309S耐热不锈钢蠕变行为及寿命预测

以工业化生产的309S奥氏体耐热不锈钢为研究对象,在不同温度和应力条件下进行单轴拉伸蠕变实验.结果表明:700℃/100 MPa蠕变断裂时间最长为480.13 h,蠕变曲线有明显的3个阶段;800℃/100 MPa蠕变断裂时间最短为4.61 h,蠕变曲线只有第3阶段,直至断裂.扫描电子显微镜(SEM)观察分析表明,蠕变...     

Creep and rupture of an ods alloy with high stress rupture ductility

The creep and stress rupture properties of an oxide (Y₂O₃) dispersion strengthened nickel-base alloy, which also is strengthened by γ′ precipitates, was studied at 760 °C and 1093 °C. At both temperatures the alloy YDNiCrAl exhibits unusually high stress rupture ductility as measured by both elongation and reduction in area. Failure was transgranular, and different modes of failure were observed including crystallographic fracture at intermediate temperatures and tearing or necking almost to a chisel point at higher temperatures. While the rupture ductility was high, the creep strength of the alloy was low relative to conventional γ′ strengthened superalloys in the intermediate temperature range and to ODS alloys in the higher temperature range. These findings are discussed with respect to the alloy composition; the strengthening oxide phases, which are inhomogeneously dispersed; the grain morphology, which is coarse and elongated and exhibits many included grains; and the second phase inclusion particles occurring at grain boundaries and in the matrix. The creep properties, in particular the high stress dependencies and high creep activation energies measured, are discussed with respect to the resisting stress model of creep in particle strengthened alloys. RICHARD M. ARONS, formerly a Graduate Student at Columbia University     

超超临界火电机组用T23钢的高温蠕变断裂行为

 通过研究和分析超超临界火电机组用T23钢持久试样的断口形貌及其在600℃高温蠕变过程中的组织演变,探讨了不同应力水平下T23钢的蠕变断裂机制。研究结果表明,T23钢在高应力条件下的蠕变断裂机制类似于常温下典型的韧性断裂,蠕变空洞主要形核于晶内的夹杂物处;而在低应力条件下的蠕变断裂机制表现为脆性沿晶断裂,蠕变空洞则主要形核于晶界第二相处。     

超超临界火电机组用关键锅炉钢性能分析

对T/P92、T/P122、S30432(Super 304H)、S31042(HR3C)、9Cr3W3Co等先进锅炉钢的性能进行了综合评述,对比分析了这几种钢管及其焊接接头的持久强度、抗腐蚀性能和常规力学性能数据,采用等温线法和Larson-Miller法外推了这些锅炉钢的10万h持久强度,探讨了这几种钢管许用应力的设定及在工程设计和应用的问题。     

Creep equations for heat resistant steels

Creep equations are necessary for the design and supervision of high temperature components. To obtain a sound basis for the development of such equations, the typical results of creep rupture tests have usually to be expanded by supplementary tests. At first a creep equation is developed for an extensively tested individual material. This equation then serves as prime solution for the corresponding steel type. The creep equation of the steel type is derived from condensed creep data which are the result of a scatter band analysis of multi-heat creep data. In the analysis, a large number of values of stress to specific strain are assessed on the basis of a time temperature parameter. The result is a family of correlated curves of stress to specific creep strain against time for all temperatures, stresses and strain values covered by the original test results. From these isostrain curves, mean creep curves are derived to form the basis for the modelling of the creep equation. For the development of this equation, a formulation is recommended which contains terms for initial plastic strain and primary, secondary and tertiary creep strain. The individual terms of this equation, which generally depend on stress and temperature, are determined in a stepwise procedure. The methods how such creep equations are developed and how their limits are fixed, are demonstrated for the examples of individual steels of type 14 MoV 6 3 and X 3 CrNiMoN 17 13 and of steel type 1 %. CrMoNiV according to SEW 555.     

