There are steel grades that pretty much won’t harden, at least in the most familiar sense. There are steel grades that are hardenable by thermal treatment. And there are steel grades and that are typically hardened by strain hardening or work hardening.
The elements of carbon and Manganese have a lot to do with where in the above categories a steel grade fits. Steel with less than around .26 carbon may be referred to as a “Mild” steel (1018, 1020). It is generally not responsive to thermal hardening or work hardening.
Examples of other grades that typically do not respond to customary heat-treating methods would be the austenitic stainless grades (typically 300 series, such as 304 and 316, and the less common 200 series). Hadfield Manganese Steel, aka 10 to 14% manganese, 11 to 13% manganese, Work Hardening Steel, and Austenitic Manganese Steel, is a somewhat unique grade that falls into a class of its own. Discovered near the end of the 19th century by a gentleman of the name Hadfield, the high content of manganese contributes to its austenitic grain structure. It shares similarities, (characteristic physical properties) of the austenitic stainless grades.
Like those grades of stainless, austenitic manganese steel in typically non-magnetic, it hardens under impact and/or load, and again, it generally does not respond to thermal hardening treatment.
What are the benefits of Manganese steel? Most manganese steel is supplied in plate form, some bars, and some shapes. The shapes may be finished parts typical to a particular field of heavy industry such as railroad applications and shot blast applications.
As supplied, most manganese is furnished at a hardness of 225 to 250bhn. Under the right cold working conditions, the steel should work-harden to a very high hardness of approximately 500 to 600bhn. A fabricator or end-user may pre-harden pieces or parts of manganese steel by vibratory peening or shot blasting. This is also true with austenitic grades of stainless steel, which like manganese has low hardness as supplied. The process is not sophisticated and requires significant trial and error. In recent years there are a diminishing number of vendors who offer this service, and I suspect that most work hardening grades are put into service soft. We have had several customers who find rails made from manganese steel do harden sufficiently to increase resistance to abrasive wear even when they are exposed only to moderate repeated load.
A major benefit of the manganese grade of steel is that, once hardened to the 500bhn range, it continues to maintain a high degree of toughness and will withstand a significant amount of impact and load.
Manganese is a very tough steel capable of withstanding terribly abusive environments. Unfortunately, it also withstands most attempts at machining and forming. Using this grade is not for the faint of heart. The rewards are significant but so are potential difficulties in machining, drilling, forming and grinding. Caution is required. You should not be afraid to use this effective grade of steel, just be sure to obtain and follow the appropriate fabrication guidelines for austenitic manganese.
-Howard Thomas January 5th, 2021