A Study On The Welding Properties Of T23 Steel

Due to the development of the technology in ultra-supercritical coal-fired power units,the efficiency of generate electricity enhanced and attain energy conservation.As the operation temperature and pressure of water wall from ultra supercritical power units increased,the materials with high temperature stability,excellent thermal conductivity and thermal processing are required.The traditional steel(T11,T12,T22)and other materials are hard to meet the mechanical properties of ultra-supercritical coal-fired power units.Alternative materials with higher alloy content,higher thermal strength and better welding performance are required.Due to no need preheating and post-welding heat treatment(PWHT)for T23 steel,that plays an important role in the water wall of ultra-supercritical thermal power units.The unique properties of T23 steel makes it widely used in the ultra-supercritical power units.However,it was found that the T23 steel applied in the ultra-supercritical power units emerged welding cracks lead to leakage issue produced,which related to the welding performance of T23.Hence,the study on the welding performance of T23 steel is significant.(1)The sensitivity of welding-cold cracking for T23 steel was studied by the experiment of maximum hardness in weld heat-affected zone(HAZ),Y-groove cracking test and welding-cold cracking test using implants..It shows that T23 steel has certain of sensitivity of welding cold cracking.When the preheating temperature exceed 50 ℃,the cracks disappeared followed.The coarse-grained heat-affected zone(CGHAZ)of T23 steel welding was hybrid structure of bainite and martensite.As the preheat temperature of T23 steel increased,the content of bainite and martensite in the CGHAZ is increasing and reducing respectively.The coarse-grained heat-affected zone(CGHAZ)hardness of the welding is decreasing as the temperature increased.Hence,the critical fracture stress of cold cracking will increase.(2)The reheat cracking sensitivity of domestic T23 steel was studied by means of Y-groove cracking test,reheat crack sensitivity plug test and reheat crack thermal simulation test.The results show that the T23 steel has a sensitivity of reheat cracking.The sensitive temperature zone range from 690 ℃to 760 ℃,the most sensitive temperature of T23 steel is 720 ℃.The fracture behavior occurred in the coarse grain HAZ,the main form of the fracture is intergranular fracture.When the reheating temperature reach to 720 ℃,the cracks,voids and chains in voids appeared in the grain boundary.As the increase of temperature,the impact energy of CGHAZ have a tendency to go down and up.The CGHAZ of T23 welded joints showed a certain reheat embrittlement sensitivity in the range of 600℃~690℃,and the most sensitive temperature is 650℃.(3)Considering the realistic industrial application of T23 steel,the temper embrittlement and stress analysis of the fillet weld seam in water wall were invested.The results show that when the temperature of heat treatment range from 600 ℃ to 650℃,the tempering brittleness of T23 steel was very obvious.The most sensitive temperature is 650℃.Hence,when deciding the post-weld heat treatment process,the heat treatment temperature should avoid the temperature range of 600 ℃~650 ℃ as far as possible.The impact toughness of T23 be of weld state is poor due to lacking post-welding heat treatment(PWHT).PWHT should be carried out to improve the impact toughness of weld seam.With the increase of welding preheating temperature for T23 dissimilar steel(T23/SA387Gr22CL1),not only the residual stress of weld seam but also the HAZ and hardness of the weld seam decreased continuously.Furthermore,it’s significant that the high temperature tempering treatment can decrease the residual stress and hardness of the weld seam.