In article FRkNe.264882$_o.217775@attbi_s71,
"Cy Galley" wrote:

The warning about using a lead pencil for marking is usually with regard to
aluminum. Lead graphite will make cracks in aluminum.

Soapstone is works for welding as it with stands the heat.


Lead pencils are not lead, but graphite (carbon). If you mark, ay, an
exhaust pipe with a lead pencil, you are introducing a localized
high-carbon area, which the metal will absorb when it gets red-hot. This
will cause localized stress concentrations, leading to cracks. An old
friend related that he was looking at the exhaust pipe on a Lockheed
Constellation and saw a crack shape "OK" on the pipe.

Carbon absorption is the reason that good engine shops banish lead
pencils from their shop.

The problem with aluminum is that the carbon sets up a battery action
with the aluminum, and subsequent corrosion.


Cy Galley - Chair,
AirVenture Emergency Aircraft Repair
A Service Project of Chapter 75
EAA Safety Programs Editor - TC
EAA Sport Pilot
"Michael Horowitz" wrote in message
...
I'm reading about welding. Here's an article about how to cut a
'fishmouth' joint (and I visualize clamping the tube in a vice, in the
meanwhile making small marks in the metal), then the pre-cleaning
(using an abrasive, another thing my A&P told me not to use when
cleaning tubing) then the actually welding.

On the other hand, sometime ago someone mentioned the harm that might
occur if one were to mark on the metal with a pencil.

In one case you're handling the material and putting in a nominal
number of nicks and scratches; on the other hand I'm told not to mark
with a pencil?

What am I missing? - MIke