Steels with large percentages of manganese (14%+) have the unique ability to
form a very hard layer on their surface when subjected to impact. The
hardening is limited to a thin surface layer and the underlying base metal
remains ductile. This quality makes them ideally suited for wear parts in
rock crushers and other ground engaging tools.
I understand the concept of strain hardening in general, but only in the
sense of the classic explanation involving a stress/strain chart. But how
does high manganese content actually enable this behavior? What is occuring
at the microstructure level and why is manganese so uniquely suitable?
Austenitic Manganese Steel
Hadfield's managanese steel, with 12-14% Mn and 1% Carbon, was one of the
first alloy steels to be discovered and produced commercially. It was
developed in 1882 by Sir Robert Hadfield at Shefield, England.
Manganese goes into solid solution, increasing the strength and hardness,
and also forms hard carbides. It lowers the critical cooling rate, thus
increasing the hardenability of the steel and giving rise to the
air-hardening martensitic steels. Mn lowers the critical range, thus
stabilising the austenite. The Mn steels may be conveniently structurally
classified (approximately) as under:
Mn type of structure
Hadfield's steel is normally heat treated by soaking at and water quenching
from 1000degC, when the carbides are taken into solid solution to give a
uniform solid solution of austenite.
In this condition the alloy is soft as measured by the normal indentation
tests. The BHN is about 200. However, the alloy has excellent
wear-resisting properties, since abrasion converts the surface layers into a
hard structure with a BHN of more than double the soft condition, to values
of around 550. This hardened surface layer is caused by the strain
distortion of the surface layer of austenite phase, into a superficial layer
of accicular structure of pseudo-martensite. Since it only applies to the
severely cold worked/abraided layer, the underlying structure of austenite
is unaffected & remains relatively soft.
The steel as stated, is widely used for rock drills, crushers, railway
points and dredging equipment, where this surface layer hardening can result
in a relatively long working life for the component. The alloy is
difficult to machine, but may be readily hot forged or hot rolled.