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What is Cold Welding? |
Cold welding also known as Thread Galling is a phenomenon that can occur during the tightening - mainly at high speed - of threaded connections.
It is caused by a combination of pressure and friction (high friction value in stainless steel). You notice it when, while tightening a bolt, the threads get stuck in a nut or tapped hole. And, once the bolt is stuck you usually can't remove it the normal way. Then it becomes grinding, drilling or tapping.
How does cold welding occur?
During the tightening of fasteners, the pressure between the contact surfaces increases. This creates friction and damages the thread surface of both the external and internal threads.
These damaged surfaces then adhere to each other. The threaded connection seizes and, in extreme cases, the bolt or screw even breaks off.
Soft materials are often more sensitive than harder materials. Cold welding usually occurs with fasteners made of stainless steel, aluminum and titanium.
How to prevent cold welding?
EXPERT'S EXPERIENCES
AND RECOMMENDATIONS ON PREVENTING GALLING OF STAINLESS STEEL FASTENERS
Surface defects in the thread
These are usually caused when rolling threads on surfaces that have already been work hardened.
They are characterized by a cracked, jagged, and flaky surface on the thread flanks. These surface
defects increase the surface area and thus the area exposed to corrosive influences.
Incorrect tool selection
Under certain circumstances, the tools used may not be suitable for processing the material. For
example, when cutting a thread with an unsuitable tool, the thread may not be cut cleanly and
completely, which can lead to cold welding during assembly. The same applies to the forming of
threads. Here, too, the aforementioned situation can occur due to incorrect or defective tools.
Hardness difference
One of the most common causes of Galling is when the hardness difference between the screw and
nut threads is too small. This difference is essential to prevent the connecting elements from
permanently bonding (galling) due to physical-chemical reactions. A hardness difference of at
least 50 HV should be aimed for in order to minimize the risk. This is usually ensured with
cold-formed fasteners of the same strength class, as the screw thread undergoes significant work
hardening in the thread flanks during the forming process. To reduce the risk when using different
materials, the delivery conditions (mechanical properties) of the starting materials should be
compared before production.
Additional safety can be achieved by using appropriate lubricating pastes, which are applied to
the screw thread before assembly. This measure is often used in so-called hot screw connections
to prevent the fasteners from welding together at high operating temperatures.
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