Heat of Hot Dip Galvanizing 2

[CTSeng]

Can the heat of hot dip galvanizing damage or alter typical structural steel, A36 or A992?

What temperature is HDG done at? Is the material quenched after being removed from the HDG bath?

Any info is appreciated

 

[unclesyd]

It will not hurt the steels you mentioned.

It is carried out around 900°F in a molten zinc bath.

Check out the following website.

http://www.hdg.org.uk/

 

[CTSeng]

I have a project where steel beams have a large amount of 'natural' camber. It just so happens that the camber is being oriented upside down due to fabrication prior to HDG. The fabricator is saying that they fabricated the beams with any mill camber up and are suggesting that the HDG process has released some internal strain hardening causing the beams to sag.

Is this possible?

 

[Rich2001]

At 900 F you are below the stress releiveing range of carbon steel. However, heating in tyhe galvaize bath is not uniform due to the thickness of the material, for example the web of a beam being thinner will heat up to 900 F faster than the flanges causing thermal strain in the part. If there is any resistance to the strain one experiences stress.

The chemical composition of the galvanizing bath has a lot to do with the heat transfer rate. A tenth of one percent of bismuth, lead, tin or nickel will significatly change the heat transfer rate (up to 30%).

Quenching when the material is removed from the bath? depends on the Galvanizer, I have seen both.

If there are any plates welded to the weld I suggest that a Magnetic Particles test be performed around the welded section, look for cracks in the welds and plates and rolled sections. ( MT - using dry powder, AC). The steel may crack under the galvanizing and not be visible without MT. I have been investigating this Liquid Metal Assisted Cracking problems for quite some time now. I know of cases throught North America and Western Europe.

 

[CWIC]

Rich provided some good insight on this subject.

Many DOT projects that utilize HDG weldments now get extra attention for reasons noted by Rich. Fine cracks have been found after the galvanizing process due to restraint within the members. I'm told by one local galvanizer an entire jumbo built-up member had failed almost the entire length. I believe these are documented failures w/photos and much more info is available on this subject.

 

[unclesyd]

Rich2001,CWIC,
I've recently examined several failures/cracks on HDG structures/weldments and found the main culprit to be poor welding. The biggest problem was lack of fusion, undersized welds, or a combination of both. I wasn’t able to do a complete metallurgical analysis on any of the cracks/failures, but it was quite obvious what the problem was. On one rather large structure I was able to chisel out a large section of a grove weld in ½" plate. As you would expect the atmosphere was rather contentious and the only information gleaned from the fabricator was that they were MIG welded, no other details.
Rich2001,
You can stress relieve C/S at 900̊F but it will take several hours. If you left C/S in molten zinc bath that long you would have a vat full of dross. I’ve never seen any liquid metal cracking from HDG a part. I’ve seen it on parts that shouldn’t have been HDG and later cracked at high temperature. Also seen S/S in contact with HDG parts that cracked. A retired foreman from a very large HDG facility told me that they are having a lot more trouble with HDG parts now. He attributes that to the lower lead content of the Zinc (US). This might be factor in your investigation.

 

[bklauba]

The problem appears to be one of dimensional instability, rather than cracking. You could try stress relieving prior to HDG. If thermal is inpractical, think about vibration, which has a good track record solving dimensional instability problems.

 

[Rich2001]

unclesyd, I too have seen problems caused by poor welding techniques, however I am discounting these.

I have been performing 100% MT on HDG structural shapes, plates and weldments, during my investigation. I have been finding many cracks in these pieces that are in the base material not the weld metal or the HAZ area. I have done SEM analysis of the crack surfaces. The problem apears to be that the low metal alloying elements in the galvanizing bath are wetting the grain boundaries.

I have all the certs, mechanical and chemical analysis of the base matels, welder certs and filler metal certs. Representative samples of the material certs were verified by an outside lab. Each item was 100% MT prior to and after HDGing.

Most of the cracks that I have discovered are not visible, but are readily apparent with the use of MT or ACFM NDT techniques.

Steels such as ASTM A36 have far less cracking problems than ASTM A572 Grade 50 of stronger material.

This is such a problem that during the last TECHFORUM (AGA meeting)

http://www.galvanizeit.org/resources/files/TF 03 Activities Brochure.pdf

several presentation were presented on HDG cracking.

In Europe it is fairly common that the Galvanizer utilizes an preheater to heat the steel prior to galvanizing, hence reducing the thermal induced stresses. This is typically not done in North America.

 

[unclesyd]

I'll have to admit that I didn’t check very far outside the HAZ as the cracks in most places were quite obvious. The failures that I looked at were all A-36 (quote), not verified.
It kinda interesting that the cracking was more pronounced on the higher strength steel. This wasn’t perchance EX-TEN was it. The reason for the EX-TEN question was we had some welding problems with the material at one time due to heat input. The difference in tensile strength of A-36 and A-572-50 isn’t really that much.

Are you just going to check production run material are will you be able to piggyback stressed/normal coupons with the production items? If you publish the data let everybody know you results. If you find anything I hope you will be able get the data out as quick as possible as a lot of people are looking at HDG as the price of Nickel continues to go up.

Not being too curious I would like to know how the material was prepped for the HDGing. I’ve encountered some very wild pickling solutions that would do a number on a piece of steel. We had some materials HDG that had relatively a deep etch prior to galvanizing which we found when we had to modify the structure. I’m pretty sure that this was just etching as we recalled some parts that had been pickled but not galvanized that had the etching.

Please don’t find too much wrong with HDG. That’s about all we’ve got left in my price range.

 

[CWIC]

Hi unclesyd:
The weldments that I noted in my 1st reply to this post were all welded using proper technique and workmanship. Some of these were UT'd and found to be satisfactory. Many of these parts where somewhat complex in geometry as far as structural welds go. They were of heavy/medium weight materials which comprised a relatively small weldment (about 40" in length) with many inches of CJP/PJP groove and fillet welds.

The fillet and groove welds were found to have microfissures in the throat of the welds after HDG. Stress relieving prior to HDG had been suggested but not implemented. The parts were simply galvanized at another facility and the problem seemd to go away. I think much had to do with the transistion temp. period between facilities. The 2d galvanizer lowered parts into the bath at a slower rate than the 1st allowing the parts to "preheat" before being fully submersed.

I've seen simple hand rails end up with cracked welds after HDG. As noted by Rich, the problem can be secondary-process related (HDG), and as noted by yourself, it can be primary process related (welding).

On a similar note regarding material specs, many materials now have a dual or multi-grade status. The A 36 is comliant with A 572 Gr. 50, A 992, etc. The HSLA steel is now carbon steel (or is that the other way around?) due to microalloying. Several of my past projects did not go well as the stong column-weak beam design philosophy went out the window with the mill certs.

 

[chicopee]

Why hot dip galvanize structural steel? just spray paint your zinc base that way you will not have thermal stresses developing.