Optimizing the heat treatment of Ni-based superalloy turbine discs

The heat-treatment processes for nickel-based superalloys continue to change due to the development of new alloys, new requirements, and subsequent new manufacturing facilities. Nickel-based superalloys are continuing to evolve to meet emerging applications, while new alloys are also being introduced for advanced applications. These new materials are also being optimized for numerous mechanical and physical properties, making the selection of heat-treatment parameters increasingly challenging. New processing facilities and methods are also being implemented to allow tailoring of heat-treating parameters to meet these new challenges. For example, the Ladish SuperCooler technology allows engineering and control of all aspects of the heat-treatment process for nickel-based components, resulting in never-before possible disc properties. For more information, contact D.U. Furrer, Ladish Company, P.O. Box 8902, Cudahy, WI 53110-8902; (414) 747-3063; e-mail dfurrer@ladishco.com.     

增压涡轮用镍基高温合金的凝固特性和热裂倾向性

研究了增压涡轮用镍基高温合金K418和K419的凝固特性和热裂倾向性。同时研究了合金元素的偏析行为和析出相。结果表明,凝固末期多种强枝晶间偏析元素在液相中的偏聚导致K419合金的凝固行为较K418复杂。多种元素在枝晶间剩余液相中的偏聚导致K419合金的液相线极低。K419合金凝固过程中漫长的剩余液相期的存在严重削弱了枝晶间结合力,增加了其热裂倾向性。基于一种热裂敏感区模型提出热裂倾向性系数判据,K419合金的热裂倾向性系数高于K418合金。     

Formation of low angle boundaries in Ni-based superalloys

Abstract

An experimental study has been used to determine the influence of liquid undercooling ahead of dendrite tips on dendrite growth and the accompanying evolution of misorientation resulting in formation of low angle grain boundaries during solidification of a Ni-base superalloy, CMSX-4. The experiment was designed whereby two dendrite envelopes formed by initial branching of the primary front converged along horizontal platforms perpendicular to the withdrawal direction. The magnitude of the undercooling was increased by increasing the length of the platform, ranging from 10 to 60 mm. The position of the dendrite envelope was calculated using a computational thermal model, ProCAST. Misorientation, measured using Electron Back Scattered Diffraction (EBSD), was generally <2·5°, which was consistent with previous studies of dendrite growth in the constrained condition. However, in the longest platform a monotonic increase in misorientation in one dendrite envelope developed leading to formation of a grain boundary of 10°. This monotonic increase was not dependent on the local undercooling ahead of the dendrite envelope and was instead caused by mechanical moments arising from extensive lateral growth of unsupported tertiary dendrite branches growing laterally across the platform.     

Global heat transfer model and dynamic ray tracing algorithm for complex multiple turbine blades of Ni-based superalloys in directional solidification process

High-quality solidification microstructure during directional solidification relies on precise temperature gradient control, so accurate calculation of the temperature field is critical. In this study, a 3D transient global heat transfer model of directional solidification by the Bridgman method based on the finite difference method is developed. The radiation heat in this model is calculated by the discrete transfer method, and a modified method of external surface area for irregular geometric models is proposed to reduce the zigzag shape caused by finite difference grids. Considering the radiative heat transfer between any surface elements of all materials in the directional solidification furnace, a dynamic ray tracing algorithm is developed to simulate the entire process of directional solidification. Then, the simulated results are compared with the theoretical results and experimental results, respectively. Finally, based on the present model and method, the simulation program developed is applied to the directional solidification of actual castings. The simulated results are in good agreement with the experimental results, which indicate that the model and method developed in this study is effective and practical.     

A combined mapping process for the development of platinum-modified Ni-based superalloys

Heat-resistant Ni-based alloys have been used extensively for components in turbine engines for the past 50 years. Platinum group metal (PGM) alloying additions (i.e., platinum and iridium) have been found in past studies to increase the oxidation and the hot corrosion performance of Ni-based alloys. The basis for this investigation was to develop a PGM-modified superalloy having high-temperature strength, thermal stability, and excellent oxidation/hot corrosion performance.     

A physically based methodology for predicting anisotropic creep properties of Ni-based superalloys

This paper is focused on developing suitable methodology for predicting creep characteristics(i.e., the minimum creep strain rate, stress rupture life and time to a specified creep strain) of typical Ni-based directionally solidified(DS) and single-crystal(SC) superalloys. A modern method with high accuracy on simulating wide ranging creep properties was fully validated by a sufficient amount of experimental data, which was then developed to model anisotropic creep characteristics by introducing a simple orientation factor defined by the ultimate tensile strength(UTS). Physical confidence on this methodology is provided by the well-predicted transitions of creep deformation mechanisms. Meanwhile, this method was further adopted to innovatively evaluate the creep properties of different materials from a relative perspective.     

