First Aspect
Active Member
More generally, steel is an alloy of iron plus low percentages of other elements, to modify the physical and/or chemical behaviour.Basically, steel is iron with carbon atoms introduced into the metal's crystalline structure by smelting in a blast furnace together with a high-grade form of coal called coke. The percentage of carbon that's incorporated into the molten iron will depend on the what the resultant steel is to be used for.
What the carbon does, is add toughness and "workability" to the metal, simply by acting as a handbrake in the crystalline structure and stopping the iron atoms sliding over each other too easily. Pure iron is very soft and malleable in its wrought form, and brittle when cast.
Not being a metallurgist, I have limited insight into the details, or the extent to which the changes can be correlated or predicted, based on what is added. We chemists tended to think of our metallurgist cousins as the kids in remedial school learning how to use a divining rod, so I don't think it's all that susceptible to an ab initio approach.
But very broadly, atoms of another element is smaller or larger than iron and it acts as both a focus for an a terminus of defects that propogate through the crystal structure under stress. Hence modification of mechanical properties.
Chemical changes I was less clear on, but it's basically the same idea - you disrupt the spacing and also the chemical potentials, so water, principally, interacts in a different way. Or not at all, ideally.
An alloy, incidentally, is a solid solution basically. So iron is the solvent.