propane forge or OA torch hot work

[Thomas Powers]

A lot of that depends on the alloy used; you can't quench harden mild steel for instance, (Blacksmiths usually breakdown heat treating into separate processes exp: annealing, normalizing, hardening, tempering---instead of combining hardening and drawing the temper into a single word.)

[Konstantin the Red]

Yeah, in mild steel the answer is usually no, for having to normalize.

Normalizing, like a complete heat-treat, relieves internal stresses made by working the metal. The stresses fatigue the metal, normalizing relaxes them.

Now let's swim in the deeper waters. It's not scary at all.

Normalizing is the annealing heat, but cooled much faster than a total anneal. Yet not so fast as the second step in a heat-treat, which is quenching, to lock the martensitic structure in the steel crystals. Tempering is a re-heat after that quench, to reduce the martensite in the metal to whatever degree desired, and this is closely controllable by how hot you reheat the steel. You can take steel down from full hard by baking it in your kitchen oven -- good time & temp control there. The oven does not get hot enough to completely soften a piece down from full hard.

Martensite, bainite, and pearlite are various crystal orders in the structure of steel, and how much of each is in there determines if the steel is hard or soft, springy or completely malleable. Technical stuff; no marks off if you don't happen to remember just everything about them. They form at various temperatures, which is why you have to get steel red hot to make martensite, and chill it fast to keep any of it in there. Cool it slowly and the martensite sort of melts out and its crystalline structure becomes bainite. I think. Softer. The carbon atoms have been wandering around in the crystal lattices -- having influence.

Mild steel doesn't come to full hard, regardless. Doesn't have enough carbon in the mix. Doesn't take much carbon -- high grade, or high carbon, steel is going on 1% C content, and that amount will make a razor. Or a machine tool or a file. Thulsa Doom could have told Conan the Secret, the Riddle, Of Steel was, well, soot. Just a spoonful.

There's a fly in the ointment: everything happens harder and trickier in steel fabrication as the amount of carbon in it increases. It's less chemically stable and can actually be made to burn, looking like a yellow sparkler when it does (completely ruins the steel that did that, too), rusts quicker and worse, workhardens more and faster. All extremely interesting, the way clearing a minefield is extremely interesting -- do it right and everything's just jake; do it wrong and not so good. And, steel that is full hard is also brittle. You can break it in pieces dropping it on the floor. Even in this state it makes excellent material for machining softer steel and other metals. But you can destroy such a machine tool by sitting it on the concrete shop floor and bashing it with a hammer.

Buuuuut -- soften the steel down again from full hard and it becomes much tougher, more steel like. So you can temper high-carbon steel from machine-tool hardness down to for use as a knife, a little softer and springier yet for use as a sword, perhaps slightly softer than that for an axe or a spring, and a little more for your own handmade hammer head.

Low-carbon, or mild, steel is from about 0.06% C to maybe 0.25-30% C. This is our era's wrought-iron, and used for all the tasks it once was. It's cheap and strong, and tough. Easy to work, easy to weld, easy to forge in a reducing flame. (It does not oxidize the steel and waste it.)

Medium-carbon makes springs, axes, swords and large knives. It can be heat treated to be harder, balancing hardness with toughness as desired. Being harder, its strength is greater than mild's. Examples of medium carbon steels are the designators 1050 and 4130. 1050, a plain and simple carbon steel without alloying elements in significant amounts, has 0.50% C to it; 4130 has about 0.30. Medium carbon ranges from 0.30-0.60% C.

High carbon steel makes knives, files, machine tools. It is the most expensive steel. 1095 steel is superb for a cutting blade, sharpens to a razor and holds that fine edge quite a while; 0.95% C -- I reckon you see a pattern here. High carbon starts at 0.70% C. High carbon steel can be tempered the very hardest. It can make springs. Like all the plain ole carbon steels from high-grade to mild low-carbon, it can be annealed down to very soft. It also burns easily so don't get it even twenty degrees hotter than you need it, and it workhardens a lot, to the point where at any given time you can only push it so far and no farther; it fights back. Then it has to go back in the fire and get softened and stress-relieved again. Demanding stuff to work; very high performance too.

These fractions of a percent are known in the trade as "points of carbon," ten points of carbon being a tenth of a percent C. Many steel numbers and indicators show at least roughly how much carbon is in the steel and hence give a general idea of its strength/toughness ratio. Other alloying metals mixed in do fancier stuff, tailoring steel to particular purposes, like extreme resistance to wear, corrosion resistance as in the stainlesses (17% or more of chromium melted into it), molybdenum to make it more machineable and still strong... all the clever things they've come up with beginning in the previous century.

