ISO 3506: Mechanical properties of stainless steel screws
The ISO 3506 series of standards specifies the mechanical properties of fasteners made of corrosion-resistant, stainless steel Part 1 of the series deals with screws with coarse and fine threads and defines the permissible steel grades, strength classes, and associated test methods for these. The standard thus forms the basis for stainless steel screws. counterpart to ISO 898-1This standard applies to fasteners made of carbon steel. For technical purchasing, quality assurance, and testing laboratories, ISO 3506-1 provides the basis for the traceable testing and documentation of the strength values of stainless steel screws.
the essentials in brief
ISO 3506-1 defines the mechanical requirements for screws made of stainless steel and specifies how their strength is verified in a tensile test. The following overview summarizes the key points of the standard.
- The standard applies to Screws with metric ISO thread and a nominal thread diameter of up to 39 mm.
- The test is performed in an ambient temperature range between 10ºC and 35ºC.
- They are recorded austenitic, martensitic, ferritic and austenitic-ferritic duplex steel grades.
- The strength of the screws is determined by the Classes 50, 70, 80 and 100 divided.
- The tensile test on the entire screw provides the characteristic values yield strength Rp0,2, tensile strength Rm and elongation at break A.
- The series of standards is divided into several parts: ISO 3506-1 for screws, ISO 3506-2 for nuts, Additional parts for threaded studs, sheet metal screws and other fasteners.
What does ISO 3506 regulate?
ISO 3506-1 specifies the strength classes, permissible steel grades, marking, and mechanical and physical properties of screws with any head shape whose nominal thread diameter does not exceed 39 mm. The threads must conform to the dimensional specifications of DIN ISO 68-1, DIN ISO 261, and DIN ISO 262. The test itself is carried out within a defined temperature range of 10 °C to 35 °C to ensure comparable results.
The ISO 3506 series of standards comprises several parts: ISO 3506-1 covers screws, ISO 3506-2 covers nuts with specified steel grades and property classes. Other parts address, among other things, set screws and fasteners not subjected to tensile stress, as well as sheet metal screws. In the 2020 version, the structure of ISO 3506-1 was aligned with that of ISO 898-1. Furthermore, austenitic-ferritic duplex steels, additional steel grades, and a new property class 100 for austenitic steels were included in the standard.
Steel grades and strength classes
ISO 3506-1 distinguishes four groups of stainless steels, which differ in microstructure, hardenability and corrosion resistance.
| Steel group | Identification letter | Characteristics |
|---|---|---|
| Austenitic | A1 to A5, A8 | Not hardenable, high toughness, good corrosion resistance. Higher strength is achieved exclusively through work hardening. |
| Martensitic | C1 to C4 | Hardenable by heat treatment, higher achievable strength, lower corrosion resistance than austenitic steels. |
| Ferritic | F1 | Magnetic, moderate strength, good resistance to stress corrosion cracking. |
| Austenitic-ferritic (duplex) | D2, D4, D6, D8 | A combination of high strength and good corrosion resistance. Part of the standard since the 2020 version, available in strength classes 70, 80 and 100. |
The designation of a stainless steel screw consists of the steel grade and the strength class. In the designation A4-70, A4 stands for the steel grade and 70 for the strength class, which indicates one-tenth of the nominal minimum tensile strength in MPa. A screw of class 70 must therefore achieve a minimum tensile strength of 700 MPa. The material 1.4404, as documented in the test certificate for steel grade A4-70, belongs to the austenitic group and is one of the corrosion-resistant chromium-nickel-molybdenum steels commonly used in industry.
Testing method according to ISO 3506-1
The mechanical properties are determined in a tensile test on the entire bolt. In method B15, documented in the test certificate, the bolt is tested with a strain gauge The test allows for the precise determination of tensile strength, elongation at break, and yield strength (Rp0,2). Since the strength properties of stainless steel screws are partially achieved through cold forming, testing the entire screw, not a turned specimen, is crucial.
