Effect of pre- and post-weld heat treatment on metallurgical and tensile properties of Inconel 718 alloy butt joints welded using 4 kW Nd:YAG laser

The effects of pre- and post-weld heat treatments on the butt joint quality of 3.18-mm thick Inconel 718 alloy were studied using a 4 kW continuous wave Nd:YAG laser system and 0.89-mm filler wire with the composition of the parent metal. Two pre-weld conditions, i.e., solution treated, or solution treated and aged, were investigated. The welds were then characterized in the as-welded condition and after two post-weld heat treatments: (i) aged, or (ii) solution treated and aged. The welding quality was evaluated in terms of joint geometries, defects, microstructure, hardness, and tensile properties. HAZ liquation cracking is frequently observed in the laser welded Inconel 718 alloy. Inconel 718 alloy can be welded in pre-weld solution treated, or solution treated and aged conditions using high power Nd:YAG laser. Post-weld aging treatment is enough to strengthen the welds and thus post-weld solution treatment is not necessary for strength recovery.     

Residual stress driven cracking in superalloy weldments

Superalloy weldments are normally given post weld heat treatments to homogenize the weld metal microstructure, relieve residual stress, and precipitate strengthening phases. The relationship between microstructure and post weld heat treatment is easily studied; it is less straightforward to study the effects of post weld heat treatment on residual stress relaxation. Using a self-restrained testing procedure, a relatively simple approach was used to investigate the effects of microstructure and post-weld heat treatment on cracking during residual stress relaxation. Candidate superalloys for Advanced Ultra Supercritical steam plants were studied. It was found that cracking due to residual stress relaxation is primarily dependent on grain size, and in cases of intermediate grain size, intragranular precipitation is a controlling factor. These results are in agreement with traditional stress relaxation cracking theories.     

Single pass laser joining of Inconel 718 superalloy with filler

Abstract

Microstructure and mechanical property of CO₂ laser beam welded IN 718 superalloy were studied by electron microscopy and hardness testing. The use of a welding filler wire produced a sound fusion zone with no cracking but grain boundary microfissuring occurred in the heat affected zone (HAZ) and was observed to be significantly influenced by pre-weld heat treatment and laser welding speed. Crack-free weld was produced by a pre-weld heat treatment that minimised non-equilibirum grain boundary boron segregation and inhibited grain growth. While post-weld heat treatment (PWHT) reduced the difference between the hardness values of the base alloy, HAZ and the fusion zone, it resulted in increased HAZ cracking, which was likely aided by pre-existing cracks. The PWHT cracking was, however, avoided by subjecting pre-weld material to the heat treatment condition that produces crack-free weld during welding process.     

Effect of pre- and post-weld heat treatments on the mechanical properties of electron beam welded Ti-6Al-4V alloy

This paper represents a summary of experimental work carried out to find the effect of various pre- and post-weld heat treatments on Ti-6Al-4V alloy. In the as-welded state the samples exhibit about 80% of the tensile ductility and about 90–95% of the impact/fracture toughness of the base metal. Low temperature stress relieving or ageing carried out subsequent to the welding operation improves the tensile properties but decreases the toughness at the fusion zone. Solution treatment followed by welding and ageing or the post-weld solution treatment and ageing treatment leads to only a marginal increase in tensile strength at the expense of toughness at the fusion zone. High temperature annealing of the welded samples does not increase the tensile ductility but improves the toughness at both the fusion zone and the heat affected zone. The above facts and a special burst-pressure test conducted on a gas bottle in the as-EB welded state show that Ti-6Al-4V components can be used without subjecting them to any post-weld heat treatments.     

An experimental study of the residual stresses,and their alleviation,in tube to tube-sheet welds of industrial boilers

Micro-crack development in welds, including repair welding, of the tube to tube-sheet region of industrial boilers is not an uncommon problem. Often such cracking is exacerbated by stress corrosion cracking or thermal fatigue, in which the residual stresses associated with welding can play a major role. In this paper, a custom built, but robust, air abrasive centre hole drilling facility was used; which induced negligible machining stresses. This system was modified to attach to the (vertical) tube sheets of boilers on site, and to sample residual stresses in the tube to tube-sheet welds, of both conventional circumferential fillet welds of protruding tubes as well as recessed ‘J’ configuration type. The experimental test programme evaluated transverse and circumferential profiles of the residual stresses. Global post-weld heat treatment (PWHT) of a whole boiler, indicated substantial (75%) reductions in the tensile peak residual stress. A localised PWHT technique, using a 1 kW incandescent light source inserted into the tube, easily achieved the requisite temperatures and heating rates, but led to significant increases in local residual stresses. The implications of such localised PWHT techniques on residual stress and consequent service life, are therefore of vital importance with regard to plant integrity and safety.     

