[TowerTalk] galvaniizing heat treated steel

[Grant Saviers]

On 12/28/2012 7:15 AM, Jim Lux wrote:
> On 12/27/12 1:49 PM, SPWoo wrote:
>> Hi Jorge,
>>
>> It's easier to stick to a 2" OD.  I went with 2" OD but with a 3/8" 
>> wall 4130 chromoly.  The cost difference between 1/4" and 3/8" is not 
>> significant.  I want to make sure the mast outlasts me!  Attached 
>> below are the details of my mast and the cost.  I had to pay another 
>> vender to galvanize it for me for an additional $150.  So a total 
>> cost of $650.  Not cheap but when you look a the big picture it's 
>> cheap insurance.  Using the windload calculator this mast is good for 
>> 135mph for my two yagis.  We do get 120mph winds here once in a 
>> decade.  GL and 73.
>>
>> Description: 4130 CDS TUBING SR, ASTM A 519
>> 2.000 OD X .375 W S/C 15'0" ( 180.0")
>> Heat: 001M59442            Item: 100549
>>
>> ----------------------------------------------------------------
>>                    MECHANICAL PROPERTIES
>> ----------------------------------------------------------------
>>                  YLD STR     ULT TEN    %ELONG   %RED   HARDNESS
>> DESCRIPTION       PSI        PSI      IN 2 IN  IN AREA     BHN
>>                    104628.0   122695.0  18.7                 101
>> ----------------------------------------------------------------
>> EDDY CURRENT: OK
>>
>> ------------------------------------------------------------
>
> With that yield (and hardness number!), it's definitely been heat 
> treated..
I see some suppliers offer HRC45 heat treating of 4130 which takes the 
yield strength to over 200,000 psi - that is about 6x low carbon!
>
> Whether the hot-dip galvanizing changes the yield is hard to say. I've 
> been looking through my (very old) textbooks and steel company 
> application notes and most seem to say that the temperature of the 
> molten zinc is far enough below the temperature of the original heat 
> treat that it's not a big issue.
> I did find one reference that says that hot dip galvanizing reduces 
> the fatigue resistance (i.e. the steel fails earlier from fatigue 
> cracking).
Interesting, I was aware that electroplated zinc might cause hydrogen 
embrittlement which accelerates fatigue.  Normalizing for 4130 is done 
at 1600d F and molten Zn is probably 860d F, so the reduction in 
hardness/strength is probably minor for the amount of time in the bath.
>
> In the ham antenna situation, I have no idea if fatigue is an issue. 
> Wind loads are repetitive, but generally quite small, so they may not 
> get up to where fatigue is an issue.
I think the rule of thumb is that fatigue life is unlimited if the 
cyclic stress doesn't exceed 25% or so of the tensile stress.  At 50% 
stress, life is about 10,000 cycles from a reference I found, which 
could happen in a mast.  One time high stress substantially reduces the 
fatigue life.  So designing with a big safety factor is a good idea.
>
> One would need to consult someone who actually knows this stuff to 
> know for sure.
Agree, I'm not that person, so YMMV.
>
> Yes, a lot of folks invest in strong steel for their masts, and it 
> works just fine, but from an engineering standpoint, one wonders if 
> it's worth the extra expense.  Putting up cold rolled 1020 at half the 
> ultimate yield might have survived just as well.  Or, alternately, 
> spending a few hundred bucks to not worry about it might be worth it.
Previous posts have emphasized the benefit of increased diameter for a 
mast.  Generally, going thicker than 0.25" wall is not a good investment 
and has excess weight.  A 2.5" od mast with 0.25" wall has more than 2x 
resistance to bending and increase in strength with a 29% weight 
increase vs 2.0"od and .25" wall .  A 2.0" od by .375" wall only 
increases strength by 27% with 50% more weight.

Also, heat treating and 4130 costs money.  Normalized 4130 has a yield 
strength of 63,000psi and is tough to machine, yet is available heat 
treated to more than 3x that, so if cost is no object or weight/diameter 
is a constrained, then heat treating 4130 is a good choice.  OTOH, A513 
steel has a yield of 72,000psi and is about 40% the cost of normalized 
4130, so for me it is my choice for high strength DOM (drawn over 
mandrel) tube.
>
> It's not like people instrument their masts to measure the actual loads.
I think the mast calculators are pretty decent estimates of loading.  
There are also numerous web applets for calculating stresses and 
deflections in beams with almost any load configuration that a mast will 
have.  It is instructive to see how much a mast can bend without damage.

Grant KZ1W
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