Page 222 - Materials Science and Engineering An Introduction

194 • Chapter 6 / Mechanical Properties of Metals

Table 6.6a Rockwell Hardness Scales Table 6.6b Superficial Rockwell Hardness Scales
Scale Symbol Indenter Major Load (kg) Scale Symbol Indenter Major Load (kg)
A Diamond 60 15N Diamond 15
B 16 1 -in. ball 100 30N Diamond 30
C Diamond 150 45N Diamond 45
1
D Diamond 100 15T 16 -in. ball 15
1
E 1 8 -in. ball 100 30T 16 -in. ball 30
1
F 16 1 -in. ball 60 45T 16 -in. ball 45
G 16 1 -in. ball 150 15W 1 8 -in. ball 15
H 1 8 -in. ball 60 30W 1 8 -in. ball 30
K 1 8 -in. ball 150 45W 1 8 -in. ball 45

Semiautomatic techniques for measuring Brinell hardness are available. These
employ optical scanning systems consisting of a digital camera mounted on a flex-
ible probe, which allows positioning of the camera over the indentation. Data from
the camera are transferred to a computer that analyzes the indentation, determines
its size, and then calculates the Brinell hardness number. For this technique,
surface finish requirements are normally more stringent than those for manual
measurements.
Maximum specimen thickness and indentation position (relative to specimen edges)
as well as minimum indentation spacing requirements are the same as for Rockwell
tests. In addition, a well-defined indentation is required; this necessitates a smooth, flat
surface in which the indentation is made.
Knoop and Vickers Microindentation Hardness Tests 17
Two other hardness-testing techniques are the Knoop (pronounced nup) and Vickers
tests (sometimes also called diamond pyramid). For each test, a very small diamond
indenter having pyramidal geometry is forced into the surface of the specimen.
Applied loads are much smaller than for the Rockwell and Brinell tests, ranging
between 1 and 1000 g. The resulting impression is observed under a microscope and
measured; this measurement is then converted into a hardness number (Table 6.5).
Careful specimen surface preparation (grinding and polishing) may be necessary
to ensure a well-defined indentation that may be measured accurately. The Knoop
and Vickers hardness numbers are designated by HK and HV, respectively, 18 and
hardness scales for both techniques are approximately equivalent. The Knoop and
Vickers techniques are referred to as microindentation-testing methods on the basis
of indenter size. Both are well suited for measuring the hardness of small, selected
specimen regions; furthermore, the Knoop technique is used for testing brittle mate-
rials such as ceramics (Section 12.11).
The modern microindentation hardness-testing equipment has been automated by
coupling the indenter apparatus to an image analyzer that incorporates a computer and
software package. The software controls important system functions, including indent
location, indent spacing, computation of hardness values, and plotting of data.

17 ASTM Standard E92, “Standard Test Method for Vickers Hardness of Metallic Materials,” and ASTM Standard
E384, “Standard Test Method for Microindentation Hardness of Materials.”
18 Sometimes KHN and VHN are used to denote Knoop and Vickers hardness numbers, respectively.