Hardness Testing

Author:  Dr. David Kirk | Coventry University
Source:  The Shot Peener magazine, Vol 38, Issue 1, Winter 2024
Doc ID:  2024006
Year of Publication:  2024
Abstract:  
INTRODUCTION Fortuitously, for shot peeners, their hardness testers don’t measure hardness! Classically, hardness is defined as the resistance of a material to abrasion. Tests such as that of the Mohr’s Scale, arranged substances according to their ability to scratch any material below it. Hence diamond heads the scale with a value of 10 whilst talc, with a value of 1, is at the foot of the scale. Methods have later been developed that have much greater precision and accuracy. These, however, are based on the size of indent produced using a known force to an indenter. The applied force divided by the surface area of the indentation gives the so-called hardness value. The Brinell test, devised in 1900, involves pressing a hardened steel ball into the test piece’s surface. Brinell hardness is then given by Applied Force/Surface area of impression. The Vickers Hardness Test uses a diamond in the form of a square-based pyramid. This does not deform to the same extent as does a steel ball. For a given applied force, Vickers hardness value increases as the diagonals of the indentation decrease. Ludvik invented the first differential depth hardness tester in 1908. The Rockwell differential depth hardness tester, devised in the USA in 1914, was aimed at rapid routine testing of samples. This is because the Rockwell value is displayed directly on a scale, without the need for operator intervention. Different combinations of indenter and applied force became available. All of the methods rely on resistance to indentation—which is at the heart of shot peening control. This article concentrates on the applications of the Rockwell test. A central problem arises when different companies test nominally identical samples such as batches of Almen strips. Proper comparison can only be achieved if the test method employed is precisely identical: ASTM E-18 (USA) and ISO 6508 (International) are appropriate standards.