409 is often considered “the barely stainless steel,” or affectionately the most humble of the stainless steels. Stainless steel must have a minimum of 10.5% chromium to be stainless steel. 409 Contains a minimum of 10.5% chromium, thus the moniker barely stainless steel.
In addition to minimal chromium content, 409 stainless has three additional properties that make it an attractive product for substitution: it is the lowest cost stainless, it has good oxidation resistance and excellent formability.
Middleweight Corrosion Fighter
According to AK Steel, 409 gets specified where oxidation and corrosion requirements go beyond carbon steel and some coated steels. North American Stainless suggests, “it is not as resistant to corrosion or high-temperature oxidation as the higher-alloyed stainless steels (430 or 304), but it is still far superior to mild steel and low alloy corrosion resisting steels and most coated mild steels.”
And not to ignore the other main US producer of 409, Allegheny Technologies explained its usage in automotive mufflers, “The good fabricability of this alloy, combined with its basic corrosion resistance and economy have significantly broadened the utility of ATI 409HPtm stainless.”
As most MetalMiner readers know, alloy substitutions in stainless steel have typically occurred when an alloying element such as nickel has increased in price. When nickel becomes volatile, manufacturers have sought options with less nickel or no nickel that have sufficient properties to make the final product without compromising quality. Both 304 and 316L are readily available and could be considered the path of least resistance in terms of specifying stainless steel; however, in some cases, these alloys may exceed the necessary properties for the final application.
Most consider 304 or 316L the “old standby” grades, but that thinking contains a few misconceptions. For example, stainless is stainless because it has at least 10.5% chromium (some would say 11% chromium), not because it contains nickel. Stainless can be both magnetic and non-magnetic. In commercial food service equipment — NSF specifies, for food zones, stainless needs to have a minimum chromium content of 16% and has nothing to do with whether or not it is magnetic.
In the early 2000s, product substitution meant a new push to inform the manufacturer that type 430 has 16% chromium and is, thus, NSF 51-compliant. In many cases, a transition occurred in which buying organizations switched from 304 to lower nickel-bearing grades such as 301 or 201 before the switch to 430 occurred. In cases in which 430 could not be substituted for 301 or 201, the next wave of substitution came from higher-chromium ferritic grades such as 439 or 441. Both alloys were developed for the automotive market in which weldability and formability were necessary along with added corrosion resistance from the basic 409 automotive grade.
In residential appliances, the major manufacturers became reticent to move to magnetic stainless grades due to a perception that magnetic equated to not “real” stainless steel.
The irony? Many people loved the stainless look but were saddened that they couldn’t put their kids’ artwork on the fridge because it was not magnetic.
We will continue covering other alloy histories in subsequent posts.