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Leaving dies glass hard?


Aug 19, 2021
Hey team.
My die making, lapping and polishing is really starting to pick up and I’m really starting to make some nice parts. Lapping bores with 3 micron paste is annoying though… haha.

So I did have a question to refine my process… the dies we use are 4140 or O1 - I normally harden them, then temper them in an oven then final polish to remove scale etc. I’m wondering if there’s any reason they can’t just be left glass hard?untempered? They aren’t under any impact and I’m curious if the surface will be a better finish due to its crystalline structure.
So yeah, is the full hard untempered die going to provide a better surface finish? It would also save me heaps of time not having to polish out the scale twice. The forces we use we won’t blow them
Up in out presses.
Hi Lplates:
Although I don't KNOW, I suspect it's not going to make enough difference to notice.
I believe your choice of steel and the quality of the heat treat is going to matter far more.

Heat treating in a vacuum or in an inert atmosphere (or even in a foil wrap) will make worlds of difference.
Picking an air hardening steel (A-2, D-2, S-7) over an oil hardening steel (4140, O-1) also gains you a lot.

I'd be looking mostly there for my gains.

The other bad thing about leaving the steel hard and un-tempered is the risk of spontaneous cracking or cracking with little provocation.
The stress release from tempering does much more than just make it a bit softer...it also makes it much more stress tolerant.


Depending on the material they may crack from residual stresses. I doubt you will see much difference in polishing if they are tempered at 300F.
My old boss told me about some plates they made for very large oil field pump jacks. The drawings and heat treat specs came from an engineer that copied another brand of jacks. When they took the spring plates to the heat treater, they were told then needed to change the specs as the parts would not make it to the truck without shattering. The engineer changed the specs and the heat treater said the parts might make it to the truck with the new specs. They did not, the truck driver had to get many stiches in his thigh when the one he was carrying exploded.
Whilst the surface is glass hard due to the properties of steel the inside isn’t, there’s a test ( well several) to show the hardening, joimny is one, the steel is heated to above the lower critical limit A1, then a jet of water impinges on it to cool as quickly as possible, a section shows what’s what inside, the outside skin, mainly cementite, then Bain it’s or Martinsite or whatever to perlite ferrite at it’s core, it’s considered impossible to through harden thick sections as it takes time to remove the heat, hence TTT graphs, time temperature transformation, so leaving it probably won’t cause an explosive failure, thin bits may crack though!
You should do at least a minimal temper to get the material away from glass hard, and with most tool steels you should do it right way. With A2, the mills recommend starting the tempering process as soon as the part gets down to 150F in its slow air quench.
Do the old-school trick of sealing the quenched part in a stainless foil envelope along with a bit of paper to reduce scaling during tempering. If you change over to an air-hardening tool steel (A2, D2), you can leave the part in the stainless foil envelope throughout both the hardening and tempering heat treating. This will give you much cleaner parts, even if you don't have a controlled atmosphere oven.
Hi sfriedberg:
I am a fan of sealing the part in foil during the quench rather than just during the temper.
As you point out, sealing in a bit of paper to consume the residual oxygen in the bag is super helpful, so is making the bag several layers thick if you're going to harden your part in it.
Critical temperature for most hardenable steels will eat up the first layer or two of stainless foil.

So I foil it bring it to critical temp, hold it and then quench it in the bag in forced air, right down to where a 150 degree Tempil stick will just melt on the surface of the bag.
It then goes in the temper oven and sits there for 2 hours or so for a small part.
I cool in still air after the temper and I often (usually) draw at least twice if I need it to be tough and stable.

I get super clean parts...they look like they just came out of a vacuum oven or were descaled with glass beads.


I don't have experience with foil-bagging oil hardening steels (the OP's 4140 and O1). Do they quench fast enough through the bag? I've read of people bagging them, then cutting the orange-hot bag open to drop the parts in the quench tank, but I've never had a helper to provide the necessary extra hand(s). So I harden them naked and drop them directly in the tank.
For air-hardening steels, yes, keep it bagged the whole time it's going through hardening, quench and temper. Lovely results.
Hi again sfriedberg:
I have no experience trying to bag oil hardening steels either, but I also don't really see the point of trying.
They all dirty up so badly from the quench that there's nothing to be gained from bringing them up to temperature in a shielded environment, and then soiling the crap out of them when they're hardened.
I suppose you could argue there's a bit less surface decarburization and less oxidation, but that's not much of a benefit.

This brings me to the point...O-1 and 4140 are crappy candidates for mold or die components in almost all circumstances...only our OP knows why they were chosen, but I'm not a fan of the choice.
Any cost savings in the material is so quickly swallowed up in the handling and extra processing cost that the benefit is trivial if any, and they are inferior steels in just about every respect other than the cost and the fact that Homeshop Harry can torch harden them and get a usable result.

For these applications, (whatever they are) give me an air hardening steel any day.


Could you go with a nitriding steel, high surface hardness, little or no scale or pitting, can’t remember the grade, it was a chrome molybdenum one, topped 65 on the hardness, took a pounding well ( stuffed in a 2500 ton press) 40 cdv 12 or somthing
Machined well, but cost a bit, I can remember 220mm round billot about 4’ long was a good holiday in the Bahamas! ( joking btw)