[wfw]

To oversimplify considerably, a particular piece of steel's mechanical properties are determined by its heat treatment. The reason is that steel forms different crystalline structures at different temperatures, and those crystalline structures have wildly varying physical properties. The heat soak (letting a part sit for a while at elevated temperature) allows the crystals to set in whatever arrangement is characteristic for that steel at that temperature, and the quench (rapid cooling) freezes or traps the crystals in that arrangement. A subsequent temper (a relatively mild reheat) then allows a partial stabilization of the crystal structure to some other configuration, with further modification of the steel's physical properties. The phase diagrams for iron show crystalline phase (austenite, ferrite, pearlite, martinsite, etc) as functions of temperature vs. percentage of various alloying elements.


There are other hardening mechanisms. Precipitation hardening is not based so much on crystal structure, but depends on distributions of elements which precipitate out of the alloy into microscopic clumps. This happens because, for example, carbon is not soluble in certain crystalline phases of iron. The layman finds the concept of non-solubility to be peculiar when applied to solids, but it's true nonetheless. Precipitation hardening is the only heat-treatment hardening available with most aluminum alloys, but its commercial use in steel is relatively rare. The AISI series 15 and 17 wrought stainless steels are precipitation hardened, and I think that's about it.


Another crafty and very old trick is case hardening, in which the carbon content of the steel part is increased locally. That higher-carbon region then reacts differently to the subsequent heat-treatment process than the lower-carbon regions of the part. Typically the part is immersed in graphite and subjected to a heat soak. At elevated temperature the carbon in the graphite is soluble in the steel and is therefore absorbed into its surface. From there it can diffuse into the steel to some considerable depth (in practice, only a few thousandths of an inch). Then when quenched, the high-carbon regions are much harder than the rest of the part. Hence, the hard "case" around the part. This is good for, say, carpenter's hammers. The face can be case-hardened so it isn't diggered up by nail heads. If through-hardened, the hammer head would shatter under impact. The combination of hard face and more ductile body is far more durable (in this particular application, anyway) than any other obvious combination. Case-hardening was the process used in a grander scale to make the armor (Harvey process and Krupp process) in dreadnoughts. The soak times for carbon diffusion could run for weeks, to get a good thick hard surface to the armor plate. So much for those who think "case hardening" is just for the fancy colors.


Another hardening mechanism is work hardening. This can occur naturally as a part is used, or it can be done deliberately. Some common alloys of, say, copper, can only be hardened by this process. A simple example of work hardening in steel is a bend in a paper clip. As the wire is bent back and forth in the same place repeatedly, it begins to harden, to the point that it eventually becomes (locally) brittle, and fractures. In a real engineering "strength of materials" class, this would be illustrated with a suitable stress-strain diagram. Cold-rolled metals are sometimes work hardened; drawn wire usually is. Some metals, such as molybdenum, when cold-rolled, are work-hardened to an extreme degree. Even thin sheets tend to delaminate into two layers, as the outer rolled surfaces are much harder than the interior of the sheet. Work hardening in any metal can be made to disappear by annealing, which is just another heat soak which allows the crystals, all broken up by the working processes, to reform.


In engineering, all of this is very old-hat. Generally, "modern processes" make metal processing cheaper, not technically "better" (as in, more suitable for a particular application).


Now for specifics. Medium-alloy steels like 4130 and 4140 are used in tremendous quantities in applications like, say, aircraft engine mounts. Ultimate tensile strengths are a bit better than those of low-alloy steels of similar carbon content (1030 or 1040), and fatigue resistance is better (hence the aircraft application). Airplanes constructed of tube and fabric (there are still some being made like that) use 4130 tubing more often than not. It welds easily, and the heat treatment doesn't require exotic equipment. Wrought 4100 series stock is commonly available as tubing and cold-rolled bar, sheet, and plate. Cold-rolled was more popular than hot-rolled in the past, as the surface finish was much better (the hot-rolled stuff was all scaly), but nowadays hot-rolled is supplied with a pickled surface (picking is a corrosive bath which eats off the scale).


The strength requirements for magazines are not very stringent. (I would call the inside of a jet engine a stringent environment). Any decent spring steel (say, AISI 1080 - iron with 0.80 percent carbon added) should be just about indestructible. Depending on the part and how one wants to fabricate it, it can be made of annealed material and then hardened, or it can be made directly of hardened material. If one was limited to a garage-type workshop, fabrication (cutting, bending, welding) followed by hardening would probably be suitable. More complex parts (that is, almost anything else besides magazines) would tend to call for more complex processes, and more expensive materials (such as air-hardening alloys, which avoid the dimensional distortions commonly induced when quenching - not so good for precision parts).