ISO 3506-4 Mechanical Properties of Stainless Steel Tapping Screws
ISO 3506-4 specifies the mechanical and physical properties of corrosion-resistant stainless steel tapping screws. The standard addresses the performance requirements of stainless steel fasteners used in applications where corrosion resistance and reliable fastening are important. It covers different stainless steel material groups, with requirements established according to the material classification and applicable screw specifications.
The mechanical performance of stainless steel tapping screws depends on the selected material, manufacturing process, and screw geometry. Properties such as hardness and torsional strength are important for evaluating the screw’s ability to withstand installation stresses without premature failure. ISO 3506-4 provides a standardized basis for assessing these properties and verifying compliance with the specified requirements.
Surface Hardness for Martensitic grades:
Surface hardness is an important parameter for evaluating the resistance of martensitic stainless steel tapping screws to surface deformation and wear. The specified hardness requirements help ensure that the screw surface can withstand the stresses encountered during installation. The following table summarizes the applicable surface hardness requirements for martensitic stainless steel tapping screws in accordance with ISO 3506-4.
| Stainless steel grade | Hardness class | Surface hardness, HV 0,3 min. |
|---|---|---|
| C1, C3 | 30H | 300 |
| C1, C3 | 40H | 400 |
Core Hardness:
Core hardness indicates the hardness of the material beneath the surface and provides information about the screw’s overall material condition. For stainless steel tapping screws, the applicable requirements depend on the material group and grade specified in ISO 3506-4. The following table presents the relevant core hardness requirements for the applicable stainless steel grades.
| Stainless steel grade | Hardness class | Core hardness, HVᵃ min. |
|---|---|---|
| A2, A3, A4, A5, A8 | 20H | 200 |
| A2, A3, A4, A5, A8 | 25H | 250 |
| F1 | 20H | 200 |
| F1 | 25H | 250 |
| D2, D4, D6, D8 | 20H | 200 |
| D2, D4, D6, D8 | 25H | 250 |
| D2, D4, D6, D8 | 30H | 300 |
Torsional Strength:
Torsional strength measures the ability of a tapping screw to withstand twisting forces during installation without fracturing. This property is particularly important because excessive installation torque can cause screw failure before the fastening operation is completed. ISO 3506-4 specifies the applicable torsional strength requirements according to the relevant screw dimensions and material classification. The following table summarizes the corresponding requirements.
| Thread | Minimum breaking torque, MB,min (Nm) — 20H | 25H | 30H | 40H |
|---|---|---|---|---|
| ST2,2 | 0,38 | 0,48 | 0,54 | 0,60 |
| ST2,6 | 0,64 | 0,80 | 0,90 | 1,00 |
| ST2,9 | 1,00 | 1,20 | 1,40 | 1,50 |
| ST3,3 | 1,30 | 1,60 | 1,80 | 2,0 |
| ST3,5 | 1,70 | 2,2 | 2,4 | 2,7 |
| ST3,9 | 2,3 | 2,9 | 3,3 | 3,6 |
| ST4,2 | 2,8 | 3,5 | 3,9 | 4,4 |
| ST4,8 | 4,4 | 5,5 | 6,2 | 6,9 |
| ST5,5 | 6,9 | 8,7 | 9,7 | 10,8 |
| ST6,3 | 11,4 | 14,2 | 15,9 | 17,7 |
| ST8 | 23,5 | 29,4 | 32,9 | 36,5 |
Dimension Standard:
The dimensions of hexagon socket countersunk head cap screws are specified according to the applicable metric or inch standard. For metric sizes, DIN 7991 and ISO 10642 define the principal dimensions, including thread diameter, length, head diameter, head height and hexagon socket dimensions. For inch-series screws, ASME B18.3 specifies the corresponding dimensional requirements for socket countersunk head screws.
