Stainless steels are most commonly used for their corrosion resistance. However, another common reason for their use is for high temperature applications in which high temperature oxidation resistance may be necessary and/or high temperature strength may be required. The high chromium content of stainless steels not only provides the benefit with respect to wet corrosion resistance but also provides the benefit of high temperature strength and resistance to scaling at elevated temperatures.
A stainless steel contains a minimum of 10.5% chromium to provide the formation of a stable, transparent, passive, protective film that enhances corrosion resistance. In addition, such a chromium level provides resistance to oxidation at elevated temperatures. In many instances, nickel is also added to the composition for a stainless steel to promote a stable austenite microstructure. Austenite is stronger and more stable at higher temperatures than ferrite. A common problem that may be experienced in stainless steels is the formation of sigma phase in high temperature applications. The effect of the formation of this phase is the phase may cause the steel to be extremely brittle and failure can occur because of brittle fracture. When nickel is added to a chromium stainless steel, the microstructure of the steel changes from ferritic to austenitic. Thus, the addition of nickel suppresses the formation of sigma phase and promotes the formation of austenite. In addition, nickel also increases resistance to oxidation, carburization, nitriding, thermal fatigue, and strong acids.
Nickel is an expensive raw material and suffers from a large fluctuation in price, which has become increasingly volatile. The high cost of nickel directly impacts the cost of producing the grade of stainless steel. Therefore, efforts have been made to replace the nickel with other alloying elements such as carbon, manganese, nitrogen, and copper. Such elements are lower in cost than nickel and may still promote the formation of austenite.
Accordingly, it would be beneficial to produce an alternative alloy for use in applications requiring the use of stainless steel that is capable of withstanding high temperatures and promoting the required microstructure, while minimizing the amount of nickel to keep costs low and more predictable.