Was this embrittlement, or what?

I have just made a new plate to cover a hole in an engine cooling system. Flat 2mm galvanised steel, 6.5mm holes round the edge, and a 32mm hole in the middle for the bolt-on feed pipe. The last operation was brazing the two bolts to hold the feed pipe, and as I expected, the zinc plating was partially destroyed in the process. No problem, as it will be painted both sides anyway.

But, half an hour after it had cooled naturally, a crack was seen about 20mm long each side of one of the bolt-holes. And wide enough to get a visiting card in.

My guess is embrittlement, but I thought this only occurred in high-tensile steels, not 'ordinary' plate. And then, only where tensile loads were involved (but that might have been the result of uneven flame heating?)

Any thoughts?

I think that you will find that what has happened is zinc embrittlement of the steel. Have you heard of Flixborough, a nylon making plant on the lower river Trent, which was destroyed in an explosion some fifty years ago. The explanation was embrittlement of hot pipes by the galvanised netting holding the insulation in place. Zinc migrated into the steel where the netting touched it.

Early nuclear reactors with mild steel pressure vessels had to be kept above about 100 degrees C to avoid the danger of embrttlement cracks in the irradiated steel.

High temperatures are not needed for embrittlement, but brazing temperatures are high enough to act fairly rapidly..

Google Flixborough, and you may find out more.

Regards. Mike Potts.

Liquid metal embrittlement? In this case molten zinc penetrating into the grain boundaries during brazing?

Thats interesting.

I have to weld some galvanised tube together. Light structural duties not fluid or gas transport. How much parent metal do I need to remove in the weld zone and how far back from the weld do I need to go to avoid zinc embrittlement?

I do know how to deal safely with zintec (coated steel) but there is lots less zinc involved in the coating and the technology is different. I do know how to set-up to deal with the zinc fumes too.

Not an ideal situation, long way from it in fact, but needs must.

Clive

My (limited) understanding of the process of embrittlement relies on the atoms of hydrogen worming their way along the stressed crystal boundaries until they separate. Like a drop of water will cause the collapse of a sugar cube.

Problems (for me):
- Zinc has much bigger atoms than hydrogen, so clearly the process is not of the same order at all.
- I have no knowledge of any related process happening with brazing of steel parts, in which red-hot zinc is a major ingredient.
- In my experience the time between plating and failure has been short - hours only. And it has always involved 'strong' steel, and tensile loads (as in fully tightened socket screws). This steel was ordinary (mild) and weeks, perhaps months after it was plated.

Can anyone offer answers?

If I have another go, with plain un-plated steel, should I also use un-plated bolts to fix to the plate? And are there brazing or flux compounds I should avoid?
NB the brazing is needed because there is no access behind the plate in service, and the bolts must be firm and water-tight.

Tim

You may find this interesting, Tim [16 pages] : **LINK**

https://www.voestalpine.com/division_stahl/content/download/52624/655464/file/VAST-W17008A%20White%20Paper%20Sierlinger.pdf

It’s about spot-welding rather than brazing, but I think some of it is probably relevant

Subject is Liquid Metal Embrittlement

MichaelG.

I wouldn't be as worried about the atom size as the WHS risks of welding or brazing zinc-coated steel. The fumes are toxic so it is something best avoided. Or the zinc ground off the entire area likely to get hot in the process. So, no I would not use zinc coated bolts on your next try. Use plain mild steel. You will get a cleaner joint without all that nasty zinc flowing around in the mix too.

For corrosion resistance you could spray the whole job with cold galvanizing from a spray can afterwards. Not as good as real galv but better than just ordinary paint.

Hydrogen embrittlement not be confused with liquid metal embrittlement.

During electrolytic qalvaning you do not only coat the substrate with a layer of metal but can also “pull” hydrogen into the substrate to be coated. This atomic hydrogen diffuses to higly stressed areas in the steel and there induce hydrogen induced cracking (hic). Hydrogen seems then to follow the tip of the growing crack. By the end the part fails because of a to large crack.

Hydrogen induced cracking should not be onfused with stress corrosion cracking in which a crack grows under stress in a corrosive medium.

