EFFECT OF HEAT HISTORY ON INITIAL TOUGHNESS OF CARBON-0.5-PERCENT MOLYBDENUM STEEL

Although carbon-0.5% molybdenum (C-0.5% Mo) steel is widely used as a material for pressure vessels in petroleum refining and related proces s plants, there have been instances of reactor vessels rupturing due t o the extremely low initial toughness of the material. Hence, the obje ctive of the current research has been to investigate the effects of h eat history on the initial toughness and ultrasonic attenuation charac teristics of C-0.5% Mo steel. As a result, it has been found that init ial toughness depends greatly on aluminum content and the rate of cool ing from normalizing temperature, and that the smaller the ferrite gra in diameters and the higher the bainite content due to faster cooling from the normalizing temperature, the higher the toughness of the mate rial. Furthermore, a relationship between ultrasonic attenuation and t oughness is evident whereby the higher the ultrasonic attenuation coef ficient of the material, the lower the toughness. This indicates the p ossibility of deriving a nondestructive method for evaluating toughnes s.