Why steel grade matters
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.
Carbon steel
- 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.
Alloy steel
- 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.
Stainless steel
- 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.
How to choose
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.
FAQ
Can 304 stainless be hardened?+
Not by heat (it’s austenitic); it work-hardens from cold forming. For real hardness, choose a martensitic grade.
What steel are grade 10.9 bolts?+
Mostly SCM435 alloy steel, quenched and tempered to the grade’s hardness and toughness.
