Precipitation of Fine Niobium Carbide Particles in Low Carbon Steels

Abstract

The preci/Jitation oj fin e carbide particles in the low carbon steels containing small amount oj niobium has been investigated mainly by means oj transmission electron microscopy. It was confirmed that the fine particles responsible Jar the hardening were NbC with Jace-centered cubic structure and were related to theJerrite in the way similar to the V,C 3 lJerrite orientation relationship obtained by Baker. In the specimens transformed ill the range 800° to 650°C, the Iliobium carbide particles Jormed initially at the allstmite graill boundaries and on the dislocations in austenite, and then the Jerrite nucleated at the austenite grain boundaries where the carbon atoms were delJleted by the NbC precipitation. The interphase preci/Jitation started at the tip oj these growing Jerrite grains. The strengthening is thought to be due to the friction stress Jar the dislocation to cut the particles though there exist some evidences that the Orowan mechanism was as well operative. III the tempered martensite, the NbC particles Jormed priferentially at the lath boundaries and on the dislocations. These particles contribute to the strengthening by both the precipitation hardming and the retardation oj the .ferrite recrystallization.