Effects of heat-affected zone peak temperatures on the microstructure and properties of 2090 Al alloy

The influence of the heat-affected zone (HAZ) thermal cycles on the microstructure and mechanical properties of 2090 Al alloy has been investigated. Emphasis is placed on the effects of the HAZ peak temperatures on the T3 and T8 tempered 2090. The HAZ thermal cycles were simulated using a Gleeble, and tensile tests were conducted at 293 and 77 K. The difference in properties between the HAZ and the base metal is primarily caused by the dissolution of strengthening phases. At 578 K, degradation in strength occurs due to dissolution of δ’ phase and, at 773 K, due to dissolution of T₁ phase. The thermal cycle to 648 K leads to an observable growth of T₁ phase, but the volume fraction is too low to be an effective strengthener. As a result, at the 293 K test temperature, the base metal (BM) tempers become irrelevant with the peak temperatures above 578 K, and the ultimate tensile strength (UTS) curves of both the BM-T3 and BM-T8 coincide and follow the same trend up to 853 K. At the 77 K temperature, the strength and elongation of the BM and the thermally cycled specimens in the T3 and T8 tempered conditions follow the trend seen at 293 K and exhibit higher strengths and elongations. Formerly Graduate Student Research Assistant Graduate Student Research     

Kinetics of grain growth in the weld heat-affected zone of alloy 718

Grain-boundary liquation occurs in the weld heat-affected zone (HAZ) of the Ni-base superalloy 718 at locations where the peak temperatures are greater than about 1200 ‡C. The evolution of the grain structure at these HAZ locations depends upon the interaction between the grains and the grain-boundary liquid. The evolution of grain structure in the presence of grain-boundary liquid was simulated by subjecting samples to controlled thermal cycles using resistance heating. A measurement of grain size as a function of isothermal hold at two peak temperatures of 1200 ‡C and 1227 ‡C indicated that in alloy 718, the kinetics of grain growth depended upon the prior thermal history of the alloy. In the solution-treated alloy, the presence of grain-boundary liquid did not arrest grain growth at either peak temperature. In the homogenized and aged alloy, a grain refinement was observed at the peak temperature of 1227 ‡C, while an arrest of grain growth was observed at a peak temperature of 1200‡C. Liquid film migration (LFM) and subgrain coalescence, either acting alone or simultaneously, are shown to explain most of the observed microstructural phenomena and the kinetics of grain growth in the alloy. Formerly Research Associate, with the Department of Materials Science and Engineering, University of Alabama at Birmingham, Birmingham, AL     

Monte Carlo simulation of grain boundary pinning in the weld heat-affected zone

A methodology for obtaining a one-to-one correlation between Monte Carlo (MC) and real parameters of grain size and time is described. Using the methodology, and the MC grain growth algorithm, the grain structure in the weld heat-affected zone (HAZ) of a 0.5 Mo-Cr-V steel has been simulated. The simulations clearly show that the kinetics of grain growth can be retarded by the presence of steep temperature gradients in the weld HAZ. Additional pinning due to the formation of grain boundary liquid near the solidus temperature has also been simulated. It is shown that in order to accurately predict the observed grain size in the weld HAZ of the 0.5Cr-Mo-V steel, the retardation in growth kinetics due to temperature gradients as well as liquid pinning should be considered.     

Microstructural evolution in the heat-affected zone of a friction stir weld

The microstructure and crystallographic texture spanning the soft region at the thermomechanically affected zone/heat-affected zone (TMAZ/HAZ) boundary of a friction stir weld in 2519 Al were systematically investigated to determine their contributions to the properties of that region. The microstructure was shown to be the primary cause of softening at the TMAZ/HAZ boundary. During welding, fine ϑ′ precipitates responsible for much of the strength in this alloy coarsen and transform to the equilibrium ϑ phase in the HAZ and into the TMAZ, accounting for the observed softening through the HAZ region. The higher temperatures achieved in the TMAZ partially resolutionize the precipitates and allow the subsequent formation of Guinier-Preston (GP) zones during cooling. These two processes are responsible for the variation in microhardness observed in the TMAZ/HAZ region. Texture analyses revealed significant differences in the crystallographic texture across this region that were primarily due to macroscopic rigid-body rotations of the grains, but do not account for the observed softening. The effect of the observed microstructural evolutions on the friction stir welding (FSW) deformation field and on the fracture behavior of the weld are also discussed.     

