Experimental Study of the Creep Behaviour of the Cold‐Drawn 304L Stainless Steel

Abstract: In this article, the material and physical parameters for the creep constitutive equations of cold‐drawn 304L stainless steel have been determined using experimental data. Austenitic stainless steel 304L is used mostly in power generation and petrochemical industries because of its high‐temperature creep resistance even at above yield stresses. Test samples have been obtained from cold‐drawn bars, and the material conforms to ASTM A276‐05a specifications. The creep behaviour and properties have been examined for this material by conducting uni‐axial creep tests. Constant temperature and constant load uni‐axial creep tests have been carried out at three temperatures of 680,700 and 720 °C, subjected to constant loads which produce below and above yield initial stresses of 200, 250, 320, 340 and 360 MPa. The experimental data have been used to obtain the creep constitutive parameters using numerical optimisation techniques. In addition, the temperature and stress dependency of the creep properties for this alloy have been investigated using Larson–Miller and Monkman–Grant parameters.     

Determination of material constants in creep continuum damage constitutive equations

Experimental creep and creep rupture data are presented for copper at 150, 200 and 250°C and for 0.5Cr0.5Mo0.25V steel at 640°C. Creep continuum damage constitutive equations have been shown to be capable of accurately representing the creep behaviour of both materials. A six variable optimisation program, which was developed to efficiently obtain the material constants, is described.     

Micro–macro creep damage simulation for welded joints

Abstract

Assessment of Type IV creep damage in pipe welds is important for residual life prediction of fossil power plants. Actual creep damage is firstly microscopic, such as the initiation and coalescence of small defects, and lastly macroscopic, such as the propagation of crack-like defects. In this paper, an outline of the micro – macro combined creep damage simulation on the basis of the grain-boundary-fracture-resistance model is shown, and is applied to the creep damage simulation of both low-alloy steel welds and high-chromium steel welds. Firstly modelling of the simulation for low-alloy steel welds such as 2.25Cr –1Mo steel (ASME P-22) is discussed, and application examples are shown such as creep rupture specimens in the laboratory and welds in actual power piping. Secondly a trial is carried out to reproduce the number density of small defects in the heat-affected zone of welded joints of modified 9Cr –1Mo steel (ASME P-91). The possibility of predicting the microscopic damage is demonstrated.     

Mechanistic approach for prediction of creep deformation,damage and rupture life of different Cr–Mo ferritic steels

Abstract

A mechanistic approach based on finite element analysis of continuum damage as proposed by Kachanov has been used to assess and compare creep deformation, damage and rupture behaviour of 2·25Cr–1Mo, 9Cr–1Mo and modified 9Cr–1Mo ferritic steels. Creep tests were carried out on the steels at 873 K over a stress range of 90–230 MPa. Modified 9Cr–1Mo steel was found to have highest creep deformation and rupture strength whereas 2·25Cr–1Mo steel showed the lowest among the three ferritic steels. Creep damage in the steels has been manifested as the microstructural degradation. 2·25Cr–1Mo steel was more prone to creep damage than 9Cr–steels. Finite element estimation of creep deformation and rupture lives were found to be in good agreement with the experimental results.     

底注钢锭模设计及特种Cr-Ni、Cr-Mo钢锻件的试制

指出了Ｃｒ Ｍｏ钢、Ｃｒ Ｎｉ钢等特种钢锻件生产中存在的工艺问题。设计了专用的底注钢锭模,试制出符合ＡＳＴＭＡ１８２－７８标准要求的Ｃｒ９Ｍｏ、１．２５Ｃｒ０．５Ｍｏ、Ｃｒ２５Ｎｉ２０钢锻件,解决了这些特种钢的原材料供应难题。     

Prediction of deformation and failure of modified 9Cr–1Mo steel under creep-fatigue interaction

Abstract

Plant components often undergo loadings which have aspects of both fatigue and creep by experiencing repeated start-up/shut-downs between steady-state operation at high temperature. In such a case, structural materials show inelastic deformation different from that observed under pure fatigue or pure creep conditions. This finding prompted the development of “unified” constitutive models in which cyclic and creep deformations are treated in a unified way. The author’s group has been developing such a unified model for modified 9Cr–1Mo steel which is widely used in ultrasupercritical fossil power plants. Although the latest model has been demonstrated to be capable of describing deformation behaviour under various loading conditions, including cyclic and steady-state loadings, the model still has room for improvement, such as the need to represent accelerated deformation in the tertiary creep regime. In this study, an attempt is made to improve the capability of the model from various perspectives. A failure prediction model which can deal with various failure modes is also formulated. The deformation and failure lives predicted using these models show reasonable agreement with the results of various load sequence tests obtained on modified 9Cr–1Mo steel.     

