Diffusion Bonding of a Cold-Worked Ni-Base Superalloy

Diffusion bonding was conducted on cold-worked Alloy 600. Cold-work of 50 % was applied prior to diffusion bonding in order to incite recrystallization and limit grain growth. Tensile testing was conducted at room temperature and 550 °C for evaluation of joint efficiency, while premature brittle failure at the bond-line was observed for most diffusion bonding conditions. It was found that such premature failure was related to a planar bond-line that indicated lack of grain boundary diffusion across the bonding surfaces. Additional application of post-bond heat treatments did not result in significant bond-line migration. Microstructural analyses revealed the existence of Cr-rich carbides and Ti-rich precipitates along the bond-line, which prevented bond-line migration by acting as pinning points.

Microstructures of Grade 91 Steels Under Long-Term Creep Conditions

Type IV damage has been found at several ultra-supercritical (USC) plants that used high-chromium martensitic steels in Japan, and the assessment of the remaining life of the steels is important for electric power companies. The assessment of the remaining life needs long-term creep data for over 10 years, but such data are limited. We have attempted to assess the remaining life by creep tests and by microstructural observation of Grade 91 steels welded pipes which were used in USC plants for over 10 years. Following the results of microstructural observation of USC plant pipes, we find that microstructures, especially distribution of MX precipitates, have large effect on the creep life of Grade 91 steels.

Development of Creep-Resistant, Alumina-Forming Ferrous Alloys for High-Temperature Structural Use

This paper overviews recent advances in developing novel alloy design concepts of creep-resistant, alumina-forming Fe-base alloys, including both ferritic and austenitic steels, for high-temperature structural applications in fossil-fired power generation systems. Protective, external alumina-scales offer improved oxidation resistance compared to chromia-scales in steam-containing environments at elevated temperatures. Alloy design utilizes computational thermodynamic tools with compositional guidelines based on experimental results accumulated in the last decade, along with design and control of the second-phase precipitates to maximize high-temperature strengths. The alloys developed to date, including ferritic (Fe-Cr-Al-Nb-W base) and austenitic (Fe-Cr-Ni-Al-Nb base) alloys, successfully incorporated the balanced properties of steam/water vapor-oxidation and/or ash-corrosion resistance and improved creep strength. Development of cast alumina-forming austenitic (AFA) stainless steel alloys is also in progress with successful improvement of higher temperature capability targeting up to ∼1100°C. Current alloy design approach and developmental efforts with guidance of computational tools were found to be beneficial for further development of the new heat resistant steel alloys for various extreme environments.

Creep Rupture Analysis of Candidate Steels for the Traveling Wave Reactor

TerraPower has developed sophisticated computational analysis tools to support the design and fabrication of high temperature components to be used in the Traveling Wave Reactor (TWR). One of the key material properties required to predict material damage and remaining lifetime of key in-reactor components is the thermal creep rupture time. Although TerraPower optimized ferritic-martensitic (FM) HT9 steel has shown consistent improvement in yield stress and creep rupture strength through uniaxial tensile tests, extrapolations of existing test data are still needed to fully support the complex analysis used in the TWR design. Traditional Larson-Miller analysis for creep rupture was used to compare the TerraPower optimized HT9 steel to the existing historical database. The results of the Larson-Miller analysis were compared to the results from the Wilshire analysis to explore the relative advantages and disadvantages of each method. The best estimate values for fitting constants and activation energies were determined for both methods, taking into account the effects of the higher yield stress observed in TerraPower optimized HT9 compared to historic HT9. Likewise, the best estimate creep rupture stresses for TerraPower optimized HT9 at various times and temperatures were determined by extrapolations using both the Larson-Miller and Wilshire analysis. The allowable stresses of historical and TerraPower optimized HT9 steels were compared to those of existing materials (9Cr-1Mo-V) in the ASME high temperature code. The comparison of analysis methods and rupture stresses demonstrate that TerraPower FM steel thermal creep performance and analysis methods are comparable to existing ASME qualified materials for high temperature applications.

A Proposal for Post-Assessment Test of Long-Term Creep Rupture Strength of Grade 91 Steel

Predictions as to 105 hrs creep rupture strength of grade 91 steel have been made recently. The predictions should be verified by some means, since they are based on certain assumptions. A formula for predicting long-term creep rupture lives should correctly describe long-term data points used in its formulation. Otherwise the formula cannot properly predict further longer-term creep rupture lives. On the basis of this consideration, the predictions are examined with long-term creep rupture data of the steel. In the predictions three creep rupture databases were used: data of tube products of grade 91 steel reported in NIMS Creep Data Sheet (NIMS T91 database), data of T91 steel collected in Japan, and data of grade 91 steel collected by an ASME code committee. Short-term creep rupture data points were discarded by the following criteria for minimizing overestimation of the strength: selecting long-term data points with low activation energy (multi-region analysis), selecting data points crept at stresses lower than a half of proof stress (σ0.2/2 criterion), and selecting data points longer than 1000 hrs (cut-off time of 1000 hrs). In the case of NIMS T91 database, a time-temperature parameter (TTP) analysis of a dataset selected by the multi-region analysis can properly describe the long-term data points. However, the TTP analyses of datasets selected by the σ0.2/2 criterion and by the cut-off time of 1000 hrs from the same database overestimate the long-term data points. The different criteria for data selection have more substantial effects on predicted values of the strength of the steel than difference of the databases.