Kinetics of grain growth in the weld heat-affected zone of alloy 718

Grain-boundary liquation occurs in the weld heat-affected zone (HAZ) of the Ni-base superalloy 718 at locations where the peak temperatures are greater than about 1200 °C. The evolution of the grain structure at these HAZ locations depends upon the interaction between the grains and the grain-boundary liquid. The evolution of grain structure in the presence of grain-boundary liquid was simulated by subjecting samples to controlled thermal cycles using resistance heating. A measurement of grain size as a function of isothermal hold at two peak temperatures of 1200 °C and 1227 °C indicated that in alloy 718, the kinetics of grain growth depended upon the prior thermal history of the alloy. In the solution-treated alloy, the presence of grain-boundary liquid did not arrest grain growth at either peak temperature. In the homogenized and aged alloy, a grain refinement was observed at the peak temperature of 1227 °C, while an arrest of grain growth was observed at a peak temperature of 1200 °C. Liquid film migration (LFM) and subgrain coalescence, either acting alone or simultaneously, are shown to explain most of the observed microstructural phenomena and the kinetics of grain growth in the alloy. B. Radhakrishnan, formerly Research Associate, with the Department of Materials Science and Engineering, University of Alabama at Birmingham, Birmingham, AL.     

Three-dimensional monte carlo simulation of grain growth in the heat-affected zone of a 2.25Cr-1Mo steel weld

We report here the first three-dimensional (3-D) Monte Carlo (MC) modeling of grain growth in the heat-affected zone (HAZ) of a weldment. Computed grain-size distributions in the HAZ of a 2.25Cr-1Mo steel, for different heat inputs in the range from 1.1 to 4.8 MJ/m, were compared to independent experimental results. The simulated mean grain size for different heat inputs agreed well with the corresponding independent experimental data. The mean grain size at various locations equidistant from the fusion line were different. The predicted grain size from the 3-D model matched the experimental results more closely than that from a two-dimensional (2-D) model. When the whole calculation domain was subjected to a single thermal cycle experienced by a monitoring location within the HAZ, the computed grain size was larger than that calculated at the monitoring location by taking into account the prevailing temperature gradient due to thermal pinning. The good agreement between the simulated grain structure and the corresponding experimental results indicates significant promise for understanding grain-growth phenomena in the entire HAZ by using the MC technique.     

Dispersoid-free zones in the heat-affected zone of aluminum alloy welds

The local microstructure in the heat-affected zone 1 (HAZ1) of a laser beam-welded Al-Mg-Si-Cu aluminum alloy is investigated closely. Dispersoid-free zones (DFZs), where the dispersoids of the base material (BM) are dissolved, are found in the vicinity of the fusion line (FL). They are not uniformly surrounding a grain, but oriented toward the FL. Their width can be as large as 10 μm. Detailed analysis has revealed a decreased silicon concentration as well as a decreased hardness of the oriented dispersoid-free zones (ODFZs). Two mechanisms for the formation of this welding metallurgical feature are discussed.     