Measurement of solute segregation in as-cast single crystal Ni-based superalloys

Abstract

Detailed composition measurements using X-ray spectroscopy have been conducted in a typical third generation Ni based superalloy, CMSX10K. It is shown that the measured composition is a volume average of the fraction of γ and γ′ phases within the X-ray interaction volume. The measurements were affected by the size and distribution of the γ′ precipitates constituting the microstructure and most significantly when there is an appreciable difference in solubility limits in γ and γ′ phases for an element. These results are discussed in relation to microsegregation characterisation using the commonly used point count method. Some limitations of this latter approach are highlighted.     

Preparation and cyclic oxidation of gradient NiCrAlYRe coatings on Ni-based superalloys

Arc ion plating (AIP) and chemical vapor deposition (CVD) aluminizing were combined together to produce a gradient NiCrAlYRe coating. Elements in this coating were chemically graded with Al enrichment in the outer zone and Cr enrichment in the intermediate zone. Cyclic oxidation tests were performed at 1100 °C for up to 22 cycles. The results showed that the gradient NiCrAlYRe coatings performed better resistance to spallation and Al depletion than the conventional NiCrAlYRe coatings. This favorable oxidation behavior was attributed to the high Al distribution in the near surface region of the gradient coating.     

The intermediate temperature deformation of Ni-based superalloys: Importance of reordering

A number of planar deformation mechanisms, such as microtwinning, a[112] dislocation ribbon, and superlattice intrinsic and superlattice extrinsic stacking fault formation, can operate during the intermediate temperature deformation of nickle-based superalloys. The fundamental, rate-limiting processes controlling these deformation mechanisms are not fully understood. It has been recently postulated that reordering of atoms in the wake of the gliding partial dislocations as they shear the γ′precipitates within the γ/γ′microstructure is the limiting process. Experimental evidence that substantiates the validity of the reordering model for the microtwinning mechanism is provided. A conceptual approach to study reordering at the atomic scale using ab-initio calculation methods is also presented. The results of this approach provide a clear conceptualization of the energetics and kinetics of the reordering process, which may be generically important for the aforementioned planar deformation modes.     

Solidification behavior of Re-and Ru-containing Ni-based single-crystal superalloys with thermal and metallographic analysis

To clarify the solidification behavior of Re-and Ru-containing Ni-based single-crystal super alloys, four experimental alloys with varied contents of Re and Ru were investigated by differential scanning calorimetry(DSC) and metallographic techniques. To obtain the γ′-solvus temperatures, the stepwise solution and aging heat treatments were used. DSC analysis shows that Re leads to the increase in freezing range and γ′-solvus temperature.On the contrast, Ru only has negligible influence on the freezing range, but leads to the lower γ′-solvus temperature. In comparison with Ru, Re leads to more severe segregation and higher eutectic fractions in as-cast microstructures. Furthermore, the castability and phase stability of Ni-based superalloys were analyzed by the results of DSC and metallographic analysis, such as freezing range, critical nucleation temperature,γ′-solvus temperature and eutectic fractions. It shows that Re leads to the wider freezing range and lower critical nucleation temperature, indicating the worse castability of Re-containing Ni-based single-crystal superalloys.     

汽轮机汽蚀叶片YAG激光熔覆镍基合金研究

用YAG激光器对表面汽蚀的2Cr13钢汽轮机叶片进行修复,采用同步送粉的方式进行自熔性Ni-Cr-B-Si合金粉末的激光熔覆.借助扫描电子显微镜(SEM)和能谱仪(EDS)对激光熔覆层进行组织及成分分析.研究结果表明,激光熔覆层硬度可达45～50 HRC,高于基底材料的硬度(35～40 HRC).使用YAG同体激光对汽蚀叶片进行修复可以获得与母材冶金结合且无气孔、裂纹等缺陷的熔覆层,其组织主要是γ-Ni、Cr7C3、Ni2B、Ni3B以及少量的Cr2B和Cr3Si.     

