A laboratory test procedure developed by W. Jominy in 1938 for determining the hardenability of steels and other ferrous alloys. The test, usually called the End Quench Test (ASTM A255), is the most common method of determining hardenability, the relative ability of a steel to form martensite when quenched from a temperature above the upper critical temperature.
In the test procedure, a sample of a particular steel is heated to the correct quenching temperature, assuring that the surface is protected from oxidation. After heating, the sample is quenched. The quenching water jet impinges on the end of the sample and this area is cooled very rapidly. Since the heat must travel by conduction from the sample to the quenched end, the top portion of the sample will cool very slowly. Different rates of cooling, therefore, will occur all along the sample.
The hardness of the steel at different rates of cooling is indicated by Rockwell C (HRC) hardness readings, starting at 1.6 mm (1/16 in.) from the hardened end and at 1.6 mm (1/16 in.) intervals for a distance of SO mm (2 in.).
The sample consists of a piece 10 cm (4 in.) in length. It is 25 mm (1 in.) round for a distance of 9.8 cm (3.875 in.), with a flange approximately 2.8 cm (1.125 in.) in diameter and 0.4 mm (0.015 in.) thick on one end. After the sample has been quenched, the next step is to grind a flat about 0.4 mm (0.015 in.) deep along the entire length of the sample to remove the carburized surface. It is on this flat area that the Rockwell C hardness readings are taken. The data are normally plotted as hardness (HRC) versus distance from the quenched end at which a certain hardness (such as HRC SO) is observed that may be used as an indication of hardenability.
If the hardness in the coarse-grained region of the heat-affected zone (HAZ) of a weld in a steel is matched with the same hardness on a Jominy bar of the same steel, then the cooling rates at these two positions (one in the HAZ and the other on the Jominy bar) are the same. The cooling rates at various positions along the Jominy bar are measured and tabulated. Further, the HAZ cooling rates for various welding conditions (plate thickness, joint design, initial plate temperature, current. voltage and travel speed) are measured and tabulated. Thus it is possible to select conditions that avoid the formation of brittle martensite during the arc welding of a particular steel.
Additionally, in lower-carbon quenched-and-tempered steels, conditions can be selected so that a tougher martensite forms in the heat-affected zone.