热处理对GH3128合金接头组织及力学性能的影响

 核能已经逐渐取代化石能源成为新一代能源,作为重要构件的高温气冷堆中间换热器得到了广泛关注。由于GH3128合金具有较好的焊接性、较高的高温抗氧化性能和组织稳定性,有望成为超高温气冷堆中间换热器的候选材料,但基于换热器结构复杂性以及密封性的要求,焊接是其生产和制造的关键成形手段。采用脉冲钨极氩弧焊(GTAW)对GH3128合金2 mm板材进行对接焊,研究了热处理对焊接焊接接头显微组织以及应力的影响。结果表明,在优化焊接工艺参数下,固溶态板材接头表现出最高的强塑性,室温及高温拉伸断裂位置均为母材。由于热轧态与固溶态板材接头热影响区在焊接过程中产生残余应力,导致该区硬化,在高温变形过程中残余应力诱发热影响区μ相析出,对接头持久、蠕变性能造成不利影响。焊后热处理消除了接头热影响区的残余应力,减少了持久、蠕变过程中μ相的析出,接头持久寿命得以改善。在1 200 ℃下,残余应力可为焊后热处理过程中静态再结晶提供激活能,接头热影响区发生再结晶,硬度下降,接头塑性变形能力不协调,导致室温拉伸与950 ℃拉伸断裂位置均为焊接接头。对固溶态板材试样进行不同的焊后热处理,EBSD扫描结果分析发现,接头经过1 100 ℃×10 min热处理后,残余应力明显消失,温度升高至1 140 ℃后,热影响区开始发生再结晶。     

Effect of cooling after welding on microstructure and mechanical properties of 12 Pct Cr steel weld metals

The microstructure of three 12 pct cr steel weld metals with different nickel and nitrogen contents was studied in as-welded condition and after postweld heat treatment with and without intercooling. Tensile strength and impact toughness of the weld metals were investigated in different postweld heat treatment conditions. In weld metals heat treated without intercooling, austenite decomposed by a eutectoid reaction that resulted in M₂₃C₆ aggregates around retained δ-ferrite. Two morphologies of M₂N and MN precipitates were found in a low-dislocation α-ferrite. It was concluded that these phases were also transformed from austenite. In weld metals heat treated with intercooling, M₂₃C₆ precipitates were smaller and more homogeneously distributed. Different MN precipitates were found in the tempered martensite. The fracture mode of the weld metals at room temperature was mainly transgranular cleavage with some fibrous fracture. Intercooling treatment improved Charpy impact toughness of the 12 pct Cr steel weld metals substantially. It was found that the important microstructural factors affecting the impact toughness of the weld metals which were heat treated without intercooling were the sizes of the α-ferrite grains, nonmetallic inclusions, and M₂₃C₆ aggregates. For the weld metals heat treated with intercooling, the factors which affect the toughness of the weld metals were the sizes of martensite packets and nonmetallic inclusions.     

低合金钢焊接热影响区的微观组织和韧性研究进展

对钢结构而言,诸如海洋平台、船舶、桥梁、建筑和油气管线等,焊接后的性能直接决定了其服役寿命和安全性,重要性不言而喻.在针对焊接相关问题的研究中,焊接热影响区的韧性提升一直是重点和难点.焊接热影响区会经历高达1400℃的高温,从而形成粗大的奥氏体晶粒,如果焊接参数控制不当,不能通过后续冷却过程中的相变细化组织,就会造成韧性的降低.而多道次焊接的情况更为复杂,前一道次形成的粗晶区还会在后续焊接过程中经历二次热循环,从而形成链状M-A,造成韧性的急剧下降.本文旨在对一些现有焊接热影响区的相关研究结果进行总结,探讨母材的成分、第二相及焊接工艺等因素对热影响区微观组织和性能的影响,为低温环境服役的大型钢结构的焊接性能改善提供一些设计思路.     

Influence of heat treatments on microstructure,mechanical properties,and corrosion resistance of weld alloy 625

The effects of heat treatments of the industrial type (eight-hour hold times at temperatures between 600 °C and 1000 °C) on the structural, mechanical, and corrosion resistance characteristics of weld alloy 625 have been studied. During the heat treatment, the mean concentration ratios of Nb, Mo, Si, Cr, Ni, and Fe elements between the interdendritic spaces and dendrite cores show little evolution up to 850 °C. Beyond that temperature, this ratio approximates 1, and the composition heterogeneity has practically disappeared at 1000 °C. An eight-hour heat treatment at temperatures between 650 °C and 750 °C results in increased mechanical strength values and reduced ductility and impact strength linked to the precipitation of body-centered tetragonal metastable intermetallic γ″ Ni₃Nb phase in the interdendritic spaces. An eight-hour treatment in the temperature range between 750 °C and 950 °C has catastrophic effects on all mechanical characteristics in relation with the precipitation, in the interdendritic spaces, of the stable orthorhombic intermetallic δ Ni₃(Nb, Mo, Cr, Fe, Ti) phase. At 1000 °C, the ductility and impact strength are restored. However, the higher the heat treatment temperature, the weaker the mechanical strength. Heat treatments have no effect on the pitting resistance of weld alloy 625 in sea water. The comparison of the results of this study on weld alloy 625 with those previously obtained on forged metal 625 shows that heat treatments below 650 °C and above 1000 °C are the sole treatments to avoid embrittlement and impairment of the corrosion resistance characteristics of alloy 625.     

