20CV composition varies from spec

A

attila.

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Hello everyone. I've never posted in this section, but since I recently got some CPM 20CV from Niagara Specialty Metals (NSM), I have a couple questions that I need help with. NSM included a chemical analysis of the chunk I got. It has the following composition in percentages:

C 1.920
Si 0.670
Co 0.040
S 0.013
Mn 0.330
Cr 20.850
W 0.630
P 0.016
V 4.040
Ni 0.150
Fe balance

I'm specifically wondering about how this variation from the standard spec might behave during heat treat and subsequent use. I have yet to heat treat any stainless steel, so I'm actually starting at the point of looking at datasheets. The Co @ 0.040, Cr @20.850, and Ni @ 0.150 are what intrigue me most.

For those who understand stainless steel metallurgy as affected by small variables like these, how would you expect this steel to differ in a standard heat treat from the published specs?

I am leaning towards the following:
2100° austenitizing,
-100° cryo,
600° tempering.

Without more experience, I think this is the best course of action to start.

With regard to expected hardness, should I expect this specific alloy to come out slightly harder or softer than the published data indicates?

Beyond hardness, toughness, and wear resistance, I am curious about corrosion resistance. I know it will be excellent regardless, but with that extra 0.850 of Cr and 0.150 of Ni, is it reasonable to expect improved corrosion resistance? I understand that it will be an immeasurable difference in use, but I'm still curious for understanding's sake.

Thank you in advance to any and all who can help clarify the situation or make helpful recommendations.

Attila
 
Since the data sheets don’t guarantee a composition range for any of the variables we can’t say if it is “out of spec.” The cobalt and nickel are likely the result of melting scrap. The cobalt will do little at that level, the nickel will have a small improvement to toughness and corrosion resistance but probably imperceptible to the knife maker or end user. However, we don’t know what the typical nickel level is so it might not be an “improvement” but simply an average value.
 
Timely thread. I'd like to heat treat some 20CV I have. It looks like from the crucible data sheet this is what they recommend:
http://www.crucible.com/PDFs//DataSheets2010/Datasheet CPM 20CV.pdf

HARDENING:
Preheating: Heat to 1400-1450°F (760-788°C) and equalize.
Austenitizing (High Heat): Heat rapidly from the preheat to a temperature to within 1960-2150°F (1071-1177°C). A lower austenitizing temperature will maximize impact toughness. A higher austenitizing temperature will maximize wear resistance and corrosion resistance. Soak at the austenitizing temperature for 30 minutes.
Quenching Pressurized gas or warm oil.

For pressurized gas, the furnace should have a minimum quench pressure of 4 bars. A quench rate of approximately 400 °F (222°C) per minute to below 1000°F (538°C) is critical to obtain the desired properties.

For oil, quench until black, about 900°F (482°C), then cool in still air to 150-125°F (66-51°C).

Then cryo treatment then temper.
It looks like your 600 degree temper should give you 61RC with the 2100 austenitizing temp + cryo that you're planning.

Are you going to oil quench? I was always under the impression that this was an air hardening steel.
 
There is always a range for composition !! The melt foremen are fantastic when it comes to making an alloy but adjusting the composition [by the shovel full for tons of steel ] Usually the high or low end of the range can be found but a range is there!
The big part of cryo IS the tempering and that is 300-400 F . If you then temper above that [some have tried 900 F !! ] you destroy the cryo you just did .Cryo gives you eta carbides extra hardness which will be destroyed at higher temps ! Lower LN temps [below the -100 temps sub-zero ] don't give significantly less RA . The extra hardness is from eta carbides and the associated strain fields [cohesion ] !
 
Thank you all for the replies.

Larrin,
I am aware that there are allowable ranges in the specifications of alloys. I am just trying to grasp how differently this sample would behave as compared to an alloy chemically more "pure" with regard to the intended composition. Since we don't know the regularly-present Co and Ni content, it may be a question without answer.

I also figured the Co would have a negligible effect at that low amount, but the Ni and Cr looked potentially impactful. I was initially worried that the increased Cr might reduce toughness, but that could be balanced by the Ni. I won't ever actually know what effects these variations all have, but as I said earlier, this is a learning exercise for me.

Robert,
I am unsure about the quenching portion of it. I have 11-second Mcmaster quenching oil, but the datasheet doesn't specify quanchant speed, so that's still a bit weird for me. I am still looking for data about air quenching, but I want to read through M390 and CTS 204P data sheets or make some calls to get more relevant data on this class of steel before making a decision.


Mete,
I'm not looking for absolute hardness, and the cryo for me is just for reducing retained austenite. I chose the high aust temp to maximize corrosion resistance and the 600° tempering to reduce hardness (increase toughness a tiny bit). Above 700° is where I don't want to go with tempering due to losing corrosion resistance at higher tempering temps.

With regard to "destroying carbides," I thought that shouldn't happen at such low temperatures. The steel's hardness is reduced, but the carbides should still be there.

