70 KD

Oil tempered SiCr-alloyed clutch and transmission spring wire

70 KD is especially intended for the manufacture of clutch and transmission springs and similar moderately high fatigue stressed springs. 70 KD S is a shaved version of 70 KD intended for applications with higher surface requirements. The material has good relaxation properties. The wire is manufactured in sizes from Ø 0.50 to 7.00 mm. Other wire sizes and shapes on request.

Chemical composition

C (%) Si (%) Mn (%) P max. (%) S max. (%) Cr (%)
0.50 - 0.60 1.20 - 1.60 0.50 - 0.80 0.025 0.020 0.50 - 0.80

Cleanliness in steel

Non-metallic inclusions:
Max. level 2 according to DIN 50602-M.
Corresponding standard: ASTM E-45-A.

Mechanical properties

1) Other wire sizes on request.
2) Ovality, i. e. the difference between the largest and smallest dimension of a cross section, is maximum half the tolerance range.
3) Conversion from tensile strength to hardness values can be calculated in standard ISO EN 18265. The tensile strength Rm within one coil does not vary more than 60 N/mm2.
4) Torsion test is carried out for assessing deformability. The fracture of the torsion test piece shall be smooth and perpendicular to the wire axis. The rupture shall show no longitudinal cracks.

For round wire

Diameter (mm) Tolerance (± mm) Tensile Strength (N/mm²) Torsions (l=300 mm, min. revs) Reduct. of area (min. %)
0.50 - 0.80 0.010 2080 - 2210 6
0.81 - 1.30 0.015 2080 - 2210 5 50
1.31 - 1.40 0.015 2080 - 2210 5 50
1.41 - 1.60 0.020 2080 - 2210 5 50
1.61 - 2.00 0.020 2010 - 2160 5 50
2.01 - 2.50 0.020 1960 - 2060 5 50
2.51 - 3.00 0.020 1910 - 2010 4 50
3.01 - 3.20 0.020 1910 - 2010 4 45
3.21 - 3.50 0.025 1910 - 2010 4 45
3.51 - 4.50 0.025 1860 - 1960 4 45
4.51 - 5.00 0.025 1810 - 1910 3 45
5.01 - 5.60 0.030 1810 - 1910 3 40
5.61 - 6.00 0.035 1760 - 1860 3 40
6.01 - 7.00 0.035 1760 - 1860 40

Yield point

The proof stress Rp0.2 is min. 0.9 x tensile strength of the wire.

Surface conditions

Surface condition

Surface condition – non-destructive testing
In the standard size range 2.00 - 6.00 mm the wire is tested continuously in Eddy Current equipment ; for 70 KD a level of 60 microns and for 70 KD S of 40 microns.

Surface condition – end sample test
The wire is end sample tested by means of etch testing and binocular inspection as well as microscopical inspection of the material structure. Max. permissible depth of partial surface decarburization and surface defects, 1.3% x wire diameter. No complete decarburization allowed.

Physical properties

E and G modulus of elasticity

206 kN/mm²

E and G modulus of shear

79.5 kN/mm²

Recommendations

Heat treatment

As soon as possible after coiling, the springs should be stress relieved.

Hot presetting

After shot peening, the springs should be hot preset or stress relieved. In order to reach optimum fatigue and relaxation properties, the springs must be preset at an appropriate stress.

Shot peening

In order to obtain optimum fatigue properties, the process time should be adjusted to get a complete treatment. Size of shots should be adapted to wire dimension, pitch and shot peening equipment. Shot peening of the inside of the spring coils is particularly critical.