Hot Deep Galvanized

Date 11-29-2016 17:51

Hot dipped galvanizing, not hot deep galvanizing.

The professional organization for hot dipped galvanizers has a standard that "regulates" the percentage of zinc, aluminum, etc. permitted in the molten bath.

Al

By jwright650 (*****)

Date 11-29-2016 20:14

I got a galvanizer all worked up when I asked for the bath analysis. I was experiencing an issue with cope cracking. I suspected too much of a particular element that was known to increase the opportunity of cope cracking, so I asked for the bath analysis and those folks went off on me telling me that I had no right to that information. Just telling you this so if you go that route to expect some resistance.

By 803056

Date 11-29-2016 20:45

There may well be some resistance, but the purchaser has a right to know if the requirements of the standard are being met and whether the contractor is performing the QC functions required by the standard invoked by the customer.

I've had the same feed back when I asked for copies of the CMTRs as per the project specifications. "No one ever asks for that information." is the standard response to many questions and requests made by the third party inspector.

The other stand by, "That's the way we've always done it."

Sometimes we have to stand our ground. Sometimes the financial ramifications are very hard on the contractor that has "always done it that way."

Best regards - Al

By jwright650 (*****)

Date 11-29-2016 20:58

Agreed

By 803056

Date 11-30-2016 01:48

John;

Did you find out what the chemistry was?

I suspect it was a problem with the hydrogen introduced by the acid pickling (rich in hydrogen ions) done before HDG, especially with the ASTM A992 and other high strength low alloy steels.

What conclusions did you draw?

Best regards - Al

By jwright650 (*****)

Date 11-30-2016 02:12

I was suspicious of the tin that was added. It was obvious that it was happening while in the bath because the zinc was inside the crack. Tin melts at a much lower temp and stays molten longer as the piece is cooling inviting the liquid metal embrittlement.

By 803056

Date 11-30-2016 02:58 Edited 11-30-2016 03:13

I would expect the zinc to fill the crack as well as the tin. Consider BCuZn, typical brass brazing rod, it wets clean iron and steel rather effectively. Depending on the composition, ratios of zinc to tin, the melting point of the alloy would be depressed relative to either pure zinc or pure tin. Granted, the melting temperature of the tin is lower than zinc, but in the liquid state, both would be in solution and both would begin to melt at one temperature and be fully liquid at some higher temperature. The plot of an alloy that consists of two constituents in solution forms a lazy S-curve when plotting temperature versus time. Between the solidus and liquidus temperatures, there exists a mushy phase consisting of some solids in suspension in liquid phase.

I suspect the hydrogen ions in the pickling acid is the root of the issue because the problem is mitigated if the hardened HAZ associated with touch cut edges are preheated before cutting and ground to bright metal. Ref. clause 5.16.2 and C-5.16.2. I suspect the carbon in the martensite, ferrite supersaturated with carbon, combines with the hydrogen ions to form methane, CH4. That puts a significant strain on the atomic lattice which can cause the initiation of a crack. The area that was heated to a temperature above about 300 degrees F has exceeded the yield strength of the steel, thus the residual stress in that region is in tension. The material has yielded and is now in tension, therefore a prime location for crack initiation. That would explain why sufficient preheat before thermal cutting potentially eliminates the formation of martensite and grinding to bright metal goes a long way in eliminating any martensite that may have formed along the thermally cut edge.

Good conversation.

Al

By jwright650 (*****)

Date 11-30-2016 12:53 Edited 11-30-2016 12:56

I might agree with you about the effects of the cutting torch, but that doesn't explain how this tube cracked because it was cut with a band saw,

and other trials where we drilled the cope with a drill and sawed back to the drilled hole, sent it off to the galvanizer and that too came back with cope cracks.

When we had galvanized pieces, we tried all sorts of ideas that were thrown out by the AISC, and AGA to try to mitigate this expensive repair work on galvanized pieces.

Here is one that we burned with the torch, ground back to bright material beyond any ill effects that the torch could have produced, yet it came back with cope cracks too.

I found that it was only one of several galvanizers that we used where this took place on a regular basis....this was the place that gave me such a fit about the request to see the bath analysis.

I find it funny (actually alarming) that other fabricators were laughing at me because I took the time to bring all galvanized material back and MT (magnetic particle) the usual places where this "cope cracking" happens. They all said they never experienced anything like this, yet none of them ever brought any material back to inspect, so how did they know that their materials were crack free? They all sent the materials straight to the jobsite from the galvanizer and relied purely upon the galvanizer's visual inspection of their materials. As you know Al, many of these indications are not seen with the naked eye, in fact many are covered over and filled in with zinc and until a little magnetism and colored iron powder are applied they are invisible to the naked eye. PT (dye penetrant) won't even find these.

Now I agree with your points to some degree(melting point of zinc 790°F and bismuth 520°F), but with the tin, it can stay fluid longer due to the low melting temp(450°F) and can enter the grain boundaries while the steel is at an elevated temperature, then when the steel cools the grains try to tighten back up as it shrinks, except now it has other elements trapped inside and stresses overcome the material and cracks form. These extra elements are used for wetting and spangle of the galvanized but if used in larger quantities, cracking is more likely. ODOT had contacted me years ago when I expressed my concerns in this forum regarding these problems. They were having the same thing with galvanized bridges and they had found when tin was present in larger percentages in the bath, the cracking was also present.

By 803056

Date 12-01-2016 02:36

Interesting points John. You seem to have a handle on the issue, i.e., the amount of tin in the melt. An interesting experiment would be to hot dip a few samples in pure tin to see if there is a significant increase in the occurrence of cracking.

I read an article that hypothesized a Soviet submarine sunk because some braze filler metal landed on the main steam line and solidified. So far, so good. The braze filler metal that had landed on the steam line melted when the steam came on line and came up to operating temperature of about 1200 degrees F. The temperature was hot enough to melt the braze filler metal, which in turn was able to wet the grain boundaries of the nickel alloy of the steam line. The braze filler metal wetted the grain boundaries and embrittled the area which lead to a rupture. I can only imaging the hell that resulted when superheated steam was released in the engine compartment.

I like your reasoning John. Now we need data to back up the hypothesis.

Best regards - Al

By Lawrence (*****)

Date 12-01-2016 13:48

I wish every cope I saw was blended that nicely (forget the crack for a minute)

By SWN1158 (***)

Date 12-01-2016 22:13

maxilimiano,

The new galvanizing specification for threaded fasteners is ASTM F2329 (no longer A153), and the minimum zinc content should be the same as what was stated in ASTM A153... "The molten metal in the working volume of the galvanizing bath shall contain not less than an average value of 98.0% zinc by weight." ASTM A123 makes that same statement.