12Cr1Mo和10CrMo910钢蠕变寿命研究的电阻法

在研究了湿度载荷对蠕变损伤的影响的基础上,对耐热负蠕变形提高了新的认识,建立了蠕变损伤与材料电阻特性变化之间的关系,提出了以材料电阻特性变化为主,综合显微组织结构特性及硬度变化的耐热钢材料寿命评定的综合评定方法。     

低合金耐热钢蠕变断裂韧性

试验表明,耐热钢和合金在高温长期应力作用下,由于内部组织变化会出现蠕变脆性现象。事实表明:对耐热钢和合金的高温断裂寿命评定,特别是在缺口蠕变断裂时应采用持久强度和高温蠕变断裂韧性相结合的性能指标,但目前尚缺乏能反映耐热材料在高温长期应力作用下脆性特性指标。蠕变断裂过程实际上是蠕变裂纹萌生和扩展过程。文献[5～8]认为:当裂纹尖端区蠕变损伤积累     

高温环境下蠕变疲劳交互作用损伤力学研究

应力控制下的疲劳、蠕变及其交互作用损伤实际上是循环蠕变、静蠕变引起的材料延性不断耗竭的过程，本文在延性耗竭理论和损伤力学有效应力概念的基础上．对疲劳蠕变交互作用损伤演化进行了研究，提出了一个新的疲劳蠕变交互作用损伤模型．采用非弹性应变能密度变化作为损伤参量定义损伤变量。通过耐热钢1.25Cr0.5Mo光滑试样高温环境下应力控制的梯形波加载试验．验证上述疲劳蠕变交互作用模型，最终得到了1.25Cr0.5Mo钢540℃下不同最大应力、不同应力幅组合条件下的损伤演化统一表达式，试验损伤点与该模型的损伤演化规律符合较好．表明该模型和损伤变量适合于疲劳蠕变交互作用下的损伤描述．     

铁素体钢蠕变强度与微量元素的关系

本文通过试验方法，以弄清铁素体基体蠕变强度的控制因素的目的，分析了碳钢的蠕变强度特性，在研究微量合金元素对基体蠕变强度特性的同时，与耐热钢蠕变断裂特上比较 进行了研究。     

P92钢蠕变本构方程测试研究

高温构件因蠕变损伤而引起失效断裂的现象已引起研究人员的重视,针对新型耐热钢P92钢高温蠕变特点,考虑实际工程应用,采用蠕变拉伸试验得到适合P92钢母材蠕变第二阶段的蠕变本构方程,为进一步研究P92钢焊接接头的寿命评估提供了基础数据.     

超临界CO2动力循环高温材料腐蚀研究进展

超临界CO2动力循环具有效率高、结构紧凑、噪声小等优点,可应用于舰船动力、核电、光热发电等领域.该循环的长时运行依赖于工质与材料的相容性与化学稳定性.超临界CO2中典型耐热钢及合金的腐蚀特性方面已形成部分理论共识:铁素体耐热钢的抗腐蚀性能劣于奥氏体耐热钢及合金;超临界CO2中C的扩散反应形成复杂碳化物,在一定程度上降低了耐热钢及合金的高温强度;超临界CO2中的微量杂质H2O会加速耐热钢及合金的腐蚀;表面预氧化、渗铝和涂层等处理可提高耐热钢及合金的抗腐蚀性能.应力作用机制、蠕变疲劳交互作用、渗碳无损检测、定量诊断和腐蚀寿命预测将成为未来超临界CO2动力循环高温材料腐蚀深入研究的重点方向.     

电站锅炉用改进型9Cr-1Mo耐热钢高温蠕变行为研究

对电站锅炉用改进型9Cr-1Mo耐热钢进行了高温蠕变试验,获得了其在565℃和650℃条件下的蠕变应变-时间曲线。研究了不同蠕变温度和应力水平对材料蠕变寿命的影响规律。基于蠕变试验数据,分析了最小蠕变速率与应力、蠕变断裂时间之间的关系。结果表明:在565℃和650℃下应力指数n的值分别为20. 02和9. 44。通过分析试验数据,对其蠕变寿命进行了预测。通过金相、断口等分析,研究了蠕变过程中的微观组织演变规律。     

基于Ω方法的T/P91耐热钢蠕变持久寿命预测

利用Ω方法预测原始态和服役态T/P91耐热钢的蠕变持久寿命,并将其与利用等温线法和Larson-Miller参数法得到的结果进行对比。提出一种基于Ω方法的加速试验方案,该方法仅需一根试样即可快速预测给定温度和应力参数下的蠕变持久寿命。结果表明:与等温线法和Larson-Miller参数法相比,加速Ω方法的预测精度更高;对于服役态T/P91耐热钢,不同蠕变加速试验参数下得到的Ω较接近,蠕变断裂时间预测值均与试验值较吻合。     

