Larry list account writes:
>
> There has been a lot of talk about the various methods of welding; MIG, TIG
> and OA. During those discussions I picked up on an issue I've heard others
> talk about; welding chrome moly tubing and normalizing it after welding.
> Can someone enlighten me on this? Here is what I think I've heard. After
> welding chrome moly tubing the tubing needs to be normalized. The
> normalization process consists of heating and cooling the chrome moly .
>
> Here are my questions.
Apart from what else has been said, be sure to check the rules governing
materials for cages. Some rules do not allow 4130. It is stronger, but also
somewhat more brittle. If there's sufficient loading to break the cage, 4130
tends to snap into pieces rather than deform, which mild steel will do. It's
just a function of the elongation of that particular material, which is
quite small. For most steels, the ability of the material to deform under
load decreases as the tensile strength increases.
As for normalizing big objects, the general rule of thumb is to normalize
the tubing one foot beyond the weld area in all directions. It's not
necessary to normalize the entire cage--it's only necessary to do the
portion of the tubing affected by welding heat. Doing it a foot in all
directions from the weld area provides some insurance that there won't be
any portion with stressed areas from welding heat.
As for normalizing itself, I wonder if the term is being used improperly
here--normalizing is a stress-relieving operation, but usually as a
pre-treatment to hardening. It requires a higher temperature than annealing,
and requires fairly careful temperature control when cooling from
normalizing temperature. Normalizing normally precedes hardening (in the
case of 4130, at a temperature about 50-100 deg. higher than the hardening
process itself--1650-1700 deg F). If you achieve normalization temperature
and then air-cool without controls on the rate of cooling, you'll end up
with irregular brittle spots.
I believe you are actually talking about annealing, which for 4130 should
done at 1550 F for a time long enough to allow thorough heating of the
section size and area, then the area is cooled at a rate of less than 50 F
per hour down to 900 F, followed by air cooling from 900 F. For large
objects too big for furnace control of the cooling, I think there are
ceramic blankets available for that.
If you have any question about achieving the correct temperature for
annealing, ask your welding supply for Tempilstik. It's a high temperature
indicator in a variety of ranges and can be applied as a stick-on dot, or as
a crayon--changes color when the temperature is achieved.
All advantages and disadvantages taken together, I'd stick to making a cage
in 1020.
Cheers.
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