A new formula BL70 has been developed for estimating HAZ Hmax in a steel bead weld. A backward logistic curve Hmax=H∞+K/(1+exp (a (Y-Y5))) has proved to approximate with excellent accuracy to the measured relation between Hmax and Y(=log t8/5, where t8/5 is the cooling time between 800 and 500°C). The values of material constants K, a, and Y5 were determined for each of seventy test steels (HT50 to HT100 of C%=0.017 to 0.33) by simple regresison analysis on measured values of Hmax and Y.
Stepwise multiple regression analyses were performed for the material constants, K, aK and Y5 on eleven alloying elements as independent variables, and finally BL70 formula was obtained. The formula has proved to be more reliable than other formulae thus far proposed by Beckert, Yurioka, Düren, and Terasaki, respectively.
Relationship between carbon equivalents which are related with Hmax are 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 t8/5 are needed for estimating Hmax with satisfactory accuracy. It has satisfactory accuracy very close to BL70 formula.
For other Hmax data besides the BL70 steels, the simplified BL70S formula seems to be more accurate than the original BL70 formula. In conclusion, BL70S formula is more recommended than BL70 for future general use. However, for low-Mn-low-Pcm type TMCP (CLC) HT50 steels, BL70SM formula should be used.
A revised. formula CEm is proposed to substitute the IIW carbon equivalent CE; CEm=CE+(Si/24+ 15B).