Precipitation behaviour and stability of nitrides in high nitrogen martensitic 9% and 12% chromium steels

Precipitation behaviour and stability of nitrides has been investigated in two high nitrogen, high vanadium containing martensitic 9-12% chromium steels under different heat treatment conditions. Microstructural evolution during the heat treatments was studied by hardness measurements, optical microscopy, transmission electron microscopy and energy dispersive X-ray analysis. Nitrides with f.c.c, structure were found to precipitate very uniformly and densely during aging in the austenite (ausaging) as well as during aging in the ferrite (tempering) at temperatures between 823 and 973 K. Microstructural evolution during tempering was influenced by pre-precipitation of the nitrides in the austenite prior to the martensitic transformation. It is shown that the hardness of martensite and tempered martensite depend strongly on the size and distribution of nitrides. Niobium is dissolved in primary vanadium nitrides. Dissolution and reprecipitation of these nitrides in the austenite was found to be influenced by the presence of niobium. Ausaging treatment df a niobium containing alloy at 898 K has a more significant effect on the martensite hardness than a niobium free alloy. The stability of dense precipitation states produced in the austenite prior to the martensitic transformation is discussed in terms of the development of non uniform precipitation states during tempering of the martensite. The uniformity of the precipitation states is stabilized by a pre- overaging treatment in the austenite. It is shown for a martensitic 12% chromium alloy, that a pre-ausaging treatment retards the formation of the rapid coarsening hexagonal chromium nitride which dissolves a high amount of vanadium during tempering at 973 K. As a result the tempering resistance becomes improved by maintaining an increased density of face centered cubic nitrides precipitated in the austenite.