Dispersion hardening by niobium carbonitride precipitation in ferrite

Abstract

The formation offine carbonitride distributions in an experimental, low carbon, high niobium microalloyed steel transformed at various cooling rates was accompanied by TEM observations, tensile tests, and hardness measurements. Interphase precipitation or carbonitride formation on ferrite dislocations were observed to be the only precipitation modes which were responsible for the nucleation of new particles inferrite. An increase of only ∼ 90 MPa in yield strength was found as the result of carbonitride precipitation on dislocations that occurred in an acicular ferrite microstructure with a dislocation density of (3–5) × 10¹⁰ cm^-2. A yield strength contribution of ∼ 200 MPa was associated with interphase precipitation in polygonal ferrite formed during slow cooling of undeformed austenite. However, interphase precipitation was encountered only occasionally and did not contribute to the strength in the presence of carbonitride particles nucleated during austenite deformation.