How To Stop Thread Galling On Stainless Fasteners
By fastener expert Joe Greenslade:
A few times each year we receive calls from fastener suppliers who are in
conflict with their customer over the quality of stainless steel bolts and
nuts. The customer's complaint is that during installation the bolts are
twisting off and/or the bolt's threads are seizing to the nut's thread. The
frustration of the supplier is that all required inspections of the fasteners
indicate they are acceptable, but the fact remains that they are not working.
This problem is called "thread galling." According to the Industrial Fastener
Institute's 6th Edition Standards Book (page B-28),
Thread galling seems to be the most prevalent with fasteners made of stainless
steel, aluminum, titanium, and other alloys which self-generate an oxide
surface film for corrosion protection. During fastener tightening, as pressure
builds between the contacting and sliding thread surfaces, protective oxides
are broken, possibly wiped off, and interface metal high points shear or
lock together. This cumulative clogging-shearing-locking action causes increasing
adhesion. In the extreme, galling leads to seizing - the actual freezing
together of the threads. If tightening is continued, the fastener can be
twisted off or its threads ripped out.
Carpenter Technologies, the fastener industry's largest supplier of stainless
steel raw material, refers to this type of galling in their technical guide
as "cold welding." Anyone who has seen a bolt and nut with this problem
understands the graphic nature of this description.
The IFI and Carpenter Technologies give three suggestions for dealing with
the problem of thread galling in the use of stainless steel fasteners:
1. Slowing down the installation RPM speed will frequently reduce, or sometimes
solve completely, the problem. As the installation RPM increases, the heat
generated during tightening increases. As the heat increases, so does the
tendency for the occurrence of thread galling.
2. Lubricating the internal and/or external threads frequently eliminates
thread galling. The suggested lubricants should contain substantial amounts
of molybdenum disulfide (moly), graphite, mica, or talc. Some proprietary,
extreme pressure waxes may also be effective. You must be aware of the end
use of the fasteners before settling on a lubricant. Stainless steel is
frequently used in food related applications, which may make some lubricants
unacceptable. Lubricants can be applied at the point of assembly or pre-applied
as a batch process similar to plating. Several chemical companies offer
anti-galling lubricants. One such source, EM Corporation, suggests their
Permaslik¨ RAC product for use at the point of assembly. They suggest
Everlube¨ 620C for batch, pre-applying to stainless steel fasteners.
3. Using different stainless alloy grades for the bolt and the nut reduces
galling. The key here is the mating of materials having different hardnesses.
If one of the components is 316 and the other is 304 they're less likely
to gall than if they're both of the same alloy grade. This is because different
alloys work-harden at different rates.
Another factor affecting thread galling in stainless steel fastener applications
is thread roughness. The rougher the thread flanks, the greater the likelihood
galling will occur. In an application where the bolt is galling with the
internal thread, the bolt is usually presumed to be at fault, because it
is the breaking component. Generally, it is the internal thread that is causing
the problem instead of the bolt. This is because most bolt threads are smoother
than most nut threads. Bolt threads are generally rolled, therefore, their
thread flanks are relatively smooth. Internal threads are always cut, producing
rougher thread flanks than those of the bolts they are mating with. The reason
galling problems are inconsistent is probably due largely to the inconsistencies
in the tapping operation. Rougher than normal internal threads may be the
result of the use of dull taps or the tapping may have been done at an
inappropriately high RPM.
Fortunately, stainless steel bolt and nut galling problems do not occur everyday,
but when they do it usually creates a customer crisis. Knowledge of why this
occurs and how to remedy it can save the supplier much grief and many headaches.
Below are the questions that should be asked and the suggestions that should
be made immediately when you are confronted with a customer's complaint about
1. Are you using the same driver RPM you have used in the past to install
these stainless fasteners?
If they say they are driving them faster than in the past or if they say
this is a new application, suggest they immediately try slowing the driver
RPM and see if the problem goes away. In general, a stainless bolt of a given
size should be driven slower than a steel bolt of the same size.
2. Are the bolts and/or internal threads lubricated?
If they say, "no", suggest they try lubricating the bolts and/or internal
threads with one of the lubricants listed above. If this eliminates the galling,
you might want to batch lubricate the remainder of the order to eliminate
the extra work of applying lubricant at the point of assembly.
In applications where galling is a repetitive problem, it is advisable to
supply the fasteners with pre-applied lubrication to eliminate future problems
3. Are you using the same grade of stainless steel for the bolts and nuts?
If the answer is, "yes", you can suggest changing one or the other to a different
Be sure the suggested grade meets their corrosion needs and changing the
material does not cause a procurement problem.
When thread galling occurs in stainless steel bolt and nut applications,
don't panic. Try the suggestions listed above. One, or a combination of these,
will probably resolve the problem immediately.