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Uses of Jominy data: Measurement of hardenability

Data from the Jominy end quench test can be used to determine whether a particular steel can be sufficiently hardened in different quenching media, for different section diameters.

For example, the cooling rate at a distance of 9.8 mm from the quenched end is equivalent to the cooling rate at the centre of an oil-quenched bar with a diameter of 28 mm. Full transformation to martensite in the Jominy specimen at this position indicates that a 28 mm diameter bar can be through hardened, i.e. hardened through its full thickness.

A high hardenability is required for through hardening of large components. This data can be presented using CCT (Continuous Cooling Transformation) diagrams which are used to select steels to suit the component size and quenching media. Slow quenching speeds are often chosen to reduce distortion and residual stress in components.

CCT diagram for an alloy steel

CCT diagram for an alloy steel

Slower cooling rates occur at the core of larger components, compared to the faster cooling rate at the surface. In the example here, the surface will be transformed to martensite, but the core will have a bainitic structure with some martensite.


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