304H Stainless Spring Wire - What the H Suffix Actually Guarantees

304H stainless spring wire is the high-carbon variant of the familiar 304 grade. The single letter H is not a marketing term - it is a specification-controlled carbon minimum that changes how the wire behaves in elevated-temperature service. This page explains, in plain technical terms, exactly what the H suffix guarantees and where it matters for spring and forming applications.

Product Specifications

What the H Suffix Guarantees

In ASTM A240 and A276, the H designation requires a minimum carbon content of 0.04 percent together with verified high-temperature mechanical properties. The key guarantee is not simply more carbon - it is that the material will retain usable creep and stress-rupture strength at service temperatures where standard 304 would relax.

Carbon Content and the H Designation

Standard 304 allows carbon up to 0.08 percent but sets no minimum. 304H is controlled to a carbon range of 0.04-0.10 percent, so every heat is verified to meet the lower bound. That guaranteed minimum is what the spec commits to.

Elevated-Temperature Strength

The higher carbon raises the material allowable stress in the 500-800 C range and slows stress relaxation in hot springs and fasteners. For room-temperature springs, 304H offers no advantage over 304 and is not required; the H grade is specified only when the component sees sustained heat.

Applications

• Springs and spring washers operating at elevated temperature
• Valve trim, seals, and gaskets in thermal systems
• Fasteners and retaining rings exposed to heat
• Wire forms in furnace and exhaust environments
• High-temperature forming and coiling work

Product Advantages

• Specced carbon minimum of 0.04 percent verified per heat
• Better creep and stress-rupture resistance than 304 at heat
• Same corrosion resistance as the 304 family (18/8)
• Consistent diameter tolerance for automatic coiling
• Available soap-coated for draw-down or bright annealed

Quality Control

Each coil is supplied against a material test certificate (EN 10204 3.1) recording heat analysis, including the verified carbon value. Diameter is checked across the length, and surface is inspected for draw marks and contamination before packing.

Selection and Ordering Notes

1. Confirm the service temperature - specify 304H only if heat is involved.
2. State the wire diameter and the tolerance class you require.
3. Choose soap-coated for further drawing or bright annealed for direct use.
4. Indicate the packing format (spool, coil, or cut length).
5. Request the 3.1 material certificate if your QA requires heat-level data.

If your spring or formed part runs warm, the H suffix is the line on the certificate that proves the carbon floor - ask for it by name and we will match the spec to your drawing.