Understanding the Jominy End-Quench Test

The Jominy end-quench test is used to measure the hardenability of a steel, which is a measure of the capacity of the steel to harden in depth under a given set of conditions. This article considers the basic concepts of hardenability and the Jominy test.

Knowledge about the hardenability of steels is necessary to select the appropriate combination of alloy steel and heat treatment to minimize thermal stresses and distortion in manufacturing components of different sizes. The Jominy end-quench test is the standard method for measuring the hardenability of steels. This describes the ability of the steel to be hardened in depth by quenching. Hardenability depends on the chemical composition of the steel and can also be affected by prior processing conditions, such as the austenitizing temperature. It not only is necessary to understand the basic information provided from the test, but also to determine how the information obtained from the Jominy test can be used to understand the effects of alloying in steels and the steel microstructure.

Hardenability

Hardenability is the ability of the steel to partially or to completely transform from austenite to some fraction of martensite at a given depth below the surface when cooled under a given condition from high temperature. A quench-and-temperheat treatment uses this phase transformation to harden steels. Tempering the martensite microstructure imparts a good combination of strength and toughness to the steel. Without tempering, martensite is hard but brittle.

To select a steel for a component that will be heat treated, it is important to know its hardenability. Both alloying and microstructure affect the hardenability, allowing the correct steel and quenching rate to be selected. Prior processing of the steel also affects the microstructure and should be considered.

Hardening of steels can be understood by considering that the austenite phase of the steel can transform to either martensite (Fig. 1a) or a mixture of ferrite and pearlite (Fig. 1b) on cooling from high temperature.

The ferrite/pearlite reaction involves diffusion, which takes time. However, the martensite transformation does not involve diffusion and is essentially instantaneous. These two reactions are competitive, and martensite is obtained if the cooling rate is fast enough to avoid the slower formation of ferrite and pearlite. In alloyed steels, the ferrite/pearlite reaction is further slowed down, which allows martensite to be obtained using slower cooling rates. Transformation to another possible phase (bainite) can be understood in a similar way.

Hardenability describes the capacity of the steel to harden in depth under a given set of conditions. For example, a steel of a high hardenability can transform to a high fraction of martensite to depths of several millimeters under relatively slow cooling, such as an oil quench. By comparison, a steel of low hardenability may only form a high fraction of martensite to a depth of less than 1 mm, even under quite rapid cooling, such as a water quench.

Steels having high hardenability are required to make large high-strength components (such as large extruder screws for injection molding of polymers, pistons for rock breakers, mine-shaft supports and aircraft undercarriages) and small, high-precision components (such as die-casting molds, drills and presses for stamping coins).

The slower cooling rates that can be used for high-hardenability steels can reduce thermal stresses and distortion. Steels having low hardenability may be used for smaller components, such as chisels and shears, or for surface-hardened components, such as gears, where there is a desire to maintain a ferrite/pearlite microstructure at the core to improve toughness. The Jominy end-quench test is the standard method to measure the hardenability of steels.[1]

Testing Particulars

The test sample is a 100 mm (4 inch) long x 25.4-mm (1-inch) diameter cylinder (Fig. 2a). The steel sample is normalized (to eliminate differences in microstructure due to previous hot working) and then austenitzed usually at a temperature of 800-925°C (1470-1700°F). The test sample is quickly transferred to the test fixture (Fig. 2b), which quenches the steel by spraying a controlled flow of water onto one end of the sample (Fig. 2c). The cooling rate varies along the length of the sample, from very rapid at the quenched end where the water strikes the specimen to slower rates that are equivalent to air cooling at the other end.

The round specimen is then ground flat along its length on opposite sides to a depth of at least 0.38 mm (0.015 inch) to remove decarburized material. Care should be taken that the grinding does not heat the sample because this can cause tempering, which can soften the steel.

Hardness is measured at intervals from the quenched end, typically at 1.5 mm (0.062 inch) intervals for alloy steels and 0.75 mm (0.031 inch) for carbon steels, beginning as close as possible to the quenched end. The hardness decreases with distance from the quenched end. High hardness occurs where high-volume fractions of martensite develop. Lower hardness indicates transformation to bainite or ferrite/pearlite microstructures.

Measurement of hardness is commonly carried out using a Rockwell or Vickers hardness tester.[1-3] Conversion charts are available to relate the different hardness scales[4,5] if necessary, but care should be taken to use the correct charts for steel. Rockwell and Vickers hardness tests deform the metal differently, and the results are affected by work hardening. The hardenability is described by a hardness curve for the steel (Fig. 3) or more commonly by reference to the hardness value at a particular distance from the quenched end.

