4.8 · 8.8 · 10.9 · 12.9 — How to read steel bolt property classes
First number is nominal tensile strength, second the yield ratio. Nominal values versus the standard's minimums, how old 4T/8T marks relate, materials and hydrogen embrittlement, and why tapping screws are excluded.
The property class of a steel (carbon or alloy steel) bolt or screw is two numbers separated by a dot. For 8.8:
- first 8 → nominal tensile strength 800 N/mm² (number × 100)
- second .8 → nominal yield strength is 80% of nominal tensile → 640 N/mm²
One mark carries the nominal relation between tensile and yield. The standard is ISO 898-1, which covers carbon and alloy steel bolts, screws and studs with metric threads. Stainless steel (A2-50 etc.) has its own system, and tapping screws are not covered by these classes either; their mechanical properties come from separate standards (ISO 2702 for heat-treated steel, ISO 3506-4 for stainless, KS B 0251 in Korea, etc.).
Class table
Nominal values and the minimums the standard actually requires are different, so the table shows them separately.
| Class | Nominal tensile | Min. tensile | Min. 0.2% proof Rp0.2 | Proof stress Sp | Hardness (HV) | Heat treatment |
|---|---|---|---|---|---|---|
| 4.8 | 400 | 420 | — (Rpf 340*) | 310 | 130–220 | None (as cold headed) |
| 6.8 | 600 | 600 | — (Rpf 480*) | 440 | 190–250 | None |
| 8.8 (≤ M16) | 800 | 800 | 640 | 580 | 250–320 | Quenched and tempered |
| 8.8 (> M16) | 800 | 830 | 660 | 600 | 255–335 | Quenched and tempered |
| 10.9 | 1,000 | 1,040 | 940 | 830 | 320–380 | Quenched and tempered |
| 12.9 | 1,200 | 1,220 | 1,100 | 970 | 385–435 | Quenched and tempered |
N/mm² (= MPa). Basis: ISO 898-1:2013 Table 3. Separately from the maximum hardness, hardness measured at the end of the fastener is 250 HV maximum for classes 3.6 to 5.8. * For 4.8 and 6.8, Rpf (stress at non-proportional elongation of the full-size fastener) is noted in the standard as under investigation, given for calculating the proof stress ratio and not a test value. The "340" for 4.8 should therefore not be read as a minimum yield strength.
Korea has related standards, KS B 0233 (based on ISO 898-1:1999) and KS B ISO 898-1, which may differ in edition from the ISO 898-1:2013 used here. For delivery and inspection, check the standard and edition specified in the contract and drawing.
A property class states the mechanical properties required of the material and product, but some products cannot carry the full tensile load because of their head geometry (countersunk or low heads), so check the relevant product standard as well.
The difference between 4.8 and 8.8 is heat treatment
Classes 4.8 and 6.8 are generally non-heat-treated carbon steel products. Their strength comes from cold forming (heading and rolling); with no heat-treatment step they cost less and ship faster. Our carbon steel machine screws and flange bolts are in this group.
From 8.8 upward, parts are quenched and tempered, and the steel changes to one that responds to heat treatment — boron steel (51B20 etc.) or chromium-molybdenum alloy steel (SCM435). The standard permits carbon steel, boron-added carbon steel and alloy steel for 8.8 and 10.9. Class 12.9 needs high hardenability and strict control of material and heat treatment; alloy steel is mostly used, and carbon steel with additives is also permitted under the conditions the standard sets. So the actual material of high-strength classes is often alloy steel. Material, heat treatment and inspection all differ, making it a different product even at the same size — a stock 4.8 bolt does not become 8.8 by being made "a bit stronger".
4T · 8T · 10T · 12T — the old JIS marks
The T marks still common on site (4T, 8T, 10T) are old JIS strength marks and do not exist in the current standards (ISO 898-1, KS B 0233). The number before T times 10 gives tensile strength in kgf/mm², so 4T is about 40 kgf/mm² (about 392 N/mm²). The mark carries no yield information.
| Old mark | Current class to compare | Note |
|---|---|---|
| 4T | 4.6 or 4.8 | Tensile level comparison; not a direct equivalence |
| 6T | 6.8 etc. | Confirm the applicable standard |
| 8T | 8.8 | Customary correspondence |
| 10T | 10.9 | Customary correspondence |
| 12T | 12.9 | Customary correspondence |
This table compares the approximate tensile level of old marks and current classes. Because the systems differ, they are not automatically equivalent. If a drawing or purchase specification uses a T mark, confirm the applicable standard and required mechanical properties before deciding on a substitute. Combined marks such as "4.8T" also appear; on drawings, certificates and purchase documents we recommend the current class (4.8 etc.), which has a standard behind it.
