by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
2005 by George Langford
Surface treatments of steel can greatly reduce the cost of a finished part, because the hardness changes to the surface improve properties such as resistance to wear or bending without requiring large quantities of alloying elements or more expensive melting practices.  Three distinctly different treatments are illustrated in my photomicrographs of the specimens in the M.I.T. collection here:
(a) Induction hardening;
(b) Carburization; and
(c) Nitriding.
The first two treatments give martensitic surface layers which can be quite thick.  Hardenability is rarely a problem.
Nitriding combines a surface hardening treatment with the tempering of a piece which has already been quenched to martensite, so hardenability is required to be appropriate for the section thickness. 
Hardening by resistance (I^2*R) heating from magnetically induced electrical currents is useful because an appropriate frequency of the alternating current will heat the specimen to the specified depth by the skin effect, in which the induced eddy currents fall off to 1/e of their surface intensity at a depth which increases as the frequency of the alternating magnetic field decreases.  Time is also a factor, since the heat flows radially inward towards the core of the workpiece as well as being radiated from its surface.
Carburization permits the carbon content to be controlled both at the surface, by choice of carbon activity or chemical potential of the usually gaseous fluid medium, and at depth, by time- and temperature-controlled diffusion of carbon in the austenite.
Nitriding in an atmosphere of dissociated ammonia (3NH3 --> H2 + N2 + N) gives an exceptionally hard and wear resistant case on tempered martensitic workpieces - or even on ferritic/pearlitic steels such as crankshafts - but can be overdone.  The specimens illustrated here were used in the pioneering work by the late Professors Carl Floe and Michael Bever at M.I.T.
This lesson is also useful for studying the effects of carbon gradients and temperature gradients on the microstructures.
Allow plenty of time to study and to take good notes about each specimen.  About two hours per lesson would be appropriate.  You will be expected to interpret some of these specimens during the final examination.  Feel free to use the Internet to find additional information about the alloys and applications mentioned here.
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