In order to prevent the brittle fracture accident, minimum design metal temperature of ferrite steel should be limited. After the minimum design metal temperature curve in American Society of Mechanical Engineers VIII-2 (2007) was proposed, much related research has been done in recent years. In this paper, firstly the theoretical basis of four methods used to determine the minimum design metal temperature was introduced. Secondly, the mechanical properties of Q345R was measured by tensile test, Charpy v-notch impact test and fracture toughness test Thirdly, minimum design metal temperature curve of Q345R that determined by four methods were obtained. There are obvious difference between the curves of Q345R that determined by four methods. It can be concluded that low temperature fracture toughness of Q345R is underestimated when classifying Q345R into exemption curve A in American Society of Mechanical Engineers VIII-2 (2007).

工程实践表明现代钢材质量与焊接技术足以保证结构钢在更大厚度时免除焊后热处理,相关厚度值远高于现行规范源于工程经验的最大焊后热处理(Post weld heat treatment,PWHT)豁免厚度规定.从压力容器防脆断的角度出发,基于断裂力学原理,研究了材料最大PWHT豁免厚度的影响因素,结合Q345R钢性能数据,评价了具体厚度值的工程适用性,分析指出保守的防脆断条款是导致我国压力容器规范最大PWHT豁免厚度小于欧美规范的根本原因.

Low alloy ferritic steels are widely used to manufacture critical components that operating at elevated temperatures in the process industry, but a better understanding of the creep responses of their weldments are still needed to provide guidance for the design of high-temperature welded structures. In the present paper, the heterogeneous creep behavior of vanadium-modified 2.25Cr1Mo ferritic steel weldments was investigated by using the digital image correlation (DIC) technique. Three-dimensional plot of creep strain across the weld joint was constructed to intuitively represent the spatio-temporal evolution of creep strain for the base metal (BM), weld metal (WM) and the heat affected zone (HAZ). Combining the analysis of strain contour evolution during creep with the microstructural examination, it is found that creep strain was primarily concentrated in the HAZ, resulting in the creep cavitation at grain boundaries of the coarse-grain bainitic HAZ. In addition, BM exhibited a faster deformation rate than WM during creep. In order to quantitatively describe the heterogeneous creep deformation, a sub-region extensometer method based on the construction of virtual extensometers was proposed to obtain the representative creep strain data of individual regions of weldments. Furthermore, a parameter identification procedure combining a global optimization method (i.e. genetic algorithm) with an initial value determination scheme, was developed to determine the model parameters of BM, WM and HAZ for the hyperbolic-sine creep constitutive model. The results show that the heterogenous creep behavior of the V-mod 2.25Cr1Mo steel weldment can be well characterized by combining the sub-region extensometer method, the hyperbolic-sine creep constitutive model and its genetic algorithm-based parameter identification method.

低温低应力工况是压力容器用碳钢低合金钢在低温领域应用的重要保障,我国压力容器规范低应力工况仅为欧美规范的特例,其工程应用受限.分析了ASMEⅧ与EN 13445中低应力工况温度调整曲线理论背景,研究了板厚与强度对温度调整值的影响规律.在此基础上,建立基于主曲线法的低应力工况温度调整准则,发现压力容器规范缺乏科学合理的最低设计温度确定方法是限制我国压力容器用钢在低温下服役的根本原因.

The brittle fracture prevention model is of great importance to the safety of pressure vessels. Compared to the semi-empirical approach adopted in various pressure vessel standards, a model based on Master Curve technique is developed in this paper. Referring to ASME nuclear code, the safety features including the lower bound fracture toughness and a margin factor equal to 2 for the stress intensity factor produced by primary stress are adopted in the new model. The technical background of the brittle fracture model in ASME VIII-2 has been analyzed and discussed, and then its inappropriate items have been modified in the new model. Minimum design temperature curves, impact toughness requirements, and temperature adjustment for low stress condition are established on the basis of new model. The comparison with the relevant curves in ASME VIII-2 is also made. The applicability of the new model is verified by the measured fracture toughness and impact toughness data of several kinds of pressure vessel steels. The results suggest that the minimum design temperature and the impact test requirements derived by the new model are compatible with each other. More testing data of different steels to check this model is necessary for further engineering application.

Q345R steel is the most commonly used material in fabrication of the pressure vessels and boilers in China, due to its excellent properties. In 2010, ASME code case 2642 accepted Q345R steel for use in construction of pressure vessels. The code case specified impact test exemption curve A for the impact test requirements for Q345R. However, this provision severely limits the application of this material at low temperature, since most of the minimum design metal temperature (MDMT) of curve A is above the freezing point. In this paper, a series of tests (such as uniaxial tensile test, impact test, and fracture toughness test) were carried out at low temperature to investigate the mechanical properties of Q345R steel plates with thickness of 36–80 mm. This study of low temperature usage of Q345R steel was conducted using the fracture mechanics assessment procedure of API 579-1/ASME FFS-1. The fracture toughness is given by master curve (MC) method in the transition regime. The results show that Q345R can be used at lower temperature and that classifying Q345R steel into curve D is appropriate.