Post-weld Heat Treatment Cycles Research Articles

The effect of weld bead heat input on the performance of thick 3CR12 stainless steel welds

3CR12 is a low cost special stainless steel containing 12% chromium. It is widely used in material handling, road transport, rail transport, power generation and petrochemical applications due to its favorable corrosion performance. 3CR12 is a weldable material, but it has challenges such as poor ductility, poor toughness of 3CR12 stainless steel welds, unfavorable microstructure and cross-sectional properties of the weld bead. This is due to the welding heat input and heat transfer across beads, which causes cracking due to property changes. When welding a thick stainless steel section, the hardest heat affected zone (HAZ) is created due to uneven heating and cooling in the weld metal and base material, which is susceptible to cold cracks and residual stresses, resulting in welding defects and unfavorable material properties. Welding thick 3CR12 stainless steels can be accomplished by using a variety of welding techniques. In order to restore undesirable material qualities, the majority of welds require a post-weld heat treatment (PWHT). However, critical PWHT factors such as duration, weld geometry, and unsupported loads during the hot phase of the weld do not always allow the process to be effective. This process results in prolonged downtime since the holding time in the PWHT cycle can be very long, especially with thick-walled 3CR12 steel members. There are other methods that are currently used to weld or repair thick 3CR12 steels. Among these other methods, the use of controlled temper bead welding technique which potentially could mitigate the above mentioned problems is mentioned. The approach involves using the heat input of the weld bead as a way of introducing the heat into the previous weld bead. This is achieved by varying the heat input in order to improve the mechanical performance of the previous weld bead and thus the overall weld integrity. Welding procedures for 3CR12 recommend the use of minimum heat to avoid weld defects and unfavorable material properties. This study was carried out to investigate the effect of the heat input of the weld bead on the mechanical properties of thick welds made of 3CR12 stainless steel. By utilizing inert gas (MIG) welding, two heat input levels were used in the process: low heat input averaged at 1.53 kJ/mm and high heat input averaged at 2.1kJ/mm. The heat input was varied by changing the welding parameters, for example the average welding voltage ranging from 28V to 30V, the average welding current from 120 A to 150 A and the average feed rate between 1.77mm/sec to 2.0mm/sec.

Post-Weld Heat Treatment of S690QL1 Steel Welded Joints: Influence on Microstructure, Mechanical Properties and Residual Stress

During the manufacturing of welded structures, some degree of residual stresses occurs. The classic approach to residual stress reduction is Post-Weld Heat Treatment (PWHT). In the case of structural grade mild steels, the thermal process is well established. In case of S690QL1 High Strength Steel (HSS), which is manufactured using the Quenching and Tempering (QT) process considered in this paper, only limited PWHT treatment is possible without deterioration of mechanical properties. Since this steel grade is susceptible to subsequent heat input, the challenge is to establish adequate PWHT parameters, achieving residual stress reduction while retaining sufficiently high mechanical properties. The paper considers X joint welded HSS steel plates with slightly overmatching filler metal. The welded coupon is prepared and subjected to PWHT treatment. The research on the influence of heat treatment was performed using the four different PWHT cycles and initial As-Welded (AW) material condition. The authors proposed those PWHT cycles based on available resources and the literature. Process holding temperature is considered the variable parameter directly related to the behaviors of material properties. The methodology of welded joint analysis includes experimental testing of mechanical properties, metallographic examination, and residual stress quantification. Testing of mechanical properties includes tensile testing, Charpy V-notch impact testing, and hardness testing in scope of complete welded joint (BM + HAZ + WM). Metallographic examination is performed in order to characterize the welded joint material in relation to applied PWHT cycles. In order to quantify residual stresses, all heat-treated samples were examined via the X-ray diffraction method. Mechanical properties testing determined that an increase in PWHT cycle holding temperature leads to degradation of tested mechanical properties. For specific zones of the welded joint, the decreasing trend from AW condition to Cycle D (max. 600 °C) can be quantified. Based on representative specimens comparison, strength values (BM ≤ 5.7%, WM ≤ 12.1%, HAZ ≤ 20%), impact testing absorbed energy (BM = 17.1%, WM = 25.8%, FL = 12.5%, HAZ = 0.6%), and hardness values (BM = 4.1%, WM = 3.2%, CGHAZ = 16.6%, HAZ = 24.2%) are all exhibiting decrease. Metallographic examination, using the light microscopy, after the exposure to PWHT thermal cycles, did not reveal significant changes in the material throughout all specific welded joint segments. Average relative reduction in residual stress in correlation with PWHT temperature can be observed (AW = 0%, Cycle A (max. 400 °C) = 72%, Cycle B (max. 530 °C) = 81%, Cycle C (max. 550 °C) = 93% and Cycle D (max. 600 °C) = 100% stress reduction). It can be concluded that S690QL1 HSS welded joints can generally be subjected to PWHT, while adhering to the limits of the material and process. In the authors’ shared opinion, it is advisable to use the PWHT Cycle C (max. 550 °C) with 93% RS reduction, while mechanical properties retain high values.

