This is a good thread packed with excellent information. As I've gained experience over the years, like many others, I've found some favorite metals that can do most of the jobs we need.
One thing on metals cost - we generally work on a very small scale. If I'm welding up a flat bed trailer, I'm concerned about materials cost. But for a small engine, I'd rather have a superior metal, even if it costs a bit more. Especially if a part is more complicated, as a superior metal can contribute to fewer mistakes and less scrapping.
Maybe already mentioned - watch out for the stresses in steel when you have an asymmetric part to machine. Let's say you want to machine a "U" shape using a mild steel flat stock. That C1018 cold-drawn flat stock looks really nice and clean, but cut out the inner portion of the "U" and it'll warp like a pretzel. Use hot-roll instead. You can tell the difference in that hot-roll has a black, scaly surface, while cold drawn (cold roll) steel has a pleasing, clean surface. This is all very applicable to crankshafts especially, and hot roll 4140 or 4340 is the best choice.
OK, I like cold rolled for it's nice, smooth surface appearance. I don't have to work to make it look nice. So, if I use hot-rolled, and pickle off that scaly surface, how do I make it smooth and nice looking? I don't have a surface grinder. I don't want to machine a flat bar just to make it nicely finished, so what do I do?
I am making brackets for DRO's on my mill, that do not require much machining or strength, just some threaded holes and a large cutout along 1 side. Or do I go with 7075 or even 6061? Then with Al. I have to paint it, which is OK.
The black mill scale can be removed from hot roll or heat treated steel with a pickling solution. Works like a charm. Salt & Vinegar, diluted muriatic (HCl) acid, sulfuric acid, all work. But watch out for hydrogen embrittlement.
For mild steel, why use 1018 when 12L14 is available? 12L14 is glorious stuff.
With aluminum, I now rarely buy 6061 when 7075 is available. Yes 7075 is expensive and scarce, but what a material. Machines like a dream, and I will readily replace a mild steel component with 7075-T6, as the aluminum is both harder AND stronger than C1018.
Tool steel... for me, it's all A-2, all the time. For years, I used O-1 which is good steel, but the heat treatment... what a pain! Coat with a borosilicate glass like keep-bryte, or watch 0.008" fall off the part as ugly scale. Instead, the A-2 parts go into a stainless foil envelope. The envelope is heat treated, removed from the furnace with tongs, and swished through the air or set on a drill press table. The parts come out grey, purple, blue, and squeaky-hard. And A-2 tempering temperatures are very high, typically 700 f or more to get the same hardness as O-1 would see at 350. This means A-2 parts can see service under higher temperatures.
Brass/bronze - if given a choice, give me bronze, purely for cosmetic reasons! What is more lovely than aged bronze, buffed occasionally with a soft cloth, rather than a nasty brass that corrodes more readily and can turn green?
That's all I can think of for now. Materials can be very complex, especially when dealing with hardening carbon steels. A good furnace is a huge boon to a home shop, and opens up all sorts of possibilities for advanced work.
Mild steel: 12L14
Med. carbon steel: 4140
Aluminum: 7075, 2024, 6061 in order (keep welding and/or anodizing in mind, that may force a choice)
Tool steel: A-2 (O-1 or W steels are fine if no heat treatment is needed.
Stainless: 303, she's for me... ;D