Creep and stress rupture of a mechanically alloyed oxide dispersion and precipitation strengthened nickel-base superalloy

The creep and stress rupture behavior of a mechanically alloyed oxide dispersion strengthened (ODS) and γ′ precipitation strengthened nickel-base alloy (alloy MA 6000E) was studied at intermediate and elevated temperatures. At 760 °C, MA 6000E exhibits the high creep strength characteristic of nickel-base superalloys and at 1093 °C the creep strength is superior to other ODS nickel-base alloys. The stress dependence of the creep rate is very sharp at both test temperatures and the apparent creep activation energy measured around 760 °C is high, much larger in magnitude than the self-diffusion energy. Stress rupture in this large grain size material is transgranular and crystallographic cracking is observed. The rupture ductility is dependent on creep strain rate, but usually is low. These and accompanying microstructural results are discussed with respect to other ODS alloys and superalloys and the creep behavior is rationalized by invoking a recently-developed resisting stress model of creep in materials strengthened by second phase particles. The analysis indicates that at the intermediate temperature the creep strength is controlled by the high volume fraction of γ′ precipitates and the contribution to the creep strength from the oxide dispersion is small. At the elevated temperature, the creep strength is derived mainly from the inert oxide dispersoids. Formerly at Columbia University.     

NiAl-28Cr-5.5Mo-0.5Hf-0.012P合金的蠕变性能研究

研究了热等静压态NiAl-28Cr-5．5Mo-0.5Hf-0．012P合金的高温蠕变行为。结果表明：实验合金具有较短的减速蠕变阶段和相当长的稳态蠕变阶段以及很高的蠕变应变；在研究的实验条件范围内，合金的蠕变变形机制为低温高应力下的位错粘滞滑移控制和高温低应力下的位错攀移控制；蠕变后合金的显微组织变化不大，表明蠕变断裂受孔洞及裂纹的形成和扩展所控制，而且蠕变断裂行为符合修正后的Monkman-Grant规律：Intf＋0.775lnε=1.104。     

Tensile, Fatigue, and Creep Properties of Aluminum Heat Exchanger Tube Alloys for Temperatures from 293 K to 573 K (20 °C to 300 °C)

Since automotive heat exchangers are operated at varying temperatures and under varying pressures, both static and dynamic mechanical properties should be known at different temperatures. Tubes are the most critical part of the most heat exchangers made from aluminum brazing sheet. We present tensile test, stress amplitude-fatigue life, and creep–rupture data of six AA3XXX series tube alloys after simulated brazing for temperatures ranging from 293 K to 573 K (20 °C to 300 °C). While correlations between several mechanical properties are strong, ranking of alloys according to one property cannot be safely deduced from the known ranking according to another property. The relative reduction in creep strength with increasing temperature is very similar for all six alloys, but the general trends are also strong with respect to tensile and fatigue properties; an exception is one alloy that exhibits strong Mg-Si precipitation activity during fatigue testing at elevated temperatures. Interrupted fatigue tests indicated that the crack growth time is negligible compared to the crack initiation time. Fatigue lifetimes are reduced by creep processes for temperatures above approximately 423 K (150 °C). When mechanical properties were measured at several temperatures, interpolation to other temperatures within the same temperature range was possible in most cases, using simple and well-established equations.     

P92钢的蠕变损伤容许量系数及蠕变断裂机理

利用P92钢在595、610、640、670℃的高应力试验条件下的蠕变试验数据,得出其Norton应力指数,依据Norton应力指数的大小判定其蠕变机理为位错蠕变。同时结合1种新的蠕变变形及断裂模型,引入将蠕变损伤看作1个内在的阶段变量的蠕变损伤容许量系数,根据蠕变损伤容许量λ=2.94,判断其蠕变变形和断裂是位错运动控制的。微观组织的观察也表明,蠕变后的试样中位错密度大大降低,高密度位错是P92钢持久强度高的原因,伴随着位错密度的下降,P92钢持久强度降低直至断裂。     

