interface.

Enhancement of mechanical properties and failure mechanism of electron beam welded 300M ultrahigh strength steel joints

In this study, four post-weld heat treatment (PWHT) schedules were selected to enhance the mechanical properties of electron beam welded 300M ultrahigh strength steel joints. The microstructure, mechanical properties and fractography of specimens under the four post-weld heat treatment (PWHT) conditions were investigated and also compared with the base metal (BM) specimens treated by conventional quenching and tempering (QT). Results of macro and microstructures indicate that all of the four PWHT procedures did not eliminate the coarse columnar dendritic grains in weld metal (WM). Whereas, the morphology of the weld centerline and the boundaries of the columnar dendritic grains in WM of weld joint specimens subjected to the PWHT procedure of normalizing at 970°C for 1h followed by conventional quenching and tempering (W-N2QT) are indistinct. The width of martensite lath in WM of W-N2QT is narrower than that of specimens subjected to other PWHT procedures. Experimental results indicate that the ductility and toughness of conventional quenched and tempered joints are very low compared with the BM specimens treated by conventional QT. However, the strength and impact toughness of the W-N2QT specimens are superior to those of the BM specimen treated by conventional QT, and the ductility is only slightly inferior to that of the latter.

Properties of T/P92 weld metals for ultra super critical (USC) power plant

The introduction of new power generation technologies, such as ultra super critical (USC) plant, has resulted in the development and application of a series of advanced Cr-Mo creep resistant steels. Among these new alloys, T/P92 steel has creep strength 25%-30% higher than the currently widely used T/P91 steel and it has become one of the major alloys for the construction of USC plant. In order to obtain a good understanding of the microstructural characteristics and properties of the T/P92 weld metals, investigation was conducted to welds from SMAW, SAW and FCAW processes using matching filler metals. The important phase transformation temperatures, Ac1, Ms and Mf, were measured. The mechanical properties at ambient temperature and creep properties of the weld metals were evaluated. Procedural factors that influence the performance of T/P92 weld metals were discussed and recommendations for suitable practical welding and post-weld heat treatment procedures were presented.

The effect of post-weld heat treatment on the mechanical properties of 2024-T4 friction stir-welded joints

In this study, the effect of post-weld heat treatment (PWHT) on the mechanical properties of friction stir-welded 2024 aluminum alloys in the T4 temper state was investigated. Solution heat treatment and various ageing treatments were given to the welded joints. The PWHT procedures caused abnormal coarsening of the grains in the weld zone, which resulted in a drop in micro-hardness at the weld zone compared to the base material of the joints. T6 (190 °C – 10 h) ageing treatment after welding was found to be more beneficial than the other heat treatments in enhancing the mechanical properties of the 2024-T4 joints. However, the T6 (190 °C – 10 h) heat treatment led to significant ductility deterioration in the joint.

Use of the Master Curve to Investigate the Effect of Post-Weld Heat Treatment on ASTM A106B

ASTM A106/B is commonly used for pressure piping in alumina refineries. Due to the problem of caustic cracking in alumina refineries, piping must be stress relieved after welding, usually through a post-weld heat treatment (PWHT) process at a temperature of 635°C. However, multiple PWHT procedures tend to degrade the mechanical properties of steel and design standards have set limits on the length of time PWHT can be carried out before steel should be replaced. In this study, the effect of PWHT time on the properties of A106/B parent metal was examined, with particular emphasis on the effect on toughness. PWHT was carried out for 1, 8 and 40 hours and the results were compared with those for as-received material. Impact transition temperature and room-temperature tensile results were determined using standard tests. The reference temperature, To, was determined using ASTM E1921 with arc-shaped tension specimens. The results showed that the ductile-to-brittle transition temperature increased significantly as PWHT time increased and the implications of this to a fracture mechanics analysis of plant operations are discussed.