Down and dirty method for heat treating 4140 part

[fastline12]

We have some parts that our customer must have by Sunday. We will not have time to get them to a heat treater. The part is not structural but requires really only a case harden for wear resistance. I know, 4140 is a bad choice but not our call.

I am curious what method we can use to Get in the HRC 45-50 range with a part that will not be super brittle? I was considering a oxy acet torch to bring them up to a target "color", quench in a decided bath, then possible try to temper a bit with a torch.

I realize this is not exact science but not a super critical part. Part is about 1.5x1.5x.50. Flame might even work to our benefit by not heating the core as much?

 

[redpicker]

Your best bet is to let your customer know they are un-hardened and let them decide if they want them. Unless you have experience with flame hardening, you are not likely to be successful and are likely to produce a part that will fail quickly.

I am not sure why you say 4140 is a bad choice. Depending on the application, just like any other material, it can be an excellent choice (or a horrible one, it all depends on information you have not provided).

Are you trying to harden the 0.5 wide surface, or one of the 1.5 wide surfaces? How deep do you want to harden?

If you want to try and flame harden, you will need some kind of temperature measurement to let you know when you reach around 1700F. Going for "color" is going to be difficult if you don't have considerable experience. The problem is, if you are too low, it will not harden at all; all you will do is soften. If you are too high, you will likely through-harden and it will likely crack.

If you do manage to heat the surface to the proper temperature, I think I'd "quench" with air (high speed, high volume). For a part this size, a large air nozzle on a decently sized air compressor should be fine. For tempering, again, without any temperature control, using a torch would be difficult. Since the part is small, a kitchen oven set at 450F for a couple hours may do some good.

The best choice is to wait until you can get to a heat treater. If your customer has to have something now, give him one unhardened and tell him when he can expect ones that are made correctly.

rp

 

[fastline12]

I am curious why you would select air for a quench? Is that to increase the quench time and try to avoid cracking? Not being familiar with quenches, what I have read indicates thinner oils tend to cool much faster than thicker oils? Water would be the fastest but also the most problematic for cracking?

How long do you usually have from HT to temper? Can they be brought to room temp?

 

[redpicker]

Yes, I'd use air to reduce the tendency to crack. Oil if fine, but unless you have access to a decent quench oil, and the equipment to hold it in the proper temperature range and provide adequate agitation, you are not likely to get reproducible results, not to mention the fire hazard. For surface hardening 4140, a high velocity air stream should be adequate, particularly for the size you mention. A lot depends on part geometry and the depth you are trying to harden.

Leaving as-quenched parts laying around is asking for trouble. How long and to what temperature is dependent on many factors. I've seen some processes where the material is not allowed to cool below 300F before tempering, others where delays up to two hours are acceptable.

All of these are reasons I recommend leaving heat treating to the professional heat treaters. They have the equipment, tools, knowledge, and experience to do the job. Sure, a guy with a sharp knife, a table, and a good lamp can perform an appendectomy, but if you need one, you're better off getting to a hospital.

rp

 

[fastline12]

Well, the job (or damage) is done. Needed to move ahead and due to the use of these parts, we saw it as an opportunity. I used Peanut oil for the quench. There was no flame going in, no stick, no smoke. Parts were hardness tested going in and coming out. We hit right at HRC55 as quenched. There was some decarb but comes off easily. Parts were quenched for about 15min, then wrapped and taken to temper within 1 hr while still warm but probably not more than 150F.

Temper was done at 450F for 2hrs. Parts have not been tested from temper yet. Probably some ignorance on my part but I studied a 4140 hardness graph when deciding on the temper but after the fact, actually found a toughness graph that differs wildly showing that I might have actually LOST toughness. The graph showed a dip in the area of 180-400C when tempered. That concerns me a bit because the whole reason for the temper was to improve the part.

I did not get a change to properly test a sample but I just threw one in a vise to see if it would bend at all before breaking. NOPE, part snapped but took considerable force. Way more than could ever be applied to the part. This was before temper though.

I probably should have researched the temper more but I know the part will work as is. Regardless, the toughness graph confused me.