Choosing the material family, steel and heat treatment, stainless, and aluminum — and avoiding corrosion and cracking
How to choose a material for a part — family, grade, heat treatment, corrosion resistance — with the reasoning, the risks, and a confidence level. Source stock and specialty materials from the Materials & Chemicals supplier directory, and see the full engineering guides index.
Use carbon/alloy steel for strength and stiffness at lowest cost where corrosion is managed by coating. Use aluminum for light weight, good machinability, and corrosion resistance where absolute strength isn't critical. Use stainless steel for corrosion resistance and hygiene. Use titanium for the best strength-to-weight with corrosion resistance where cost is justified (aerospace, medical, marine). Use engineering plastics for low weight, electrical insulation, chemical resistance, and no corrosion.
Selection balances strength/stiffness, weight, corrosion, temperature, machinability, and cost. Steel gives the most strength and stiffness per dollar but rusts. Aluminum is ~1/3 the density with a self-protecting oxide, but lower stiffness and strength. Stainless buys corrosion resistance at higher cost and worse machinability. Titanium is strong, light, and corrosion-proof but expensive and hard to machine. Plastics win on weight, chemistry, and insulation but not on stiffness or high temperature.
Carbon steel in a wet/corrosive service without coating rusts. Aluminum where stiffness governs deflects (its modulus is ~1/3 of steel, regardless of alloy). Over-specifying titanium or stainless where coated steel would serve wastes cost and machining time. Match the property that actually governs the part, not habit.
Choose the grade by whether you need hardenability and strength (alloy steels like 4140/4340), machinability (12L14, 1018), or wear surface. Specify heat treatment to the requirement: through-hardening (quench & temper) for uniform strength/toughness, case hardening (carburize/nitride/induction) for a hard wear surface over a tough core, and always temper after hardening to trade a little hardness for the toughness that prevents cracking.
Carbon content and alloying set how hard and how deep steel can harden; heat treatment sets the final structure. Quench-and-temper gives strength through the section for shafts and structural parts. Case hardening puts a hard, wear- and fatigue-resistant skin on a shock-tolerant core — ideal for gears, pins, and cams. Tempering is not optional: as-quenched martensite is hard but brittle, and tempering relieves stress and restores toughness at a controlled hardness.
Skipping temper leaves a brittle part that cracks in service. Through-hardening a gear tooth that needed a tough core makes it snap; case-hardening a part that needed core strength leaves it soft underneath. A low-hardenability grade won't reach spec hardness in a thick section (quench cracks or soft core).
Use 304 austenitic as the general-purpose stainless. Step to 316 (added molybdenum) for chloride/marine/medical/chemical environments where 304 pits. Use a martensitic 400-series (410/420/440) when you need a stainless that can be hardened for wear or cutting edges — accepting lower corrosion resistance. Passivate after machining regardless of grade.
Chromium gives the passive oxide layer; the alloying beyond it sets the trade. 316's molybdenum resists chloride pitting that eats 304 in seawater, de-icing salt, and many process fluids. Austenitic grades (304/316) can't be hardened by heat treatment (only work-hardened), so when a stainless must be hardened for an edge or wear surface, you move to a martensitic 400-series — which has less chromium in solution and so corrodes more easily. There is no single "best" stainless; it's a corrosion-vs-hardenability trade.
304 in a chloride environment pits and stains despite being "stainless." Expecting to harden 304/316 by heat treatment fails — they don't respond. Using a 440 blade grade in a wet corrosive service rusts faster than expected. And unpassivated stainless rusts from machining-smeared free iron.
Use 6061-T6 as the versatile default — good strength, weldable, corrosion-resistant, machinable. Use 7075 for the highest strength (aerospace structure) where weldability and corrosion resistance are secondary. Use 2024 for high strength and fatigue resistance (also poor weldability/corrosion). Use 5052/5083 for formable, marine-grade, weldable sheet. Match the temper (T6, T651, etc.) to the strength and stress-relief the part needs.
Aluminum alloys trade strength against weldability, corrosion resistance, and formability, and the temper sets how much of the alloy's strength is developed. 6061-T6 sits in the sweet spot for general machined and structural parts. The high-strength 2000/7000 series buy strength by alloying with copper/zinc — which hurts corrosion resistance and weldability, so they're used where strength-to-weight rules and joining is mechanical. The 5000-series marine alloys prioritize corrosion resistance and formability over peak strength.
Welding 7075 or 2024 cracks and destroys their properties (they're generally not weldable). Using 6061 where 7075's strength was needed under-builds the part. Ignoring temper (e.g. annealed vs T6) leaves the part far below expected strength. Marine service in a copper-bearing alloy corrodes.
Accelerated corrosion at the joint between two different metals — the less-noble metal (e.g. aluminum, steel) corrodes rapidly where it contacts a more-noble one (e.g. stainless, copper), especially in the presence of moisture or salt. Pitting, white/red corrosion product, and loosening fasteners.
Two metals far apart on the galvanic series in electrical contact with an electrolyte (moisture, salt) present, and an unfavorable area ratio (a small anode feeding a large cathode corrodes fastest) — e.g. steel fasteners in an aluminum structure, or aluminum against stainless.
1) Pick metals close on the galvanic series where they must contact. 2) Electrically isolate dissimilar metals — insulating washers, sleeves, gaskets, coatings. 3) Seal out the electrolyte (paint, sealant) so no continuous moisture path exists. 4) Favor a large anode / small cathode area ratio, not the reverse. 5) Use sacrificial coatings (galvanizing) or compatible fasteners (e.g. stainless in aluminum with isolation).
Sudden, brittle fracture of a high-strength part — often a fastener or spring — sometimes hours or days after installation, with little warning and no obvious overload. Cracks appear under sustained tensile stress in high-hardness steel.
Hydrogen absorbed during electroplating, pickling, or corrosion, then trapped in high-strength steel (roughly ≥1000 MPa / ≥~40 HRC is most susceptible), combined with sustained tensile stress. Stress-corrosion cracking is the related failure where a specific corrosive environment plus tensile stress cracks a susceptible alloy over time.
1) Bake plated high-strength parts promptly after plating (e.g. ~190 °C for several hours) to drive out absorbed hydrogen. 2) Prefer non-embrittling finishes (mechanical plating, certain coatings) on critical high-strength fasteners. 3) Keep the hardest grades below the susceptibility threshold where the design allows. 4) For SCC, avoid the alloy/environment pairings known to crack (e.g. certain stainless in chlorides) and reduce sustained tensile stress. 5) Follow the governing spec for plating + bake on high-strength hardware.