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.     

Cyclic overaging heat treatment for ductility and weldability improvement of nickel based superalloys

Abstract

A new cyclic overaging heat treatment for ductility and weldability improvement of advanced nickel based superalloys has been investigated. The heat treatment consists of intermittent heating cycles during the cooling stage to check the nucleation of fine γ' in the material during the overaging. Using Rene 80 alloy as a model material, the effects of intermittent heating rate and duration were studied and optimised. Quantitative metallography was carried out to compare the microstructures produced by the proposed heat treatment with those produced by other conventional pre-weld heat treatments. Tensile tests and welding tests both showed that the proposed cyclic over-aging heat treatment is effective in improving the tensile ductility and weldability of Rene 80 alloy.     

Analysis of laser beam weldability of Inconel 738 superalloy

The susceptibility of pre-weld heat treated laser beam welded IN 738 superalloy to heat affected zone (HAZ) cracking was studied. A pre-weld heat treatment that produced the minimal grain boundary liquation resulted in a higher level of cracking compared to those with more intergranular liquation. This deviation from the general expectation of influence of intergranular liquation extent on HAZ microfissuring is attributable to the reduction in the ability of the base alloy to accommodate welding tensile stress that accompanied a pre-weld heat treatment condition designed to minimize intergranular liquation. Furthermore, in contrast to what has been generally reported in other nickel-based superalloys, a decrease in laser welding speed resulted in increased HAZ cracking in the IN 738, which can be attributed to exacerbated process instability at lower welding speeds.     

钛合金薄板激光和钨极氩弧焊残余应力测试研究

采用小孔释放法对钛合金薄板激光焊和钨极氩弧焊(TIG焊)的焊接残余应力进行了测试，并分析了焊接方法、焊接线能量和焊后热处理对残余应力分布规律的影响。研究结果表明：激光焊残余应力分布规律与普通熔焊方法相似，但其分布区域较窄；在热影响区内，激光焊残余拉应力值比TIG焊的约低100MPa；在焊缝及其熔合线附近，激光焊残余应力却比TIG焊的高。对于不同线能量激光焊接，线能量越大，焊缝越宽，热影响区的残余应力也越大。焊后真空热处理能降低残余应力90％。     

反应堆压力容器接管安全端焊接残余应力模拟及其焊接参数优化研究

目的 针对反应堆压力容器接管安全端焊接残余应力较大易导致应力腐蚀开裂的问题,探究焊接工艺参数对焊接残余应力的影响,并寻找最佳工艺参数。方法 利用有限元参数化建模方法建立反应堆压力容器接管安全端的三维热-力耦合模型,模拟其焊接过程,研究焊接残余应力的变化情况。采用正交试验设计法分析了焊接电流、焊接速度及层间冷却时间对焊后最大等效应力的影响,建立了焊后最大等效应力与焊接电流、焊接速度及层间冷却时间的二次回归模型,基于该模型利用遗传算法寻优焊接参数。结果 焊接残余应力峰值靠近熔合区位置,残余应力较高,超过了材料的屈服应力;各参数按对焊后等效残余应力峰值的影响由大到小的顺序依次为焊接速度、焊接电流、层间冷却时间。正交试验所得最佳工艺参数如下：焊接电流为610 A,焊接速度为20 mm/s,层间冷却时间为400 s,经遗传算法进一步优化后所得的最佳参数如下：焊接电流为610 A,焊接速度为23 mm/s,层间冷却时间为427 s。通过仿真验证遗传算法优化结果,得到焊接残余应力的峰值为373 MPa,比未优化前减小了44 MPa。结论 优化后的工艺参数有效降低了焊接残余应力,提高了反应堆压力容器接...     

The effects of pre- and post-weld heat treatment variables on the strain-age cracking in welded Rene 41 components

Rene 41, a precipitation-hardened nickel-base superalloy, is mostly used for hot section parts of jet aircraft engines. When cracks occur in these parts, welding is frequently utilized for crack repair to extend their service life. Rene 41 is very susceptible to strain-age cracking during welding and post-weld heat treatment because of its strengthening mechanism. The strengthening mechanism depends on the existence of gamma prime (γ′) phase and the morphology of carbides both of which are affected by heat treatment. In this study, the effects of pre- and post-weld heat treatment variables such as heating rate, holding temperature and time, and cooling rate on the strain-age cracking of Rene 41 during welding and post-weld heat treatment were investigated. An optimum combination of heat treatment variables was investigated to obtain a microstructure that would be less susceptible to strain-age cracking during welding and post-weld heat treatment. Pre- and post-weld solution heat treatments at 1080 °C for 30 min with 20 °C/min heating and 34 °C/min cooling rates produced optimum microstructures, and no cracks were observed in the Rene 41 samples after welding. MC carbides were dissolved lightly and distributed homogenously within the grains. M₆C carbides precipitated as globules within the grains and in the grain boundaries. M₂₃C₆ carbides precipitated as chains of globules in the grain boundaries.     

