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Design and Fabrication of Functionally Graded Transition Joints to Replace Failure-Prone Dissimilar Metal Welds.

机译：功能梯度过渡接头的设计和制造，以替代失效点异种金属焊接。

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摘要

Dissimilar joints between two different materials find applications in a wide variety of industries. In the power generation industry, thousands of dissimilar metal welds (DMWs) are used to join ferritic low-alloy steels to austenitic stainless steels used for waterwall tubing in the superheater and reheater sections of each plant. It is widely known that these welds can be susceptible to failure at less than half of their design life. The high operating temperatures and thermal cycling during service in combination with thermal expansion mismatch, differences in chemistry, and hardness and strength gradients produced by carbon migration in the two steels, produce accelerated creep failures. In fact, premature failure of waterwall tubing is the leading cause of forced outages in power plants resulting in approximately ;Functionally graded materials have potential for joining dissimilar materials in many applications. Building off of the concept of functionally graded materials (FGMs), a transition joint that gradually changes from "pure" austenitic stainless steel to "pure" ferritic low-alloy steel could replace the one dissimilar weld with two similar welds. By continuously grading the joint composition, the sharp changes in microstructure and properties of traditional DMWs would be extended over the whole length of the component, preventing many of the causes of premature failure.;In this work, models have been developed and utilized for designing functionally graded transition joints for joining ferritic and austenitic alloys. Finite element models were used to optimize the grade length and geometry of the transition joint in order to minimize stresses due to thermal expansion mismatch. Thermodynamic and kinetic models were employed to determine the length of grade needed to reduce chemical potential gradients and carbon migration. Results from the finite element simulations of a conventional dissimilar weld demonstrate that localized stresses as high as ∼ 240 MPa can exist at 650°C when a nominal tensile stress of ∼ 32 MPa is applied. The high local stress is due primarily to CTE mismatch between the ferritic and austenitic alloys. Mechanical property mismatch between the two alloys plays a much smaller role. Similar FE model results from graded joints demonstrate that these local stresses can be reduced significantly to ∼ 40 MPa for a 120 mm grade length that consists of at least 30 layers within the transition zone. Thermo-Calc model results of the chemical potential of carbon in a T22-Alloy 800-347 graded transition show that the chemical potential gradient is steepest between the T22 and Alloy 800 and is due to the large differences in chromium content between the two materials. Results from kinetic simulations demonstrate that a 25 mm grade length should significantly reduce carbon migration at 500°C. Higher operating temperatures will require increased joint lengths to provide similar reductions in carbon migration. These results are useful for fabricating optimized graded transition joints to replace failure-prone dissimilar metal welds (DMWs) in the power generation industry.;These models