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What is heat treatment?
Heat Treatment
is a technique used to alter the physical (and sometimes
chemical) properties of a material. Heat treatment involves the
use of heating or chilling, normally to extreme temperatures, to
achieve a desired result such as hardening or softening of a
material.
Metallic materials consist of a
microstructure of small crystals called "grains" or
crystallites. The nature of the grains (i.e. grain size and
composition) determine the overall mechanical behavior of the
metal. Heat treatment provides an efficient way to manipulate
the properties of the metal by controlling rate of diffusion,
and the rate of cooling within the microstructure. In carbon and
low alloy steels, fast rates of cooling result in a high degree
of hardness.
Quenching is the act of rapidly
cooling the hot steel to harden it, a metal (in
our case 416 steel) must be heated into the austenitic crystal
phase and then quickly cooled. Depending on the alloy and other
considerations (such as concern for maximum hardness vs.
cracking and distortion), cooling may be done with forced air or
other gas (such as nitrogen), oil, polymer dissolved in water,
water, or brine. Upon being rapidly cooled, the austenite will
transform to martensite, a hard brittle crystalline structure.
Most applications require that quenched parts be tempered (heat
treated at a low temperature, often three hundred degree
Fahrenheit or one hundred fifty degrees Celsius) to impart some
toughness. Higher tempering temperatures (may be up to thirteen
hundred degrees Fahrenheit, depending on alloy and application)
are sometimes used to impart further ductility, although some
strength is lost.
The lab is intended
to accomplish five goals:
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To introduce the
Johnson Heat Treatment Furnace, (a basic tools used in
manufacturing shops) and its related terminologies to the
students.
-
To learn the
importance of the concept of heat treatment in manufacturing.
To practice the
team work concept through a hands-on practice.
To practice
safety related to working in a manufacturing environment.
To provide an
experiment through which students recognize and see the
concepts of hardness by heat treatment explained in the textbook.
The Lab
Each group will
test each of the four (4) specimens. The smallest of the four is
to be used as your original. The three remaining pieces will be
heated (record the ~temperature), quenched (record the ~temp of
the quenching agent -- water, brine, or oil).
Test each of the
four samples in the hardness tester
and perform at least three (3) separate tests on each specimen. You will average the
test hardness ratings for each specimen. From your
collected data, note the distinctions and the effects of each of
the quenching agents. Submit your data in table format in your report.
Submit each of
the four samples marked to identify your group to the
instructor. These samples will be used in the next lab. |
