52031

Has hot dip galvanizing weakened our mounting brackets?

April 30, 2009

Sir
We have a component manufactured for the Wind Mill Tower, which is a mounting bracket (10 mm Thick)used to mount the Platforms at the various levels of the tower. This bracket is having material grade S355 equivalent Indian Standard IS-2062.It is a mild carbon steel.This bracket is having a 90 degree bend and gets clamped at both ends at a rt angle. This material is Hot Dip Galvanized. We are finding cracks at the assly stage from the bending line (Bending is having a radius of R16. We have done a testing at our end by loading the component as done at the ass'y stage, the pieces without galvanizing did not fail, but the pieces after galvanizing tested after 3 days of galvanizing failed from the radius. Is the galvanization process playing the key roll for the failure??

Regards
Benny Michael

Benny Michael
Fabrication - Hyderabad, Andhra Pradesh, INDIA

Is the actual steel failing or just appears to be cracked because the galvanize cracked.
If the part went into the zinc vertically, I can not see a good reason for it to crack.
Dr. Cook will probably have technical reasons in a day or so.

James Watts
- FL

Hi Benny,

I infer that you are doing the bend cold (less than around 600 degrees C), otherwise your problem wouldn't occur. The hot dip galvanizing is causing your problem, because the bend radius is too tight for a cold bend.

Onesteel Australia have published an FAQs document dealing with concrete reinforcing bar, including the galvanizing of cold-bent bar. Here is the relevant FAQ cut from the document (which does not carry a copyright note):

Quote
Q. Does galvanizing have any detrimental effects on bent bars?
A. Bent rebars that are subsequently galvanized can be affected in a number of ways. Heating of the steel to 450 degrees C will induce some degree of strain ageing into the cold worked section of the steel. Immersion in a pickle bath to clean the steel may induce some hydrogen embrittlement in the steel. Immersion of steel in a molten metal zinc bath can cause liquid metal embrittlement due to absorption of zinc into the steel grain boundaries.

For these reasons bars should not be bent around pins of less than 5 db for bars up to 16 mm and 8 db for bars greater than 16 mm. It is preferable to bend bars after galvanizing in order to overcome any potential for small surface folds, that may be formed in the bar surface during
bending around small pins, to become a surface cracking problem in bent and galvanized rebars. We do not recommend that pre-bent and galvanized bars be rebent on site.
End of quote

If we apply the Onesteel radius limit for bar, to your plate, you would have a limit of R25. Maybe the limit should be higher than that because of the different geometry of plate vs bar. Either way your R15 is too tight.

Assuming that you need the R15, and assuming that you don't need the additional strength at the bend brought about by the cold work, you could do the bend then temper at around 650 degrees C then go to hot dip galvanizing. The temper would remove the effects of the cold work.

Or you could heat the strip to around 900 degrees C with a torch and bend it hot then let it cool naturally, which would give you no cold-work effects at all since you would not be doing any cold work.

Or you could hot dip galvanize the strip first, then bend it cold. Obviously you would need to run some test pieces first to check how tight the bend could be without the zinc being damaged. A matter of absolutely major importance is the silicon content of the steel, because certain silicon contents in the steel can lead to brittle zinc coatings which would crack on a tight bend (although their corrosion resistance is normal). This post is already quite long enough without going into the silicon question in detail, but we can deal with it if you wish.

Bill Reynolds    consultant metallurgist Ballarat, Victoria, Australia

Sir:

I take a different point of view about your problem. Since the ungalvanized bracket does not break, I believe that is proof enough that the galvanizing caused the problem. If you post publically, or e-mail me privately the composition of the galvanizing bath in which these brackets were galvanized, I would be happy to comment.

Regards,

Dr. Thomas H. Cook, Galvanizing Consultant Hot Springs, South Dakota, USA

That's what I said in my first paragraph: "The hot dip galvanizing is causing your problem...". Dr Cook, what are you disagreeing with?

