Heat treating high alloy and high-speed steel tools

[HennieL]

Hennie, is this a case of buy-and-use-immediately? Or can you store it for a (presumably short) time?

Yes on both counts... Because I buy such a small amount (1-1.5 liters), and because I transport and store it in a coffee flask, it evaporates completely within about 36 hours if not used. As soon as one places a piece of metal into the coffee flask, the LN boils very rapidly, and that obviously speeds up the loss. I try to buy the LN on the same day that I plan to use it, and with 2-3 knives, or (say) two of the gauges that I've shown in the first post immersed in the LN there is perhaps 1/10th of the original volume left in the flask after 24 hours. This is still good enough, as the cold is still conducted through the whole of the part, and the knives/tools are uniformly cold when they are removed from the flask.
For interest, here are two photos showing the inside of the flask with LN and knifes about 24 hours after inserting the knifes into the LN (apologies for the bad quality...), and one showing one of the knifes after it was removed from the LN. The "smoke" coming off the knife is actually condensing oxygen vapor, I'm told.

 

[HennieL]

Oh, one more thing that might be of interest, while we're talking HT and LN - cooling down the higher alloy steels does two things:

• It sets up the structure of the steel for retained austenite (RA) (the soft stuff that we don't want) to transform into martensite (Ms) (the hard stuff that we do want) with the next tempering cycle. I underlined the last part, because that's crucial - one must always temper the cryo'd steel after the cold treatment, else nothing will happen... This transformation is basically just an extension of the quenching process, and must be done as soon as possible after quenching from red heat. Depending on the type of steel, one also don't need to go down to the temperature of LN - just sticking your tool into the wife's deep freezer will already have some benefit, and cooling it down to around -80°C is sufficiently cold for all types of steel. You also don't need to keep the steel at this cold temperature for very long to set up for the transformation of the RA - just reaching the temperature is sufficient, but I would suggest keeping it there for at least 10 minutes, just to be sure that the temperature has evened out properly.
• Keeping your steel in LN for extended periods (24 - 72 hours) has a secondary effect that merely cooling it for a few minutes, or only cooling it to the temperature of dry ice, or to that of your freezer (~-15°C to -20°C), does not have... and that is grain refinement. The science behind this is still not folly understood, and many people still claim that this is not true, but the majority of metallurgists seem to agree that this effect does happen, and I'm quite happy to subscribe to this little bit of "black magic" without actual proof. What I can attest to is that I can achieve an additional 1-2 Rockwell C hardness increase above that of non-cryo'd steel treated at the same temperatures, in the same oven, at the same time - and that's good enough for me

 

[willray]

Oh, one more thing that might be of interest, while we're talking HT and LN - cooling down the higher alloy steels does two things...

Hennie,

Would you be so kind as to suggest how this information might be integrated with your suggested recipe for trying to salvage my well-cooked HSS tooling? If grain growth from baking can potentially be corrected by cryo-soaks, it seems like I should give the process every possible opportunity to rescue utility in the tooling. LN is cheap enough.

Also - side question from the "right way" approach to HSS heat treating - expecting that the surface has lost some carbon due to the extended bake in my fire (though the tooling was pretty well buried in carbon most of the time, and thankfully much of it was in closed drill indexes and such, so at least partially oxygen-excluded), do you think it would be of any advantage to try to supplement surface carbon during the normalization or heat-treating steps? Charcoal pack? Kasenit? something like that?

 

[timo_gross]

To add to Henniel's safety warnings re. liquid nitrogen, suffocation by nitrogen gas is quite insidious as the brain only reacts to an increase in carbon dioxide, NOT a reduction in oxygen- hence there is absolutely no warning of suffocation by excess nitrogen in the air- one moment you are awake, the next you are unconscious on the floor. And as the nitrogen that put you there is heavier than air, without outside help you will rapidly become an ex- model engineer......

A factory was using Nitrogen for the pneumatic transport. In one room the Oxygen level dropped so much that it became deadly. Two workers got killed when they entered the room for maintenance. No one had thought about this danger.
One customer told me during a trip that his co-worker suffered from permanent brain damage. He was 1 min in a Nitrogen filled pipe during a maintenance inspection.
I remember in the mid 90ties gas monitors for furnace workers ( mandatory wearing in some countries ) costing the equivalent of 1100 Euros and needed recalibration every month or so. In 2013 they could be bought for less than 200 Euros. ( devices from Draeger )
A cheap chinese uncertified can be had for just under 100 USD. Better than none, basic funcition can be tested by using some welding gas a plastic bag for the O2 and a ordinary cigarette for CO. I should follow my own advice and get one before I play around with my gas torche again.

Greetings Timo

p.s. such device checks CO and O2 levels.

 

[HennieL]

Would you be so kind as to suggest how this information might be integrated with your suggested recipe for trying to salvage my well-cooked HSS tooling? If grain growth from baking can potentially be corrected by cryo-soaks, it seems like I should give the process every possible opportunity to rescue utility in the tooling. LN is cheap enough.

Will, yes - if it was me, I would certainly treat the re-hardened tools with LN. I would not try to reduce the grain growth through multiple quenchings, as I mentioned earlier, as that would likely create even more carbon loss, without any guarantee of grain refinement.

Also - side question from the "right way" approach to HSS heat treating - expecting that the surface has lost some carbon due to the extended bake in my fire (though the tooling was pretty well buried in carbon most of the time, and thankfully much of it was in closed drill indexes and such, so at least partially oxygen-excluded), do you think it would be of any advantage to try to supplement surface carbon during the normalization or heat-treating steps? Charcoal pack? Kasenit? something like that?

Case hardening requires time at temperature, and with HSS you don't want to keep the tools at temperature for more than a minute or two to restrict further grain growth (and possible further loss of carbon). Having said that, I don't think that your carbon loss (if any) would be critical - firstly, carbon can diffuse from the interior of the tool to the "depleted zone" close to the surface, making up for some loss with a slight reduction deeper into the steel. Secondly, the HSS contains a lot of other alloying elements that are probably still present in the steel as carbides (tungsten carbide, chromium carbide and vanadium carbide), and they would still play a role in keeping the red-hardness close to that of the original steel.

As I said earlier, though - I'm not a metallurgist and what I said above is more a matter of "gut feel" than applied science - perhaps clockworkcheval can ask his resident metallurgist for a professional opinion? Also, there won't be much loss if you try to harden a few tools and see what happens, and if you can have the hardness tested before and after the re-hardening it will certainly go a long way towards providing actual answers, rather than just opinions...

Good luck with this operation.

 

[willray]

Will, yes - if it was me, I would certainly treat the re-hardened tools with LN....

Good luck with this operation.

Thank you sir. This is a bit of a long-term project, as most of my space and time is currently occupied with trying to build a decent door for my secure-storage space and get a couple other doors and windows into the new shop building. Once I have room and time to experiment, I will definitely keep you posted on how it goes.

Will