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.

Identification of ductile to brittle transition temperature by using plane strain specimen in tensile test and correlation with instrumented Charpy impact test: experimental and numerical study

“…The improved mechanical properties are attributed to the sub-cell structure introduced in austenite by processing HTMT and inherited martensite. In a tempered martensitic steel (0.3C -0.5Si -2Mn -0.001P -0.001S), Takebayashi et al [4] showed that refining microstructural parameters such as the prior austenite grain size improved the USE going up to 30J, but no significant effect of carbide size distribution was mentioned in this study. Im et al [5] showed that increasing molybdenum and reducing the carbon content can improve the USE, by enhancing precipitation of M 2 C-type alloy carbides instead of cementite, whose size is much lower than the critical carbide size to initiate fracture.…”

“…In the second part of the curve, the load decreases steadily or abruptly up to the end of the test, due to crack propagation and final fracture. As reviewed in [4], the shape of this curve strongly depend on the test temperature. Abrupt load drops were attributed to brittle, unstable crack propagation, whereas ductile fracture generally induces a stable decrease in the load vs. displacement curve.…”

“…Following the analysis of fracture energy in terms of initiation and propagation parts for martensitic steels proposed by Takebayashi et al [4], the amount of energy absorbed during initiation and propagation stages are plotted against test temperature in Figs 8a and 8b, respectively. The main tendencies were as follows: (i) there iwas no dependence of the initiation energy with temperature, even if initiation energy of M specimens tend to be superior to that of P specimens at a given test temperature ; (ii) for both microstructures, the change in fracture mechanism from brittle to ductile was correlated to a transition in propagation energy, the higher USE of M specimen being attributed to the propagation part of the ICIT curve.…”

Study of the Effect of Multiple Tempering on the Impact Toughness of Forged S690 Structural Steel

“…This evidence is in accordance with literature results, obtained on different steels. In detail, Takebayashi et al [20] found that carbide size and distribution significantly influence impact toughness of martensitic steels.…”

The Relationship between Strength and Toughness in Tempered Steel: Trade‐Off or Invariable?

“…Therefore, the carbide precipitation would be the major factor in promoting TME in this study. Lee and Takebayashi reported a linear relation between the carbide size and the critical distance for cleavage crack initiation, revealing that the coarse carbides are detrimental to the impact toughness. Several researchers thought that the distribution of carbides along the martensite lath and prior austenite grain boundaries causes embrittlement of materials, promoting rapid fracture.…”