Study on Relationship between Impact Fracture Surface and Toughness of Low-carbon Steels

Abstract

The brittle fracture surfaces of steels are closely related to the microstructures, and it has been shown that there is a good correlation between the unit crack path (l_c) and the impact transition temperature (vTrS) ^4-6.
In this study, it is confirmed that the unit crack path in the ferrite-pearlite structure can be used as the factor controlling the toughness of steel; there is a linear relation between vTrS and log l_c-1/2, as previously obtained in the quenched structures. The relationship among the austenite grain size, the ferrite grain size (l_a), and the unit crack path in the ferrite-pearlite structures is determined. The relation l_c=2l_a is obtained, as expected in the previous paper⁴.
The fracture appearance in the brittle-ductile transition range of the ferrite-pearlite and the quenched structures are also invesitgated. In increase of the temperature from the brittle range in Charpy impact test, the ductile networks enclosing the brittle fracture facet are observed. The brittle fracture facet itself becomes ductile with further increase of temperature. However, the unit crack path is found to be unchanged, accordingly, this value is considered as a characteristic of the structure. The areal amounts of the ductile fracture measured by the scanning electron microscope are closely related to the Carpy impact values