Uses of Hardenability Values

Data from the Jominy end-quench test can be used to determine whether a particular steel can be sufficiently hardened in different quenching media, for different section diameters. For example, the cooling rate at a distance of 10 mm (0.390 inch) from the quenched end is equivalent to the cooling rate at the center of an oil-quenched 28-mm (1.1-inch) diameter bar. Full transformation to martensite in the Jominy specimen at this position indicates that a 28-mm-diameter bar can be through-hardened (i.e., hardened through its full thickness).

A high hardenability is required for through-hardening of large components. This data can be presented using CCT (continuous-cooling transformation) diagrams,[6] which are used to select steels to suit the component size and quenching media (Fig. 4). Slower cooling rates occur at the core of larger components, compared with the faster cooling rate at the surface. In the example in Fig. 3, the surface will be transformed to martensite, but the core will have a bainitic structure with some martensite. Slow quenching speeds often are selected to reduce distortion and residual stress in components. Reference 6 contains further information on the heat treatment and properties of steels.

Effects of Alloying and Microstructure

The Jominy end-quench test measures the effects of microstructure, such as grain size and alloying, on the hardenability of steels. The main alloying elements that affect hardenability are carbon; a group of elements including Cr, Mn, Mo, Si and Ni; and boron.[7] Reference 7 contains further information about the microstructure and metallurgy of steels.

Carbon

Carbon controls the hardness of the martensite; increasing carbon content increases the hardness of steels up to about

0.6 wt.% carbon. At higher carbon levels, however, the critical temperature for the formation of martensite is depressed to lower temperatures. The transformation from austenite to martensite may then be incomplete when the steel is quenched to room temperature, which leads to retained austenite. This composite microstructure of martensite and austenite results in a lower steel hardness, although the hardness of the martensite phase itself is still high (Fig. 5).

Carbon also increases the hardenability of steels by retarding the formation of pearlite and ferrite. Slowing down this reaction encourages the formation of martensite at slower cooling rates. However, the effect is too small to be commonly used for control of hardenability. Furthermore, high-carbon steels are prone to distortion and cracking during heat treatment and can be difficult to machine in the annealed condition before heat treatment. It is more common to control hardenability using other elements and to use carbon levels of less than 0.4 wt.%.

Other Alloying Elements

Cr, Mo, Mn, Si, Ni and V retard the phase transformation from austenite to ferrite and pearlite. The most commonly used elements are Cr, Mo and Mn. The retardation is due to the need for redistribution of the alloying elements during the diffusional phase transformation from austenite to ferrite and pearlite. The solubility of the elements varies between the different phases, and the interface between the new growing phase cannot move without diffusion of the slowly moving elements. There are quite complex interactions between the different elements, which also affect the temperatures of the phase transformation and the resultant microstructure. Alloy steel compositions are, therefore, sometimes described in terms of a carbon equivalent, which describes the magnitude of the effect of all of the elements on hardenability. Steels of the same carbon equivalent have similar hardenability.

Boron

Boron is a very potent alloying element, typically requiring 0.002-0.003 wt.% to have the equivalent effect of 0.5 wt.% Mo. The effect of boron is independent of the amount of boron, provided a sufficient amount is added. The effect of boron is greatest at lower carbon contents, and it is typically used with lower-carbon steels.

Boron has a very strong affinity for oxygen and nitrogen, with which it forms compounds. Therefore, boron can only affect the hardenability of steels if it is in solution. This requires the addition of “gettering” elements, such as aluminum and titanium, to react preferentially with the oxygen and nitrogen in the steel.

Grain Size

Increasing the austenite grain size increases the hardenability of steels. The nucleation of ferrite and pearlite occurs at heterogeneous sites such as the austenite grain boundaries. Therefore, increasing the austenite grain size decreases the available nucleation sites, which retards the rate of the ferrite/pearlite phase transformation (Fig. 6). This method of increasing the hardenability is rarely used because substantial increases in hardenability require large austenite grain size, which is obtained through high austenitizing temperatures. The resultant microstructure is quite coarse, with reduced toughness and ductility. However, the austenite grain size can be affected by other stages in the processing of steel. Therefore, the hardenability of a steel also depends on the previous stages used in its production.