Alloy development for high Cr martensitic steel

Steam power plants burning fossil fuels (coal, oil, natural gas) generate most of the global electricity, and in countries such as China, India, Russia and Poland fossil fired power plants generate 80% or more. Though huge efforts are made to transform power production to renewable energy such as wind and solar, fossil power will still have roles in coming decades to maintain weather-independent power supply and to provide cheap and reliable electrification in developing countries.

The efficiency and harmful emission from steam power plants are highly dependent on the operating parameters, such as steam temperature and pressure, which are limited by the properties of engineering materials used in high temperature components such as turbines, pipes and castings. Creep resistant martensitic 9-12% Cr steels are preferred materials because of their excellent combination of high creep strength, corrosion resistance and relatively low cost.

The strongest commercial steels available today are 9% Cr steels, which allow steam tempera-tures up to 620 °C. Efforts are ongoing to increase steam temperatures to 650 °C by improving the creep strength and corrosion resistance of the martensitic steels. Corrosion resistance is achieved by increasing of the Cr content up to 11-12%.

However, so far the alloy concepts for creep resistant 11-12% Cr steels have failed, since they relied on precip-itation strengthening from fine MX ((V,Nb) (C,N)) carbonitrides, which transform into coarse Z-phase (Cr(V,Nb)N) nitrides causing a loss of precipitation strengthening and creep strength breakdown.

In the present Phd project a promising alloying concept combining reduced nitrogen and added boron to provide microstructure stability of 9-12% Cr martensitic steels is investigated. The aim of the research is to improve the understanding of strengthening mechanisms in low nitrogen martensitic steels.

Investigations were made in two directions: microstructure characterization of new experimental 11% Cr martensitic steels and study of boron behavior in various 9-10% Cr martensitic steels. Four 10-11.2% Cr tempered martensite steels alloyed with nitro-gen, titanium and boron were investigated after long-term aging and creep.

Their microstructure evolution was characterized by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). It was found that slight differences in the contents of nitrogen and titanium significantly affect the microstructure stability and creep behavior. When all nitrogen in the steels is tied up as TiN the creep strength is low.

If free nitrogen is available after TiN formation high creep strength and microstructure stability can be achieved for 10% Cr steels, presumably by the formation of fine nitrides. In 11.2% Cr steels with free nitrogen available after TiN formation, Z-phase formation led to creep strength breakdown after 3,000-5,000 h at 650 °C.

The studies of boron comprised development of a site-specific sam-ple preparation technique using focus ion beam milling, which allowed speeding up of successful sample fabrication for atom probe tomography (APT). Detailed investigations of boron segregation behavior during nor-malizing from 1100 › were made on three martensitic steels containing 10 ppm, 70 ppm and 100 ppm of boron.

Fine M2B borides (15×70 nm) were found to form on prior austenite grain boundaries during normaliz-ing of the 100 ppm boron steel. Lower boron content did not show such boride formation. APT measurements showed that boron segregated to austenite grain boundaries in similar concentrations (0.8 at.%) in the 10 ppm boron and the 70 ppm boron steels, but significant amounts (25-40%) of the added boron remained in the grain center after nor-malization.

Half (45-55%) of the added boron in the 10 ppm boron and 70 ppm boron steels was located by the APT measurements. In the 10 ppm boron steel the remaining boron was found to form boronitrides. The measured concentration profiles near boundaries did not fit to profiles predicted from theoretical models of diffusion limited equilibrium segregation or non-equilibrium segregation.

The present quantitative study of boron segregation provides data, which are believed to be highly useful for the optimization of boron additions to the creep resistant martensitic steels. It should be followed up with further quantitative studies of boron behavior during tempering and creep.