Quick 4140 Heat Treat Question

sargentcrunch · Feb 18, 2020

When you guys heat 4140, do you put the part in the cold oven and bring up to austenitizing temperature? Or do you preheat the oven to austenitizing temperature and then put the part in, and wait for the part to reach temperature. Or do you preheat the oven below austenitizing temperature, put the part in, let it equalize, then ramp up?

TDegenhart · Feb 18, 2020

The proper way is to preheat the part to 1200F for 10-15 min or until uniformly heated, then bring it up to the austenizing temp 1570F. The method of reheat is not important but it is not good practice to put a completely cold part in a hot furnace. Prewarm the part first to 200-300F.

Tom

sargentcrunch · Feb 18, 2020

TDegenhart said:
The proper way is to preheat the part to 1200F for 10-15 min or until uniformly heated, then bring it up to the austenizing temp 1570F. The method of reheat is not important but it is not good practice to put a completely cold part in a hot furnace. Prewarm the part first to 200-300F.

Tom

Ok thank you! So just to verify, you put the prewarmed part in, then ramp up to 1200f and let the part 'equalize' so to speak. Then ramp up to austenitizing temperature? Sorry, asking a lot of questions, but I always like to know exactly what I am doing when I do something new, makes situations go smoother haha

Forrest Addy · Feb 18, 2020

The mass and surface area of the part also have to be considered. The material I've read instructs one to start with the work in a cold furnace, ramp at so many degrees per hour to process temp, hold, etc. This assumes your furnace is equipped with a process controller but nudging the temp control knob has worked for many years before fancy electronics came along.

Speaking of: process temp controllers and thermocouples are quite cheap on eBay and simple to retrofit to a cheap electric pottery kiln re-purposed for heat treating..

Knife makers can usually avoid the fuss because their work is usually small, thin, and of large surfact to volume ratio. Torch heat if carefully controlled is sufficient for all but the most sensitive steels.

cyanidekid · Feb 18, 2020

sargentcrunch said:
Ok thank you! So just to verify, you put the prewarmed part in, then ramp up to 1200f and let the part 'equalize' so to speak. Then ramp up to austenitizing temperature? Sorry, asking a lot of questions, but I always like to know exactly what I am doing when I do something new, makes situations go smoother haha

There isn’t one answer. If for example you are making a small thin part that has to have a good surface and you are trying to do it without an atmosphere controlled furnace, the least time at temp Is the obvious way to go. ( yes use tool wrap )

If on the other hand, you are doing a 300lb part with some thin flange or other projection then you would want to go slow whenever you have the option.

If it’s just a chunk o something and it’s getting surface ground it just doesn’t matter.

Time at temp is obviously going to influence grain size, but time up to temp, an it’s oxidation, and chance of differential heating and possible distortion that is the consideration I’d think?

sargentcrunch · Feb 21, 2020

It seems like from my research, hold at austenitizing temperature for 1 hour per inch of thickness. So for a .25 wall thickness tube, I would hold for about 15 minutes from what I gather.

guest · Feb 21, 2020

Depends on the furnaces too. We never shut ours down.

The temp is going to fall when the door opens. We held the basket in the vestibule to preheat it, then transfer into the furnace. Start timing when it's back up to temp. I think we usually did something like 1 hour at temp plus 1/2 hour per inch.

These were internal quench atmosphere furnaces.

sargentcrunch · Feb 21, 2020

guest said:
Depends on the furnaces too. We never shut ours down.

The temp is going to fall when the door opens. We held the basket in the vestibule to preheat it, then transfer into the furnace. Start timing when it's back up to temp. I think we usually did something like 1 hour at temp plus 1/2 hour per inch.

These were internal quench atmosphere furnaces.

How big were the parts you were doing? The first part I have to do is essentially a 4 inch long tube with .25 inch wall thickness. My biggest concern is overcooking it and ruining the part. Granted, its a really easy part to make, but I would rather not remake it anyway lol

guest · Feb 22, 2020

sargentcrunch said:
How big were the parts you were doing? The first part I have to do is essentially a 4 inch long tube with .25 inch wall thickness. My biggest concern is overcooking it and ruining the part. Granted, its a really easy part to make, but I would rather not remake it anyway lol

Much larger cross sections than what you are working with. Our stuff went up to about 8" in diameter and 60" long, sometimes longer,

You mentioned .25 wall, but not the diameter.

If it's not too big, it might be a good candidate for induction hardening. If you're gonna do it in a furnace, it's not going to take much time to come up to temp. 1/2 hr at temp should be good- I'd probably double that if it's SS wrapped and no atmosphere.

I didn't work on that side of the company, I ran the manufacturing side. I learned a lot, but I'm not a heat treating expert, more like I know enough to be dangerous...

sargentcrunch · Feb 22, 2020

guest said:
Much larger cross sections than what you are working with. Our stuff went up to about 8" in diameter and 60" long, sometimes longer,

You mentioned .25 wall, but not the diameter.

If it's not too big, it might be a good candidate for induction hardening. If you're gonna do it in a furnace, it's not going to take much time to come up to temp. 1/2 hr at temp should be good- I'd probably double that if it's SS wrapped and no atmosphere.

I didn't work on that side of the company, I ran the manufacturing side. I learned a lot, but I'm not a heat treating expert, more like I know enough to be dangerous...

Oh my bad, its 1 inch diameter, .25 inch wall thickness. Its going to be done in a furnace, I am not going to use SS foil but ATP anti scale coating so I don't have to fumble around with foil before trying to quench haha

Nerdlinger · Feb 22, 2020

We put parts from room temp right into the up-to-temp furnace. A trick an old Italian die maker that I used to work with used regularly - wrap the parts in newspaper. The paper will burn and use up the oxygen in the furnace that would otherwise react with the surface of the part. I cannot say if it really worked but it *sounds* like a good idea. Also, our furnace is tiny...maybe 12” x 12” x 12”. Good luck!

sargentcrunch · Feb 22, 2020

Nerdlinger said:
We put parts from room temp right into the up-to-temp furnace. A trick an old Italian die maker that I used to work with used regularly - wrap the parts in newspaper. The paper will burn and use up the oxygen in the furnace that would otherwise react with the surface of the part. I cannot say if it really worked but it *sounds* like a good idea. Also, our furnace is tiny...maybe 12” x 12” x 12”. Good luck!

The newspaper thing is interesting, I was thinking about something like that earlier, might give that a try on a test piece to see. Thanks!

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