the Jominy test
/ JOM-in-ee /
How do you put a number on hardenability, on how deep a steel hardens, without machining and sectioning dozens of bars? The Jominy end-quench test is the clever standard answer: cool one end of a single standard bar fast and let the far end cool slowly, all in one shot, so a single specimen samples the whole range of cooling rates at once.
The procedure is neat. Take a standard round bar (25 mm diameter, 100 mm long), heat it to austenite, stand it up, and spray water on just the bottom end. The quenched end cools fastest; each point farther up cools progressively slower because it is farther from the water. Then you grind a flat along the side and take a row of hardness readings marching away from the quenched end. Plot hardness versus distance and you get the hardenability curve: a steel that stays hard far up the bar has high hardenability; one whose hardness drops off quickly near the quenched end has low hardenability. The distances (in units of 1/16 inch, called Jominy distances) map to known cooling rates, so a lab can predict the hardness at the center of a real quenched part of any size.
The Jominy test matters because it turns how deep does it harden into a repeatable curve you can compare between steels and hand to a designer. Two steels with the same maximum hardness at the quenched end can have completely different curves further along, revealing which one will harden through a thick shaft. Its limit is honesty about scope: it characterizes one bar geometry under one cooling condition, and translating its curve to an oddly shaped real part still takes engineering judgment and correlation charts.
On a Jominy plot, a plain 1040 steel might read about 55 HRC right at the quenched end but fall below 30 HRC only 10 mm away; an alloy 4340 stays above 50 HRC far up the bar, visibly proving its far greater hardenability.
One bar, many cooling rates: near the sprayed end is fast (hard), the far end is slow (soft). The shape of the drop-off is the hardenability.
The Jominy test measures hardenability (depth), not a material's peak hardness, two steels can start at the same hardness at the quenched end yet differ hugely further along the bar. Read the whole curve, not just the first point.