抄録
In order to find a clue to establish the relationship between fracture toughness and metallurgical factors, macroscopic energy (GIC) for intergranular fracture of temper embrittled 0.3%C-1%Mn-1%Cr-0.02%P-0-3%Ni steels was investigated and it was compared with the surface energy of a crack (γ: true surface energy, γp: plastic strain energy in front of a crack) introduced from Griffith-Orowan's thermodynamic and/or microscopic criterion. To obtain the exact fracture toughness (GIC) corresponding to intergranular fracture due to temper embrittlement, it is required not only to exclude the influence of stretched zone formation or normal rupture in front of fatigue pre-crack, but also to consider the influence of pre-crack path on fracture toughness. Fractographic studies were carried out in detail with a scanning electron microscope to find the exact energy for intergranular fracture mentioned above.
It was found that fracture toughness (GIC) corresponding to intergranular fracture due to temper embrittlement was in the order of 103 J. m-2. GIC was much bigger than γS (surface energy) and γB (grain boundary energy), and it was found to be close to γP. If it is assumed that GIC corresponds to γP, the reasons for the increase of degree of embrittlement (ΔFATT) and the decrease of GIC with Ni content in embrittled steels (P content is constant) are considered as follows. Intergranular fracture due to temper embrittlement is mainly caused by the decrease of γP, and the decrease of γP is considered, from AES analysis, to be due to the segregation of impurity elements and to the segregation of alloying elements such as Ni and Mn to austenite grain boundaries.
