In this column, I will discuss the different types of heat-resistant alloys used in heat treating.
Introduction
Heat-resistant alloys are crucial to heat-treating operations. They are used in a wide variety of applications, such as support grids, alloy baskets, retorts, muffles, radiant tubes, hearth rails, recirculating fans, and many other applications in heat treating.
These alloys in general have chromium contents greater than 10 percent, and often have chromium contents much higher. These alloys are designed to operate at temperatures above 650°C and have sufficient mechanical strength and ductility to meet the desired high-temperature properties. These alloys must also have a good surface film stability for oxidation and chemical resistance in both oxidizing, reducing, and carburizing atmospheres commonly found in heat treating. Typical compositions of cast heat-resistant alloys are found in Table 1.
There are three predominant groups of alloys used in heat treating or high-temperature applications. These are chromium and iron based; Chromium, nickel, and iron based; and nickel, chromium, and iron alloys.
Chromium-iron alloys are those alloys that have a limited nickel content (up to 7 percent) but have chromium as the dominating alloying element. These alloys are mostly ferritic and have poor high temperature strength. Generally, these alloys are limited to temperatures less than 750°C. They are most used for high sulfur-containing atmospheres, such as found in the petrochemical industry.
Chromium-nickel-iron alloys have high-temperature strength, with good ductility when hot or cold. These alloys have 8-22 percent nickel, and 18-32 percent chromium. These alloys are predominantly austenitic; however, some alloys are mixtures of ferrite and austenite. Carbides are present and improves the high temperature strength.
Nickel-chromium-iron alloys are the most used in the heat-treating industry. These alloys are fully austenitic, and can be used up to approximately 1,150°C. These alloys contain 25-70 percent and 10-26 percent chromium. They are typically not prone to sigma-phase formation, which can embrittle other alloy groups. The largest total production of heat-resistant castings is type HT, with nearly 15 percent of the total production [1]. HT has good resistance to oxidation, has good thermal shock resistance, and good strength at furnace temperatures.
These alloys have excellent hot strength, and resistance to carburization and thermal fatigue. They are most often used for heat treating baskets and other parts where there are large thermal gradients, and cyclic temperature loading.
Effect of Alloying Content
The various chemical constituents play different roles in providing temperature resistance.
Nickel
Nickel is present in heat-resistant alloys to almost 70 percent. Nickel promotes the formation of austenite, which is more stable at elevated temperatures than ferrite. Nickel provides strength to the matrix, as well as providing ductility. It also contributes to carburization resistance during processing.
Chromium
For heat-resisting alloys, the chromium content varies from roughly 10-30 percent. Chromium contributes to strength at elevated temperatures by the formation of chromium carbides in the matrix. This contributes to high temperature creep and rupture strength by pinning grain boundaries and providing resistance to grain boundary sliding. Most importantly, chromium forms an adherent oxide film on the surface of the part.
Other Elements
Carbon provides the source for carbides in the structure. While increased carbon can increase the susceptibility to sensitization, it increases the high temperature strength and creep resistance.
Silicon improves high-temperature oxidation resistance. It can also form sub-scales of silica beneath the surface complex chromium oxides, which further improve oxidation resistance. Generally, silicon is kept at concentrations lower than 1.5 percent, because greater amounts can negatively affect the high temperature creep and rupture properties.
Effect of Microstructure
In general, alloys for service up to 650°C have mixed microstructures of ferrite and austenite. For service greater than 650°C, the matrix is austenitic. The presence of ferrite at temperatures greater than 650°C is avoided to prevent the transformation of ferrite to sigma phase. Sigma phase seriously impacts the toughness at room temperature. The predominant source of strength is the solid solution strengthening of austenite by additions of nickel and chromium.
Carbides also increase high temperature strength, by pinning grain boundaries and interfering with dislocation movement. Iron carbides tend to redissolve in the matrix at service temperatures, but additions of nickel and chromium retard dissolution of the carbides in the matrix.
Castings are stronger than wrought products at similar compositions [2]. This is due to the typically greater carbon content in castings over wrought products. Further, the presence of dendrites in castings tend to reduce grain boundary sliding and provide an interlocking effect to reduce creep.
High Temperature Properties
As with most metals, the strength of a material decreases with increasing temperature. The purpose of heat resistant alloys is to slow down the decrease in properties as the temperature is increased.
At high temperatures, stressed metals experience elastic deformation, as well as slow plastic deformation. Exposure time at temperature is an important factor that is not reflected in room temperature tensile testing. Creep strength and the stress-rupture properties are the properties that are used in design for elevated temperature service.
Creep strength is the slow deformation that occurs under load at elevated temperatures. Creep properties are obtained under constant load and temperature. For furnace part design, a value of 0.0001 percent per hour is used for comparison purposes. An example comparing the creep strength of RA330 and ACI HT (similar composition) is shown in Figure 1.
Stress-rupture properties are determined under constant load and constant temperature. It is useful for approximating the time to fracture for a specific loading condition and service temperature. Usually stress-rupture properties are provided as the stress to rupture within a specific time, such as 100 or 1,000 hours.
Conclusions
In this brief article, I have described the different types of heat-resistant alloys and provided an overview of the chemistry and microstructure of this class of materials.
Should you have any questions regarding this column, or have suggestions for further articles, please contact the writer or editor.
References
- Nickel Development Institute, “Heat and Corrosion Resistant Castings: Their Engineering Properties and Applications.”
- Steel Founders Society of America, “Steel Castings Handbook Supplement 9: High Alloy Data Sheets Heat Series,” Steel Founders Society of America, 2004.
