Development of Novel Alloys for Use in Ethylene Cracking Furnaces

Cast chromia-forming austenitic stainless steels such as HP-type (25Cr-35Ni) and 34Cr-45Ni alloys are used in a wide range of industrial applications such as high temperature furnace components, radiant burner tubes, and ethylene cracking furnaces that demand high temperature microstructural stability, corrosion resistance, and creep strength. In particular, alloys used in ethylene cracking furnaces should possess good high temperature mechanical properties, coking resistance, must be fabricable using centrifugal/static casting, and must be weldable. Alloys such as cast HP-type alloys and 35Cr-45Ni achieve their environmental protection through the formation of a chromium-rich oxide scale. Recent work has shown that alloys that form a thin alumina-scale show better coking resistance in ethane steam cracking environments [1]. Although alloys that form alumina scales offer better corrosion protection and coking resistance at relevant temperatures, developing cast austenitic alloys that form a stable alumina scale and achieve creep strength comparable to existing cast chromia-forming alloys is challenging.

Cast alumina-forming austenitic stainless steels have recently been developed at ORNL for use from about 750°C to 1150°C [2]. In a recent study Muralidharan et al discussed the effect of composition on the alumina-forming capability and creep properties of cast alloys of composition Fe- 25 wt.% Ni- 14 wt.% Cr-3.5 wt.% Al and with varying levels of Nb, Si, and C [3,4]. It was observed that alloys in this composition range containing about 25 wt. % Ni may be limited to operation at temperatures of about 800°C - 850°C depending on the water vapor levels in the environment due to inadequate oxidation resistance at higher temperatures. In a more recent study, Brady et al. [5] studied the ability of Fe–(25–45)Ni– (10–25)Cr–(4–5)Al–1Si–0.15Hf–0.07Y–0.01B wt% base alloys with and without Nb, Ti, and C additions to form a protective alumina scale at 1100 °C in air with 10% water vapor. It was found that Fe–35Ni–25Cr–4Al with additions of Nb, C showed promise as an alloy base for future development to achieve a cast alumina-forming austenitic alloy for use at temperatures exceeding 1100°C.

Oak Ridge National Laboratory has been working with industrial partners Duraloy Technologies, and MetalTek International and University of Wisconsin, Milwaukee to develop cast austenitic stainless steel compositions for use in ethylene cracking furnace applications. This presentation will outline the methodologies used in the development of the cast Fe-Ni-Cr-Al austenitic stainless steels, their microstructure, and highlight their high temperature oxidation resistance. In addition, the ability of these alloys to resist coking in an ethane steam cracking environment has been evaluated. This talk will outline the experimental setup, testing methodologies, and results from cyclic coking-decoking tests conducted on the developmental alloys. These results were compared to results from baseline 35Cr-45Ni alloys.

Research sponsored by ARPA-E, US Department of Energy, under Contract DE-AC05-00OR22725 with UT-Battelle, LLC.

1. E. Munoz Gandarillas, K. M. Van Geem, Marie-Francoise Reyniers, Guy B. Marin, “Influence of the Reactor Material Composition on Coke Formation during Ethane Steam Cracking,” Ind. Eng. Chem. Res. 2014, 53, 6358-6371.
2. G. Muralidharan, Y. Yamamoto, and M. P. Brady, “Cast Alumina Forming Austenitic Stainless Steels,” U.S. Patent 8, 431, 072 B2, April 30, 2013.
3. G. Muralidharan, Y. Yamamoto, M. P. Brady, D. Leonard, B. A. Pint, D. Voke and R. Pankiw, (2015) “Development of Cast Alumina-forming Austenitic Stainless Steel Alloys for use in High Temperature Process Environments,” NACE Paper C2015-6114, Houston, TX, presented at NACE Corrosion 2015, Dallas, TX, March 2015.
4. G. Muralidharan, Y. Yamamoto, M. P. Brady, L. R. Walker, H. M. Meyer II, and D. N. Leonard, “Development of Cast Alumina-Forming Austenitic Stainless Steels” JOM, First Online: September 6, 2016, DOI: 10.1007/s11837-016-2094-8.
5. M.P. Brady, G. Muralidharan, Y. Yamamoto, and B. Pint, “Development of 1100°C Capable Alumina-Forming Austenitic Alloys,” Oxid Met, DOI 10.1007/s11085-016-9667-3.