The example of a M6x40 socket head cap screw made of material 1.4404 (steel grade A4-70) shows the variation in test results within a batch:
| Sample No. | Rp0,2 (MPa) | Rm (MPa) | Elongation at break A (mm) |
|---|---|---|---|
| 1478879 - 25250 | 539 | 743 | 8,6 |
| 1478879 - 25254 | 469 | 716 | 8,5 |
| 1478879 - 25252 | 576 | 704 | 6,6 |
| 1478879 - 25253 | 521 | 708 | 6,5 |
| 1490501 - 16392 | 590 | 732 | 8,3 |
| 1490501 - 16391 | 476 | 732 | 8,6 |
All six samples achieve a tensile strength of at least 700 MPa, thus meeting the requirements of strength class 70. The yield strength Rp0,2 varies considerably more between the samples than the tensile strength, which is typical for work-hardened austenitic steels. The elongation at break is between 6,5 and 8,6 mm for all samples, confirming ductile fracture behavior.
Related standards
- ISO 898-1: Mechanical properties of fasteners made of carbon steel and alloy steel, the equivalent of ISO 3506-1 for non-corrosion-resistant materials.
- ISO 3506-2: Mechanical properties of nuts made of corrosion-resistant, stainless steel.
- ASTM F606: US standard for testing the mechanical properties of fasteners.
- ISO 6892-1: General basis for the tensile test, to which ISO 3506-1 refers for the test procedure.
Requirements for modern testing machines
For the tensile test on complete bolts according to ISO 3506-1, a testing machine is required that covers different force ranges depending on the thread size. Smaller threads, such as the M6 in the documented example, can be tested with comparatively low test forces, while larger thread diameters require significantly higher maximum forces. In the present test report, a Galdabini Quasar 600 was used, a testing machine with a maximum force of 600 kN.
In addition to force measurement, a precise strain gauge or... EXTENSOMETER This is necessary to reliably determine the yield strength Rp0,2. Suitable clamping devices for the screw holder are also a prerequisite for results that comply with standards.
Factors influencing the measurement results
- According to the standard, the ambient temperature must be between 10 °C and 35 °C, as temperature fluctuations affect the strength values.
- Testing the entire screw, rather than a turned sample, is crucial, as the strength is partly due to cold forming in the thread area.
- Regular calibration of the testing machine and the strain gauge is a prerequisite for traceable and comparable results.
FAQ: Frequently asked questions about ISO 3506
Why do stainless steel screws require a separate testing procedure?
Austenitic chromium-nickel steels, unlike carbon steels, cannot be hardened by heat treatment. Their strength results solely from work hardening that occurs during manufacturing, for example, through thread rolling. This difference in material behavior necessitates a separate testing standard, as the relationships between deformation, strength, and elongation behavior differ from those of carbon steel according to ISO 898-1. In addition to mechanical strength, corrosion resistance plays a crucial role in stainless steels, which is why the steel grade and strength class are always specified together in the marking.
What does ISO 3506 regulate?
ISO 3506-1 specifies the steel grades, strength classes, marking, and mechanical and physical properties of screws made of corrosion-resistant, stainless steel.
What does the designation A4-70 mean?
A4 designates the steel grade, in this case an austenitic chromium-nickel-molybdenum steel. The number 70 indicates the strength class and corresponds to a minimum tensile strength of 700 MPa.
What are the differences between ISO 3506 and ISO 898-1?
ISO 898-1 applies to fasteners made of carbon steel and alloy steel, while ISO 3506-1 applies to fasteners made of corrosion-resistant stainless steel. Since austenitic stainless steels are not hardenable, the strength mechanisms and therefore parts of the testing methodology differ.
How is the yield strength of screws determined according to ISO 3506-1?
The yield strength Rp0,2 is determined in a tensile test on the entire screw using an extensometer, since the strength properties are partly achieved through cold forming.
