Rockwell Hardness Tester
The Rockwell
hardness test method, as defined in ASTM E-18, is the most commonly used hardness
test method. You should obtain a copy of this standard, read and understand the
standard completely before attempting a Rockwell test. The Rockwell test is
generally easier to perform, and more accurate than other types of hardness
testing methods. The Rockwell test method is used on all metals, except in
condition where the test metal structure or surface conditions would introduce
too much variations; where the indentations would be too large for the
application; or where the sample size or sample shape prohibits its use.
The
Rockwell method measures the permanent depth of indentation produced by a
force/load on an indenter. First, a preliminary test force (commonly referred
to as preload or minor load) is applied to a sample using a diamond indenter.
This load represents the zero or reference position that breaks through the
surface to reduce the effects of surface finish. After the Preliminary
test load, an
additional load, call the major load, is applied to reach the total required
test load. This force is held for a predetermined amount of time (dwell time)
to allow for elastic recovery. This major load is then released and the final
position is measured against the position derived from the Preliminary
test load, the
indentation depth variance between the Preliminary
test load value and major load value. This
distance is converted to a hardness number.
Hardness is a characteristic of a material, not
a fundamental physical property. It is defined as the resistance to
indentation, and it is determined by measuring the permanent depth of the
indentation. More simply put, when using a fixed force (load) and a given indented, the smaller the indentation, the harder the material. Indentation
hardness value is obtained by measuring the depth or the area of the
indentation using one of over 12 different test methods. Click here to learn
more about hardness
testing basics.
A
variety of indenters may be used: conical diamond with a round tip for harder
metals to ball indenters ranges with a diameter ranging from 1/16” to ½” for
softer materials.
When selecting a Rockwell scale, a general guide is to select the scale that specifies the largest load and the smallest indenter possible without exceeding defined operation conditions and accounting for conditions that may influence the test result. These conditions include test specimens that are below the minimum thickness for the depth of indentation; a test impression that falls too close to the edge of the specimen or another impression; or testing on cylindrical specimens. Additionally, the test axis should be within 2-degress of perpendicular to ensure precise loading; there should be no deflection of the test sample or tester during the loading application from conditions such as dirt under the test specimen or on the elevating screw. It is important to keep the surface finish clean and decarburization from heat treatment should be removed.
When selecting a Rockwell scale, a general guide is to select the scale that specifies the largest load and the smallest indenter possible without exceeding defined operation conditions and accounting for conditions that may influence the test result. These conditions include test specimens that are below the minimum thickness for the depth of indentation; a test impression that falls too close to the edge of the specimen or another impression; or testing on cylindrical specimens. Additionally, the test axis should be within 2-degress of perpendicular to ensure precise loading; there should be no deflection of the test sample or tester during the loading application from conditions such as dirt under the test specimen or on the elevating screw. It is important to keep the surface finish clean and decarburization from heat treatment should be removed.
