Deboronization in Boron Steel and a Model for Its Calculation

Abstract

Deboronization behavior during austenitizing of 0.2%C-0.85%Mn boron-treated steels was investigated in the various atmospheres. A considerable amount of boron removal was observed during the heat treatment in the air, and neither an atmosphere of argon nor a nickel-plating on metal surfaces had detectable effect in preventing this phenomenon. Deboronization could be eliminated only in the vacuum of 10^-4 Torr with a zirconium oxygen getter.
Boron concentration profiles after deboronization occasionally showed some anormalous appearances which were unexplainable by a diffusion control mechanism. One was a two-step fashion in a boron concentration profile and the other was an enhancement of deboronization at a certain temperature range. The latter, for instance, involved a lower residual boron concentration at 850°C rather than at 950°C.
A probable model for deboronization kinetics was discussed taking a dissolution of fixed boron as precipitates together with a diffusion of free boron into consideration. In addition, the effect of formation of ferrite owing to a simultaneous occurrence of decarburization was also taken into consideration. A finite-difference method was applied to the calculation of boron profile and it was shown that the above-mentioned anormalous phenomena were predictable.