Cheap and strong. Corrosion resistant. Different jobs.

Carbon steel contains mostly iron. Iron reacts with oxygen and moisture to form iron oxide (rust). No protective barrier — rust flakes off exposing fresh iron. Stainless steel contains chromium (10%+) which forms a thin protective chromium oxide layer that self-heals when scratched. This prevents further corrosion. The passive layer is what makes stainless "stainless".

Yes, widely. Zinc plating (galvanization), powder coating, paint, phosphate conversion — all protect carbon steel from rust. Protected carbon steel is the most cost-effective approach for most structural applications. However: protection can fail (scratches, chips, coating breakdown), requires maintenance, limits service life vs inherently corrosion-resistant stainless. For applications requiring 20+ year service without maintenance, stainless preferred.

Austenitic stainless (304, 316L) work hardens rapidly during machining. The surface becomes harder as it deforms, increasing cutting force and tool wear. Low thermal conductivity means heat concentrates at the cutting edge. Solutions: sharp carbide tooling, aggressive feeds (keep ahead of work hardening), flood coolant, rigid setup. Machining time: typically 2-3× equivalent carbon steel. Cost premium: 30-50% above carbon steel for machining.

304: general stainless, cheapest, excellent for most corrosion applications, food equipment, architectural. 316L: 2% molybdenum added, better chloride/saltwater resistance, required for marine and high-chloride service. Cost: 316L 15-25% more than 304. Rule: for indoor or non-chloride, 304 works. For coastal, marine, high-chloride food (sauces, brines), specify 316L. When in doubt, 316L is safer choice.

Carbon steel (4140, 4340, 8620): through-hardens to 50-62 HRC via quench and temper. Wide range of strength options via heat treatment. Stainless 304/316L: cannot be heat-hardened (austenitic — no phase transformation). Work hardening only. Stainless 17-4 PH, 15-5 PH: precipitation hardening stainless — can heat-treat to 1100 MPa yield. Martensitic stainless (420): can be hardened like carbon steel but sacrifices corrosion resistance.

Austenitic stainless (304, 316L) non-magnetic in annealed state — slightly magnetic after cold work. Ferritic and martensitic stainless magnetic. Carbon steel always magnetic. For applications requiring non-magnetic (MRI environments, sensor equipment, compass proximity), specify austenitic stainless or aluminum/titanium.