If an amount of cold-work exceeding some threshold level is done prior to galvanizing, then the subsequent galvanizing will cause a problem in that cold-worked material.

Possible solutions are (1) reduce the amount of cold work if the application permits (that is, use a larger bend radius), or (2) bend hot so that no cold-working is done - subsequent galvanizing will then not damage the bend zone.

Or (3) galvanize first then bend, but be aware that the silicon content of the steel is important, to avoid a brittle zinc layer that might crack when bent.

Bill Reynolds    consultant metallurgist Ballarat, Victoria, Australia

Bill,

I found your technology on bending quite interesting and this may well be exactly what is causing the problem.

I was thinking that the galvanizer of these brackets may be using more than 0.3% tin (Sn) in his zinc which has caused many, many cracking problems in Europe and North America. Unfortunately I could not get information about the zinc in the kettle to verify my thinking.

Galvanizers have found that tin in their zinc reduces zinc usage, perhaps by lowing the freezing point of the zinc allowing better zinc drainage back into the kettle. Unfortunately this tin content, above about 0.3% causes cracks in stressed steel and MANY failures have been reported including:

1)Hanging wires breaking in galvanizing plants in the zinc.
2)Overhead highway signs cracking.
3)Stressed steel bridges failing.
4)Pipe pilings spliting sometimes like a banana being peeled.
5)A stadium roof structure cracking and needing braces.
6)Rectangular welded structures cracking and breaking.
7)Scaffolding cracking and failing.

One company in a patent recommended 0.5% tin and since has lowered this recommendation to 0.2%. Another company recommended above 1% tin and is contained in their patent. The second company has been successfully sued.

Tin causing the cracking has been carefully studied and PROVEN to be the problem. In any case of failure the failed galvanized product can be tested tin can be proven to be the cause.

About eight years ago I became aware of the problem and sent two letters to ASTM recommending a limit on the use of tin in hot dip galvanizing. My understanding is that no action was taken by ASTM because "Cook lacked sufficient proof." I have both dated letters and I have the full scientific paper which proves that tin is the problem.

Regards,

Dr. Thomas H. Cook
Dr. Thomas H. Cook, Galvanizing Consultant
Hot Springs, South Dakota, USA

Dr Cook, thanks for your extra information.

I agree entirely with you that tin in the zinc would be damaging. Steel and molten tin make a very unhappy partnership. Even when the tin is in dilute solution in another molten metal, intergranular penetration by the tin can effectively form microcracks in the steel surface which can then easily propagate when lightly stressed. (The successful use of tinned steel sheet for cans is another story - highly polished steel, very soft steel, different steel microstructure, low service stress, etc etc - and tinplate was made by hot-dip tinning for very many years before electrolytic plating)

But surely the use of tin in the kettle would be unusual? It's not a constituent of zinc blocks from the zinc refinery, so would have to be a deliberate addition. Was it just for the purposes represented by the patents you mentioned?

The information that I quoted from Onesteel in fact deals primarily with the temperature cycle imposed on the steel by the galvanizing process, rather than details of the zinc itself. Similar degradation of the steel properties would occur if a furnace was used to impose the same temperature cycle as occurs in the kettle, on the same amount of work-hardening, in the complete absence of zinc or tin or any other metal.

So there are evidently two distinct paths, each of which leads to premature service failure - excessive cold work followed by galvanizing which imposes a damaging temperature cycle on the bulk steel, or excessive tin in the zinc which causes degradation of the steel surface.

I sympathise with your attempts to get something new into an ASTM spec - it's bad enough trying to get an editorial correction made of an obvious error!

Bill Reynolds    consultant metallurgist Ballarat, Victoria, Australia

The use of tin added to the zinc melt started to be an increasingly common practice in some European countries 10 or so years ago. It has all but stopped I think, perhaps as a result of the problems mentioned above.
The purpose was to change the appearance of the coating, the thickness and the consumption of zinc as a result of that.

Geoff Crowley    galvanizing &    powder coating shop Glasgow, Scotland