J. Hawk, P. Jablonski
National Energy Technology Laboratory,
Keywords: steel, high temperature, heat
Summary:Alloys used in fossil energy power generation must be stable for extended times with some components having expected lifetimes of up to 30 years. For temperatures in excess of 600C, advanced 9% Cr steels are used. The 9% Cr steels possess martensitic structure and are strengthened by a variety of mechanisms at different length scales. Overall microstructural stability is provided through a 3-dimensional network of carbide that hold together the many structural sub-elements during use. As such the approach used at NETL to design and manufacture heat resistant advanced 9% Cr steels for creep strength and microstructure stability will be discussed with the results on 9% Cr steel in both wrought and cast forms presented. In particular, creep capability and the steel’s resistance to corrosion will be discussed in detail and compared against similar results from commercial and developmental 9% Cr steels. We find that our close attention to detail with use of scalable manufacturing techniques leads to microstructural control and long term stability and thus a long useful life. Steps including alloy design, source material selection, melting, homogenization, hot working (in the case of wrought materials) and finally microstructural and property evaluations are important. These efforts have lead recently to a US patent: 9,181,587 for a creep resistant high temperature martensitic steel. This new alloy, called NETL-CPJ7, has comparable or better creep performance as the best commercial alloys in this class at a 50F higher temperature (as shown in the figure below). This is a significant game changer. The cost of the alloy is not expected to be significantly different from established alloys since there are not large quantities of expensive elements included and the processing is conventional. This alloy has the potential to reduce the cost of a variety of advanced energy systems, as the increase temperature capability may allow for substitution of more expensive austenitic steels or even Ni-alloys.