SPRING 2019

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THE DISTRIBUTOR’S LINK
Rob LaPointe AIM TESTING LABORATORY
Rob LaPointe is a noted authority in materials and fastener technology. With extensive experience
in the management and science of materials testing laboratories combined with master’s degrees
in physics and education, he excels at bringing solutions to the client. Working specifically in the
fastener testing industry, he has developed expertise in mechanical, nondestructive, metallurgical
and chemical testing. With a background of 20 years in physics education, Rob is effective at
communicating complex ideas in a simple and understandable manner, communicating well with
clients enabling them to make informed decisions about their products and business.
METALLOGRAPHY FOR FASTENERS
Modern fasteners have significant materials science
behind them and are used to assemble nearly all of our
technology from bridges and infrastructure to aircraft and
deep-space probes. Technical applications of fasteners
demand that they be produced and tested to high quality
standards to ensure they function as they’re intended.
Critical application fasteners undergo many tests to
verify that they have the performance characteristics
needed for the job. Most people with some fastener
industry knowledge are familiar with some of the tests
that are required on today’s products such as tensile
and hardness, but the more critical applications, such as
aerospace, require significantly more poking and prodding
to see if they’ll make the grade.
Product specifications written for critical application
fasteners usually include required metallographic tests
such as macrostructure and microstructure. If the
fastener is made from alloy steel, decarburization testing
is often required. These tests, as well as many others,
are classified as metallographic testing. Metallography is
the science of understanding the structure and properties
of metal as well as the tests that insure this structure
and performance. Metallographic tests generally require
microscopic observation and are often documented
with high resolution photography. Sometimes confused
with metallurgy which is the science of crafting alloys to
achieve desired properties, metallography is the study
of the products that metallurgical scientists produce
to confirm the materials can do what they’re designed
to do. This article will highlight the most-often required
metallographic tests that support the quality of the
products the industry provides.
TECHNICAL ARTICLE
Macrostructure is the most fundamental test for
determining the quality of metal the mill is producing and
for determining the quality of cold or hot forming processes.
Acceptable grain flow ensures that the forming process
didn’t compromise the native strength of the metal.
Macrostructure is typically viewed at low magnification.
Usually between 1 X and 10 X. Macrostructure exams are
routinely done by the metal producer in the mill to insure
the quality of the melt and ingot or the continuously-cast
forming process. These exams reveal issues with the
material such as inclusions (unwanted material mixed in
with the metal) or discontinuities such as voids or laps.
For finished products,
testing standards for highstrength
fasteners often
require a macrostructure
exam to determine that
acceptable grain flow is
maintained during the
FIGURE 1. HOT FORMED BOLT upsetting process of
SHOWING GRAIN FLOW. HCL +
H 2 O AT 140°F FOR 45 MIN head forming. Figures 1
(1 X MAGNIFICATION).
and 2 show the grain flow
distortion created by a hot forming process. Macrostructure
exams for a product, such as a socket-head cap screw,
require a longitudinal (along the long axis) cross-section
showing the areas of interest. Specifically, the head and
thread sections need to be examined to see that the
grain flow in the regions upset by the forming process.
This requires the fastener to be cut, sanded smooth
(sometimes polished) and etched to reveal the grain. A
mixture of 50 % hydrochloric acid (HCl) and water (H ²
O) is
often the etchant of choice.
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