In-situ X-ray difraction analysis of reverted austenite in supermartensitic stainless steel weld deposits

To improve the mechanical properties in supermartensitic stainless steel weld deposits, suitable post weld heat treatments (PWHT) are required, since these properties are controlled by the resulting microstructure. Some aspects of localized corrosion resistance could also be enhanced by the PWHT. In this sense, the austenite transformation or stability is a key issue. The aim of this work is to “in-situ” analyze the evolution of austenite during the thermal cycle associated to a post weld inter-critical heat treatment in a supermartensitic stainless steel weld deposit, by means of a thermomechanical simulator integrated on a synchrotron X-ray diffraction line. An all-weld metal coupon was welded with a semi-automatic process. Dilatometry measurements were performed at different heating rates (1, 10 and 100 K/s) in the thermomechanical simulator, to determine the critical transformation temperatures (Ac1 and Ac3) of the all-weld metal. To study the phase transformation during the applied thermal cycle (heating to 938 K at 1 K/s, maintenance during 15 min and cooling to room temperature at 2 K/s), in-situ synchrotron X-ray diffraction measurements were carried out in the facilities of National Laboratory Synchrotron Light (Campinas, Brasil). The microstructural evolution was discussed in terms of volumetric phase fractions, micro-deformation and crystallite size. The techniques used allow to detect the critical transformation temperatures, phase transformations and their kinetics, monitoring particularly the austenite evolution during the post weld heat treatment cycle. At the inter-critical temperature 71% of austenite was formed, after the 8.5 min from the 15 min of permanence. Finally, at room temperature 29% of stable reverted austenite was obtained. Both martensite and austenite there did not show significant variations during the thermal cycle both for microstrain and crystallite size.

A comparative study on microstructural and mechanical behaviour of spot welded Ti-6Al-4V alloy in as-welded and solution treated condition

ABSTRACT The current experimentation emphasises on the diversification in metallurgical transformation under solution treatment, which governs the solidification rate and can trigger the final mechanical behaviour of the welded structure. In the present work, a thin sheet (750 μm) of Ti-6Al-4V alloy is joined using resistance spot welding process in overlap configuration at varied weld time cycles of 30, 50, and 70. Further, in order to explore metallurgical aspects and associated influence on mechanical behaviour, post-weld heat treatment (PWHT) is performed. The weld pad is heated up to -transus temperature (~915°C) for 60 min, consequently cooled by water quenching method. It is opined that the welding heterogeneity that arises at welding can be homogenised by gradual heating and cooling cycles in the PWHT cycle. Upon increasing weld time from 50 → 70 cycles, mechanical properties are improved. Therefore, 70 cycles is regarded as the optimised weld time. Coarsening of -lath within the prior -grain boundary is confirmed in the weld zone of as-welded specimens. On the other hand, application of solution treatment found is to be advantageous since it changes the mode of failure from interfacial to pullout, which is always desirable. Improved tensile-shear load bearing capacity (TSLBC) and extension are indicated by pullout failure accompanied by solution treatment.

