Cheap and strong. Or corrosion-proof. Very different prices.

Raw material: 1018 carbon steel ~$0.80-1.20/kg, 304 stainless ~$3-5/kg, 316L stainless ~$4-6/kg. For finished CNC machined parts: carbon steel parts typically 40-60% cost of stainless equivalent due to both material savings and easier machining (stainless work-hardens, requires slower speeds). For typical 1kg bracket: carbon steel $30-50, stainless $50-90 finished.

Yes — properly painted or coated carbon steel lasts 10-20 years outdoors. Required process: primer + topcoat paint (commercial quality), hot-dip galvanize (for fencing and structural), powder coat (for cosmetic outdoor), Cor-Ten weathering steel (rusts to stable patina). For salt-air coastal or road-salt exposure, choose hot-dip galvanize + powder coat combination, or upgrade to stainless.

Heat-treated 4140 (Q&T to 30-32 HRC): 950 MPa yield, 1000 MPa tensile. 17-4 PH H900 stainless: 1170 MPa yield, 1310 MPa tensile. For absolute maximum strength with corrosion resistance, 17-4 PH wins. For maximum cost-effective strength where corrosion isn't critical, 4340 (ultra-high-strength alloy steel) reaches 1400 MPa yield at half the cost of 17-4 PH.

Yes for specific applications. MRI-compatible components: require non-magnetic (austenitic 304, 316L). Sensor housings near magnetic sensors: non-magnetic preferred. Speaker driver magnets: carbon steel magnetic yoke needed. Electromagnetic isolation: non-magnetic stainless or aluminum. For standard mechanical applications, magnetism is irrelevant — strength and cost dominate decisions.

Carbon steel: cutting speeds 80-150 m/min, readily machinable. Stainless 304: 60-100 m/min, work-hardens (must cut continuously), tool wear faster. For equivalent part, stainless machining time is 1.5-2x carbon steel. This multiplies the material cost difference — stainless finished parts typically 1.8-2.5x carbon steel part cost. Budget accordingly when specifying stainless.

Cor-Ten (A242) develops a stable rust patina over 2-3 years, then self-protects from further corrosion. Used for architectural, sculpture, exterior structural. Cost similar to carbon steel. Limitations: cannot be used where rust runoff would stain surroundings, requires 2-3 year weathering before cosmetic appearance is final. Good middle option for outdoor where stainless cost isn't justified but rust concern exists.