Effects of ramp rate on heat treatment

[Matt Rochester]

So I did my first double batch of knives Saturday. I typically can only do 7 knives at a time and had 10 to do so I split them up and did 2 batches of 5 knives each. The steel is cpm 154 and I equalized at 1400 and then full ramp to 1950. The initial ramp took about 25 mins to go from 1400 to 1950. Now, on the second batch I let the oven cool to 1400 again and equalized then ramped to 1950 only this time it only took 10-15 mins. With the ramp time being cut in half would this make any difference metallurgically? I am waiting on a on a friend to call me back so I can test the hardness so I don't have that yet.

 

[Stacy E. Apelt - Bladesmith]

Not an issue. Ramp time was probably faster because the refractory was fully soaked.

 

[JTknives]

I have found that if I ramp the oven with the steel in it then it over shoots by quite a bit. My oven heats very fast so I have to tone it down to around 1000°/hr. If I'm tempering I just let it soak for an hr befor inserting blade. I found this out by axadent on a finished Damascus blade. Did my first temper after the oven was hot. Then I left the blade in over night and in the morning I woke up and did the second and just fired it up to 400° for 2hrs and when done I pulled it out and it was blue. It sucks as its sitting at around 57-58rc but not a total loss as the knife I sadly have to say is for looking at, customer will not use it so he is ok with the slightly softer blade.

 

[Matt Rochester]

Thanks. I checked them today and they came on at 60.5-61.5rc so it didn't make a difference in terms of hardness. I guess the first batch got a little longer soak time since the austenitizing temp is 1900-2000 and I soak at 1950 for 30 mins. Anyway, thanks for the responses as I was just curious.

 

[Darrin Sanders]

I never do a pre-heat. I let the furnace equalize at the Aus. temp. I will be using, put the blade/blades in, let the furnace cycle back to temp., & start my soak. I have no problem hitting my target hardness and my knives perform as expected. Personally, I believe that pre-heat cycles are for objects thicker than knife blades.

 

[Matt Rochester]

I never do a pre-heat. I let the furnace equalize at the Aus. temp. I will be using, put the blade/blades in, let the furnace cycle back to temp., & start my soak. I have no problem hitting my target hardness and my knives perform as expected. Personally, I believe that pre-heat cycles are for objects thicker than knife blades.

My understanding is it helps with warping but since I plate quench I don't know that it is necessary.

 

[Stacy E. Apelt - Bladesmith]

The pre-heat brings the steel into the austenite range and allows it all to become austenite fully before heating to the range where the alloying gets distributed. Since austenite is far more flexible than martensite or pearlite, this allows the blade to release stresses that may cause the blade to warp as it heats to 1900-1950F. Placing directly into a 1900+ oven could possibly cause part of the blade to be austenite while other parts are still martensite or pearlite. It won't ruin the blade, and if you use quench plates, it will be straightened out, but metallurgically, it is a sound practice. If hardening complex shapes or parts, the pre-heat is very important to avoid distortion.