Internal stresses in a clad pressure vessel steel during post weld heat treatment and their relevance to underclad cracking

A potential connection between the residual stresses generated by a cladding operation, the thermal stresses generated during post weld heat treatment (PWHT), and the propensity for underclad cracking was investigated. As-deposited residual stresses were measured at ambient temperature, using the contour method and neutron diffraction. Measurements were performed in 20mm thick steel substrates that were partially clad with AISI grade 309 (layer 1) and grade 308 (layer 2) stainless steels, using the submerged-arc welding process. Neutron diffraction measurements were also performed after heating the clad samples to 200 and 325°C. The mechanical properties in each region within the steel heat affected zone (HAZ) were determined, at those same temperatures, by subjecting coupons to simulated cladding thermal cycles and then to tension tests. Substantial tensile residual stresses in the HAZ were measured after cladding. These persisted during subsequent heating to 325°C: a temperature that is approximately at the mid-point of the heating ramp in a typical PWHT cycle. The tensile results, however, indicated that each microstructural zone within the HAZ is likely to retain sufficient ductility to prevent fracture, and that underclad cracking is unlikely to occur during the early stages of the PWHT ramp.

Shielded Active Gas Forge Welding of an L80 Steel in a Small Scale Shielded Active Gas Forge Welding Machine

The Shielded Active Gas Forge Welding (SAG-FW) method is a solid-state welding technique in which the mating surfaces are heated by induction heating or direct electrical heating before being forged together to form a weld. In this article, an API 5CT L80 grade carbon steel alloy has been welded using the SAG-FW method. A small-scale forge welding machine has been used to join miniature pipes extracted from a large pipe wall. The welding was performed at three different forging temperatures, i.e., 1300 °C, 1150 °C and 950 °C, in some cases followed by one or two post weld heat treatment cycles. In order to qualify the welds, mechanical and corrosion testing was performed on miniature samples extracted from the welded pipes. In addition, the microstructure of the welds was analysed, and electron probe microanalysis was carried out to control that no oxide film had formed along the weld line. Based on the complete set of experimental results, promising parameters for SAG-FW welding of the API 5CT L80 grade steel are suggested. The most promising procedure includes forging at relative high temperature (1150 °C) followed by rapid cooling and a short temper. This procedure was found to give a weld zone microstructure dominated by tempered martensite with promising mechanical and corrosion properties. The investigation confirmed that small scale forge welding testing is a useful tool in the development of welding parameters for full size SAG-FW welding.

Microstructure and mechanical property evolution of post weld heat treated bainitic steel welds

Abstract The achievement of excellent combination of high tensile strength and good ductility has been the key factor that facilitates the Advanced High Strength Steels (AHSS) to be the candidate materials for automotive applications. Carbide free bainitic steels are a class of AHSS that primarily have microstructure consisting blend of fine bainitic laths and interwoven retained austenite in the interlath boundaries of bainite. Solid state welding techniques are regarded to be the most sought after fabrication processes as they result in retention of basic microstructural constituents, especially nonequilibrium phases like bainite. Keeping this fact in view, friction stir welding was carried out on 3 mm thick carbide free bainitic steel plate with varying tool rotational speeds. The temperatures of stir zones were recorded. The microstructural changes and phase analysis from base metal (BM) to Thermomechancally Affected Zone (TMAZ) and Stir Zone (SZ) were studied. The microhardness profiling was done across the weld covering the BM, TMAZ and SZ. The abrupt increase in the hardness of stir zone and microstructure stability i.e., retention of bainitic microstructure along with the retained austenite were observed. The tool rotational speed of 150 RPM has been optimized. Post weld heat treatments were undertaken at different temperatures and increased soaking time on the welded samples of optimized rotational speed to facilitate bringing down the stir zone hardness to the base metal. Basing the phase analysis and EBSD data obtained, the post weld heat treatment cycle conducted at 500OC with a soaking time of 1 h has been optimized.

