Effects Of Welding Parameters On Cryogenic Properties Of Welded Joint Of 16MnR Steel And Safety Assessment

The welded structure of the pressure vessels are used to be affected by welding parameters,the defects exist inevitably in the welded joint and increase the brittle fracture tendency,easily lead to brittle fracture .On the other hand,the pressure vessels due to the impact of low temperature will inevitably increase the sensitivity of welded joints defects which tend to increase brittleness,and these will be harmful to the safety of pressure vessels.At present,the safety assessment of pressure vessels based on"Fitness for service"principle has been widely developed both at home and abroad,and many new theories are developed. Accordingly,many new standards are established,such as SINTAP and GB/T19624-2004 and so on. The foundation of"Fitness for service"principle is the scientific method based on the theory analysis and the experiment tests. It substituted for"perfect was complete"method based on the experience,and can objectively guarantee the pressure vessel without any known mechanism expiration accidents.In this research,pressure vessel steel-16MnR were commonly used in our country through carrying out for cryogenic mechanical properties tests:uniaxial tension test,impact toughness test of the welded joint,fracture toughness test were made at -40℃. OM and SEM were employed to investigate the microstructure and fracture surface of welded joint. It was showed that the tensile properties,impact properties and fracture toughness of A welding parameters at low temperature were better than B,C,which mainly because the microstructure of the HAZ were coarse and non-uniform,and the emergence of large widmanstatten structure,in addition,the fracture surface clearly showed the characteristic of cleavage fracture,which made the mechanical properties of HAZ at low temperature decrease more obviously than weld metal. The results of crack tip opening displacement tests showed that the crack resistance of 16MnR weld metal was much better than that of the HAZ,which proved that the 16MnR had good crack arrest property.This research was based on the project investigation and repeated discussion. The Chinese standard"Safety assessment for in-service pressure vessels containing defect"(GB/T19624-2004)was selected to assess the safety of girth surface flaw in weld metal and HAZ of 16MnR at -40℃. The results of the assessment showed that all assessment points were located within the failure lines of analysis simplified assessment and traditional assessment. So welded joint of the pressure vessel was safe. Maximum tolerable equivalent flaw size was calculated according to the above assessment results. It was showed that the welded joint's safety at -40℃welded by A was the best,and the risk to surface flaw of of same size in HAZ was relatively dangerous than in weld metal.The study is of great significance in terms of engineering and theory significance and provides research ideas for laying the foundation of application of"Safety assessment for in-service pressure vessels containing defect"to pressure vessels structure assessment.