631, S17700, 1.4568 spring stainless steel strip
631, S17700, 1.4568 is a precipitation hardening, austenitic stainless steel of the 17Cr-7Ni PH type, with a high tempering effect and good structural stability. The grade is characterized by:
- High tempering effect, up to 400 MPa
- Low relaxation at high stresses and elevated temperatures
- Higher fatigue strength
- Very small distortion during tempering
- High service temperatures
Service temperature: up to 350°C (660°F)
631, S17700, 1.4568 is suitable for complex, formed springs with high demands on spring force and service life. It is also suitable for use at elevated temperatures.
Standards
- ASTM: 631
- UNS: S17700
- EN Number: 1.4568
- EN Name: X 7 CrNiAl 17-7
- SS: 2388
Chemical composition (nominal)
Chemical composition (nominal) %
| C | Si | Mn | P | S | Cr | Ni |
| 0.08 | 0.5 | 0.9 | ≤0.030 | ≤0.015 | 16.5 | 7.5 |
Others: Al=1.0
Applications
631, S17700, 1.4568 is a most suitable grade for springs or other high strength components. It has good spring properties including corrosion resistance, mechanical strength and fatigue resistance, making it an excellent material in most situations. The greatest benefit can be found in applications where low relaxation properties are required even at elevated temperatures. Due to the high ageing effect of this precipitation hardenable grade, high strength can also be reached for complicated shapes or heavy gauge components.
Corrosion resistance
It is very important to avoid corrosion in spring applications so as not to impair spring properties. 631, S17700, 1.4568 is an austenitic stainless steel and has sufficient corrosion resistance in most spring applications. The corrosion resistance is almost the same as ASTM 304 and, compared to other low alloyed spring steels, 631, S17700, 1.4568 has superior performance. However, all austenitic steels of this type are susceptible to stress corrosion cracking (SCC) when in contact with chloride solutions at elevated temperatures.
Bending
The values given below have been obtained by bending according to Swedish standard SS 11 26 26 method 3 (in a 90° V-block with a 25 mm die opening, a sample of 35 mm width, turned so that the burrs of the blanked edges face into the bend). They can be used as guidance for the smallest recommended bending radius.
| Nominal tensile strength | Thickness | Min. bending radius as function of thickness *) | |
| Rm | t | ||
| MPa | mm | ⊥ | // |
| 1300 | 0.25 | 1 t | 4 t |
| 1300 | 0.50 | 1 t | 6 t |
| 1300 | 0.75 | 1 t | 7 t |
| 1300 | 1.0 | 1 t | 7 t |
| 1500 | 0.25 | 1.5 t | 7 t |
| 1500 | 0.50 | 1.5 t | 7.5 t |
| 1500 | 0.75 | 1.5 t | 8 t |
| 1700 | 0.25 | 3 t | 10 t |
| 1700 | 0.50 | 4 t | 11 t |
∗)⊥ Bend transverse to the rolling direction
// Bend parallel to the rolling direction
Forms of supply
631, S17700, 1.4568 is supplied, as standard, in the cold rolled condition. Strip steel can be supplied in coils, bundles, on plastic spools or in lengths. The edges can be either slit, deburred or smoothly rounded.
Dimensions
The following range of thicknesses and widths can be supplied as standard. Please contact Alleima if other dimensions are required.
| Thickness, mm | Width, mm | Thickness, in. | Width, in. |
| 0.015 – 3.00* | 2 – 360 | .0006 – .118 | .079 – 14.2 |
* Depending on requested tensile strength.
Tolerances
The thickness and width tolerances are +/- tolerances to the nominal size. The normal tolerance classes for most of our strip products are T2 and B1. Tighter tolerances as well as other tolerance limits can be offered upon request.
Stock standard
The following combinations of tensile strength and thickness are available from stock. Other combinations can be supplied on request.
| Condition | Tensile strength, Rm | Thickness | |
| MPa | ksi | mm | |
| Cold rolled | 1300 | 189 | 0.20/0.50/1.5/2.0 |
| Cold rolled | 1500 | 218 | 0.75/ 0.80 |
| Cold rolled | 1700 | 247 | 0.15 |
Heat treatment
631, S17700, 1.4568 is a precipitation hardenable steel and, compared to standard austenitic stainless spring steels e.g. type AISI 301, has a more pronounced tempering (ageing) effect. Consequently, it can be supplied with a comparatively low tensile strength in order to provide good forming properties, but still reach a high tensile strength after forming, by a simple heat treatment. Tempering also improves fatigue and relaxation resistance and, furthermore, it will decrease the internal stresses present in the spring after forming.
For maximum increase in mechanical strength, tempering of 631, S17700, 1.4568 should be carried out at 480°C (900°F) for 1 hour. It will give a significant increase in strength for initial tensile strengths above approximately 1300 MPa (189 ksi). This increase can be between 150-400 MPa (22-58 ksi) and is higher the greater is the initial tensile strength. Because of its high structural stability in the precipitation hardened condition, the good spring properties are also retained at elevated temperatures, up to about 350°C (662°F).
To avoid discoloration, parts should be carefully cleaned before heat treatment. Tempering in open air furnaces gives a harmless brownish oxide on the surface.
