79 Hightemperature Alloys

The need for high-temperature materials is encountered in a wide variety of modern industries such as in aerospace, metallurgical, chemical, petrochemical, glass manufacture, heat treatment, waste incinerators, heat recovery, advanced energy conversion systems, and others. Depending on the condition of chemical makeup and temperatures, a variety of aggressive corrosive environments are produced, which could be either sulfidizing, carburizing, halogenizing, nitriding, reducing, and oxidizing in nature or a combination thereof. All high-temperature alloys have certain limitations and the optimum choice is often a compromise between the mechanical property requirement constraints at maximum temperature of operation and environmental degradation constraints imposed due to the corrosive species present.

Alloys designed to resist high-temperature corrosion have existed since the beginning of the twentieth century. Generally high-temperature metal degradation occurs at temperatures above 1000°F (540°C), but there are few cases where it can also occur at somewhat lower temperatures. Carbon steel, a very useful and the workhorse material of construction in many industries, is attacked by H2S above 500°F (260°C), by oxygen or air above 1000°F (540°C) and by nitrogen above 1800°F (980°C). Chromium and molybdenum containing low-alloy steels significantly extend the range of usefulness of carbon steel. However, the severity of the processes as encountered in modern-day industries (chemical, petrochemical, refineries) and the new technologies of thermal destruction of hazardous and municipal waste, fluidized-bed combustion, coal gasification and chemical from coal processes, and the use of "dirty feedstock," such as heavy oil and high-sulfur coal, coupled with demands for higher efficiency and tougher environmental regulations, have necessitated the use of higher alloy systems of iron base, nickel base, and cobalt base alloys. Today alloy systems have not only to provide reliable and safe performance in a cost-effective manner but must have sufficient versatility to resist changing corrosive conditions due to starting feedstock changes.

Optimal material selection for high-temperature applications requires a thorough understanding of the mechanical requirements at the temperature of operation including upset conditions and mechanical degradation due to high-temperature corrosive attack. These specific property requirements are:

Mechanical Corrosion Resistance

High-temperature Strength Oxidation

Stress rupture strength Carburization and metal dusting

Creep strength Nitridation

Fatigue Sulfidation

Thermal stability Halogenation

Thermal shock Molten salt corrosion

Toughness Liquid metal corrosion

Others Ash/salt deposit corrosion

These requirements will vary and be different for various industries such as heat treatment, aerospace, power generation, metallurgical processing, petrochemical and refineries, heat recovery, waste incineration, and others.

In nickel base alloys, the major alloying elements for imparting specific property or a combination of properties are tabulated in Table 7.3. These alloying elements can be classified as follows:

• Protective scale formers: Cr as chromia, Al as alumina, and Si as silica

• Solid solution strengtheners: Mo, W, Nb, Ti, Cr, Co

• Age hardening strengtheners: Al + Ti, Al, Ti, Nb, Ta

• Carbide strengtheners: Cr, Mo, W, Ti, Zr, Ta, Nb

• Improved scale adherence (spallation resistance): Rare Earths (La, Ce), Y, Hf, Zr, Ta

Most high-temperature alloys have sufficient amounts either of chromium with the addition of either aluminum or silicon to form the protective oxide scales for resisting high-temperature corrosion. Table 7.12 gives the typical chemical composition of several common high-temperature alloys in commercial use today. Optimization of the various alloying elements led to a new alloy for service

TABLE 7.12 Metallurgical Optimization of Alloy 602CA Nominal Chemistry Comparison to Other High-Temperature Alloys






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