The development of titanium nitride strengthened creep resistant ferritic steels
Economic and environmental considerations have resulted in a worldwide drive to increase the cycle efficiency of fossil fired power plant. Boiler designs able to achieve significant efficiency increases already exist; the limiting factor is the performance of materials. As a result, much effort is currently being focussed on the development of enhanced materials to increase their operating temperature and/or pressure. The requirement that such materials should possess good thermal fatigue performance in addition to good creep performance dictates the selection of ferritic and martensitic steels for many components. Thus, most of the development effort in this field is currently focussed on martensitic steels that can operate beyond the current maximum plant design of 290 barg/580°C up to 335barg/630°C or even beyond. The most advanced conventional ferritic steels such as E911, P92, P122 and NF12 are 9-12% Cr martensitic steels and gain their creep strength from the tempered martensite structure and the precipitated carbides and nitrides. Their long term creep performance is ultimately limited by the rate at which these precipitates coarsen or otherwise transform over time at elevated temperature. This research work presents the development of an alternative alloy which aims to increase the high temperature long term creep performance by replacing the relatively low stability carbides and nitrides present in conventional ferritic steels with a thermodynamically more stable dispersion of titanium nitride particles. To overcome the solubility limitation on precipitating a significant level of fine titanium nitride and to remove the dimensional constraints of gas phase nitriding, the innovative technique being developed here is one of solid state nitriding using a nitride donor. The microstructure and properties of the titanium nitride strengthened steels have been assessed at each stage of the alloy development using a range of optical and electron microscope examination techniques and hardness, tensile and creep mechanical assessment techniques. The results have shown that the processing route plays an important role in the development of the titanium nitride particles and these in turn play an important role in the development of the grain structure. The initial evaluation of the creep rupture properties found them to be very poor, below that of the base material. This was due to two factors; relatively coarse titanium nitride particles and very fine grain size (due to the titanium nitride particles pinning) which resulted in extensive grain boundary sliding. This research, therefore, investigates the development of the entire processing route, including the development of powder metallurgy and spray forming procedures with the aim of achieving a homogeneous dispersion of fine titanium nitride particles to resist dislocation creep and the development of a coarse interlocking grain structure to resist grain boundary sliding. The achievements in the creep properties are presented in comparison with conventional ferritic creep resistant steels and advanced ferritic steels such as PM2000. The properties achieved are discussed, not only in relation to the beneficial aspects such as creep strength and the effect this has on boiler cycle efficiency, but also in relation to deleterious effects that are a consequence of reduced creep ductility. Finally, possible mechanisms to improve the properties as well as methods of reducing the production costs are assessed with a view to achieving the overall objective of developing a commercially viable material.