Thermal diffusivity measurement of 2.25Cr-1Mo steel with internal friction

In this work, the internal friction of 2.25Cr-1Mo steel measured on a dynamic mechanical analyzer (DMA) was used to evaluate its thermal diffusivity at elevated temperatures. With the dual cantilever clamp, the internal friction of 2.25Cr-1Mo steel at high temperatures was obtained, which was mainly composed of the thermoelastic relaxation with a peak of 15 Hz and the broad spectrum of relaxation processes especially at low frequencies. According to the deconvolution of these results, the thermoelastic internal friction of 2.25Cr-1Mo steel at high temperatures was achieved. Then, the thermal diffusivity of 2.25Cr-1Mo steel at high temperatures was quantitatively estimated, which confirmed the validity of present method.     

The effect of stress relaxation loading cycles on the creep behaviour of 2.25Cr-1Mo pressure vessel steel

This paper investigates the effects of repeated stress relaxation loadings and post stress relaxation creep to assess the stress relaxation-creep interaction and microstructural evolution of 2.25Cr-1Mo steel. Prior to creep testing, the microstructure of the material subjected to stress relaxation exhibited a structure which was non-conservative in predicting the remaining creep life of the material. The results obtained in the test program showed that the damage due to the effects of stress relaxation was crucial and had a significant effect on the creep life of this material. The study has also shown that the extent of metallurgical degradation, due to stress relaxation, may not be evident through microstructural assessment. Consequently, established life assessment procedures may not represent conservative estimates of remaining life because the microstructural indicators of damage, due to stress relaxation, are not evident.The effects of stress relaxation on the creep properties of ferritic pressure vessel steel are life limiting for critical high temperature power generation plant. In this study a comprehensive test program has been undertaken to assess the interaction of stress relaxation with creep and microstructural evolution in 2.25Cr-1Mo steel.     

Testing of welded 2.25CrMo steel, in hot, high-pressure hydrogen under creep conditions

The standard 2.25Cr-1Mo steel, has been used for most of the hydroprocessing reactors built during the 30 years up to the end of the 1990’s. Under service conditions, the steel, and in particular its weldments, may experience degradation by “hydrogen attack”, or high-temperature hydrogen attack (HTHA), which is responsible for several failures and even casualties occurring in the history of the industry. The selection of the structural steels is still based on the empirical Nelson curves which, among other limitations, do not provide information on the behaviour of weldments. This causes uncertainties in the definition of the safety boundaries for operation of pressurized components in the petrochemical industry. The results of the present study confirm that weldments are a weak point of the pressurized components and suggest that the HAZ, and in particular the ICHAZ, is the most sensitive metallurgical zone to hydrogen attack.     

2.25Cr-1Mo钢后续热处理中的磷偏聚行为

根据溶质晶界偏聚理论和原子扩散原理研究了2.25Cr-1Mo钢服役12年后磷晶界偏聚行为及脱脆处理工艺。采用俄歇电子能谱实验对溶质原子的晶界偏聚程度进行了测试分析。结果表明:对于已脆化的2.25Cr-1Mo钢,存在临界脆化温度,且脆化程度越高,临界脆化温度越低;高于临界脆化温度数小时的保温即可消除脆化现象,即2.25Cr-1Mo钢的脱脆速率远大于脆化速率。     

Application of Electron Energy Loss Spectroscopy and Energy Filtering Transmission Electron Microscopy for Microchemical Studies in 2.25Cr-1Mo Steel

Electron enregy loss spectroscopy (EELS) and energy filtering transmission electron microscopy (EFTEM) investigation on 2.25Cr-1Mo steel was carried out to understand the nature of evolution of secondary carbides. The filtered images obtained from two different ageing treatments indicate that the steel evolves to a more stable carbide namely M23C6 in comparison to M2C. Microchemical information was generated from EELS spectra. Suitable choice for estimating the microchemical state was discussed. To evaluate the behaviour of ageing an elemental ratio of Fe to Cr is employed.     

Creep-fatigue life evaluation for weldments of modified 9Cr-1Mo steel at boiler temperatures

Creep-fatigue tests were conducted to verify long-term structural integrity for weldments of modified 9Cr-1Mo steel at boiler plants. As a result, it was proved that the weldments were fractured mainly at the HAZ and the fatigue lives were reduced by strain hold. This appeared to be due to acceleration of crack initiation by creep strain concentration and acceleration of crack growth by void generation in the HAZ. A method for predicting the life is proposed based on the fracture mechanism, and its accuracy was confirmed to be within a factor of two.     

