The Fundamentals of Rockwell Hardness Test Method
The Rockwell
test method, as defined in ASTM E-18, is the most commonly used hardness
test method. It is generally easier to perform and more accurate
than other types of hardness testing. It is used on all metals except
in conditions where the test metal structure or surface conditions would
introduce too much variation, where the indentations would be too large
for the application or where the sample size or shape prohibits its use.
The Rockwell
method measures the permanent depth of indentation produced by a force
on an indenter. First, a preliminary test force (pre-load or minor load)
is applied to a sample using a diamond indenter. This is the zero or reference
position that breaks through the surface to reduce the effects of surface
finish. Then, an additional test force (or major load) is applied to reach
the total required test force. This force is held for a predetermined
amount of time to allow for elastic recovery. The additional test force
is then released and the final position is measured against the preliminary
position and converted to a hardness number. Preliminary test forces range
from 3 kg (used in "Superficial" Rockwell scale) to 10 kilograms
(used in "Regular" Rockwell scale) to 200 kilograms (macro scale
- not part of ASTM E-18; see ASTM E-1842). Total test forces range from
15 through 150 kilograms (superficial & regular) to 500 through 3000 kilograms
(macro). A variety of indenters may be used: a conical diamond with a
round tip for harder metals, and ball indenters ranging from 1/16"
to 1/2" for softer and softer materials. Here
is a complete list of Rockwell hardness
scales Also
refer to the General Hardness Training Guide: Part
1 Part 2 Part
3.
As a general guide to
selecting a Rockwell scale, the operator should select the scale that
specifies the largest load and smallest indenter possible to do the job
without exceeding defined operating conditions and accounting for conditions
that influence the test result. These influencing conditions include
test specimens which are below the minimum thickness for the depth of
indentation (see chart); a test impression
that falls too close to the edge of a specimen or another impression (indentations
should be spaced greater than 3x indentation diameter and greater than
2-1/2x diameter from the edge of the specimen); or testing on cylindrical
test specimens (see chart). Some important
operating conditions are: the axis of test be within 2 degrees of perpendicular;
there should be no deflection of the test sample or tester during the
load application (from conditions such as dirt under the test specimen
or on the elevating screw; surface finish should be kept ; surface conditions
such as decarburization from heat treatment should be removed.
Special
cases: sheet metal can be too thin and too soft for testing on a particular
Rockwell scale without exceeding minimum thickness requirements and potentially
indenting the test anvil. In this case a diamond anvil can be sued to
provide a consistent influence of the result. Another special case in
testing cold rolled sheet metal is that work hardening can create a gradient
of hardness through the sample so any test is measuring the average of
the hardness over the depth of indentation effect. In this case any Rockwell
test result is going to be subject to doubt, there is often a history
of testing using a particular scale on a particular material that operators
are used to and able to functionally interpret.
Users should obtain the ASTM standard
for Rockwell Hardness Testing, ASTM E-18, before beginning actual testing.
Selecting a Newage Rockwell Hardness Tester
Rockwell
scale testing often presents some challenges for users. Poor operating
conditions, inexperienced users, tedious high-volume testing and demanding
applications can all work to reduce accuracy and repeatability. Newage
testers provide answers to these problems.
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