44Si2CrV钢弹簧的室温蠕变

试验结果得出,44Si2CrV钢弹簧经淬火+中温回火处理并除掉表面脱碳层后,在室温下所受应力超过临界蠕变应力值时即发生蠕变;钢的抗拉(或剪切)强度越高,临界蠕变应力越高。44Si2CrV钢900℃淬火,500℃回火,临界蠕变应力 τ_cc =700 MPa;900℃淬火,380℃回火, τ_cc =800 MPa。蠕变开始时的塑性变形速率较大,之后急剧减小,在一定应力下保持一定时间后蠕变停止。经长时间压缩,蠕变变形已经停止的弹簧,于200℃保温2 h后空冷,弹簧的自由高度伸长,并且在原来的应力下加载,弹簧蠕变又重新开始。     

Evaluation of structural stability and creep resistance of 9-12% Cr steels

During creep exposure of modified chromium steels lowering of solid solution strengthening due to precipitation of Laves phase as well as coarsening of all precipitates causes degradation of creep resistance. Two distinct domains of the stress dependence of creep rate and time to rupture have been observed in precipitation strengthened modified chromium steels. The stress characterizing the transition between these domains was found to be closely related to the Orowan stress. This stress consists in these steels of the contribution from large particles on subgrain boundaries (mainly M₂₃C₆ and during the limited time also Laves phase) and from small precipitates (Nb(C,N) and VN) inside subgrains. This has to be considered when measuring the interparticle spacing and calculating Orowan stress. Larson-Miller parametric equation is used to elucidate the necessity of long-term creep testing. By means of two heats of CrMoVNbN steel it is shown that reliable extrapolation of creep properties is possible only in a stress and temperature domain in which only one creep creep rupture mechanism operate. In the high stress domain Larson-Miller constant C_LM is well above 30 while in the low stress domain this constant does not exceed 25. When the extrapolation is based mainly on short-term creep tests, the C_LM constant is close to that valid in the high stress domain and therefore it overestimates long-term creep strength.     

Elevated temperature creep-rupture behavior of the single crystal nickel-base superalloy NASAIR 100

The creep and rupture behavior of [001] oriented single crystals of the nickel-base superalloy NASAIR 100 was investigated at temperatures of 925 and 1000 °C. In the stress and temperature ranges studied, the steady state creep rate, time to failure, time to the onset of secondary creep, and the time to the onset of tertiary creep all exhibited power law dependencies on the applied stress. The creep rate exponents for this alloy were between seven and eight, and the modulus-corrected activation energy for creep was approximately 350 kjoule/mole, which was comparable to the measured activa-tion energy for Ostwald ripening of the γ′ precipitates. Oriented γ′ coarsening to form lamellae perpendicular to the applied stress was very prominent during creep. At 1000 °C, the formation of a continuous γ-γ′ lamellar structure was completed during the primary creep stage. Shear through the γ-γ ' interface is considered to be the rate limiting step in the deformation process. Gradual thickening of the lamellae appeared to be the cause of the onset of tertiary creep. At 925 °C, the fully developed lamellar structure was not achieved until the secondary or tertiary creep stages. At this temperature, the γ-γ′ lamellar structure did not appear to be as beneficial for creep resistance as at the higher temperature.     

P92钢的蠕变行为研究

在不同温度和压力条件下完成P92钢的蠕变及持久试验,采用SEM、TEM研究P92钢的强化机制及退化机制。持久试验外推强度同欧洲蠕变委员会公布的数据基本相同。Norton应力指数数值表明,高应力阶段的蠕变机制为位错蠕变。组织观察结果表明:P92钢的主要强化机制为位错强化及弥散强化,淬火得到的马氏体内部有高密度的位错,板条间的碳化物M23C6及弥散分布碳氮化物MX是P92钢热强性高的原因。随着位错密度的降低及析出相的粗化,P92钢的高温耐热性也降低。     

High-temperature rupture of microstructurally unstable 304 stainless steel under uniaxial and triaxial stress states

Specimens of 304 stainless steel were tested to failure under two different stress states, uniaxial tension using smooth bar specimens and triaxial tension using notched bar specimens. The tests were conducted at a temperature that gives rise to carbide particle growth which, in turn, leads to microstructural softening. Rupture times are compared for uniaxial and triaxial stress states with respect to multiaxial stress parameters that are directly related to physical mechanisms. The success of the parameters is judged according to how well the rupture times of notched specimens can be predicted using the rupture data for specimens under uniaxial tension. The data indicate that the rupture time is not governed by deformation processes, despite evidence for substantial softening by particle coarsening. The results further suggest that the creep rupture process is dominated by cavitation that is coupled with localized shear deformation along the inclined grain boundaries.     

