OTEVA 75 SC, OTEVA 75 SC PLUS

Oil tempered SiCrV-alloyed high tensile valve spring wire

OTEVA® 75 SC is a Super Clean steel, especially intended for the manufacture of clutch/ transmission springs with extremely high fatigue properties and good relaxation properties at moderately increased working temperatures.

Manufactured as standard in shaved condition in sizes from Ø 1.60 mm to 6.50 mm, or in egg or elliptical shape corresponding to round cross section 2.50 mm to 6.50 mm. Other wire sizes and shapes on request.

OTEVA® 75 SC PLUS is intended for manufacture of valve springs and other springs requiring extremely high fatigue properties and good relaxation properties at increased working temperatures.

Further information about the approval process for OTEVA SC PLUS is found here.

Chemical composition

C (%) Si (%) Mn (%) P max. (%) S max. (%) Cr (%) V (%)
0.50 - 0.70 1.20 - 1.65 0.50 - 0.80 0.020 0.020 0.50 - 1.00 0.05 - 0.20

Cleanliness in steel

The presence of non-metallic inclusions in the wire rod is inspected for every heat of OTEVA® 75 SC in accordance with the Suzuki Garphyttan method by the steel supplier.

Before release for production, Suzuki Garphyttan performs non-metallic inclusion inspection for every fifth heat. The criteria for supplier inspection and releasing inspection are the following;

For wire rod samples: Inclusion size max. 15 µm down to 1 mm below surface. Inspection area: 1 000 mm2.

Inclusion size, surface 5-10 µm >10-15 µm >15 µm
Max. number of inclusions 50 7 0

For OTEVA 75 SC PLUS, every heat is inspected including a SEM-EDS analysis of inclusions > 10µm to verify a Super Clean composition.

As stated by IVSWMA, International Valve Spring Wire Manufacturers Association, it is likely to find occasional inclusions in valve spring quality steel of a size larger than 30 µm.

Mechanical properties

For round wire

Diameter (mm) Tolerance (± mm) Tensile Strength (N/mm²) Torsions (l=300 mm, min. revs) Reduct. of area (min. %)
1.60 - 2.00 0.020 2160 - 2260 5 45
2.01 - 2.50 0.020 2110 - 2210 5 45
2.51 - 3.20 0.020 2060 - 2160 5 45
3.21 - 4.00 0.025 2010 - 2110 4 45
4.01 - 5.00 0.025 1960 - 2060 3 45
5.01 - 5.60 0.030 1910 - 2010 3 40
5.61 - 6.00 0.035 1910 - 20100 3 40
6.01 - 6.50 0.035 1910 - 2010 35

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 to a surface level of ≥ 40 microns. Other wire sizes on request.

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 % x wire diameter. In shaved condi-tion; for diameters <=2 .00 mm 10 µm, for diameters > 2.00 mm 0.5% x d.

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.

 

 

 

 

 

Spring Conditions For Tests

Spring conditions for fatigue and relaxation tests (specially designed test spring) Diagram 1 and 2:
Wire size Ø 4.00 mm
Diameter external 28.00 mm
Spring length, l0 59.5 mm
N active 4.80
Spring index 6.0
Stress relieving
Temperature 420 ±5°C (790 ±10 °F)
Time 30 minutes
Shot peening (double shot peening)
1st treatment Speed 56 m/sec. for 20 minutes, size of shots 0.8 mm
2nd treatment Speed 30 m/sec. for 10 minutes, size of shots 0.8 mm
Hardness of Shot-peening grit (shot): 610-670 Hv
Aim for Almen arc-height Min 0.45 mm
Hot presetting
(theoretically set)
1300 N/mm2
Temperature 200°C (max. 250°C)
Time 10 minutes

Relaxation And Fatigue Properties

In diagram 1 the fatigue properties of this grade are illustrated in a Goodman-diagram, based on a special test spring design.

Diagram 2 shows the relaxation properties (loss of load) of springs made from OTEVA® 75 SC wire subjected to static compression at different temperatures.