Boron-Containing Precipitates Formed on the Prior Austenite Grain Boundaries in High Strength Low Alloy Steels

Abstract

This paper deals with the identification of boron constituent and its transition to other ferrous boroncarbides during heat treatment. In order to observe boron constituent clearly, the present authors adopted a new technique by which boron constituent on intergranular fractured surface is observed without performing etching.
Boron constituent forms at austenite grain boundaries by step quenching, but this boron constituent begins to transform to fine M₂₃(CB)₆ particles from the periphery during holding below A_C1. Boron constituent never forms in carbon free steels, and thus it must contain carbon atoms.
It is considered that the boron constituent is not identical with M₂₃(CB)₆ bacause of 1) the above mentioned transition during heat treatment and 2) the difference in response to etching between them.
Globular M₂₃(CB)₆ directly forms from ferrite by reheating at temperature below A_C1 after the specimen is quenched with so high cooling rate that martensite is produced from austenite without forming boron constituent.