Experimental Study of the Creep Lifetime of the 1.25Cr 0.5Mo Steel Pipes

Abstract: In this paper, physical parameters for the creep constitutive equations of the low alloy ferritic steel 1.25Cr0.5Mo have been determined using experimental data. This alloy is used mostly in power generation and petrochemical industries because of its high temperature creep resistance. Test samples have been obtained from a new super‐heater pipe wall of a steam‐generating boiler in Tabriz Petrochemical Plant according to the ASTM standards. By conducting creep rupture tests for 1.25Cr0.5Mo steel, creep behaviour and creep‐rupture properties were examined for this material. Creep rupture tests have been carried out at four temperatures of 700, 725, 750 and 800 °C, under applied uni‐axial stresses of 30, 35, 40 and 50 MPa. The experimental data have been used to obtain the constitutive parameters using numerical optimisation techniques. Also the temperature and stress dependency of the creep lifetime for this alloy has been investigated using Larson–Miller and Monkman–Grant parameters. The results show good agreement with other test data such as ASTM and API. Finally, these constitutive equations have been used to study the creep behaviour of the super‐heater pipe. The results show that the super‐heater tube has been over designed in terms of the creep lifetime and this is in accordance with the in‐plant observations.     

Behavior of AISI 316L Steel Subjected to Uniaxial State of Stress at Elevated Temperatures

This paper presents an experimental investigation on an AISI 316L stainless steel regarding mechanical properties and short uniaxial creep tests at elevated temperatures. The short time creep tests were carried out under different but constant stresses. The obtained data of ultimate tensile strength, yield strength, creep curves and effects of elevated temperatures on mechanical properties were presented. For a selected rheological model,material parameters were obtained. As a justification, such rheological model is implemented in the finite element procedure for an uniaxially stressed specimen in selected environmental conditions.     

Structural instability of cold worked alloy 304 in 650 °C service

A heavily worked 304 stainless steel wire basket recrystallized and distorted while in service at 650 °C (1200 °F). This case study demonstrates that heavily cold worked austenitic stainless steel components can experience large losses in creep strength, and potentially structural collapse, under elevated temperature service, even at temperatures more than 300 °C (540 °F) below the normal solution annealing temperature. The creep strength of the recrystallized 304/304L steel was more than 1000 times less than that achievable with solution annealed 304H. These observations are consistent with limitations (2000 Addendum to ASME Boiler and Pressure Vessel Code) on the use of cold worked austenitic stainless steels for elevated temperature service.     

RATCHETTING IN PRESSURISED PIPES

Abstract— The plastic deformation of thin-walled cylinders has been experimentally examined for the loading conditions of ±1% axial strain with hoop stresses of approximately 0, 1/4, 1/2 and 3/4 of the initial uniaxial yield stress.
Two materials similar to those used in the pipework of PWR nuclear plant in the U.K. have been tested, namely 304S11 stainless steel and En6 low-carbon steel. The results of the tests were to be compared with the allowable stresses and deformations specified in the ASME Boiler and Pressure Vessel Code, Section III. The code specifies that a prescribed combination of primary stresses must not exceed 1.5 S _m, where S _m is a stress value defined for each material.
The results indicate that the limit of 1.5 S _m is excessively low for both materials and that in particular, the stainless steel could tolerate 5 S _m. Although the En6 steel is more prone to ratchetting than the stainless steel, the results suggest that it too could tolerate a higher primary stress than the code allows. Both materials are shown to satisfy the proposed ASME ratchet strain limit of 5% hoop strain after 10 cycles of ±1% axial strain range, for any value of internal pressure.     

Super304H钢拉伸流变特性的研究

采用电液式万能试验机测试了奥氏体不锈钢Super304H在20～600℃、不同应变率下的流变应力和应变的关系。实验结果表明,应变速率和变形温度的变化强烈地影响Super304H钢的流变应力,在高温下出现明显的动态软化。根据得到的流变应力曲线,拟合出了JC模型和其修正的JC模型中的相关参数。经与实验对比验证,修正的JC本构模型能够很好地描述Super304H钢的动态力学性能,为生产工艺的制定和钢管质量的控制提供了重要的材料参数。     

S30408奥氏体不锈钢高温力学性能试验研究

不锈钢材料高温力学性能是不锈钢结构抗火设计与数值模拟分析的重要参数。相对于碳素钢,不锈钢材料具有强非线性、比例极限低、无明显屈服平台、各向异性、应变硬化显著等特点,高温下两种材料的力学性能存在着较大的差异。该文利用MTS810试验系统对S30408奥氏体不锈钢进行了常温、高温稳态和高温瞬态试验研究,并将试验结果与《欧洲规范》以及已有的研究结果进行了对比分析。利用稳态试验结果,对影响高温下不锈钢材料力学性能的硬化指数n_θ和m_θ进行了修正,提出高温下不锈钢材料本构关系表达式,并给出了高温下不锈钢材料的初始弹性模量、屈服强度和极限强度的折减系数。对比分析了稳态试验结果与瞬态试验结果,结果表明：通过两种试验方法获得的高温下不锈钢材料应力-应变曲线存在一定的差异;在温度600℃范围内,差异不明显,当温度超过600℃时,两者差异随温度升高而逐渐增大。     

