Effect of W-addition on low cycle fatigue behavior of high Cr ferritic steels

A study was done to investigate the effect of tungsten (W) addition on the microstructure, tensile properties and low cycle fatigue (LCF) properties of 9Cr-lMo steels at 298K and 873K. Four steels containing different amounts of W (0W, 1.2W, 1.8W and 2.7W) were normalized at 1323K for 1 hour and tempered at 1023K for 1 hour. Microstructural analysis revealed that no significant differences wete observed in their tempered martensitic microstructure of 0W, 1.2W and 1.8W alloys, but d-ferrite was observed to form at the prior austenite grain boundaries of the 2.7W alloy. With the increase in W content, yield and tensile strength increased at both temperatures. Low cycle fatigue life also increased with the W content up to 1.8%, but decreased in the 2.7W alloy, which was primarily due to the presence of soft d-ferrite acting as the crack initiation site. The fatigue life at 873K was reduced compared to that at 298K, due not only to the decrease in strength at high temperature but also to the formation of oxide layers along the slip bands, which increases slip irre-versibility during cyclic deformation. This article based on a presentation made in the symposium “The 1^st Korea—Japan Structural Dynamic Symposium”, held at Sunchon National University, Sunchon, Korea, August 5–8, 1999 under the auspices of The Korean Insitute of Metals and Materials and Research and Development Center for Automobile Parts and Materials.     

固溶氮原子对不锈钢单轴及多轴低周疲劳特性的影响

对３１６Ｌ和３１６ＬＮ不锈钢进行了单轴拉压及多轴非比例加载低周疲劳实验研究,分析了固溶的氮原子对单轴及多轴非比例加载低周疲劳特性及其微结构的影响。结果表明,固溶的氮原子增大了不锈钢铁的单轴循环软化程度及非比例循环附加强化程度,延长了不锈钢的单轴拉压低周疲劳寿命,但却缩短了不锈钢的多轴非比例加载低周疲劳寿命,固溶的氮原子对单轴及多轴低周疲劳密度位错结构的形成具有明显的抑制作用,利用Ｍｏｅｓｓｂａｕｅ     

EXPERIENCE WITH THERMOMECHANICAL FATIGUE UNDER SERVICE-TYPE LOADING

The thermomechanical fatigue behaviour of different high temperature alloys has been investigated and is under investigation respectively. The creep-fatigue behaviour of heat resistant steels was investigated by long-term service-type strain cycling tests simulating thermomechanical fatigue (TMF-) loading conditions at the heated surface of e.g. turbine rotors. Single-stage as well as three-stage cycles leads to similar results at the application of the damage accumulation rule. Life prediction which simulates typical combinations of cold starts, warm starts and hot starts has been established successfully for isothermal service-type loading and will be exceeded for thermomechanical loading. Long-term thermomechanical fatigue testing of Thermal Barrier Coating systems show typical delamination damage. An advanced TMF cruciform testing system enables complex multiaxial loading.     

ON THE METHOD OF ACCELERATING HIGH-TEMPERATURE CREEP TEST BY AN ELEVATION OF TEMPERATURE

用四种耐热钢分别在同一应力不同温度下进行了一系列的蠕变试验.又分析了文献上四种耐热钢和五种镍基合金的蠕变及蠕变断裂数据.所有自己的试验温度以及文献上原始数据的试验温度都在每一种耐热钢或高温合金的0.45T_m(T_m是熔点的绝对温度)以上.根据自己的试验以及分析文献数据的结果,指出在所研究的温度范围内,对于所研究的不同类型的耐热钢和高温合金说来,高温蠕变过程中存在激活能是一个普遍的现象.文中指出了通过激活能用提高温度法缩短蠕变试验时间的可能性,并讨论了这种外推方法所适用的温度范围和它的误差.     

