"Steel needs to be rested"

[PEU]

So I was watching a video on YouTube with an interview to Vincent, the sharpener at the famous Korin store in NY when I stumbled upon the post title statement and how Mizono, a kitchen knife japanese brand, ages its steel before shipping it to customers.

It sounded like BS, but since you never stop learning, maybe this is true, so guys, please tell me if this smells or not please...

Skip to 7:00 for the comment about "knives needs to be rested"

Pablo

 

[DevinT]

It’s true, steels grow when heat treated, especially carbon steels. They will continue to move for a year. It will show up most in laminated blades because the low carbon cladding does not move as much as the core.

This falls under “distortion in heat treating”, it’s something that tool and die makers deal with. One reason tool steels are cryogenically treated is to stabilize them against movement. Retained austenite wants to convert to fresh martensite which expands.

Whole books are written on the subject.

Hoss

 

[Joel Mercier]

Can we safely assume that a cryo treatment will do as much, if not more, than letting the blade rest for a year?

 

[ten-six]

steels grow when heat treated

How much growth are we talking about here? Could it be measured with something like a mikrokator indicator or is this at the atomic level?
I can understand that precision would be imperative to a die maker but to a knife maker this seems like overkill, big time.

 

[DevinT]

How much growth are we talking about here? Could it be measured with something like a mikrokator indicator or is this at the atomic level?
I can understand that precision would be imperative to a die maker but to a knife maker this seems like overkill, big time.

It’s about .0015-.002 per inch for carbon and low alloy steels. On a single bevel blade with only two layers it can cause severe warping.

Hoss

Edit: a ten inch blade would grow .020

 

[DevinT]

Can we safely assume that a cryo treatment will do as much, if not more, than letting the blade rest for a year?

It will eliminate most of the movement but not all.

Hoss

 

[weo]

On a single bevel blade with only two layers it can cause severe warping.

In your opinion, Devin, when using 1080/15N20 (and/or O1), what is the minimal number of layers to make this a non-issue. Or would you recommend makers who forge their own damascus do a cryo treatment to minimize warping in future years due to this movement?

 

[DevinT]

In your opinion, Devin, when using 1080/15N20 (and/or O1), what is the minimal number of layers to make this a non-issue. Or would you recommend makers who forge their own damascus do a cryo treatment to minimize warping in future years due to this movement?

It’s a non-issue unless there are welding flaws.

Hoss

 

[weo]

Thanks Devin. One less thing to worry about. What do I owe you?

 

[PEU]

Thanks Devin, now I wonder, if this is a real/common issue, why I don't keep receiving calls from old customers about warpage... My knives are almost all thin, in the 2mm / 0.08" range.

Pablo

 

[DevinT]

Thanks Devin, now I wonder, if this is a real/common issue, why I don't keep receiving calls from old customers about warpage... My knives are almost all thin, in the 2mm / 0.08" range.

Pablo

It’s only a problem with ni-mai and san-mai laminated blades. Mono steel blades properly heat treated won’t be a problem.

Hoss

 

[tkroenlein]

So the amount of movement is relative to the % of RA?

 

[DevinT]

Austenite is smaller than tempered martensite is smaller than newly formed martensite. Some steels with large amounts of retained austenite grow very little when hardened.

Hoss

 

[coldsteelburns]

I remember learning about this in an older Murray Carter video, although I thought it mostly only applied to ni mai blades. Always interesting to see the methods and processes used by bladesmiths in other cultures.

Here's the one I'm talking about, where it's discussed a little @ about 10:00 in:

~Paul
My Youtube Channel
... (Some older vids of some of the older knives I made)

 

[PEU]

It’s only a problem with ni-mai and san-mai laminated blades. Mono steel blades properly heat treated won’t be a problem.

Hoss

One of the steels I like the most is VG10 and one of the variants I use is a 1.5mm (1/16") sanmai from Takefu, warpage is never a problem, you can straight it cold because the sides do not harden, it only takes patience.

Pablo

 

[JTknives]

Can we safely assume that a cryo treatment will do as much, if not more, than letting the blade rest for a year?

Not sure if this holds water so to speak but I can bend an aebl blade after a cryo quench by hand. Some times I have to tweak blades by hand after cryo and befor tempering. Aebl is the only steel I will do this to.