WINTER 2016

114 THE DISTRIBUTOR’S LINK
CARMEN VERTULLO TO BAKE OR NOT TO BAKE? from page 26
Taking it from the top, let’s answer some of those
questions.
Why Do We Bake?
Applying heat to any material causes greater mobility of
its constituent atoms and molecules, especially those that
are not bonded to others, such as atomic hydrogen. When
heat is applied the hydrogen becomes highly mobile in the
metal matrix and eventually finds its way to the surface or
some other area in the matrix where it becomes trapped.
The trapped hydrogen is no longer available to become a
cause of HE. The hydrogen that reaches the surface will
escape to the atmosphere. Basically, baking removes the
hydrogen that is produced in the plating process and
renders the otherwise HE susceptible fasteners safe to use.
At What Temperature?
When you think of HE relief baking temperatures think
of baking a cake or bread. It is not a very high temperature
compared to the heat treating process. The higher the
temperature the faster the HE will get out, but just like we
don’t want to burn the bread, we don’t want to damage the
material or the coating of the fastener. Baking for HE relief
is not a heat treatment and does not in any way change the
mechanical properties of the material in terms of strength
or hardness. We must not heat the material above the
tempering temperature of its heat treatment and we must
not risk degrading the plating metal, with a reasonable
safety margin. As an example zinc melts at 419°C and
cadmium melts at 321°C. With that in mind, specification
established baking temperatures vary but are generally 190-
220°C (375-430°F) with cadmium on the lower side due to
its lower service temperature. Most platers use 375--400°F.
What Fasteners Require Baking?
This is the question that brings the most concern and
confusion. When I first came into the industry about 25
years ago it was SAE J429 Grade 8, ASTM Alloy Steel
Socket Screws, ISO 898-1 PC (property class) 10 .9 and
12.9 bolts and screws, tapping screws with a high hardness
and anything hard and springy like roll pins and lock
washers. This was based on the belief that steel with a core
hardness of about HRC 36 and above was where HE
susceptibility becomes an issue. We now know that HRC 39
and below products have very low susceptibility to HE so
grade 8 and PC 10.9 that are in accordance with their
specifications do not require baking. We also know that core
hardness is the controlling factor and fasteners with high
surface hardness but controlled core hardness are less
susceptible. This is reflected in current plating
specifications such as ASTM F1941/F1941M.
In terms of where the most risk is, without question it
is with ASTM A574 and ISO 898-1 PC 12.9 Socket Head
Cap Screws. These are the products where we see the
highest incidence of HE. Other socket screw configurations
in this strength range such as button heads and flat heads
are equally susceptible and certainly should be baked.
However, we see fewer HE failures with these screws
because they are most often not used in high stress
applications. The same is true of some tapping screws
which may be incapable of achieving the stress necessary
to activate the HE process before stripping. A baking
decision based on the application if the application is well
understood may also be part of an effective and
economical HE risk management strategy.
It is important to know what the hardness of the
actual fastener material is. Just because the specification
says the product is supposed to be HRC 39 maximum
does not mean that the particular lot of fasteners you are
plating, or every fastener in that lot is HRC 39 maximum.
What Fasteners Do Not Require Baking?
Take the list of all fasteners and subtract those that
require baking and you have the list of those that do not,
except for those that do. This means that there are some
fasteners that do not require baking normally, but because
of an obsolete specification or an uninformed customer,
baking is specified. Some OEM’s and some plating
specifications may require baking non-susceptible
products and even those with much lower hardness. The
supplier should not deviate from these requirements
without first consulting the customer. There are also
customers and suppliers who simply do not want to risk
any chance of HE so they bake everything that gets plated,
or they bake Grade 8 and PC 10.9 and above. Strangely,
these same customers are unaware of the proper way to
manage HE risk and they think just throwing money at the
problem will make it go away. They apply a half-measure to
all fasteners and fail to apply a full measure when it really
matters.
Which Ones, If Any, Are Borderline?
There are a few fasteners where the answer to the HE
risk question is “it depends”, and so a decision has to be
made. An example would be Grade 8 bolts where the
supplier does not have confidence that the product is fully
within the specification limitation of HRC 39 maximum for
Grade 8 (you might ask why are they selling this product in
which they do not have confidence? – that’s the topic of a
future article). Another is self-drilling screws, which
depending on the material and the coating may or may not
present a risk of HE.
CONTINUED ON PAGE 160