Investigation the effect of crystal orientation of nickel‐based single crystal superalloys on bending creep tests

The relationship between the three points bending creep test and the uni‐axial creep test on the single crystal superalloy was investigated by using crystal plasticity slip theory with a three‐dimensional (3D) finite element model. The purpose of the present work is to build the relationship between bending creep and conventional uni‐axial tensile creep in determining crystallographic creep parameters for face centered cubic (FCC) nickel‐based single crystal superalloys. To this aim, the bending creep performed on [001]‐, [011]‐, and [111]‐oriented nickel‐based single crystal superalloys were respectively investigated, and the data was compared with those obtained with uni‐axial tensile creep counterparts. It shows that the determination of crystallographic creep stress exponent is independent of crystallographic orientations, and the results agree reasonably well between bending creep test and uni‐axial tensile creep test. The findings may shed some light on understanding of the crystal structures and its time‐dependent deformation mechanisms with the bending creep method.     

Some factors exerting an influence on the coaxing effect of austenitic stainless steels

The present paper describes some factors exerting an influence on the coaxing effect of austenitic stainless steels. Particularly, the influence of prestrain was investigated in detail. The materials used were austenitic stainless steels, type 304 and 316. Type 304N2 was also used to examine the properties of the stabilized austenitic phase in type 304. Two types of rotating bending fatigue tests, i.e. the conventional constant amplitude tests and stress‐incremental tests, were performed using the specimens subjected to the several tensile‐prestrain levels. Under the constant amplitude tests, the fatigue strengths of type 304 and 316 increased with increasing prestrain. Under the stress‐incremental tests, type 304 showed a remarkable coaxing effect, where the fatigue failure stress significantly increased regardless of the prestrain level. The coaxing effect in the unprestrained specimens was larger than those of the prestrained ones. Type 304N2 showed lower coaxing effect than type 304. In addition, the strain‐induced martensitic transformation did not occur because of the higher stability of austenitic phase in type 304N2. In type 316, the coaxing effect was dependent on the prestrain level, i.e. below 15% prestrain the coaxing effect became smaller with increasing prestrain, whereas above 25% prestrain the coaxing effect reappeared. Based on the tests results, it was considered that the coaxing effect in austenitic stainless steel was due to the mechanisms such as work hardening, strain ageing and strain‐induced martensitic transformation. The contribution of these mechanisms to the coaxing effect was different among type 304, 304N2 and 316.     

A computational comparison of the inelastic constitutive models of hart and miller

Summary The uniaxial response behavior of Hart's and Miller's nonelastic constitutive equations is compared. These models have been selected because they are fully developed and have been applied on the uniaxial nonelastic response behavior of different materials. Among these for stainless steel AISI 316 the complete set of material parameters for both models has been published. Based on these parameter sets a comparison of both models is performed including monotonic strain controlled tensile tests, creep tests, load relaxation tests and cyclic tests. The predictions of both models are compared with available experimental data.Both models can not describe the whole range of experimental data. For Hart's model one essential flow parameter had to be adjusted to obtain a reasonable simulation of creep experiments. Further it gives unrealistic predictions for strain cycling. The incorporation of a so-called negative strain rate sensitivity severly restricts the practical applicability of Miller's model. Additionally in the high temperature regime the response curves for load relaxation tests deviate considerably from the experimentally observed ones at low strain rates. Both models have to be improved for practical applications.With 11 Figures     

An approach to modeling time-dependent creep and residual stress relaxation around cold worked holes in aluminium alloys at room temperature

Creep behaviour of aluminium alloys is also observed at room temperature. As a result, a relaxation occurs of deliberately introduced beneficial residual stresses around fastener holes, before the relevant structural component is subjected to exploitation. Therefore, to adequately asses the life-time of the component with cold worked holes, it is necessary to quantify this relaxation. In this paper a combined iterative approach for building a time-dependent creep constitutive model of aluminium alloys at room temperature has been developed in order to be used in finite element (FE) simulations of the cold hole working process. The approach is based on an experimental study of the change in diameters of cold worked holes through mandrel cold working method and a subsequent series of FE simulations of the cold working process and of the following creep behaviour to determine the necessary equivalent stresses in the constitutive model. The obtained creep constitutive model has been founded on the power-law model. The model parameters A, n and m have been determined on the basis of a developed by the authors algorithm. The approach has been illustrated on D16T aluminium alloy widely used in the airspace industry. The material behaviour in the plastic field has been described by the nonlinear kinematic hardening model, obtained through a uniaxial tensile test. Both constitutive models have been used in FE simulations of the cold working processes and of subsequent residual stress relaxation around the cold worked open holes due to creep at room temperature. On the base of the FE results, mathematical models describing the residual stress relaxation have been obtained. Thus, the residual stresses are adequately evaluated immediately before introducing the structural component in operation.     