are applied in fabricating graded transition joints for joining ferritic and austenitic alloys. A TIG welding system employing dual wire feeders was used to construct T22-800, T22-347, and T22-IN82 joints. All three joint combinations were characterized to determine compositional variation, microhardness profile, and microstructure evolution. Additionally, tensile tests were performed to evaluate the high temperature mechanical properties of the joints. In all cases, a smooth transition in composition was achieved over ∼ 50 mm, an increase of three orders of magnitude relative to traditional DMWs. Hardness peaks were observed in all joints, corresponding to the formation of martensite, as predicted by the Schaeffler diagram and confirmed through light optical microscopy. Tensile testing revealed that graded joints exhibit mechanical properties that closely match the nominal alloys between 20 -- 650°C. Tensile failure occurred in the section of the joint with the lowest tensile stress, which was seen to vary with temperature for T22-347 joints. In the case of T22-800 joints, solidification cracking in the austenite region was observed, and thus use of this joint combination is not recommended. Future work includes long-term creep testing to evaluate the service life of the graded joints compared to DMWs.;The use of functionally graded joints represents a novel solution to the problem of joining dissimilar materials. Overall, results indicate that a significant reduction in stress levels and sharp gradients seen in traditional DMWS, as well as an increase in service life, is possible. These graded joints should find applications in a wide range of industries, including replacement of failure-prone DMWs in the power generation industry.
机译：两种不同材料之间的异种接头可用于多种行业。在发电行业中，成千上万种异种金属焊缝（DMW）用于将铁素体低合金钢与用于每个工厂的过热器和再热器部分中的水冷壁管的奥氏体不锈钢连接。众所周知，这些焊缝在不到其设计寿命的一半时就容易失效。维修期间的高工作温度和热循环，以及热膨胀失配，化学差异以及碳在两种钢中的迁移所产生的硬度和强度梯度，均会加速蠕变破坏。实际上，水冷壁管的过早故障是发电厂被迫停运的主要原因，导致大约200％的损坏；功能梯度材料在许多应用中具有连接异种材料的潜力。在功能梯度材料（FGM）概念的基础上，过渡接头从“纯”奥氏体不锈钢逐渐变为“纯”铁素体低合金钢，可以用两个相似的焊缝代替一个不同的焊缝。通过对接缝成分进行连续分级，传统DMW的微观结构和性能的急剧变化将扩展到组件的整个长度，从而防止了许多过早损坏的原因。在这项工作中，已经开发了模型并将其用于设计用于连接铁素体和奥氏体合金的功能梯度过渡接头。有限元模型用于优化过渡接头的坡度长度和几何形状，以最小化由于热膨胀失配引起的应力。使用热力学和动力学模型来确定减少化学势梯度和碳迁移所需的梯度长度。常规异种焊缝的有限元模拟结果表明，当施加约32 MPa的标称拉伸应力时，在650°C时可能存在高达〜240 MPa的局部应力。高局部应力主要是由于铁素体和奥氏体合金之间的CTE不匹配。两种合金之间的机械性能不匹配所起的作用要小得多。渐变接头的相似有限元模型结果表明，对于120 mm渐变长度（在过渡区内至少30层组成），这些局部应力可以显着降低至〜40 MPa。在T22合金800-347梯度过渡中碳的化学势的Thermo-Calc模型结果表明，T22和800合金之间的化学势梯度最陡，这是由于两种材料之间的铬含量差异很大。动力学模拟的结果表明，梯度长度为25 mm可以显着减少500°C下的碳迁移。更高的工作温度将需要增加接头长度，以实现碳迁移的类似减少。这些结果对于制造优化的渐变过渡接头以替代发电行业中易失效的异种金属焊缝（DMW）很有用。这些模型被用于制造渐变过渡接头以连接铁素体和奥氏体合金。使用双送丝机的TIG焊接系统用于构造T22-800，T22-347和T22-IN82接头。所有三个关节组合的特征在于确定组成变化，显微硬度分布和显微组织演变。此外，进行了拉伸试验以评估接头的高温机械性能。在所有情况下，在约50 mm的范围内均能实现平滑过渡，相对于传统DMW而言，增加了三个数量级。如Schaeffler图所预测并通过光学显微镜确认的，在所有接头处均观察到了硬度峰，对应于马氏体的形成。拉伸测试表明，梯度接头的机械性能与20-650°C之间的标称合金非常匹配。拉伸断裂发生在具有最低拉伸应力的节段中，对于T22-347接头，这会随温度而变化。在T22-800接头的情况下，在奥氏体区域观察到凝固裂纹，因此不建议使用此接头组合。未来的工作包括长期蠕变测试，以评估与DMW相比渐变坡道的使用寿命。功能渐变坡道的使用代表了一种解决异种材料连接问题的新颖解决方案。总体而言，结果表明，可以显着降低传统DMWS中的应力水平和急剧的梯度，并可以延长使用寿命。这些渐变接头应在广泛的行业中找到应用，包括在发电行业中替换容易失效的DMW。

著录项

  • 作者

    Brentrup, Gregory J.;

  • 作者单位

    Lehigh University.;

  • 授予单位 Lehigh University.;
  • 学科 Engineering Materials Science.
  • 学位 M.S.
  • 年度 2011
  • 页码 110 p.
  • 总页数 110
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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