[TowerTalk] Welding on Rohn tower

[Mark .]

<< ...I beg to differ! People have built aircraft for decades using much 
thinner tubing thicknesses than Rohn 25 or 45 have. Naturally, thin-walled 
tubing is a bit more challenging to weld, but the tubing sides of 25 and 45 
are plenty thick by comparison...Bill Coleman, AA4LR...>>

Hello all,
I changed the subject on this offshoot of tower repair thread for future 
searches.

I think welding on galvanized metal releases some nasty, toxic, fumes, so 
more precautions must be taken. Although it certainly is possible to weld to 
thin sections, the following must be taken into account:

1) The heat from welding will be hot enough to change or completely destroy 
the heat treatment of the metal. It may even take it as far as fully 
annealing (essentially softening) the local metal adjacent to the weld. 
Annealing will reduce a heat-treated metal's yield stress, while at the same 
time, making it more ductile. That means if will fail with less load, but 
tend to bend more and absorb energy before breaking.

2) Sometimes, it is possible to preserve some amount of heat treatment by 
quenching the welded area with ice water immediately after welding, but the 
results are quite variable. This is similar to the original hardening of the 
metal. You can then scrape a bright patch on it (if it is steel) and 
carefully re-heat the area *gently* with a torch and read the color change 
on the metal to temper it. When a metal is fully hardened, it has great 
yield strength bu is more brittle/less ductile. Tempering to various 
temperatures (also called 'drawing') after hardening simply strikes a 
balance betwwen the two conditions of hard/brittle and soft/ductile.

3) I don't know the extent of the heat tretment of the steel in Rohn towers. 
We already know is that it is certainly ductile enough to absorb energy and 
bend to some degree, thankfully for our friend with the damaged tower. 
Welding on the section could possibly leave the welded area softer and more 
ductile than the original condition if it is heat-treated.

4) I would not recomment welding on a section that is highly stressed unless 
it was supported or unless the welding were carried out in carefully 
controlled portions to prevent the entire section from becoming red hot and 
soft at one time.

If anyone knows the actual heat treatment of Rohn's steel, please jump in 
and help steer this thread. Perhaps it's on one of their drawings.

I used this effect recently to bend some aluminum rod in tight, 90 degree 
bends for an antenna mount for my bicycle (good shop tip coming up).

How many of you have tried to bend aluminum, especially 6061-T6 (hardened) 
only to have it crack? I heated the area I wanted to bend with a torch, 
destroying the teat treatment in the bend area, and making the metal more 
ductile. I made the bends (crack free), then reheated them and quenched them 
with ice water.

So go forth and weld carefully!

--...MARK_N1LO...--


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