Martensite is an equilibrium phase, but is extremely hard and brittle and requires a further process to make the steel usable as a crankshaft. This process is called tempering, and involves getting the steel moderately hot in the case of a crankshaft (somewhere between 500 and 700 degrees C (approximately 930 – 1300 degrees F) for a nitriding steel depending on the exact grade and the level of strength desired). Steels for other applications will temper at higher or much lower temperatures (for example, common gear steels are tempered at around 150 degrees C). As for stabilisation (stress-relief), tempering depends on a combination of time and temperature. If the temperature in service (or during the rest of the manufacturing process) exceeds the tempering temperature, there is a danger of loss of mechanical properties.
Nitride hardening can affect the manufacture of crankshafts in this way. Lower tempering temperatures, in general, leads to higher hardness and strength and lower ductility. If one was to specify a high hardness, requiring a low tempering temperature, it may be that the core material would be softened by the nitriding treatment. This effect can be mitigated by careful choice of alloy if high core strength is felt to be a requirement.
However, it is quite common for crankshaft blanks to be ordered pre-hardened and tempered to the desired core hardness – careful manufacturing minimises the amount of stress-relief required, and proper specification of the nitriding process poses no danger of diminishing the required properties.
Martensite is an equilibrium phase, but is extremely hard and brittle and requires a further process to make the steel usable as a crankshaft. This process is called tempering, and involves getting the steel moderately hot in the case of a crankshaft (somewhere between 500 and 700 degrees C (approximately 930 – 1300 degrees F) for a nitriding steel depending on the exact grade and the level of strength desired). Steels for other applications will temper at higher or much lower temperatures (for example, common gear steels are tempered at around 150 degrees C). As for stabilisation (stress-relief), tempering depends on a combination of time and temperature. If the temperature in service (or during the rest of the manufacturing process) exceeds the tempering temperature, there is a danger of loss of mechanical properties.Nitride hardening can affect the manufacture of crankshafts in this way. Lower tempering temperatures, in general, leads to higher hardness and strength and lower ductility. If one was to specify a high hardness, requiring a low tempering temperature, it may be that the core material would be softened by the nitriding treatment. This effect can be mitigated by careful choice of alloy if high core strength is felt to be a requirement.However, it is quite common for crankshaft blanks to be ordered pre-hardened and tempered to the desired core hardness – careful manufacturing minimises the amount of stress-relief required, and proper specification of the nitriding process poses no danger of diminishing the required properties.
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