超期服役F12耐热钢断裂失效与组织结构的相关性

采用金相、SEM和XRD等方法,对原始及不同服役阶段的F12马氏体耐热钢试样的性能和组织结构进行了分析和比较.结果表明:随着服役时间的延长,试样的力学性能明显下降;在不同服役阶段,F12马氏体耐热钢试样的显微结构发生了明显变化,不仅出现原奥氏体晶界处碳化物的粗化、基体组织中α-Fe的晶格常数减小,且断裂态马氏体组织完全分解,并在晶界粗化的碳化物处形成蠕变孔洞.马氏体组织分解、碳化物粗化和基体组织合金元素贫化是直接导致材料性能下降的原因.     

舰用燃气涡轮叶片材料的蠕变寿命模型研究

本文在金属材料典型蠕变过程的基础上,分别研究了蠕变各个阶段的本构模型。针对金属材料在较高应力下没有蠕变稳定阶段的现象,提出了金属材料高低应力值的判定准则并根据高低应力的不同特点推导了不同的耦合损伤本构模型。通过对舰用燃气轮机涡轮叶片材料的蠕变试验结果与模型预测结果进行比较可以看出：本文提出的蠕变模型近似地模拟金属材料的蠕变全过程,准确地预测金属材料的蠕变寿命。     

Creep behaviour of 12Cr-Mo-V steel weldments

This paper describes an investigation into the creep behaviour of weldments in 12Cr-Mo-V steel, with particular emphasis on the effect of welding conditions and type of filler metals. At 600°C, the creep lives of all the weldments fall below the DIN 17175 scatterband and are not influenced by a variation in welding regime, number of postweld heat treatments and type of filler metal. This is attributed to the premature creep damage in the ‘type IV’ zone, the outer part of the fine grained zone, which possesses the poorest creep resistance.     

Creep deformation characterization of heat resistant steel by stress change test

To evaluate the creep deformation mechanism of heat resistant steel, stress change tests were conducted during creep tests for Modified 9Cr–1Mo Steel and 2.25Cr–1Mo Steel. In this study it was confirmed that the dislocation behavior during the creep tests was in viscous manner because of no instantaneous plastic strain observed at stress increments. Transient backward creep behavior was observed after stress reduction for these steels in this work. Mobilities of dislocation were evaluated by observed backward creep behavior after stress reductions. Internal stresses were evaluated by the changes of creep rate in stress increments. And mobile dislocation densities were evaluated with the estimated mobilities of dislocation and the changes of creep rate in stress increments. It was found that the variation of evaluated mobile dislocation densities during creep deformation showed the same tendency as the variation of creep rate. Therefore mobile dislocation density is the dominant factor that influences the creep rate variation in creep deformation of these steels in this work. The mobilities of dislocation showed a good correlation with 1/T and related with solute amount of Mo that is a solution hardening element.     

Creep degradation in welds of Mod.9Cr-1Mo steel

Creep life assessment technology has not yet been developed for Mod.9Cr-1Mo steel welded joints due to the unique degradation and failure mechanisms in the heat affected zone. Nevertheless, there is strong demand from power plant operators for the development of nondestructive damage detection and life assessment technology for weldments of this steel. In this study creep rupture testing using a large-size welded joint was conducted, and creep and damage detection tests were carried out to elucidate the degradation mechanism. Also the microstructure and hardness changes in the heat affected zones of the welds were investigated to clarify the degradation mechanisms of such welds in comparison with base metal during creep. In general, the changes in hardness distribution along the welds were difficult to correlate with the life consumption, although degradation in welds due to creep was successfully detected. Accordingly, a new approach to degradation evaluation and creep life assessment by hardness measurement method for Mod.9Cr-1Mo steel welds is proposed.     

Creep and fracture behaviour of English Electric Mark III transition welds: Part (b) Ex-service weldments

The microstructure and high temperature behaviour of an ex-service weldment have been assessed. Exposure at elevated temperature resulted in some carbide development in the niobium stabilised buttered layer. However, significant decarburisation in the low alloy steel was not detected. Cross-weld uniaxial creep testing invariably led to low ductility failures in the heat affected zone on the 2.25 Cr1Mo steel side of the weld. These failures were associated with the nucleation, growth and link-up of cavities on prior austenite grain boundaries. Assessment of tests interrupted at selected life fractions indicated that the development of cavitation followed a sensible trend with creep exposure. The cavitation behaviour of the transition weld was in general agreement with data obtained from creep tests undertaken on material heat treated to simulate heat affected zone structures. The level of creep damage in these welds can be assessed by evaluation of local strain accumulation or through monitoring the presence of cavities.     

