For the steel DT containing low carbon, Ni, Mo, V and B, of which ultimate tensile strength is about 90 kg/mm
2 in the state quenched from 930°C and water cooled after tempering for 1hr at 630°C, as well as USS "T-1" steel, the continuous cooling transformation diagram in case of maximum heating temperature 1350°C was constructed. Then this continuous cooling transformation diagram was compared with that obtained in case of maximum heating temperature 1093°C by Nippes, Savage and Allio.
The following results were obtained :
1. The A
3-transformation point of steel DT is extremely low. In the continuous cooling transformation diagram, the time at the Ac
3 temperature 833°C was adopted as 0 sec of a datum point.
2. Critical cooling times of steel DT are determined as follows ; C
z'=6 sec, C
f'=18 sec, C
p'=56 sec, C
e'>2000 sec. The cooling time corresponding to 50% martensite is 18.5 sec, namely, near the value of C
f'.
3. The positions of the beginning curves and the ending curves of each consitituents of microstructures in the authors' continuous cooling transformation diagram in case of maximum heating temperature 1350°C, remove to the longer cooling time and lower temperature side, compared with those in the continuous cooling transformation diagram in case of maximum heating temperature 1093°C constructed by Nippes, Savage and Allio.
4. In the author's continuous cooling transformation diagram, critical cooling time C
z', C
f', C
p', 50% martensite cooling time and others are determined, but in that diagram by Nippes, Savage and Allio, C
z' critical cooling time was only determined and 4.6 sec of the value is smaller than 6 sec of the author's value.
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