Jominy Test
Does Jominy measure hardness?
It measures hardenability, which is the ability of steel to harden to a given depth under a given quench condition. It does not measure surface hardness alone.
Why is Jominy useful for part design?
It helps predict whether a real component section will harden through its cross-section or only near the surface, which is critical for selecting steel grades and estimating heat-treat response.
What standards govern the Jominy test?
It is standardized under ASTM A255 and ISO 642.
The Jominy Test is a standardized end-quench hardenability test for steel that measures how far hardness can be retained along a specimen after one end is quenched with a controlled water jet following austenitizing. It is used to evaluate hardenability, not simply surface hardness. The test generates a hardness-vs-distance curve from the quenched end to assess the steel's response to cooling.
In manufacturing, the Jominy test guides alloy selection and heat-treat recipes so steel parts harden predictably through their section thickness, reducing distortion, quench cracking, and uneven properties in shafts, pins, dies, and gears. A standard cylindrical specimen, about 25.4 mm by 100 mm, is austenitized, end-quenched, ground flat, and hardness-tested at set distances from the quenched end to build a hardness-vs-distance curve. This curve shows whether a production part of a given size will transform to martensite deeply enough under a real quench, or whether a higher-hardenability alloy is needed. For a CNC or machine shop, that data matters before final design release or heat-treat planning: low-hardenability steels may machine well in the annealed state yet fail core hardness, while highly hardenable grades can generate residual stress and dimensional movement. It balances machinability, hardness depth, and dimensional stability.
Soft core despite acceptable surface hardness: A part passes surface checks but still has a soft core when the selected steel lacks sufficient hardenability for the section size. Premature wear, yielding, or gear-tooth deformation follows after service.
Quench condition drift: If water temperature, nozzle distance, flow rate, or fixture setup deviates from the standard, the cooling gradient changes and the curve no longer matches published Jominy data. This leads to misleading heat-treat decisions and uneven hardening.
Poor specimen preparation: If decarburized material is not removed by grinding flat surfaces before hardness testing, readings near the quenched end are artificially low and distort the curve. A good steel may be rejected while a marginal steel is approved.