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.
| Download PDF |
---|