Comparison of Three Microturbine Primary Surface Recuperator Alloys

Extensive work performed by Capstone Turbine Corporation, Oak Ridge National Laboratory, and various others has shown that the traditional primary surface recuperator alloy, type 347 stainless steel, is unsuitable for applications above 650°C(∼1200°F). Numerous studies have shown that the presence of water vapor greatly accelerates the oxidation rate of type 347 stainless steel at temperatures above 650°C(∼1200°F). Water vapor is present as a product of combustion in the microturbine exhaust, making it necessary to find replacement alloys for type 347 stainless steel that will meet the long life requirements of microturbine primary surface recuperators. It has been well established over the past few years that alloys with higher chromium and nickel contents than type 347 stainless steel have much greater oxidation resistance in the microturbine environment. One such alloy that has replaced type 347 stainless steel in primary surface recuperators is Haynes Alloy HR-120 (Haynes and HR-120 are trademarks of Haynes International, Inc.), a solid-solution-strengthened alloy with nominally 33 wt % Fe, 37 wt % Ni and 25 wt % Cr. Unfortunately, while HR-120 is significantly more oxidation resistant in the microturbine environment, it is also a much more expensive alloy. In the interest of cost reduction, other candidate primary surface recuperator alloys are being investigated as possible alternatives to type 347 stainless steel. An initial rainbow recuperator test has been performed at Capstone to compare the oxidation resistance of type 347 stainless steel, HR-120, and the Allegheny Ludlum austenitic alloy AL 20–25+Nb (AL 20–25+Nb is a trademark of ATI Properties, Inc. and is licensed to Allegheny Ludlum Corporation). Evaluation of surface oxide scale formation and associated alloy depletion and other compositional changes has been carried out at Oak Ridge National Laboratory. The results of this initial rainbow test will be presented and discussed in this paper.

Comparison of Three Microturbine Primary Surface Recuperator Alloys

Extensive work performed by Capstone Turbine Corporation, Oak Ridge National laboratory, and various others has shown that the traditional primary surface recuperator alloy, type 347 stainless steel, is unsuitable for applications above 650°C (∼1200°F). Numerous studies have shown that the presence of water vapor greatly accelerates the oxidation rate of type 347 stainless steel at temperatures above 650°C (∼1200°F). Water vapor is present as a product of combustion in the microturbine exhaust, making it necessary to find replacement alloys for type 347 stainless steel that will meet the long life requirements of microturbine primary surface recuperators. It has been well established over the past few years that alloys with higher Chromium and Nickel contents than type 347 stainless steel have much greater oxidation resistance in the microturbine environment. One such alloy that has replaced type 347 stainless steel in primary surface recuperators is Haynes Alloy HR-120, a solid-solution-strengthened alloy with nominally 33 wt.% Fe, 37 wt.% Ni and 25 wt.% Cr. Unfortunately, while HR-120 is significantly more oxidation resistant in the microturbine environment, it is also a much more expensive alloy. In the interest of cost reduction, other candidate primary surface recuperator alloys are being investigated as possible alternatives to type 347 stainless steel. An initial rainbow recuperator test has been performed at Capstone to compare the oxidation resistance of type 347 stainless steel, HR-120 and the Allegheny Ludlum austenitic alloy AL 20-25+Nb. Evaluation of surface oxide scale formation and associated alloy depletion and other compositional changes has been carried out at Oak Ridge National Laboratory. The results of this initial rainbow test will be presented and discussed in this paper.

Identification of non-martensitic transformation products(NMTP) in a carburized 8620 steel

Additions of Cr and Mn to steel are intended to increase hardenability (the ability to form martensite on cooling from austenite) and thereby improve the mechanical properties. However, conventional gas carburization of alloy steels containing Cr and Mn as well as Si will produce oxides of these elements (internal oxidation) near the surface (due to the oxygen potential of the carburizing atmosphere). Additionally, the oxidation of these elements locally reduces the hardenability of the alloy, creating the potential to produce non-martensitic transformation products (NMTP) near the surface. These oxides and the associated NMTP have been shown to decrease the fatigue resistance of carburized steels if they are not removed by subsequent machining operations. The objective of this study is to characterize the oxides, identify the NMTP and assess the degree of local alloy depletion in a carburized 8620 steel (0.21 C, 0.93 Mn, 0.023 S, 0.12 Si, 0.49 Cr, 0.38 Ni and 0.17 Mo).