Decided to make a die for forging bowls and came across a 6" diameter 3" thick piece of 4140 cheap.  I machined the top and bottom for different  size top dies. Having never heat treated anything this large, I've got a few questions.....How long a soak at high heat (1550F) ?   Should I water quench or use the six gallons of aviation hydraulic fluid that I have?  Is 6 gallons enough??  Any other suggestions are appreciated.............

Generally you are looking at 1 hour per inch of thickness. A big smooth piece probably won't crack with a water quench. How are you going to control scaling? That long without an atmosphere can wreak some havoc.6 gallons will be pushing it in my opinion. I would use a 55 gallon drum of water myself. Better to have too much than not enough. What is the flash point of the oil you have? Hate to see you on the Prayers List anytime soon.

Why Heat Treat?? What are you expecting a 2" thick piece of 4140 to deform into??

Not being nosey, Just asking.

 

Neil

I would wrap in stainless foil to minimize scaling. Biggundoctor hit the other salient points - although I would try oil before water or brine because you can always HT again if too soft.

The hydraulic fluid is fire resistant mil spec given to me by a bladesmith. Flash point is 501 deg F. I want to do this right so I don't have to do it twice LOL. Where can I get SS foil?? Thanks for the help....Keith

MSC is one place to get heat treating foils. Not inexpensive, but one way to control scaling. I use it in the small oven at work, and I put some paper in with the parts to consume any oxygen in the pouch as it burns. You could use anything that will take the heat as well as seals out the oxygen. Some old techniques used an iron box filled with charred bone,salts,etc. The whole idea is just to keep any oxygen from the surface while it is at temp. For small parts that I have to HT with a torch I use silver soldering flux to keep them clean.

6 gallons should  be just barely enough.  Make sure it is in a steel container with a steel lid.  I would guess that heavy a piece will heat the oil up to 350-450F. 

Heat for 3 hours. I wouldn't work about the scaling. Let it happen and grind it off. You won't be able to get the stainless foil off before you quench and it will act as an insulator counteracting the effect of the quench. Plus it is expensive. For a 19# piece you really need much more quench fluid. I'd suggest at a minimum 20 gallons. For 4140 I'd quench in a light weight oil. Something on par with vegetable or peanut oil. If you have a lot of work already in this die, just go pay a commercial heat treater to do the job. They typically have a minimum fee then charge by the pound. For your job you'll probably end up paying just the minimum and that should be in the neighborhood of $50. That's a small investment to know the job was done properly.

 

Patrick