Hardness Testing - How Is It Done?
The determination of the
Rockwell hardness of a substance consists of first applying a small load
succeeded by applying a large load, and after that observing the level of
penetration, that is to say finding the hardness number as indicated on a dial.
A harder substance shows a larger number. The primary benefit of Rockwell
hardness is definitely its capability to exhibit hardness valuations right away,
effectively eliminating the mind-numbing calculations required with various
other hardness measurement methods.
The application is usually
In order to obtain a
dependable measurement, the particular thickness of the actual test-piece needs
to be a minimum of 10 times the thickness of the resulting indentation.  Furthermore, readings need to be obtained via a
flat perpendicular area of the test material, due to the fact that rounded
surfaces provide lower results. A modification factor needs to be employed when
the particular hardness has to be calculated for a rounded surface.
Rockwell Hardness Test
The Rockwell hardness test
is the most generally employed commercial hardness test. Its almost universal
acceptance is attributable to its speed, virtually error free results, its
capability to differentiate smaller hardness variations in hardened steel, as
well as the small area of the actual indentation, which means that the finished
heat-treated components can almost always be tested without causing any
This specific test engages
the depth of indentation, beneath continuous tension, as a gauge of hardness. A
small stress of 10 kg is initially used to seat the component. This lowers the
degree of work surface preparation required and also decreases the particular
inclination for ridging or perhaps sinking in by the indenter. The primary load
is then utilized, and the degree of indentation is effortlessly registered on a
regular dial gage that reports the arbitrary hardness result.
The dial consists of a
hundred sections, each section symbolizing a penetration of 0.00008 in (0.002
mm). The dial is normally reversed which means that a high hardness, which
compares to to a little penetration, returns a high hardness number. This is
actually in agreement with the various other hardness numbers defined
previously, however in contrast to the Brinell and Vickers hardness
classifications (that have units of MPa), the Rockwell hardness numbers are
Large loads of 60, 100, and
150 kg are utilized. Considering the fact that the Rockwell hardness is reliant
on the load in addition to the indenter, it is definitely essential to identify
the combination that is applied. This is accomplished simply by prefixing the
hardness number using a letter revealing the specific blend of load and indenter
pertaining to the hardness scale employed. A Rockwell hardness number absent the
letter prefix is in fact worthless.
Hardened steel is usually
subjected to testing on the C scale using the diamond indenter and a 150-kg
major load. The practical spectrum for this scale is from about RC 20 to RC 70.
Less rigid components are generally tested on the B scale having a
1/16-in-diameter steel ball and a 100-kg major load. The viable range connected
with this scale is from RB 0 to RB 100. The A scale (diamond penetrator, 60-kg
major load) provides the most extended Rockwell hardness scale that is
appropriate for elements from annealed brass to cemented carbides.
Numerous other scales are offered intended for special applications.
The Rockwell hardness test
is an incredibly practical and reproducible one as long as a handful of
easy to understand guidelines are followed. The majority of the factors
listed below pertain every bit as well to the various other hardness tests:
The indenter and anvil need to be washed and
The surface area to be analyzed should
preferably be clean and dry, smooth, as well as totally free from oxide. A
rough-ground work surface is generally satisfactory for the Rockwell test.
The work surface ought to be flat and
additionally vertical with respect to the indenter.
Tests using round surfaces will definitely
produce low readings, with errors based upon the curvature, load, indenter,
and hardness of the substance. Theoretical and scientific modifications with
regard to this particular effect currently have been written and published.
The thickness of the tested item ought to be
such that an impression or bulge is never visible on the opposite side of
the piece. It is advisable that the thickness be at the very least 10 times
the depth of the indentation. The space between indentations need to be 3 to
5 times the diameter of the indentation.
The speed of the application of the load needs
to be consistent. This is accomplished by modifying the dashpot on the
Rockwell tester. Differences in hardness can easily become significant in
extremely soft substances unless the pace of applying the load is carefully
PMPA’s Designer’s Guide: Heat
http://www.pmpa.org/technology/design/heattreatment.htm, retrieved 2009-06-19 .
(Click Values and Standards
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