Q: How does Manganese promote work hardening in "Hadfield's Manganese"

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?
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Jon Juhlin wrote:

High manganese additions to iron result in metastable austenitic steels. Plastic deformation leads to the formation of martensite.
HTH
Michael Dahms
f'up2 s.e.metallurgy
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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 0-2% pearlitic 2-12% martensitic 12-100% austentitic
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.

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