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首页> 外文学位 >Heat Affected Zone Cracking of Allvac 718Plus Superalloy during High Power Beam Welding and Post-weld Heat Treatment.
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Heat Affected Zone Cracking of Allvac 718Plus Superalloy during High Power Beam Welding and Post-weld Heat Treatment.

机译：大功率梁焊接和焊后热处理过程中，Allvac 718Plus合金的热影响区开裂。

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摘要

The present dissertation reports the findings of a study of cracking behavior of a newly developed superalloy, Allvac 718Plus during high power beam welding and post-weld heat treatments. Microstructures of the base alloy, heat affected zone (HAZ) and fusion zone of welded and post-weld heat treated coupons were examined by the use of standard metallographic techniques involving optical microscopy, analytical scanning electron microscopy (SEM) and analytical transmission electron microscopy. Moreover, non-equilibrium segregation behavior of boron atoms to the grain boundaries of the alloy during pre-weld heat treatments was evaluated using secondary ion mass spectroscopic system.;Although HAZ cracking was prevented by welding with a high heat input laser, it resulted in a significant damage to the parent microstructure through its extensive liquation. Therefore, such a technique is not usually recommended in order to ensure integrity of weld structures. The use of low heat input welding, which will minimize the microstructural damage is desirable. Thus, it is necessary that another approach be employed to minimize or eliminate HAZ cracking of the alloy which occurred during welding using low heat input. The use of suitable pre-weld heat treatment to engineer pre-weld microstructure capable of reducing or eliminating cracking during welding has been widely reported for other nickel-base superalloys. Therefore, during the second phase of this research, the effects of pre-weld thermal processing on the cracking response of 718Plus were investigated. The alloy was thermally treated at different temperatures and cooling rates before welding. Results from the quantification of the cracking response of the alloy showed that HAZ cracking during welding may be significantly reduced or eliminated through an adequate selection of pre-weld thermal cycle.;In the third stage of this research, crack-free welds of 718Plus were post-weld heat treated using standard thermal schedules recommended by the manufacturer of 718Plus. This was meant to relieve welding stresses as well as homogenize the microstructure of the welds. However, a significant solid state cracking of the alloy occurred during the post-weld heat treatment. This appears to constitute a major difference between the weldability of gamma' strengthened 718Plus and that of its parent alloy, 718, which is strengthened by gamma' precipitate that has a sluggish precipitation kinetics. It is hypothesized that post-weld heat treatment cracking of 718Plus occurred due to the combined presence of aging contraction stresses, due to fast precipitation of gamma' in HAZ, at a period when the grain boundaries in the zone have been embrittled. SEM and TEM analyses revealed a significant precipitation of gamma' phase during post-weld heat treatment of the welds. Also, the grain boundaries that mostly cracked during post-weld heat treatment were outlined by Laves phase and extensive needle-like precipitates of delta phase. The Laves