Which Steel Grades Harden — and How Far
Pick your grade to see the right process, realistic hardness, and what to put on the order — based on the small parts we harden every day.
S45C
AISI 1045 / DIN C45Carbon steel · C 0.42–0.48%- Recommended process
- Quench & temper (Q&T)
- As-hardened surface
- 50–58 HRC
- Typical in-service
- 40–55 HRC
Medium-grade bolts and nuts, pins, small shafts and general machine parts
The most common hardening grade in Thailand; service hardness tunes over a wide band via tempering temperature.
Not sure of your grade, or want the spec confirmed before ordering? Send us your drawing for a free check — reply within 24 h.
What determines how hard a steel can get
Grades respond to hardening very differently. Carbon content sets the ceiling (more carbon → harder), while alloying elements such as chromium, molybdenum or boron improve hardenability — how deep the hardness reaches. Below roughly 0.3% carbon, direct hardening barely works; the standard route is carburizing, which adds carbon to the surface first.
As-quenched hardness and in-service hardness are not the same number. Freshly quenched steel is at its hardest but brittle, so it is tempered — trading some hardness for toughness. The "in-service" column shows the ranges drawings actually specify.
The numbers on this page are typical for small parts (screws, nuts, bolts, springs, pins), which through-harden easily. Real results still depend on section size, quench medium and heat chemistry — send us your drawing and we confirm the spec before any work starts.
Grade-by-Grade Hardness Chart (13 grades)
Typical ranges for small parts (screws, nuts, bolts, springs, pins) after the recommended process.
| Grade | Carbon (%) | Recommended process | As-hardened surface (HRC) | Typical in-service (HRC) | Typical uses |
|---|---|---|---|---|---|
| S20CAISI 1020 / DIN C22 | 0.18–0.23 | Carburizing (case hardening) | 55–62 | 55–60 | Self-tapping screws, machine screws, rivets and parts needing a wear-resistant case over a tough core |
| S45CAISI 1045 / DIN C45 | 0.42–0.48 | Quench & temper (Q&T) | 50–58 | 40–55 | Medium-grade bolts and nuts, pins, small shafts and general machine parts |
| SCM435AISI 4135 / DIN 34CrMo4 | 0.33–0.38 | Quench & temper (Q&T) | 48–55 | 33–44 | Grade 10.9 high-tensile bolts, studs and load-bearing screws |
| SCM440AISI 4140 / DIN 42CrMo4 | 0.38–0.43 | Quench & temper (Q&T) | 50–58 | 28–48 | Grade 12.9 bolts, high-load pins and torque-carrying parts |
| SCr440AISI 5140 / DIN 41Cr4 | 0.38–0.43 | Quench & temper (Q&T) | 50–57 | 28–45 | Load-bearing bolts and pins, small automotive parts |
| SWCH10B21AISI 10B21 | 0.18–0.23 + B | Quench & temper (Q&T) | 40–48 | 28–40 | Cold-headed self-tapping screws and grade 8.8–10.9 bolts |
| SWCH10B33AISI 10B33 | 0.30–0.36 + B | Quench & temper (Q&T) | 45–52 | 32–42 | Grade 10.9 bolts, wheel nuts and cold-headed high-strength screws |
| SUP9AISI 5155 / DIN 55Cr3 | 0.52–0.60 | Quench & temper (Q&T) / Austempering | 50–58 | 40–50 | Leaf springs, coil springs, spring washers and automotive spring parts |
| SK85 (SK5)AISI 1084–1086 / DIN C85 | 0.80–0.90 | Quench & temper (Q&T) / Austempering | 58–64 | 45–60 | Flat springs, clips, spring washers and small blades |
| SUJ2AISI 52100 / DIN 100Cr6 | 0.95–1.10 | Quench & temper (Q&T) | 60–64 | 58–64 | Small bearings, rollers, hardened pins and gauge pins |
| SKS3AISI O1 (近) / DIN 100MnCrW4 | 0.90–1.00 | Quench & temper (Q&T) | 58–63 | 56–62 | Small punches, dies, gauges and small cutting tools |
| SUS420J2AISI 420 / DIN X30Cr13 | 0.26–0.40 | Quench & temper (Q&T) | 48–55 | 45–52 | Stainless parts that must be hard: special screws, small blades, small valve stems |
| SUS304AISI 304 / DIN X5CrNi18-10 | ≤ 0.08 | Not hardenable by heat treatment | — | — | General stainless nuts, screws and washers where corrosion resistance matters more than hardness |
Figures are typical ranges for small, thin-section parts, based on JIS specifications and our production experience. Actual results depend on section size, quench medium, tempering temperature and the heat's actual chemistry — treat as guidance, not a guarantee.
Steel Grade & Hardening FAQ
Can SUS304 be hardened?
No — SUS304 is austenitic stainless; its structure does not transform to martensite on quenching, so heat treatment will not harden it. It only work-hardens from cold forming. If you need a hardenable stainless, use a martensitic grade such as SUS420J2 (about 48–55 HRC).
Why can't low-carbon steels like S10C–S20C be hardened directly?
Below roughly 0.3% carbon, too little martensite forms and the hardness gain is minimal. The standard route is carburizing — diffusing carbon into the surface before quenching — giving a 55–62 HRC case over a tough core. Ideal for self-tapping screws and wear parts.
What should a hardening order specify?
At minimum: steel grade, target hardness as a range (e.g. 38–44 HRC), where it is measured (surface or core), case depth for carburizing work, and quantity or weight. A drawing or photo helps — send it through our quote form.
How far can actual hardness deviate from this chart?
It depends on section size, quench medium, tempering temperature and the heat's actual chemistry. These ranges assume small parts that through-harden easily. We confirm the spec together before accepting the job and ship in-house lab readings with every lot.
What we can tell you that the calculator cannot
A grade reaching a hardness in a table and your part reaching it on our line are different claims: section size, prior condition and the tolerance band on the drawing all move the result. Send the drawing and we will check it against the processes we actually run. We reply to quote requests within 24 hours.