Jominy end quench test / Jominy test
The Jominy test, also referred to as Jominy end quench test, is a standardized method according to ISO 642 and ASTM A255 for determination of the hardenability of steel.
Hardenability is the ability of the steel to increase hardness superficially or pervasively through the formation of martensite or an intermediate structure.
Specimen preparation and test sequence Jominy test hardness determination method Jominy hardness testing instruments
Jominy end quench test sequence to DIN EN ISO 642
The test sequence to DIN EN ISO 642 can be broken down into four steps:
- Creation of the mold and preparation of the specimen
- Heating of the specimen to a specified temperature in the austenitic range for a defined duration
- Quenching of the specimen on the front surface with a water jet under predefined conditions
- Testing the hardness at defined points in the longitudinal direction of the ground specimen test surface
1. Specimen preparation
First, the specimen, which consists of a round bar, must be machined to a diameter of 25 mm and a length of 100 mm. The front surface that is not quenched, must have a diameter of 30 mm to 32 mm or 25 mm, depending on whether it is the flange or groove type (see illustrations). These shapes also help with quick centering and positioning of the specimen for the quenching procedure.
Before the specimen is machined and quenched, it must be normalized. The surface of the specimen must be finely turned and the front surface to be quenched must be finely machined. The front surface must be free of burrs.
2. Specimen heat treatment
After the specimen has been prepared, it must be uniformly heated to the temperature specified in the standard for at least 20 minutes and then held at the determined temperature for at least 30 minutes and at most 35 minutes. Carburization or decarburization of the specimen should be kept to a minimum and oxidation with scale formation should be avoided.
3. Specimen quenching
The time frame between removing the specimen from the furnace and starting the quenching process may take no longer than five seconds. A test fixture is used (see illustration) that allows the water jet to abruptly strike the front surface of the specimen to be quenched. The distance between the opening of the water supply tube and the front surface of the specimen to be quenched is 12.5 mm, with a tolerance of ±0.5 mm. During the entire quenching process, which takes at least 10 minutes, the specimen must remain centered over the opening of the water supply tube and be protected from splashing water. After the quenching process is complete, the specimen can be submerged in cold water so it can cool down completely.
4. Specimen hardness test
For hardness testing of the Jominy specimen according to Rockwell (HRC), two test surfaces are offset by 180° and ground in the longitudinal direction (approx. 0.4-0.5 mm in depth). Under certain conditions, the Rockwell hardness test HRC can also be replaced by the Vickers hardness test HV30.
To ensure that no softening has occurred during fine grinding, the specimen is immersed in 5% nitric acid. The specimen should be uniformly blackened. Discolorations indicate soft spots and two new test areas must be created as described above.
In the next step, the specimen must be well secured and may not move during the hardness test. The first eight points of the HRC test are performed at specified intervals, relative to the front surface: 1.5 – 3 – 7 – 9 – 11 – 13 – 15 mm. After that, a test point is set every 5 mm. The hardness results are plotted in a diagram over the specimen length. This results in a hardness progression curve, from which the hardenability of steels can be compared or the effective hardening depth can be read.
Method for determining hardness in the Jominy test
Determination of one or two limit curves for the hardness depth in the Jominy test:
- Lower limit curve
- Upper limit curve
- Upper and lower limit curve
Determination of specific points on the hardness curve:
- Upper limit point
- Lower limit point
- Upper and lower limit point
The image displays the hardness progression curves of a Jominy specimen with upper and lower limit curves.
