Papers by Keyword: Creep Properties

Abstract: The operational integrity of supercritical steam power units necessitates comprehensive understanding of welded joint behaviour under high-temperature service conditions. Advanced steam cycle technology requires meticulous periodic evaluation of pressure-bearing components to ensure structural integrity throughout extended service periods. This requirement is particularly critical for components subjected to the most severe operational parameters, including superheater tubes, main steam pipelines, and steam collectors operating above critical temperature. For pressure components not directly exposed to exhaust gases, microstructural degradation represents the primary degradation mechanism governing component lifespan.P92 (X10CrWMoVNb9-2) steel, characterized by a tempered martensitic microstructure with 9% chromium content, has been extensively utilized for high-pressure applications in supercritical steam power generation systems. This advanced creep-resistant steel demonstrates superior mechanical properties, including exceptional high-temperature strength retention, oxidation resistance, and creep rupture strength under prolonged thermal exposure.Welded joints of pressure elements in steam boilers are potentially the weakest points when assessing their service life. These joints exhibit enhanced susceptibility to microstructural degradation and mechanical properties deterioration, particularly within the heat-affected zone (HAZ), during extended high-temperature operation. Therefore, systematic material characterization of welded joints relative to base material performance is essential for understanding long-term degradation mechanisms.This investigation presents systematic creep testing methodology and experimental results for P92 welded joint specimens subjected to annealing at temperatures of 600°C and 650°C for durations up to 10,000 hours. Both abridged and long-term creep tests were performed with the results of determination of creep strength and creep speed in steady state.

Abstract: The effect of the primary α content and precipitate on the creep resistance of a high-temperature titanium alloy with a small amount of Hf addition were studied. The microstructures with different primary α contents were prepared by the heat treatment of 920~1010 °C /1 h+700 °C/5 h, and the creep test (600 °C/150 MPa/100 h) was carried out. The interaction between the precipitation phase and the dislocation configuration was analyzed. The results showed that with the increase of solution temperature, the volume fraction of primary α phase decreased from 44.9% at 920 °C to 0% at 1010 °C, and the steady state creep rate of the alloy decreased from 60.60×10^-4%/h to 3.72×10^-4%/h, indicating that the creep property was significantly improved with the decrease of solution temperature. The basket structure with optimal creep resistance was obtained under the heat treatment of 1010 °C/1 h+700 °C/5 h. It is believed that during the high temperature creep test, the precipitated α₂ phase and the hafnium-containing silicide hinder the dislocation motion in α crystal and the phase boundary, thereby improving the creep resistance of the alloy.

Abstract: The creep rupture behavior of weld joint of modified 9Cr-1Mo martensitic heat-resistant steel is investigated by conducting creep tests. The creep strain-time curves of 838K under different stress levels are obtained. The effects of different stress conditions on the creep rupture lifetime are analyzed. Based on the creep test data, the relationship between minimum creep rate and stress, creep rupture time is analyzed. The test results show that the creep rupture occurred in the fine-grained heat-affected zone. The microstructure evolution during creep process is studied by metallographic analysis and fracture analysis.

Abstract: Intermediate temperature creep properties are considered a key indicator of single crystal superalloys used for turbine blades of aircraft engines. The interrupted and ruptured creep tests were carried out in a second generation single crystal superalloy under the conditions of 760°C/785MPa. The creep rupture life as well as minimum creep rate were also in the same level of those in CMSX-4 and PWA1484. The microstructural evolution at different creep stages were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed the γ’ phases kept the cuboid morphology mostly until the creep rupture, and super lattice stacking faults (SSFs) extended along [-1 1 0] and [-1-1 0] orientations within the γ’ precipitate were the typical dislocation configuration.

Abstract: In the early 1980’s, the small punch technique was developed in USA and Japan. This technique has been applied to nuclear reactors, electric power plants for safety assessments. European researchers have carried out the pioneer work on SP creep testing in the 1990’s. European Code of Practice (CoP) documents have become available in 2006, which provides a guide line to perform small punch tests for metallic materials, and introduces methods for estimation of tensile properties, fracture toughness and creep properties. Since then several conferences and symposiums have been held to exchange ideas and experiences, and several efforts have been made to improve the approaches for interpreting the material properties from test results. However, due to the complexity of the small punch test itself and lack of systematic test data for verification, especially in the creep region, up to now there is no common acceptable method to interpret creep properties. This paper discusses some of the problems commonly encountered in the small punch creep test (SPCT) analysis, such as the uniqueness in reverse creep analysis for identification of material parameters, how to consider effects of large deformation and strain hardening, how to separate deflection into a part caused by loading and a part caused by creep, and how to estimate the elastic-plastic properties of a material in the creep temperature region. Based on these considerations, several existing approaches for interpreting creep properties have been re-evaluated. In addition, a software package, based on the optimization toolbox of Matlab has been developed for identification of material parameters automatically. Verifications are performed by checking the agreement between properties derived by small punch test and uni-axial tests. Discussions on problems of existing approaches and how to improve them further, are described in the paper. Experimental data are provided by JRC Petten, Netherlands, and Henan Electric Power Research Institute, China. Recently a working group led by JRC Petten is engaged in upgrading the CoP to a European standard. It is expected that in the future more data will become available for further verification.

