4140 Annealed parts. Need to decide on HT procedure.

[fastline12]

We have some parts that look like stars, are made from a chuck 1.5x1.5x.75", have a hole in the middle as well as some small features machined into the top about .150" diam. We used to make these parts from 4140HT which is HRC28-32. We had problems pushing the parts around on the machines and the hardness, while it did work, should have been harder.

So we are remaking from annealed, and primarily just need about .030 case hardening to near max hardness but I know that would be difficult with these parts. I am curious what procedure might be preferred to achieve the highest surface hardness while still retaining some "chewy inside"?

Even if they will ultimately through harden due to the geometry and alloy grade, we need to decide how hard we can practically go without problems. From the charts, it looks like HRC50 is acheivable all day long. Would really like to see HRC60 but I understand the carbon issues or lack there of.

Also need a procedure that will be simple and cheap for our heat treat source. Need to be under a buck/part for this.

 

[swall]

You have boxed yourself in with the stated price goal of "under a buck". About all you will be able to get at that price would be to through harden by austenitizing at 1550F, oil quenching and tempering at 400F. The entire part will be hard and possibly too brittle for the intended purpose.You could temper at 800F for a tougher part, but then hardness on the wear surfaces will suffer.If you are willing to absorb some tooling costs, you could have the wear surfaces of the part induction hardened and possibly meet your piece price target.

 

[fastline12]

What kind of hardness will we be looking at as a through harden? As I stated above, the HRC28-32 was and is working but mating components are 8620 that are cased to HRC58-62 so the higher the better within reason. I kind of figured through hardening was all we could get on these but I guess that makes the engineering easier..lol

I think the temper was where I was getting confused. The higher 800F would pull back the hardness a bit but would at least resist any concerns of cracking. This part operates in a high vibration environment but otherwise shock loads and impacts are minimal.

 

[redpicker]

You will have problems with these if they are over 40 HRC or so. Even at 40 HRC, they will likely be too brittle to be useful. Your best bet is to use the same material and processing as the mating components; 8620 carburized 58/62 HRC with a 0.030" case.

With the 4140, you can try to flame (or induction) harden the surfaces you need to have hard. The cost for this is going to be highly dependent on quantity, since you will have a high set-up cost. You should be able to get a 1/8" flame hardened layer to hold 50 HRC or so. If the annealed material is too soft to support the anticipated loads, you may need to Q&T the parts to 28/32 HRC before flame hardening.

rp

 

[swall]

With a 400F temper you would be looking at about Rc 50 surface hardness. 800F would take you back to about where you were with the pre-heat treated 4140. You mention 8620 for the mating parts--I think that would have been a better choice for this part.

 

[fastline12]

We were unable to get flat bars in 8620 so choice was made years ago to switch to 4140. Probably should have shopped more on this to find a more suitable material.

I thought carburizing was expensive anyway? Or IIRC, this involves a vacuum furnace and intro a high carbon gas like Methane? IE, about as easy as heat and quench?

 

[swall]

Yes, there is vacuum carburizing, but the more tradional method is just enriched endo gas in a furnace, then oil quench. Basically, the amount of furnace time dictates the resultant case, so your major cost is in furnace time, say 6-12 hours depending on where you want the case depth.