WINTER 2016

160 THE DISTRIBUTOR’S LINK
CARMEN VERTULLO TO BAKE OR NOT TO BAKE? from page 114
Nuts are generally immune to HE as there are no
commercial nuts with specified core hardness at the
susceptible level, however I have seen HE failure of zinc
plated nuts which have been improperly heat treated. While
nuts are not technically tensile loaded fasteners, there are
very high radial tensile loads throughout the nut cross
section. Some nuts, such as clinch nuts, have no specified
hardness but need to be hard in order to function and the
specification (e.g. NASM 45938) requires baking if hardness
is above HRC 39.
Socket set screws are certainly at the hardness level
where they are HE susceptible if plated, but they are strictly
compression fasteners and should not experience the type
of loading that leads to HE. Once again though, I have seen
HE failures in plated socket set screws.
ASTM F436 hardened flat washers have a specification
hardness of HRC 38-45, and even though they are always in
compression, in some applications (uneven surfaces or
across a slot) they may be put into bending stress which is
severe loading, so unless there is good knowledge and
control of the application which does not include bending
stress, these washers should be baked. It would be
impractical to try to control the hardness below HRC 40, but
depending on the application a hard washer that is not so
hard, say HRC 36 may be suitable for a specific application
and that could be chosen instead of the F436 hardened flat
washer.
What Processes Require Baking?
It will be the product that determines if baking is
necessary before the process comes into play. Once we have
susceptible product (HRC 39 and above) then we ask if the
process generates hydrogen and if it does, does the
hydrogen enter the product and if it does enter the product
can it get out on its own or is it constrained in the material
by the coating? Assuming we have a susceptible part, at the
top of this list are electroplated coatings such as zinc and
cadmium because they generate hydrogen, it gets in the
steel and it cannot get back out through these coatings.
Electroplated zinc is a particularly impermeable coating.
Coatings such as zinc-nickel are more permeable and while
HE susceptible fasteners with this coating may still be baked,
they are far less likely to hold the hydrogen in.
What Processes Do Not Require Baking?
Much effort and research has been spent on developing
new coatings for high strength fasteners that do not induce
HE, and in determining which standard coatings and
processes do not cause HE. Any process that does not
produce hydrogen or prevents it from entering the steel is a
safe bet. Unfortunately the literature and the specifications
are not clear on which is which. At the top of the list of non-
HE inducing coatings are mechanical zinc, dip-spin coatings,
black oxide, phosphate coatings, and electroless nickel -
which does not generate hydrogen. Even if the coating or
cleaning process exposes the fastener to hydrogen and it
gets into the steel it will diffuse out of the product on its own
in a short time without the need for baking because the
coatings (except for electroless nickel) are very permeable to
hydrogen. That is why some processors require that
fasteners with these processes be held for 24 hours after
coating before shipping or use. HE risk can be further
reduced with the use of plating and cleaning processes that
generate less hydrogen. A combination of strategies can
provide an overall process that proves to be non-HE inducing
thus precluding the need for baking.
For How Long Do We Need To Bake?
Right next to “bake or not to bake” this is the money
question. The answer is - however long it takes to get enough
of the hydrogen out of the product to eliminate the HE risk.
We have seen requirements of from 2 hours to 48 hours,
usually based on the hardness of the product. The
permeability of the coating is also an important factor –
possibly the most important factor in the baking time, and
relates directly to the coating thickness. Most specifications
only use material hardness or tensile strength as a baking
time criteria and do not consider coating thickness, but their
requirements will usually accommodate the thicker coatings.
Thinner coatings may allow for a shorter baking time but
must be qualified by testing (more on that later). Current
electroplating specifications ASTM 1941/F1941M-15
require a minimum of 14 hours for products with a specified
hardness of over HRC 39 and up to HRC 44, and 24 hours
minimum for fasteners with a specified hardness over HRC
44. In the absence of specification guidance the
requirements for baking must be agreed upon by the
customer and the supplier, or the supplier is responsible for
determining baking times.
The size and configuration of the fastener may also
affect baking times, but there is no verifiable research at this
time to indicate which way to go. Small fasteners with very
thin coatings (e.g. smaller than .25” or 6 mm, .0001” thick
coatings) may be effectively baked at lower times if verified
by testing. Very low baking times such as 4 hours are not
effective and are a waste of time and money. It is not difficult
with some thought to develop a comprehensive baking time
strategy based on the product and the coating process.
CONTINUED ON PAGE 173