Rockwell Standard Revealed
February 22, 2008
The long awaited ASTM E18-07 Rockwell Hardness Standard has been released. While it is technically similar to the older version, some changes affect both users and service providers.
Here is a brief summary of the major changes:
IndentersAll indenters require certifications and serial numbers. In the case of diamond indenters, there are now three classes, each class having different tolerances.
In the past, to comply with E18, diamonds were required to have their shape verified on an optical comparator, and then undergo performance verification on test blocks. E18-07 now requires that each indenter have its geometrical attributes (angle and radius) physically measured to ensure compliance. This eliminates the go/no-go measurements allowed by the old standard.
What this means is that unless a manufacturer has a certification for a given diamond that states that it was physically measured and is compliant with the tolerances for geometry and performance outlined in E18-07, the machine cannot be certified to ASTM E18-07.
Indenter balls and indenter holders also will require independent certification, with indenter holders demonstrating that the indenter holder was performance verified, and that there is at least 0.3 millimeter protrusion of the ball from the ball cap. The indenter balls are certified for diameter, material and hardness. The new standard also ties an indenter to a machine. This means that an indenter must be verified, using an abbreviated indirect verification, and tied by serial number to a given machine, before it can be used to measure parts. The indenter must be verified on two test blocks for each scale that it is intended to be used on- for example, HRA, HRC or HR15N. A certificate, similar to the ones generated when a machine is certified, must be generated and kept. Users are allowed to perform this verification themselves.
Direct VerificationsDirect verifications are a means of inspecting individual critical attributes of a hardness tester. In a Rockwell hardness tester that includes measuring the force at preload, total force and force at elastic recovery (the reading position). The displacement measuring device must be isolated and verified. Additionally, the timing and machine hysteresis must be verified.
Direct verifications have always been done by testing machine manufacturers when machines are manufactured. They also were performed when machines are rebuilt, and as a diagnostic tool when the force, or measuring systems are suspect. The new standard now states a direct verification (less the timing) must be performed “When adjustments, modifications or repairs are made that could affect the application of test forces, the depth measuring system or the machine hysteresis.”
To comply with E18-07, if adjustments are made that affect any of these key elements, service technicians can no longer just rely on performance of the machine on test blocks. A direct verification must be done to ensure that all of these elements are in tolerance, and are not simply correct by “canceling errors.”
In other words, there may be cases where an instrument reads the test blocks within tolerance, but fails a direct verification. Under the new revision, these machines cannot be deemed “certifiable” unless the machine can be adjusted/repaired to pass both a direct and an indirect verification.