Coupled model for carbon partitioning, diffusion, Cottrell atmosphere formation and cementite precipitation in martensite during quenching

Abstract

High-strength low alloy martensitic alloy steels are produced by quenching methods to achieve a martensitic microstructure. The carbon supersaturation of the martensitic structure serves as a driver for autotempering, which has advantageous effects on the physical properties of the steel and may take place even at very high cooling rates. So far, the precipitation kinetics during the quenching of low alloy martensitic steels have been modelled with by assuming no carbon loss due to diffusion from martensite into the inter-lath austenite, and the partitioning and diffusion has been modeled without considering the precipitation, although previous thermodynamic calculations show both precipitation and partitioning occur at similar rates, and thus should be modeled concurrently. In addition, the segregation of carbon to the dislocations needs to be taken in to account. The aim of this work was to develop such a coupled model that can predict these phenomena concurrently in the context of martensite formation during rapid quenching. By comparing the model predictions with experimental data on two steel grades austenized and subsequently quenched at two cooling rates (120 °C/s and 1000 °C/s), it was found that the calculated maximum radius of the precipitates as well as their number distributions were in good agreement with experimental observations. In further work, it is possible to extend the model to account also for more complex heat cycles.