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New revision to ASTM E-18
for Rockwell Scale Hardness Testing
How will it affect you?
Hardness Standards?
For years, users of
Rockwell Scale Hardness Testers have been attempting to achieve correlation
between customer and vendor, machine to machine, and operator to operator.
In many cases, this simple expectation has been an unrealistic goal. The
major source of this dilemma has been the lack of a good, consistent hardness
standard.
Subsequently when
Heat Treater and customer disagree on their hardness results on a given
part, each one refers to his trusty test blocks. Each satisfied that his
machine reads his testblocks correctly, the dance begins. Who's right?
Most likely both! Herein lies the problem.
In North America,
hardness standards have been maintained over the years by the test block
manufacturers, without any real standardization of methods, indenters,
or underlying fundamentals. All claim to standardize blocks in accordance
with the loosely written section of ASTM - E18 that deals with test block
manufacturing. The result of this "free for all" has been shifting standards
and a lack of correlation between test block manufacturers.
In many countries
in Europe and Asia, nationally traceable hardness standards have been
around for years. Why not in N. America? Well, for many years, users assumed
that the commercial "keepers of the values" were doing that. As this assumption
began to be dispelled by the problems discussed above, a movement came
about to get the then National Bureau of Standards (now NIST) involved.
This was no small feat, as NBS had never been involved in Hardness Standardization,
and didn't even have a Standardizing Machine.
NIST
to the Rescue
With pressure mounting
from industry to standardize Rockwell values, and the active involvement
of NIST's Dr. John Smith, NIST took action in the early 90's, and purchased
a Rockwell Hardness Standardizing Machine. This precise instrument utilizes
dead weights for load application and a laser interferometer for the measurement
of depth. All critical aspects of the test can be controlled, such as
indenter contact speed, rates of loading and unloading. Machines such
as this are used by the Standardizing agencies of other countries, and
typically cost in excess of $200K. Because of lack of funding the newly
purchased machine sat idle for a number of years before "swinging into
action".
In the interim, representatives
from NIST became active in the ASTM E 28.06 Indentation Hardness Testing
Task Groups (E-28.06.07 Traceable Standards deals with the new E18 specification
and traceability). It was then determined that once funding became available,
the HRC scale would be the first logical scale to address.
Commercially available
test blocks were first evaluated to ascertain the magnitude of the problem.
NIST found more than a 1.0 HRC difference between domestic manufacturers
of test blocks. A shift of almost 1.0 HRC also was realized versus standards
from other countries. This truly reinforced the need for standardization.
After analyzing the
various parameters of the test including loads, rate of loading, time
at load, elastic recovery, and indenters, NIST then established a testing
procedure for calibrating the SRM (Standard Reference Materials), or what
industry has called NIST Traceable Test Blocks. By now, Sam Low of NIST
assumed responsibility for Hardness at NIST, and has since been a tireless
supporter of Hardness Standardization.
NIST then standardized
blank test blocks purchased from commercial vendors. Utilizing this procedure
with it's extraordinarily tight tolerances (especially on the diamond
indenter) produced results that were more closely aligned with the results
of foreign National Laboratories than with the values that were being
used in North America. The most dramatic change in values comes at the
upper end of the HRC Scale (HRC 59-63) values shifted upward by 0.5-0.8
points HRC, while from HRC 46-58 the shift is from 0.2-0.49. Below HRC
46, the shift is insignificant.
These HRC SRM's (HRC63,
HRC45, HRC25) were made available to the public through NIST'S SRM Group,
at a price of $638.00 ea. Secondary traceable standards are available
from commercial test block manufacturers at prices ranging from $45.00-$75.00.
It is important to
note that currently there are no specifications that require the use of
NIST traceable test blocks.
ASTM-18
NIST's involvement
in ASTM E 18 brought a new level of interest in standardization of the
entire test method, and not just Hardness values. (As a clarification,
it is important to note that NIST is represented at the ASTM level in
the same manner other as representatives of other companies. While their
opinions are highly valued, they are of the same weight as those of commercial
members.)
What started our as
a simple change to add NIST traceability to the process through the use
of these SRMs turned out to be a virtual overhaul of the specification.
Towards the end of global conformity of hardness test methods, attention
has also given to comply with established ISO specifications.
The new E18 is in
the final stages of draft, and will be balloted before the end of 1999.
Some of the significant changes being proposed are as follows:
1. Time at load
New dwell times
have been proposed for preload, full load, and elastic recovery.
Current Revision
E18-98
Dwell time at preload: None
Dwell time at total force: < 3.0 seconds
Dwell time at elastic recovery: None
Proposed Changes
Dwell time at preload: 2.5 seconds +/- 2.0 seconds
Dwell time at total force: 4.0 seconds +/- 2.0 seconds
Dwell time at elastic recovery: 2.5 seconds +/- 2.0 seconds
2. Verifications
Indirect Verifications:
Indirect verifications must be performed utilizing the Calibration Agency's
Class A indenter (an indenter that has a tighter tolerance than the
standard Class B indenter), to insure the consistency of the instrument,
and then repeat the procedure with the customers Class B indenter. Additionally,
users can only utilize indenters that have been verified on their machine
by their calibration service.
