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.     

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。     

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℃.     

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.     

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.     

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.     

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.     

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.     

Damage mechanism in weldment of 2.25Cr-1Mo steel under creep-fatigue loading

The purpose of this study is to clarify damage process of “Type IV cracking” in weldment of a 2.25Cr-1Mo steel and to propose a micro damage prediction method. From continuous observation under a creep-fatigue test, it was found that spherical shape voids initiated and grew on grain boundaries in fine grain region and these voids grow continuously by changing their shape to crack-like. Both spherical and crack-like void growth rate equations were derived from the proposed void growth model. It was indicated that measured void growth rate under the creep-fatigue loading was well predicted by the growth rate equations.     

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.     

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.     

Cr9Mo铸钢组织和性能的研究

研究了Cr9Mo铸钢组织和性能之间的关系。依据试验结果,指出了热处理工艺变化对其组织、性能的影响,并与Cr5Mo铸钢作比较。     

高温回火热处理对9Cr-1Mo钢焊缝组织韧性的影响

以ESAB公司的三种9Cr-1Mo焊条的焊缝金属为研究对象,以Formastor-D全自动热膨胀记录仪分析了焊缝金属的连续冷却曲线,用MTS试验机分析了焊缝组织的断裂韧性.研究结果表明,Cr、Ni、C和Mn等元素导致OK1和OK3的Ms点应比OK2的低,奥氏体稳定性升高.710℃高温回火处理后,OK2焊缝组织的韧性较高.回火组织中碳化物沿晶界或相界析出使裂纹扩展更加容易从而使9Cr-1Mo焊缝组织的韧性降低.     

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。     

Optimization of Mold Inverse Oscillation Control Parameters in Continuous Casting Process

This paper introduces an approach towards selection of optimum control parameters of mold oscillation in the continuous casting process. A new model was applied to optimize the control parameters of the inverse oscillation control model. Optimization was aimed at reducing the depth of oscillation marks and was performed within constraints imposed by process parameters that influence the quality and productivity of continuous casting. Optimization generated an optimum control parameter set that provided better performance than that offered by Baoshan Iron & Steel Co., Ltd., and was verified by testing results.     

热处理工艺对Y1Cr17Mo钢组织和性能的影响

Y1Cr17Mo钢属于铁素体-马氏体钢的范畴,其显微组织随热处理保温时间和温度的不同而发生变化,显然这也将影响到材料的力学性能。主要介绍了热处理工艺对Y1Cr17Mo钢显微组织的影响,摸索淬火温度、保温时间与显微组织、硬度之间的关系,以便为实际生产提供科学指导,从而得到所需的力学性能。     

An investigation on oxidation/carburisation of 9Cr-1Mo steel heat exchanger tube in an AGR environment

Abstract

9Cr–1Mo steels have been used extensively in the power generation industry. In this study, a wide range of experimental samples exposed at different times and temperatures in a CO₂ environment were analysed to look at the development of the metal and oxides over time. The main objective of this work was to obtain a better understanding of the carburisation and oxidation behaviour of 9Cr 1Mo steels as a function of temperature/time, with special attention paid to the transition from protective to breakaway oxidation. In addition, experiments were also carried out to investigate any links between oxidation transition and carburisation behaviour of these materials.     

Cu合金化对Cr9Mo1钢耐腐蚀及力学性能的影响

为了研究Cu元素对含Mo高Cr钢耐腐蚀性及力学性能的影响,本文通过添加Cu元素和热轧工艺,制备了含有0.2%(质量分数)Cu元素的两组不同含C量的Cr9Mo1钢.利用拉伸试验机、光学显微镜(OM)及盐雾试验箱等,对比研究了添加Cu元素前后,Cr9Mo1钢力学性能、组织形貌及耐盐雾腐蚀性能的变化.结果表明,添加Cu元素均可显著提高Cr9Mo1钢的耐腐蚀性,但对力学性能及组织形貌无显著影响.低C钢组添加Cu元素后耐腐蚀性显著提高,强度和延伸率仅略有提高;高C钢组添加Cu元素后耐腐蚀性显著提高,强度略有提高,延伸率有所降低.热轧后的低C钢组的基体组织为混晶铁素体,高C钢组的基体为马氏体组织和少量δ铁素体.C含量的多少主要影响钢的力学性能,对耐腐蚀性的影响无显著差异.相较于含C量低的Cr9Mo1钢,含C量高的Cr9Mo1钢强度较高、塑性较差,但耐腐蚀性基本相同.