Heat treating 440C looking for info

R

Rebel_Blades

Member
This worked well the last time, so lets see if it does again. I am sure this question has been asked and answered here before. Anyway here goes. I got a piece of 440C free from New Jersey Steel Baron, and ground out a knife. Everything is good, till I got to heat treating. I am sure this is my fault, I just need to know what is the right way. I placed the knife in the oven, then punched in 1900 degrees and let it go. It takes my oven about an hour and a half to get to 1900. Once there, I waited 10 minutes then took it out. I used an oil quench, I have since found out this is not needed with 440C. Anyway I ended up with a crispy critter that is non magnetic.

I have found that I need to wrap the knife with foil. Fine not a problem I ordered some wrap last night.
I have also found that I do not need to oil quench, air quench is fine for 440C
What I need to know is the correct procedure to heat a knife made of 440C ? Knowing that my oven takes an hour and a half to get to 1900 degrees ?

Thanx in advance !
 
Wrapping or coating with something will reduce or prevent scaling. I took a quick trip over to Alpha Knife Supply and took a look on their site for heat treating instructions. It says to preheat to 1400° long enough to make sure it's evenly heated throughout then ramp the temperature to 1850-1950°. Oil, plate, or air quench to harden. I'm at a loss to say why you ended up with non-magnetic steel. For grins and giggles, is there a way you can put that in a bath of acetone and dry ice to see if you can drive retained austinite through conversion to untempered martensite? Maybe even a liquid nitrogen bath?

I don't work with stainless, but if I did, I would heat the kiln to 1400° empty and then put the knife in when the temperature was reached and let the kiln recover the it's temp and hold for about 5 minutes then ramp it up to 1900° degrees with the blade inside. Because a knife is thin and irregularly shaped, I would, as a general rule, quench at the slowest speed that would adequately harden the blade, in this case that would be air or plate quenching.

Hopefully someone who works with 440C can give you a better idea of how they handle it.

Doug
 
Doug,
I am done with this knife, I totally ruined it beyond any kind of salvage. It is warped and twisted, the metal appears to have started to melt actually. I did put it on the belt grinder just to see if it had retained anything like the finish I put on it before hand, and that is where I found the small melt marks. I know it has to do with the amount of time it was in the oven. I am just unsure of how long or where to put it in the oven to start. To be honest 1400 seems a tad low as it takes the oven 30 minutes or more to make it to 1900 from there.
But if I were to use 1400 as a start point, I would probably need to punch in the 1900 right away ........ I would think anyway.

How long do other ovens/kilns take to reach 1900 ??
 
Is there any possibility that the steel could have come into contact with a heating element? You shouldn't have anything that looks like melting unless the steel got a lot hotter than 1900°.

The reason behind preheating to 1400° and then proceeding on up to 1900° is to let all the steel in the mass austinize at close to the same time and not from the outside in, which could set up some cracking with the two areas of the steel being at different phases. This could be less important with something as thin as a knife blade.

To know whether or not your kiln is slow we would have to know what size it is and what it rated for electrically.

Doug
 
Doug, Well I did not get as many replies as last time that is for sure !! LOL Anyway the kiln is one I built it measures inside 4 inches square opening, and right at 17 inches long. It is running 110. I may have to bump that up to 220 but I am gonna try and make this work before I do. The kiln goes to about 1500 with no problem within half an hour to 45 minutes. Then takes another half hour to 45 minutes to get to 1900. Maybe with it moving slowly I am shooting too high ? Maybe I should start to count my soak at 1850 ?
I am thinking I am gonna go with a 1500 degree start point then use the 1850 as my finish. That should keep the knife from being in the heat too long. What are your thoughts ? Just bouncing ideas around now ........
I think before I do this I will give NJ Steel Baron a call as well.
 
There is nothing wrong with 90 minute heat up time. That's close to par for the Evenheat 120V kilns. You do need decarb protection - preferably foil. If you put that foil in a 1400f oven, air expansion could be so violent as to pop the envelope. I put my wrapped blades in a cold oven - at least not over 500f. Ramp straight to full temp - no need for a presoak with knife blades. I use 1875 for 440C, but 1900 is fine. It should soak at temperature for at least 20 minutes and better 30 -40. If you can leave it in the envelope and quench between thick plates, you will get cleaner blades. If no plates, you can remove it from the envelope to air cool. It will aquire some oxides that can be cleaned up later. Cryo if you are going to and then double temper.

I am concerned about your temperature accuracy. Have you verified it?

Rob!
 
I have no idea how to verify 1900 degrees. I have however verified the thermocouple is accurate at ambient temp.

How would I verify 1500 or 1900 ?

I do have another thermocouple .......... But it would be difficult to change it out.

The controller is digital ..... info in output out
 
If you have a second thermocouple, I take it that it's rated for at least 2000°, all you will need is to get a second pyrometer or PID unit and run both to check the units against each other. I've gotten pyrometers for less than $60 on Ebay.

Doug
 
I keep a separate pyrometer to test kilns, but that may be a bit overkill for you. If you know someone who is a welder, you may be able to borrow a couple tempilstiks. There is a thread on here soomewhere about various temperatures at which some things melt. Your thermocouple is likely fine, but thats a pretty tight chamber and I'm wondering if there may be hot and cool spots. I agree that the blade was likely trash after all that time unprotected, but your description of 'melting' is of concern.

Rob!
 
I will go out and grind on this thing some more to see if what I thought was melting was actually melting.

Hot and cold spots ? I seriously doubt it. I have a double row of coils running down both sides. There may be a small cold spot up towards the door, since that is the only place unsealed with furnace cement/mortar. The blades go in handle out. The part that has me is the non-magnetic.

I will try again with the foil.
 
Ok I now know what the melting was .......... It was not melting, however it did loose metal on both sides of the blade. I used a piece of fire brick with slots cut in it to support the blade. I am guessing that while in the oven the brick heated up and pulled more carbon out of the steel ??? Anyway the fix for that is make it thinner and move it back to where the handle is in case it does it again.

The steel is still non magnetic however and I have no explanation for that ...........
 
I'll bite. You've got good info on the foil. My guess is that you'd be better off at 1850 and then to use a dry ice or nitrogen cryo. Austentite is non magnetic. Lowering the temp a little and adding cryo will take care of the austentite, and therefore the magnetism.
 
Looks like a nice build. I'm sure it will work better with a good foil envelope. The only thing to watch with your fire brick blade rack is the possibility of the envelope expanding and getting stuck in the brick. :34: That leaves you pulling out a hot blade and envelope - stuck in a blade holder brick - which is sure to fall on the floor - where it will either land on the paper owners manual for your left handed frammis - or on the toe of your running shoe where it will stick to the rubber.

Please don't ask how I came to this conclusion.
 
Point taken. But that would be another very good reason to move it back so the handle is where it gets supported. I could if need be use something to hold the rack down with. I have already widened the slot in anticipation of that exact problem. Still need to make it thinner however.
Thanx for all the help on this.
 
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