The most important point about steel is that, although steel is by definition an alloy, the term alloy steel refers to the use of one or more of the metallic alloying agents (chromium, vanadium and molybdenum). The other very important point about steel is that just carbon steel with no other alloying agents other than carbon can be a really extremely tough and strong material when it is produced with precise manufacturing techniques.
Structural Carbon Steel
Work Hardening, Tempering and Case Hardening
What most people think of if the word steel is used alone is mild steel. Mild steel is a carbon steel with a slightly low carbon content which dramatically reduces the strength of the material. What mild steel also is though is malleable, which makes it easy to use for a variety of reasons. Mild steel is easy to drill, easy to cut and easy to machine. Even a hand crank drill can be used to drill through mild steel, although it is still a lot of hard work and requires a specialty drill bit geometry. Mild steel can also easily be bent into shape. This is significant for the use of stamping and bending machinery in a production setting, but the malleability of mild steel is also very significant for smaller scale operations. Custom parts made out of mild steel can be hammered and bent into position relatively easily.
Parts and machinery made out of mild steel tend to be heavier and somewhat less functional than similar designs in other materials, but mild steel does have some very significant functional advantages. Most importantly mild steel tends to just bend out of shape when it fails, where other materials break and fail catastrophically. If something made of mild steel is accidentally subjected to excessive loads it often just bends out of shape and can be brought back into a serviceable condition just by bending it back. Even when parts made out of mild steel do actually break the failure tends to be much more localized and easier to repair than when parts made out of other materials break.
Structural carbon steel is also an alloy of just iron and carbon, but it has the precise proportion of carbon to attain the highest tensile strength. The key piece of information for understanding structural carbon steel is that the lattice structure of carbon and iron atoms is not a perfect crystalline structure. What this means is that there is actually a small range of proportions of carbon that yield very strong structural carbon steel with somewhat different properties. If the high end of the range of carbon proportions is used then the structural steel is very hard and even slightly brittle. If the lower end of the range of proportions of carbon is used then the structural steel is not quite so hard, but still just about as strong. There is a precise carbon proportion for the absolute strongest carbon structural steel, and this yields a very strong material that is in fact so tough that it has often been confused for other types of materials.
Even slightly harder carbon steel can be made with a higher carbon content, but these hardest carbon steels are not as strong and are extremely brittle. If even more carbon is used then what is produced is cast iron, which is not nearly as strong and is not even as hard as high carbon steel.
Another consequence of the fact that the lattice structure of carbon steel is not a perfect crystalline structure is the importance of how the structural steel is put together. Forged carbon structural steel is considerably stronger than any carbon structural steel cast at atmospheric pressure. The reason that forging, or some other high pressure process, is important is that it forces the carbon and iron lattice structure uniformly into the closest approximation of a crystalline structure attainable. The way this is often described is that the "grain" of the steel becomes aligned to produce a uniform material.
Rolling steel can produce higher levels of lattice alignment, so sheet and other rolled steel products are often somewhat tougher than castings made out of the same material. High pressure casting where the molten steel is pumped into a pressure mold has also been used to attain higher levels of lattice alignment and a finished product more similar to forged steel.
Cast or rolled carbon steel can be made harder by work hardening, tempering and case hardening. When any carbon steel that is at all malleable is bent back and forth it will get harder and slightly stronger before it breaks. This is why it is more difficult to straighten a bent part the second time it gets bent out of shape than the first time it got bent out of shape. Work hardening is however usually a destructive process, the piece is nearly always less useful after work hardening than before work hardening.
Tempering is a general term that can refer to work hardening, quenching or hammering. It is sometimes useful to harden steel by heating it and then quenching it in water. This rapid cooling changes the alignment of the lattice structure and yields a significantly harder material, at least near the surface. Heating steel and then hammering it obviously tends to produce a material more similar to forged steel, and in fact the term "forged" itself originally referred to hammering. Case hardening is usually a process of quenching heated steel in oil to increase the carbon content of the surface layer of material.
The metallic alloying agents, chromium, vanadium and molybdenum, can be used to produce a variety of alloy steel products with considerably different properties than are attainable with just carbon steel. Most significantly there is at least one perfect crystalline structure attainable with rather large quantities of one or more of the alloying agents. This perfect crystalline structure is much stronger than the lattice structure of any of the carbon steels. The strongest alloy steel is really extremely strong, the toughest stuff on the planet really.
Alloy steels are however expensive and undesirable to work with for a variety of reasons. First and foremost the alloying agents are both much harder to come by than iron and carbon and also pose more severe environmental and public health hazards than just iron and carbon. Both of these factors contribute to the metallic alloying agents and the finished alloy steels being rather expensive.
Alloy steels are much more of an environmental and public health hazard at just about every stage of the production process. Mining the metallic alloying agents tends to cause much worse environmental problems when done haphazardly, and likewise carless material handling procedures can result in exposure of workers to unacceptable levels of toxic materials. The production facilities where alloy steel is produced are also more of a potential environmental hazard than steel mills where only carbon steel is produced. And again producing alloy steel tends to be more of a health hazard to the workers than producing just carbon steel. Grinding of alloy steel in manufacturing facilities also poses both an environmental and public health hazard. Grinding carbon steel makes a bit of a mess, but it is easy to clean up and poses only very minimal environmental and public health risk. Grinding alloy steel on the other hand tends to produce a variety of nasty materials that make workers sick and contaminate dirt and water.
Despite the higher cost and greater difficulty in producing alloy steel there are applications where the much higher strength or other desirable properties are overwhelmingly desirable. The important distinction though is that forged carbon structural steel can in fact be so extremely strong and tough that it seems sufficient for nearly any application. Where alloy steels tend to seem more indispensable is where high corrosion resistance is required.
Forged carbon structural steel is in fact much more corrosion resistant than mild steel, and this is very significant. Where high corrosion resistance is required in a malleable material though 316-L stainless steel is practically indispensable. Bronze and brass can work for some applications, but are not as strong as steel and also have very poor cycle lives in demanding applications. It should be noted that brass is an easy to work with material but does not have anywhere near the corroseion resistance of bronze. Likewise aluminum alloys can be somewhat corrosion resistant but perform very poorly is applications where large amounts of bending or flexing are required. Despite the high cost of 316-L stainless steel it still ends up being the cheapest and easiest material to use when high levels of corrosion resistance are required.
The top of the line material is of course titanium, which is not only very strong and totally corrosion resistant but also a whole lot lighter than any steel. "As light as aluminum and as strong as steel!" is the traditional way of describing the amazing properties of titanium. The reality is that titanium is quite a bit denser than aluminum alloys and also is not nearly as strong as alloy steel. The fact that titanium is as strong as mild steel and very resilient while being much lighter does make it a highly desirable structural material. Titanium makes the best parts for just about any application where high hardness is not required, but it is difficult to work with and of course extremely expensive.