Stainless Steel Low temperature Carburizing 7

[gciriani]

I'm working with a supplier that Carburizes our 316L parts, to improve the surface finishing. The parts are mirror polished before the treatment (Pionite process by Air Water Ltd.), which is an S-phase carburizing. The steel increases its hardness from 250 Hv to about 800 Hv0.05 with a thickness between 20 and 30 micrometers. The material is then cleaned in two subsequent baths, to eliminate a black film that forms during carburizing, and give it back its shine.

Yet we experience a problem, because of the appearance of orange peel effect. The microdefects that appear seem to be the same size of steel crystals (100-200 micrometer) and rise slightly from the surface (about 0.15 micrometers). Is this consistent with the expanded Austenite phenomenon? Or do you think it is more related to the probable etching that is performed afterwards (to eliminate the black film)?

 

[mcguire]

Austenitic stainless steels are notorious for etching at the grain boundaries more rapidly than at the interior of the grain. That's why they don't make very good mirror finish via bright annealing.

If there is hydroflouric in the etchant, try reducing its concentration. Or you may want to use a sulfuric/phosphoric electropolish to clean up the surface.

If you can adjust the 316L chemistry, try stronger de-oxidation than the normal Mn/Si.

And, refine the grain size.

 

[EdStainless]

How are you developing the bright surface? Part of the problem may be from smearing or distrotion of the surface prior to carburize. I think that you are right though, this is related to the slight differences in C uptake and resulting expansion.
If you need a very flat polished surface your only option is to final polish after Carb.

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

 

[gciriani]

Mcguire, thanks for the tip. Regarding the grain size, do you think my problem is consistent with noticing it more on stamped parts, than on parts made from extruded profiles? That is, if the grain is bigger in stamped parts because of the stamping (think of a burger pattie), then the problem would be more noticeable. At least this is what I'noticing. Finally what can be done to refine the grain size? Work with the 316L supplier? Re-heat the parts once they have been machined?

EdStainless, thanks for your tip too. The parts are polished prior to carburizing, on a wheel with brush, cloth and polishing compounds. So I guess the mirror polish is obtained thanks to some smearing of the metal. But what is the mechanism of distortion of the crystal during smearing? Does it mean that if a grain is more covered than its neighbor it would absorb C more slowly? But if the smear is only 316L, the absorption rate should be the same through both grain and smear, resulting in the same depth and same swelling. Perhaps the different swelling means that the smear has some other substance mixed in it that changes the diffusion rate. Does it make sense? It is difficult to polish after carb, because first the hardness has increased a lot, and then it risks to take away too much material, i.e. the 20 micrometers would disappear in the blink of an eye at edges.

 

[mcguire]

Superficial plastic deformation has the effect of enhancing diffusion, probably because of the huge increase in dislocation density it causes.
As far as grain size refining, the producing mill is capable of controlling to a finer range, if your business is worth it to them. You can't refine grain size because you can't put the necessary cold work into the steel which is what triggers recrystallization.
The differential carbon uptake may simply be that grain boundary diffusion, which is predominant at these temperatures, is "draining" the carbon deeper via the grain boundaries than can be done across the garins, so the grains swell.
Sounds like a potential Ph.D. thesis.

 

[gciriani]

McGuire, regarding diffusion enhancement, would you say that there is more superficial plastic deformation in a stamped part or in a drawn profile?

Regarding increase in dislocation density, what would you say for the same two types of parts?

What's the order of magnitude of the space between grain boundaries (interstices)?

Also the supplier states that, if the parts being treated for low temperature carburizing are laying on a tray, the points of contact are treated as well. I have my doubts.

 

[mcguire]

gciriani, the surface deformation is most likely greater for drawn parts than for stamped. One would need specifics to say with ceratinty. That means the dislocation density is greater for the drawn part.
The size of grains usually is on the order of 10 to 50 microns in austenitic stainless.
I, too, do not quite believe that having parts in contact will not affect carbon uptake.
I would try to get the steel supplier to go with a low temperature anneal to get grain size down to ASTM 9 or 10.
Maybe you could put superficial cold work into the surface by abrasion to make the uptake less dependent on grain boundary diffusion which predominates at these temperatures.
This may also speed up the carburization.

