This is the conclusion of David Pye's blog on the time-temperature-transformation (TTT) curve. Read parts 1 and 2 here.

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.


In Conclusion

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.