The variation of simulated heat affected zone (HAZ) toughness with nitrogen content in titanium containing thermomechanically controlled rolled steel was investigated and interpreted in terms of its microstructure and the amount of free nitrogen present. Measurement of the amounts of titanium and aluminium in precipitates extracted from the HAZ showed that 13 ± 45%TiN and 72 ± 79%AlN were dissolved at 1400°C peak temperature. The measured amount of titanium was in good agreement with the amount calculated by means of thermodynamic analysis, indicating that such analysis can be used to estimate the amounts of TiN and free nitrogen in the HAZ at a given peak temperature. Simulated HAZ toughness was in¯uenced not only by the microstructure, in turn in¯uenced by the amount of undissolved TiN at the peak temperature, but also by the amount of free nitrogen present after the formation of BN during the cooling cycle. Multiple regression analysis of the simulated HAZ toughness showed that the detrimental effect of free nitrogen was much greater than the bene® cial effect of TiN in this experimental condition.MST/5042
In this study, to predict the microstructure in weld HAZ of low alloyed steel, prediction model for the phase transformation considering the influence of prior austenite grain size and cooling rate was developed. For this study, six low alloyed steels were designed and the effect of alloying elements was also investigated. In order to develop the prediction model for ferrite transformation, isothermal ferrite transformation behaviors were analyzed by dilatometer system and 'Avrami equation' which was modified to consider the effect of prior austenite grain size. After that, model for ferrite phase transformation during continuous cooling was proposed based on the isothermal ferrite transformation model through applying the 'Additivity rule'. Also, start temperatures of ferrite transformation were predicted by A r3 considering the cooling rate. CCT diagram was calculated through this model, these results were in good agreement with the experimental results. After ferrite transformation, bainite transformation was predicted using Esaka model which corresponded most closely to the experimental results among various models. The start temperatures of bainite transformation were determined using K. J. Lee model. Phase fraction of martensite was obtained according to phase fractions of ferrite and bainite.
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