Research On Microstructure And Properties Of CGHAZ And Hot Cracking Sensitivity Of 12Cr1MoVG Steel

12Cr1MoVG steel belongs to Cr-Mo low-alloy pearlite heat-resistant steel.It has good oxidation resistance,corrosion resistance,heat resistance and stress rupture plasticity under 580=C.So it is widely used in manufacturing the high temperature parts such as power plant boiler superheater,reheater and steam pipe.But the weld crack and joint embrittlement often occur in 12Cr1MoVG thick-walled pipe welding process.So it is necessary to study the microstructure,properties and the failure pattern of coarse grain heat affected zone(CGHAZ),and provide reference for optimization of welding and heat treatment process of 12Cr1MoVG steel.In this paper,the samples of CGHAZ of 12Cr1MoVG steel were acquired by thermal cycle simulation.The influences of thermal cycles and reheat temperature on microstructure and properties of CGHAZ were studied by mechanical properties test,microstructure analysis and fracture scanning analysis.The influences of electrodes and the constraint degree on hot cracking sensitivity were studied by the method of FISCO welding hot cracking test.The results showed that:(1)The reheat temperature had a remarkable effect on the impact toughness and hardness.With the reheat temperature increasing,the impact absorbing energy first decreases and then increases,but the trends of the hardness done the opposite.When the reheat temperature reached 630℃,the impact absorbing energy had the lowest value and the microhardness had the highest value.The main reason for decline in material toughness was the intragranular directional carbide precipitation and poor alloy layer by separating out carbide particles in boundary.(2)When the different thermal cycles samples of CGHAZ tempered,the low carbon martensite and strip bainite will decompose into tempered structure which consists of ferrite,granular bainite and carbide.The grain size also didn’t change significantly.With the thermal cycling times increasing,the impact absorbing energy decreases gradually,but the trends of the hardness done the opposite.The main reason for decline in material toughness was the intragranular directional carbide precipitation.(3)Compared with R317L electrode,CHH317 electrode has good hot cracking resistance properties,and with the constraint degree increasing,the hot crack rate increases.The main reason for the influences of electrodes on hot cracking sensitivity was the microstructure of weld zone.The obvious directional columnar crystal will increase the hot crack rate.