The conversion issue becomes even
more problematic for materials that
are not covered by the standard conversion tables. Many people use ASTM
E140 Tables 1 and 2 for hardness conversions for materials that are not covered in any of the tables in E140. For
example, assume a specification (such
as one of the NACE sour service standards) calls for a particular maximum
Rockwell C hardness for a duplex
stainless steel (such as 28 Rockwell C),
and the hardness for the part is reported in Brinell (e.g., 286 Brinell). The
existing ASTM E140 Table 1 for non-austenitic steels would indicate a conversion of 286 Brinell = 30 Rockwell
C, which would cause rejection of the
material. However, some private test-
ing indicates that 286 Brinell actually
converts to less than 28 HRC in at
least one duplex stainless-steel material. Unfortunately, verified and standardized tables of conversion values for
duplex stainless steels do not exist.
This results in false rejection of materials, leading to increased costs and
equipment delivery delays.
In summary, hardness conversion is
a very complex subject. Conversion of
readings from one scale to another or
one method to another should be performed only when absolutely necessary,
and with great care and consideration.
Furthermore, hardness requirements
for materials should be specified using
methods and scales that are most
appropriate for the material (e.g.,
1. ASTM E140-07 Standard Hardness Conversion Tables for Metals Relationship Among Brinell
Hardness, Vickers Hardness, Rockwell Hardness,
Superficial Hardness, Knoop Hardness, and Scle-roscope Hardness, ASTM International, West
Brinell for large castings instead of
Rockwell B or C). This approach eliminates the need for conversion and the
issues that can result. VM
DON BUSH is a principal materials engineer at
Emerson Process Management-Fisher Valve
Division ( www.emersonprocess.com). Reach him
at Don.Bush@Emerson.com. The author wishes
to acknowledge the assistance of Thomas
Spence, director of materials engineering of
Flowserve Corporation ( www.flowserve.com).
COMING IN THE
FALL 2008 ISSUE OF
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• Controlling Quality in a Global Environment
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• Beyond Valves: Flowmeters
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