If it is not possible to accomplish the final tempering temperature (or even a snap temper), then at least keep the steel warm without allowing it to go down to room temperature. Even holding the fully hardened steel in boiling water, you will reduce the risk of cracking. (Or you can keep the fully martensitic part warm on top of an operating furnace.)
Once you have a furnace available at the appropriate tempering temperature, then you are able to temper the components safely. But make sure that they do not grow cold. The aforementioned suggestion does not apply to aerospace heat-treated and fully hardened components. It would be seen as a “major finding” by an auditor discovering extensive “gaps” in the total process procedure.
If the components are allowed to achieve room temperature for any length of time (provided they do not crack), depending on how complete the austenite to martensite transformation was, it will become increasingly more difficult to achieve the particular as-tempered hardness using the normal time-at-temperature rule of 2 minutes per 1 mm of maximum cross section (60 minutes per 1 inch of maximum cross section).
The main purpose of the temper procedure is to develop a good combination of both hardness (which is resistant to indentation) and toughness (resistance or improvement to impact strength). It is most important to note that tempering gives the freshly formed martensite steel the desired metallurgical and mechanical properties.
When the tempering is applied to fully martensitic steel, the previously formed martensite will transform to what is generally known as tempered martensite. This is, in reality, a structure consisting of very fine carbide particles in ferrite that forms as a result of the tempering process (dependent on the selected tempering temperature).
Another very important thing to remember is that the data given by the steelmakers and the steel reference books will be based only on a 25-mm maximum (1-inch) cross-sectional area and not on diameters greater than 25 mm (1 inch). This means we have two probable choices:
- Observe the hardenability based on the Jominy end-quench test. The given hardness and phase transformation on the hardenability test is based on a bar with a 1-inch cross section and 4 inches long being quenched only on the 1-inch exposed face. This is almost like quenching an 8-inch-diameter bar.
- Develop your own tempering diagrams based on the component geometry, steel chemistry, austenitizing temperature, quench medium, quench-medium agitation and the as-quenched hardness.
You will be surprised as to how quickly you will develop your own tempering diagrams, which will be fairly accurate.
The effects of tempering will be discussed in our next installment.