03. Nickel-rich austenitic stainless steels
An Achilles heel of austenitic stainless steels is the susceptibility to stress corrosion cracking (SCC). However, when the nickel concentration exceeds about 20% considerable improvement in the resistance to stress corrosion is observed (Fig 1). Nickel-rich austenitic stainless steels (NiASS), therefore, deserve to be treated as an own family. In fact, for nickel concentrations beyond about 30% the resistance to stress corrosion is comparable to that of duplex and ferritic stainless steels.
Nickel is an expensive alloying element and used only when it is justified with regard to the performance. However, in certain applications, when resistance to SCC in combination with an austenitic structure is desired, nickel is indispensable. For instance, this is the case in high temperature applications when creep resistance may be crucial. As in the case of traditional austenitic steels, twin boundaries are distinctive features of NiASS owing to the low stacking fault energy (see Fig 2).
A limited selection of NiASS is listed in Table 1. The super austenitic stainless steels 254SMO and 654SMO are designed for the oil and gas industry. Typical applications are equipment for seawater cooling, pulp bleaching and hydraulic and instrumentation tubing.
Sanicro 25, a 22Cr-25Ni alloy, is tailored to be used in boilers at 700°C and above. Because of its good creep rupture strength in combination with high temperature corrosion resistance it is a suitable material in super heaters and reheaters. In fact, the creep rupture strength of Sanicro 25 is superior to most austenitic stainless steels in the temperature range 600–750°C.
|Alloy 800||0.07||0.6||0.6||20.5||30.5||-||-||-||-||-||-||Ti: 0.5
|Alloy 825||0.03||≤0.5||0.8||20||38.5||2.6||-||-||1.7||-||-||Ti: 0.7|
|Alloy 625||0.03||≤0.5||≤0.5||21||bal.||8.5||-||-||-||-||-||Nb: 3.5 Fe: 3|
|Alloy 690||0.02||≤0.5||≤0.5||30||60||-||-||-||-||-||-||Fe: 10|
|Alloy 600||0.05||0.4||0.8||16.5||72.5||-||-||-||≤0.5||-||-||Fe: ≤10|
In extremely corrosive acid environments Sanicro 28 is often a suitable choice. It is used for high strength downhole production tubing, casing, and liners in sour gas wells. Other applications include heaters, pumping systems, pumps and vessels in wet process phosphoric acid plants and in super phosphoric acid plants.
The versatile Alloy 800 is used in the temperature range 550°C to 1100°C in applications where excellent creep properties are required in combination with good resistance to high temperature corrosion and structural stability at high temperatures. It has also found applications as pigtails and headers in plants for the production of ammonia, methanol, and town gas. It is also a good choice for furnace tubes in the production of vinyl chloride and ethylene. Other applications include heat-exchanger tubing for fluidized bed combustion, radiant tubes, and components for heat-treatment furnaces, e. g. muffle tubes and thermocouple protection tubes.
The 25Cr-35Ni alloy 353MA, designed to be used in applications where carburization and nitrogen uptake is a potential problem, is a suitable material as cracker and reformer tubes in the processing of synthetic gas. Despite the fact that many alternative materials contain more chromium, 353MA is often superior. One of reasons is the element cerium, which contributes to a very stable surface oxide layer.
The major application of Alloy 690, with 60% nickel, is as tubing in steam generators in nuclear power plants (Fig 3). The operating temperature is typically 365°C, where intergranular stress corrosion cracking is a potential problem. Alloy 690 has been found to be almost immune to corrosion under the given service conditions and has, therefore, become the preferred alloy.
As a matter of curiosity, it deserves to be mentioned that the nickel-rich austenitic steel 254SMO has been used also in arts. “God Father on the Rainbow”, created by Carl Milles and situated on the southern shore of the sea-approach to Stockholm at Nacka Strand (Fig 4) was inaugurated in 1995. It is about 23m in height and is a well-known sight to the large number of seafarers passing by. The surrounding water is brackish. Chloride induced corrosion is an apparent risk and a bow of a low alloy steel would rust almost instantaneously. The high strength super-austenitic stainless steel 254SMO was found to be suitable and provided by Avesta Sheffield AB.
This column brings my series of the seven families of stainless steels to an end. The next column, which will appear in the December issue will be devoted to medical applications of stainless steels and special alloys.
This article was first published in Stainless Steel World Magazine in December 2017.