Welding prep, cutting notches and stress risers

ORVAL FAIRAIRN wrote:
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

Basically nothing but here's a couple of hints. I am not an expert but
the first thing to buy is the Monster Garage's book, "How to Weld Damn
Near Anything". It is by Richard Finch and includes alot of material on
aircraft welding and tube fitting. It is a very good reference easy to
read and lots of good clear pics. The other hint is that you can use a
Sharpy brand marker to mark tubing. A Sharpy is the only thing Pratt
will allow to be used on an jet engine case.It is also mentioned in the
book for use on tubes.

Frank M.Hitlaw
Jakarta,Indonesia

4130 is not notch sensitive like Plexiglas or aluminum. The occasion light
scratch will not cause any problem.


"Ernest Christley" wrote in message
...
mhorowit wrote:
What about all the scratches made by clamping it in the vice and using
an abrasive to clean before welding? arn't those sources of 'stress
risers'? - Mike


You don't need to clean back any further than you will be welding.
'Scratches' should be melted away. Use a slightly worn out abrasive pad
(sand paper on foam), or 120 or greater grit paper. It will give you a
mirror finish. A 'scratch' has to be a significant depth into the parent
material to be of consequence.

'Significant' in this case is a weasel word without definite meaning. The
precise depth is different for various materials. Consult 43.13B for
specific details.

--
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