Quantitative evaluation of softened regions in weld heat-affected zones of 6061-T6 aluminum alloy—Characterizing of the laser beam welding process

In welding 6061-T6 aluminum alloy, softening caused by the dissolution of strengthening β″ (Mg₂Si) precipitates occurs in heat-affected zones (HAZs). Laser beam welding is advantageous in view of narrower softened regions. The width of the softened region in a laser beam weld with a welding speed of 133 mm/s is 1/7 that of a tungsten inert gas (TIG) weld with a speed of 5 mm/s. The hardness distributions and width of softened regions in the HAZ have been quantitatively predicted to characterize the laser beam welding process. To this end, a kinetic equation describing the dissolution of age precipitates has been established and has been applied to 6061-T6 aluminum weldments. The hardness profiles and the width of softened zones have been successfully predicted in both welding processes. Prediction of the width of softened regions with varying power inputs and welding speeds reveals that a high energy density and a high welding speed in laser beam welding result in significantly narrower softened regions, in which the width is insensitive to variations in welding parameters compared to that of TIG welding.     

Metallographic examination of the heat-affected zone of a test weld in a type X 20 CrMoV 12 1 steel

Metallographic examination of a similar test weld in a type X 20 CrMoV 12 1 steel revealed a martensitic phase within the heat-affected zone of the base metal adjacent to the fusion boundary. As a consequence, the heat-affected zone displayed an excess microhardness compared with the unaffected base metal. It is shown that the occurrence of this martensitic phase can be correlated with segregations in the base metal. Due to its martensitic nature, this phase can be transformed to α-ferrite and carbide by an additional tempering treatment subsequent to the standard post-weld heat treatment. By this additional tempering treatment the uniformity of the microstructure of the heat-affected zone can be considerably improved.     

成分和组织对中锰钢模拟飞溅区腐蚀行为影响

 采用回火调质处理制备出含逆转变奥氏体的中锰钢,并对逆转变奥氏体的形貌进行了观察,通过周期浸润腐蚀试验研究了海洋平台用中锰钢在海洋飞溅区的腐蚀行为,主要研究了中锰钢中合金元素和微观组织对腐蚀行为的影响。结果表明,耐蚀元素中铬的防腐作用最显著,在内外锈层中间处出现了含铬化合物,改善了锰元素对腐蚀过程的负面作用并稳定了腐蚀速率。试验钢的微观组织也对其腐蚀行为产生影响,腐蚀过程中,回火马氏体优先溶解,逆转变奥氏体对腐蚀起到修正的作用。     

Effects of high temperature fatigue on microstructure and mechanical properties of a nickel-base precipitation hardened alloy

Effects of high temperature strain controlled push-pull fatigue on the microstructure and mechanical properties of a nickel-base precipitation hardened alloy were studied. The fatigue deformation alone at 700 °C did not impair the mechanical properties of this alloy; however, a hold period ranging from one minute to one hour at tension-peak decreased the tensile ductility and the fracture toughness significantly. This was mainly attributed to grain boundary cavitation. Continuous fatigue resulted in dislocation bands, whereas hold-time fatigue caused a coherency loss iny’ precipitates. Implications of these microstructural changes for the residual mechanical properties are discussed. Formerly with the Brookhaven National Laboratory, Upton, NY 11973. Formerly with the Brookhaven National Laboratory.