Effect of Structure on Ductile–Brittle Transition in Steels with BCC-Lattice

Abstract

The effect of structural characteristics (ferrite, ferrite + pearlite, sorbite) including morphology, size and distribution of nonmetallic inclusions of different morphology, sizes and distribution, on ductile–brittle transition in structural steels is considered. It is shown that an elementary ductile microcrack is initiated by particles of critical size. For steels with ferrite-pearlite and sorbite microstructure, the particle size (carbide, nitride and nonmetallic inclusions) is almost the same. The ductile–brittle transition is caused by competition of the size of the brittle and ductile (pits) microcracks. As a result, the critical brittleness temperature T_c in simplified form looks like \( {T}_c={T}_c^0+B{\Lambda}_d^{1/2}/{\Lambda}_b, \) where elementary microcracks Λ_b and Λ_d correspond to the size of a brittle crack of transcrystalline cleavage (Λ_b) and the size of ductile crack (Λ_d), Т_c⁰ reflects the contribution of different strengthening mechanisms to metal yield strength: dislocation, solid solution, grain size, precipitation hardening, and others, and B is a coefficient taking into account the stress-strain state in the fracture zone. As part of this model of failure, it becomes clear why for hardened steel the grain size does not change up to the recrystallization temperature, and yield strength and the value of T_c have a complex dependence on the tempering temperature. Since the steel has particles of different sizes, they initiate the occurrence of pits of different sizes. The larger the particle, the sooner a micropore originates around it and pit growth is faster. A wide range of pit sizes arises that causes a wide range of T_c values. A close connection is demonstrated for the width of the ΔT_c ductile–brittle transition temperature range with a change in the distance between particles, including between pearlite colonies. This relationship can be positive and negative, which is confirmed by experiment.