Heat treating during powder coating test

[Ian]

OK, it's under way.

Alloy is COWW + 1% Sn. Normally this alloy air cools to about 14 bhn for me after a week or 10 days.

I took 60 gas checked .30 caliber bullets (Lee C312-155) and 60 gas checked .22 caliber bullets (NOE 225-67 MX3), pre-sized, powder coated, and baked at 390F for 27 minutes and turned it down to 375 for the last four, total of 31 minutes in the oven, which was preheated to 400.

Straight into 85 degree water (it's hot in Texas).

They all tested same as right after casting, 8.5 BHN for both calibers. I put some pre-flattened test slugs in the mix and dispersed them throughout the tray to get an average. All tested the same except one which was just a little softer than the rest, likely due to a 10 degree difference in temperature across my oven left to right. Coated and uncoated tested exactly the same, three bullets each in each caliber tested, so the powder coating isn't affecting the Lee hardness tester at all.

I'll let the little dendrites chooch and report what happens in a week or so. I'm shooting for 16-18 bhn for this batch.

 

[JonB]

85º water ...ain't you got no ice ?

 

[Rcmaveric]

You only get so much ice out of dollar store ice trays. That's my excuse. I make ice blocks for hunting and fishing. So that's a thought.

 

[Ian]

One of the things we're going to find out is if the water temp matters that much for the mid-range hardening I'm trying to achieve.

 

[Brad]

On water temp and if it matters. Is the hardening because we heated to a specific temp then cooled quickly below a certain temp? Is it merely the temp differential?

If the temp differential matter then yes, colder water makes for harder bullets.
If it is merely a matter of cooling below a specific temp then as long as the water is below that temp then the water temp doesn't matter.

I don't know enough about the metallurgy involved to say

 

[Ian]

I'm pretty certain that cooling below a certain temp in a certain time is the key. Colder water takes more BTUs to boil than warmer water, and we know from the sizzle that the water boils at the surface of the bullet, so it stands to reason that colder water will cool the bullet faster, hence make it harder.

If I were going for maximum heat treat, cold water would probably be a factor. I'm not because the loads I'm going for don't require it or desire it, and I'm going for a sort of quick 'n' dirty method that doesn't take forever to do or require anything special like ice or a thermometer in the bucket, so I'm not concerned about it yet. If these bullets don't get as hard as I want them I know either the time soak was too short, the temperature was too low, or the water was too warm. Since all the bullets, including those cast a week ago and air cooled got completely annealed, I'm confident they got some heat treatment from the quench. The only question is how much.

 

[Brad]

Big question is how much.
If the water is 40° one time and 80° the next will the difference be noticeable? Can we correlate a water temp vs heat treat temp over a range of temps?
In short if the water is 40° warmer do we need a higher heat treat temp for same end hardness?

 

[popper]

If the water is 40° one time and 80° the next will the difference be noticeable? Yes. I use ~420F for an hour and tap water ~60F for 40sw pistol, ice water for 308W HV rifle. ~380F and ice drop, about the same as tap temp. The 30F cooking diff. is harder to get without slump.

 

[Ian]

So there's a zone where colder quench can make up for lower cook time and yield similar end results? 380°/ice water same as 420°/tap temperature water?

 

[Ian]

Seven days in, last night I tested the bullets again and they are approaching air-cooled hardness of around 15 bhn. The coated ones both .22 and .30 cal are about 12 bhn and the bare ones are both about 14. Some pc air cooled from same alloy cast and coated a week prior are 15.4 bhn and stable, were tracking about the same as the later batch.

I've been testing every day to note trends but the hardening was incredibly erratic an impossible to chart after day zero until about day five.

So far it appears that 30 minutes isn't long enough, the water needs to be cooler, or the oven hotter.

 

[popper]

colder quench can make up for lower cook time Not that I can tell.

 

[Brad]

Cook time, no
Cook temp, probably

I really believe an hour is required for proper heat saturation and time for crystalline change in the metal

 

[Ian]

I tend to agree, Brad, an hour is what I always used because everyone said so. But, since the hardness of previous heat treatment can be zapped out almost immediately upon reaching the minimum annealing temperature, I was curious if the same would be enough to re-harden. Probably it is not, but I cannot be sure since I used warm water and not a very high heat treat temperature....and still have another week before I can expect the alloy to settle down based on the air-cooled samples from before.

 

[KeithB]

I don't know the ins and outs of heat treating lead alloys nearly as well as I know about heat treating ferrous alloys, but there are some basic principles that I think apply across the spectrum.

1. When changes in microstructure depend on phase changes (transformation from one crystalline structure to another) then (a) you have to heat or cool the item above/below the phase change temperature, (b) you have to hold it at that temperature long enough for complete phase change, (c) unstable structures typically require rapid temperature changes to accomplish, and (d) if the material isn't in the right starting condition, it won't end up consistently in the proper ending condition.

2. Microstructure changes that depend on precipitation hardening (the relatively slow change in structure caused by migration of atoms through solid state diffusion) are also time and temperature dependent, but the time scale is distinctly longer.

Big fat heavy atoms usually diffuse slower than smaller atoms, requiring a longer soak to get a complete phase change and a longer precipitation hardening time (days and weeks vs a few seconds). Precipitation hardening is also very highly dependent on composition, some alloys simply will not change hardness with time.

If I harden steel I have to get it above the transformation temperature. One degree too cool and it won't change phases into the proper starting microstructure. Even if I get it hot enough if I don't let it soak at that temperature long enough for 100% of the structure to transform into the proper starting condition I won't get 100% hardening. And if I don't get it cool enough quickly enough some of the material turns into something other than the hardest phase possible. And without the right composition precipitation hardening isn't possible.

I always remember three or four rules

1. Heat (above transformation temperature.
2. Hold (long enough to get complete transformation)
3. Quench (quickly enough to trap atoms in harder but unstable structure)
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4. Temper (for steel only, to restore ductility) Don't think lead alloys need this.