Vacuum heat treating at home

[Storm_Eagle]

Vacuum heat treatment is used not only because vacuum is good at preventing oxidation, but also because it enables use of gas quenching

But does not that require gas quenchable steel? What i have now is "silver steel" that i think is pretty much the same as O1 tool steel in the US.

And i was thinking the vacuum was good for isolation as well. Because it will be no convective heating going from the hot part to the tube and endcaps, just a little radiative heating. So i could get away with making the end caps out of aluminium, and maybe even just use borosilicate glass for the tube.

I think i will try to make it to final dimension before heat treatment and see how it turns out. And if it gets warped i will have to buy some inserts to try hard turning.

 

[Flynth]

But does not that require gas quenchable steel?

No it doesn't, because any quenchant can be approximated with pressurised gas/vacuum furnace by tweaking process parameters.

For example you can introduce the gas at higher pressure and achieve a faster quench (simulating water) or introduce it slowly and simulate air quench. Big furnaces also have gas heat exchangers and fans, they can control the speed of cooling of the gas too.

There are air quenched steels, but they don't even need any pressurised gas/vacuum furnace. You can pull it out of the furnace, put it away and it will quench in free air.

What i have now is "silver steel" that i think is pretty much the same as O1 tool steel in the US.

Not quite

Silver steel I believe is water hardening steel (for example EU 1.2210, US 115CrV3), while O1 is an oil hardening steel.

And i was thinking the vacuum was good for isolation as well. Because it will be no convective heating going from the hot part to the tube and endcaps, just a little radiative heating. So i could get away with making the end caps out of aluminium, and maybe even just use borosilicate glass for the tube.

At higher temperatures more and more energy is given off as radiation. At yellow heat (820C) quite a bit of energy is lost as radiation. Consider how hot your hand gets in front of a radiative heater that glows yellow. It's definitely not going to be "a little radiative heating". Borosilicate glass will be fine for the tube as long as the heat is not going to be on too long. The end caps... Hmm, they will get hot if near the part, but the other side will be in air so it could be OK. I would worry more about aluminium expansion ratio being different than your glass and cracking the tube.

I would forgo the vacuum in the first place. This way the whole thing becomes a lot easier, also with gas quench you can tweak a lot if process parameters.

I think i will try to make it to final dimension before heat treatment and see how it turns out. And if it gets warped i will have to buy some inserts to try hard turning.

I would love to hear how it turns out. I think that's the first time I hear anyone trying a vacuum quench in oil. I'm curious myself.

 

[GeneT45]

I thought "silver steel" was "drill rod" - which is to say the entire class of O, W, A - x hardenable steels. I am quite certain that it is used that way in at least some casual talk.

GsT

 

[whitmore]

Hi. I want to make a custom ball bearing and want to make the race my self. So i was thinking about making it from tool steel and harden it in a vacuum tube to avoid oxidation.

I've done heat treatment with a tube furnace (alumina ceramic tube) in vacuum, but not for 'oxidation', rather
because the alloy was a vacuum-killed specialty item (lessens the creep under stress by eliminating
nitrogen). For anti-oxidation, just cover the part with boric acid.

 

[Flynth]

I thought "silver steel" was "drill rod" - which is to say the entire class of O, W, A - x hardenable steels. I am quite certain that it is used that way in at least some casual talk.

GsT

Indeed, the term is often used incorrectly

Silver steel is another name for W1, 1.2210 or 115CrV3. It is a water hardenable steel for cold work(paper punches, tools that don't get too hot generally) . This steel has over 1% of carbon, some manganum, and no molybdenum. It makes it's appearance a lot more "silvery" than O steels for example. Also in contrast with O steels it is offered(at least here in EU) primarily in form of rods including some quite thin(is that why "drill rod" term is used? ) . One of its properties is very good quench cracking resistance.

O type steels( O1, 1.2842 etc. ) or are also cold work steels, but there is a lot less carbon and additional molybdenum makes their appearance more grey. Also they have far better dimensional stability during heat treatment. That's why they are(used to be?) used for precision tools like v blocks etc. O steels crack a lot more easily that's why they are considered oil hardening steels.

As for A steels like A2. Unlike O and W they are chromium steels. They are also used for cold work, but they are considered "premium" over O and W steels due to many excellent properties. Some are, good abrasion resistance, very low dimensional stability etc.

About that "drill rod" term. I don't know where it came from. Perhaps from W1 being available in small rod sizes? In general any hardenable steel can be used for drills, but hot work steels will make much more resilient drills!

