Abstract
Weldability of the HT80 steel was investigated by making the SH-CCT diagram, heat cycle simulating test and the slit type cracking test in order to make clear the influence of the welding procedure on the notch toughness, the hardness and the crack sensitivity in the weld heat-affected zone. The results of these test are summarized as follows.(1) The impact value (vE-15) of simulated HAZ steeply fails when the peak temperature of single heat cycle exceeds the ACI point (710°C). And the lowest impact value is indicated when the peak temperature is made above 900°C and transformed to upper bainite during contenuous cooling.(2) The impact value of simulated HAZ will be remarkably increased when cooling time of the heat cycle being shorter than the critical cooling time of upper bainite transformation and then postheated at 600°C, 3 hours with the cooling rate after postheating is more than 100°C/h.(3) It has been found that temper brittleness is occured in the range of 450-550°C.(4) In weld joint tests, the effect of cooling time on the impact value of HAZ agree well with the ten-dency in the results of the heat cycle test.(5) In the slit type cracking test, when the bainite structure of HAZ is made approximatly more than 45 % (preheating temp. is higher than 150°C), HAZ cracking can be prevented even if the diffusible hydrogen in weld metal is increased from 2 cc/ 100 g to 12.7 cc/ 100 g.
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