300 vs 200 Series Stainless Steels, Which Alternative Suits?
Stainless steel is a quality material. When specified correctly, it delivers structural durability, exceptional corrosion resistance and visual appeal. It is the most cost-effective solution in a wide range of applications and environments. This is the first in a series of communications comparing possible alternative grades to 300 series stainless steels.
There are hundreds of stainless steel grades to choose from. More often than not, a 300 series stainless steel is the right choice for its availability, ease of fabrication, exceptional performance and long life. Recent market volatility might prompt specifiers to consider alternative grades to the standard 300 series (304 and 316L). However, grade substitution is not always the answer. Poor design and grade selection can lead to corrosion or structural failure, and other substantial consequences, ultimately resulting in increased costs, and sometimes replacement.
Sadly, a few will make poor choices, underestimating the required performance. When considering grade selection, it’s important to remember that quality is long remembered after cost is forgotten. Correct specification is key to delivering desired outcomes and performance.
You may be thinking about using 200 series stainless steels. These stainless steels are often brought up as an alternative to 300 series. Both have an austenitic structure, with manganese and often nitrogen used instead of some of the nickel. They look and feel like 300 series materials.
The original 200 series alloys were first produced in North America in the 1950s. They had 16.0% chromium, versus the 18.0% of 304, and about half the nickel content. Beginning around 2005, other 200 series alloys rapidly increased in volume in Asia. To reduce the nickel content even further, producers had to further lower the chromium content, in some cases as low as 12%. Consequently, these alloys are much lower in corrosion resistance. In some cases, a slightly lower level of corrosion resistance is acceptable. In other cases, a higher rate of cold working is a benefit in applications such as hose clamps, some structural applications and reinforcing bar with low magnetic response.
Many producers also added copper to their alloys to reduce the very high rate of cold working. Often these low chromium alloys are not standardized and are banned for food contact use in China. In the food industry, often the most corrosive conditions are not the food, but the sanitising chemicals used. The 300 series alloys are approved, standardized, and are the food contact materials of choice around the world.
Low chromium 200 series alloys are also susceptible to delayed cold cracking after forming, sometimes months later. Welding can be more challenging, especially in thicker sections. The following picture is from a newly welded tank, where a low chromium 200 series alloy had been purchased rather than 304L. The tank cracked in many areas adjacent to the welds when filled with water and had to be rebuilt using 304L.
There are many legitimate applications for these low chromium 200 series alloys, especially where there are lower demands on corrosion resistance. The selection of an alloy should be guided by careful examination of the needs of the application. Before making a switch, it’s important to fully investigate an alloy’s strengths, weaknesses and applicability to your structure.
The Nickel Institute offers a free technical inquiry service to help you when using nickel – https://inquiries.nickelinstitute.org
Many publications for users and specifiers are also available free-of-charge on our website. Of particular relevance is The Nickel Advantage, Nickel in Stainless Steels, available in English, French, Japanese and Chinese. https://nickelinstitute.org/library/ (put Advantage in the search engine)