The rate of cooling significantly affects the microstructure of steel.
Isothermal transformation (TTT) diagrams map these effects. When a
thick section of steel is quenched the cooling rate varies with the
distance from the surface.
Hardenability refers to the depth to which martensite is formed. The
Jominy end-quench test is the standard method to determine
hardenabilty. The cylindrical specimen is heated to the austenitizing
temperature. The bar is placed in the Jominy tank fixture,
suspended by a bolt in the end. The other end is cooled by a water jet
in the recirculating water tank. Since one end is quenched and the
other is in room temperature air, the cooling rate varies along the
|Jominy Quench Tanks
After cooling in the Jominy tank, the specimen is cleaned using a grinding wheel wire brush, flats are ground on two sides with a belt sander, and hardness values are measured along the length using the Rockwell hardness testers.
The hardness data are plotted vs. length to generate hardenability curves.
Specimens that have been sectioned, polished and etched are examined with the metallurgical microscope.
The change in microstructure with respect to the distance from the
quenched end is noted. These features are correlated with isothermal
transformation diagrams using cooling rates at various distances from
the quenched end. The hardness data are related to the proportion of