Effects of the Aged Structure and the Prior Austenite Grain Size on Fracture Toughness of 18Ni Maraging Steels

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Abstract
A study has been made of the effects of the aged structure and the prior austenite (γ) grain size on plane strain fracture toughness (KIC) of 18 Ni 300 and 350 grade maraging steels. The relationship between KIC and tensile properties or Chrpy V notch absorbed energy (CVN) has been also investigated. KIC is strongly affected by the structure aged at different temperatures. It is found that Kic of the specimen with high temperature aged structure is superior to those with low temperature aged and overaged structures, and the factors which affect KIC of the three different aged structures are discussed. KIC is scarcely affected by the prior γ grain size. This phenomenon is in contrast to those of tensile ductility and CVN which markedly decrease with increasing the prior γ grain size. Therefore, any relationship between KIC and tensile properties or CVN can not be observed. This may be due to the change of the fracture mode, with increasing the prior Therefore, any relationship between KIC and tensile properties or CVN can not be observed. This may be due to the change of the fracture mode, with increasing the prior γ grain size, from dimple in the grain to dimple along the prior γ grain boundaries or quasi-cleavage. grain size, from dimple in the grain to dimple along the prior γ grain boundaries or quasi-cleavage.