Three alloy steels with typical microstructures, low alloying ultra-high strength steel 40CrNiMo, high strength and toughness steel CF62 and medium carbon pearlite-ferrite steelS34MnV, were selected for study of the cleavage fractures.
The 40CrNiMo steel was designed to have dislocation strengthened martensite with dispersed ε-carbide and little amount of retained austenite between the laths. The tempered embrittlement was studied by Charpy V notch impact test for the quenched and tempered samples. The hydrogen embrittlement behavior was investigated in the quenched tensile samples. The effects of the carbide precipitation on the hydrogen embrittlement and characteristic distance in the impact cleavage fracture were discussed
The effect of grain size on the cleavage fracture of the S34MnV steel was studied by reasoning the connection between the effective grain size and the cleavage characteristic distance. The strength and toughness after the second simulated normalization were analyzed from three aspects of the effective grain size, the pearlite content and the lamellar spacing.
Three CF62 steels, named Steels A, B and C, with similar composition, except the Ti and N levels, were prepared by the same rolling processes. The steels have the resultant microstructure of granular bainite. However, they showed totally different impact toughness. By eliminating the possible factors, such as effective grain size, martensite-austenite component (MA) and the microalloy precipitation, the coarse TiN inclusion was finally found to be the right reason of deteriorating the toughness of Steels B and C by using scanning electron microscopy (SEM). The distribution and morphology of TiN inclusions were analyzed by using transmission electron microscope (TEM) and SEM. It was found that in Steel A, Ti and N elements dispersed in the matrix in form of fine TiN precipitates, while in Steels B and C there were high density of TiN inclusions, which nucleated at the deoxidization Al2O3 particle. The different precipitation behaviors of TiN in steels were analyzed by means of alloy thermodynamics. The effect of TiN inclusions on the ductile-brittle transition temperature (DBTT) was phenomenologically explained. The crack growth mechanism of TiN inclusion initiating cleavage fracture was explained by taking the stress distribution ahead of the notch into account.
At last, the general rules of impact cleavage fracture were extracted from the cleavage behaviors of these three alloy steels with different microstructures and strengths.
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