 

[mcguire]

gciriani
Can you email me some information about this pionite process? MFM304@aol.com is my address. I couldn't find a website under the names you gave. I would like to use it on 304 for a non-stick, non-scratch surface. Do you recommend it for that purpose? Is there a alternative? Thanks

 

[unclesyd]

mcguire,
Here is another process for SS. I haven't checked it much further than this ad. I have recommended it for some parts in an extrusion process.

http://www.industrialheating.com/CD...ures/BNP__Features__Item/0,2832,99594,00.html

gciriani,
Part of your problem may be differential polishing of the SS. If you can get with your consumables supplier of the polishing materials and see what they have to say.

 

[gciriani]

McGuire, the Pionite process is a patent of Air Water Ltd. (formerly Daido Hoxan Inc.). You can find its description in the US Patent Office web site, under patent 5,556,483. If you have difficulty accessing it, let me know, I can e-mail the text description. However, it is patented, therefore not freely usable for commercial purpose, unless you are licensed.

 

[mcguire]

I found the patent. Thanks. Do you know a licensed vendor of this process who would make samples?

 

[unclesyd]

Here is another link to a new process from Swagelock. I haven't run anything down on this process.

http://www.asminternational.org/Con...wsArticle.htm?SMContentIndex=5&SMContentSet=0

 

[gciriani]

Vendors: I looked at UncleSyd's link, and it looks like Swagelock is a potential vendor for a treatment similar (at leats the results) to the one I'm working on.

I have a good article describing characteristics of competing low-temperature nitriding and carburizing processes. It appeared in the June-2002 issue of "Advanced Materials & Processes", with the title "Stainless Steel Low Temperature Hardening", by Thomas Bell and Chen X. Li. I only have a copy, unfortunately, which I could fax to you if you wish; just e-mail your fax number to Ciriani@Promotion-Spa.Com.

The various treatments mentioned by the article are by: Birmingham University, UK; Nitruvid, France; Bodycote, Netherlands, France and USA; Air Water, Japan; Nihon Parkerizing, Japan. It is interesting to note, in a comparative table contained in the article, the characteristics of carburizing and nitriding: the hardness profile of carburizing changes gradually, which makes the load bearing capacity much better than that in nitriding (where the hardness profile changes abruptly); however, nitriding results in harder surfaces, which results in very good wear resistance. In my case I have to protects my surface from accidental dings, therefore I opted for carburizing.

 

[gciriani]

On a recent thread "Grinding and Corrosion Resistance" there were interesting statements by McGuire on "... polishing stainless steel ...", and by EdStainless on "... smeared metal ...", affecting corrosion resistance etc.
In the problem I'm trying to resolve, orange peel results after low-temperature carburizing and subsequent electro-chemical cleaning. An industry veteran (stainless steel luxury-goods with finishing by polishing) suggested that metal smeared by the polishing may be the culprit.

This person suggested that polishing smears the microscopic peaks of metal into the microscopic valleys of the steel surface. This would result in a surface with smeared metal all around. Then the hardening process takes place in an oven, and the unfortunate side effect is that the parts lose their shiny appearance. To counteract it, they are sent to an electrochemical bath to be cleaned again and regain the shine. Lastly a slight buffing touch (no smearing here should take place).

The person asserts that during the electrochemical bath all or most of the smeared metal is eaten away by the bath and this in turn would uncover the valleys previously covered (100 micrometer width, depth 0.1-0.2 micrometers, i.e. 4 mils wide x 0.004-0.008 mils deep, ). Does this makes sense? And, if true, what could be done to regain the shine without uncovering the valleys?

I've read and heard around my industry that ideally, polishing works with periferal wheel speeds around 1000 m/minute. Is that consistent with heating and softening the metal enough, to be able to smear it?

 

[mcguire]

We seem to have overlooked the obvious. Why mirror polish before treatments that ruin the mirror polish? You can do a semi finish before treatment, then take it to a mirror finish after s-phase carburizing. This may allow you to omit the chemical baths.

The smearing of metal during polishing is a fact. As you go from coarser to finer grit, you should be removing the damage from the previous grit until with a mirror finish there is no damage left. Therefore, it would seem that the valleys come from differential attack in the post-carburize treatment or from the treatment itself, i.e. differential carbon pickup.

Polishing last would address the problem directly.