The Rockwell Hardness Test

THE ROCKWELL HARDNESS TEST The Rockwell hardness number represents the additional depth to which a test ball or sphero-conical penetrator is driven by a heavy (major) load beyond the depth of a previously applied light (minor) load. High hardness numbers that are obtained from hard materials indicate a shallow indentation while low numbers found with soft materials indicate deep indentation. The increment of penetration depth for each point of hardness on the Rockwell scale is 0.00008 inch. For example, if a piece of steel measures Rockwell C 58 (extremely hard) at same point and C 55 at another, the depth of penetration would have been 0.00024 inch deeper at the softer spot. Hardness is not an absolute measurement and hardness can only be compared when the penetrators, loads, and types of materials tested are the same. Several scales are available to the metallurgist. A major load of 150 kg and sphero-conical brace penetrator is used for the C scale for example. This scale is most convenient in testing heat treated and hardened steels. For the C scale, hardness numbers are indicated on the black figured scale of the dial of the analog hardness tester. Brass, bronze, unhardened steel and cast metal usually are tested on the B scale using a 100 kg load and a 1/16" diameter ball. The results are then read from the red scale of the analog hardness tester. Scale Type of penetrator Standard Scales B C Major Load (Kg) 1/16” Ball Brale diamond 100 150 Brale diamond Brale diamond 1/8” Ball 1/16” Ball 1/16” Ball 1/8” Ball 1/8” Ball 60 100 100 60 150 60 150 Special Scales A D E F G H K ** Calibrations are made using a hardness standard with known hardness number. When results from more than one hardness tester are to be compared, such calibration blocks must be used.