The Jominy Test: How Alloying Elements Increase Steel Hardenability

After individually analyzing the alloying elements (Carbon, Chromium, Molybdenum, Nickel, Vanadium, etc.), it’s time to see their concrete effect on the hardenability of special steels, meaning their ability to generate a martensitic structure in depth during quenching. To evaluate this aptitude, one of the most widely used and universally recognized tests is the Jominy test.

The Jominy test is the standardized method (UNI EN ISO 642) that evaluates how steel behaves in response to heat treatment by checking the penetration of hardness in depth. The test involves a cylindrical specimen of Ø 25 × 100 mm, which is austenitized and then cooled at one end with a water jet (at specified pressure and temperature).

The cylindrical surface of the specimen is then ground to allow for the measurement of hardness progression using the HRC scale, every 1.5 mm.

The graph of “hardness (HRC) vs. distance from the quenched end (mm)” describes the hardenability of the steel under examination.

Elements like Chromium (Cr), Molybdenum (Mo), and Nickel (Ni) slow down the transformation to pearlite/bainite, allowing for the formation of martensite at greater depths. In general, all elements, with the exception of cobalt, each with their own characteristics, increase hardenability.

As clearly shown by the graphs above, as the amount of alloying elements increases, the hardenability of special steels also increases, meaning the depth of hardening penetration.

An application example is represented by pre-hardened steels used in molds for plastics processing. Beyond a certain thickness (approximately 300 mm), steel 1.2311 (40CrMnMo7) gives way to 1.2738 (40CrMnNiMo8-6-4), which, thanks to its 1% Ni content, guarantees hardening penetration even in the center of the bar.

The Jominy test concretely demonstrates how alloying elements can improve the hardenability of special steels, making it possible to design large-section components without sacrificing core hardness.

Bonomi Acciai, a partner of Swiss Steel Group, supports designers in choosing the steel with the appropriate hardenability for each specific application. Contact us for a dedicated consultation.