The diaphragm spring that disengages your clutch and the disc springs that cushion it are stamped from spring steel strip in the 2.0–4.0 mm range — hardened and tempered to 42–50 HRC before the blank ever hits the die.
The diaphragm spring flexes millions of times, holds a load set that the driver can feel, and sits in an oil bath at operating temperature. That combination — fatigue, load stability, and surface integrity — is the definition of spring steel strip, and the grade choice is really a fatigue-budget decision.
Chromium and vanadium refine the quench structure and pin the grain; this is the grade for diaphragm springs that must survive 1,000,000+ cycles with a tight load window. Costs more, earns it back in warranty terms.
Choose 50CrV4 when the load window and fatigue life are contractual.Manganese (0.90–1.20%) buys deeper hardening in heavier 3–4 mm sections where lean 1078 would run soft in the core. The standard spring steel of Chinese and much of Asian transmission production.
Choose 65Mn for thicker discs and through-hardened sections.No alloy, just carbon. For thin disc springs and wave washers in the 2.0–2.5 mm range it hardens fully and costs the least; the fatigue envelope is adequate where stress levels are modest.
Choose 1078 for thin discs and cost-driven programs.Indicative values from production clutch programs; the final spec is agreed at RFQ against your spring drawing and transmission duty.
| Parameter | Typical value | Why it matters |
|---|---|---|
| Strip thickness | 2.0 – 4.0 mm | Diaphragm springs run the heavy end; disc springs and wave washers run thin |
| Delivery condition | Hardened & tempered (H&T) | Spring rate and hardness delivered with the strip; blank, stress-relieve, assemble |
| Hardness (delivered) | 42 – 50 HRC (per grade and spec) | Sets the load at deflection — the number the clutch feel is built on |
| Hardness uniformity | ± 2 HRC across coil and heat | Keeps the spring rate inside tolerance from one part to the next |
| Fatigue life | ≥ 1,000,000 cycles (system target) | Material + edge + surface must hold the load window for transmission life |
| Decarburization | Controlled, measured per coil | A soft surface layer lowers the fatigue limit and shifts the load |
| Surface / edge | Clean, edge condition agreed (rolled or slit) | Blanked edges are stress raisers; edge quality is a fatigue input |
| Width / coil form | Slit to progressive-die width | Blanking of annular springs wants exact strip width with controlled burr |
Fatigue is verified by the spring maker's endurance rig; the strip contributes uniform hardness, low decarb and a clean microstructure. All three are documented per coil.
All three supplied hardened-and-tempered with a confirmed HRC band, or spheroidized if you harden in-house. Choose by section, fatigue budget and governing standard.
| Grade | Typical C % | Why it is specified | Typical use in the clutch |
|---|---|---|---|
| 50CrV4 | 0.47 – 0.55 + Cr/V | Best fatigue life and load stability; the premium diaphragm choice | Diaphragm springs, main clutch springs |
| 65Mn (GB) | 0.62 – 0.70 + Mn 0.90–1.20 | Better hardenability in 3–4 mm sections; standard in Asian transmission programs | Disc springs, cushion springs, thicker plates |
| SAE1078 | 0.72 – 0.85 | Lean high carbon, lowest cost, hardens fully in thin sections | Thin disc springs, wave washers, shims |
| C67S (EN) | 0.65 – 0.73 | European spring strip per EN 10132-4; good cold formability for stamped springs | EU-spec clutch and transmission springs |
| 60Si2MnA (GB) | 0.56 – 0.64 + Si/Mn | Silicon spring steel for higher strength where the design allows lower toughness | Heavy-duty disc springs, high-load cushions |
Chemistry shown is the typical band; confirm the delivered heat on the MTC. C67S values are indicative of the EN 10132-4 family — refer to the latest revision of the standard for the governing limits.
These are the failure modes that show up in spring programs, in the order transmission engineers care about them.
If hardness wanders, the load at engagement wanders and the clutch feel changes car to car. Fix: H&T delivery with a tight HRC band and ±2 HRC uniformity confirmed per coil.
Cracks start at the blanked edge or a soft surface layer. Fix: clean microstructure, measured decarb, and edge condition agreed at RFQ — plus shot peening on your side.
Springs that relax under sustained deflection fail the clutch. Fix: adequate tempering and stable carbide structure — the reason 50CrV4 is specified where load stability is contractual.
2.0 mm H&T strip is hard to blank; die wear and edge cracking show up fast. Fix: consistent hardness and gauge so the die runs one window; we confirm both on the certificate.
The clutch assembly uses more than one steel family. Match your component below and confirm at RFQ.
50CrV4 / 65Mn / SAE1078 H&T strip for diaphragm and disc springs — the core of this page. SAE1078 strip →
Spheroidized medium carbon for clutch plates and hubs that are blanked then hardened. Fine blanking range →
For non-spring load plates and separators where fatigue is secondary. High carbon family →
A typical single-plate clutch diaphragm spring, described the way it runs.
The part is a diaphragm spring about 240 mm across, 3.0 mm thick, with a continuous outer ring and radial fingers machined or blanked into the inner disc. The drawing calls 50CrV4 strip, 3.0 mm, hardened and tempered to 44–48 HRC, hardness uniformity ±2 HRC, with the load at a defined deflection stated as a tolerance. The strip arrives slit to width, is blanked to the outer ring, fingers are cut, edges are deburred and rounded, then the spring is shot-peened, stress-relieved, and load-tested at the assembly line.
The load test is the moment of truth: every spring is measured at the engagement deflection, and the scatter of the batch has to fit the assembly tolerance. That scatter is inherited from the strip — its hardness band, its uniformity, its surface. When the strip is right, the line's job is just to not disturb it; when the strip wanders, no amount of peening fixes the load. Our job is the former: 50CrV4, H&T, ±2 HRC, measured decarb, and an MTC that documents all of it.
If your spring sits in the 2.0–4.0 mm envelope, send the drawing and we confirm grade, HRC band, uniformity and a trial-coil plan.
Related reading: what actually drives fatigue life in hardened spring strip, and how decarburization is controlled on annealed high-carbon strip.
These grade families share material, processing or application territory with the topic on this page.
65Mn / 50CrV4 / 60Si2Mn grades with high elastic limit and fatigue resistance.
SAE1078 / 65Mn / SK5 grades for hardened parts, springs and cutting edges.
40Cr / 42CrMo / 20CrMnTi grades with through-hardening and toughness for loaded components.
Send the spring drawing or part number, thickness × width, HRC band and quantity. Our engineers reply within one working day with grade match, uniformity confirmation and a trial-coil option.