Elastic Recovery in the Rockwell Hardness Testing by the Steel Ball Indentor

In a theoretical consideration of the Rockwell hardness, the value of elastic recovery of indentation depth which occurs on removal of major load, becomes an important problem. The effects of shape of anvil, and elastic deformation of a frame and an indentor of a testing machine were examined. The true value of elastic recovery of indentation depth could be derived experimentally from the apparent readings of elastic recovery of the testing machine. Experimental results of elastic recovery of indentation depth agreed with the theoretical value calculated by the Hertz's equation. Therefore it may be considered that the elastic recovery of indentation in the case of Rockwell hardness testing is an identical phenomenon of elastic contact expressed by the Hertz's equation. The elastic recovery of indentation depth becomes larger when the major loads is larger, the diameter of the steel ball indentor is smaller, the testing specimen is harder and the Young's modulus of the specimen is smaller. The proportion of the elastic recovery to the total depth of indentation under major load becomes larger when the major load is smaller, the diameter of the steel ball indentor is larger, the testing specimen is harder and the Young's modulus of specimen is smaller.