GH30和GH34合金的蠕变－疲劳交互作用

研究了ＧＨ３０和ＧＨ３４合金的蠕变－疲劳交互作用，观察到两种负交互作用：ＧＨ３０合金在先蠕变后疲劳的条件下的负交互作用和ＧＨ３４合金在连续循环蠕变的条件下的负交互作用。前者的交互作用项中疲劳损伤指数远大于蠕变损伤指数。因而负交互作用主要是由于预蠕变部分地抑制了后续疲劳的损伤。后者的交互作用项中的蠕变损伤指数远大于疲劳损伤指数。因而负交互作用主要是由于疲劳部分地消除了蠕变损份。还用微观观察的结果分析了这两种负交互作用的微观本质。     

铸造铝合金在多轴非比例载荷下的低周疲劳行为研究

主要研究了356铸造铝合金在多轴非比例载荷下的低周疲劳性能。结果表明，合金在循环变形过程中表现为循环硬化特征，循环硬化的程度表现出对非比例加载路径的依赖性；在相同等效应变幅值下，合金的非比例加载低周疲劳寿命小于比例加载，且非比例加载低周疲劳寿命也表现出对非比例加载路径的依赖性；在多轴疲劳寿命预测模型中考虑非比例加载路径对疲劳寿命的影响可以大大提高寿命模型预测的精确度。     

铸造A356铝合金的低周疲劳行为

研究不同加钛方式和钛含量对铸造A356铝合金常温低周疲劳行为的影响,分析合金疲劳断口的形貌特征.结果表明:4种A356合金均表现出明显的循环硬化行为,但Ti含量(质量分数)为0.14%的合金比Ti含量为0.10%的合金具有更高的循环硬化率.在低应变时,加钛方式对合金的循环硬化影响相近;而在高应变时,电解加钛A356合金...     

Chemical composition optimization for austenitic steels of the Fe-Mn-Al-C system

Alloys of the FeMnAlC system have been developed for different uses, from cryogenic temperatures up to 673 K, with specific composition recommended for each specific application. More recently, the possibility of adopting alloys of this system for structural purposes has attracted considerable attention. However, the absence of systematic criteria in the design of such compositions imposes severe restrictions on practical uses of these alloys. In this paper, we define composition limits in order to obtain an optimum microstructural state, characterized by the absence of embrittling components, and more restricted limits to obtain acceptable properties for structural applications, based on minimum values for ultimate tensile strength and impact toughness.     

Inconel 617 and Stellite 6 alloys for tooling in thixoforming of steels

Thermal fatigue and high temperature wear are the two principle failure mechanisms for thixoforming dies. Samples of Inconel 617 and Stellite 6 alloys were submitted to thermal cycling under conditions which approximate thixoforming of steels and to sliding wear tests at 750 °C. The experimental results thus obtained were compared with those of the X32CrMoV33 hot work tool steel. The Inconel 617 and Stellite 6 samples are much more resistant to oxidation and to softening than the hot work tool steel, providing a superior resistance to thermal fatigue cracking. The wear resistance of the Inconel 617 and Stellite 6 alloys at 750 °C is also markedly superior. The adhesive oxides growing slowly on Inconel 617 and Stellite 6 alloys sustain the wear action without spalling and are claimed to be responsible for the superior wear resistance of these alloys at 750 °C.     

P92钢630 ℃下的高温低周疲劳行为分析

P92钢作为新一代耐热钢,其服役温度正随着超超临界机组的不断推广而逐渐提高,而高温疲劳性能对保证电厂的安全运行具有重大作用.文中通过P92钢630℃下的疲劳试验,研究了其在不同应变幅和应变速率下的疲劳寿命以及应力—应变关系,并结合断口形貌分析了P92钢的断裂机理.结果表明,P92钢属于高温循环软化材料,其疲劳寿命随塑性应变幅的增大而呈指数下降,同时应变速率的增大会导致其疲劳寿命的增大.P92钢疲劳断口分为疲劳源区、裂纹扩展区以及断裂区.高应变幅下蠕变孔洞和二次裂纹的增加是导致其疲劳寿命降低的主要原因.     

