Time waits for no man. Time cannot be saved. Time only moves on, even as we speak.

The process temperature will determine what microscopic transformation will occur (including grain growth). Transformation is the resulting metallurgical effects of the process time and temperature selected (Fig. 1).


Fig. 1. Metallurgical transformation occurrence

As heat treaters and practicing metallurgists, we do not always fully appreciate the importance of time, temperature and transformation as they relate to the heat treatment of metals. The Triple T curve (Fig. 2) will embrace all recommended cooling rates. It tells us that no matter what time allocation we give to the metal being treated, it will affect the metal grain size and final metallurgy.

In diffusion processes, we are diffusing another element into the metal’s surface. There will be a gain in weight simply because of absorption of another element into the surface of the component/test coupon and the component being modified by the application from an external heat source. This effect occurs on all diffusion surface-treated steels. This is true even for the lower-temperature (in the order of 900°F) nitriding process, which will have the lowest amount of weight gain.


Fig. 2. Graphical illustration of principle occurrences (Triple T curve)