The process temperature selection will also determine the metal grain size. Figure 3 shows the final metallurgical condition of the complete transformation. The observation of the iron-carbon equilibrium diagram can only be a guide to the process temperature selection simply because the diagram is based only on iron and carbon without any additional alloying elements (in equilibrium). Alloy additions to the alloy steels will disturb what is observed from the iron-carbon equilibrium diagram, and no industrial process is in equilibrium.

We often tend to search for the process transformation temperature in the iron-carbon diagram data displayed in the general iron-carbon equilibrium diagram. This is not a practical solution because the iron-carbon equilibrium diagram is simply (as its title displays) based on the relationship of temperature and carbon; that is, plain iron and carbon without any influence from other alloying elements. Therefore, it makes the iron-carbon equilibrium diagram useless to predict the indicated heat-treatment procedure.

Once you begin to account for the effects of alloying elements, such as chromium, nickel, tungsten, manganese, aluminum and other alloying elements, you have a fighting chance of succeeding to obtain the desired microscopic and correct and appropriate metallurgical conditions.

Fig. 3. The iron-iron-carbide equilibrium diagram labeled in general terms