屈服准则对不锈钢弯管变形预测精度影响

通过XRD衍射及不同方向单向拉伸试验验证,经过多道次拉拔生产出的304奥氏体不锈钢管材存在有明显的各向异性现象.采用Mises各向同性、Hill1948和Barlat1991各向异性屈服准则对不锈钢弯管过程进行有限元模拟,分析弯曲后管材内外侧壁厚分布、弯曲角度及管坯截面椭圆度的变化规律,通过模拟与实验结果对比发现,当实验数据较少时,采用Hill1948各向异性屈服准则,能够很好预测304奥氏体不锈钢管材弯曲成形过程,而Barlat1991各向异性屈服准则中的一些参数经过近似后,对成形行为的预测精度明显降低.     

Torsional fatigue with axial constant stress of oligo‐crystalline 316L stainless steel thin wire

A series of symmetric torsional fatigue with axial constant stress tests, a kind of multiaxial fatigue test, was conducted on oligo‐crystalline 316L stainless steel thin wire, which was less than 3.5 grains across diameter of 200 μm. The material presents significant cyclic hardening under symmetric torsion cycling, and hardening is more obvious with the increasing shear strain amplitude. However, symmetric torsional cycle with constant axial stresses tests characterize rapid initial hardening and then gradually softening until fatigue failure. The axial stress has a great effect on torsional fatigue life. Fractography observation shows a mixed failure mode combined torsional fatigue with tensile strain because of axial tensile stress. A newly proposed model with axial stress damage parameter is used to predict the torsional fatigue life with constant axial stress of small scale thin wire.     

FATIGUE DESIGN OF SPOT-WELDED AUSTENITIC AND DUPLEX STAINLESS SHEET STEELS

The fatigue strength of spot-welded stainless sheet steels has been investigated. The main part of the fatigue tests was performed on a cold rolled austenitic stainless sheet steel (AISI304) in air at ambient temperature. For comparison, a duplex stainless steel (SAF2304) of similar yield strength as AISI304 was also incorporated into the test programme. Since the fatigue strength of spot-welded joints depends on the mode of loading, both shear-loaded and peel-loaded joints were tested. The fatigue strength of the spot-welded stainless steels was found to decrease with decreasing sheet thickness. Furthermore, the fatigue strength for peel-loaded joints is lower than that of shear-loaded joint for sheets of equal thickness.
The local loading conditions at the weld edge have been analysed in terms of finite element calculations and fracture mechanics. A design parameter derived from a fracture mechanics analysis was defined for spot-welded stainless sheet steels. It was shown to predict the fatigue life of the present steels and joint configurations in a satisfactory way.     

Classification of metallic alloys for fatigue damage accumulation: A conservative model under strain control for 304 stainless steels

Fully reversed uniaxial tests performed under total strain and stress control on 304 stainless steels specimens show that, under strain control the fatigue damage for High–Low (H–L) cycling is more significant than that using Miner’s rule, but under stress control opposite results are obtained. This has been attributed to opposite effects of pre-hardening under strain and stress control. Classical non linear damage accumulation models are not able to take into account this difference in sequence effect. Smith–Watson–Topper (SWT) and Fatemi–Socie (FS) criterion combined to linear damage accumulation can take into account this difference in sequence effect through the presence of maximum stress. However these models require an elastic–plastic constitutive law which is difficult to propose due to the presence of high cycle secondary hardening observed on 304 stainless steel. A conservative model for damage accumulation under variable amplitude strain control loading is thus proposed, which does not require a constitutive law. Linear damage accumulation is used, while sequence effect is taken into account using the elastic–plastic memory effect through cyclic strain–stress curves (CSSC) with pre-hardening. This modeling classifies metallic alloys in two groups for damage accumulation, with a stable (independent to pre-hardening) CSSC as for aluminum alloys and with an unstable (dependent to pre-hardening) one as for austenitic stainless steels. For the former case the modeling is identical to Miner’s rule. The modeling is approved based on a large number of tests on 304 stainless steel and is compared with SWT and FS models. In presence of mean stress the modeling permits in a qualitative way to explain the fact that tensile mean stresses in constant amplitude strain control tests are more detrimental than for constant amplitude stress control tests. Moreover it is shown that the SWT model is not always able to predict accurately the fatigue life in presence of a mean stress. Finally, it is concluded that for a 304 stainless steel, in order to take into account the mean stress in fatigue life, the mean stress effect has to be decomposed into two parts: maximum and “intrinsic” mean stress effects.     

