Ability of steel to harden depends on its section size, composition, austenitizing temperature, and quenching medium is termed as hardenability.

The Jominy’s test provides a measure of the ability of steel to harden by transforming into martensite under set conditions, i.e., a measure of the hardenability of the steel.

A standardised bar, 25.4 mm diameter and 102 mm long, is heated to the austenitizing temperature and then placed on a rig in which one end of the rod is quenched by a standard jet of water.

This results in a progressive decrease in the rate of cooling along the bar from the quenched end. The depth from the quenched end, over which martensite is obtained, is then the measure of hardenability.

The austenitizing temperature of steel is 800 to 900 degrees.

After cooling completely the Rockwell hardness is calculated at various points from bottom end and a graph is plotted between hardness and length form quenched end of specimen.

The hardness decreases from quenched end as we move ahead along the length of specimen and giving result of formation of martensite form quenched end and slowly reducing to perlitic & ferritic phase.