Microstructure and mechanical performances of CGHA

Authors: GuoLi Liang ShanWu Yang HuiBin Wu XueLi Liu

Publish Date: 2013/04/23

Volume: 32, Issue: 2, Pages: 129-133

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Abstract

Microstructure and mechanical performances of the coarse grain heataffectedzone CGHAZ for oil tank steel with different Ti content were investigated through Gleeble3500 scanning electron microscopy transmission electron microscopy and energy dispersive spectrometer The results show that the strength and lowtemperature toughness of base material are significantly improved for the high titanium content steel but the impact toughness of CGHAZ is seriously deteriorated after the high heat input welding and declined sharply with the heat input increasing while the effects of heat input on impact toughness are very weak for the low titanium content steel impact toughness of which is gradually larger than that of high titanium content steel with the welding heat input increasing because of the granular bainite increasing TiN particle coarsening and Ti Nb N composition evolution during the high input welding for high titanium content steelHighstrength lowalloy steel has been widely applied to build the largescale structures such as oil tank because it has a huge volume up to 10–20 million m3 wherein a lot of welding works are required in the construction 1 2 3 In order to save cost and improve construction and efficiency high efficient automatic welding methods such as submerged arc welding and electrogas arc welding must be employed However the high heat input welding tends to cause a considerable reduction in the strength and toughness of HAZ especially for the coarse grain heataffectedzone CGHAZ The CGHAZ is adjacent to the fusion line where the peak temperature is close to the melting point of steel which results in austenite grain obviously coarsening and occurrence of coarse grained embrittlement Meanwhile the continuous rapid cooling after the peak temperature also spurs the formation of upper bainite and granular bainite resulting in microstructure brittleness 4 5 6The toughness of CGHAZ is significantly improved by adding Ti Nb and other microalloyed elements in the lowalloy high strength steel this is because of the action between Ti Nb and C N in the steel to form the fine dispersed carbonitride These carbonitrides block the growth of austenite grains and improve the secondary microstructure of CGHAZ However these secondphase particles dissolve grow and reprecipitate when they experience the welding thermal cycle the size number and distribution of the second particles are affected in the welding process and then the ability to block austenite grain growth and the toughness of CGHAZ are also affected 7 8 Therefore it is very important for us to study the effects of Ti content on the highstrength oil tank steel when they experience the high heat input welding At present study the impact toughness and hardness of steel with different Ti content in the welding thermal cycle were investigated by metallographic microscope scanning electron microscopy SEM and transmission electron microscope TEM

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