How can carbon harden 316L & improve corrosion resistance? 7

[rnd2]

In thread 330-49696 TVP mentions a surface hardening process called Kolsterising.
Apparently tough corrosion resistant material like 316L can surface harden to 75 HRC with Kolsterising.
Apparently corrosion resistance is improved, particularly if molybdenum is present; the hardening agent is carbon which is infused into the metal surface and the infusion process ensures carbon does not react with chromium to form chrome carbides.

1) Can anyone list the conditions where this process improves the corrosion resistance of 316L?

2) Can anyone list the conditions(if any)where this process inhibits the corrosion resistance of 316L?

 

[Carburize]

For more information on the Kolsterising process visit

http://www.kolsterising.bodycote.com

 

[rnd2]

Thanks Carburise
I already visited the website.
I was unable to find an explanation why corrosion resistance is improved. What is the mechanism?
I work with salt water and if someone proposed to join carbon laminate to 316L stainless steel without electrically isolating the two materials they would be laughed out of town, yet here is a process that apparently improves the surface hardness of a previously only work hardenable ss by infusing carbon directly into the material through the surface and claims corrosion will either not be affected or may actually improve!
I have a project involving 316L operating in seawater just below the splash zone that would certainly benefit from the hardening process if it were cost effective and I were convinced of the corrosion claim.

 

[Carburize]

I'm sure you can get a sample prepared by Bodycote which you could use for a salt spray test to get a quick answer as to whether the claims are correct.

 

[rnd2]

Thanks again Carburize. I sent an inquiry and await their response.

 

[rnd2]

Shortly after my last post we sent actual parts made of 316L for the hardening treatment. They were duly treated and returned.
Initialy, other than a significant increase in hardness, the treated parts look identical to the un-treated parts. Now, after some months of room temp air exposure, unsightly brown specks have appeared on the surface of the treated parts, but not the un-treated parts. Aside from the surface hardening treatment itself, the hardened parts have been treated no way different to the un-treated parts. I would very much appreciate information about what may be happening here.

 

[EdStainless]

A process that uses the Cr and/or Mo form the alloy in a hardening process must reduce it corrosion resistance. You have tied some of the Cr and Mo up as carbides so they are not avaialble to help the general alloy. Now, if the carbides are very evenly distributed and do not favor grain boundaries, then the impact will be minimized.
Could you also passivate these parts in order to remove some Fe from the near surface in order to improve corrosion resistance?
316 will not stand up in sea water unless there is agressive cathodic protection. You need a better alloy for seawater service.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be slowed down.

 

[rnd2]

Ed, thanks for your reply. This process delivers infused carbon via a proprietary method that apparently does not interfere with the chrome. It is claimed corrosion resistance actually goes up, especially if Mo is present. How does the process work?
The hardness and toughness is OK for our application. To prove the corrosion resistance one way or the other there is a test piece in service in the Pacific but the info on that won't be available for several months yet.
The unsightly brown specks that have emerged on the remaining air exposed pieces are very concerning. What do they mean?
Regular 316L corrosion-wise is just fine for this ocean water application. Un-treated parts continuously emersed for 12 months remain in pristine condition.
Mechanical-wise it would be beneficial to have 316L harder and at the same time maintain its toughness and corrosion resistance.

 

[EdStainless]

If you are staying below the carbide forming temps and just using the carbon as an interstitial strengthener then there should be no impact on corrosion resistance. It will also be very limited in how much the strength can be increased. Your discoloration must be due to surface cleanliness issues.
Could you use cold worked 316 for this part? That would be stonger. The other option is to look at duplex 2205 which has a much higher strength.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be slowed down.

 

[YLWEA]

I agree with EdStainless - Alloy 2205 provides much better strength than Type 316 and I would not be surprised if the cost is lower than doing the special surface treatment on Type 316. Another option might be Nitronic 60 (see

http://www.hpalloy.com/DataSheets/ni60.htm)

which is claimed to have chloride pitting resistance as good as Type 316 and twice as much strength, with very good galling and wear reistance. You probably need to verify that the good chloride resistance is maintained in the very high strength versions of the alloy, though.

 

[metengr]

rnd2;
The following post in the Metal and Metallurgy Engineering forum might have some additional info regarding the surface hardening treatment for 316ss. The claim made is that there is no affect on corrosion resistance.

thread330-92279

 

[kclim]

If the kolsterising process introduced compressive stresses into the surface, wouldn't this decrease the susceptibility to stress corrosion cracking and/or corrosion fatigue? Just a stab in the dark...