Investigation on Cracking in the Surfacing Welding Layer of Ni_3Al Based Alloy

Investigation has been made into the causes of cracking in the Surfacing welding layer of Ni3Al based alloy by analysing both the liqu id-to-solid transformation in the molten pool and the distribution of thermal stress within the surfacing welding layer. The results show that cracking in the surfacing welding layer is directly related to the producing of eutectic phase β' (NiAl) in the interdendritic region and high thermal stress within the surfacing welding layer. When the process of electric arc surfacing welding is changed from along straight line to along" Z" pattern, cracking in the surfacing welding layer of Ni3Al based alloy is prevented due to being reduced of both the cooling rate of liquid in the molten pool and the moving speed of the heat source. Reducing the melting volume of the substrate material by lowering the output power of electric arc welding would make the content of iron atoms in the molten pool decrease. and this also can reduce the trend of the eutectic reaction in the interdendfitic region and is helpful to Suppress cracking in the surfacing welding layer.     

Failure Analysis of Welded Radiant Tubes Made of Cast Heat-Resisting Steel

Radiant tubes made of cast heat-resisting steels were cracked after 4 years of operation at 1020 °C temperature in hydrocarbon cracking furnace. Optical microscopy of the tubes showed that there was extensive precipitation and intermetallic compound formation especially as brittle networks with progressive reduction in toughness and resistance to thermal and mechanical stresses. SEM and EDS analysis proved both decarburization and oxidation on interior and exterior surfaces. Apart from cracking due to long-term heating, the tubes experienced high temperature creep. HAZ cracking after welding of cracked and/or creeped tubes due to formation of brittle carbide networks was overcome by localized solution heat treatment followed by fast dry air cooling. Localized dissolution of carbide networks and intermetallic compounds resulted in lower strain microstructures and enhanced resistance of parts to thermal and mechanical stresses during repair welding. It is evident that localized solution heat treating other than lowering strains can cause the precipitates to be more uniformly and finely distributed. Fast dry air cooling rate after solution heat treating and similar cooling after welding can help to control precipitation of carbides. Detailed non-destructive testing after welding along with tensile testing proved that post-weld cracking was controlled.     

Investigations on welding residual stresses in penetration nozzles by means of 3D thermal elastic plastic FEM and experiment

Recent discoveries of stress corrosion cracking (SCC) in weldments including penetration nozzles at pressurized water reactors (PWRs) and boiling water reactors (BWRs) have raised concerns about safety and integrity of plant components. It is well known that welding residual stress is an important factor resulting in SCC in weldments. In the present work, both experimental method and numerical simulation technology are used to investigate the characteristics of welding residual stress distribution in penetration nozzles welded by multi-pass J-groove joint. An experimental mock-up is fabricated to measure welding residual stress at first. In the experiment, each weld pass is performed using a semi-circle balanced welding procedure. Then, a corresponding finite element models with considering moving heat source, deposition sequence, inter-pass temperature, temperature-dependent thermal and mechanical properties, strain hardening and annealing effect is developed to simulate welding temperature and residual stress fields. The simulation results predicted by the 3D model are generally in good agreement with the measurements. Meanwhile, to clarify the influence of deposition sequence on the welding residual stress, the welding residual stress field in the same geometrical model induced by a continuous welding procedure is also calculated. Finally, the influence of a joint oblique angle on welding residual stress is investigated numerically. The numerical results suggest that both deposition sequence and oblique angles have effect on welding residual stress distribution.     

某机组高压外缸开裂原因分析

采用残余应力测试、力学性能测试、断口分析和金相检验等方法分析了某机组ZG15Cr1Mo1V-B2钢高压外缸在泵水时开裂的原因。结果表明:开裂始于高压外缸内表面的补焊区,该区为缩松缺陷密集区,材料塑性指标不符合技术条件要求,导致缺陷处的承载能力下降,应力集中加剧;另高压外缸补焊区的残余应力较大,硬度与基体之间的差异也较大;可见,汽缸在补焊过程中未能将铸造缺陷彻底清除干净,焊接质量不高和焊后去应力处理不充分是造成该高压外缸在泵水时开裂的主要原因。     

POST WELD HEAT TREATMENT EFFECTS ON FATIGUE CRACK PROPAGATION IN AN ELECTRON BEAM WELDMENT

This paper studies the effect of two post-weld heat treatment processes on the fatigue behaviour of an electron beam weldment in 9 mm AISI 4130 steel. Electron beam tempering, in a vacuum chamber, immediately after welding and a traditional furnace tempering treatment were compared. Fatigue crack propagation resistance was assessed by a linear elastic fracture mechanics analysis. The resistance to fatigue crack growth was improved with post weld heat treatment due to residual stress relief and the existence of a toughened tempered microstructure. The specimens with an electron beam post-weld heat treatment showed better fatigue properties than those of furnace-treated specimens. An electron beam post-weld heat treatment causes the fatigue crack growth rate to decrease with increasing energy input and decreasing micro-dot-pattern width. For a furnace post-weld heat treatment, the fatigue crack growth rate decreases with increasing tempering temperature.     