Effect of Nd:YAG pulsed laser welding process on the liquation and strain-age cracking in GTD-111 superalloy

The weldability of GTD-111 nickel-based superalloy by Nd:YAG pulse laser with an average power of 250 W was studied using several pre- and post-weld heat treatment cycle, and the characteristics of liquation, solidification and strain-age cracks were also investigated. The results revealed that liquation cracks in the GTD-111 Nickel-Based Cast Superalloy were associated with the constitutional liquation of γ′ particle, MC carbides, inter-dendritic γ-γ׳ eutectic and melting of Cr-rich boride. Also, cracking during welding occurred at the places that the concentration of Al and Ti is high. Crack-free laser welds were observed in the MT1 conditions (1200 ℃ for 2 h) owing to dissolution of deleterious phases before welding. γ′ phase had little effect on the incidence of cracking in the MT1 condition. An analysis of the microstructures indicated that the cracking was caused primarily by liquation in the as-welded condition and was exacerbated by post-weld heat treatment cracking during the subsequent heat treatment. aging heat treatment of samples which were undergone aging heat treatment before welding, resulted in the formation of strain-age cracking due to γʹ and γ-γ׳ eutectic phases precipitation. The results of micro-hardness indicated that the hardness of fusion zone (FZ) was higher than that of HAZ and base metal zone (BMZ). The liquation cracking in the heat affected zone (HAZ) was observed to be affected by the hardness of base-alloy.

Optimization of post weld heat treatment cycle of fiber laser welded bainitic steel

Automobile industry has always been in look out for advanced materials that would account for greater crash resistance, high fatigue strength, optimum ductility and longer service life despite heavy mechanical loads applied on these engine components. These critical requirements are met through maintaining the complex microstructures and optimum phase constituents. The retention of microstructural constituents has always been a key parameter while fabricating these advanced automobile materials by fusion welding process. Carbide free bainitic steels are emerging out to be the candidate materials for automobile applications. Owing to their microstructure consisting fine bainitic ferrite laths that are interwoven with retained austenite in their lath boundaries. The fine Bainitic laths provide enough strength and the retained austenite phase facilitates the desired ductility. The current paper critically discusses the microstructural and microhardness variation across the zones during Fiber Laser welding of bainitic steel sheets. Keeping the phase transformations during welding in view, post weld heat treatments were undertaken. The welded steel was austenitized at 820 OC, rapidly cooled to 390 OC, and soaked at different durations before furnace cooing. The microstructure variation and microhardness profiling were done at all these heat treatment conditions. Basing on the analyses, the heat treatment cycle has been optimized.

Effects of pre- and post-weld heat treatment cycles on the liquation and strain-age cracking of IN939 superalloy

Performing heat treatment cycles on precipitation-hardened nickel-based superalloy before and after welding imposes significant contributions to reduced welding cracks. In this study, different pre- and post-weld heat treatment cycles were performed on IN939 alloy to study their effects on microstructure and mechanical properties of the weldments. Three types of pre-weld heat treatment cycles were performed to over age the microstructure and reduce the hardness of the base alloy. Results indicated that hardness of over-age heat-treated samples decreased to about 300 to 315 HV (∼35%). Standard post-weld heat treatments (PWHT) were applied on weldments for strain-age cracking examination. Microstructural investigations showed that the above-mentioned treatments impose significant effects on reducing the liquation and strain-age cracking in heat-affected zone (HAZ) of IN939 weld. Slow furnace cooling heat treatment was found to be the optimal pre-weld heat treatment for the tungsten inert gas (TIG) welding of IN939. Based on the results obtained upon mechanical and microstructural analyses, the four-stage PWHT cycle was found to be a suitable one for IN939 welding. A joint efficiency of 79% was achieved in the optimum pre-and post- weld heat treatment cycles.

Microalloyed steel welds by HF-ERW technique: Novel PWHT cycles, microstructure evolution and mechanical properties enhancement

Novel post-weld heat treatment (PWHT) cycles consisted of multiple austenitizing, normalizing, quenching and tempering steps had been introduced and examined in order to improve the toughness of high-frequency electrical resistance welded (HF-ERWed) microalloyed line pipe steel joints. Comparison was made to commercial PWHT route of API X60 grade steel which has been used widely for pipeline parts. Effects of applied cycles on microstructural evolution were investigated by optical and scanning electron microscopy techniques. Meanwhile, Vickers hardness and Charpy V-notch impact toughness tests were conducted to evaluate the mechanical properties of the treated weldments. Based on the obtained results for the proper quenching and tempering heat treatment at 600 °C for 30 min, both hardness and ultimate strength remained unchanged related to the classic treatment (172 Hv and 535 MPa); however, it eventuated the absorbed energy enhancement to nearly two-times greater than that for the commercial treatment (33 J versus 17 J at −25 °C). While the mechanical properties of one-step normalizing treatment satisfied the API specification, the two-step quenching and tempering PWHTs were recommended for API X60 grade steels.

