>From a metallurgist, heat treating will help a lot with the strength.
Fatigue load is about 50% of the ultimate. However, this is on a perfectly
polished bar. Any sort of oxidation or other flaws, such as a nick will
reduce the life by a lot. Easily determined from any strength of materials
or fatigue book. Galvanizing will also reduce the fatigue strength
considerably. You need to look at the loading of the mast to determine
appropriate size, loading, etc. Fatigue is generally not an issue.
A single overload is not generally damaging to fatigue life - on the
contrary, it is often beneficial and it introduces some plasticity.
Regarding hydrogen embrittlement, it usually comes from corrosion. Failure
mechanism is from static loading or very low strain rates. A good plater or
galvanizer would do a simple hydrogen embrittlement relief - 350F for 24
hours to preclude this from occurring from the galvanizing or plating
operation. However, should the coating be damaged, then the zinc acts as an
anode, and hydrogen is formed. If the steel has a hardness of greater than
36 hrc, then it will be prone to hydrogen embrittlement.
Most steels - especially 4130 are "killed" meaning that they have a bit of
alumina added during the steel making process.
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Jim
Lux
Sent: Friday, December 28, 2012 1:41 PM
To: SPWoo
Cc: towertalk@contesting.com
Subject: Re: [TowerTalk] galvaniizing heat treated steel
On 12/28/12 10:28 AM, SPWoo wrote:
> Good questions Jim. I called the tube supplier and they told me the
> tube is stress relieved but not quenched as in traditional heat
> treatment. The 'SR' designation in the description denotes stress
> relieve. I did a google on 4130 stress relieve and it appears it's
> done at about 1200F. Hot dipped galvanizing is done at 830F. I did
> some research on galvanization of high strength steel and it appears
> that the process is highly dependent on the chemical composition of the
steel.
> Steels with yield strength higher than 150ksi encounters
> 'hydrogenembrittlement'. To get an answer on this from an expert I
> sent an e-mail to a person at the American Galvanizers Associastion at
> the recommendation of my galvanizing supplier. My galvanizing
> supplier uses the same process for all steels with yield strength less
than 150ksi.
> For steels more than 150ksi they use a different process.
>
Very interesting.. so it's very much a matter of making sure that everyone
involved knows what's going on. That is, you wouldn't want to take that
tubing down to Bob's Corner Galvanizing and tell them "throw it in the tank
with the rest of the stuff"
> You are correct if some of this is overkill. In my case I do get a
> lot of wind here and I'd rather be safe than sorry. Another local ham
> lost an antenna with the same mast that the 4130 replaced. That mast
> is the M15R sold by US Towers. The material for that mast is low
> carbon structural steel with a yield strength of only 36kpsi. For me
> it's better safe than sorry. 73.
Exactly... for a few hundred dollars on a $10,000 installation, it might not
be worth agonizing..
And, going to 120ksi from 30ksi would double the "wind speed rating" of the
system. I'm not sure, in fact, that you could even get the same failure
load on a solid 2" bar of 30ksi as you could with 1/4" walls of
120ksi: the middle just doesn't carry that much of the load.
In fact, I just ran a simple calculation.. a solid 2" bar is only about bit
more than twice the strength of a 2" OD tube with 1/8" walls of the same
material. For 1/4" walls, you only pick up 60% by going to solid.
There's more to be gained by going bigger in diameter and thinner walls.
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