Weldability and properties of martensitic/austenitic stainless steel joints

Abstract

A series of studies has been carried out to examine the weldability and properties of dissimilar steel joints using martensitic and austenitic stainless steels F6NM (OCr13Ni4Mo) and AISI 347, respectively. This type of joint requires good mechanical properties, corrosion resistance, and a stable magnetic permeability in addition to a good weldability. Weldability tests include weld thermal simulation of the martensitic steel to investigate the influence of weld thermal cycles and post-weld heat treatment (PWHT) on the microstructure and mechanical properties of the heat affected zone (HAZ); implant testing to examine the tendency for cold cracking of martensitic steel; and rigid restraint testing to determine hot crack susceptibility of the multipass dissimilar steel joints. The simulation results indicated that the toughness of the martensitic steel HAZ did not change significantly after the weld thermal cycles. The implant test results indicated that welds produced using nickel based filler show no tendency for cold cracking, whereas welds produced using martensitic or ferritic filler show such a tendency. Based on the weldability tests, a welding procedure (tungsten inert gas welding for root passes with HNiCrMo-2B wire followed by manual metal arc welding using ENiCrFe-3B coated electrode) was developed and a PWHT at 600°C for 2 h was recommended. Joints produced using the developed welding procedure are not susceptible to hot and cold cracking. After PWHT the joints exhibit both satisfactory mechanical properties and stress corrosion cracking resistance.

MST/1955     

大厚度奥氏体不锈钢筒体填丝激光焊接变形和应力分析

目的 研究大厚度奥氏体不锈钢筒体填丝激光焊接,优化结构设计和工艺设计。方法 建立大厚度奥氏体不锈钢筒体填丝激光焊接数值分析模型,通过数值模拟的方法,定量分析大厚度奥氏体不锈钢筒体焊接变形和应力。结果 零件下部38 mm厚焊缝位置处的最大径向收缩量为1.2 mm;零件下部60 mm厚焊缝位置处的最大径向收缩量为2.0 mm;零件中部60 mm厚焊缝位置处的最大径向收缩量为1.9 mm;零件上部60 mm厚焊缝位置处的最大径向收缩量为1.8 mm。填丝激光焊接轴向收缩量为0.55 mm。焊接残余应力最大值在450 MPa左右,应力主要分布在焊缝附近。热处理后,焊接残余应力都有明显降低,最大残余应力从450 MPa左右降低到200 MPa左右,焊接残余应力范围存在一定程度减小;焊接残余变形变化较小,热处理后某些位置的变形略微有所增大。结论 模拟结果表明,大厚度奥氏体不锈钢筒体填丝激光焊接变形和应力在可接受范围内,焊后热处理对释放残余应力有重要作用。     

HMn58-2铜制冷却器开裂失效分析

某HMn58-2铜制冷却器在库房存放4a后开裂,采用光学显微镜、力学性能试验机和布氏硬度计对开裂冷却器进行了检验和分析。结果表明:由于焊接工艺或者焊接操作不当造成的热影响区晶粒粗大和α-Cu相自β相晶界大量析出形成魏氏组织形态,降低了材料的韧性、塑性和强度;此外,冷却器铜板冷卷后没有及时进行退火处理,使得材料内部存在较大的内应力;两方面共同作用导致冷却器在存放过程中于焊缝处萌生裂纹并不断扩展最终造成开裂失效。     

Improved properties of friction stir-welded AZ31 magnesium alloy by post-weld heat treatment

Mechanical properties and microstructure of friction stir-welded AZ31 based on variety post-weld heat treatment (PWHT) temperatures were evaluated, and an optimal PWHT condition was identified. At rotational speed of 1200?rev?min^?1 and welding speed of 300?mm?min^?1, the average yield tensile, tensile strength and elongation of friction stir-welded joints was 92.5?MPa, 199.1?MPa and 7.3%, respectively. It was found that (300°C – 1?h) heat treatment after welding was more beneficial than other heat treatments in enhancing the mechanical properties and homogenising grain size. The maximum yield and tensile strength was 139.9 and 238.4?MPa, respectively, tensile longitudinal and compressive transverse residual stress could be effectively eliminated, and the fatigue strength increased 34.2% comparing with as-welded joints.