Influence of joint thickness on Type IV cracking behaviour of modified 9Cr-1Mo steel weld joint

Effect of joint thickness on Type IV cracking behaviour of modified 9Cr-1Mo steel weld joint has been investigated. Creep tests on multi-pass double-V cross weld joint flat specimens of the steel having thicknesses in the range 1–17 mm have been carried out at 923 K (650°C) and 50 MPa stress. Creep rupture life of the weld joint was found to increase with thickness and reached a maximum value around 10 mm of thickness followed by decrease with further increase in thickness. Failure in the weld joints occurred in the soft intercritical region of the heat-affected zone (HAZ). Creep strain localisation was observed at the fractured location and was more in the thinner weld joints than in the thicker weld joint. Creep cavitation in the intercritical region of HAZ close to the unaffected base metal was more extensive at the mid-location of the weld pass, where the HAZ width was relatively larger and hardness was lowest. The type IV cavitation in intercritical HAZ was more extensive in thicker joint, whereas creep strain concentration in the intercritical HAZ was more in thinner weld joint. Creep cavitation in the joint was more pronounced at near mid-thickness locations than those beneath the specimen surface. Joints of intermediate thickness possessed higher creep rupture life because of relatively less accumulation of creep deformation coupled with lower creep cavitation in the intercritical region of HAZ.     

An improvement in creep strength of thermo-mechanical treated modified 9Cr-1Mo steel weld joint

Modified 9Cr-1Mo steel weld joints generally experience the type IV premature failure in the intercritical region (ICR) of HAZ under long term creep exposure at high temperature. Possibility of improving the resistance of this joint to type IV cracking through thermo-mechanical treatment (TMT) of the steel has been explored. Weld joints have been fabricated from the TMT and conventional normalized and tempered (NT) steels using electron beam (EB) welding process. Creep tests have been carried out on NT and TMT steels joint at 923 K (650°C) and 110–100 MPa applied stress. Creep rupture life of the TMT weld joint was significantly higher than the NT steel weld joint. Significant variations of microstructural constituents such as M₂₃C₆ precipitate; lath structure and hardness across the joint have been examined in both the joints. The coarser M₂₃C₆ precipitate and lath, and subgrain formation in the ICR resulted in the soft zone formation and was predominant in the ICR of NT steel joint. The enhanced MX precipitation through TMT processing and reduction in coarsening of M₂₃C₆ precipitate under thermal cycle resulted in improved creep rupture strength of TMT steel weld joint.     

Fatigue life estimation of notched specimens of modified 9Cr-1Mo steel under uniaxial cyclic loading

Accuracy in the estimation of low cycle fatigue life of modified 9Cr-1Mo steel notched specimen by different analytical methods such as linear rule, Neuber’s rule, strain energy density method and numerical method such as finite element analysis have been studied in this investigation. The fatigue tests on notched specimens having notch radius of 1.25 mm, 2.5 mm and 5.0 mm were carried out at 823 K with net stress amplitudes of 250 MPa, 300 MPa and 350 MPa. The fatigue tests on smooth specimens were carried out with strain amplitudes ranging from ±0.3% to ±0.8% with a strain rate of 3 × 10^?3 s^?1 at 823 K to evaluate the fatigue life of notched specimen through strain-life approach. In order to predict the cyclic stress response of the material, Chaboche non-linear hardening model was employed considering two back stress components. Predicted hysteresis loops for smooth specimen were well in agreement with experimental results. Estimated fatigue lives of notched specimens by analytical methods and finite element analysis were within a factor ±16 and ±2.5 of the experimental lives respectively.     

2.25Cr-1Mo-O.25V低合金耐热钢中贝氏体的形成机制研究

采用真空冶炼炉和可逆式轧机制备出试验材料,将其在940℃奥氏体化后分别水淬到550℃、650℃和750℃,再空冷至室温,然后采用扫描电镜和透射电镜观察材料的显微组织,发现分别主要为下贝氏体、上贝氏体和粒状贝氏体.上贝氏体组织和下贝氏体组织均以扩散机制形成,而区域冷却速率的差异是造成粒状贝氏体组织基体形貌差异的主要原因.     

