Jae Hoon Jang

Interphase precipitation in Ti–Nb and Ti–Nb–Mo bearing steel

Abstract The interphase precipitation of carbides has been studied in two microalloyed steels containing Ti and Nb, but only one of which contains molybdenum. The precipitates obtained are, therefore, (Ti,Nb)C and (Ti,Nb,Mo)C respectively. It is found that molybdenum significantly reduces the size of the carbide precipitates and also strongly retards their coarsening behaviour during subsequent heat treatment. This is because it enhances the nucleation rate by reducing the interfacial energy, but it is not thermodynamically favoured within the (Ti,Nb,Mo)C, so the later stages of growth involve its partitioning into the matrix, thus explaining the reduction in coarsening rate relative to the molybdenum free steel.

Modelling coarsening behaviour of TiC precipitates in high strength, low alloy steels

Abstract Some of the most modern automotive sheet steels rely on a dispersion of fine precipitates based on TiC, generated during the major phase changes that occur as the rolled material is cooled to the coiling temperature. The coils themselves cool extremely slowly, thus leading to the coarsening of the precipitates and a loss of strength. Beginning with a calculation of the interfacial energy, the precipitate coarsening kinetics are modelled as a function of the stoichiometry of titanium and carbon. The purpose was to assess the influences of interface energy and Ti/C stoichiometry which limit the rate at which the dispersion coarsens by the diffusion of solute from the small to the larger particles. It is found that Ti/C ratio plays a critical role; a titanium concentration which is slightly less than required to combine with carbon leads to a dramatic reduction in the coarsening rate.

First-Principles Calculations and the Thermodynamics of Cementite

Thermodynamic data for the substitution of silicon and manganese in cementite have been estimated using first-principles methods in order to aid the design of steels where it is necessary to control the precipitation of this phase. The need for the calculations arises from the fact that for silicon the data cannot be measured experimentally; manganese is included in the analysis to allow a comparison with its known behaviour. The calculations for Fe3C, (Fe11Si4c)C4, (Fe11Si8d)C4, (Fe11Mn4c)C4 and (Fe11Mn8d)C4 are based on the total energy all-electron full-potential linearized augmented plane-wave method within the generalized gradient approximation to density functional theory. The output includes the ground state lattice constants, atomic positions and bulk moduli. It is found that (Fe11Si4c)C4 and (Fe11Si8d)C4 have about 52 and 37 kJ greater formation energy when compared with a mole of unit cells of pure cementite, whereas the corresponding energy for (Fe11Mn4c)C4 and (Fe11Mn8d)C4 is less by about 5 kJ mol1. These results for manganese match closely with published trends and data; a similar comparison is not possible for silicon but we correctly predict that the solubility in cementite should be minimal.

Segregation of phosphorus to ferrite grain boundaries during transformation in an Fe–P alloy

Abstract A binary alloy of iron containing 0.17 wt.% of phosphorus has been heat treated under a variety of conditions in order to see whether the segregation of phosphorus to grain boundaries can be controlled. The alloy transforms fully into ferrite. It is found that the majority of solute found at the ferrite grain boundaries has its origins in the temperature range where phase transformation occurs, in other words, phosphorus that is accumulated and dragged with the growing ferrite–austenite transformation front. As a consequence, it cannot be suppressed using cooling rates as high as 400 K s−1.