Heating up austenitic stainless steels

Materials World magazine
1 Jul 2007

Researchers at Oak Ridge National Laboratory, USA, claim to have developed inexpensive austenitic stainless steels that can withstand higher temperatures while maintaining creep strength and oxidation resistance.

The drive towards higher operating temperatures is with the aim to improve the efficiency of energy-producing, chemical and processing systems that employ austenitic stainless steels in turbine recuperators, heat exchangers, piping and tubing.

Conventional materials use nano carbide or nitride precipitates to achieve creep strength. Chromia surface scales are formed for oxidation resistance at up to 800-900ºC. But these scales are only effective in clean dry air – volatilization in water vapour-containing environments can restrict the maximum operating temperature to as low as 600-650ºC.

Michael Brady of the R&D team at Oak Ridge explains, ‘Water vapour is present in virtually all combustion environments and is therefore relevant for energy production. Chemical and process industries also involve exposure to aggressive carbon and sulphur-containing species under low oxygen conditions – this can interrupt the formation of a protective chromia scale and result in accelerated rates of oxidation and corrosion.’

Alumina coatings, which are currently used to protect the materials under these conditions, can increase costs and performance concerns. Replacing iron-based austenitic stainless steels with corrosion-resistant nickel-based alloys or super alloys, meanwhile, can be 5-10 times more expensive due to the high cost of nickel and the need for special casting procedures.

The team at Oak Ridge sought to address this by introducing aluminium into the stainless steel alloy composition itself to form protective alumina scales. Brady says, ‘Alumina scales offer greater thermodynamic stability than chromia and are highly stable in water vapour, thus [providing] protection at higher temperatures.’

He adds, ‘This has been tried on and off for the past 30 years. Efforts tended to rely on intermetallic phase strengthening or high levels of aluminium (four to five weight per cent (wt.%) of the material) such that the austenitic matrix was destabilised. Alumina is a strong body centred cubic (bcc) stabiliser, [resulting] in a loss of creep resistance. Body centered cubic iron exhibits poor high temperature strength.

‘To our knowledge, we are the first to succeed in combining alumina formation and creep resistance in the ~600-800ºC range, [and] at a comparable cost to existing stainless steel alloys [due to the] relatively low nickel level (less than 20-25wt.%).’

By adjusting the chromium and nickel levels in the material, and using smaller quantities of aluminium, researchers at Oak Ridge have created a single-phase stable austenitic matrix. The new alloy is based on Fe 20Ni 14Cr 2.5Al wt.% – iron (Fe), nickel (Ni), chromium (Cr) and aluminium (Al).

Creep resistance is optimised using conventional MC-type nano carbide precipitates (MC equals niobium, vanadium or titanium). If used, only low levels of vanadium and titanium are added (alone not together), as they impact on the material’s ability to form alumina scales.

Creep-rupture lifetime now exceeds 2,000 hours at 750ºC and 100 megapascals in air, comparable to advanced austenitic stainless steels, such as NF709, used today. Oxidation resistance in water vapour-containing air has been demonstrated at up to 800ºC.

The team now hopes to exploit this research in a range of iron-based alloys and are testing these materials to determine application areas, such as under sulphidising and carburising conditions.

Brady adds, ‘Early indications [demonstrate that] these alloys should be scalable for industrial production. They also show good weldability. We plan to pursue collaboration with industrial partners [for] practical use.’