Creep damage in small punch creep specimens of Type 304 stainless steel

Small punch creep tests on Type 304 stainless steel have been performed at 650 °C. Based on these tests, a finite element model, with modified Kachanov–Rabotnov creep damage constitutive equations, was established. The variation of central deflection and creep strain with time and the evolution of creep damage under constant loads were analysed by using the finite element model. The central creep deflection curves in the specimens were obtained at different loads in both tests and simulations and have three different stages, similar to conventional creep tests. There is good agreement between experimental results and simulation data. The creep damage at the central part is high, and localization of damage is obvious. Initial failure occurs at the bottom surface, about 0.8 mm away from the centre which agrees well with the finite element mode observation.     

Evaluation of creep damage in heat affected zone of thick welded joint for Mod.9Cr–1Mo steel

Mod.9Cr–1Mo steel has been used for boiler components in ultra-supercritical (USC) thermal power plants. The creep strength of welded joint of this steel decreases due to the formation of Type IV cracking in heat affected zone (HAZ) at higher temperatures. The present paper aims to clarify the damage processes and mechanisms of the welded joint for Mod.9Cr–1Mo steel. Long-term creep tests of base metal, welded joint and simulated fine- grained HAZ were conducted at 550, 600 and 650 °C. Creep tests using thick plate welded joint specimen were interrupted at several time steps, and evolutions and distributions of creep damages were measured quantitatively using laser microscope. It is found that creep voids initiate at early stage of creep life (0.2 of life), the number of creep voids increases until 0.7 of life, and then voids coalesced into the macro crack at the later stage of life (0.8 of life). Creep damages concentrate mostly at a quarter depths of the plate thickness within the fine-grained HAZ of the present welded joint. The experimental creep damage distributions were compared with the computed results by using the FEM analysis. Both creep strain concentration and high stress triaxiality in fine-grained HAZ of welded joint are considered to accelerate the creep void formation and growth.     

Heat-to-heat variation in long-term creep strength of some ferritic steels

Heat-to-heat variation in creep life has been investigated for some ferritic steels, mainly 12Cr steels, using long-term creep data in the NIMS Creep Data Sheets. The tempered martensitic plain 12Cr and 12Cr–1Mo–1W–0.3V steels exhibit large heat-to-heat variation in creep life, as shown by about one order of magnitude difference in time to rupture or more between the strongest and weakest heats. On the other hand, low-Cr steels of tempered bainitic 1Cr–1Mo–0.25V, ferritic–pearlitic 2.25Cr–1Mo and ferritic 9Cr–1Mo steels exhibit small heat-to-heat variation in creep life. The heat-to-heat variation in long-term creep strength is correlated with the degradation behaviour at long times, which depends on initial strength and concentrations of Al, nitrogen and Cr. The present results suggest that taking the mechanisms responsible for the heat-to-heat variation in creep life into account, quality of heat resistant steels as well as reliability of remaining life estimation can be further improved.     

A CDM-based study of fatigue–creep interaction behavior

Under stress control mode, the damage evolution during fatigue, creep and their interaction behavior actually is a ductility exhaustion process in response to cyclic and static creep. Based on the ductility dissipation theory and effective stress concept of continuum damage mechanism (CDM), a new fatigue–creep interaction damage model has been developed in this paper, where the change of the inelastic strain energy density is used to define the damage variable. To assess this damage model, high temperature fatigue–creep interaction experiments have been carried out for 1.25Cr0.5Mo steel under stress control mode employing a trapezium waveform. The damage evolution laws of 1.25Cr0.5Mo steel under various combinatorial conditions with different maximum stresses and stress amplitudes are derived. Results indicate that the damage parameter and damage model presented in this paper are applicable to describe the damage evolution for fatigue–creep interaction.     

MODELING OF DAMAGE EFFECT ON HEAT TRANSFER IN TIME-DEPENDENT NONHOMOGENEOUS SOLIDS

This article demonstrates a concept of coupling between the heat transfer equation and the damage evolution in solids subject to creep at elevated temperatures. The second-rank damage tensor, which appears in the constitutive equations of creep and damage and, on the other hand, in the combined heat flow-radiation rule, plays a role of the variable introducing feedback. As examples, axisymmetric cylinders and disks subject to creep damage under combined mechanical and thermal loadings are considered