The Jominy end-quench test represents a quandary. On one hand, everyone seems to know what the test is, but on the other hand, many don't understand how to use the data in a meaningful way in their everyday jobs. To help clarify this situation, consider the difference between the concepts of hardness and hardenability, which sometimes are used interchangeably.
Hardness vs. hardenability
Hardness simply is the measure of the resistance of a material to an applied force and involves the use of an indenter of fixed geometry under static load. The ability of the material to resist plastic deformation depends on the carbon content and microstructure of the steel. Therefore, the same steel can have different hardness values depending on its microstructure, which is influenced by the cooling (transformation) rate.
Hardenability, by comparison, is used to describe the heat treatment response of steels using either hardness or microstructure, both of which are interrelated. Hardenability is a material property, independent of cooling rate and dependent on chemical composition and grain size.
When evaluated using hardness testing, hardenability is defined as the capacity of the material under a given set of heat treatment conditions to harden "in depth." In other words, hardenability is concerned with depth of hardening, or the hardness profile obtained, not the ability to achieve a particular hardness value. When evaluated using microstructural techniques, hardenability is defined as the capacity of the steel to transform partially or completely from austenite to some percentage of martensite at a given depth when cooled under known conditions [1]. The reason why it is important to measure the hardenability of a steel is to ensure that you are making the right material choice for a specific engineering application.
Adding to the confusion is a common misconception associated with the Jominy test that the hardness values achieved can be used directly for actual parts being oil quenched. This is not true. An example of this difference in results can be seen in Fig. 1a and 1b, which relate equivalent cooling rates for round bars quenched in water and oil [2].
The results of the test allow comparing steels to determine their equivalent hardenability. In other words, Jominy curves can be used to predict the expected hardness distribution obtained on hardening steels of different dimensions in various cooling media [3].
The Jominy end-quench test, as well as a number of other testing methods, provide a cost- and time-effective way to determine the hardenability of steel. These tests were developed as alternatives to the creation of continuous cooling transformation (CCT) diagrams.
Determining hardenability
The Jominy end-quench test is the standard method used to measure the hardenability of steels. (For a discussion of the basic aspects of the test, see "Understanding The Jominy End-Quench Test," August 2001, IH).
One criticism of the Jominy test is that it is not discriminating when applied to steel of low hardenability. For such steels, the S-A-C test is considered more reliable. The test sample also is a cylinder, but has a length of 5.5 in. (140 mm) and a diameter of 1 in. (25.4 mm). After normalizing and austenitizing above the Ac3 (the temperature at which that transformation of ferrite to austenite is completed during heating), the specimen is quenched by submerging in a water bath.
After quenching, a 1-in. long cylinder is cut from the test specimen and the end faces are ground very carefully to remove any tempering effects induced by the cutting operation. Rockwell C hardness measurements are then made at four positions on the original cylinder face, and the average hardness provides the surface, or S, value. Rockwell testing is carried out along the specimen cross section from the surface to the center and provides a hardness profile similar to that shown in Fig 2. The total area under the curve provides the area, or A, value in units of "Rockwell-inch" and the hardness at the center gives the C value.
For example, a steel value reported as 65-51-39 indicates a surface hardness of 65 HRC, an area value of 51 Rockwell-inches and a core hardness of 39 HRC. The advantage of the S-A-C test is that the three numbers provide a good visual image of the hardness distribution curve. Surface hardness is influenced by the carbon content and is important in placing the hardness distribution curve, and the area under the curve provides both a knowledge of the extent of hardening and a comparative index of various steels [4].
Other testing methods include the Jominy-Boegehold end quench test used to measure the hardenability of the carburized portion of carburized and hardened steels, the Cone Test (Post, Greene and Fenstermacher) used for shallow hardening steels having hardenabilities in the same range as that of carbon tool steels (typically 1.10%C) and the Shepherd P-F and P-V tests used for shallow hardening steels. In the P-F test, P stands for penetration of hardening (hardenability) and F stands for fracture grain size. P-V indicates penetration of hardening on a V-shaped test piece [5].
More recently, Liscic and Filetin, Wunning, Tensi, and others developed processes to evaluate quenching severity (intensity). The Liscic technique is based on a method using a test probe to determine the heat flux (the rate of heat transfer across a given surface) at the surface of a part during quenching. In addition to the heat flux data obtained from the probe, a second specimen with the same dimensions as the probe and a Jominy specimen of the same steel are quenched under identical conditions. The samples are evaluated and a database is established for various quenching conditions and for determining equivalent Jominy distances [6].
Although the Jominy end-quench test is relatively easy to perform, the procedure must be followed exactly to obtain good results. Perhaps the single most important aspect of the test is consistency. Each test must be done under identical conditions; that is, austenitizing temperature; transfer time (from oven to quench); handling method and water temperature, flow and pressure. Water is an excellent and inexpensive quenchant, but its heat removal characteristics are highly variable, being dependent on temperature and the relative velocity between the water and specimen.