Experimental investigation on microstructure and mechanical properties of activated TIG welded reduced activation ferritic/martensitic steel joints

In the present study activated tungsten inert gas welding (A-TIG) process has been attempted for welding of 6mm thick reduced activation ferritic/martensitic (RAFM) steel. Comparative studies on the effect of single (760°C for 2h) and double (760°C for 2h+760°C for 2h) post-weld heat treatment (PWHT) cycles on microstructures, microhardness, tensile strength, impact strength and ductile to brittle transition temperatures (DBTT) of these welded joints have also been evaluated. It was inferred from the studies that ductility was improved for the weldments after undergoing double PWHT cycles. The toughness values increased after double PWHT cycles and were comparable to that of base metal values. DBTT values of −5°C and −11°C were obtained for the welded samples undergone single and double PWHT cycles respectively. This study also attested the detailed structure–property relationship of A-TIG weldments using optical microscopy, scanning electron microscopy, energy dispersive spectroscopy (EDS) techniques.

Efecto del tratamiento térmico sobre las propiedades mecánicas y microestructura de un acero para tubería API 5CT J55

The effects of two different post-weld heat treatment cycles on the microstructure and mechanical properties of welded API 5CT J55 steels were investigated in the present work. Experiments were carried out based on a Taguchi experimental design. Ortogonal arrays (L9) of Taguchi and statistical analysis of variance (ANOVA) were employed to determine the impact of the heat treatment parameters on the microstructure and mechanical properties of experimental steel. From the results of ANOVA, there were obtained the empirical equations for optimizing the heat treating conditions that lead to the best mechanical properties.

Evaluation of post weld heat treatment quality of modified 9Cr–1Mo (P91) steel weld by magnetic coercive force measurements

This paper presents a novel technique for evaluation of quality of post weld heat treatment (PWHT) of modified 9Cr–1Mo (P91) steel weld by correlation with its magnetic coercive force (MCF). The MCF of modified 9Cr–1Mo steel was studied in respect of different PWHT cycles involving ordered variations of soaking temperature unto 770°C and soaking time unto 140min. It was observed that MCF of the weld metal reduced with increase in both the soaking temperature and soaking time of PWHT. The drop in MCF of welds after PWHT was found to be dependent on the drop in the hardness of the weld and on the degree of carbide precipitations seen in the weld microstructure. Based on the study, a criterion was evolved for evaluation of heat treatment quality of the weld.

Effect of post-weld heat treatment (PWHT) time and multiple PWHT on mechanical properties of multi-pass TIG weld joints of modified 9Cr-1Mo steel

The effect of post-weld heat treatment (PWHT) time and multiple PWHT on mechanical properties of modified 9Cr-1Mo steel weld joints has been studied using multi-pass tungsten inert gas (TIG) weld joints fabricated from 12.5-mm-thick plates using matching composition ER90S-B9 filler wire. Radiographically qualified weld joints were subjected to single PWHT at 760 °C for different hold times, viz. 1, 3, 4, 8 and 12 h, as also to multiple PWHT cycles of (1 h + 3 h) and (4 h + 4 h) for comparing with single PWHT of 4 h and 8 h, respectively. Transverse-weld tensile strength of the weld joints (as also the base material) decreases marginally with increasing heat treatment duration. In all heat-treated conditions, the Charpy V-notch impact toughness of more than 200 J is obtained for the weld and base materials. Multiple PWHT is found to have no adverse effect on the tensile properties and impact toughness of the weld joint, which has been corroborated by microstructural examination and hardness measurements.