Mechanical properties
Static strength
Nominal values at 20°C (68°F)
| Condition1) | Tensile strength | Proof strength | Elongation | ||
| Rm | Rp0,2a) | A11,3 | |||
| MPa | ksi | MPa | ksi | % | |
| C | 1300 | 190 | 1150 | 165 | 12 |
| CT | 1550 | 225 | 1400 | 203 | 6 |
| C | 1500 | 220 | 1400 | 203 | 6 |
| CT | 1800 | 273 | 1650 | 240 | 2 |
| C | 1700 | 245 | 1650 | 240 | 2 |
| CT | 2050 | 295 | 1950 | 285 | 1 |
1) C = Cold rolled, CT = Cold rolled and tempered, 480°C (896°F)/1 h. See further under section ‘Heat treatment’.
- a) R2 corresponds to 0.2% offset yield strength.
1 MPa = 1 N/mm2
Fatigue strength
Nominal values at 20°C (68°F) in a normal dry atmosphere. The fatigue limit is defined as the stress at which 50% of the specimens withstand a minimum of 2 million load cycles.
Reversed bending stress
Average stress = 0
Bending transversal to rolling direction.
Comparison made for different thicknesses and tensile strength levels.
| Tensile strength | Fatigue limit | Fatigue limit | |||
| Rm | MPa | ksi | |||
| Thickness, mm | Thickness, in. | ||||
| MPa | 0.50 | 0.75 | ksi | 0.020 | 0.030 |
| 1300 | – | ± 405 | 189 | – | ± 58.8 |
| 1500 | ± 555 | – | 218 | ± 80.5 | – |
| 1700 | ± 610 | ± 525 | 247 | ± 88.5 | ± 76.2 |
| 1900 | ± 620 | – | 276 | ± 90.0 | – |
Fluctuating tensile stress
Minimum stress = 0
Specimens parallel to rolling direction.
Comparison made for different thicknesses and tensile strength levels.
| Tensile strength | Fatigue limit | Fatigue limit | |||
| Rm | MPa | ksi | |||
| Thickness, mm | Thickness, in. | ||||
| MPa | 0.50 | 0.75 | ksi | 0.020 | 0.030 |
| 1300 | – | 340 ± 340 | 189 | – | 49.3 ± 49.3 |
| 1500 | 390 ± 390 | 360 ± 360 | 218 | 56.6 ± 56.6 | 52.2 ± 52.2 |
| 1700 | 425 ± 425 | 410 ± 410 | 247 | 61.7 ± 61.7 | 59.5 ± 59.5 |
| 1900 | 445 ± 445 | 435 ± 435 | 276 | 64.6 ± 64.6 | 63.1 ± 63.1 |
Physical properties
The physical properties of a steel relate to a number of factors, including alloying elements, heat treatment and manufacturing route, but the following data can generally be used for rough calculations. These values refer to cold rolled material, at a temperature of 20oC(68oF) unless otherwise stated.
Density 7.9 g/cm3 (0.29 lb/in3)
Resistivity 0.9 μΩm (35.5 μΩin.)
Modulus of elasticity
´as delivered´: approx 180 000 MPa (26 100 ksi)
tempered: approx 190 000 MPa (27 550 ksi)
Shear modulus
as delivered: approx 70 000 MPa (10 150 ksi)
Specific heat capacity 500J/kg °C (in the temperature range 50-100°C)
Thermal expansion 1)
| Temperature, °C | per °C | Temperature, °F | per °F |
| from 20 – 100 | 13 | from 68 – 200 | 7 |
| from 20 – 200 | 13.5 | from 68 – 400 | 7.5 |
| from 20 – 300 | 14 | from 68 – 550 | 8 |
1) mean values in temperature ranges (x10-6)
Thermal conductivity
| Temperature, °C | W/m °C | Temperature, °F | Btu/ft h °F |
| 20 | 15 | 68 | 8.5 |
| 100 | 15 | 210 | 8.5 |
| 300 | 19 | 570 | 11 |
Welding
631, S17700, 1.4568, like most austenitic stainless steels, has good weldability. Welding, however, introduces excess heat into the material closest to the weld that breaks down the structure formed by cold working. As a consequence, this will decrease the mechanical properties of the welded area. The lowest practical heat input, <1,0 kJ/mm, and interpass temperature for multipass welding, <100°C (210 oF), is recommended.
In most cases, the TIG(GTAW) method is preferable. It can be used either autogenously (without filler metal) or with filler metal. In both cases, pure argon (99,99%) should be used as a shielding gas.
When filler metal is used the recommendation is as follows: according to standard ISO 14343-A: 19 9 L; AWS A5.9/ASME SFA-5.9: ER308L; W.Nr.: 1.4316 and ISO 14343: 19 9 L Si; AWS A5.9/ASME SFA-5.9: ER308LSi; W.Nr.: 1.4316
Due to the high carbon content of 631, S17700, 1.4568, there is also a risk of carbide precipitation at the grain boundaries of the material in the heat affected zone (HAZ), which may decrease the corrosion resistance of the material in certain environments.
