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Research Papers: Materials and Fabrication

# Developing New Cast Austenitic Stainless Steels With Improved High-Temperature Creep Resistance

[+] Author and Article Information
Philip J. Maziasz, John P. Shingledecker, Neal D. Evans

Oak Ridge National Laboratory, Oak Ridge, TN 37830

Michael J. Pollard

Caterpillar Technical Center, Peoria, IL 61656

J. Pressure Vessel Technol 131(5), 051404 (Sep 02, 2009) (7 pages) doi:10.1115/1.3141437 History: Received April 30, 2008; Revised September 19, 2008; Published September 02, 2009

## Abstract

Oak Ridge National Laboratory and Caterpillar (CAT) have recently developed a new cast austenitic stainless steel, CF8C-Plus, for a wide range of high-temperature applications, including diesel exhaust components and turbine casings. The creep-rupture life of the new CF8C-Plus is over ten times greater than that of the standard cast CF8C stainless steel, and the creep-rupture strength is about 50–70% greater. Another variant, CF8C-Plus Cu/W, has been developed with even more creep strength at $750–850°C$. The creep strength of these new cast austenitic stainless steels is close to that of wrought Ni-based superalloys such as 617. CF8C-Plus steel was developed in about 1.5 years using an “engineered microstructure” alloy development approach, which produces creep resistance based on the formation of stable nanocarbides (NbC), and resistance to the formation of deleterious intermetallics (sigma, Laves) during aging or service. The first commercial trial heats (227.5 kg or 500 lb) of CF8C-Plus steel were produced in 2002, and to date, over 27,215 kg (300 tons) have been produced, including various commercial component trials, but mainly for the commercial production of the Caterpillar regeneration system (CRS). The CRS application is a burner housing for the on-highway heavy-duty diesel engines that begins the process to burn-off particulates trapped in the ceramic diesel particulate filter (DPF). The CRS/DPF technology was required to meet the new more stringent emissions regulations in January, 2007, and subjects the CRS to frequent and severe thermal cycling. To date, all CF8C-Plus steel CRS units have performed successfully. The status of testing for other commercial applications of CF8C-Plus steel is also summarized.

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## Figures

Figure 9

The new Caterpillar regeneration system housing exhaust component using CF8C-Plus cast stainless steel. CF8C-Plus steel was chosen for the CRS housing used as part of the diesel particulate filtration system in July, 2006, and has been employed on all heavy-duty highway truck diesel engines since the end of 2006 to meet the new emissions regulations in January, 2007. The adjacent turbocharger housing is made from SiMo cast iron. Exhaust from the turbocharger is injected with fuel to begin the combustion that cleans the particulate from the DPF, so the CRS is subjected to high temperatures and rapid thermal cycling. To date, over 27,215 kg (300 tons) of CF8C-Plus steel have been cast for this application.

Figure 8

Creep-strain versus time plot for creep-rupture testing in air at 750°C and 65–85 MPa for CF8C-Plus and CF8C-Plus Cu/W austenitic stainless steels for specimens taken from sand-cast keel blocks. Creep-rupture tests approach or exceed 20,000 h, and the CF8C-Plus Cu/W clearly exhibits more creep resistance than CF8C-Plus steel, due mainly to a very low creep-rate in the secondary-creep regime.

Figure 7

Out-of-phase thermomechanical fatigue testing in air, with a thermal cycle ranging from 300°C to 760°C for CF8C-Plus cast stainless steel, but a 300–700°C thermal cycle for SiMo cast iron. Nf is the number of cycles to failure. A relatively slower heating/cooling rate of 50°C/min was used for both alloys. CF8C-Plus has significantly more TMF resistance than SiMo cast iron, despite being subjected to a higher maximum cycle temperature.

Figure 6

CF8C-Plus steel back-scattered SEM (a) and (b) and XEDS (c) microstructural analysis after creep-testing at 850°C/35 MPa for 23,598 h. No sigma phase is found in the creep-tested specimens, and some Cr-rich M23C6 carbides are found nucleated adjacent to coarse NbC particles found in the interdendritic regions of the cast structure.

Figure 5

Comparisons between CF8C (a)–(d) and CF8C-Plus (e) and (f) materials in either the as-cast condition, or aged for 3000 h at 750°C. (a) and (b) show the XEDS spectra obtained from analysis of particles of delta-ferrite and sigma phase, respectively, found in standard CF8C steel. By contrast, the CF8C-Plus steel has no delta-ferrite in the initial as-cast structure and hence has no sigma-phase formation after aging. (c)–(f) are back-scattered SEM images of metallographically polished and unetched specimens.

Figure 4

Rupture elongation versus time for CF8C, CF8C-Plus, and CF8C-Plus Cu/W steels, all creep-tested between 650°C and 850°C at 35–200 MPa. CF8C-Plus and CF8C-Plus Cu/W do not form embrittling grain boundary phases during creep, resulting in excellent rupture ductility compared with that of CF8C steel, which initially contains 15–25% delta-ferrite that transforms to sigma phase at elevated temperatures. Test data in this figure correspond to the same creep-tested specimens used to generate the data in Fig. 3. In the legend, EL stands for total elongation, and RA stands for reduction-in-area.

Figure 3

Larson–Miller parameter comparisons between CF8C-Plus steel, CF8C-Plus Cu/W steel, and standard CF8C steel. The top abscissa gives the estimated temperature (°C) for rupture in 100,000 h. The creep strength of CF8C-Plus steel is much better than that of standard CF8C steel, and the CF8C-Plus Cu/W shows a strength boost relative to the CF8C-Plus steel. Most data come from static-cast keel blocks or centrifugally-cast rings, but these data also include recent tests on thinner sections in step-cast ingots.

Figure 2

Larson–Miller parameter plot showing CF8C-Plus steel compared with various high-strength wrought austenitic stainless alloys and to Ni-based alloy 617. The top abscissa gives the estimated temperature (°C) for rupture in 100,000 h. The creep strength of CF8C-Plus steel is as good or better compared with the best commercial wrought stainless steels.

Figure 10

A gas-turbine end-cover component cast with CF8C-Plus austenitic stainless steel. The 6700 lb casting was made by MetalTek International for Solar Turbines, as a prototype test for their Mercury 50 industrial gas-turbine engine. Pictured is one of the co-authors, Mike Pollard, at the Caterpillar Technology Center. Inspection of the surface and sectioned pieces showed this test component to be crack/defect free.

Figure 1

Larson–Miller parameter plot showing candidate diesel exhaust alloys. CF8C-Plus steel shows superior creep strength compared with current alloys.

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