Investigation of the hardness–toughness relationship of a welded joint after different heat treatment cycles

Due to the increasing use of high-strength steels, the requirements for filler materials are also growing. To investigate the toughness properties of the corresponding welded joints, a new testing method has to be developed. This new test method uses subsized Charpy-V specimens (abbr. KLST samples) which permit the analysis of welded sheets with a thickness of approx. 5 mm. For the development and evaluation of the new method, a high-strength steel (S690) and a high-strength filler metal (T 69 5 ZMn2NiCrMo M M1 H5) were used. Physical simulations of various post-weld heat treatment cycles and multipass welding were conducted in a dilatometer. After the heat treatment, the toughness of the specimens was tested by Charpy impact testing and the hardness with Vickers hardness measurements. Additional metallographic investigations were also carried out. The results show that the method provides useful results for a wide range of parameters and it is appropriate for the simulation of post-weld heat treatment cycles and multipass welding on welded joints.

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

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

FCAW process to avoid the use of post weld heat treatment

Post weld heat treatment (PWHT) is the most common technique employed for relieving residual stresses after general repair welding. Besides, the primary purpose of reducing the effect of stresses induced by welding, PWHT is also intended to temper the metallurgical structure of the heat-affected zone (HAZ). Unfortunately, there are significant difficulties in carrying out post weld heat treatment such as; the complexity of weld geometry, the possibility of distortion in the case of any mechanical loads, difficulty in heating symmetrically, and also PWHT may cause degradation of the material properties (especially creep and tensile strength in the case of multi PWHT cycles). Most of the repairs in industry are performed with manual metal arc welding (MMAW), however, the benefits of the flux cored arc welding (FCAW) process have been appreciated by industry for many years. Guidelines in the current welding standards in addressing the issue of temper bead welding (TBW) when fully automated flux cored arc welding is used are very limited. This paper reports research work carried out to investigate, whether a fully automated flux cored arc welding process with bead tempering can be used in repair welding instead of manual metal arc welding in order to eliminate the use of post weld heat treatment. The paper also examines different percentages of bead overlaps and studies their effects on the mechanical properties as well as the microstructures. The results show that desirable microstructures and hardness values can be obtained using flux cored arc welding when 70% bead overlap is used.

Effect of PWHT cycles on fatigue crack growth and toughness of quenched and tempered pressure vessel steels

Transportable pressure vessels, commonly manufactured from quenched and tempered (QT) steels, require mandatory postweld heat treatment (PWHT) regardless of plate thickness. During their life, transportable pressure vessels may have up to 4 PWHT cycles at temperatures between 540 and 590 ◦ C, and concerns have emerged about possible effects on the mechanical properties of the base (parent) plate. This paper reports work on the resistance to fatigue crack growth, impact toughness and CTOD fracture toughness of two types of QT steels. The parent metal region was examined for both steels, and then exposed to temperatures and times in the PWHT range. Although there was a marginal decrease in the resistance to fatigue crack growth for up to four postweld heat treatment cycles, there was a significant decrease in impact toughness and CTOD fracture toughness. © 2004 Elsevier B.V. All rights reserved.

Toughness Evaluation of a Shielded Metal Arc Carbon-Manganese Steel Welded Joint Subjected to Multiple Post Weld Heat Treatment

This study was part of a program to investigate the influence of multiple post weld heat treatment (PWHT) on the fracture toughness and defect tolerance of a welded joint. The present work reports base metal data obtained for a quenched and tempered BS7191 Grade 450EM steel (0.10wt%C-1.08wt%Mn), weld metal data for a ferritic multipass weld obtained by shielded metal arc welding using an AWS E-9018M type electrode, and heat affected zone (HAZ) data obtained using a modified bead on groove technique for different PWHT conditions. The effect of the repeated heat treatment cycles on the mechanical properties was evaluated by hardness tests and toughness testing assessed by Charpy V-notch and crack tip opening displacement (CTOD) techniques. The characterization of the microstructure was undertaken utilizing optical and electron microscopy. As fabrication codes for new equipment do not allow more than three PWHT cycles, the application of more cycles is only justifiable for old equipment when a fitness for purpose criterion is applied and these restrictions are not applicable. The results obtained are currently applied in repair work and revamps of pressure vessels and gas storage tanks.