On Sun, 04 Jul 2004 05:07:36 -0400, Gary Coffman
inadvertantly omitted:

On Sun, 04 Jul 2004 02:36:09 GMT, Seppo Renfors
wrote:

---- snip ----

"It is the sudden impact pressure that causes the
molecules to move rapidly, that causes FRICTION, which in turn causes
heat and if sufficient sudden pressure is applied (eg hammer blow to
already hot metal) it CAN melt the material. To "weld" something by
definition requires bringing part of it to a liquid state - ie melted
in the portion being welded.

.... when he quoted my response to it.

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.

You are discussing the underlying welding mechanism. The point is
that, in those circumstances, welding occurs without either heat or
significant pressure, irrespective of whether it is due to Van der
Waals forces, the Casimer force, atmospheric pressure or whatever. I
do know that if such gauges are left in contact for sufficiently long
it is virtually impossible to separate them.

A true weld is as strong as the parent materials. (up to 200,000 PSI for
tool steel gage blocks).

That is very rarely the case.

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.

It depends upon how long you leave them together.

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.

So?

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.

But we (Seppo and I) were discussing welds at ambient temperature. See
http://www.twi.co.uk/j32k/protected/.../ksedn002.html



Eric Stevens