The authors performed a series of researches on continuous cooling transformation diagrams of structural steels used for welding and reported them successively in Journal of Japan Welding Society (1957-1959, vols. 26-28). In these reports, the continuous cooling transformation diagrams were obtained with small specimens in the case of rapid-heating to maximum temp. 1350°C, cooling by various processes immediately after reaching its temp. and without holding at its temp. They gave fundamental data for tha sake of selecting the weldable steels and determined the conditions when welding these steels. Therefore, such continuous cooling transformation diagrams were presumed very important and useful.
In the hresent report, on the basis of the results of the above researches, further determinations were made on the continuous cooling transformation diagrams for high tensile steels 2H which were of Mn-Si type and showed the tensile strength enhanced to about 60kg/mm2 by the heat-treatment of water-quenching and tempering.
For those 2H steels could be adopted also, a diagram that was plotted to show the relation between critical cooling-time Cz', Cf' or 50% martensite cooling-time and the carbon equivalent of mild steels and Mn-Si type high tensile steels as rolled or normalized. And then, using this diagram the critical cooling-time Cz', Cf' and 50% martensite cooling-time and the carbon equivalent were determined with mild steels and Mn-Si type high tensile steels as rolled or normalized And then, based on this diagram the critical cooling-time Cz', Cf' and 50% martensite cooling times of arbitrary steel grades could be predicted from the carbon equivalent Ceq= (C+1/12Mn+1/24Si) % of those steels.
Next, the various types of high tensile steel having 60kg/mm2 tensile strengths were compared with each other, From stand points of the buctility and crack-sensitivity of the weld heat affected zone, the critical cooling-time Cf' which began to reveal existance of primary ferrite was important, and the steels having smaller value of Cf' were found desirable. For this purpose, even with any type of steels, it was necessary that the carbon content of a steel was lowered to a mimimum.