304 Spring Wire - Tensile Strength Over Hardness Numbers

Product Introduction

TOPONE supplies 304 stainless steel spring wire in 0.15-12 mm diameters, delivered in the standard drawn tempers of EN 10270-3 and tested by tensile strength - not by hardness. This page explains why tensile testing is the correct acceptance method for spring wire and how to compare hardness figures correctly when a drawing calls for them.

Product Specifications

Why Wire Is Sold by Tensile Strength

A hardness indentation needs a flat, thick surface to be valid. Spring wire is round, thin and often under 2 mm - an indentation deep enough to measure would be distorted by the curved surface and by the work-hardened gradient left by drawing. A tensile test, by contrast, uses the whole wire cross-section and reports the property that actually governs spring performance. That is why EN 10270-3, ASTM A580 and JIS G4314 all specify tensile ranges per diameter, and why a certificate quoting only hardness on fine wire should be questioned.

Converting Between Tensile and Hardness - With Care

Standard tables (ASTM E140, ISO 18265) give approximate conversions:

• 1800 MPa tensile corresponds to roughly 530 HV or 51 HRC.
• 1400 MPa corresponds to roughly 430 HV or 44 HRC.
• 1000 MPa corresponds to roughly 310 HV or 32 HRC.

These are estimates, not equivalence. Cold-drawn austenitic wire is heavily textured and non-homogeneous through the section, so conversions can be off by 10 percent or more. Use them for screening, never for acceptance.

What This Means for Your Drawing

• Specify tensile strength and diameter, not hardness, for spring wire.
• If hardness must appear on the drawing, state the tensile range it came from, so the mill can test what is meaningful.
• For flat or thick sections cut from wire, hardness testing becomes valid again.
• Ask for the test method standard (ASTM A370 or EN ISO 6892) to be stated on the MTC.

Applications

• Compression, extension and torsion springs
• Wire forms and retaining rings
• Spring clips and electrical contacts
• Weaving and brushing wire where resilience matters

Product Advantages

• Tensile tested per batch on the actual wire diameter
• Tempers aligned to EN 10270-3 classes for predictable spring rate
• Tight diameter tolerance for automatic coiling
• 3.1 MTC with chemistry and mechanical data for every coil lot

Quality Control

Tensile tests follow ASTM A370 / EN ISO 6892 on full-section wire specimens. Diameter is verified with calibrated micrometers along the coil, and results are reported on the mill test certificate shipped with each batch.

Selection and Ordering Notes

1. Give the wire diameter and required tensile range, or the spring standard and class.
2. State whether loads are static or dynamic, to help select temper.
3. Confirm surface finish: bright, soap coated or pickled.
4. Confirm coil weight and packaging.
5. Request tensile data per lot on the MTC.