Corrosion of stainless steel 303 (1.4305) after nitriding process 2

[bukowski]

Dear all.

Problem:
- surface corrosion occurrence of nitrided 303 rod after being exposed to 85°C and 85%RH test (total duration 1000h, corrosion occurrence already in 24h)
- corrosion occurs on non-nitrided area of the rod.

Requirements:
- rod dimensions: dia. 4 mm, length 55 mm, nitrided area only 5 mm in length
- material: non-magnetic - in that case we have chosen the austenite stainless steel
- one of rod ending is shaped into the sphere in order to push a lever in application. This sphere has to be hardened because it moves the lever with the impact. Nitriding process was chosen due to the material.
- surface roughness (Ra => 0,4 um, Rz =< 1,6 um)


Process of manufacturing of rods:
- CNC machining - machine and tools used for different kind of metals (free-iron contamination possible)
- Washing with ultrasound before nitriding
- plasma nitriding at 500°C, 12h or 520°C, 16h

Analyses and results:
1. Material analysis:
- supplier influence - we change them - same problem occurred
- type of material influence - we tried several different austenite SS, also 316L - corrosion resistance improved but still same problem occurs

2. Process:
a) Washing before nitriding:
- we tried gasoline, ultrasound, others
- washing does not influence on corrosion occurrence

b) Nitriding:
- tried to lower the temperature below 400°C and at the same time prolonged the time to 30h where the level of corrosion lowered down and the thickness of the nitriding layer (logically) was thinner.
- So, slight corrosion still occurred but the rod is inadequate because we have an impact on the rounded end of the rod and it deforms completely the material.
- If 520°C, 16h material does not deform.

c) Post-nitriding treatments - just for the tests:
- tried electropolishing - corrosion occurrence on nitriding area - (it not electropolished, corrosion occurs on non-nitrided area)
- passivation - same problem with corrosion on non-nitrided area
- pickling and passivation - corrosion of nitrided area + surface roughness is NOK

Are there any other solution of post treatments or hardening process which could solve the issue and to meet the requirements?

BR

 

[EdStainless]

Select an alloy with the proper level of corrosion resistance.
303 has very poor corrosion resistance.
It is apparent that you are damaging the non-nitrided surface if EP or pickling gives you good corrosion resistance.
If the surface was still non-oxidized then pickling should not change the surface roughness at all.
Perhaps your pickle was too aggressive?
You should not pickle or EP the nitrided portion.
I would suggest that you look into the low temperature carburization or nitriding processes that are done near 400C. These have actually been shown to improve corrosion resistance. They result in no formation of carbides or nitrides in the structure.

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P.E. Metallurgy, Plymouth Tube

 

[TugboatEng]

No expert here but if carbide precipitation is a problem how about a stabilized stainless steel such as 321 as the cost is not far off the other alloys listed. This is more of a question for myself than a solution to OP's problem.

 

[EdStainless]

If you want to preserve corrosion resistance you must nitride or carburize at a temp low enough to not form carbides or nitrides.
If you are using a conventional higher temp treatment you need to have a much higher Cr alloy to start with.

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P.E. Metallurgy, Plymouth Tube

 

[bukowski]

@EdStainless:

Material:
As mentioned we tried the austenite 303, 304, 316L and in all cases the result was corrosion. 316L was best in terms of corrosion amount, but it was still there. 303 or 304 is chosen due to CNC machining and price and going to 316L did make no sense due to the tests and CNC machining difficulties.

Pickling:
Is it even possible to make it non-influential to the surface roughness? I'm not an expert for the pickling process, so what would be the correct way?

Quote: "You should not pickle or EP the nitrided portion."
May I ask why? Because it ruins the structural integrity or due to the corrosion occurrence?

Low temperature nitriding:
Quote: "I would suggest that you look into the low temperature carburization or nitriding processes that are done near 400C. These have actually been shown to improve corrosion resistance. They result in no formation of carbides or nitrides in the structure."

We tried the Low temperature nitriding (S-phase), but the layer thickness was so small that when we performed impact tests with the lever it has completely deformed the material. You are correct, the level of corrosion was significantly lower, but it was still present on the non-nitrided area.





@TugboatEng:
321 could improve the corrosion resistance, but we already performed humidity tests with 304 and there was a lot of corrosion present, so we expect a minor improvement with 321. Downside for 321 is also the machining process which in 321 case gets more demanding and the price of the part goes up.

 

[EdStainless]

Are you working from cold finished rod or annealed product?
I can't imagine you getting a strong enough case on annealed SS to hold up, you need a stronger core and you should be working from cold drawn bar. (It also machines cleaner, takes more power but cuts better)
The case should be to provide local wear resistance, the strength needs to come from the bulk part.

Pickling should just remove the oxide layer and the thin layer of Cr depleted material below it. If the surface is not oxidized and your pickling solution is not too aggressive (20-25% Nitric, 4-5% HF, room temp)then the surface shouldn't change. When you pickle the treated portion because it is multi-phase you will get significant localized attack and pitting.

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P.E. Metallurgy, Plymouth Tube

 

[bukowski]

Annealed. Core hardness = 300 HV0,3. After nitriding >1100HV0,3 and 80-100 um layer thickness which serves for the application just fine.

before pickling Ra <0,4 um and Rz < 1,6 um and after pickling both increased e.g. Rz = 5 um. We added passivation after pickling and yes, you are correct the nitrided part started to corrode already within 24h of humidity testing.

So, what would be the most adequate approach to use?
Should we use nitrocarbonizing or any other surface hardening (I know we are limited with austenite type of material)? Or is there any right combination of the post-treatment for the selectively nitrided rod?

Thank you very much for the help!

 

Nitrogen will react preferentially with chromium (and molybdenum if present), depleting it in the matrix near the surface where corrosion happens. Similar problem with carbon.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

Work from cold drawn bar and use low temp S phase treatment.That will give you strength, wear resistance, and corrosion resistance.

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P.E. Metallurgy, Plymouth Tube

 

[bukowski]

Do you maybe know what would be the level of Vickers achieved?
In addition, I'm worried about the surface roughness in cold drawn bar...

 

[bukowski]

by S phase you mean temperature 380°C, 30h and 95% to 5% N vs H?