Creep strain prediction of 2 1/4 Cr 1Mo steel—A model based approach

The paper examines the potential of a model based approach for predicting creep behaviour of 2 1/4 Cr 1Mo steel. Effects of prior strain and thermal aging on the shape of the creep curve have been experimentally established. Particle coarsening has been identified as the dominant mechanism of creep. Creep curves theoretically simulated using particle coarsening model have been compared with those obtained for a variety of initial microstructural conditions.     

Isochronous creep rupture loci on deviatoric plane and its application to P92 steel creep behaviour under multiaxial stress state

The transformation relationship of the coordinate variables between principal stress space and deviatoric stress plane has been deduced and the isochronous creep rupture loci of disparate criteria have been described on deviatoric stress plane so as to analyze the creep behaviour under multiaxial stress state. The creep experiments of P92 steel smooth and notched specimens subjected to various stresses at 650 °C have been conducted. A modified constitutive model for the creep of P92 steel has been proposed and used to simulate the creep of P92 steel notched specimens with FEA software. The FEA results were consistent with the experimental data and the fracture morphology observation. It was found that the Hayhurst criterion had the best correlation with the experimental results of P92 steel under multiaxial stress state than other criteria through the comparison of the isochronous creep rupture loci on deviatoric plane.     

Creep property prediction of the high chromium ferritic steel 1Cr10NiMoW2VNbN at 600°C

Abstract

High chromium (Cr) ferritic steel 1Cr10NiMoW2VNbN was the first material developed in China for ultra-supercritical power stations. The creep property of high Cr ferritic steel 1Cr10NiMoW2VNbN has been studied under constant loading conditions at 600°C. The Theta projection concept applied to the creep curves was found to yield accurate predictions when interpolating and predicting creep data under service conditions. Four Theta parameters were obtained by Theta projection concept from the creep curves. The shape of the creep curves, as well as the minimum creep rate, the time to reach a limiting strain, and the time to rupture were considered with a wide range of stresses at 600°C of the high Cr ferritic steel.     

Creep behaviour of parent,weld and HAZ materials of new,service-aged and repaired 1/2Cr1/2Mo1/4V: 2 1/4Cr1Mo pipe welds at 640°C

Abstract

The results of a series of creep tests on the materials of CrMoV weldments, in main steam pipes, are presented in this paper. The tests were performed at 640°C, using uniaxial, notched, impression and cross-weld creep test specimens. The materials involved include service-aged and ‘as-new’ 1/2Cr1/2Mo1/4V parent materials, service-aged and new 2 1/4Cr1Mo weld metals and the HAZ materials in the new, service-aged and fully repaired welds. Based on the test data obtained, material constants in creep continuum damage constitutive equations for the parent, weld and HAZ materials were determined. Direct comparison of the test data allows the creep deformation and rupture behaviour of the parent, weld and HAZ materials to be identified. The material properties of these CrMoV weldment materials can be used in numerical FE modelling to contribute to the understanding of the performance of the new, service-aged and repaired welds in similar materials.     

Comparison of creep behaviour of 2.25Cr–1Mo/9Cr–1Mo dissimilar weld joint with its base and weld metals

