We recently received a block back from a client with a note expressing concern that it was two Rockwell units softer than stated on its surface. Naturally this is something we take very seriously so a Rockwell technician was given the block to examine. First, he took the block exactly as it was when it arrived back at our Head Office, placed it on our master calibration machine (which had been checked that morning) and, without any of the usual preparatory actions, made an indentation. He was not surprised to find that the block was apparently on the soft side, as the customer claimed, and that the first of five indentations he made was softer than the remainder. He then cleaned the anvil, cleaned the block with a rapid-evaporation solvent (and the contaminants he removed from the block are clearly visible in the photo), cleaned the indenter and repeated the process (after pressing the block down onto the anvil by indenting a softer block resting on it). The results agreed with the block’s stated hardness.
Block cleanliness is critical in Rockwell indenting. Any contamination under the block or on the anvil surface will give a falsely soft reading by ‘giving’ as the indenter is driven into the block, thereby permitting further indenter travel than would occur in the block material alone. Lubricant contamination is obviously especially problematic but dust and oxides are also a threat to accuracy. A block with an underside like Picture 1 will produce a false reading so it is best to thoroughly clean the base with a lint free cloth and solvent. The same applies to the test surface though contamination here gets noticed more often! The anvil should also be cleaned by gentle application of lint-free cloth dampened with solvent and the indenter itself should be gently wiped at intervals throughout the test session. Another place where contaminants can build up (easily producing an error of two Rockwell points) is the mating face where the indenter holder is inserted into the test head of the machine (see Picture 3). It’s obviously also essential that the anvil mount cannot budge under the indenting load. One further check worth making on every block is that it hasn’t been dropped and landed on a corner of the underside. This can sometimes leave a small burr which will prevent the block sitting flush on the anvil and wreck any chance of correct readings.
If the first indentation on a block suggests a lower hardness than the remainder there is a chance that air was trapped underneath. The first indentation usually drives any air out but the hardness reading will be falsely soft as the air has allowed the indenter to travel further than should have been the case. Placing a block that is softer than the test material on the test block and putting one indentation into it before commencing the tests will eliminate this problem (see Picture 6, where an HRBW 50 block is being used to drive out the air from under an HRBW 90).
Test blocks should, ideally, be stored in airtight cases to reduce the rate at which oxides form on their surfaces. Better still, wrap them in rust-reducing paper as well.
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