Cryogenic Treatment effect on Hardness

[mewhg]

Let's say I have specimen of carburized 8620 steel, .020 case depth, 58RC hardness (89R15-N)at the surface of the case.

If this sample was cryogenically treated (soaked in liquid Nitrogen for 1 hour) would the case hardness increase as retained Austenite was converted to Martensite? If hardness increased about how much could I expect?

Many Thanks,
Bill

 

[swall]

The answer is "it depends". At Rc 58, assuming this is after a 350F temper, you probably don't have much retained austenite, so any hardness increase after cryo would be minimal. On samples with, say 40% retained austenite, you could see an increase of 3 or 4 Rc points after a cryo treatment. So, if the case hardness was Rc 54, with 40% retained austenite, you could end up with Rc 58 or so after cryo treat. These hardnesses are "ballpark figures", btw.

 

[arunmrao]

To begin with for 8620 steel why should you end up with 40% retained austenite. The heat treatment process should be selected to minimize the retained austenite.

I have found cryo treatment beneficial in tool steels and high carbon high chromium grades castings.

 

[TVP]

Bill,

Normal carburizing of 8620 results in fairly low retained austenite, probably 10-20%. Subzero treatments will convert this and raise the hardness somewhat, but I do not have any data to indicate by how much. I would speculate that it might increase 1-2 HRC at the most. Perhaps Carburize can provide some information based on his experience.

 

[metman]

Bill,
As mentioned above, any potential gain in hardness is minimal. If you really need some extra surface hardness, then maybe you should be looking for a different process like Nitriding or maybe the carbon potential in the carburizing atmosphere is low since you should max out in the low HRC 60's with max carbon percent without pushing into carbide network unless you have excessive mass/cross-section that your quench cycle is not achieving adequate quench rate.

If a tiny bit extra surface hardness will really work for you, the cryo treatment does not require an hour. Transformation to Martensite is not time dependent rather it is only temperature dependent.

 

[mewhg]

Gentlemen,
Thanks for the replies. Let me explain exactly what I am doing and why I am asking this question.

I have an 8620 lever arm that is spring loaded in a mechanism. At times the lever rotates through an arc and with a tip velocity of 13 fps it strikes a much more massive part. This lever is carburized to about 58 RC on the case, .015" RC50. I am getting wear on one location of the lever that rubs against another. I am thinking I need another 2-3 Rockwell points (lubrication, nitriding and hard coatings I can't consider). I was just wondering if I cryo treat I can get the extra hardness or do I need to perhaps increase the case depth a bit and temper to 60RC. I would love to use a through hardening alloy (like A2 or D2) but the part has some thin sections and I am worried about fracture failure; the part has to have impact resistance with a hard surface. I will test and let everyone know the results.

Bill

 

[JHossom]

You might want to look at a grade like Crucible's CPM-3V which is both shock (several times A2) and wear (similar to D2) resistant at Rc60/61

 

[Tmoose]

Why no dry lube option? Vacuum, temp, ?

 

[mewhg]

Tmoose,

My customer requested no lubrication. The application will be exposed to the elements and must work with little to zero maintenance.

JHossom, is the CPM-3V a powder metal product? I forgot to add that I am limited to the investment casting process in the part's fabrication.

Constraints can be a pain sometimes.

Bill

 

[CdotS]

Try some PVD hard coatings such as titanium nitride on a hard steel (Q+T) surface. This should give sufficient wear resistance when it is sliding against a smooth hard surface. There are many job shops for PVD coatings available in the USA. Try a google search for PVD coaters.

 

[mewhg]

Cdots,

The wear area is sort of an internal feature and I thought PVD coatings need direct access as it is a thermal spray. This might be my ignorance talking.

I will look into your suggestion.

Bill

 

[JHossom]

CPM-3V is a powder based steel. You might want to talk with Crucible in NY. Investment casting is a whole 'nother thing.

http://www.crucibleservice.com/

 

[Carburize]

Sorry I missed this one - been doing some personal sub-zero treatment on the ski slopes in Colorado.
I agree that 8620 should not show any significant change in hardness due to a sub-zero treatment the case should be pretty well fully tranformed at room temperature and transformation of the remaining ~10% austenite will not produce any major increase in hardness. Even if there is a 1 or 2 point Rockwell increase this will be reduced by the temper needed after the sub-zero treatment.

 

[mewhg]

Carburize,

Does a part definitely need a post temper after sub-zero treatment? What happens if that step is forgone?

Bill

 

[NickE]

"Does a part definitely need a post temper after sub-zero treatment? What happens if that step is forgone?"

If the cryo treatment is done to remove RA then definately, other wise teh untempered martinsite will cause problems.

 

[Carburize]

Any fresh martensite produced by transformation of retained austenite is "as-quenched" and very brittle and the standard practice requires a second temper at around 25F above the first temper.

 

[mewhg]

Ok, understood. Thanks for all the help.
Bill

 

[metman]

Could you use a shock redsisting tool steel like S7?

I'm not sure this would solve even your problem if you are showing wear at HRC58, it would seem you need to be much harder than even HRC65 to help much and since it is impact loading, Nitriding does not apear an option.

Can you redesign the lever to act like a spring and absorb energy so that the impact results in less contact (compressive) stress?