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This is a tool steel which was abusively cut with an
abrasive saw. The photomicrograph at left was made at 500X with a
Nital etch. There is a hard spot resulting from local austenitization from the frictional heat. The hard spot ruined a cutting tool during subsequent machining. Traces of the cutting tool were deposited on the workpiece at this hard spot. The trace is the light etching wedge to the right in the upper frame. |
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As can be seen in the second image at right (1000X, same
etch) the darker etching hard spot is martensitic. It is darker etching because the fresh martensite was tempered by the heat remaining in the metal from the cutoff sawing. |
| SUMMARY: These specimens show
the results of failure to pay attention to the kinetics and equilibria
of phase transformations and to the effects of time and temperature
treatments on the morphology of microconstituents such as the
spheroidization of pearlite. In ancient times, all knowledge was
empirical, and timid blacksmiths dared not deviate from established
practice, lest their hours of labor be wasted by incorrect heat
treatments. This attention to every detail of processing and heat
treatment resembled a religious ritual. We now are occasionally
simply too lazy or cocky to bother looking up the actual kinetic data,
because we think that understanding the science of the phase
transformations permits us to rush them. Nature hasn't changed,
nor has its its kinetics. |