Why Add Other Metals to Steel? Changing What It Can Do
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Steel is iron with a small amount of carbon, and adding further elements in modest quantities transforms its hardness, toughness and resistance to corrosion. The alloys in use are designed for specific failure modes.
What carbon already does
Iron by itself is soft and of limited use, and adding carbon up to roughly two per cent produces steel, whose properties depend strongly on how much carbon is present and on how the metal was cooled. Carbon atoms sit between the iron atoms and obstruct the planes along which the lattice would otherwise slide, which is what makes the material harder and stronger and also less ductile. Rapid cooling traps the carbon in an arrangement that is very hard and brittle, and reheating gently afterwards trades some of that hardness for toughness, which is the basis of the whole heat treatment craft. Everything further is a matter of adding other elements that interact with the iron, with the carbon or with each other.
What the common additions do
Each element is added for identifiable effects:
- •Chromium for corrosion resistance, forming a transparent protective oxide layer, and for hardness
- •Nickel for toughness, particularly at low temperature, and for stabilising a more ductile crystal structure
- •Manganese for strength and to counteract the harmful effects of sulphur
- •Molybdenum for strength at high temperature and resistance to a form of embrittlement
- •Vanadium and niobium for fine grain size, which raises strength and toughness together
- •Tungsten for hardness retained at the temperatures a cutting tool reaches
- •Silicon for strength and for particular magnetic properties in electrical steels
How stainless works
The corrosion resistance of stainless steel is not a coating and is worth understanding correctly. Chromium above roughly eleven per cent reacts with oxygen to form an extremely thin, transparent and tightly adherent oxide layer over the whole surface, which stops further oxygen reaching the metal beneath. The layer is only a few atoms thick, is invisible, and reforms immediately if scratched, provided oxygen is available, which is why the material is self-repairing and why it corrodes in environments that exclude oxygen or that attack the layer chemically, including some chloride solutions. Different grades trade corrosion resistance against strength and workability, and the numbers used to identify them encode composition. The material was developed independently in several countries in the early twentieth century.
How steel is actually made
The chemistry is only useful if the material can be produced at scale, and the production history explains why steel arrived when it did. Iron smelted directly from ore in early furnaces emerged as a spongy mass requiring extensive hammering, and carbon content was controlled by experience rather than measurement, which made consistent steel expensive and rare. Blast furnaces produce liquid iron with too much carbon, so the problem became removing it, and the process patented by Bessemer in 1856 did that by blowing air through the molten metal, burning out carbon in minutes rather than hours and collapsing the price. Modern practice uses oxygen rather than air for the same purpose, or melts scrap in an electric arc furnace, and alloying elements are added at the end with composition verified by analysis before casting.
Designing against failure
Alloy selection is generally a response to a specific way the part might fail, which explains why so many grades exist. A component that must hold an edge needs hardness retained at working temperature. A pressure vessel needs toughness so a crack does not propagate catastrophically, and the sensitivity of toughness to temperature is why several catastrophic failures of ships and structures occurred in cold conditions. A part under repeated loading needs resistance to fatigue, which begins at surface defects. A part in a chemical plant needs resistance to the specific chemistry present, since no material resists everything. A part that will be welded needs a composition that does not embrittle in the heat affected zone. Each requirement narrows the choice, and the resulting grade is a compromise rather than an optimum.
The takeaway
Carbon obstructs the planes along which the iron lattice would slide, which hardens it, and heat treatment trades hardness against toughness. Chromium, nickel, molybdenum and vanadium each address a specific property. Stainless resistance comes from a self-repairing oxide layer a few atoms thick, which fails where oxygen is absent or chlorides attack it.