抄録
In order to clarify the origin of the excellent toughness of 9% Nickel low carbon steels at low temperatures, the change in mechanical properties with particular reference to the notch toughness on the isothermal tempering at various temperatures was investigated in connection with the precipitation of austenite, the change in microstructures, and the fracture characteristics in a 0.1%C 9%Ni steel carefully prepared by vacuum melting. The main results are as follows: (1) The high value of 25∼27 kg-m/cm in the Charpy 2 mm V-notch test is obtained at −196°C in a specimen quenched from 800°C, tempered at 550°C for 20∼60 min and water-quenched. (2) The temper brittleness characterized by the intercrystalline fracture is observed when tempered below 525°C. (3) The intercrystalline fracture is also induced by the segregation of non-metallic impurity atoms such as carbon when a specimen is cooled at slow rates after tempering. (4) The embrittlement characterized by the cleavage fracture observed in specimens tempered above 550°C is ascribed to the precipitated austenite, which is stable even when quenched into liquid nitrogen but by impact can be transformed to martensite; the relation found between the impact value and the amount of austenite does not necessarily support the generally accepted view that the existence of the austenite improves the toughness of this steel. (5) The excellent toughness is achieved by tempering at 525∼550°C at which lattice defects such as dislocations in martensite mostly annihilate, and the coarsening of the precipitated austenite does not occur. The rapid cooling after tempering is also necessary for the good toughness.
