Nickel is known to be an expensive alloying element and is essential in some applications where both stress corrosion resistance and austenite structure are required. For example, creep resistance is important in high temperature environments, where austenitic stainless steels are needed. Similar to the traditional austenitic stainless steels, the twin boundary is a significant feature of the nickel-rich austenitic stainless steels because of the lower stacking fault energy. Austenitic stainless steels are prone to stress corrosion cracking (SCC). However, the stress corrosion resistance is greatly improved when the nickel content exceeds 20%. The effect of nickel on the stress intensity of stress corrosion threshold (105℃, 22% NaCl aqueous solution) in Fe-Ni-Cr alloys containing 16%~21% chromium is studied. Nickel-rich austenitic stainless steel (NiASS) can be considered as a separate class of stainless steel. In fact, the stress corrosion resistance of biphasic and ferrite stainless steels is comparable to that of biphasic and ferrite stainless steels when the nickel content exceeds 30%. Several limited grades of nickel-rich austenitic stainless steels are listed in the table below. Super austenitic stainless steels 254SMO and 654SMO are designed specifically for the oil and gas industry. Typical applications are seawater cooling, pulp bleaching, and hydraulic and instrument piping equipment.
Ni-Austenitic stainless steels grades
SANICRO 25, a 22Cr-25Ni alloy, is designed for use in boilers up to 700 °C. It is a material suitable for superheaters and reheaters due to its good creep fracture strength and high temperature corrosion resistance,. In fact, the creep fracture strength of SANICRO 25 is superior to that of most austenitic stainless steels in the range of 600~750℃. In a highly corrosive acidic environment, The Sanicro 28 is usually the best choice. It is used in high-intensity drilling Wells with tubing, casing and acid gas lining, and other applications include heaters, pump systems, and pumps and containers in wet phosphoric acid plants and super phosphoric acid plants.
Alloy 800 is often used in the environment range from 550 to 1100℃, which requires excellent creep resistance, good high-temperature corrosion resistance and high-temperature strength of materials. These alloys are also used in the inlet and outlet ports of the production of ammonia, methanol and civil gas, as well as in the furnace tubes used in the production of vinyl chloride and ethylene. Other applications include heat exchange tubes and radiation tubes for fluidized combustion beds and parts of heat treatment furnaces, such as muffler tubes and protective sleeves for thermocouples.
The 25Cr-35Ni alloy 353Ma is designed for use in cracking furnaces and reforming tubes where synthetic gases are treated in environments where carburizing and nitrogen absorption are potentially problematic. Although there are other alternatives that contain more chromium, 353 MA is the best choice. One reason is that it contains the element Ce, which helps form a very stable surface oxide layer.
Alloy 690 contains 60 percent nickel and is used mainly in the piping of steam generators in nuclear power plants. The operating temperature is 365℃, at which the stress corrosion crack between grains is a potential problem. Under given service conditions, alloy 690 is almost free from corrosion, making it the preferred alloy.
It is interesting to note that nickel-rich Austenitic stainless steel 254SMO is also used for art. “God, Over the Rainbow” sculpture by Carl Milles was installed in 1995 on the south coast of the Nak Strand in Stockholm. The sculpture is about 23m high and is a famous scenic spot where a large number of sailors pass by every day. The surrounding seawater contains salt, chloride is very easy to cause surface corrosion, high strength super austenitic stainless steel 254SMO is very suitable for this environment.