Applicability of C parameter in assessing Type IV creep cracking in Mod. 9Cr-1Mo steel welded joint

A welded joint of Mod. 9Cr-1Mo steel, whose Type IV cracking behavior is an important issue to be assessed, was subjected to a series of creep crack propagation experiments in order to clarify the applicability of existing standard ASTM E1457-98. Standard 1T-C(T) specimens made of base metal (BM) and welded joint (WJ), in which the heat affected zone (HAZ) was set to be the crack plane, were subjected to the experiments under 600, 650, and 700 °C, and with a few load level conditions. While the crack planes of BM specimens were fairly flat, those in WJ specimens showed bumpy surfaces following the shape of multi-path weld beads. The cracks in WJ specimens were of typical Type IV cracking, and their crack passed through nearby the interface of BM and weld metal. There are the fairly good relationships between the creep crack propagation rate (da/dt) and C^* parameter. All the BM and WJ data fallen in each one C^*-da/dt relationship for BM and WJ, respectively, regardless of the temperature and load level. The C^* parameter used here is defined for the homogeneous material and does not give a physically correct C^* for WJ, nevertheless all the WJ data still tends to gather each other on single C^*-da/dt relationship. This fact suggests that the geometrical limitations of E1457-98 standard also can act well as the limitation for the inhomogeneity of weld structure and may eliminate the effect of large scale inhomogeneity due to the combination of BM and weld metal. The da/dt of WJ were about 3-10 times faster than that of BM for the same C^* value. This difference can be attributed as the effect of difference in triaxiality, the relative constraint between the weld metal and the base metal, or the difference in resistance for creep crack propagation in HAZ material.     

Creep behaviour leading to Type IV cracking for service-exposed 1.25Cr-0.5Mo steel welds

To ensure reliability of elevated temperature components, the creep behaviour of weldment must be predicted since the ultimate failures mostly take place at this tiny region. In the case of low alloy ferritic steels, the most likely failure mode of equipment operated for long hours should be Type IV cracking, which is defined as preferential damage evolution at the Intercritical HAZ (ICZ). Despite the importance of this phenomenon, there have been some uncertainties remained unsolved. In order to elucidate the cause and accelerating factors of Type IV cracking, creep behaviours of cross-weld and the ICZ microstructure have been examined in the present work using service-exposed 1.25Cr-0.5Mo steel.Onset time to Type IV failure significantly reduced when tested by spirally notched cross-weld specimens as a result of concentrated damage accumulation at the root of a vee notch, revealing that multiaxaial stress state could play a key role in Type IV failure.The feature of creep damage suggests that grain boundary damage leading to Type IV cracking is caused by the sliding of grain boundaries around fine grains which are considered to be the products of partial transformation during welding. Heterogeneous damage evolution to the level of facet cracking surrounded by damage free grains raises the fundamental question on the validity of a generally accepted assumption, namely, that stress of grains associated with a grain boundary cavity will be off-loaded. As a matter of fact, a clear evidence that grain boundary cavitation accelerates the strain rate at the tertiary regime has not been observed in creep curves of simulated ICZ specimens, owning a bimodal microstructure expected at the ICZ in whole gauge length.Difference in the susceptibility to Type IV cracking has been found in materials with the same alloying elements and the vulnerability of the ICZ microstructure is not necessarily dependent upon creep strength of parent material.Considerable metallurgical factors to shorten the onset time to Type IV damage and the effectiveness of strain rate measurement as a potential technique for the life assessment shall be discussed.     

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.     

Prediction of Type IV creep failure of a seam-welded mod. 9Cr-1Mo elbow based on microscopic damage simulation

Utilising the random-fracture-resistance model of grain boundaries, micro-macro combined creep damage simulation was applied to the prediction of the distribution of small defects in the FGHAZ (fine-grained heat-affected zone) of longitudinal welds in an actual-size elbow of modified 9Cr-1Mo (9Cr-1MoVNb) steel subject to internal pressure at 923 K. Based on the simulation results, a prediction scheme for the final rupture life of welds was considered using the damage mechanics concept together with effective stress. The applicability of nonlinear fracture mechanics was also discussed, assuming the initial crack length determined from the microscopic simulation results. The results thus obtained are summarized as follows: As the simulation results showed, the peaks of small defect density in the subsurface could be predicted, corresponding well with the observed results. Final failure life prediction based on the damage mechanics concept was found to be applicable, by considering both the final failure surface connecting the weakest grain boundaries and the effective stress against this surface. The fracture mechanics approach was also found applicable when assuming the initial crack length from the high peaks of the simulated small defects in the last stage of creep life.     