Optimization and verification of creep equations for heat resistant steels

A direct creep curve assessment of multi-heat creep data delivers condensed creep curve bands of a steel type for a limited number of stress groups. The mean creep curves of these stress groups are the basis for an examination and if necessary optimization of a creep equation which was originally based on a time temperature parameter assessment. This procedure is demonstrated for creep equations of steel types 10 CrMo 9 10 and X 21 CrMoV 12 1. The creep equations are available in the form of a subroutine Kara FW which serves as a user subroutine for finite element programmes. The scatter band covered by such a creep equation can be described by strain factors for initial plastic strain and for creep strain. A critical strain was developed to indicate creep rupture or local crack initiation. This critical strain is the uniform elongation which appears at rupture. It can be determined by finite element analyses of the specimens tested. Further, the optimized creep equations are verified by the recalculation of different service type tests using the programme Abaqus and the subroutine Kara FW.     

热穿孔温度对T91钢管持久强度的影响

研究了φ75mm管坯穿孔温度对T91锅炉钢管高温力学性能的影响。结果表明，1200℃穿孔温度能获得持久强度和持久断裂塑性较好配合的综合性能。     

Microstructure,creep, and tensile deformation in Ti-6Al-2Nb-1Ta-0.8Mo

The effects of microstructure, temperature, and stress level on the creep response of Ti-6211 have been investigated. A variety of microstructures simulating the heat affected zone of a weld (HAZ), as well as the as-received structure, were tested in a temperature range of 298 K to 873 K. At stress levels below the tensile yield strength, creep curves saturate in the ambient temperature regime. The colony type Widmanstätten alpha + beta as-received structure exhibited the highest creep strains at ambient temperatures. Long slip lengths associated with the large colony size, and sliding along various interfaces account for the relatively high creep strains to saturation. Planar arrays of straight dislocations operating on a single slip system were observed for samples crept at 298 K while thermally activated cross slip was observed for samples crept at 453 K. Beta-annealed martensitic micro-structures displayed enhanced creep resistance, out-performing other recrystallized HAZ structures. Above 778 K the activation energy of creep is close to that for self-diffusion in titanium, suggesting that diffusion-controlled dislocation mechanisms are the rate-controlling processes at elevated temperatures. Creep rupture at elevated temperatures occurred by microvoid nucleation and growth. Fracture occurred along colony boundaries in the as-received structure but appear to be intergranular with the crack propagating along G. B. alpha/matrix interfaces in the equiaxed microstructures. Sliding along alpha/beta interfaces, colony boundaries, prior beta grain boundaries, and slip traces contributed to the creep strain and rupture process. Cyclic creep with a loading-unloading sequence was also performed at room temperature and cyclic creep acceleration was observed.     

The impact of welding on the creep properties of advanced 9–12% Cr steels

The term “long-term creep properties” for heat resistant steels is mainly reflected by the 100.000 hour creep rupture strength at elevated testing temperature. Often, results of high stress, short-term creep tests are extrapolated to this 100.000 hour target value. Results of long-term creep tests are rather rare because of high testing costs and the time consuming testing procedure. Especially, long-running crossweld creep tests have not been performed in a sufficient extent so far, although, the heat-affected zone of crosswelds of ferritic chromium steels is known as a possible weak point. The long-term creep properties of crosswelds is linked to microstructure of the heat-affected zone of 9–12% chromium steels. The formation of heat-affected zone microstructures is studied by dilatometry, in-situ X-ray diffraction using synchrotron radiation, optical microscopy as well as most advanced electron microscopic methods. Results of crossweld creep tests up to duration of 40,000 hours are directly linked to the heat-affected zone microstructure at the location of fracture. The most predominant failure mechanism at lower stress levels and long term duration is Type IV cracking in the fine-grained heat-affected zone region of crosswelds. The failure mechanism is discussed in detail in this paper.