Precipitation kinetics of niobium carbide and copper in a low carbon, chromium-free steel

Co-precipitation of niobium carbide and BCC copper is being investigated as a basis for making a high strength, easily weldable, low carbon, chromium-free structural steel as an alternative to tempered martensite. Theoretical curves were calculated for the nucleation and growth rates of NbC and BCC Cu precipitates in ferrite as a function of temperature for various austenizing temperatures. Calculated nucleation and growth rates for NbC in ferrite were significantly influenced by austenizing temperature while Cu precipitation kinetics were not effected. Nucleation and growth rate curves for copper and the nucleation rate curve for NbC showed the typical inverse “C” type behavior. For all austenizing temperature, the BCC Cu nucleation rate was orders of magnitude faster than NbC. Aging treatments selected on the basis of the computed curves are in approximate agreement with them.