Allowable Compressive Stress Rules in the ASME Boiler and Pressure Vessel Code, Section VIII, in the Creep Regime

This paper outlines several procedures for developing allowable compressive stress rules in the creep regime (time dependent regime). The rules are intended for the ASME Boiler and Pressure Vessel codes (Sections I and VIII). The proposed rules extend the methodology presently outlined in Sections I, II-D, and VIII of the ASME code for temperatures below the creep regime into temperatures where creep is a consideration.

High-Entropy Alloys: Formation and Properties

This paper presents ongoing research at NETL aimed at gaining fundamental understanding of high-entropy alloys (HEAs) formation and their properties, and developing highperformance HEAs for high-temperature fossil energy applications. First-principles density functional theory (DFT), Monte Carlo simulation, and molecular dynamics simulation are carried out to predict enthalpy of formation, the entropy sources (i.e., configurational entropy, vibrational entropy, and electronic entropy), and elastic properties of model single-phase HEAs with the face-centered cubic, body-centered cubic and hexagonal closed-packed structures. Classical elastic theory, which considers the interactions between dislocations and elastic fields of solutes, has also been used to predict solid solution strengthening. Large-size (∼7.5 kg) HEAs ingots are produced using vacuum induction melting and electroslag remelting methods, followed by homogenization treatment resulting in greater than 99% homogeneity. Subsequent thermomechanical processing produces fully-wrought face-centered cubic microstructures. The tensile behavior for these alloys have been determined as a function of temperature, and based on these results screening creep tests have been performed at selected temperatures and stresses.

Estimation of Creep Strength of Grade 91 Steel Welded Joint in Time Region Over 100,000 Hours

A creep strength of welded joint of ASME Grade 91 steel in a region exceeding 100,000 hours was examined in this work. Creep tests were conducted on the steel used at USC plants for long-term, and remaining creep life of the material for operating condition was calculated on a fitting curve using Larson-Miller parameter. Total creep life of the material, which means a creep life at initial state, was presumed to be a summation of the service time at the plants and the remaining creep life. The estimation was conducted for welded joints used at five plants for long-term, and all results lay within 99% confidential band by the creep life evaluation curve of the material proposed by Japanese committee in 2015, while a significant heat-heat variation of creep strength was found even in the region exceeding 100,000 hours. Creep tests on base metals related to each welded joint were also conducted, and the estimation results of the total creep life of the base metals were compared to those of the welded joints. It was suggested that the heat-heat variation of the welded joints eminently depends on the creep life property of the corresponding base metal.

Techniques for Simulating Creep Damage Evolution at Welds With Emphasis on Evaluating Longitudinal Seam Peaking in High-Temperature Piping Systems

Realistically simulating the creep response of welded components can help quantify the risk associated with operating inservice, high-temperature equipment and can validate new component designs in the power generation and petrochemical industries. Detailed finite element analysis (FEA) is employed in this study and is coupled with generalized, non-linear creep simulation techniques to investigate the elevated temperature response of welds. Depending on original heat treatment, creep damage progression is known to be accelerated by the mismatch in properties of the base metal, weld deposit, and heat affected zone (HAZ). This mismatch results in stress intensification that can accelerate creep damage near a weldment (typically in or adjacent to the HAZ). In this paper, the effect of implementing an elastic damage parameter that adjusts the stiffness of the material as a function of creep damage is examined. This type of damage mechanics model has a significant impact on the predicted damage evolution near weld deposits and can realistically mimic observed in-service failures. Additionally, commentary on different creep damage failure criteria is provided. The simulations presented utilize the Materials Properties Council (MPC) Omega creep methodology, with particular emphasis on the behavior of high-temperature, low chrome (1-1/4 Cr 1/2 Mo) piping with longitudinal weld seam peaking. Application of these techniques to high-temperature, low chrome piping is relevant as there have been numerous related catastrophic failures in the power generation and petrochemical industries attributed to weld seam peaking. Commonly, weld peaking occurs during fabrication due to angular misalignment of rolled plate. Furthermore, for many fusion-welded piping fabrication standards, no tolerance for peaking is specified. Local peaking can induce significant local bending stresses, and for components that operate in the creep regime, the presence of peaking can lead to an increased risk for creep crack initiation, propagation, and eventual rupture. An overview of some well-known historical low chrome piping failures is provided in this paper, and a literature review on existing creep analysis and peaking measurement methodologies is offered. Additionally, the remaining life of low chrome piping systems is estimated and the sensitivity in results to variations in key parameters is highlighted; these parameters include operating temperature, magnitude of peaking, and the effect of heat treatment. The simulation techniques discussed in this paper are not only valuable in estimating remaining life of in-service components, but detailed analysis can help establish recommended weld seam peaking fabrication tolerances, appropriate manufacturing practices, and practical inspection intervals for high-temperature piping systems.