Fastener Steel Grades: S45C, SCM435, SUS304

The grade sets peak hardness, toughness and corrosion resistance. Pick the wrong steel and the part either won’t harden or turns brittle, so understand the basics first.

The number in a steel name often gives carbon content — S45C is about 0.45% carbon.

• SWCH / low-carbon — easy to cold-form for general screws; low carbon, so it needs case hardening (carbonitriding).
• S45C (medium ~0.45% C) — moderately through-hardenable; good for general bolts and studs.
• High-carbon — high hardenability; for springs, lock washers and wear parts.

• SCM435 (chromoly, 34CrMo4) — chromium-molybdenum alloy with good hardenability and high strength; the main feedstock for grade 10.9/12.9 bolts.
• SCr / SNCM — alloy steels for high-load, high-fatigue automotive components.

• SUS304 (A2) — good corrosion resistance for general/outdoor/food use, but austenitic, so not heat-hardenable (hardens by cold work).
• SUS316 (A4) — higher corrosion resistance, salt-tolerant; for marine/chemical work.
• SUS410/420 (martensitic) — hardenable but less corrosion-resistant than 304; for screws and pins that must be hard.

High load + hardening → SCM435; low-cost general work → case-hardened carbon steel + zinc plating; corrosion focus without high hardness → SUS304; both hard and fairly corrosion-resistant → martensitic stainless. Our team advises by real use.