Experimental evaluation of the interaction effect between plastic and creep deformation

An experimental study of plasticity-creep interaction effects is reported. The combined stress tests are performed on thin wall tubular specimens of SUS 304 stainless steel at room temperature and high temperature (600° C). The plastic behaviors subsequent to creep pre-strain and creep behaviors subsequent to plastic pre-strain are obtained for loading along straight stress paths with a corner. The inelastic behaviors including both plastic and creep deformations are experimentally investigated. The interaction effects between plastic and creep deformations are quantitatively estimated with the equi-plastic strain surface.     

基于材料全程本构关系对延性断裂韧性的数值模拟方法与应用

利用ANSYS APDL编程语言,采用FAT (Finite-element-analysis Aided Testing)方法获取26NiCrMoV11-5汽轮机转子材料、A508-III压力容器钢和304不锈钢材料全程单轴拉伸本构关系曲线。借助ABAQUS软件中GTN (Gurson-Tvergaard-Needleman)损伤模型,采用有限元逆向法对3种材料漏斗试样单轴拉伸试验过程进行数值仿真,标定相应的GTN模型参数,获得了较好的结果。在此基础上将3种材料GTN模型参数用以模拟紧凑拉伸(Compact Tension,CT)试样的准静态裂纹扩展,基于载荷分离法实现了对26NiCrMoV11-5、A508-III和304不锈钢3种材料J阻力曲线的良好预测。     

Experimental study of remaining creep life of SA-304L stainless steel using small punch creep test

In this paper, the remaining creep life of SA-304L stainless steel at elevated temperatures is studied. The small punch creep tests (SPCT) were performed on SA-304L virgin and aged materials at constant loads. Times to fracture and the minimum deflection rate in SPCTs were recorded, and the time-temperature parametric analysis was performed, based on experimental results. The constants of the Larson–Miller parameter and the Monkman–Grant relationship were obtained for different consumed creep life ratios, and eventually the equations were established to explain the variation of constants of the Larson–Miller parameter and the Monkman-Grant relationship with respect to the consumed creep life ratio.     

Variable amplitude cyclic deformation and fatigue behaviour of stainless steel 304L including step,periodic, and random loadings

This paper discusses cyclic deformation and fatigue behaviours of stainless steel 304L and aluminium 7075‐T6 under variable amplitude loading using strain‐controlled as well as load‐controlled tests. Load sequence effects were investigated in step tests with high‐low and low‐high sequences. For stainless steel 304L, strong hardening induced by the first step of the H‐L sequence significantly affects the fatigue behaviour, depending on the test control mode used. For periodic overload tests of stainless steel 304L, hardening due to the overloads was progressive throughout life and more significant than in H‐L step tests. For aluminium 7075‐T6, no effect on deformation behaviour was observed due to periodic overloads. However, the direction of the overloads was found to affect fatigue life, as tensile overloads led to longer lives, while compressive overloads led to shorter lives. Deformation and fatigue behaviours under random loading were also studied for the two materials. To correlate a broad range of fatigue life data for a material with strong deformation history effect, such as stainless steel, it is shown that a damage parameter with both stress and strain is required. The Smith‐Watson‐Topper parameter as such a parameter is shown to correlate the data reasonably well under different control modes and loading conditions.     

Effects of thermal creep of prestressed steel on post‐tensioned concrete slabs in and after fire

The effects of thermal creep of prestressed steel on post‐tensioned concrete slabs in and after fire were investigated based on an existing thermal creep model and calibrated parameters in this paper. A nonlinear finite element model was built up employing ABAQUS package, taking into account frictionless contact behaviour between prestressed steel tendons and surrounding concrete. The nonlinear material behaviour of concrete and prestressed steel at elevated temperatures was taken into account, where three material models for prestressed steel were adopted with or without considering thermal creep, and based on the model from EN 1992‐1‐2. The finite element model developed was verified against experimental results from the literature, showing that the model considering thermal creep was more accurate. Then the fire resistance period and responses of post‐tensioned concrete slabs in and after fire were investigated based on the verified model. Ignoring thermal creep underestimated the fire resistance period but overestimated the residual tendon stresses. The model from EN 1992‐1‐2 achieved nearly the same effects as the model considering thermal creep in fire but might yield inaccurate evaluation of residual tendon stresses. The model considering thermal creep worked well under fire and in the post‐fire conditions yielding reasonable predictions.