Technology

Fundamentals ofHardness Testing ofFasteners
Although it is likely that few individuals outside of the fastener industry possess an appreciation forfasteners, the reality is that fasteners, even ones we consider “simple” are sophisticated engineered devices.
Since fasteners are so crucial in holding everything together, it is important that suppliers and users can verify
that the fastener will perform the way that it is intended. To this end, fasteners that have been heat treated
to meet desired performance requirements must be examined to verify that they meet the standards and will
fulfill these specific requirements. One of the more important verification methods is hardness testing.
This article will explore the fundamentals of hardness testing of fasteners. In addition to gaining insight
into what hardness is and why it is a useful test, we will explore the different methods that may be employed toconduct hardness testing.

What is Hardness?
Hardness is a measure of a material’s resistance to plastic deformation when subjected to indentation. It is also commonly
considered as the ability to resist scratching, abrasion, cutting, or deformation when a load is applied. The harder a material is thebetter it can resist these sorts of damage. Hardness is often a function of multiple variables and other material properties such as
ductility, stiffness, strength, toughness, and viscoelasticity. In simpler terms, however, we generally recognize materials as being hardor soft, and when it comes to fasteners, we often associate the degree of hardness with the presence or absence of manufacturing processes
such as annealing, quench and tempering, or case hardening. We recognize, for example, that high strength, quench and tempered parts are
harder than lower strength, annealed parts.

Why do we measure hardness?
With many materials, and specifically steels, there is a good correlation between hardness and tensile strength. Strength, as a general
concept, is defined as the material’s capacity to handle applied stress. Tensile strength is the maximum amount of tensile stress a material
can withstand before it breaks. Although hardness testing does not as accurately correlate to yield strength and ductility, hardness testing
conducted properly and found within specification becomes a good predictor that the other material properties will also be in specification.
When verifying that a fastener can meet its minimum tensile strength requirements, it is best to conduct a tensile test. However,with parts that are too small to be properly fixtured in a tensile test frame, a hardness test becomes the only reliable predictor of
the fastener’s tensile strength ability. Thus, hardness testing on fasteners is quite common, as so many fasteners are simply not able to betensile tested. Additionally, hardness testing is fast and efficient, so that it can be successfully deployed as an in-process monitoring step. In
other words, fastener manufacturers and their heat-treating vendors may employ hardness testing during the process to ensure that parts arestaying within specification.

Hardness Testing Types
Hardness testing is conducted by pushing a uniquely shaped indenter with a very well-defined load into the part. These test methods
fall into two broad categories: macro hardness testing and microhardness testing. Macro hardness tests are conducted with large
indenters and loads and can usually be seen by the naked eye. On the other hand the use of light loads and small indenters results in274

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an indentation invisible to the naked eye. These are referred toas micro hardness tests.
For general purpose testing, macro hardness testing is what
is usually employed. There are multiple different test methods
including Rockwell B and C, Macro Vickers, and Brinell that
commonly get employed on fasteners. In these tests, the size ofthe indenter and the load applied are large and leave a mark that isvisible to the eye.
When delicate or very thin sections need testing, micro
hardness testing is employed. This includes hardness tests on
the outer case sections of surface hardened fasteners. Often thesecase-hardened sections are very thin, and the loads exerted by amacro hardness test would be too severe and the indenter would
pierce through the area of interest resulting in errant or false
measurements. Micro hardness is also often deployed duringexperimental research where multiple groupings of test sites withinsmall areas are desired. Micro hardness testers use light loads, andthe indentations must be analyzed under magnification.

Hardness Testing Methods
In addition to these two distinct categories of hardness testing,there are multiple different methods. Each method usually has
its own unique scale associated with the obtained test results.
Although there are conversion tables between these different
methods, it is not always exact and anytime that one method must
be converted to a different method, care should be taken.

Rockwell Hardness:
In North America, the mostcommon hardness testing method
for fasteners is the Rockwell test.
Although Rockwell scales run
from A to G, steel fasteners areevaluated exclusively with either
the Rockwell B scale or Rockwell
C scale. The Rockwell B Scaleuses a 1/16” diameter round balland 100 kgf load. The Rockwell CScale uses a 120° Diamond Coneand 150kgf load. Obviously theheavier the load the more capable
it will be to indent harder parts and thus, obtain an accurate
hardness measurement. For this reason, the Rockwell B scale may
be used on annealed and un-heat-treated parts while the Rockwell
C scale is used on heat treated parts.
In the Rockwell test a small preload referred to as the “minorload” is applied to the indenter. The minor load is about 10kgf, asmall percentage of the total applied load and effectively seats thetip of the indenter into the part. The remaining part of the load, the“major load”, is then applied. Rockwell measures depth and takes
just the distance between the depths of the minor and major load
and applies an algorithm to convert the depth reading into a value
along the appropriate Rockwell scale.
Rockwell also has an N Scale. This is the Superficial Hardness
Scale, meaning that it is specifically for measuring case hardened
parts. There are three N variations: 15N, 30N, and 45N. Each uses
a diamond cone indenter and a 3kgf minor load. Thus, the majorload for 15N is 12kgf, for 30N is 27kgf, and for 45N is 42kgf.

Vickers Hardness Method:
Outside of North America, the Vickers method tends to befavored for hardness testing of fasteners. This method is considered
to be the “easiest” of the testing methods because the calculations
are independent of the size of the indenter. In fact, the same
pyramidal diamond indenter can be used for all materials regardlessof the hardness of the part. The Vickers test results in a Vickers
Pyramid Number (HV). This test is most often performed withlight loads as a micro hardness test, but larger loads are possible forthe macro test. Loads can range between 1gf and 100kgf.
W hereas the Rockwell test determines the depth of theindentation, the Vickers test measures the length of the diagonals
obtained by the indentation. These are measured and applied to an
equation to determine the HV value.

Knoop Hardness Method:
The Knoop hardness method is a microhardness test utilizing
a pyramidal diamond indenter and 100gf load. With its indenter
geometry, the Knoop test is perhaps the best method for very
brittle or thin layer testing. Like the Vickers test the indentation ismeasured, but unlike Vickers which uses both diagonals, the Knoop
calculation uses only the long side of the indentation.

Brinnell Test Method:
Although the least
common method forfasteners, the Brinnell
method is sometimes
still employed. The
Brinnell indenter isa very large 10mm
diameter ball and30 0 0kgf load. T heresulting indentationis measured andapplied to an equation to generate the resulting Brinnell Number.

Fastener Testing
Hardness testing is most commonly employed as an in-process
method during the heat treating of parts or as a final inspection toolat either the heat treating, fastener manufacturing, or end user. The
processes that trigger a hardness check include heat treatments suchas a strengthening quench and temper, case hardening, or annealing
process. Quench and tempered parts will normally receive a
Rockwell C or macro-Vickers check. Case hardened fasteners
require a microhardness method, such as Knoop or micro-Vickers.
Annealed fasteners usually receive a Rockwell B test.

In Summary
Hardness testing is an important and everyday test that all
fastener suppliers employ. Regardless of the chosen method,they are all governed by consensus standard organization
specifications, in particular ASTM and ISO methods. Although
the test methods are common and straight forward, they must
be conducted closely with these standard methods to assure theaccuracy and quality of the test.

Article by Laurence Claus / Copyright owned by Fastener World

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