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 The Jominy
End Quench Test measures Hardenability of
steels. Hardenability is a measure of the capacity of a steel
to be hardened in depth when quenched from its austenitizing temperature.
Hardenability of a steel should not be confused with the hardness of a
steel. The Hardness of a steel refers to its ability to resist deformation
when a load is applied, whereas hardenability refers to its ability to be
hardened to a particular depth under a particular set of conditions. Information gained from this test
is necessary in selecting the proper combination of alloy steel and heat
treatment to minimize thermal stresses and distortion when manufacturing
components of various sizes.To
perform the Jominy Test: First, a sample
specimen cylinder either 100mm in length and 25mm in diameter, or
alternatively, 102mm by 25.4mm is obtained. Second, the
steel sample is normalized to eliminate differences in microstructure due to
previous forging, and then it is austenitised. This is usually at a
temperature of 800 to 900°C. Next, the specimen
is rapidly transferred to the test machine, where it is held vertically and
sprayed with a controlled flow of water onto one end of the sample. This
cools the specimen from one end, simulating the effect of quenching a larger
steel component in water. Because the cooling rate decreases as one moves
further from the quenched end, you can measure the effects of a wide range
of cooling rates from vary rapid at the quenched end to air cooled at the
far end.
 Next,
the specimen is ground flat along its length to a depth of .38mm (15thousandths of
an inch) to remove decarburized material. The hardness is measured at intervals along its length beginning at the quenched end. For alloyed steels
an interval of 1.5mm is commonly used where as with carbon steels an
interval of .75mm is typically employed.
And finally the Rockwell or Vickers hardness values
are plotted versus distance from the quenched end.
The Jominy Test data illustrates the effect of
alloying and microstructure on the hardenability
of steels. Commonly used elements that affect the hardenability of steel are
carbon, boron, Chromium, Manganese, Molybdenum, Silicon, and Nickel.
Carbon is primarily a
hardening agent in steel, although to a small degree it also
increases hardenability by slowing the formation of pearlite and ferrite.
But this affect is too small to be used as a control factor for
hardenability. Boron can be an
effective alloy for improving hardenability at levels as low as .0005%.
Boron is most effective in steels of 0.25% Carbon or
less. Boron combines readily with both Nitrogen and Oxygen and in so doing
its effect on hardenability is sacrificed. Therefore Boron must remain in
solution in order to be affective. Aluminum and Titanium are commonly added
as "gettering" agents to react with the Oxygen and Nitrogen in preference to
the Boron. Slowing the phase
transformation of austenite to ferrite and pearlite increases the
hardenability of steels. Chromium, Molybdenum, Manganese, Silicon, Nickel and Vanadium all effect the hardenability of steels in this manner.
Chromium, Molybdenum and Manganese being used most often. |
