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%).