Nickel and Cobalt Alloys for High Temperature Service
Rodger Seeley - Haynes International Corp.First we will address "What are high temperature alloys?" Why are Crome Nickel and Molybdenum key alloying elements?
High temperature alloys are generally used above 1000F (538C). They are specifically designed to withstand heat. Key properties for such applications are 1) High Temperature Strength, 2) Limited Scaling (corrosion), and 3) good thermal stability over time.
Specific properties considered for high temperature applications include Fatigue, Creep, Tensile, High Temp corrosion, Out-of-service corrosion, erosion, thermal stability, and physical properties.
The three basic families of high temperature alloys are Nickel base, Cobalt base, and Iron base. The Nickel base alloys are further classified as Solid Solution Alloys and Age Hardening Alloys. The elements Cr, Si, and Al are used in high temperature alloys to impart corrosion resistant properties. Al, Ti, and Nb are used in the age hardenable alloys to form strengthening percipitates. W and Mo are added to increase strength in solid solution alloys. Rare earth elements, (such as La and Y) are added to stabilize scales. Nb, Ta, Ti, and Zr are added to stabilize carbon. Finally the elements carbon and nitrogen are used to increase creep strength.
Nickel base solid solution alloys are noted for excellent resistance to carburization, chloride and oxidants attack, and also for good high temperature strength. The Nickel base precipitation hardening grades offer the highest strengths but are limited to intermediate temperature service. Cobalt based alloys offer excellent wear resistance, creep strength, and best sulfidation resistance. The iron based alloys are easier to fabricate, have good creep strength, and are also sulfidation resistant.
Some applications for high temperature alloys include rotary kiln parts, calcinators, refuse burners (particularly in superheater portions), fans for high temperature and corrosive gasses, and gas turbine combustors.
Some material properties become particularly important when considering high temperature applications. One of these is creep. Creep is the deformation of the metal with both temperature and TIME considered. It occurs in three stages. Stage 1 is an initial high rate deformation. Stage 2 is characterized by a uniform rate of deformation over a relative long period of time. Stage 3 is similar to regular tensile failure with rapidly increasing deformation until final rupture. Creep has three methods of measurement. 1) "Rupture life" is the TIME, STRESS, and TEMPERATURE which result in a rupture. 2) "Creep Rate" - is the rate (change in length per unit time) of deformation during Stage 2 of the creep. 3) "Creep Life" is the TIME, STRESS, and TEMPERATURE needed to cause a defined amount of deformation (short of a failure).
Fatigue is another property which has special implications in high temperature service. Low cycle fatigue in measured by the repeated application of a specified amount of strain at a stated temperature until failure. The cycle count is the value reported. High cycle fatigue is usually measured by repeated stress to failure, and is more significant in lower temperature applications. It is generally reported as a the stress level which reaches a certain cycle count limit.
Thermal Stability is yet another property very important to high temperature service. It is measured by common ductility measurement methods except after prolonged high temperature exposure periods.
Haynes has not found that simple weight gain or weight loss provide a good measure of High Temperature Corrosion by oxidation. Instead we employ a metallographic technique whereby metal loss from both surfaces is added to either the average penetration of oxides into the metal, or the maximum penetration of oxides into the metal. The first measure is referred to as "Average Metal Affected", and the later is "Maximum Metal Affected".
(Editor's note: Mr. Seeleys talk included numerous charts and graphs illustrating the above characteristics and comparing numerous alloys. For copies of this extensive data, please contact him at Haynes Inernational Inc.) rseeley@haynesintl.com