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Elevated temperatures effect on quenched and age hardened stainless steels

Elevated temperatures effect on quenched and age hardened stainless steels

Elevated temperatures effect on quenched and age hardened stainless steels

(OP)
Not sure if Metallurgy or this forum would have the best resource but here goes:

I’m not an expert so please correct any inconsistencies I have in my theory.

I have a few questions regarding the effect of titanium nitriding and/or aluminum titanium nitriding that I'll refer to as their acronyms from here on out.

Traditionally we manufacture medical instrumentation from a fairly narrow band of stainless materials. For cutting instrumentation I prefer to use 420/440A/440C due to the edge retention properties, reasonably good corrosion resistance, and availability. However, we have a new requirement that some instruments I would use this material for have to be AlTiN’d for the purposes of reducing reflectivity—my understanding is that this is performed at temperatures greatly exceeding the temper temperature generally used to achieve a 50+ HRC in any of these materials. Does this not adversely effect the material properties in a way that would compromise its strength and long term function? I believe I have some historical evidence of instrument fractures in TiN coated 440A that were analyzed and found microstructure changes—though I wasn’t involved so I can’t speak to the veracity.

On the flip side we often use precipitation hardened steels (mostly 17-4/455/465) for almost all other instruments that need to have more strength than 304/316. In this case sometimes they are TiN coated for cosmetic or labeling purposes (e.g. “push the gold button”). It is my understanding that there are low and high temperature TiN coatings available but that generally they are all done below the age hardening temperature of these steels (usually 800/850 F). As long as the temperature remains below the age hardening temperature should you expect any change to the microstructure/mechanical properties?

One last inquiry to tie this together—in the case of edge retention I’ve always been of the opinion that PH steels are not as optimal because while their hardness on a macroscopic level is usually very high, the matrix the martensitic precipitates is in is generally not so much (I’ve certainly seen evidence for this in damaged 17-4 instruments). This is why in order to solve my question #1 above I’m hesitant to switch to a PH grade and AlTiN coat it because ultimately the coating is thin and subject to the limits of its substrate. Am I wrong in this line of thinking? Because, if so, I imagine that would be simplest solution.

Thanks for any direction you can give me, it would be nice to be able to advise others at my workplace as I think there is some misconception about how TiN and AlTiN coatings are performed and how it might affect the effectiveness of their parts.

RE: Elevated temperatures effect on quenched and age hardened stainless steels

(OP)
I made a typo, that should read "900/950 F" in the second main paragraph.

RE: Elevated temperatures effect on quenched and age hardened stainless steels

I understand you using such coatings for cosmetic reasons, but there are other coatings for wear applications. TiC is a common one. I have seen it used on 440C, the process temp is the same as tempering for this alloy. This coating is done by reacting with the C in the alloy so it becomes part of the material not just a surface coating.
What temper temp are you using on 440C? Isn't it near the 900F that you use for aging the PH grades?
While a carbide containing alloy may be superior in wear resistance if you coat the surface all that really matters is that the base alloy is strong enough to support the coating. With hard coatings I would expect blades of 440C and 17-4 (at the same strength) to have identical performance, limited by the coating.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

RE: Elevated temperatures effect on quenched and age hardened stainless steels

(OP)
Thanks for the response, I think I must have mixed units or misunderstood something as I think you are right that normally 440C would be tempered above the PVD temperature in almost all cases.

I want to preface the following by saying I didn’t design the instrument I mention tongue. The instrument fracturing was not 440C but 440A so I researched and found out the temper temperature was 325 F to achieve the spec 55-58 HRC. However, it was then TiN coated at temperatures exceeding 350 C (662 F). Tests of the fracture sites at the core measured 44/47 HRC and microstructure showed carbide precipitation at the grain boundaries. So basically it appears the wrong grade was chosen as it appears that 440C could have used a temper temperature above the PVD process to achieve the same hardness.

Do you know of a resource where I can find “common” PVD temperatures for these coatings? I’ve seen a low temp TiN as low as 375 degrees F but usually it doesn’t seem to be advertised what the process temperature is.

I will have to look into TiC and any ramifications on biocompatibility studies. Thanks for all your help Ed, I’ve read many a reply from you and learned valuable information over the years.

RE: Elevated temperatures effect on quenched and age hardened stainless steels

Hi ,

If I understood, you want to reduce reflectivity, keep sharpness and improve corrosion resistance by TiN coating.

With 440C we tried chrome coating and it works fine. Mostly, No effect on the microstructure.
did you look at cold spray coatings.

Did you ever try 17-7PH? looks with intresting properties, but available for lessthan 0.150''

Regards,

RE: Elevated temperatures effect on quenched and age hardened stainless steels

Bipolarmoment, have you considered Kolsterising process . This is case hardening of stainless steels.

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

RE: Elevated temperatures effect on quenched and age hardened stainless steels

The deposition temperature is a trade-off of a number of factors, including base material of the target, type of equipment being used, and desired performance of the coating. In general, higher deposition temperatures promote better adhesion. For the common nitride type coatings, there isn't a defined deposition temperature. Take a look at some of these links for more information:

http://en.wikipedia.org/wiki/Physical_vapor_deposi...

http://www.ionbond.com/dynasite.cfm?dsmid=83917

http://www.richterprecision.com/pvd-coatings.html

http://www.oerlikon.com/balzers/en/industries/solu...

RE: Elevated temperatures effect on quenched and age hardened stainless steels

(OP)
TVP,

Thanks for some leads--I hadn't found the last two links previously and the richter precision one has some handy info.

I was aware that there's some variables in the temperature but knowing what's available and some basics is helpful.

RE: Elevated temperatures effect on quenched and age hardened stainless steels

I would contact Questek as they have a steel that make an excellent knife with great edge holding ability.
I’ve mad many a knife fro BG42 and all are excellent in all respects.
Also you might look at Ion-Nitriding improve edge retention.

http://www.questek.com/

http://www.matweb.com/search/datasheet.aspx?matgui...

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