Introduction
Nitride [1, 2], carbide [3] and carbonitride [2]
precipitates in steels pin grain boundaries. Depending on the chemical
composition of the steel, above certain temperatures during heat
treatment, rolling and/or welding, the austenite grains tend to coarsen
because the precipitates are taken into solution, leaving the grain
boundaries unimpeded [4-6]. Formation and coarsening of nitride and
carbonitride precipitates, particularly in microalloyed steels, have
been extensively studied experimentally due to their influence on the
mechanical properties of products [1] and the formation of cracks
during shaping of steels [7, 8].
Titanium Nitride (TiN) precipitates in microalloyed steels extend the
temperature range over which the grain boundary pinning is effective,
due to a significantly higher solution temperature of these precipitates
compared to other precipitates that are known to restrict grain growth
[1,2].
Therefore, extensive efforts have been made to optimise the Ti/N ratio
as a major microstructural control factor for a variety of steels [1,
2, 9-11]. A high concentration of Ti and N promotes the formation of
coarse TiN precipitates that deteriorate toughness. Consequently, a
reduced content of Ti and N is suggested while simultaneously retaining
a minimum Ti/N ratio to obtain suitable hardenability [1].
It should be recognised that the dissolution and growth of TiN
precipitates and austenite grains are kinetically controlled phenomena.
Therefore, time is a governing factor for the size and distribution of
precipitates. However, the formation temperatures of precipitates can be
determined thermodynamically as a function of chemical compositions.
This information guides the design of experiments and consequently
minimises the experimental data required to optimise the Ti/N ratio for
various compositions. In the current study, the effect of Ti, Niobium
(Nb) and N concentrations are assessed on precipitation of common
linepipe steels as a case study using CALPHAD (CALculation of PHAse
Diagrams) method. The approach presented here can be applied to various
steel compositions for more efficient design of experiments.
METHOD
Several industrial compositions of linepipe steels were adopted to
calculate the phase diagrams and study the composition and formation
temperature of nitride and carbonitride precipitates, aiming to explore
the possibility of calculated methods to minimise the number of
experiments required to develop the optimum Ti/N ratio and microalloying
element concentrations. The ThermoCalc software package [12], using
the thermodynamic databases of TCFE7 [13], was employed to study the
composition and formation temperature of nitride and carbonitride
precipitates in titanium and niobium microalloyed linepipe steels. This
software package is based on minimizing the Gibbs free energy of the
individual phases in the equilibrium state. It uses the CALPHAD method
to extrapolate thermodynamic descriptions for use in an n-component
system, based on the assessment of binary and available ternary and
quaternary experimental data stored in the thermodynamic database. The
thermodynamics of the liquid phase is described by a regular solution
model and the solid phases by sub-lattice models. The phase equilibria
are calculated by a free-energy minimization determined by a
Newton-Raphson technique. The databases are used to calculate the phase
fractions, phase compositions and transformation temperatures under
thermodynamic equilibrium conditions [14, 15], providing an
efficient calculation technique to predict the equilibrium amount and
composition of the stable phases of complex system under a given set of
conditions [16]. Phase diagram sections can be determined with up to
five independent variables in a very complex multi-component and
heterogeneous system (up to 40 elements and 1000 species) [13].
RESULTS AND DISCUSSION
Table 1 summarises the chemical composition of a number of
common linepipe steels with various Ti/N ratios. The linepipe steels
contain elements such as phosphorous, sulphur, calcium and aluminium in
trace levels, which have marginal impact on the composition and
temperatures of the precipitates within agreed compositions. These
elements were disregarded in the calculation of phase diagrams to
minimise the complexity of calculations and improve the feasibility of
achieving reliable results from the calculated phase diagrams. All
calculations were performed above 900 K where the diffusion of the
interstitial elements in the steel matrix is fast enough to reduce the
kinetic effect on nucleation and the chemical composition of
precipitates. For the Nb and Ti microalloying elements to diffuse one
micrometre at 900°C (1173K), it takes approximately 12 minutes and 14
seconds respectively, according to the diffusion coefficients for Nb
[17] and Ti [18] in steel. Therefore, the calculated phase
diagram should be treated with extra care at lower temperatures.
Two sets of calculations were performed, based on fixed Ti
concentrations and varying N contents. The compositions are referred to
as Low-Ti and High-Ti, according to the Ti concentration in the steels
indicated in Table 2.
Table 1 Chemical composition of common linepipe steels (wt%).