METAL PROPERTIES, CHAR, USES, AND CODES - OD1643 - LESSON 1/TASK 2
reflection will show that these various indications of hardness are
not the same. The following subparagraphs describe the processes for
the performance of various hardness tests.
b.
Brinell Hardness Test.
One of the most common methods of
measuring the hardness of a metal is to determine its resistance to
the penetration of a nondeformable steel ball.
This is done by
determining the depth to which such a ball will sink into metal under
a given load. This test is made by forcing the hardened steel ball
into the test material by the weight of a known load.
The ball is
usually 10 millimeters in diameter and has an applied pressure of 500
kilograms for soft materials such as copper and brass, and 3000
kilograms for materials such as iron and steel.
Once the load has
been applied, the diameter of the resulting impression is measured
with a small microscope. The hardness number of the metal is found
by dividing the load applied by the area of the impression and
comparing the results of the division with a standard hardness
conversion table, which provides the hardness number of the metal.
c.
Rockwell Tester. This test is based upon the difference between
the depth to which a test point is driven into a metal by a light
load and the depth to which it is driven by a heavy load. The light
load is applied first and then, without moving the piece being
tested, the heavy load is applied.
The hardness number is
automatically indicated on a dial. In this test, a 120 diamond cone
for hard metals or a 1/16 inch steel ball for softer materials is
impressed into the surface to be tested by a deadweight acting
through a series of levers. The hardness is indicated on a dial gage
graduated in the Rockwell "B" and "C" scales. The harder the piece,
the higher the Rockwell number will be.
For example, machinable
steel should not show a reading of more than 30 to 35 on the Rockwell
"C" scale, while a hardened high-speed cutter would show a reading of
63 to 65. When testing hard steel, the diamond point should be used
and should be read on the "C" scale.
For nonferrous metals, the
steel ball should be used and read on the "B" scale.
d.
Vickers Hardness Test.
The Vickers hardness testing method is
very similar to the Brinell method.
The penetrator used in the
Vickers machine is a diamond pyramid rather than the round steel
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