Direct Verifications:
Currently the draft specification requires a Direct Verification of
all instruments at an interval not to exceed 36 months. As in the past,
direct verifications must be performed on all new or rebuilt machines,
or where there is suspicion that the load or measuring device is inaccurate.
This new requirement is strictly time based regardless of performance
on an indirect verification. Machines that pass an indirect verification
but fail a direct will be deemed non-certifiable. An indirect verification
must always follow a direct verification.
Gain vs Pain
No one can dispute
the need for Standardization of Hardness Values. We've all cursed the
current situation, and although long overdue it's good to know that relief
is on the way. Additionally, as the world becomes a smaller place, global
trading dictates the need for International compliance through organizations
such as ISO. However there are some real costs to these benefits:
1. Machines will
need to be recalibrated, and testblocks and indenters must be purchased
to reflect the new standard. Millions of dollars will be spent on this
effort over a very short period of time.
2. With the shift
in the HRC scale, hardness specifications will have to be changed to
reflect the new values. This will be a time consuming and costly process.
Millions of dollars will be spent over a long period of time.
A. As stated previously,
the HRC scale is the only scale that has been addressed thus far However,
many users utilize conversions to other hardness scales, such as HRA,
HRN, HV and HK scales. Presently no accommodations have been made
for accurate conversions from or to the "new " HRC values.
B. In machines
that are not calibratable by individual scales, a separate indenter
will be required for the HRC scale in order to properly read the "new"
values.
3. During implementation
of the NIST values there will be a "dual" standard. Vendors and customers
must align themselves with which blocks they are using, either the new
NIST values or the old values. Without this communication, the standardization
concept adds a variable, not a standard.
4. Test cycle times
will increase with addition of dwell times at preload, total force (current
revision of E18 stated "less than 3 seconds", new version states 4 second
+/- 2 seconds) and at elastic recovery (no time stated in current revision,
2.5 second +/- 2 seconds in proposed revision). Productivity will fall.
Costs will vary depending upon test cycle, burden, and volume of testing.
This will increase the test cycle time by 30-100%!!!
5. Users will be
forced into expensive direct verifications regardless of their machines
performance on NIST traceable standards. Proponents of this requirement
contend that canceling errors can produce correct readings in most materials,
including test blocks, but in some exotic materials these errors can
manifest themselves into inaccurate Rockwell readings. (While theoretically
possible no specific examples of such scenarios could be given by these
proponents)
6. Indirect verifications
will become more expensive as a result of the "double verification "
process utilizing the Class A and the Class B indenters. The need to
have spare indenters verified will add cost and time to the process.
Additionally, reading the test blocks properly may not be enough if
in a direct verification cycle. Many machines will pass an indirect
verification, but fail a direct verification. This will necessitate
repairs, or the machine will have to be taken out of service.
Summary
As a manufacturer
of hardness testing instruments, we welcome the standardization of hardness
values. This will certainly help to eliminate a lot of the controversies
that arise on a daily basis across North America.
We also welcome any
changes to E 18 that can prove themselves viable in bettering the test
method. However, changes that add cost to the test, but little in quality
are detractors to the effort. Some commercial heat treaters have estimated
that their annual costs for hardness testing will double as a result of
some of the proposed changes!
As manufacturers,
we can build instruments that perform in accordance with any reasonable
specification, including the one being proposed. Quite frankly, there
is nominal effect to any of the manufacturers making new machines comply.
Unfortunately, the same cannot be said for the users. If allowed to pass
unchanged there is the potential that many machines may wind up being
taken out of service for failure of direct verifications, even though
they may pass an indirect verification. The long-term effects will be
in the increased calibration costs, indenter inventories, and productivity
losses. Without minimizing the effort and resources required, the issue
of NIST test blocks is a "one time" correction that is calculable.
Currently the ASTM
E18 Task Group is made up primarily of manufacturers of Hardness Tester,
Test Blocks and Diamonds, and as discussed above, the manufacturers could
be the ones to potentially benefit from these changes. Only a few are
free of "personal motivations." Very few actual users attend these important
meetings, yet its is this group that is most effected by these changes.
I strongly encourage
you to join ASTM and become a member of the E28.06.07 task group. Dan
Schulz is the staff manager at ASTM for the E28 Mechanical Testing Committee,
under which E-28.06 falls. ASTM can be reached as follows:
ASTM
100 Barr
Harbor Drive
West Conshohocken PA. 19428-2959
Phone 610-832-9500 Fax: 610-832-9555
Email: service@astm.org
Website: www.astm.org
Help evoke change
for the good, not for the sake of change.
(A version of this article was printed in the October 1999 issue of ASM's Advanced Materials & Processes)
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