Influence of Microalloying Elements (Ti, Nb) and Nitrogen Concentrations
on Precipitation of Linepipe Steels - A Thermodynamic Approach
A CALculation of PHase Diagrams (CALPHAD) approach was used to study the
formation temperatures of nitride and carbonitride precipitates in
nominated linepipe steels. The calculated results were in good agreement
with relevant experimental data reported in the literatures, where the
optimum titanium to nitrogen ratio and austenite grain growth was
studied in similar steel compositions. The niobium concentration up to
0.12 wt% showed no influence on the formation temperature of nitrides
in a common linepipe steel composition, while significantly increased
the precipitation temperature of niobium carbides up to 0.03 wt%.
Nitride precipitates contained a high concentration of titanium while
niobium contributed mostly to the formation of carbide/carbonitrides.
Although the dissolution and growth of precipitates are controlled
kinetically, the thermodynamic calculation approach can be used to
efficiently predict the equilibrium amount and composition of the stable
phases in chemically complex systems. This results in a more accurate
design of experiments, to minimise the number of tests required to
obtain optimum chemical compositions and heat treatment procedures.