INTRODUCTION - Lesson 4Surface 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
|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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