The Jominy test is done by heating a round bar specimen, taken from the actual steel garde, to the hardening temperature of the steel with a holding time of usually 20 min. The diameter of the specimen is 25 mm and the length 100 mm. The end-face of the specimen is quenched by spraying it with a jet of water, see figure below.
Hereby the rate of cooling decreases progressively from the quenched end along the length of the bar. When it is cool, two diametrically opposites flats, 0,4 mm deep and parallel to the axis of the bar are ground and the hardness is measured along the flats. The hardness values are plotted in a diagram against their distance from the quenched end. The variation of the chemical composition, within the limit, for the steel gives a maximum and a minimum Jominy curve. The diagram below is for a carburizing steel.
Each hardness value is noted in the form; J15 = 35 HRC. The hardness 35 HRC is measured at 15 mm from the end.
Instead of doing this Jominy test it exist some formulas which can be used for calculating the Jominy curve. One of the equations is developed by Just1, and is shown below.
where
J = Jominy hardness
E = Jominy distance
K = ASTM grain size
As cited by Bozidar Liscic in Steel Heat Treatment Handbook, ISBN: 0-8247-9750-7