A New Formula for Estimating HAZ Maximum Hardness in Welded Steel :

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A new formula BL7O has been developd for estimating HAZ Hmax in a steel bead weld. A backward logistic curve Hmax=H_∞ + K/(1 + exp(a(Y - Y_5))) has proved to approximate with excellent accuracy the measured relation between Hmax and Y (= log t_<8/5>, where t_<8/5> is the cooling time between 800 and 500℃). The values of material constants K, a, and Y_5 were determined far each of seventy test steels (HT50 to HT100 of C% = 0.017 to 0.33) by simple regression analysis. Stepwise multiple regression analyses were performed for the material constants, K, aK and Y_5 on eleven alloying elements as independent variables, and finally BL7O formula was obtained. The formula has proved to be more reliable than other formulae thus far proposed by Beckert, Yurioka, Dtiren and Terasaki. Relationship between various carbon equivalents which are related with Hmax is also discussed. As the material constants can be expressed as linear functions of C% and Pcm%, a simplified formula BL70S also has been developed, in which only three variables, C, Pcm and t_<8/5> are needed for estimating Hmax with satisfactory accuracy. For other Hmax data besides the BL70 steels, a simplified BL70S formula seems to be more accurate than the original BL70 formula. In conclusion, BL70S formula is more recommendable than BL70 far future general use. However, for low-C-low-Mn-low-Pcm type TMCP HT50 steels, BL70SM formula should be used.

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