Abstract

Evaluation of the creep behaviour of 2.25Cr–1Mo and 9Cr–1Mo ferritic steel base metals, 9Cr–1Mo steel weld metal, and 2.25Cr–1Mo/9Cr–1Mo ferritic–ferritic dissimilar weld joints has been carried out at 823 K in the stress range 100–260 MPa. The weld joint was fabricated by shielded metal arc welding using basic coated 9Cr–1Mo electrodes. Investigations of the microstructure and hardness variations across the joint in the as welded, post-weld heat treated (973 K/1 h), and creep tested conditions were performed. The heat affected zone (HAZ) in both the steels consisted of a coarse prior austenitic grain region, a fine prior austenitic grain region, and an intercritical structure. In the post-weld heat treated condition, a white etched soft decarburised zone in 2.25Cr–1Mo steel base metal and a black etched hard carburised zone in 9Cr–1Mo steel weld metal around the weld fusion line developed. Hardness troughs also developed in the intercritical HAZ regions of both the steels. The width of the carburised and decarburised zones and hardness differences of these zones were found to increase with creep exposure. The 9Cr–1Mo steel weld metal showed higher creep strength compared to both the base metals. The 9Cr–1Mo steel base metal exhibited better creep resistance than the 2.25Cr–1Mo steel base metal at lower applied stresses. The dissimilar joint revealed lower creep rupture strength than both the base metals and weld metal. The creep strain was found to concentrate in the decarburised zone of 2.25Cr–1Mo steel and in the intercritical HAZ regions of both the steels. Creep failure in the stress range examined occurred in the intercritical HAZ of 2.25Cr–1Mo steel even though this region showed higher hardness than the decarburised zone. Extensive creep cavitation and cracks were observed in the decarburised zone.     

The 1989 European Conference on high temperature materials in engineering

Creep testing has been made on a service-exposed header of 2.25Cr1Mo steel. Four series including parent metal, weld metal and cross-weld combinations were studied. A metallographic investigation of cavitation development on ruptured samples has also been performed. A model for the stress state in a thick-wailed welded pipe has been derived. Results obtained from creep testing and metallographic investigation have been analysed and compared with the model.     

Creep deformation and rupture behaviour of directionally solidified GTD 111 superalloy

The creep behaviour of directionally solidified (DS) Ni‐base superalloy GTD 111 has been investigated at various temperatures (649 °C to 982 °C) and stresses (124 MPa to 896 MPa). Specimens machined in longitudinal and transverse directions with respect to the grain orientation from three batches of the material were tested. The specimens in the longitudinal direction consistently exhibited higher creep rupture life and creep ductility than specimens from the transverse direction. There were some systematic variations in creep deformation and rupture behaviour among specimens from different batches. Optical and scanning electron microscopy investigations were conducted to understand the creep rupture behaviour. Various deformation and rupture models were evaluated for representing the creep behaviour of the alloy and a neural network model was applied to creep rupture data to assess its predictive capability.     

34Cr2Ni2Mo 合金结构钢热本构方程研究

目的基于实测的流动应力曲线,构建可用于热成形模拟的34Cr2Ni2Mo合金结构钢高精度本构方程。方法采用热模拟试验测试该材料的流动应力曲线,在动态再结晶的条件下,构建了基于物理机制的热本构方程,通过曲线拟合获得了本构方程参数。结果热模拟试验测试的流动应力曲线具有明显的动态再结晶现象,构建的本构方程包括流动应力、屈服/饱和/临界/稳态应力、发生50%再结晶的时间等内变量计算方程,在参数拟合后对其误差分析表明,本构方程计算的流动应力偏差控制在±15 MPa以内。结论 34Cr2Ni2Mo合金结构钢本构方程能够较为准确的描述该钢在热成形过程的流动应力变化特征,具有较强的数值稳定性和外延拓展性。     

High temperature stability of 2.25Cr-1Mo steel during creep

The creep rapture behaviour of 2.25Cr—1Mo steel in air and in a salt mixture was studied. The salt coating, which can form a liquid phase at the test temperatures, increased the creep rate and reduced the rupture life of the material. The coating reduced the available cross-section of the material by removing the surface layers, thereby resulting in a reduction of the rupture life. Cross-sections of coated samples showed an outer oxide layer comprising oxide of the metal and precipitates of sulphide at the metal/oxide interface. This subsurface penetration of the corrodants was responsible for the early failure of the coated samples. This is typical of hot corrosion mechanisms. The formation of various carbides like M₂₃C₆ and M₆C, as observed by transmission electron microscopy, during creep reduced the creep strength of the material both in air and in the coated state. Increasing temperature enhanced the formation of these carbides with a consequent decrease in creep strength. Applied stress did not seem to play much of a role in the degree of carbide precipitation.     

Exploring the applicability of the LICON methodology for a 1%CrMoV steel

Abstract

The results of a series of creep crack incubation (CCI) tests have been used to examine the effectiveness of the LICON methodology for predicting long duration uniaxial rupture properties of a 1%CrMoV steel at 550°C. It has been demonstrated that good predictions can be made using this approach but that, for this to be possible, a knowledge of additional information is also required including uniaxial creep strain data, uniaxial and multi-axial rupture time properties and the results of finite element analysis.

