JOMINY HARDENABILITY TEST
Objectives
1. Introduce students to the concepts of hardenability
2. Demonstrate hardenability in a steel
3. Display the effects on microstructure of the hardening process
4. Provide instruction for a full lab experiment on hardenability (if equipment is available)
1.Introduction
The hardenability of a steel is defined as that property which determines the depth and distribution
of hardness induced by quenching ftom the austenitic condition. The dependence of hardness upon
quenching rate can be understood ftom the time-temperature-transformation characteristics of
steel, and, for a particular steel, can be estimated from the T-T-T diagram.
A part may be hardened by quenching into water, oil, or other suitable medium. The surface of the
part is cooled rapidly, resulting in high hardness, whereas the interior cools more slowly and is not
hardened. Because of the nature of the T-T-T diagram, the hardness does not vary linearly from the
outside to the center. Hardenability refers to capacity of hardening (depth) rather than to maximum
attainable hardness.
The hardenability of a steel depends on
(1) the composition of the steel,
(2) the austenitic grain size, and
(3) the structure of the steel before quenching.
In general ,hardenability increases with carbon content and with alloy content. The most important
factor influencing the maximum hardness that can be obtained is mass of the metal being quenched.
In a small section, the heat is extracted quickly, thus exceeding the critical cooling rate of the specific
steel and this part would thus be completely martensitic. The critical cooling rate is that rate of
cooling which must be exceeded to prevent formation of nonmartensite products. As section size
increases, it becomes increasingly difficult to extract the heat fast enough to exceed the critical
cooling rate and thus avoid formation of nonmartensitic products. Hardenability of all steels is
directly related to critical cooling rates.
Procedure
Sample of medium carbon steel machined to the shape shown in Fig.1. It is a cylindirical bar with a 25
mm. diameter and 100 mm. length. The specimen is placed in the furnace at 900 0 C for about 1 /2
1
hour. The water flow rate is adjusted so that the water column is approximately the distance 50 mm
above the end of the pipe, when water is flowing freely. After the sample has been austenitized, it is
removed from the furnace and placed directly into the quenching apparatus. A jet of water is quickly
splashed at one end of the specimen. After the entire sample has cooled to room temperature, the
scale oxidation is removed; two opposite and flat parallel surfaces are ground along the length of the
bar. Rockwell C hardness measurements are then made every 2 mm and these readings are
recorded. Results Plot a hardenability curve of Rockwell hardness vs. distance from the quenched
end.
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2.Tests and Results
a. Evaluate the hardenability of the steels used in this experiment using the plotted
hardenability curve.
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3. Discussion and Analysis
a. What was the change in hardness from the surface to the core of the material at the
end of the experiment? What is the reason for this difference? Explain it by using
TTT/CCT diagrams
b. What is the difference between the hardness distance change of normalized and end
quenched samples? Explain
c. If cooling oil was used instead of the cooling water you used, what would be the
change from the surface to the core of the material as a result of this experiment?
What is the reason of this? Explain.
d. What would be the difference if the diameter of the sample you used in this
experiment was smaller? Explain.
e. What would be the difference if the sample you used in this experiment was made of
a higher alloy steel? Explain.
4. Answer the following questions
a. How is the role of carbon and various alloy elements on the hardenability of steels
(Give examples of different hardenability curves ).
b. What is hardness? When is hardness important?
c. What is hardenability?
d. Do all steels have the same hardenability? Why or Why not?
References 1. Shackelford, IF, Introduction to Materials Science 2. Smith, W.F., Principles of Materials
Science and Engineering 3. ASM, Heat Treater's Guide S
