Completed notes of the course

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1 Chiara Moreschini NOTES OF APPLIED METALLURGY Prof. C. Mapelli – AA 2021/22 0. INTRODUCTION STEEL STRUCTURAL CONSTITUENTS à See chapter 2.1 of the book The diagram shows the equilibrium phases of steel, which are: a) Austenite (𝛾) b) Ferrite (𝛼) c) Cementite (Fe3C) d) Perlite (𝛼 + Fe3C) 2 The main temperatures to be remembered on the phase diagram are: - A1 equilibrium temperature of alpha to gamma phase; - A3 alpha to gamma transformation temperature, when we reach this line the austenite starts to decompose to form ferrite; - ACM equilibrium temperature between austenite and cementite. Non-equilibrium phases: bainite and martensite The diagram only shows the equilibrium phases of steel, but there are also non-equilibrium phases reached when the cooling rate of the steel is too high, such as: a) Bainite (lower and upper) b) Martensite à Bainite is formed when the CR (cooling rate) of austenite is too high to allow the formation of the lamellae which characterize the pearlite equilibrium phase, so we have the formation of a phase still composed of Fe3C and ferrite but this time we have Fe3C particules inside a matrix of ferrite instead of lamellae. In case of bainite we have still nucleation and growth of the grains because the CR is high but not critical (diffusion of carbon allowed even if small). à The formation of martensite is instead characterized by a critical CR which do not allow any diffusion of carbon inside the steel, so we have a diffusionless transformation in which any nucleation and growth happen but we have the instantaneous formation of the new phase. The formation of martensite is the only transformation with no diffusion, while all the oters (also bainite formation) have at least a little C diffusion. N.B. • Carbide: compound composed of carbon…