Opinions vary about the best way to measure the hardness profile (Rockwell, Brinell or Vickers). Choice of measurement depends to a great extent on the specimen dimensions and hardness. Rockwell C typically is used if the hardness is greater than 20 HRC. Brinell is used when the hardness is below 20 HRC. Vickers is selected if a microhardness profile is required. Standard hardness testing rules for minimum distance between readings and specimen thickness must be strictly followed.
For interactive training, the website How is hardenability evaluated and how are the effects of a large number of quenching media on hardness distribution evaluated without the time-consuming approach of quenching a series of round bars into various quench media? The answers to these questions were first developed years ago by Grossman and Bain and subsequently by a number of other researchers. Two parameters involved are critical size and ideal size.
Critical size, or critical diameter, can be defined as the largest size bar that, after being quenched in a given medium, contains greater than 50% martensite. That is, no portion of the core is considered unhardened after quenching. The choice of the 50% value is no accident. Both metallographic etching and mechanical fracture methods can be used to evaluate the depth of hardening at the 50% martensite level. Etching clearly differentiates the hardened surface of the bar from the unhardened core; i.e., a clear delineation between the 50% martensite-50% pearlite zone is visible. Similarly, this zone correlates well with a transition from a very smooth fracture (associated primarily with a martensitic structure) to a rough fracture surface (associated with a ductile fracture of softer nonmartensitic transformation products). In addition, hardness changes dramatically as well.
Thus, the critical size (D) of a steel bar of a known composition is directly related to a given quench medium. The higher the quench severity, the greater the critical size.
The ideal size, or ideal diameter, of a steel can be defined as the size of bar hardened to 50% martensite in a "perfect" quench medium. In such a quenchant, the surface of the bar is assumed to cool instantaneously to the temperature of the quenching medium.
The ideal size (D1) is a true measure of the hardenability associated with a given steel composition, and it can be used to determine the critical size of the steels quenched in media having different quench severities. Three factors that affect the ideal diameter are austenitic grain size, carbon content and alloy content. Basically, an increase in any one of these factors makes transformation to martensite more likely. A "base hardenability" is established for a steel based on the carbon content and grain size. This base hardenability is then multiplied by alloying factors.
The actual critical diameter can be related to the ideal diameter to determine the quench severity (H) as shown in Fig. 3 [7]. What do the test results mean and how can they be interpreted in the real world? Steels having higher hardenability will be harder at a given distance from the quenched end than steels having lower hardenability. Thus, the flatter the curve, the greater the hardenability. A flat curve demonstrates conditions of very high hardenability, such as that achieved in air hardening tool steels or highly alloyed tool steels.
For example, most gear manufacturers have graphs that show the Jominy range for different "pitch" (gear tooth size) gears. This produces a certain hardness in the middle of the tooth at the pitch line and at the root. Typical data are shown in Fig. 4 and 5 for thin and thick section gears, respectively.
Assume you want to select the most cost-effective grade of material to manufacture gears of 4 and 6 pitch given a core hardness requirement of 30 to 40 HRC at the midtooth. For the purpose of this example, assume that steel A is more expensive than steel B. In considering Figures 4 and 5, it can be determined that, as a function of gear section thickness, a 4-pitch gear requires a Jominy value between 4.5 and 5.5. By contrast, the values for a 6-pitch gear are in the range of 3.0 to 3.25. Fig. 6 shows that 4-pitch gears require steel A to meet the minimum core hardness, but that a 6-pitch gear can be manufactured from the less expensive steel B [8].
The subject of interpreting and applying the results of Jominy testing is vast, and this discussion only covers the tip of the iceberg. The technology associated with hardenability is continually evolving. Although the principles of hardenability and a great deal of the hardenability testing remains essentially unchanged, the emphasis today is on developing more reliable and systematic hardenability data to input into computer modeling software. This will allow predicting the performance of the new generation of "engineered materials" coming into the market-materials being designed for use in a specific application. Also available are materials having modified chemical compositions, and it is important to understand their hardenability performance.
Today, work continues in the areas of shallow-hardening low-carbon steels, and high-carbon and boron steels. The development and application of an "equivalent" Jominy test using a variable pressure stream of gas instead of water for quenching is actively underway. This research has been fueled by the movement of the heat treating industry into high-pressure gas quenching, which is seriously being considered as an alternative to, or replacement for, liquid media in both atmosphere and vacuum furnaces. The references listed in this article provide excellent resources to learn more.
Evaluating hardenability
Using test results
For more information: Daniel Herring is president of The Herring Group Inc., PO Box 884, Elmhurst, IL 60126-0884; tel: 630-834-3017; fax: 630-834-3117; e-mail: dherring@heat-treat-doctor.com; Internet: www.heat-treat-doctor.com
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