Plasma Nitriding and 304 Tubbing pitting

[vajra78]

I am having a problem with pitting about .001 to .002 deep on commercial grade 304 tubbing, 4"OD X .375" wall, appearing after a plasma nitriding process. I have plasma nitriding done on a barrel I make from 17-4 but only get light pitting maybe about .0005 deep at the most. The engineer at the plasma nitriding processor said pits were caused by impurities or structual inconsistencies in the material. I am tending to agree, but cannot be certain. Wanted to make sure if anyone has expierence with plasma nitriding and if it is a process problem before I move to a solid domestic 304 to produce these parts - tubbing not readly availible in high quality forms, at least were I've looked?

 

[GrinningIdiot]

What temperature was the 304 nitrided at?
When it's done above ~450C you'll wreck the corrosion resistance since any Cr at the surface will be bound up as nitrides instead of staying in the bulk helping against corrosion.
That could be the cause of the pitting though microstructure does of course play a role too. It'd not hurt having a look at a sample under the microscope.

 

[EdStainless]

What is the S level in the material? You could just be removing inclusions.
But as has been said, check the microstructure before and after. make sure that you are not getting any grain boundary sensitization.

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Plymouth Tube

 

[vajra78]

S level is 0.030 max. Supposedly the process wasn't getting too hot, when I asked about it. Some other details, penetration is to .005 and part also had elongated swirls down the ID - like a barber shop poll.

 

[vajra78]

GrinningIdiot,

Do you have a reference for the Cr being bound up as nitride at temp over ~450C and how much is bound up and to what penetration? Thank you for the replies, I will look into checking microstructure.

 

[EdStainless]

No, you need the ACTUAL S level.
The spiral could be from the tubing being rotary straightened.

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Plymouth Tube

 

[GrinningIdiot]

Most of the references I have are in German but I can summerise them if that would help.
In the mean time I've attached a paper I found for free which covers what I was thinking about though it deals with 314L instead of 304.

Since I'm very new to this forum can someone point me to any of the rules concerning uploading journal papers to a forum? I've got a few but normally you'd have to buy them from the publisher's site.

 

[GrinningIdiot]

Just occurred to me...

I don't think I made it clear that nitriding at 450C or so isn't standard for most steels. Normally it's done at 520-590C. Nitriding to get a compound layer will normally increase corrosion resistance and should greatly increase corrosion resistance if the piece is post-oxidised.

It doesn't work out so well for stainless steel, though. Nitriding at >500C is ideal for forming Cr nitridies. Normally not a problem since the compound layer has better corrosion resistance than the base material. In the case of stainless the loss of Cr used for passivation reduces tha corrosion resitance more than a compound layer can increase it.

It could well be that the process worked fine according to the heat treater if the temperature was specified at above 500C. It'd be good if you could ask them exactly what the T should have been, and (if they are equiped for it) if they can nitride at low temp. If they can, you may even be able to get even better corrosion resistance than before with increased wear resistance to boot. The down side is a much longer process time.

Of course too high a S content could be the root of the problem.