Abstract: The tensile and creep properties of P92 steel have been studied using a precision tensile and creep test machines for miniature and conventional specimens under various stress level at 625°C. The results showed that the data stability of miniature plate specimens is high whether at room or high temperature tensile tests. Compared with the conventional plate specimens, tensile strength, yield strength and total elongation is slightly lower, the uniform elongation is higher for miniature plate specimens at room temperature. By contrast, the tensile strength and uniform elongation is slightly higher, and the yield strength and total elongation is lower at high temperature. Besides, there had similar creep curves between miniature and conventional specimens, and the creep rupture time and minimum creep rate are closer under the same stress. By comparing the power law creep index and damage tolerance factor at the second creep stage, it can be derived that the creep mechanism is identical for the micro and conventional specimens, which is controlled by the dislocation movement.

Abstract: Titanium aluminides are highly attractive for high temperature applications involving dynamic components, e.g. turbine blades or turbocharger wheels, due to their high strength-to-weight ratio. The drawback is the difficult manufacturing of this material class due to the low toughness and high sensitivity to oxygen. Selective electron beam melting SEBM shows a new approach of producing complex titanium aluminide parts without a major oxygen pick up and avoiding problems with brittleness. The high cooling rates of this process lead to a very fine microstructure, which is not fully understood up to now. The microstructure determines the creep properties and therefore defines the performance of this material in high temperature applications. In this contribution, the creep properties of Ti-48Al-2Cr-2Nb fabricated by SEBM are investigated. The influence of the processing parameters and the building direction on the microstructure and the creep properties are discussed and compared to cast material.

Abstract: In this paper microstructure and creep properties of Mg-Al-Ca-Sr, Mg-Zn-RE-Zr and Mg-Sn-Si gravity casting magnesium alloys are presented. The microstructure was characterized using light microscopy, scanning and transmission electron microscopy. Phase identification was made by SAED and XRD analysis. Creep tests were carried out in the temperature range from 180°C to 200°C at applied stress of 60 MPa. Microstructure of Mg-Al-Ca-Sr alloys composed of α-Mg grains and C36, C15 and C14 intermetallic compounds in the interdendritic regions. In case of Mg-Zn-RE-Zr alloys the dominant intermetallic compound is (Mg,Zn)₁₂RE phase also located in the interdendritic regions. Microstructure of Mg-Sn-Si alloys after T6 heat treatment consists of plate-like precipitates of Mg₂Sn phase, primary crystals of Mg₂Si phase and globular Mg₂Si phase. Among the alloys in this study, the low-cost Mg-5Al-3Ca-0.7Sr alloy has the best creep resistance. The other alloys, excluding the Mg-5Si-7Sn alloy, are characterized by a poorer creep resistance in compared to Mg-5Al-3Ca-0.7Sr alloy, however their creep resistance is better if compared to typical Mg-Al alloys. Creep resistance of Mg-5Si-7Sn alloy is very low.

Abstract: Creep properties of one-step and two-step annealed Zr-1Nb-0.12O cladding tubes were studied. Creep tests were carried out at 450~500 °C with the applied stress between 80MPa and 120MPa The creep rates of the two-step annealed Zr-1Nb-0.12O alloy were found to be slower than those of the one-step annealed Zr-1Nb-0.12O alloy. The creep rate decreased with increase of grain size with annealing for Zr-1Nb-0.12O at intermediate temperatures, suggesting the creep resistance can be enhanced by the grain size control. The creep life of two-step annealed Zr-1Nb-0.12O increased over the one-step annealed Zr-1Nb-0.12O by the factor of 18~20 despite the greater initial instantaneous strain.

Abstract: Creep behavior of a heat treated single crystal nickel base superalloy containing Re/Ru under the test condition of 1100°C/137MPa high temperatures was investigated. The experimental results showed that the segregation extent of elements in the dendrite and inter-dendrite regions of single crystal superalloy decreases by heat treatment at high temperature. The creep life of the alloy at 1100°C/137MPa was measured to be 321 h displaying a better creep resistance. Wherein, significant amount of fine cubiodal γ′ particles precipitated in the γ matrix channels are considered to be the main reason of the alloy having the better creep resistance. The deformation feature of the alloy during steady state creep is dislocations slipping in the γ matrix and climbing over the rafted γ′ phase. But in the latter stage of creep, the deformation feature of the alloy is dislocations shearing into the rafted γ′ phase. As creep goes on, the main / secondary slipping dislocations in the alloy are alternately activated to result in the initiation and propagation of the cracks along the interface of the rafted γ′/γ phase up to fracture, which is thought to be the fracture mechanism of the alloy during creep.