LOW CYCLE FATIGUE BEHAVIORS AND MICROSTRUCTURES OF Ti–2Al–2.5Zr WITH FINE GRAIN AT RT AND 77 K

对细晶Ti--2Al--2.5Zr合金进行了室温/低温(77 K)疲劳实验及微观组织观察. 结果表明: 室温低应变幅Δε_t/2(=0.5%, 1.0%)下,合金表现为循环软化; 室温高应变幅(1.5%, 2.0%)下, 则表现为循环应力饱和; 77 K时, 不同应变幅下均表现为循环硬化, 且随应变幅升高, 循环硬化程度增强. 疲劳寿命测试结果表明: 低温疲劳寿命始终高于室温. 断口SEM观察表明, 室温和低温下, 疲劳裂纹扩展区均有明显的疲劳条纹,疲劳裂纹以穿晶方式扩展, 室温下伴随有大量二次裂纹, 低温下的二次裂纹数量明显减少. TEM观察表明: 低温下孪生是合金主要的变形方式, 包括{1011}和{1121}型孪晶. 疲劳变形位错组态为: 室温较低应变幅(0.5%, 1.0%)下, 形成位错线和局部位错缠结; 室温下应变幅提高到1.5%和2.0%时,\{1010}柱面和{1121}锥面滑移同时开动, 位错组态演化为亚晶和明显的位错胞. 77 K下, 应变幅2.0%时形成沿 柱面平行分布的位错带; 77 K下应变幅升高到4.5%时, 多滑移形成相互垂直的位错线. 低温诱发形变孪晶是Ti--2Al--2.5Zr低温疲劳寿命升高的原因.     

AI-Si-Cu-Mg(-Er)铸造铝合金的低周疲劳行为

对金属型铸造Al-Si-Cu-Mg和Al-Si-Cu-Mg-Er铝合金进行了疲劳试验,并研究了其室温下的低周疲劳行为。试验结果表明:金属型铸造Al-Si-Cu-Mg和Al-Si-Cu-Mg-Er铝合金表现为循环应变硬化和循环稳定,主要取决于外加总应变的高低;稀土元素Er的加入可提高金属型铸造Al-Si-Cu-Mg合金的循环变形抗力和疲劳寿命;金属型铸造Al-Si-Cu-Mg合金的塑性应变、弹性应变与断裂时的载荷反向次数之间呈直线关系,Al-Si-Cu-Mg-Er合金的弹性应变与疲劳断裂时的载荷反向次数之间也呈直线关系,但其塑性应变与疲劳断裂时的载荷反向次数之间则呈双线性关系。     

Intergranular fracture on fatigue fracture surface of 2.25Cr-1Mo steel at room temperature in air

Low-alloy steels serving for a long time at high temperature (∼500 °C) are very sensitive to temper embrittlement due to segregation of various trace elements at prior austenite grain boundaries and/or carbide/matrix interfaces. This type of segregation in combination with various environmental effects can adversely affect the fracture resistance and fatigue crack propagation rate with subsequent change in the fracture morphology of low-alloy steels. The present work describes the effects of heat treatments on impurity element segregation and its subsequent effects on fatigue fracture behavior of 2.25Cr-1Mo steel under different environmental conditions and temperatures. It has been found that either prior impurity element segregation caused during the heat treatment or hydrogen-induced embrittlement due to the presence of water vapor in laboratory air alone cannot produce intergranular fracture on the fatigue surfaces of 2.25Cr-1Mo steel at room temperature in air. The occurrence of intergranular fracture on the fatigue surfaces results from the combined effect of impurity element segregation-induced grain boundary embrittlement and hydrogen-induced embrittlement, and that the proportion of intergranular fracture is a function of prior impurity element segregation provided that the grain boundary segregation level exceeds a certain critical value.     

Low-cycle fatigue behavior of permanent mold cast and die-cast Al-Si-Cu-Mg alloys

Fatigue failure is one of the main failure forms of Al-Si-Cu-Mg aluminum alloys.To feature their mechanical aspect of fatigue behavior,the low-cycle fatigue behavior of permanent mold cast and die-cast Al-Si-Cu-Mg alloys at room temperature was investigated.The experimental results show that both permanent mold cast and die-cast Al-Si-Cu-Mg alloys mainly exhibit cyclic strain hardening.At the same total strain amplitude,the die-cast Al-Si-Cu-Mg alloy shows higher cyclic deformation resistance and longer fatigue life than does the permanent mold cast Al-Si-Cu-Mg alloy.The relationship between both elastic and plastic strain amplitudes with reversals to failure shows a monotonic linear behavior,and can be described by the Basquin and Coffin-Manson equations,respectively.     