Characterization of Hydrogen Embrittlement in 2.25cr-1mo-0.25v Steel by Eddy Current Method

Eddy current method has been recently developed to characterize mechanical properties of materials and assess internal hydrogen content of high strength low alloy steels. The application of eddy current testing in evaluating hydrogen embrittlement state of 2.25Cr-1Mo-0.25V ferritic steel has been investigated using tensile test and electrochemical hydrogen charging test. The relationship between the embrittlement index and eddy current signal is well established. It found that there is a good linear relation between the hydrogen-induced plasticity loss and the eddy current signal. This shows eddy current testing is an effective method for evaluating hydrogen embrittlement state of 2.25Cr-1Mo-0.25V steel.     

Characterization of creep-fatigue crack growth behavior in 2.25 Cr-1 Mo steel using (C t)avg

Time-dependent creep-fatigue crack growth (CFCG) is an important consideration in the design and remaining life estimation of high temperature components. CFCG tests were carried out on compact type (CT) specimens of 2.25 Cr-1.0 Mo steel and its behavior, for hold times ranging from 10 seconds to 50 seconds, at 594°C (1100°F) was characterized using the average value of the C _t-parameter, (C _t)_avg. The trends in the creep-crack growth (CCG) data for this material are also compared with the CFCG data. The analytically estimated values of (C _t)_avg are compared with the experimental values of (C _t)_avg obtained from the measured values of load-line deflection rates. It is also shown that even in the absence of accurate creep deformation constants, accurate estimates of the measured values of (C _t)_avg can be obtained in CT specimens     

Optimization of Welding Process Parameters for 9Cr-1Mo Steel Using RSM and GA

Optimization of A-TIG welding process parameters for 9Cr-1Mo steel has been carried out using response surface methodology (RSM) and genetic algorithm (GA). RSM has been used to obtain the design matrix for generating data on the influence of process parameters on the response variables. A second-order response surface model was developed for predicting the response for the set of given input variables. Then, numerical and graphical optimization was performed using RSM to obtain the target depth of penetration (DOP) and heat-affected zone (HAZ) width using desirability approach. Multiple regression models were developed based on the generated data, and then the models were used in GA to determine the optimum process parameters for achieving the target DOP and HAZ width. GA-based models employed two different selection processes. Both the RSM- and GA-based models suggested a number of solutions in terms of process parameters, and the identified solutions were validated by experiments. GA-based model employing tournament selection has been found to be a more accurate method for determining the optimum A-TIG welding process parameters.     

The Ductile to Brittle Transition Behavior of the Modified 9Cr-1Mo Steel and Its Laser Welds

The ductile to brittle transition temperature (DBTT) of the modified 9Cr-1Mo steel and its laser welds was studied. The increase in grain size of the weld structure ascended the DBTT of the steel significantly. The transformation of retained austenite at martensite interlath boundaries into untempered and/or twinned martensite could also contribute to increased DBTTs of the steel and its welds tempered at 540℃.     

The Ductile to Brittle Transition Behavior of the Modified 9Cr-1Mo Steel and Its Laser Welds

The ductile to brittle transition temperature (DBTT) of the modified 9Cr-lMo steel and its laser welds was studied. The increase in grain size of the weld structure ascended the DBTT of the steel significantly. The transformation of retained austenite at martensite interlath boundaries into untempered and/or twinned martensite could also contribute to increased DBTTs of the steel and its welds tempered at 540℃.     

金红石对1Cr-0.5Mo合金系焊条工艺性能和熔敷金属力学性能的影响

研究了金红石对1Cr-0.5Mo合金系焊条工艺性能和熔敷金属力学性能的影响。结果表明,金红石对该合金系焊条的工艺性能和熔敷金属力学性能均有较大的影响。随着药皮中金红石加入量的增加,焊条工艺性能得到改善,熔敷金属的强度增加,-20℃夏比冲击功降低。