Method For Determining The Minimum Design Temperature Of Ferritic Pressure Vessel Steels Based On The Master Curve Approach

The ductile to brittle transition phenomenon of ferritic steel with decreasing temperature leads to pressure vessels taking the risk of brittle fracture under lower temperature. The brittle failure of pressure vessels always results in great casualties and economic losses, so the detailed brittle fracture terms developed in pressure vessel codes are used to prevent the brittle fracture. Compared with the advanced and mature method for determining the minimum design temperature based on fracture mechanics in the developed countries, the method in China pressure vessel code is only on the basis of engineering experience. The main contents and conclusions are listed as following:(1) The theoretical background of brittle fracture prevention provisions was studied and the rationality of the relevant items was analyzed. The cylinder structure with R/t=100 used in ASME VIII-2 which can cover the most dangerous situation is reasonable. The thickness 50.8mm used in temperature adjustment curve for low stress condition is too conservative for AW condition and is not conservative for PWHT condition. The minimum design temperature curve in EN 133445 is obtained by further adjustment. The temperature adjustment curve for low stress condition verified the source of the temperature adjustment in the specification, and the conservatism of the relevant provisions of the temperature adjustment of the AW condition was made clear.(2) The essence of brittle fracture prevention method in VIII-2 and EN 13445 is that the fracture toughness requirement is obtained from the rigorous fracture mechanics analysis, which is converted into impact energy requirement by empirical correlation. The fracture toughness requirements in two codes are similar with each other, and the preventing brittle fracture effect of specific provisions is almost the same.(3) Q345R tested in this paper had excellent low temperature toughness according to experiment results. ASME classifying Q345R in curve A excessively underestimated the toughness of the steel itself. The strength and toughness of the welded joint H10Mn2/SJ101 were matching with base metal, meanwhile the impact energy and fracture toughness of the welded joint H10Mn2/HJ431 were significantly lower that of base metal. Two welded joints should be distinguished in engineering.(4) The method of determining the minimum design temperature which is chosen impact energy as the material toughness requirements was developed with the help of Master Curve approach. The method based on rigorous fracture mechanics analysis, which is convenient for ferritic steel. The experience in European and American Standard does not included in this method, and the verification by large scale structure testing is not necessary. Compare with ASME VIII and EN13445, the minimum design temperature curve does not contain any empirical assumption and the form is more concise.(5) The backward brittle fracture prevention provisions in pressure vessel standard restricted the minimum usage temperature of Q345R and the loose toughness requirements in material specification led to the poor quality stability of steel, which is the reason of the ASME classifying Q345R in curve A. The method of determining the minimum design temperature based on the usage experience of material was established for the present situation of pressure vessel steel in China. Suggestions on improving the quality of pressure vessel steel were put forward.(6) The method of determining the minimum design temperature which chooses impact energy as the material toughness requirements was established. T_D based on impact energy is highly likely to be not conservative, because there is no unified relationship between impact energy and fracture toughness in the transition region, although impact energy requirements based on the empirical correlations between impact energy and fracture toughness are satisfied the requirements in ASME VIII-2. The minimum design temperature curve based on the empirical correlation between T27J and Tomc can give a conservative T_D.(7) The minimum design temperature for low stress and hydrostatic test condition were studied. It was found that the adjustment temperature of low stress condition in GB150 was reasonable, and the pressure vessel had a brittle fracture risk when the hydrostatic test conducted under the temperature specified by GB150. The low stress temperature adjustment curve is established on the basis of the brittle fracture model in this paper. There is weaker yield strength dependence in the temperature adjustment curve than that of in ASME VIII-2, and is close to that of in EN 13445. Finally, the minimum hydrostatic pressure test temperature is recommended as T_D+20?.