Cold drawn spring steel wire SWP-A and SWP-B are the two dominant Japanese JIS-standard grades for automotive and industrial springs. This guide compares the two grades on strength, fatigue performance and typical applications, with selection guidance for spring makers and OEM procurement.
Cold drawn spring steel wire per JIS G3521 (SWP-A, SWP-B, SWP-V) is the dominant specification for high-quality springs in automotive suspension, valve springs, agricultural machinery, hardware, and industrial springs. The two grades most commonly specified are SWP-A (oil-tempered, used for the highest fatigue applications) and SWP-B (hard drawn, used for general purpose springs). The grade choice drives the spring life, the fatigue limit, and the cost per kilogramme.
What SWP-A and SWP-B Specify
SWP-A is an oil-tempered (quenched and tempered) high-carbon steel wire, typically supplied in diameters from 1.0 mm to 12.0 mm, with tensile strength ranging from about 1,400 MPa for the largest diameters to over 2,000 MPa for the smallest. The oil-tempered microstructure gives the best combination of strength and ductility, and the smooth surface finish (drawn from a pearlitic microstructure) gives excellent fatigue life under cyclic loading. SWP-B is a hard drawn (cold worked, not heat treated) high-carbon wire, with slightly lower tensile strength but adequate for less demanding applications.
When SWP-A Is the Right Specification
SWP-A is the right specification for high-performance springs subject to severe cyclic loading: automotive suspension springs, valve springs, clutch springs, brake springs, and any spring where fatigue life is the design driver. The oil-tempered microstructure gives a fatigue limit typically 2–3 times higher than SWP-B for the same tensile strength, which is why SWP-A is required for almost all automotive engine and chassis springs. SWP-A also gives better relaxation resistance at elevated temperature, making it the choice for springs that see thermal cycling in service.
When SWP-B Is the Right Choice
SWP-B is the right specification for general purpose springs that see static or low-cycle loading: furniture springs, mattress innersprings, agricultural springs, hardware springs (door hinges, latches, clips), toys, and consumer goods. SWP-B is significantly cheaper than SWP-A and is the right choice when the spring life requirement is thousands rather than millions of cycles. For springs that see occasional impact or vibration but not continuous high-cycle fatigue, SWP-B gives adequate performance at lower cost.
Surface Finish, Testing and Coil Handling
Both SWP-A and SWP-B are supplied in coils of 100–500 kg, with surface finish that is smooth and free of decarburization, scratches, or wire-drawing marks. The wire is tested for tensile strength, reduction of area, and (for SWP-A) fatigue life per JIS G3521. For critical automotive springs, also specify a 100% eddy-current surface defect test. Coils should be stored in dry conditions to prevent rust before spring coiling, and should be processed within 6 months of delivery to avoid age-related cracking in some high-carbon grades.
What to Specify on the Spring Wire RFQ
State the standard (JIS G3521 SWP-A or SWP-B), the wire diameter (with tolerance), the tensile strength range or grade, the coil weight and inner diameter, the surface finish requirement, and any supplementary testing (fatigue test, eddy current test, decarburization test). For high-cycle automotive and industrial springs, default to SWP-A; for general purpose springs, default to SWP-B. Always confirm the mill supplies the test report per EN 10204 3.1 with the actual tensile strength, reduction of area, and any fatigue test results.
Related Resources from Baobin Steel
- Steel wire catalog – spring, galvanized and ACSR wire
- Mild steel overview – grades and standards
- Request a quote – send your spring spec for cold drawn wire pricing