Once a tool has been hardened, it is brittle and must be tempered before using it. Tempering is the process of reheating a tool after it has been heat treated. This reheating serves many functions. It is the method of attaining the proper hardness in the tool, relieving the stress from the hardening process and transforming any retained austenite to martensite.
Tempering is the process of reheating tool steel. This reheating is done at a low temperature range, usually from 300 to 1100 degrees F, depending on the steel grade and final desired hardness. The tempering cycle should be one hour per inch of cross section and a minimum of two hours. Since the tempering temperature is relatively low, no protection from decarburization is necessary.
The primary purpose of tempering is to relieve the stress that was induced during the austenizing and subsequent quenching processes. Stress is also introduced during the initial machining stages. This is especially true for asymmetrical or heavily machined tools. The tool is under tremendous stress and would likely crack while in service. Even an untempered tool could crack if it is allowed to cool to room temperature. Tempering should be performed before the tool cools to room temperature after quenching.
Another reason for tempering the tool is to attain the final desired hardness. Usually the end-user of the tool will require that the tool has a desired hardness range. This hardness is attained by tempering the tool. When tempering, the highest possible tempering temperature should be used. By tempering at the highest temperature to attain the proper hardness range, more retained austenite will transform to martensite. Martensite is less brittle than austenite.
Transforming any retained austenite to martensite is the third reason for tempering. After hardening the tool, as much as 15% retained austenite may be present in the tool. This retained austenite should be transformed into the desirable martensite. A properly hardened and tempered tool should have less than 5% retained austenite. By raising the temperature of the tool and cooling to room temperature, this transformation takes place.
A tool steel with high alloy content may have secondary hardening occur. Secondary hardening is the transformation of retained austenite to martensite causing an increase in hardness. This frequently occurs in high alloy tool and high speed steels due to precipitation of carbides during the tempering process. High alloy grades of tool steel may require three or four tempering cycles for complete transformation.
It should be noted that the tool must be allowed to cool to room temperature between tempers. If the tool is not allowed to cool to room temperature between tempers then the tool has not completed the tempering cycle.
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