Ductility Loss due to Dynamic Strain Aging in Heat-Affected Zone of High Strength Low Alloy Steel

Abstract

When tensile tests on mild steel are carried out at various temperatures, the serrated yielding and flow are observed at a certain testing temperature range, specially 100-300°C, and the consequent em-brittlement occurs. This phenomenon is generally known as a blue brittleness or a dynamic strain aging and is related to testing temperature, solute atom contents (carbon and/or nitrogen contents), strain rate and so forth.
On the other hand, for base metal of QT-type high strength low alloy steels in which solute atoms such as carbon and nitrogen are fixed as carbide, nitride or carbo-nitride by heat treatment, such embrit-tlement seems to be not apt to occur comparatively. But, even if QT-type steels, there is a possibility of dynamic strain aging in heat-affected zone (HAZ) because of the above carbide, nitride or carbo-nitride dissolving by welding heats, and the consequent embrittlernent.
This study was investigated about the ductility loss due to dynamic strain aging in HAZ of a 80 kg/mm² class high strength low alloy steel.
It is found by this study that dynamic strain aging and, thus, the ductility loss can occur in HAZ even if QT-type steels. The degree of ductility loss is increased in the order of upper bainite, lower bainite and martensite structure.
These results can be qualitatively interpreted by considering as interaction between dislocations and solute atoms, specifically nitrogen atoms rather than carbon atoms.
Transformed HAZ, which is heated above 1200°C and has martensite structure and, thus, a high content of free nitrogen atoms, is sensitive to ductility loss due to dynamic strain aging as well as the deterioration of grain coarsened HAZ toughness.