Effect of Carbide Size Distribution on the Impact Toughness of Tempered Martensitic Steels with Two Different Prior Austenite Grain Sizes Evaluated by Instrumented Charpy Test

Abstract

The effect of tempering temperature on the impact toughness of 0.3 mass% carbon martensitic steels with prior austenite grain (PAG) size of about 6 and 60 µm was investigated. Instrumented Charpy impact test (ICIT) was used to evaluate the impact toughness. The tempering temperature of 723 K gives the largest difference in the Charpy impact energy at room temperature between the specimens with two different PAG sizes, where the finer PAG specimen shows higher impact energy at room temperature (RT). The other tempering temperatures do not show a significant difference as compared with that shown among the 723 K tempered specimens. Investigation of the test temperature dependence of Charpy impact energy in the 723 K tempered steels shows a steep transition at around 200 K for the 6 µm PAG specimen, while it shows a continuous slow transition in a wide range of temperatures for the 60 µm PAG specimen. ICIT waveform analysis of these steels shows that the fracture propagation energy mainly controls the temperature dependence of the impact energy, while the fracture initiation energy stays nearly constant against the variation of the test temperature. The carbide size distribution in these two specimens was investigated by secondary electron microscope (SEM) and transmission electron microscope (TEM). The 60 µm PAG specimen shows distribution of coarser carbide than does the 6 µm PAG specimen, which seems to give rise to the observed difference between them in the Charpy impact energy and the other properties of impact fracture.