phase formed as a resolidification product on liquated grain boundaries in weld HAZ during welding, while the needle-like delta phase precipitated around the Laves phase during PWHT. It has been reported in previous studies on 718-type alloy that Laves phase is very brittle and has a weak interface with the gamma matrix; therefore, it may provide preferential sites for crack initiation and propagation. The extensive formation of delta needles, as observed in the present work, has been also reported to impair the strength of 718-type alloys. Thus, during post-weld heat treatment of 718Plus, the presence of the embrittling phases on HAZ grain boundaries, coupled with aging contraction stresses that are generated during aging, would result in cracking of the grain boundaries at a period when the matrix has been hardened by a considerable precipitation of gamma' phase.;In the first phase of the research, 718Plus was welded using a low and high heat input CO2 laser to assess the response of the alloy to welding. Detailed examination of the welds by analytical electron microscopic technique revealed the occurrence of cracking in the heat affected zone (HAZ) of low heat input welds, while their fusion zone (FZ) was crack free. However, both the FZ and HAZ of high heat input welds were crack-free. Resolidified constituents were observed along the cracked grain boundaries of the lower heat input welds, which indicated that HAZ cracking in this newly developed superalloy was associated with grain boundary liquation. However, despite a more extensive liquation of grain boundaries and grain interior in the HAZ of high heat input welds, no cracking occurred. This was attributed to the combination of lower welding stresses generated during cooling, and relaxation of these stresses by thick intergranular liquid.
机译：本论文报告了对新开发的高温合金Allvac 718Plus在大功率束焊和焊后热处理过程中的开裂行为的研究结果。通过使用标准金相技术，包括光学显微镜，分析扫描电子显微镜（SEM）和分析透射电子显微镜，检查了基体合金，焊接和焊后热处理试样的热影响区（HAZ）和熔合区的显微组织。此外，使用二次离子质谱系统评估了预焊接热处理过程中硼原子到合金晶界的非平衡偏析行为;尽管通过高热输入激光焊接防止了HAZ裂纹，但导致大量的液化对母体的微结构造成了重大损害。因此，通常不建议使用这种技术来确保焊接结构的完整性。希望使用低热量输入的焊接，它将最小化微观结构的损坏。因此，有必要采用另一种方法来最小化或消除在使用低热输入的焊接期间发生的合金的HAZ裂纹。对于其他镍基高温合金，已经广泛报道了使用合适的预焊接热处理来设计能够减少或消除焊接过程中裂纹的预焊接显微组织。因此，在本研究的第二阶段，研究了预焊接热处理对718Plus裂纹响应的影响。在焊接之前，对合金进行了不同的温度和冷却速率的热处理。合金开裂响应的量化结果表明，通过适当选择预焊接热循环，可以显着减少或消除焊接过程中的热影响区开裂。在本研究的第三阶段，采用718Plus进行无裂纹焊接使用718Plus制造商推荐的标准热处理时间表对焊后热处理。这旨在减轻焊接应力并使焊缝的微观结构均匀化。但是，在焊后热处理过程中发生了明显的固态裂纹。这似乎构成了γ'增强的718Plus与其母合金718的可焊性之间的主要区别，而后者的母合金718则由于具有缓慢的析出动力学的γ'析出而得到增强。据推测，718Plus的焊接后热处理开裂是由于在区域中的晶界变脆的期间，由于热收缩区中γ'的快速析出引起的老化收缩应力的共同存在而发生的。 SEM和TEM分析表明，焊缝的焊后热处理期间γ'相大量析出。而且，在熔焊后热处理期间大部分开裂的晶界由拉夫斯相和δ相的大量针状沉淀物勾勒出。 Laves相在焊接过程中在焊接热影响区的液化晶界上形成为凝固产物，而PWHT期间，针状δ相在Laves相周围沉淀。先前对718型合金的研究报道，Laves相非常脆，并且与γ基体的界面较弱。因此，它可以为裂纹的萌生和扩展提供优先位置。如本工作中观察到的那样，三角针的广泛形成也损害了718型合金的强度。因此，在718Plus的焊后热处理期间，在HAZ晶界上存在脆化相，再加上时效过程中产生的时效收缩应力，会在基体硬化时导致晶界破裂在研究的第一阶段，使用低热量输入和高热量输入的CO2激光焊接718Plus，以评估合金对焊接的响应。通过分析电子显微镜技术对焊缝进行的详细检查显示，低热输入焊缝的热影响区（HAZ）发生了开裂，而其熔合区（FZ）没有开裂。但是，高热输入焊缝的FZ和HAZ均无裂纹。在下部热量输入焊缝的裂纹晶界处观察到凝固成分，这表明这种新开发的高温合金中的热影响区裂纹与晶界液化有关。但是，尽管在高热输入焊缝的热影响区中，晶界和晶粒内部的液化程度更大，但没有出现裂纹。这归因于冷却过程中产生的较低焊接应力，以及浓晶间液体使这些应力松弛的组合。

著录项

  • 作者

    Idowu, Oluwaseun Ayodeji.;

  • 作者单位

    University of Manitoba (Canada).;

  • 授予单位 University of Manitoba (Canada).;
  • 学科 Engineering Metallurgy.
  • 学位 Ph.D.
  • 年度 2010
  • 页码 200 p.
  • 总页数 200
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

  • 入库时间 2022-08-17 11:36:56

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