Possible 304L carbon pickup in a furnace?

[robsalv]

In an earlier thread, I reported on some new 32mm thick 304L heads which were found to have extensive visual surface breaking defects at the knuckle region. These were blend ground, but replicas found a sensitised structure with IG cracking. A similar damaged grain structure was also found on the inside "as pressed" surface.

The heads have now been destructively tested.

Preliminary results show that the heads exhibited these artefacts uniformly across the surfaces to a minimum depth of 50microns where sectioned and tested.

The other interesting find was that the carbon content was highest at the extreme surfaces, up to 0.09% but soon dropped to the material spec'd 0.015% further into the material cross section.

In other words, the sensitisation, IG cracking and carbon pickup appeared to be a "skin" type artefact. Tests from the core of the material were all good.

These heads were hot formed, then solution annealed. The suspicion is that the head presser's oven, being gas fired, was the source of carbon contamination - via a reducing atmosphere. This is supposition at this stage.

Some questions I'm delving into are:

How easily would 304L pick up a uniform layer of carbon in the time a 2x2m 32mm thick plate section requires to reach temperature?

How reducing an atmosphere would be required to result in a high carbon layer in this time? A lightly or heavily reducing?

If a reducing atmosphere, wouldn't an operator notice dark or black smoke?

Any thoughts, insights, or possible lines of investigation would be appreciated.

Thanks in advance.

Rob

 

[unclesyd]

Please post time and temperature in the oven and any other information you have like the MTR of the plate prior to forming.

Personally I've never seen any C pickup in a furnace when HT L grade materials. All our SS is L grade mainly 304L.

 

[btrueblood]

Could this be baked-on lube (grease or oil)?

 

[EdStainless]

My hunch is toward surface contamination, possibly during hot working. C pickup during anneal is posible, but not easy.

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Corrosion, every where, all the time.
Manage it or it will manage you.

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[unclesyd]

btrueblood brought up a good point about the lubricant in that a graphite wash is some times used while forging. Normally it is not used on SS unless the surface is going to be machine.
I know of one or two cases that this wash along with somewhat extended time at temperature has caused problems. I'll check to see if I can get to the reports.

 

[robsalv]

Thanks for your thoughts folks.

I wish I could tell you about the HT!

The pressure vessel fabricator subcontracted the head pressing to a mob who do not have a chart recorder set up on their oven. They visually monitor a digital temperature readout according to a procedure... the QA/QC is not what you'd describe as strong.

The lubricant question is being followed up.

I still can't get my head around the IG cracking though... carbon pick up would not result in IG cracking, not even after being pressed... gross cracks in the knuckle though make sense if the surface is sensitised...

The core chemical analysis has confirmed the material certificate composition, so we're assuming the plate was good to start with... however, if the mill messed up the pickle after annealing - that could do it... but then the cert does say the mill did an IG corrosion test to ASTM A262...

At this stage of the game, I'm about to recommend the fabricator get a crate of grinding discs ready to grind away the skin of the new heads if they show the same defects!

Thanks again.

Rob

 

[btrueblood]

"I still can't get my head around the IG cracking though... carbon pick up would not result in IG cracking, not even after being pressed... gross cracks in the knuckle though make sense if the surface is sensitised..."

We've seen weird inter-granular cracks happen before with baked-on drawing lubricant in some Inco 600 tubing that was annealed (without cleaning) between drawing operations. The carbon or whatever it was seemed to concentrate along grain boundaries, and seemed to be linked to a bunch of trouble downstream, ranging from fatigue failures during ultrasonic cleaning (weakened grain boundaries acting as initiation sites?) to liquid metal embrittlement (same thing). All guesswork on our part, but getting a new lot of material in with clean i.d. made a lot of the trouble go away.