Foil wrap when heat treating?

[dbb-the-bruce]

So oil quenching a round shaft without much detail wrapped in foil would probably work ok.

 

[francist]

Most descriptions of oil quenching describe how to move a part around in a figure eight or some other way to provide constant new cool oil contact.

I believe I've heard this referred to as the "the mantle effect", where immediately following the plunge the part is surrounded by a mantle or "aura" if you like of super-heated liquid and gasses. For the most part we try to avoid it but I've run across situations where it was actually desired as a way of mitigating the initial shock from immersion. ie: let the mantle exist for a few seconds, then agitate the part to reintroduce it to the un-heated quench liquid as a kind of two-stage cooling process.

Can't say that I've tried it nor had any reason to but I thought it was an interesting nuance to what I had previously thought was a pretty basic process.

 

[dbb-the-bruce]

There is a lot of Art in the Science of metal working.

 

[pontiac428]

Pardon my French, but the art is... je ne sais quoi.

Convection, conduction, and radiant. Three types of heat transport.

To say that a 900 degree C blade in a little pseudo-airtight pouch isn't going to make contact with the quench is hard to grasp considering Boyle figured out in the 1600's that the pressure and volume of a gas are proportional to its temperature. If you drop the temp of a little foil bindle with a piece of metal inside from 900C by hundreds of degrees in the fraction of second, the volume of gas interfering with conductive cooling due to being contained in the envelope will shrink to an infinitesimal volume before the part is even fully immersed, let alone swished and sloshed.

If teeny tiny amounts of cooling rate difference caused problems, there would be a litany of rules to quenching, such as- quench the fat part of the work first, never the skinny part. Or, only plunk a blade into the oil point first, never slap it in sideways. Stuff like that. Nobody talks about things like that because it makes no difference to control it to that level.

We also don't control the quench temp to fractions of a degree, and we don't specify what temp the bath has to be (not to a great extent, anyway- there are exceptions and other reasons at play), and we don't care about how long it takes to get a part from the oven to the quench as long as it is "quickly". Where's the despair over controlling those variables, which are going to have a larger impact on the process than a little hot air poof that will disappear almost instantly?

 

[gi_984]

I DO NOT wrap (stainless foil). oil or water quenching steel.
In metallurgy class we learned to heat treat a variety of steels. This included air quench, oil, and water quench. Did not do salt bath.
Some were just cut stock. But most were our graded machined parts from the machine tool classes. We had to do Rockwell hardness testing before and after as well as measurement of dimensions. Some cut stock was subjected to tensile test (until failure) and polished and checked under microscope.
Quenching speed (missing the nose) per the chart for that specific alloy would be impossible if you wrap an oil or water quenching steel. Yes it will harden. But guaranteed to not be as hard as your recipe/goal. And it won't be consistent. Rockwell testing and the microscope doesn't lie.
First hand experience with a few projects not hardening properly and had to remake and or re-heat treat. Mostly because we worked together to put through the ovens in batches. Mix ups occurred when the parts were all the same but of different steels. Even when you arrange them in groups in the ovens. Wish there was an easy way to to post some of my pictures.

 

[RJSakowski]

I would expect that it would be more about a thin layer of air preventing efficient heat transfer. If the foil made close contact with the part, no problem but any trapped air would prolong the cooling process decreasing the ultimate hardness in that location.

 

[benmychree]

I would expect that it would be more about a thin layer of air preventing efficient heat transfer. If the foil made close contact with the part, no problem but any trapped air would prolong the cooling process decreasing the ultimate hardness in that location.

agreed, RJ!