The Properties For Welding Joints Of Ultrafine Grained Ferrite And Nano-TiC Steel

This paper investigated the effects of variant kinds of welding heat input on the microstructure and properties of welded point of super-fine grain ferrite steel which is strengthened by nanoscale interphase percipitatation, by the combination of welding thermal simulation and practical weld test. The main studies are as following:The analysis of properties of the tested steel's welded point shows that the average hardness of the welded point of tested steel is lower than the base metal,and the highest soften degree is 16.8 %; the tensile strength of the tested steel's welded joint is lower 3 % than the base metal, and the tensile fracture position is located in the coarse-grain zone; impact absorbing energy(-20 ?) of the HAZ and weld line of the tested steel is lower 20.8 % and 5.8 %, respectively, than the base metal.By measuring the SH-CCT diagram of tested steel with welding thermal simulation, the influence of cooling rate on microstructure and hardness of CGHAZ was obtained to reveal the phase transition of HAZ at different cooling rates. The results showed that: the SH-CCT diagram by iron prime transformation zone, bainite transformation region, martensite transformation; With the increase of cooling rate,the hardness of CGHAZ by 320.7 HV(50 ?/s)decreased gradually for 229.0 HV(0.3 ?/s).Single pass thermal simulation was applied to investigate the effects of the different peak temperature and cooling rates on the microstructure and properties of HAZ. The different peak temperature of thermal simulation tests shows that the CGHAZ presents the weakest toughness and the highest hardness in the HAZ due to the coarsen grain zone. The different cooling rates of thermal simulation tests manifest that the microstructure of CGHAZ are all bainite with the lath and granulous morphlogy when the cooling time(t8/5) within the range of 10~120 s. The original austenite grain size increases with increasement of the cooling time. When the cooling time is 20 s maximum impact energy and eimpact energy for 18.2 J, far less than the impact energy of the parent material; The hardness of CGHAZ is 250.4 HV when the the cooling time is 20 s. the coarsen grain zone will soften when the cooling timedecline further.The multi-pass welding thermal simulation was applied to research the effects of secondary thermal cycle on the microstructure and properties of the coarsen grain zone due to the process once welding heat cycle. It will have a phase transition and the the primitive coarsen grain zone will disappear when the peak temperature of the secondary thermal cycle is at 1080 ?. Simultaneously, the microstructure of SCR CGHAZ is fine blocky ferrite and the impact absorbing energy(-20 ?) is 35.5 J which is far higher than it of the once welding heat cycle.In the practical welding tests, the strength and hardness of the welded joint all meet the industrial demands when the heat input was 8.3 k J/cm?11.2 k J/cm?15.5k J/cm respectively. The welded joint presents favourable overal perfomance.