Stainless steels have been used for high temperature applications since their invention due to their inherent ability to form a protective chromium-rich oxide. As technology has continued to evolve, alloy compositions and processing schemes have been tailored to meet the demands of new applications, acting in synergy to affect the high temperature properties of the alloy. This paper reviews recent work at ATI Allegheny Ludlum on the development of alloys and processing to improve the high temperature properties of stainless steels. A joint project with Oak Ridge National Lab (ORNL) to improve the creep resistance of Type 347 stainless steel is discussed. In addition, the development of a creep and oxidation-resistant 20Cr-25Ni austenitic stainless steel will be reviewed. This alloy, developed in coil form in conjunction with Solar Turbines, has good high temperature strength and phase stability, while maintaining excellent oxidation resistance. The primary goal of this work has been in developing durable stainless steel foils for turbine recuperator applications. The work has since expanded to include other applications and product forms.
Austenitic stainless steels have been widely employed for high temperature service due to their combination of good creep and oxidation resistance relative to other ferrous alloys. In applications where high temperature strength is of primary concern, nickel, manganese, nitrogen and carbon are included in the composition to stabilize the creep resistant FCC austenite phase. Carbon can be balanced with the stabilizing elements niobium, titanium, and other carbide forming elements to improve weldability and creep resistance. Alloys containing a high chromium content have been employed to impart oxidation resistance in the higher temperature applications. The balance of these alloying elements, along with the processing employed, act in synergy to define the high temperature properties of the alloy.
Austenitic stainless steels have inherently good creep and rupture resistance relative to other iron based alloys, as may be seen in Figure 1. However, within the alloy family, there exists a wide range of resistance to high temperature deformation. Many alloys have been designed for low temperature aqueous corrosion resistance where creep resistance is not of primary concern. Other alloys have good oxidation resistance, but poor phase stability at high temperatures, leading to poor ductility and reduced creep resistance.
The creep resistance of austenitic stainless steels has been extensively studied, and several factors have been identified that affect creep resistance. These factors include both composition and processing. Solid solution strengthening with nickel, molybdenum, carbon, and recently nitrogen, along with other elements, is well known to have a strong effect on creep resistance. The balance of carbon and nitrogen with stabilizing elements such as niobium and titanium is well documented, but often overlooked in practice. An atomic ratio of approximately 1-1 of stabilizing elements with carbon has been documented to give optimal creep resistance through the precipitation of carbides. However, commercial alloys are typically over-stabilized in order to avoid sensitization during welding, and thus, are not optimized for creep resistance. In addition, many high temperature alloys will form sigma-phase and other intermetallic phases on prolonged exposure to high temperatures. Sigma-phase is know to decrease creep resistance, and can lead to embrittlement. Therefore, optimization of the composition should be addressed when designing components for high temperature load bearing applications.
The processing employe
