Cautionary Tale: Stainless Steel Turns 100 Years Old
by Mark Hayes
Did you know stainless steel was invented 100 years ago in Sheffield, which is the headquarters town for IST? For those interested in more information on this important spring material, there is an excellent new book written by Sheffield metallurgist Dr. David Dulieu entitled “Stay Bright.” The book describes the history of stainless steel from it invention by Harry Brearley to its current use today.
The last cautionary tale discussed the oxide which forms naturally on stainless steel and how that oxide confers corrosion resistance. This prompted the idea that the various types of stainless steel should be described. They all have a layer of chromium oxide providing corrosion resistance. There are four distinct types of stainless steel. Two are excellent for spring production, but the other two are not.
The type of stainless steel invented by Harry Brearley contained 13 percent Cr (chromium) and 0.25 percent C ( carbon), and becomes corrosion resistant after it has been hardened and tempered to a martensitic microstructure. At first sight this might seem ideal for springs because it is corrosion resistant and has high strength.
Martensitic stainless steel is used for manufacturing knives and surgical instruments, industries for which Sheffield remains famous today. However, martensitic stainless steel has a fatal flaw that makes it a very unlikely choice for making springs. It is not very corrosion resistant, so even a slight trace of corrosion which will cause this type of stainless steel to fail by stress corrosion cracking. That is the first moral of this cautionary tale – do not be tempted to use martensitic stainless unless you are certain that its corrosion resistance is good enough.
For springs, a stainless steel is needed that will repair its oxide film in the event of slight corrosion before it fails. Stainless steel with 18 percent Cr and 8 percent Ni (nickel) has better corrosion resistance than the martensitic type. It has a microstructure of austenite, which needs to be cold worked to acquire spring strength. The microstructure is austenite prior to the start of the wire drawing process, but during wire drawing some of the austenite transforms to martensite, and this is what makes this grade slightly magnetic. The predominant microstructure remains austenite though, and this type of stainless steel is the most frequently used for springs everywhere in the world today.
The 18/8 stainless steel is usually called 302 or 304 type. There are two variants in common use. One is 316 type, which has 2 to 3 percent Mo (molybdenum) added for improved corrosion resistance, especially in salt environments. The other variant is 17/7PH (type 631), which has 1 percent Al (aluminium) added for precipitation hardening, and hence a strength level higher than 302 type.
Today, there are two other types of stainless steel, again both named for their microstructure. There is ferritic stainless steel, which has very good corrosion resistance, but not the high strength needed for springs. This is the type of stainless used for car exhausts.
Finally, there is duplex stainless steel, which has a duplex microstructure of ferrite and austenite, which is very corrosion resistant especially when molybdenum ( Mo) is an alloying element, and it may be drawn to high strength levels, hence conferring excellent spring properties. IST predicts that duplex stainless steel will gradually replace 316 as a spring material because it outperforms the latter with respect to corrosion resistance and strength levels.
All of these stainless steels contain at least 12 percent chromium. It is often said that their corrosion resistance is due to the formation of chromium oxide on their surface, something that happens naturally in air at room temperature. That is certainly true. However, the very thin oxide on each type of stainless steel differs slightly, and, despite being only nanometers thick, is always made up of several oxide layers. Hence, there are differences in the corrosion resistance of the four types of stainless steel.
One thing that has always puzzled the industry is the fact that molybdenum (Mo) additions will invariably improve the corrosion resistance of stainless steels. So one might ask how the Mo affects the oxide, and X-ray photoelectron spectrographic studies have shown that there is almost no Mo present in the oxide – there is concentration of Mo below the oxide, but how does that help? This leads to the second moral of this cautionary tale – the precise explanation for the corrosion resistance of stainless steels is, as yet, incomplete. This fact should keep metallurgists employed for some years to come.
Mark Hayes is technical advisor to the Institute of Spring Technology (IST) in Sheffield, England. He is also the principal trainer for the spring training courses that the Institute offers globally. Readers are encouraged to contact IST with comments about this cautionary tale, and with subjects that they would like to be addressed in future tales, by email firstname.lastname@example.org.