We continue with our discussion on the heat treatment of fasteners by offering additional tips for the heat treater.
Tip #9: Sample
Checking the results of the heat-treatment operation in a production environment
is challenging. Proper preparation of sample mounts is critical in that one
often finds the placement of multiple fasteners in a single mount necessary.
Tips for Proper Sample Preparation
let the volume of samples or speed at which they need to be generated
catch you unprepared. Do not sacrifice quality for expediency.
grind fasteners prior to placement into the mount. Extreme care must be taken
to avoid burning (tempering back) the surfaces. Fasteners are often held by pliers
or other gripping devices, so be sure to apply even, steady pressure on a wet
grinder. Excessive sparking or discoloration of the surfaces is a clear
indication that surfaces have gotten too hot.
with 2-3% nital prior to mounting to check for grinding burns and to look for
uniformity of case (if case hardening).
not overload the mount. Mounts with too many fasteners tend to bulge and/or
have rounded surfaces, making grinding/polishing difficult and producing
scratched or poorly prepared surfaces. Some fasteners in crowded mounts
are invariably too close to the edge of the mount to be adequately prepared or
considered representative parts.
both proper grinding/polishing materials and the proper sequence/number of
steps so as to eliminate scratches from all surfaces. Do not compromise quality
test using the proper load. Too often, near-surface or shallow case depths are
measured using 500-gf loads when it is inappropriate to do so. (Remember the
“diving board” effect – as you approach the edge with too heavy a load,
deflection occurs and the reading is erroneous.)
lightly first to reveal pattern irregularities and to estimate total case
at the microstructure as well as the case depth both at 100x and higher (400-1,000x)
after proper etching.
results on permanent forms or in computer databases.
sample mounts for the appropriate length of time (typically 1-5 years).
Fastener Heat-Treating Tips (part 8)
Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance). He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center. tel: 630-834-3017; e-mail: email@example.com; web: www.heat-treat-doctor.com
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