Periodic direct verification of the individual components of a Rockwell hardness machine is an excellent tool for determining what errors exist in the measurement system and for indicating that a problem may be surfacing. Unfortunately, direct verification is rarely done in practice; instead industry primarily relies on indirect verification to assess the measurement capability of Rockwell hardness machines. This is probably due to the difficulty and cost of performing direct verifications, and the fact that ASTM and ISO currently only require a limited direct verification of the hardness machine when the machine is new or installed.
5.1.1 Applied Forces Verification
The forces applied by the Rockwell machine should be verified periodically in accordance with the test method standards and using instrumentation having the appropriate accuracy, uncertainty, and traceability to national standards. The forces should be verified as they are applied during a Rockwell test; however, longer dwell times are recommended to acquire a stable measurement.
Good Practice Recommendation
For hardness machines that have the capability of adjusting the applied force levels, the forces should be adjusted as closely as possible to the center of the tolerances. Some Rockwell hardness machines allow the applied forces to be mechanically adjusted by the operator. Following direct verification and adjustment, further adjustment of the forces by the operator should not be allowed without subsequent direct verification of the adjusted force. Otherwise, the forces can easily be adjusted out of tolerance to offset other hardness machine problems that could have developed.
5.1.2 Depth Measuring System Verification
Direct verification of the depth measurement system should be accomplished as outlined by the test method standards using instrumentation having the appropriate accuracy, uncertainty and traceability to national standards. The verification should be performed in an appropriate manner that will verify the entire working range of the measurement device.
Good Practice Recommendation
• Some Rockwell machines are capable of electronically adjusting (or correcting) the depth measurement system, or the system for displaying the hardness value, based on comparisons with reference blocks. If such adjustments or corrections have been made, they should be reset or removed prior to verifying the indentation depth measuring system. Otherwise, the verification could indicate compliance for a system that is in fact out of tolerance.
5.1.3 Hysteresis Verification
A verification of the Rockwell machine should be made to determine the
magnitude of any hysteresis in the flexure and measurement systems of the
machine as a test is made. The goal of the hysteresis verification is to perform
a purely elastic test that results in no permanent indentation. In this way, the
level of hysteresis can be determined. The recommended method for assessing the level of hysteresis is to perform repeated Rockwell tests with a blunt indenter (or the indenter holder surface) acting directly onto a very hard test piece. The tests should be conducted using the highest test force that is used during normal testing. The hysteresis test is a somewhat difficult test to carry out. The slightest inelastic deformation at the interface of the blunt indenter (or holder) and the hard test piece will act to increase the apparent hysteresis. Every effort should be made to reduce any inelastic deformation.
If there were no hysteresis in a Rockwell machine, the measurements would indicate a hardness number of 130 Rockwell units when Rockwell ball scales B, E, F, G, H, and K are used and a hardness number of 100 Rockwell units when any other Rockwell scale is used. Currently, assessing the level of hysteresis in the testing machine is not required by ASTM standards. The ISO standards specify a test to evaluate the testing machine hysteresis allowing a hysteresis value of 0.5 Rockwell units for machines that have a clamping fixture for locking the test sample against the upper part of the machine frame and 1.5 Rockwell units for machines without a locking mechanism. Allowing a hysteresis level of 1.5 Rockwell units is excessive. Hysteresis should be limited to less than 0.5 Rockwell units for all machines.
Good Practice Recommendations
• When performing the hysteresis verification tests, it is important to choose an appropriate type of material that the blunt indenter or indenter holder will act against. The material should have the lowest ductility possible; yet have sufficient strength to support the test force. Metal carbides, hard ceramics, and thick glass have been used successfully for this purpose. Some hard metal test blocks when used for this purpose have exhibited a small amount of plasticity, which adds to the level of hysteresis, and, thus, are not recommended. Also, the test should be repeated in the same location many times before the first measurement is taken. Be aware that this test does not account for a possible hysteresis effect that could occur as a result of a problem at the interface of the indenter and the indenter holder, or any hysteresis due to the Rockwell indenter itself.
• For Rockwell machines that are capable of electronically adjusting (or correcting) the depth measurement system, any adjustments or corrections should be reset or removed prior to verifying the machine hysteresis. Otherwise, corrections can increase or decrease the indicated level of machine hysteresis providing an inaccurate estimate of the true hysteresis level.