Recently there are demands to decrease the carbon equivalent of 50 kg/mm² high tensile strength steels in order to prevent cold cracking of welds on welding with a low temperature preheating or without preheating.
The carbon equivalent of 50 kg/mm² high tensile strength steels was reduced from about 0.40 per cent to below 0.30 per cent. A special heat treatment was applied to a low carbon equivalent steel thus obtained to ensure the tensile strength to be more than 50 Kg/mm². The steel produced by this method has an excellent notch toughness in addition to the specified high strength because of the extra low carbon content and very fine grain structure.
The sensitivity to cold cracking of weld HAZ was examined by the Tekken cracking test, overhead fillet welding cracking test, and so on.
No cold cracking was found at all in welding even at such a low temperature as 0°C, and the low carbon equivalent was confirmed effective.
The weak point of this low carbon equivalent steel is softening of HAZ when welded by large heat inputs. The tensile strength of weld-joint decreases about 10 per cent in a narrow width specimen.
However, the decrease of tensile strength of weld-joint is at most 3 per cent in the wide specimen with the width about 6 to 7 times the plate thickness. It was found that softening could be neglected in almost all the steel structures because they were composed of wide plates.