In order to apply the creep damage enhancement approach to the low alloy creep resistant steel, it has been necessary to establish the steady-state creep stress state in the compact tension testpiece geometry used in the investigation. The new evidence is described.     

Cyclic creep behaviour under tension–tension loading cycles with hold time of modified 9Cr–1Mo steel

Abstract

Cyclic creep behaviour of modified 9Cr–1Mo steel was investigated by a series of cyclic creep (CC) tests at 600°C, which were performed under controlled tension–tension loading cycles with the magnitude of stress ranges in a constant stress ratio (R?=?0·1). Hold time was applied for a 10 min hold at the maximum stress (σ_max) and minimum stress (σ_min). The CC properties were compared with the static creep (SC) using Norton’s power law, Larson–Miller plot, and Monkman–Grant relation, and the microstructure was examined. For the test conditions employed in the present investigation, retardation in the CC behaviour in terms of a lower creep rate and longer rupture time compared to those in the SC was obtained. The retardation was ascribed to the effects associated with anelastic recovery during the 10 min hold time at the minimum load of the cyclic loading. The creep rupture ductility decreased with a general decrease in stress, and there was no difference in the creep ductility between the CC and SC. The steel displayed a transgranular fracture characterised by the presence of dimples resulting from micro-void coalescence. Carbide precipitation was more coarsened with increasing in exposure time in the CC tests.     

Procedure of Determining Creep and Creep-Rupture Strength Parameters for Isotropic Materials under Nonisothermal Loading

A procedure has been developed for determining the parameters of creep and creep-rupture strength that appear in constitutive equations of thermoviscoplasticity for describing nonisothermal processes of deformation and damage accumulation in isotropic materials due to creep. The procedure is tried out using a high-temperature chromium-nickel alloy ÉI437.     

Creep rupture behaviour of circumferentially welded mod. 9Cr–1Mo steel pipe subject to internal pressure and axial load

The multi-axial creep strength of circumferential welds in power piping, including failure mode and failure life against a wide range of stress ratios and stress levels, was newly examined in this paper. The creep rupture behaviour of modified 9Cr–1Mo steel (9Cr–1Mo–VNb steel; ASME P91) pipe with a circumferential weld subject to combined internal pressure and axial load was experimentally investigated at 650 °C. The test results, with several kinds of stress ratios of macroscopic axial and hoop stress can be summarised as follows. Along with the increased stress ratio (axial/hoop), both failure location and failure mode changed from a base metal failure caused by hoop stress to an FGHAZ (fine-grained heat affected zone) failure due to axial stress. The stress ratio where the failure mode changed was ‘0.8’. Strength reduction in FGHAZ failure by axial stress should be considered in the structural design of circumferential welds. A series of FEM creep analysis was carried out to discuss the relation of the failure mode to both the local stress distribution and the damage. The failure mode variation along with the increase in the stress ratio is also discussed based on simplified ductile creep failure analysis.     

Artificial Neural Network for Creep Behaviour Predictions of a Parallel‐lay Aramid Rope Under Varying Stresses

Abstract: An improvement of the creep behaviour prediction of parallel‐lay aramid ropes under varying stresses is the scope of the following study in which application of artificial neural networks (ANNs) for the prediction of creep under varying stresses is presented. This qualitatively different approach assumes that the ANN can be trained to simulate time‐dependent response of the rope in the given load (stress) programme and time interval. The classic rheological constitutive equations are not needed in this case, because ANN acts as a constitutive operator trained by stresses and the corresponding creep strains from experimental data. Carried numerical experiments were divided into the following three parts: (i) searching the best ANN for a creep behaviour approximation under varying stresses, (ii) investigating the best topology of the selected neural network and (iii) investigating the best results for the creep function identification. Comparison between the experimentally observed creep strains of the parallel‐lay aramid rope under varying stresses, predicted creep strains when the linear creep constitutive equation is applied and predicted creep strains when the obtained Jordan neural network with the 3‐10‐1 topology is used confirmed that the Jordan neural network developed achieved less than half mean square error beside the existing creep constitutive analytical approach.