Plasma arc cutting 4140 that has been Heat Treated at the mill?

[terryt]

Hi:

Can you Plasma arc cut 4140 that has been Heat Treated at the mill with out any problems ?
Is it and different than any other heat treated metals. I know it has to be heat treated later.

Thanks,

Terryt

[EDG]

Your question is very wide open and does not describe the size of the material or the heat affected zone.

Heat treated 4140 is not all that hard as supplied from the mill. It can be cut with normal machine tools.

If you have to plasma cut it the heat affected zone may be very hard. If the material is not too thick that zone could be ground away and the material could be used without further heat threat. I am assumming this is plate or heavy bar stock.

If cut with a saw you simply finish machine and use as is
If you require higher properties than what the factory ships consider changing the design so no post machining heat treat is required.

[terryt]

Hi:

The material is 1/4 inch thick. I was thinking of plasma arc as otherwise I would have to use a saws-all.
I will be using this to make a Chicopee rifle action.
Thanks,

Terryt

[CastingFool]

My experience with 4140 tells me it's tough enough not to require heat treating, but then again, I wasn't making gun parts.

[leftiye]

Unless you're quite good with the plasma cutter, cleaning up the mess at the edges might be more work than the sawzall. Certainly grinding off the hardened material won't be any fun (and you need to not further harden it with the grinder).

[BABore]

4140 PH (prehard) runs about Rc 28-32 and is easily machined with carbide. If you want it cut halfway precise, without changing the hardness, get it waterjet cut. The place we use can go up to an inch thick.

[country gent]

We had a guy at work that would cut a1 drill rod with the chop saw and then wonder why he couldnt drill the end. I would expect as long as you didnt quench it after the plasma cut that the 4140 would slightly anneal alnog the cut, as annealing it is heating and soaking for a period of time at a set temperature. Oil or water quench ing may greatly harden the edges.

[oldred]

Actually carbide is not required to cut 4140HT, I use HSS end mills often for this material but carbide will of course last longer.

Terry I would not suggest 4140 for the Chicopee action because of the difficulty that will be encountered welding the parts together, 4140 is weldable but it's a bit complicated and in order to do it and achieve anywhere near the properties of the original material can be quite involved. For sure any semblance of the original pre-hard properties would be destroyed and unless done properly (proper pre-heating, maintaining proper inter-pass temperatures, proper filler material, etc) under-bead cracking and thus loss of strength can be a serious issue with 4140. In any event for a receiver welded together with this material proper stress relieving and post weld heat treating would be necessary and I think you might have problems finding a place that would do that work due to liability issues and it's not something you could do at home. IMHO a much better choice would be 8620, easily machinable, nearly as strong as the 4140 (probably even more so than a welded 4140 receiver) and easily case-hardened after finishing. Of course even 8620 is going to present some problems with welding and any welds should be attempted only by a competent welder very familiar with the properties of 8620 and the proper welding of same. A simpler choice would be something like 1026 carbon steel, very easily weldable by common welding process, TIG, MIG, Stick or even Oxy/Acetylene using almost any of the 70 series fillers. The 1026 would not require any post weld stress relief on parts of this size nor would it require any specialized procedure except for general good welding practices. While 1026 would not be quite as strong as either of the alloy materials mentioned (but actually stronger than steels used earlier, before the 1930's, for this type of receiver) it should be just fine for a rifle receiver utilizing one of the lower pressure chamberings which would include the 22 Hornet that I think you have mentioned, this material is very easily case hardened and can even be color-cased if so desired to make it durable (as can the 8620 but not 4140), overall it will be a lot simpler to work with.

[uscra112]

I thought the Chicopee was designed to be silver-soldered together. Book was lost in the last move, so can't check it.

If I ever hit the lottery, I'm buying a wire-EDM for parts like that. Hardness matters not, and accuracy and finish can be superb.

[oldred]

I thought the Chicopee was designed to be silver-soldered together. Book was lost in the last move, so can't check it.

If I ever hit the lottery, I'm buying a wire-EDM for parts like that. Hardness matters not, and accuracy and finish can be superb.


I have only recently become familiar with the Chicopee but I found it quite interesting and I have been doing a bit of research on it, unless I am mistaken the Silver-soldered parts are only for the RF version???? Could be wrong about that but in any case I would personally WELD those parts with 70XX filler, it's so simple to do with normal carbon steels and would be so much stronger I can't imagine why anyone would want to use Silver-solder?

[terryt]

Hi:

Thanks everyone.

Terryt

[MtGun44]

First off, it is not likely that the 4140 was heat treated at the mill, unless it is being sold as a specifically
HT grade - which is available. Most steels come hot rolled or cold rolled, but some are heat treated
at the mill, and 4140 would be a candidate for this. Hot rolled will leave the steel soft, not quite as soft as
annealed but close. Cold rolled will work harden the steel, and this can vary, depending on alloy and what
the mill decides to do. It will be harder than hot rolled. 4140 is quite strong and tough, even in the
annealed form, but if you want extremely high strength, you should be heat treating it after machining.

Recently made a replacement choke for a Cutts compensator from cold rolled 1.25" 4140. Machined just
dandy with HS cutters, including rough boring although my good boring bars have carbide inserts so the final
internal cuts were with carbide. All the external machining was with HS cutters, including the threading and
it cut very nicely.

If I was making an action, expecially if I was certain that the mill had heat treated the alloy and that this mill
heat treat was all that I wanted - meaning no final heat treat would be done after all machine work, then I would
absolutely NOT mess with any sort of cutting other than a sawzall, bandsaw or even hand hacksaw unless I
planned to have it heat treated after finish machining.

No question there will be a softened zone at the plasma cut, not something helpful when making a gun part.

I just looked up the yield stress for 4140 in various conditions:

Hot rolled: yield = 62,000 psi failure stress = 89,000 psi
Cold rolled: yield= 90, 000 psi failure stress = 102,000 psi
Mill heat treat: yield = 131,000 psi failure stress = 153,000 psi

So, any heating above something like 600-700F will take away some of the heat treated strength,
lowering it down towards the hot rolled condition. You can easily cut the strength in half with
a melting type temp like a plasma arc - but only at the cut for some unknown distance depending
on how hot it gets.

Bottom line - use a saw or water jet if you really do have HT or CR grade 4140. If HR, no problem.

I jus thought of a good real world situation that applies to this. Plasma arc cut AR500 steel plate targets
will withstand .223 and .308 FMJ hits for many years with no significant damage - except when they take
a hit near the edge - and then a chunk is ripped out due to the local loss of hardness, maybe 3/16 of an
inch or a bit less.

Bill

[uscra112]

Wire EDM, if you can afford it, will cut 4140 prehard right to size, needing very little work afterward. You can even cut passable gears on a wire machine.