Is it necessary to temper after completing the hardening procedure? The short, simple answer is yes. It is mandatory to temper the steel after it has been hardened. This is simply because a new phase has been created, which is martensite. Remember that it is necessary to progress into the austenite phase before martensite can be created.
The steel will be transformed into the martensite phase after quenching from the appropriate austenitizing temperature provided that all of these process parameters are met:
- The steel has the appropriate amount carbon present that will go into solution and transform to martensite.
- Process (austenitizing) temperature has been achieved.
- The steel has been soaked for the appropriate length of time (equalization of the cross-sectional temperature).
- The appropriate cooling rate has been applied to transform the austenite into a complete martensite phase.
- The steel is now in its most brittle condition.
It is important that the austenitized and quenched steel is hardness-checked to confirm its response to the applied treatment. You can even use a simple file test to give you an indication that the steel has responded to the quench procedure.
If you are processing large batches of components, it will be mandatory that all components have responded to the applied treatment. In order to confirm this (by testing), one must apply a representative hardness test based on the number of components in the load. This is a most important procedure, as it will indicate to you and your customer that both the austenitizing temperature and the quench-medium cooling speed were correct to achieve the metallurgical phase of martensite and the appropriate mechanical properties.
The difficulty in using a steel that has only been austenitized and quenched is that it is almost a full certainty that the microstructure is fully martensitic. Unless the engineering drawing calls for maximum hardness achievable, it will be necessary and mandatory to temper the hardened steel.
If you are unable to temper the steel at the appropriate temperature, proceed with what is called a “snap temper.” This is an intermediate tempering procedure only. Any temperature will be appropriate to reduce the serious risk of cracking in the as-quenched steel. Then conclude the tempering procedure at the temperature required to accomplish the mechanical properties specified by the drawing. Soak at that required process temperature based on the steel chemistry, the cross-sectional thickness of the component and the final drawing/engineering requirements.
We’ll continue this discussion next time.