On Sun, 04 Jul 2004 18:06:21 +1200, Eric Stevens wrote:
How do you explain the well known welding at ambient temperatures of
precision slip-gauges made of hardened steel? Leave them in contact
overnight and you will be lucky to get them apart in the morning.

No. While true cold welding can occur, that's not the mechanism(s)
responsible for wringing gage blocks together.

Frankly, the exact details are still in dispute. Part of it is atmospheric
pressure differential between the outside and the area where air has
been forced out from between the blocks. (up to 14 PSI) Part of it is
often due to the stickiness of oil on the blocks. (roughly 2 or 3 PSI)

But neither mechanism is strong enough to account for the amount of
force typically needed to separate the blocks. (typically on the order of
100 PSI)

Most experts believe that Van der Waals forces (the same forces that
give water surface tension, or make solder adhere) are responsible for
the bulk of the effect. Others now point to the Casmir force (a quantum
effect). Lively disputes still continue.

A true weld is as strong as the parent materials. (up to 200,000 PSI for
tool steel gage blocks) When you break a true weld, parts of the parent
materials are ripped out. That doesn't happen when separating wrung
gage blocks. So that's not an example of actual welding.

To do an actual weld, the atoms of one piece of material have to be
brought as close to the atoms of the other piece of material as the
atoms of one of the pieces are to each other. At room temperature
this requires a lot of force, on the order of the yield strength of the
material.

This is a few thousand PSI for relatively low yield materials like copper,
or more than 100,000 PSI for materials like tool steel. Of course, as
you increase the temperature, the yield strength of the material declines,
and less force is needed. When a material melts, the yield strength goes
to virtually zero, so little or no force is required to achieve a weld.

Gary