[Silvester]

Good stuff!!!

I'm planning to build a small gas forge in the next year or so and this info will be very useful!!!


Right now I'm mostly dealing with aluminum shield scraps from T-6061 sheet and street signs. How bad is it to dish, heat to orange hot, dish a little deeper and then cold planish?

[rotccapt]

thank you for that great info. well for an update on my project, i now have the beginnings of my mini forge thing, now i just have to wait till the sodium silicate cures and set up my burner and i am ready to heat some metal

[Thomas Powers]

If you heat Al to orange it will be very bad indeed!

[Kerry Pratt]

The melting point of Al is below the light radiation point. If you get it hot enough to glow then it will become liquid. I have been told (but never tried it) that you turn on your acetelyne and cover the piece with soot. When the soot burns off, the piece is at forming temp and just below melting point. This trick is used to anneal the aluminum rather than hot work it, though, as aluminum doesn't hold heat well enough to be hot worked. At least in my experience.

[Steerpike]

The "shop tip" I was taught at school for annealing aluminium alloys:smear the piece with soap *, heat until the soap goes black, quench.

* we used to use the liquid stuff from the hand washing sink, washing up liquid ("dishwashing liquid" in the US IIRC) also works well

[Silvester]

The Al never went liquid on me, but it sure was easy to move after the heat.

I did more dishing and then planishing, are the chances good that I re-work hardened it?

[Konstantin the Red]

Aluminum? You damn betcha. It's springier now, isn't it? I reckon you mean 6061 alloy, T for Temper T-6. 6061 is aluminum primarily alloyed with appreciable copper and comes in T numbers from 1 (soft and gooshy) to 10 (OMG &*##! hard!). T-6 6061 is pretty much a bear to dish or bend cold, calling for a sliproller to bend and/or a heavy deadblow hammer to accomplish any dishing at all. A steel hammer will come straight back at you about as fast as you whammed in in.

No wonder they can make airplane wings and car parts of the stuff, huh? Great stuff to make shield blanks of, likewise the simpler-bend stuff that I mentioned like mandatory kidney belt plates.

Precipitation hardening as is done in these structural aluminum alloys is a subtle thing of baking in ovens at x degrees for so many minutes and hours -- and pulling it out promptly when the oven timer dings. And the alloys age to differing hardnesses out in the open, too. You can work successfully with the stuff but you have to know your shit, and your alloy too. Down at the molecular level it's not at all like what happens tempering steel. But at the higher T numbers, strength compares well with mild steel.

Steel is still a lot simpler for a beginner to work compound curvatures into.

[Konstantin the Red]

If you get low-carbon steel that is approaching medium carbon -- 25 to 30 pts C -- you can achieve some hardening by use of a brutal quench solution called superquench. It would crack high-carbon or an oil-quenching (oil hardening) alloy steel.

Superquench is a brine quench with hand dish soap added to increase its wetting, and thus its attachment to a boiling, red-hot piece of steel. So it really sucks the heat out of a piece of metal. High-carbon steel would get so temperature stressed it would go "ping!" into pieces from such treatment; water or oil (any oil from Wesson to transmission fluid) is your quenchant for high carbon. It's kinda sensitive that way.

But highish low-carbon steel has a soft heart and can survive this, and harden appreciably, though it will never be a razor.

Treating superquenched metal afterwards with a cryo quench... anybody? Cryo seems to do a lot for higher carbon blade steels, better wear resistance to the edge without compromising the heat-treated toughness of the piece.

[Thomas Powers]

Cryo quenching works on alloys that have appreciable amounts of retained austenite, low carbon steels are not known for this issue.

[Konstantin the Red]

Aha. Thanks. I was forgetting the austenite.

[Cap'n Atli]

Forging Aluminum:

I knew some blacksmiths in Florida who, living in a corrosive saltwater/tropical atmosphere, did a lot of work for estate gates and fencing in stainless steel and aluminum. For the later they had a large firebrick gas forge, precisely tuned to the temperature for forging the aluminum alloy they preferred. They did beautiful work. So; it can be done, but you have to know what you're doing.

The other trick with aluminum, and most other nonferrous alloys in the forge, is heat conductivity. If you don't like being burned, use tongs. The heat migrates down a bar like nobody's business!