Metallurgical Factors
Two alloying elements, namely carbon and manganese are highly effective in altering the impact toughness of steel. Each 0.1% increase in carbon percentage raises the ductility transition temperature by about 14oC. Each 0.1% addition of manganese in steel reduces the transition temperature by about 5oC. For a satisfactory notch impact toughness a 3 : 1 ratio for manganese percentage to that of carbon is often suggested. Phorphorous also raises the transition temperature, and steel with high phosphorous content is not preferred at low temperature. Nickel generally has beneficial effect on notch impact property while chromium has little effect. Silicon tends to increase the transition temperature if in excess of
0.25%. Nitrogen has detrimental effect on notch toughness, though it is hard to determine its effect because of interaction with other elements. Molybdenum increases the ductility transition temperature almost as rapidly as carbon. The presence of oxygen in steel is highly dangerous as it raises the transition temperature in the higher temperature range. Deoxidation practice as followed in production of killed and semi-killed steels is highly beneficial in improving impact properties.
Grain size has pronounced effect on transition temperature. In general the transition temperature reduces with decreasing grain size. This effect is observed both in mild steel and higher alloyed steel. Since in thick hot rolled plates it is not possible to obtain uniform fine grain size throughout the thickness, the transition temperature will be appreciably higher. The loss of fracture energy in specimen of thicker section is also due to geometrical factors, such as finding a weaker point is easier in the large section than in smaller section of the same material.