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)
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
Isothermal Transformation Diagram
You may ask, “How can we then predict the final and necessary heat-treatment metallurgy?” Typically, the answer lies in what is known as the principle isothermal transformation diagram.
We should recognize the fact that isothermal transformation diagrams are based on the material manufacturers’ search with a standard analysis that should only be used as a guide. However, we must also consider the fact that the metal content analysis to be treated may not match exactly the metal to be treated because of variations in the metal analysis. This “new” diagram is known as the isothermal transformation diagram.
Another consideration is that the information given in the diagram by the metal manufacturer is that the isothermal transformation is based on a maximum diameter/cross-section greater than the selected process. It is the duty and responsibility of the heat treater to ask the client and/or metal manufacturer for a test coupon of the same metal analysis so that you are able to observe the accurate and final metallurgical results.
Remember, even with the selection criteria being met and time forgotten about, it will still exhibit grain growth if you do not account for the metal analysis. We should also remember that there will be a change in the completed surface metallurgy due to thermal conditions such as the crystal structures from the alloy steel analysis.
It is up to the heat treater to work closely with the material supplier/manufacturer to determine the appropriate heat-treatment procedure. If you don’t do it correctly, the second time is costing you (the heat treater) money. Be aware that the manufacturer of the steel will only guarantee their analysis up to 2 inches.
Be sure that your test coupon is exactly of the same analysis as the steel component and the same cross-sectional area in order to acquire the appropriate desired metallurgy. If we utilize a higher process temperature, the heat-treatment temperature will cause a shift in the crystal structure, resulting in grain growth.
All figures/graphics provided by the author.
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