Alloying Additions & Effects

Plain carbon steel — iron with 0.05–1.5% carbon — covers a wide range of properties through microstructure control, but it cannot meet many demanding engineering requirements. Hardenability is limited (large sections cannot be hardened through); strength at elevated temperature is poor; corrosion resistance is minimal; toughness at sub-zero temperatures is inadequate. Alloying elements — additions of manganese, silicon, chromium, nickel, molybdenum, niobium, vanadium, titanium, boron, copper, and others — extend the property envelope of steel dramatically.

Alloying elements act through six main mechanisms: (1) solid solution strengthening — dissolving in the iron lattice and distorting it, impeding dislocation movement; (2) precipitation hardening (secondary hardening) — forming fine carbides, nitrides, or carbonitrides that pin dislocations; (3) grain refinement — forming fine carbides or nitrides that restrict austenite grain growth, indirectly increasing toughness; (4) hardenability increase — suppressing the austenite-to-ferrite transformation, allowing martensite formation at lower cooling rates in large sections; (5) elevated temperature strength — forming stable carbides that resist coarsening; (6) corrosion resistance — passivating the steel surface.

The cost of alloying elements varies enormously: manganese and silicon are cheap; niobium, vanadium, and molybdenum are moderate; nickel and cobalt are expensive. Alloy design therefore involves balancing property requirements against cost — achieving the target properties with the minimum alloy content.