High Carbon Steel C75S +QT (1.1248)
Spring steel strip to EN 10132-4, material number 1.1248, delivered hardened and tempered (+QT). At around 0.75 % carbon it is three to four times the strength of structural steel and springs back instead of taking a set. Nothing in it resists corrosion, and at this carbon level it cannot be welded.
Typical parts: Flat and leaf springs, spring clips, detents, contact fingers, shims, wear plates and doctor blades.
Technical Properties
| Composition | Fe remainder, C 0.70 to 0.80, Si 0.15 to 0.35, Mn 0.60 to 0.90, P max 0.025, S max 0.025 (%) |
|---|---|
| Standard | EN 10132-4 |
| Material number | 1.1248 |
| Condition as delivered | +QT, hardened and tempered |
| Density | 7.85 g/cm³ |
| Tensile strength Rm | 1300 to 1900 N/mm² by gauge |
| Yield strength Rp0.2 | approx 1100 to 1600 N/mm² |
| Elongation at break A | approx 5 to 8 % |
| Hardness | 40 to 53 HRC |
| Elastic modulus E | 210 GPa |
| Thermal conductivity | 45 W/(m·K) |
| Max service temperature | 150 to 170 °C |
| Heat treatable | Already hardened and tempered |
| Weldable | No, weld and HAZ crack |
| Magnetic | Yes |
| Does it rust | Yes, readily. Keep oiled |
Available thicknesses and manufacturability
| Thickness (mm) | Weight (kg/m²) | Min. Hole Diameter | Min. Bridge Width | Min. Hole-to-Edge |
|---|---|---|---|---|
| 0.2 mm | 1.57 | 0.2 mm | 0.15 mm | 0.15 mm |
| 0.3 mm | 2.36 | 0.2 mm | 0.15 mm | 0.15 mm |
| 0.5 mm | 3.93 | 0.25 mm | 0.15 mm | 0.15 mm |
| 0.8 mm | 6.28 | 0.4 mm | 0.16 mm | 0.16 mm |
| 1 mm | 7.85 | 0.5 mm | 0.2 mm | 0.2 mm |
Max Part Size: 550 x 1150 mm · Min. Part Size: 10 x 10 mm
Calculate the weight for your dimensionsWhat a part in High Carbon Steel C75S +QT costs
Example: a mounting plate 100 × 60 mm, 2 × 2 holes (5.5 mm, for M5) in every thickness, computed when this page loads with the configurator's own formula. Price per piece incl. VAT, plus shipping.
| Thickness | 1 pcs | 10 pcs | 100 pcs |
|---|---|---|---|
| 0.2 mm | €10.09 | €3.17 | €1.49 |
| 0.3 mm | €10.26 | €3.32 | €1.63 |
| 0.5 mm | €10.66 | €3.70 | €1.98 |
| 0.8 mm | €11.25 | €4.22 | €2.44 |
| 1 mm | €11.58 | €4.52 | €2.71 |
The configurator prices other sizes, holes and shapes as you type; you pick material and thickness there next to the part. Configure this plate
Parts in High Carbon Steel C75S +QT
These parts have a page of their own with prices, limits and the questions people ask most:
Which thickness for which spring
C75S is sold as spring steel or spring strip, and older drawings call it Ck75. It arrives hardened and tempered, so it is already hard and springy and needs no treatment after cutting. We cut 0.2 to 1 mm, up to 600 mm wide:
- Shims and spacer plates: 0.2 to 0.5 mm, depending on the gap
- Contact springs, small clips, snap tongues: 0.2 to 0.3 mm
- Leaf springs, spring clips, clamps: 0.5 to 0.8 mm
- Scrapers, doctor blades, wear strips: 0.8 to 1 mm
A useful rule: spring force rises with the cube of the thickness. Twice as thick means eight times the force for the same deflection, but also much less travel before the part bends for good.
How the part arrives
The parts come dry, and C75S rusts faster than almost any other steel. Oil them straight away or store them somewhere dry.
At these thicknesses the edges are usually fine to handle, and deburring is rarely needed. Very thin parts, especially at 0.2 and 0.3 mm, bend easily when unpacked, so handle them flat.
Tolerance is ISO 2768-m: a 20 mm clip is within ±0.2 mm, a hole up to 6 mm within ±0.1 mm.
Working on it yourself
- Drilling: Hardened spring steel is hard on drills and taps. Have every hole cut by the laser; adding one later is a lot of effort.
- Cutting: 0.2 and 0.3 mm can be trimmed with sheet metal shears, but the hard steel blunts them quickly.
- Rust protection: A thin film of oil or wax is enough indoors. Coatings that cure hot, such as powder coating, would ruin the temper.
- Joining: Rivets, screws or clamping, not welding. At this carbon content the weld and the area next to it crack.
Common mistakes
- Holes and sharp inside corners where the spring bends most: That is where a spring breaks first. Give inside corners a radius and keep holes out of the most heavily bent area.
- Too thick: A spring that is too stiff barely moves and bends for good at the first real load. Try it thinner first.
- Storing it unoiled: A few days in damp air are enough for flash rust.
- Expecting it to be stainless: C75S is carbon steel. A spring that has to stay rust free needs stainless spring steel 1.4310, which we do not stock; ask through the quote form.
Questions and Answers
- What does the laser do to the temper?
- Read this one before you order. The cut drives the edge past the tempering temperature, so a heat affected zone a few tenths of a millimetre wide runs along every cut, softer than the rest of the part. On most parts nobody notices. If the cut edge is the bit that actually does the springing, tell us before we cut it.
- Bending, welding?
- Bending only to generous radii, and at 1300 to 1900 N/mm² reckon on serious springback. Welding, no. At 0.75 % C the weld and the zone around it harden and crack.
- How hot can it get in service?
- 150 to 170 C. Past that it carries on tempering and stops being a spring. That is also why there is no powder coating option here: powder cures at 180 to 200 C.
- Why so thin, and only 600 mm wide?
- +QT is made as strip, not as sheet. 600 mm is about what the mills roll, and past 1 mm it cannot be through hardened evenly. Bigger parts mean a different material, or a soft annealed condition you harden yourself after cutting.