How To Heat Treat Q.. soak time during hardening cycle

[HSC ///]

If I understand correctly, grain growth concern during a hardening cycle is a function of temperature AND time....

so if a cycle calls for a 10 min soak at some temp, does increasing the soak time to 15 minutes have an adverse effect on the steel's grain growth? Is there some other ill result?

thx
Harbeer

 

[imill3567]

Based on what I’ve been reading in knife engineering, fantastic book, if they are cycling steps to reduce grain size it’s best to keep them short but if it is the hardening step and you’re looking for higher hardness it probably won’t hurt. The higher hardness would come from carbides dissolving increasing the carbon content of the austenite and resulting martensite. Can always break some test pieces and compare grain size to the extent you can see with your eye at least.

 

[DevinT]

Temperature is the main predictor of grain growth, it requires lots of time to grow the grain. 5,10, or even 20 minutes won’t make any difference, especially if there are carbides which pin the grain boundaries.

Hoss

 

[HSC ///]

Temperature is the main predictor of grain growth, it requires lots of time to grow the grain. 5,10, or even 20 minutes won’t make any difference, especially if there are carbides which pin the grain boundaries.

Hoss

thanks, that's what I needed to know

I did read Larrin's 13 myths where he addresses this - "
Some knifemakers are afraid of holding too long at austenitizing temperature because of concerns about grain growth. However, grain growth is controlled by both time and temperature. Recommended heat treatments often provide longer hold times at lower temperatures where significant grain growth would take a very long time. The longer hold times at lower temperatures means both thick and thin parts will result in a similar heat treatment. Sometimes datasheets will provide different temperatures or hold times depending on the thickness of the piece, but that is more of an economical feature (don’t waste any more time than necessary) than one that is required for performance.

There are other dangers to using short hold times, such as the heat treatment being much more sensitive to small changes at short times."
**********************************************************
The reason I'm asking is because of inconsistent results with 52100 laminated steel that has been forged (by me)
Even running the proper cycles before, I get inconsistent and unreliable results. this is the only steel I struggle with to heat treat, but only the san mai billets, not the stock removal (i.e. coupons)

I just did the same gyuto heat treat now for the 3rd time upping the soak to 20 minutes and finally getting the hardness I want. I get that this is a function of my furnace and setup etc..

In working with Tim Wright, who as you know has been making knives a long time and doing his own heat treating, I noticed most of his soak times are quite a bit longer than some of the guidelines I read. But he hits the desired HT every time perfectly. Even last night he said to me - don't be cheap with the soak times....

Harbeer

 

[Stacy E. Apelt - Bladesmith]

I use a factor of ten to describe the difference between time and temperature.

Raise the temperature from 1470 to 1480 and it has "X" effect on grain.
Extend a 10 minute soak to 100 minutes and you would get the same "X" effect on grain.

At carbon steel HT temps time is a very minor part of grain growth. It is at temperatures above 1800°F that grain growth is rapid.

 

[Larrin]

I use a factor of ten to describe the difference between time and temperature.

Raise the temperature from 1470 to 1480 and it has "X" effect on grain.
Extend a 10 minute soak to 100 minutes and you would get the same "X" effect on grain.

At carbon steel HT temps time is a very minor part of grain growth. It is at temperatures above 1800°F that grain growth is rapid.

For high alloy steels grain growth is still slow at 1800F. It depends on the temp where the carbides are dissolving.