Steel Question

[Way-Barney]

If I have a steel where Mf is below room temperature, would the RA disappear over time at room temperature and how long could it take or is Ra fixed?

 

[Larrin]

More typically the RA stabilizes at room temperature, which also makes delayed cryo less effective. To reduce your RA you either need to cool down further or heat up enough to destabilize the austenite and thus form more martensite upon cooling.

 

[Stacy E. Apelt - Bladesmith]

AS always, Larrin is te man with te answer.

Fort this situation, use a very cold freezer, or a DI bath. If none of that is available, use the house freezer and then do three tempers, with the freezer between them.

 

[Ken H>]

If none of that is available, use the house freezer and then do three tempers, with the freezer between them.

Do I understand you're saying Quench, freezer, temper, freezer, temper, freezer, temper? 3 times in freezer and 3 tempers?

 

[Way-Barney]

Thanks for the replies,
I had realized that temperature was more important than time, but had not identified that RA stabilized and its implications. I cannot justify the purchase of a cryo system and even dry ice at £50 a shot is impracticable for the few knives I make and even fewer in stainless.
To further pick your brains, which would be the best stainless steel to use considering the above limitations, my home freezer will get to -32c. I have in stock the following
AEB-L
13c26
12c27
14c28n
N690
I also have a bit of z-wear

I will give that process a try Stacy thanks.

 

[Stacy E. Apelt - Bladesmith]

-32°C/-25°F will help lower RA in many steels.

Ken, et. al.,
The initial quench from austenitization will convert most of the austenite to martensite. The unconverted austenite is called retained austenite, or RA.
The freezer is the bottom of the cooling curve, and should be done immediately after the blade reaches room temp.
This will convert as much austenite you can at that point.
Tempering will make the new martensite less brittle. There will still be the RA.
Upon cooling from temper in the freezer some of the retained austenite will convert to new martensite as it cools to the freezer temp.
The next two cycles will reduce the retained austenite as much as will be possible at the temperature the freezer allows.
The last temper can end in the freezer or at room temp. It should not make any difference.

People worry a bit too much about small amounts of RA. It isn't as bad in a knife blade as most people think.

 

[Ken H>]

Stacy, Sorry I missed your response in this thread until this morning when I saw your post in the wood ID thread - you have a double quote with me included.

Yep, now I understand what you were saying, which is what I normally do. quench, -5F freezer for a few minutes to cool before LN. Then 1 hr (or 2) temper, cool to room temp, then a 2 hr temper. I've never bothered with that 3rd temper, just didn't seem like the tiny tad or RA was a problem. The idea of freezer before LN is so I can get blade to LN ASAP after quench without putting a warm blade directly to LN.

 

[Seedy Lot]

If you are concerned about RA you can also use a lower austenetizing temperture.

With 14C28N coupons I reached same hardness with freezer and cryo when using a 1900°F austenetizing temperture.

 

[Way-Barney]

If you are concerned about RA you can also use a lower austenetizing temperture.

With 14C28N coupons I reached same hardness with freezer and cryo when using a 1900°F austenetizing temperture.

What HRC did you get at that?

 

[Seedy Lot]

61 Rc with 300°F temper