Properties of mild steel...

[Sparks]

Good morning. I have a fairly decent understanding of mild steel's reactions to heating, but I would appreciate some out side input for a certain application. I would like to put a nice symetrical 1" depression/off set/saddle into a 4"x8" piece of 5/8" thick colled rold flat bar. My plan so far is to make a sturdy male and female die set to use in a big press. The 4x8 piece of steel would be heated evenly in a gas forge, sandwiched in the die set, and then pressed to shape in the press.

My main question is about the machinability of mild steel as it is heated. I usually go by color of material to appoximate temperature, but I haven't really come accross any charts or literature actually stating whats happending to mild steel when it's heated to given temperatures and allowed to cool back on it's own. I'm aware that certain steels have critical(or transitive) temperatures, which seem to apply to situations when hardening is desired(which is what I don't want).

I guess I'm looking for that magic temp that makes mild steel to much of a pain to drill or mill through. I'd like to get the metal as hot as possible with out causing significant hardening, which seems to happen a lot easier than I've always been told.

Edited November 23, 2008 by Sparks
spelling

[matt87]

Mild steel as a rule doesn't harden appreciably. A36 (structural) steel can do if heated above the 'critical' temperature (a cherry red) and quenched in something severe like brine. It's mainly to do with carbon content. AISI1018 typically has 0.18% carbon by weight, AISI1040 has 0.40% and so on. In general terms it is considered that steel will not harden appreciably until there is somewhere around 0.20 to 0.30% carbon present, so AISI1018 probably won't harden. This is why railroad spikes don't make for good functional knives (even the ones marked 'HC' are maximum 0.30% carbon). When other alloying elements are involved, things can get complicated. Phosphorus is an example; phosphorus iron was used in antiquity for many applications which required some hardness, even though it contained no carbon. A36 is a special case as it is specified by performance rather than composition, so it can and does vary from batch to batch, sometimes having as much as 0.30% carbon.

[irnsrgn]

Old Mild Steel was 1003, the new mild steel mostly comes from Mini Mills (remelt) which uses scrap iron containing everything you can imagine. working with the new mild steel square and round for forging, if you hot cut a piece off the end and then stick the rest in the slack tub so someone doesn't get burned by accident, it usually won't cut cold with a hacksaw or bandsaw.

Unless you are ordering and using Papered iron and steel, the product from Mini Mills can be almost anything. Quality has gone down the drain since the last real steel mills producing product from iron ore have been shut down by the EPA, OSHA, high fuel costs and the Greenies.

So called Cold Rolled which is actually Cold Drawn thru dies should have close to the actual specified Carbon Content of .18% or close to it. Cold Rolled squares and Hex bars are actually Cold Hammered from Round, then drawn thru dies to get the exact size,

My source being I actually toured a plant that makes cold rolled and wire rope. I was amazed that the lubricant used for Cold Drawing was actually a dry granular soap.

I have also had the privelige of touring a mini mill in Norfolk Nebraska and witnessed the hot rolling of structural shapes from continueous cast billets on one rolling line and reheated billets on the second neighboring rolling line. Its amazing to see a 4 by 6 billet at a yellow heat go thru the various rolls and exit the other end as 1/2 inch round doing about 60 plus miles per hour.

Edited November 23, 2008 by irnsrgn

[David Einhorn]

Sounds like the real question is how to get the steel so that it can be soft as possible for milling after hot forging under a press. That being the case, heat it as hot as you would like for the forging stage of the process, then anneal the piece by letting it cool very slowly in a container of vermiculite or bucket of scale.

[Sparks]

Thanks for the help guys. Getting to check out a large scale mill has been on my to do list for a while. It sounds like a blast, no pun intended.

For the most part, I make an effort to observe the certs on metal being used for a project involving any real machining or load. Mainly for looks and convienience, it ends up being 1018. It's pretty user friendly and looks good. The price of A36 is usually less shocking, but it's hot roll finish doesn't lend itself to projects that need the smooth precise look to well.

I think your right on the money U.F.. That might be the easy solution I've been overlooking for a long time. Experimenting with some controlled cooling sounds fun. My plan is to shoot for a temp inbetween 1100 to 1400 degrees and let it cool nice and slow.

[Woody]

Most "mild steel" is A-36. It is the most widely used structural steel around. It is not what ever they happen to throw in the remelt pot that day. A-36 stands for ASTM Standard A-36. There is a specific compositon for this steel. You can look here Guide to Buying Steel Online | Online Metals Guide to Selecting Metals for Your Project and find information on it. The compositon is C .26, Mn .75, Cu.2, P .04, S.05. As Thomas Powers pointed out a couple weeks ago you can get a much higher grade of refinement using remelted steel than starting from scratch with iron ore because scrap steel is already a much more pure product.

[ThomasPowers]

Ahhh I don't believe that was me.

Thomas Powers