The above is how I understand things, but you mention "eta carbides" twice. I don't quite understand eta carbides' structure, formation, or dissolution, so their existence muddies the water a bit more. Frankly, they are less important to me at this point because they seem to be a relatively minor benefit achieved at a lower tempering range than I'm planning on. At first I'll stick to 600°, but if I have sufficient material left over, I'll try the lower tempering range you suggest.

If you can point me towards some relevant reading material, I'd gladly learn about eta carbides. Unfortunately, scientific/industrial research documents aren't generally available to us common folk.
 
After a look at the M390 and CTS 204P datasheets, I think an oil quench to black, still air to 150°, and -100° cryo will be my quench method. I'll try my 11-second oil first, but wonder if a slower oil would work better. All three data sheets seem to agree, so I'll follow that proven method despite oil being messier.
 
me2 said:
How are you planning to heat the steel?
Click to expand...

A PID controlled electric oven, so vacuum furnaces and inert atmospheres aren't available to me.

I could route argon or nitrogen gas into the oven, but I couldn't guarantee an inert atmosphere.
 
I got 62.2HRC tested with 2065F 30 min > plate quench > dry-ice subzero for 2 hours > a couple 2 hours 400F tempering.
 
When new steels are developed, the manufacturer determines what the acceptable range of elements would be so that heat treatments and properties will be consistent across the range. If a melt falls outside the range, it is scrapped.

It is a good idea to run a few test coupons to understand where the steel responds with your furnace.

Great steel choice.

Hoss
 
shqxk,
Thank you for a great recipe.

I don't have liquid nitrogen readily available. Therefore, I will be using an acetone/dry ice bath to achieve -108°F/-78°C.
If you're using just solid dry ice, you might consider making a bath to help ensure complete coverage of your items' surfaces during immersion.

Devin,
I have a 36"x2" piece that I aim to get a couple blades out of, so I do plan to doing some testing before committing to a final blade's heat treat. It'll probably be several small kiridashi-sized mules to test edgeholding along with some strips to check corrosion resistance and toughness. It'll be a fun set of experiments for sure.
 
DevinT said:
Awesome, report your findings.

Hoss
Click to expand...

Being a hobby in the learning phase, it'll be a while yet. I'll endeavor to keep detailed notes.

As of yet, I haven't found a local place to get my work's hardness tested, so I'm unsure of how that data will be collected. I definitely want (need) to do this, but the machines are just too expensive, and sending the test pieces out could take weeks to get back. My search for local(ish) hardness testing in MD continues.
 
Finding info about cryo and eta carbides has been a difficult task even for a metallurgist :mad:
The cryo temperature tweaks the matrix and that makes room for the eta carbides to form.[at 300-400 F] That , along with the cohesion gives us the extra hardness. In a manner like the 900F temper , the high heat creates larger carbides as the as the carbides collect more carbon. At a certain size the cohesion is lost and the carbides are absorbing more carbon leaving the matrix soft. That's like the great drop in hardness with the higher than 900F.
 
shqxk said:
I got 62.2HRC tested with 2065F 30 min > plate quench > dry-ice subzero for 2 hours > a couple 2 hours 400F tempering.
Click to expand...
Thank you shqxk
FYI attila
I just heat treated a coupon using shqxk's protocol (I have an Evenheat oven) and got 64-65 after plate quench. It's in LN right now. I'll temper and retest it.
Thanks again for starting this thread, there doesn't seem to be too much info on 20CV HT other than the spec sheets.
 
Robert Erickson said:
Thank you shqxk
FYI attila
I just heat treated a coupon using shqxk's protocol (I have an Evenheat oven) and got 64-65 after plate quench. It's in LN right now. I'll temper and retest it.
Thanks again for starting this thread, there doesn't seem to be too much info on 20CV HT other than the spec sheets.
Click to expand...
I'd sure like to know the RC after LN, we're are you going to start with temper?
 
Kentucky said:
I'd sure like to know the RC after LN, we're are you going to start with temper?
Click to expand...
Took it out of LN and checked. It still read in the 64-65 range. After first 400 temper it's 61 to 62. I'll do another at 400 and check again.
 
Just a comment:
Cryo is at -300°F and lower.
Sub-zero treatment is at -100°F.
With your parameters and available equipment, I would suggest following shqux's regime. I would highly recommend quench plates over the oil quench. You don't need gas in the oven, just wrap the blade in the higher temp 309 stainless foil. You can leave it in the foil packet while doing the plate quench.
 
This is excellent info Robert, thank you!

I was curious how my intended -108° will compare to your LN cryo. Since the data sheets say to cryo at -100° (-95° for M390), I don't think I'll need to use nitrogen.

Robert & Stacey,

With regard to plate quenching, do you grind the main bevel after heat treat? I prefer grinding it before heat treat to preclude the possibility of ruining my heat treat. I ask because I don't see how plate quenching could draw heat out of a ground bevel fast enough since the plates aren't touching the ground surfaces. This is why oil was my intended heat sink.
 
I think you have a bad idea about oil, well dimensioning quenching plates makes it work far faster.
 
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