High Heat Input Welding of 60Kg/mm² Grade High Tensile Strength Steel

Abstract

The notch toughness of weld-HAZs deteriorates with the increase in welding heat input. The degradation of notch toughness is caused by the austenite grain growth and heterogeneous transformation products. 60 kg/mm² grade high tensile strength steel's weld-HAZ has a stronger tendency to increase the unfabourable transformation products, for example M-A constituent, than 50 kg/mm² grade steel's weld-HAZ, because of its higher hardnability. So it was concluded that 60 kg/mm² grade steel's weld-HAZ showed lower notch toughness than 50 kg/mm² grade steel's weld-HAZ, when the high heat input welding processes were used.
This investigation was carried out to clarify the factors which govern the HAZ toughness and to develop the 60 kg/mm² grade steel suitable for high heat input welding. The results were summalized as follows.
(1) 60 kg/mm² grade steel's weldment made by the electrogas are welding technique showed low notch toughness at the weld-HAZ. These deterioration could be recognized not only at the coarse grained region but also at the fine grained region in HAZ.
(2) When the welding heat input was higher than 60 kJ/cm, HAZ toughness was improved with decreasing the heat input. On the other hand, HAZ toughness became to deteriorate with decreasing the heat input, when the welding heat input was lower than 60 kJ/cm.
(3) Welding heat input dependency on HAZ toughness mentioned above could not be explained by metallurgical point of view. But it could be interpreted by correlation between notch position and hardness distribution. That is, when the discontinuity of strength existed near the notch, the brittle crack initiated at the notch root deviated and propagated towards weaker region. And obtained values were govrened by the characteristics of weaker region.
(4) The experiment was conducted to improve the HAZ toughness. As a result, HAZ toughness of 60 kg/mm² grade steel's weld-HAZ toughness was improved by reduction of carbonequivalent. This improvement was resulted from reduction of M-A constituent and formation of pearlite.
(5) The tensile strength of low carbon equivalent type 60 kg/mm² grade steel's weld-HAZ had a tendency to decrease at the inverse of heat input. In practice, however it was concluded that lowering the carbon equivalent in a range investigated in this study got no trouble.