超期服役的X20耐热钢主蒸汽管道疲劳性能研究

本文的研究对象为在热电厂主蒸汽管道服役长达230,000h的X20马氏体耐热钢,对其和原始材料分别选取5个应力幅进行单轴常温疲劳试验,划分了高周疲劳区、低周疲劳区以及过渡区。通过对比试验了解了X20钢的疲劳特性,推算出两种试样的Basquin公式,用于预测X20耐热钢的疲劳寿命,并利用应力-应变曲线进行了循环应变分析。     

Materials for ultrasupercritical coal power plants—Boiler materials: Part 1

The efficiency of conventional boiler/steam turbine fossil power plants is a strong function of the steam temperature and pressure. Research to increase both has been pursued worldwide, since the energy crisis in the 1970s. The need to reduce CO₂ emission has recently provided an additional incentive to increase efficiency. Thus, steam temperatures of the most efficient fossil power plants are now in the 600 °C (1112 °F) range, which represents an increase of about 60 °C (108 °F) in 30 years. It is expected that steam temperatures will rise another 50 to 100 °C (90 to 180 °F) in the next 30 years. The main enabling technology is the development of stronger high-temperature materials, capable of operating under high stresses at ever-increasing temperatures. Recently, the EPRI performed a state-of-the-art review of materials technology for advanced boiler/steam turbine power plants (ultrasupercritical power plants). The results of the review show that with respect to boilers, high-strength ferritic 9–12Cr steels for use in thick section components are now commercially available for temperatures up to 620 °C (1150 °F). Initial data on two experimental 12Cr ferritic steels indicate that they may be capable of long-term service up to 650 °C (1112 °F), but more data are required to confirm this. For higher temperatures, austenitic steels and Ni-based alloys are needed. Advanced austenitic stainless steels for use as super and reheater tubing are available for service temperatures up to 650 °C (1112 °F) and possibly 700 °C (1292 °F). Ni-based superalloys would be needed for higher temperatures. None of these steels have been approved by the ASME Boiler Code Group so far. Higher-strength materials are needed for upper water walls of boilers with steam pressure above 24 MPa (3400 psi). A high-strength 2-1/2%Cr steel recently ASME code approved as T-23 is the preferred candidate material for this application. Field trials are in progress. This paper will present the results of the EPRI review in detail, relating to boiler material. Results relating to turbine materials are presented in a companion paper as Part 2.     

High cycle fatigue behaviour of Ti–6Al–4V alloy at elevated temperatures

High cycle fatigue behaviour of Ti–6Al–4V alloy was studied at 623 K and 723 K. Fatigue strength decreased at elevated temperatures compared with at ambient temperature. In the short life regime, fatigue strength was lower at 723 K than at 623 K, but in the long life regime it was nearly the same at both temperatures. At elevated temperatures, cracks were generated earlier at applied stresses below the fatigue limit at ambient temperature, indicating lowered crack initiation resistance. Small cracks grew faster at elevated temperatures than at ambient temperature, which became more noticeable with increasing temperature. After allowing for the elastic modulus, small cracks still grew faster at elevated temperatures.     

标准差S在材料疲劳寿命分析中的作用

目前,在评价材料的疲劳强度时,往往只注重比较材料的中值疲劳寿命,而忽略了对数据分散性的比较.事实上,数据分散性,尤其是标准差S在评价材料疲劳性能时非常重要.本文通过对比几组相同试验条件下中国和美国生产的同一铝合金材料的疲劳数据发现:两个产地材料的中值疲劳寿命基本接近,但美国产材料疲劳数据的标准差S小.其对应99.99%存活率的疲劳寿命也明显高于国产材料,这表明标准差大小对高存活率下的疲劳寿命影响很大.因此,应对标准差S给予高度重视.     

300M钢疲劳热耗散的试验研究

研究了300M钢疲劳损伤过程中的热耗散和温度演化规律,测试了300M钢在热处理和未经热处理状态下力学性能和疲劳性能。试验结果显示:经过热处理的300M钢其拉伸性能和疲劳性能都有显著提高,且在拉伸以及疲劳损伤过程中,温度升高更为明显。两种处理方式的试件显现出不同的热耗散规律。根据疲劳损伤过程中温度演化规律估算了疲劳极限σ-1,与试验值吻合很好,为快速确定300M钢的疲劳极限提供了一个方法。     

CYCLIC HYSTERESIS ENERGY OF CARBON AND ALLOY STEELS

Based on the low-cycle fatigue tests of carbon and alloy steels,the cyclic properties ofhysteresis energy and its changing rules have been analysed.The mathematical formula ofcyclic hysteresis energy of the materials with different cyclic properties have been presented.The total absorbed energy to failure is associated with the variation of cyclic hysteresisenergy.