Characterization Of Impact Toughness Of Coarse-grained Heat Affected Zone Of Steel Welded Joints For Pressure Equipment

Most of the structural parts need to be welded together,so excellent weldability of the material is necessary.Impact toughness of welded joints and base metal are the basic performance of pressure equipment design and performance.The CGHAZ in the heat-affected zone of welded joints often has the lowest impact toughness,which makes the CGHAZ become the weakest part.There are no domestic standards and rules of CGHAZ impact index,and the “GB 150-2011 the pressure vessel” explicitly stipulates the lowest impact energy values in welding joint,but the impact toughness for the "heat affected zone" under test conditions do not accurately represent the impact toughness results of the weakest CGHAZ.The submerged arc welding used in industrial production was adopted in this experiment,and the welding heat input is 25KJ/cm.The steel Q345 R and 12Cr2Mo1 R for pressure vessels with a thickness of 40 mm are welded.At different positions of welding joints,a large number of impact samples are prepared.It is concluded that CGHAZ is the weakest link of welded joint.On this basis,the influence factors of CGHAZ impact toughness the influence of CGHAZ on the overall joint performance,and the evaluation method of CGHAZ impact toughness were studied.In actual welding,in order to determine the toughness of CGHAZ,FGHAZ,ICHAZ and SCHAZ,the Charpy V-type notch was sampled at a distance of 0,1,2 and 3mm from the fusion line respectively.The impact experiments were carried out at the temperature corresponding to the standard in “GB 713-2014 Steel Plates for Boilers and Pressure Vessels”,or depending on the service environment of the steel.However,in actual welded joints,it is difficult to measure the impact toughness of the coarse-grained heat-affected zone,so the impact toughness of samples containing different proportions of coarse-grained heat-affected zone is statistically measured.Based on above measured results,the impact toughness of a complete coarse-grained heat-affected zone is obtained by polynomial fitting.The results show that:(1)The microstructure of Q345 R is composed of block ferrite and pearlite.The CGHAZ in welded joints is composed of a large amount of granular bainite,a small amount of block ferrite and a very few amount of lath bainite.At-20°C,0°C,and 20°C,the impact toughness of the base metal was 39.2J,91 J,219J respectively,and the impact toughness of the FGHAZ,the ICHAZ,and the SCHAZ was higher than that of the base metal.The impact toughness of the CGHAZ was 32 J,41J,and 76 J respectively,which was reduced by 18.4%,55.4%,and 65% relative to the base metal respectively.The CGHAZ shows obvious embrittlement and become the weakest link in the whole welded joint.(2)The microstructure of 12Cr2Mo1 R consists of acicular ferrite,block ferrite,granular bainite and a large amount of lath bainite.The microstructure of CGHAZ in welded joints is composed of a large amount of granular bainite,a small amount of block ferrite and a very few amount of lath bainite.At-30°C,the impact toughness of the base metal is 223 J.The toughness of FGHAZ,ICHAZ and SCHAZ is similar to that of the base metal,while the impact toughness of CGHAZ is 65 J,which is 70% lower than that of the base metal.CGAHZ shows obvious brittleness and become the weakest link in the entire welded joint.(3)The main factors influencing the impact toughness of CGHAZ are the grain size of the original austenite and the microstructure of CGHAZ.The coarse original austenite and a large amount of coarse granular bainite lead to a serious deterioration of the impact toughness of CGHAZ,so the wider the distribution of CGHAZ at the front of the notch tip is,the lower the impact toughness of the sample is.(4)According to the compensation method and toughness compensation standard of NX4335.2 in ASEM Volume III for the heat affected zone impact toughness,both the Q345 R and 12Cr2M1 VR welding coarse-grained heat affected zones need to be compensated for toughness.The impact toughness of the heat-affected zone is replaced by the impact toughness of the coarse-grained heat-affected zone to compensate for the loss of impact toughness of the CGAHZ.However,in the actual compensation,the corresponding ?AEN cannot be directly compensated,and how much compensation needs to be further explored.This compensation method can be used as a potential compensation method for the toughness of the weakest link of the welded joint,and plays an important role in improving the weldability of the base metal and ensuring the safety of the weakest link.