So as you can see there is one commonality between all W, O, A steels. That is they are all cold working steels. This means they will get annealed over a certain temperature considered low so in a way you could refer to them all as a group, but at the same time they have very different properties. So in a thread beginners might read I thought it useful to describe them briefly.

I've done heat treatment with a tube furnace (alumina ceramic tube) in vacuum, but not for 'oxidation', rather
because the alloy was a vacuum-killed specialty item (lessens the creep under stress by eliminating
nitrogen). For anti-oxidation, just cover the part with boric acid.

I wonder if anyone has a good way to get rid of that crust formed by boric acid on parts. I used it to prevent decarbonisation of my parts when I was making my "renzetti" blocks. It formed a glass like surface on the parts that was very difficult to remove (especially in threads which I packed with a lot of it..). In the end I had to run a carbide tap slowly through those threads to get rid of it. I would use it only for smooth parts it can be flaked off easily.

My recommendation for preventing oxidation is a thin stainless steel bag and a piece of paper inside. One can buy very thin stainless sheet to make them.

 

[Storm_Eagle]

I've done heat treatment with a tube furnace (alumina ceramic tube) in vacuum, but not for 'oxidation', rather
because the alloy was a vacuum-killed specialty item (lessens the creep under stress by eliminating
nitrogen). For anti-oxidation, just cover the part with boric acid.

From what i have seen both boric acid (or any anti scaling compound) and stainless steel foil with paper packing gives varying degrees of hard to remove surface layers. I want the part to be shiny right out of the heat treatment with no post processing.

If the end of the pipe has a good finish i think i can solve both differential thermal expansion of glass and aluminium and any over pressure issue by making a silicone face seal so the aluminium end cap can move relative to the glass tube, and if there is any over pressure the end cap can just pop off the tube. And make both the induction heater and the release mechanism remote controlled so i can keep a distance in case anything happens. The part is small so i think the induction coil will be able to heat all of it without moving it up and down in the tube.

 

[Flynth]

From what i have seen both boric acid (or any anti scaling compound) and stainless steel foil with paper packing gives varying degrees of hard to remove surface layers.

Yes, there is slight discoloration (definitely not shiny surface) with foil wrap method. I haven't tried foil myself (I plan to) but from what I saw in some knife maker video even if one never removes the item from the bag and a plate quench is done. There is still some discoloration. My assumption is that it is the nitrogen in the air.

Then, there is certain discoloration vacuum heat treated parts have as shown by various makers. Again, I think it is a result of using potentially a (slightly) reactive quenching gas. (nitrogen is much cheaper than argon for example).

I want the part to be shiny right out of the heat treatment with no post processing.

Then, if you plan on using oil, you will have a hard layer of black decomposed oil residue on the part. It is much easier to remove than borax, but it is matte black not shiny.

I think you'll have hard time heat treating a part with any other quenchant than a noble gas and it remaining shiny.

Even before we get to quenching I wonder how good of a vacuum you need to pull to avoid discoloration and change in surface properties just because of heat and residual atmospheric gasses in the chamber. You can always do experiments just heating the part in vacuum briefly and letting it cool down on its own. This will give you an idea how good of a vacuum you'll need.

I think flowing the chamber with argon for a bit, then pulling a vacuum (assuming you're going down the vacuum route) will allow you to use a much cheaper vacuum pump setup than trying to achieve same results from it starting up with atmospheric gasses.

Trying to avoid discoloration on stainless and oxidation of titanium due to heat and residual reactive gasses is a known problem in welding. The best results are achieved in very good vacuum with electron beam welding, but amateurs are still getting good results with a tent full of argon and an oxygen meter. They keep a steady stream of argon through it until the oxygen meter shows sufficiently low numbers. Then they weld and I saw no discoloration and no loss of shine on titanium for example and titanium is horribly reactive.

If the end of the pipe has a good finish i think i can solve both differential thermal expansion of glass and aluminium and any over pressure issue by making a silicone face seal so the aluminium end cap can move relative to the glass tube, and if there is any over pressure the end cap can just pop off the tube. And make both the induction heater and the release mechanism remote controlled so i can keep a distance in case anything happens. The part is small so i think the induction coil will be able to heat all of it without moving it up and down in the tube.

Your silicone seal will outgas in vacuum and it will be somewhat permeable. Whether its enough to cause problems I don't know.