Study On Welding Quality Control Of The Nuclear Reactor Pressure Vessel SA508Gr4Steel

During the manufacturing process of nuclear pressure vessel, the welding quality of extra-thick steel plate affects the safety operation of the whole reactor directly. However, thetemperature field in the process of welding and the residual stress field after welding areextremely important to the whole weld zone. Research subject in this thesis is SA508Gr4steelwhich is a pressure-bearing material, the thesis calculates the temperature and stress field ofsteel plate welding process through the computer simulation technique and develops anexperimental verification. Besides, it also grasps the distribution laws of residual stress and thecondition of welding deformation,and establishes a prediction method of HAZ width,which canachieve the purpose of controlling forming qualities of material welding and forecasting thequalities of welding joint.We also assays the thermophysical characteristic of matericals which are needed in thecalculations of welding simulation about SA508Gr4steel by adopting diffrential thermalanalyzer, quenching dilatometer and thermal simulateed test machine respectively. At the sametime, we need to draw SH-CCT and analyze mechanical property under high temperature, sothat we can assess its weldability comprehensively. The SH-CCT demonstrates that duringsuccessive transformed process of welding HAZ tissue, it includes only two transition zones:martensite and bainite. When t8/5ranges from4.6s to15s, it will cool untial it becomes lath-shaped mantensite; when t8/5is over1200s, it will transform bainite tissue completely. What’smore, tensile strength decreases with the increasing of temperature within temperature800-1250℃of the SA508Gr4steel. The plasticity under high temperature is well but it exists thesecond brittle zone which is biased toward high temperature, especially when it stretches underthe temperature of1230℃,the fracture will become brittle fracture clearly and the shrinkingpercentage of fracture surface is only50%. The brittle fracture is mainly caused by the oxideseparating out along the grain boundary. When the extra-thick steel SA508Gr4is welded, the tendency of harden gets more heavily and welding gets more worse, which is necessary toguarantee its quality by controling welding technology and weld preheating over200℃.In view of the above data base,double-sided single channel welding entity model ofSA508Gr4steel and double-Vee groove was established by using the numerical simulationtechnique in this thesis, heat load in a hierarchical manner and correction coefficient of heatgeneration rate was adopt for the calculation of electric-arc welding temperature field andstress field.The results show that the both ends of test plate will upwrap for2.53mm in thethickness direction after the first welding; while the test plate will bias the lower for1.75mmafter the second welding. The measured value of residual stress accords with simulated value.The transverse residual stress which is20mm far from the center of welding line550Mpatensile stress, the center of welding line is compressive stress, while longitudinal residual stressis mainly tensile stress.Finally, we will analyze the affected regularity of heat input on overheated zone, phrasechange recrystallization zone, incomplete recrystallization zone tissue and width, which isadopt for the computer simulation of welding thermal circling curves under different heat input.And then, we can establish the prediction method of HAZ and subarea width. Through thepridiction of SA508Gr4steel’s welding joint, we discover that in the width of welding HAZ,the overheated zone and incomplete recrystallization zone which property is a little unstablehave a height advantage. So it is essential to improve its welding joint property by postweldheat treatment.