Liquid metal embriittlement is dissolving of atoms of solid metal into a liquid metal. This especially happens along grain boundaries of the solid metal. So, after cooling down, you will find grain boundaries higly enriched with e.g. zinc. These weak grain boumdaries cause cracking if the part is under stress.

Hi Tim,

If this is embrittlement, a more likely cause is hydrogen.

Suggest that when making another one you ditch the idea of plated/coated materials when brazing. That avoids the risks and dangers of metal fume. And removes a source of hydrogen associated with plated surfaces.

Brazing material brass rod possibly coated or impregnated with flux?

Oxy-acetylene heating?

Bolts being placed on the surface of the plate and tacked (not brazed) into place?

Contact your brazing material supplier or someone like The Welding Institute.

Regards

Keith

Hydrogen embrittlement not be confused with liquid metal embrittlement.

During electrolytic qalvanising you do not only coat the substrate with a layer of metal but can also “pull” hydrogen into the substrate to be coated. This atomic hydrogen diffuses to higly stressed areas (crack tips) in the steel and there induce hydrogen induced cracking (hic). Hydrogen seems then to follow the tip of the growing crack. By the end the part fails because of a to large crack.

Hydrogen induced cracking should not be onfused with stress corrosion cracking in which a crack grows under stress in a corrosive medium.

Liquid metal embrittlement is dissolving of atoms of solid metal into a liquid metal. This especially happens along grain boundaries of the solid metal. So, after cooling down, you will find grain boundaries higly enriched with e.g. zinc. These weak grain boundaries may cause cracking if the part is under (a local) stress.

Edited By Jouke van der Veen on 15/04/2021 09:15:17

Having been pointed by Michael and Jouke to Liquid Metal Embrittlement, I see the Wikipedia article says loud and clear: The practical significance of liquid metal embrittlement is revealed by the observation that several steels experience ductility losses and cracking during hot-dip galvanizing or during subsequent fabrication.

As Tim was indulging in 'subsequent fabrication' of a galvanised plate, and brazing would melt the Zinc, he's ticked both of the necessary boxes. LME seems far more likely to me than Hydrogen Embrittlement, or any other cause.

I find it very difficult to imagine the effect one element will have on the physical properties of another when the two are in solution. The effects can be dramatic, for example distilled water is an electrical insulator, but the addition of a tiny amount Salt (Sodium Chloride) converts it into a conductor. The salt also alters the freezing and boiling points.

Likewise in metals, it seems small electronic changes can have beneficial or detrimental effects on the crystalline structure of solids. Carbon dissolved in Iron makes wonderful steels and useful Cast-Iron, and both can be improved by adding certain other elements like Manganese. However, other elements, such as Phosphorous and Sulphur, have highly negative effects on Iron.

In steel, I conceive sheets of Iron atoms where a tiny amount of Carbon fills gaps to reduce slipping, while too much Carbon lubricates them. Much too simplistic, because there is no physical contract: the forces involved are electronic interactions between atoms, altering the crystal structure of the metal. And the crystals are related to orbitals, taking us straight into quantum mechanical weirdness.

Soldering and Brazing exploit the good effect of creating a solution between two metals but as we know there are many ways joints can be ruined by contamination, oxides, poor choice of metals etc. Mistakes result in a flawed solution at the boundary.

LME seems to be due to another phenomenon: a crack propagated as a result of a liguid metal causing, or following, a local weakness. Due to leverage, the forces at the front of a crack can be enormous, even if the energy is only built in stress.

Dave

for Keith Hale:

The only reason for using a zinc-coated plate was that I had some of the required thickness. The next one I now need to make will be plain steel.

Brazing rod was plain and looked like brass, but may have been a low-silver alloy. The supplier is not going to be around, as they are old stock (but flow nicely).

Heating was by propane torch.

Bolts placed, not tacked, in place.

Hope that fills in some detail for you.

Tim

A useful and informed discussion, gentlemen. Many thanks. I now suspect that the galvanised plate I started with was rather more fancy than 'just a bit of steel'. Next time I am offered a few offcuts I will take more care to ask 'Offcuts of what, exactly?'

All part of learning by experience - the only way that seems to work.

Thanks again

Tim