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Depth to which a metal is hardened after being submitted to a thermal treatment Jominy test dimensioning. Used Jominy test-piece. Example Jominy results. The hardenability of a metal alloy is the depth to which a material is hardened after putting it through a heat treatment process.
When a hot steel work-piece is quenched , the area in contact with the water immediately cools and its temperature equilibrates with the quenching medium.
The inner depths of the material however, do not cool so rapidly, and in work-pieces that are large, the cooling rate may be slow enough to allow the austenite to transform fully into a structure other than martensite or bainite. This results in a work-piece that does not have the same crystal structure throughout its entire depth; with a softer core and harder "shell".
The hardenability of ferrous alloys, i. The fluid used for quenching the material influences the cooling rate due to varying thermal conductivities and specific heats.
Substances like brine and water cool the steel much more quickly than oil or air. If the fluid is agitated cooling occurs even more quickly. The geometry of the part also affects the cooling rate: of two samples of equal volume, the one with higher surface area will cool faster.
The cooling rate will be highest at the end being quenched, and will decrease as distance from the end increases. Subsequent to cooling a flat surface is ground on the test piece and the hardenability is then found by measuring the hardness along the bar. The farther away from the quenched end that the hardness extends, the higher the hardenability. This information is plotted on a hardenability graph.
Jominy and A. Jominy served as president of ASM in
List of ASTM International standards
ASTM A255 - 10(2018)
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