David Pye's series on hardness testing continues with part 10.

Comparative Hardness Conversions

Although the practice of hardness conversion comparisons are made by comparing one table value to another table value, it is not a good practice to adopt. This is stated simply because each method of hardness testing measures in a different manner, Brinell measures by displacement, and Rockwell measures by penetration. There is a relationship, but it is not accurate to compare one value to another value.

Some engineering drawings call for both a Brinell value or its equivalent in Rockwell. It is not uncommon for product specifications to define the hardness for a case depth in the Rockwell C scale, which is a bulk test scale unsuitable for case-depth determination. This is illogical, but it is widely practiced, probably because designers are not familiar with the Vickers or Knoop scales.

Hardness conversions are developed empirically, and there is a degree of error associated with all conversions. The primary source for hardness conversions is ASTM E 140, which lists the conversions in table form and also contains equations based on the tabular data. Refer to ASTM E 140.

Simple Shop-Floor File Test

The file test is an extremely simple test to establish if (for example) the austenitize and quench procedure was successful. The test is based on the hardness of the file in relation to the as-quenched hardness of the hardened component.

It is NOT a quantifiable test or even an accepted test by ASTM or by any aerospace test specification. A simple 150-mm fine-cut file is generally hardened and tempered to approximately HRC 63. If it can file the surface of the austenitized-and-quenched component, the hardness is expected to be below HRC 63, but it is not known how much the component is below HRC 63. The test simply identifies if the heat-treat procedure has worked, and it is accomplished by “feel.”