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High-Strength Low-Alloy Steel

Abstract Growth kinetics of Widmanstätten austenite in ferrite in high-strength low-alloy steel is based on a model that describes diffusion controlled growth of precipitates with shapes approximating to needles or plates, where all the factors that may influence the precipitate growth, i.e. diffusion, interface kinetics and ­capillarity, are accounted for within one equation. The ratio between calculated and experimental values of the radius of the advancing tip is inversely proportional to the degree of supersaturation. Following this theoretical work, the tensile behaviour of high-strength low-alloy steel after tempering is discussed, and well explained in view of the interactions of mobile dislocations and dissolved carbon and n­ itrogen atoms and their effects on the strain hardening exponent. In the final section, ­splitting during fracture of tensile and impact loading is examined. Delamination does not occur in the as-rolled condition, but is severe in steel tempered in the ­temperature range of 500–650 °C. Steel that has been triple quench-and-tempered to produce a fine equiaxed grain-size also does not exhibit splitting. It is concluded that the elongated as-rolled grains and grain boundary embrittlement resulting from precipitates (carbides and nitrides) formed during reheating are responsible for the delamination.

2.1 Kinetics of the Diffusion-Controlled Ferrite to Widmanstätten Austenite Transformation 2.1.1 Growth Theory 2.1.1.1 Precipitate Plates and Needles The equation relating the Peclet number (a dimensionless velocity parameter) p  =  Vρ/2D to the dimensionless degree of supersaturation Ω0 for the growth of plates is    V ρc √ Ω0 S2 ( p) (2.1) Ω0 = π/ p exp( p)erfc( p) 1 + Ω0 S1 ( p) + Vc ρ W. Sha, Steels, DOI: 10.1007/978-1-4471-4872-2_2, © Springer-Verlag London 2013

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2  High-Strength Low-Alloy Steel

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where V is the lengthening rate, D is the diffusion coefficient of solute in the matrix phase, erfc is the complementary error function, S1 and S2 are functions involved in the growth of plate. Other parameters can be calculated as below

ρc =

σ ν 1 − cα cα RT cβ − cα c0 − cα

(2.2)

where σ is the interfacial free energy per unit area between precipitate and matrix, ν is the molar volume of the precipitate, R is the gas constant, T is the temperature (in Kelvin), cα is the solid solubility of the controlling element in the parent phase α, cβ is the concentration of the controlling element in the new phase β, c0 is the concentration in the matrix before precipitation. The radius of curvature ρ  = 2f, where f is the focal distance of the parabola of either plate or needle (Fig. 1.1c), defined uniquely for a parabola lengthening along the Z direction and thickening along the X direction. In Eq. 2.1, Vc is the velocity or the lengthening rate of a flat interface during interface controlled growth (i.e. when almost all the free energy is dissipated in the transfer of atoms across the interface so that the concentration difference in the matrix vanishes)