 

[rnd2]

Thanks for the replies.
Ed
Your comment about surface cleanliness issues is relevant and it is a fact that every man and his dog examined the treated parts. The problem is the untreated parts have been examined by the same people. Only the treated parts have developed rust-like specks.
metenger
I looked at Thread330-92279. The information therein is partly our reason to trial the treatment. There is no mention of brown surface specks developing. I am concerned what they mean long term.
Ed & YLWEA I am checking alloy 2205 as a possible alternative but at this early stage I believe it may still need to undergo the surface hardening treatment.
kclim
Something has caused the brown surface specks to appear on the treated parts and not the un-treated parts. Whether it is the treatment itself or the original batch of 316L is something that needs finding out.

 

[YLWEA]

can you tell us what degradation mechanism is of concern, i.e., metal-to-metal galling, sliding wear, high velocity liquid erosion, cavitation, solid particle impact etc?

 

[rnd2]

YLWEA
The brown specks under X 8 magnification appear to surround tiny holes. The holes are common to all the treated parts although some treated parts are more affected than others. None of the untreated parts are affected.
It appears the holes have been caused simply after regular storage. At the moment they are simply unsightly. The concern is later, should the part see service fully immersed in seawater, is there potential for more severe corrosion? What do the brown specks signify?
Part A is a cast part made from 316L
Part B is a part machined from stock bar
Part C is a part machined from stock bar larger than Part B
Part D is a part machined from stock bar larger than Part C
In other words there are 4 different parts which do not share a common source stock material that when treated all exhibited brown specks after a few months.

 

[YLWEA]

What I was trying to get at was why surface hardness is considered a valuable attribute? I'm trying to get some feel for why you think perhaps the high strength of 2205 won't be good enough in the absence of a hard surface. No big deal, I was just curious and the answer might help some posters provide some guidance for other alternatives. As for the brown spots, I can't comment because I don't know enough about the coating process.

I can say that my prior experience with different types of diffusion coatings on steel and stainless substrates has been disappointing in immersion service. My own testing indicated that they might be great for wear and hardness but they did not often meet expectations for improvements in corrosion resistance. Inclusions in the material can result in localized anomalies in the coating that act as intitiation sites for subsequent corrosion. If your base metal is not extremely clean (low inclusion content) then I would guess that there is good likelihood of the coating having localized imperfections.

 

[rnd2]

YLWEA
A high surface hardness around HRC70 is desirable because occaisionally the parts will shear against sub-surface wire cable and become damaged. However, this is rare. Over the long term it is far more desirable the parts do not corrode.

 

[YLWEA]

in that case I would probably investigate the suitability of one of the Nitronic stainless steels rather than 2205. They seem to be intended specifically for galling, rubbing, etc.

 

[rnd2]

The achilles heel of this type of surface treatment appears then to be inclusions that act as corrosion accelarants.
Therefore for any given stock material once treated, the greater the inclusion density, the worse the potential for corrosion. This seems to be consistent with different concentrations of brown specks observed on our treated parts. Each part was originally produced from a completely different batch of 316L and each part showed different concentations of brown specks.

Neither 2205 nor the Nitronic 50/60 are heat treatable so to harden them they would have to rely on a surface hardening treatment like we are trialing for 316L.
Can anyone comment about ss alloy 440A or equivalents for long term seawater corrosion resistance? I know very little about it, have never used it but it seems to have plenty of chrome, carbon and Mo. Composition & some specs below.

C .60-.75, Mn 1.0, Si 1.0, Cr 16-18, Mo .75, Ni .50, S.03, Ph .04
Stainless Steel - Martensitic
Specifications
Tensile Strength ksi. (MPa) 104 (717)
0.2% Yield Strength ksi. (MPa) 62(427)
Elongation % 20
Hardness 57-60 Rc

 

[unclesyd]

Don't use 440A SS for sea water corrosion resistance.
We did use 416 SS or a few 430 SS for all vertical pump shafts for many years and still have some in service. The service is tidal and with periods of low rainfall our traveling screens catch shrimp.

Take a look at the Aquamet and Aqualoy shafting materials. Be careful about the duty cycle as long periods of inactivity in salt water are important considerations in the selection of materials. We have essentially changed to these materials for water pumps.

http://www.aquamet.com/index.cfm

http://www.aqualoy.com/

I’ve never used Kolestering, but have used the QPQ (liquid nitriding) process by Kolene with excellent results. I just checked with the pump shop and the some QPQ spare parts (17/4 SS sleeves) for split case pump shafts that have been on the shelf for over 25 years in a covered storage area are still black and shinny, no sign of spotting. We did use some 303 material nitrided for wear application and I don’t ever remember any spotting problems, but if any, they wouldn’t have been a problem in our application.