Hf对抗热腐蚀镍基高温合金微观组织和力学性能的影响

研究Hf对抗热腐蚀镍基高温合金组织和力学性能的影响;探讨组织演化和力学性能的关系,对于Hf的添加对γ筏形化和/γγ'错配度的影响也进行讨论.结果表明:与无Hf合金相比,含0.4%Hf(质量分数)合金的组织晶界上存在较少的大块状MC碳化物,而细小M23C6碳化物的含量更多,这对蠕变性能有利;Hf的添加有助于使MC碳化物的数量在蠕变过程中保持较高的水平;在长期时效过程中,晶界上碳化物分解为细小离散分布的M23C6碳化物,沿着晶界形成γ'层,无Hf合金的晶界粗化现象更为明显;Hf的添加可以提升合金长期时效后的高温拉伸性能;同时,Hf对提高低应力条件下的蠕变强度作用明显.     

A new method to strengthen turbine disc superalloys at service temperatures

TMW® superalloys have superior mechanical properties up to 725 °C. Numerous annealing twins are present because these alloys have relatively low stacking fault energies. Nanoscale deformation twins are the dominant mechanism during creep and tensile testing at 725 °C. In addition to traditional strengthening mechanisms enhanced by higher Ti addition, twinning structures enhanced by higher Co addition also play an important role in strengthening coarse-grained alloys at high temperatures. The effects of twin strengthening are discussed.     

Quantification of growth kinetics and adherence of oxide scales formed on Ni-based superalloys at high temperature

Cyclic and isothermal oxidation behaviors of first and fourth-generation superalloys AM1 and MCNG were investigated to evaluate the ability of the scratch test to quantify the adhesion of multi-layered oxide scales. Effects of sulfur content and of scale thickness were studied independently. Available models lead to large discrepancies in the calculated work of adhesion values with the evaluation of the residual stress being the largest source of error. Nevertheless, models can assess the effect of sulfur content and the scratch test can be used to correlate the long-term cyclic oxidation behavior and the adhesion of oxide scales.     

Development of two rhenium- containing superalloys for single- crystal blade and directionally solidified vane applications in advanced turbine engines

A team approach involving several turbine engine companies using the concepts of simultaneous engi-neering has been used to successfully develop CMSX-4 ® alloy for turbine blade applications. CMSX-4 al-loy is a second-generation, single-crystal cast nickel-base superalloy containing 3% Re and approximately 70% volume fraction of γ. The high level of balanced properties determined by labora-tory evaluation has been confirmed during field testing of the Solarγ Mars T-14000 industrial gas turbine with CMSX-4 single-crystal (SX) blades in both the coated and bare condition. A similar collaborative ap-proach has resulted in the successful development of CM 186 LCγ alloy for complex, directionally solidi-fied (DS) columnar grain vane segments. CM 186 LC alloy is a second-generation DS columnar grain cast nickel-base superalloy containing 3% Re and approximately 65% volume fraction of γ. Excellent com-ponent producibility and quality is demonstrated. Turbine engine testing is scheduled to commence by the end of 1993.     

基于ANN的涡轮盘等温成形过程模拟电路模型

为提高锻件质量和成品率,有必要建立一种适合于实时控制的锻件成形过程模型.利用有限元模拟技术对涡轮盘的等温成形过程进行了虚拟正交试验,通过对成形过程的载荷--行程曲线的分析,建立了粉末高温合金涡轮盘件等温成形过程的人工神经网络(ANN)模型,并将其映射成模拟电路模型.以此模拟电路模型为参考模型,应用于模型参考自适应控制(MRAC)系统,对涡轮盘件等温成形过程进行控制.结果表明,所建立的ANN模型及其模拟电路模型对粉末高温合金涡轮盘件等温成形过程的拟合精度很高,且控制参数始终与模型输出相吻合,为实现盘件成形过程的实时控制奠定了基础.     

Influence of service-induced microstructural changes on the aging kinetics of rejuvenated Ni-based superalloy gas turbine blades

Rejuvenation of Ni-based superalloy gas turbine blades is widely and successfully employed in order to restore the material microstructure and properties after service at high temperature and stresses. Application of hot isostatic pressing (HIP) and re-heat treatment can restore even a severely overaged blade microstructure to practically “as-new” condition. However, certain service-induced microstructural changes might affect an alloy’s behavior after the rejuvenated blades are returned to service. It was found that advanced service-induced decomposition of primary MC carbides, and the consequent changes of the γ-matrix chemical composition during the rejuvenation, can cause a considerable acceleration of the aging process in the next service cycle. The paper will discuss the influence of the previous microstructural deterioration on the aging kinetics of rejuvenated gas turbine blades made from IN-738 and conventionally cast GTD-111 alloys.