Jominy End-Quench Hardenability Test

The end-quench hardenability test developed by Jominy and Boegehold is commonly referred to as the Jominy test. It is used worldwide, described in many national standards, and available as an international standard. This test has the following significant advantages:

1. It characterizes the hardenability of a steel from a single specimen, allowing a wide range of cooling rates during a single test.
2. It is reasonably reproducible.

The steel test specimen (25 mm diameter x 100 mm) is heated to the appropriate austenitizing temperature and soaked for 30 min. It is then quickly transferred to the supporting fixture (Jominy apparatus) and quenched from the lower end by spraying with a jet of water under specified conditions as illustrated in Figure 1. The cooling rate is the highest at the end where the water jet impinges on the specimen and decreases from the quenched end, producing a variety of microstructures and hardnesses as a function of distance from the quenched end. After quenching, two parallel flats, approximately 0.45 mm below surface, are ground on opposite sides of the specimen and hardness values (usually HRC) are measured at 1=16 in. intervals from the quenched end and plotted as the Jominy hardenability curve (see Figure 2). When the distance is measured in millimeters, the hardness values are taken at every 2 mm from the quenched end for at least a total distance of 20 or 40 mm, depending on the steepness of the hardenability curve, and then every 10 mm. On the upper margin of the Jominy hardenability diagram, approximate cooling rates at 700 C may be plotted at several distances from the quenched end.

Jominy specimen and its quenching conditions

Figure 1: Jominy specimen and its quenching conditions for end-quench hardenability test.

Measuring hardness on the Jominy specimen

Figure 2: Measuring hardness on the Jominy specimen and plotting the Jominy hardenability curve. (From G. Krauss, Steels Heat Treatment and Processing Principles, ASM International, Metals Park, OH, 1990.)

Figure 3 shows Jominy hardenability curves for different unalloyed and low-alloyed grades of steel. This figure illustrates the influence of carbon content on the ability to reach a certain hardness level and the influence of alloying elements on the hardness distribution expressed as hardness values along the length of the Jominy specimen. For example, DIN Ck45, an unalloyed steel, has a carbon content of 0.45% C and exhibits a higher maximum hardness (see the value at 0 distance from the quenched end) than DIN 30CrMoV9 steel, which has only 0.30% C. However, the latter steel is alloyed with Cr, Mo, and V and shows a higher hardenability by exhibiting higher hardness values along the length of the specimen. The Jominy end-quench test is used mostly for low-alloy steels for carburizing (core hardenability) and for structural steels, which are typically through-hardened in oils and tempered. The Jominy end-quench test is suitable for all steels except those of very low or very high hardenability, i.e., D1< 1.0 in. or D1> 6.0 in. The standard Jominy end-quench test cannot be used for highly alloyed air-hardened steels. These steels harden not only by heat extraction through the quenched end but also by heat extraction by the surrounding air. This effect increases with increasing distance from the quenched end.

Jominy hardenability curves

Figure 3: Jominy hardenability curves (average values) for selected grades of steel (designations according to German DIN standard). (From G. Spur (Ed.), Handbuch der Fertigungstechnik, Band 4-2, Wa¨rmebehandeln, Carl Hanser, Munich, 1987, p. 1012.)

The reproducibility of the standard Jominy end-quench test was extensively investigated, and deviations from the standard procedure were determined. Figure 4 shows the results of an end-quench hardenability test performed by nine laboratories on a single heat of SAE 4068 steel. Generally, quite good reproducibility was achieved, although the maximum difference may be 8–12 HRC up to a distance of 10 mm from the quenched end depending on the slope of the curve. Several authors who have investigated the effect of deviations from the standard test procedure have concluded that the most important factors to be closely controlled are austenitization temperature and time, grinding of the flats of the test bar, prevention of grinding burns, and accuracy of the measured distance from the quenched end. Other variables such as water temperature, orifice diameter, free water-jet height, and transfer time from the furnace to the quenching fixture are not as critical.

Reproductibility of the end-quench hardenability test

Figure 4: Reproductibility of the end-quench hardenability test. Hardenability range (hatched area between curves) based on tests by nine laboratories on a single heat of SAE 4068 steel. (From C.A. Siebert, D.V. Doane, and D.H. Breen, The Hardenability of Steels, ASM International, Cleveland, OH, 1997.)

Reference: George E. Totten, Steel Heat Treatment: Metallurgy and Technologies, CRC Press, 2007, pp. 228-230.

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