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Model carbonitride precipitation kinetics in Ti-microalloyed HSLA-type steel

Opiela M.

Archives of Materials Science and Engineering

2016

Vol. 77, nr 2

78--84

Purpose: The work presents the model of precipitation kinetics of M(C,N) carbonitride in microalloyed steel, containing microaddition of Ti, under conditions of isothermal holding in austenite. The model has been based on the classical theory of nucleation, proposed by Kampmann and Wagner. Design/methodology/approach: In order to analyse precipitation kinetics of Ti(C,N) carbonitride in steel, containing 0.31% C, 0.0043% of N and 0.033% Ti, after austenitizing at the temperature of 1150°C with successive isothermal holding at the temperature of 900°C for 600 s, the CarbNit_kinet computer program has been used. The following physical data have been used for calculations: solubility products of carbides and nitrides, density of carbides and nitrides, parameters of mutual interaction of elements present in austenite, parameters of crystalline structure of carbides and nitrides, energies of interfacial boundaries between the precipitation and the matrix and diffusion coefficients of elements included in carbonitrides in austenite and ferrite. Findings: The calculated results contain data concerning: distribution of particle’s radius, nucleation rate as a function of time, particle’s number within the volume unit as a function of time, chemical composition of the matrix as a function of time and the per cent volume of the particles as a function of time. For given conditions, nucleation of Ti(C,N) carbonitrides in the investigated steel starts after 1 s and is finished after approximately 10 s. Conducted analysis revealed that the coagulation process of precipitations, in which the quantity of precipitations decreases in unit volume, occurs after more than 200 s. Whereas, distinct changes in the content of Ti, C and N dissolved in austenite was observed after the time of 8 s, subsequent to which the process of increase of precipitations begins. Research limitations/implications: Presented model of kinetics enables the analysis of precipitation process of only simple M(C,N) type carbonitrides. Calculation results strongly depend on physical parameters of the model, and in particular, on energy of interfacial boundaries. Practical implications: With the use of applied model, it’s possible to evaluate the content of precipitations of carbonitrides and distribution of their size, calculated on the basis of chemical constitution of steel and parameters of the manufacturing process. Originality/value: The model enables to distinguish specific stages of the precipitation process: nucleation, growth and coagulation. Despite the simplifications, the model makes it possible to predict changes occurring in the precipitation kinetics caused by changes in chemical composition.