Choosing a class
- 4.8 — sheet, covers and electronics, where high preload is not needed
- 8.8 — structural joints and machine frames
- 10.9 · 12.9 — socket head cap screws needing high preload. Check the strength of the mating thread (nut or tapped hole) too — a 12.9 bolt in a weak mating part fails the mating part first.
Whatever the class, proper tightening conditions matter; tightening torque is set from the required preload and friction (Tightening, loosening and breaking torque).
High-strength bolts and plating — hydrogen embrittlement
When hard steel bolts such as 10.9 and 12.9 are electroplated, hydrogen taken up during plating can later cause sudden fracture (delayed fracture). ISO 4042 sets measures to reduce the risk of hydrogen embrittlement according to property class and plating process, with stricter control the higher the strength, as for 12.9. We bake plated 10.9 and 12.9 parts after plating. Baking reduces the risk but does not guarantee that embrittlement can never occur, so process control in line with the standard and customer requirements is also needed.
In corrosive service, review the material and property class together. High-strength steel bolts can face higher risks of corrosion, stress corrosion cracking and hydrogen embrittlement depending on the environment. Switching to stainless, however, can lower strength considerably (A2-70 minimum tensile 700 N/mm² against 1,220 N/mm² for 12.9). Check that the required preload can still be reached, whether A4 suits a chloride environment better than A2, and the risk of galling — see A2-50 · A2-70 · A2-80 for stainless property classes.
How classes appear in our catalogue
| Product group | Material | Property class |
|---|---|---|
| Cross recessed machine screws · hexagon flange bolts · SEMS (carbon steel) | Carbon steel (10A) | 4.8 |
| Socket head cap screws · socket head SEMS (high strength) | SCM435 etc. (quenched and tempered) | 10.9 / 12.9; 8.8 made to order |
| Stainless machine screws and bolts | A2 (STS304Cu) | A2-50 (stock) — A2-70 and A2-80 made to order |
| Tapping screws (steel and stainless), precision screws | — | No property class shown — tapping screws are outside the ISO 898-1 and 3506-1 classes |
The class is shown in the "Property class" field on each product page.
FAQ
What does the 8 in 4.8 mean?
The 8 means the yield-related strength ratio to nominal tensile strength is 0.8, so the nominal value is 400 × 0.8 = 320 N/mm². Unlike 8.8, however, class 4.8 has no minimum 0.2% proof stress in ISO 898-1, so this value should not be read as a minimum yield strength. The required minimum tensile strength is 420 N/mm² (ISO 898-1:2013).
Are 4T and 4.8 the same?
The tensile levels are similar, but the systems differ, so they cannot simply be treated as equivalent. 4T is the old JIS mark and states only tensile strength, about 40 kgf/mm² (≈392 N/mm²); 4.8 is the ISO class with nominal tensile 400, a 0.8 yield ratio and minimum tensile 420. On site 8T≈8.8, 10T≈10.9 and 12T≈12.9 are used customarily, but if a drawing or specification uses a T mark, confirm the applicable standard and required properties before substituting.
I need 8.8 but only 4.8 is in stock — can it substitute?
No. 4.8 and 8.8 differ in material, heat treatment and required mechanical properties. 8.8 is quenched and tempered and is quoted as made-to-order.
Can a 12.9 bolt be zinc plated?
Yes, but hydrogen embrittlement is a risk, so the risk-reduction measures of ISO 4042 apply. We bake plated 10.9 and 12.9 parts after plating; baking reduces the risk but is not a complete guarantee. In corrosive service, also check that changing material does not cut strength too far (stainless A2-70 has a minimum tensile of 700 N/mm²).
Do tapping screws have a property class like 4.8?
General tapping screws do not use ISO 898-1 classes such as 4.8 or 8.8. Heat-treated steel tapping screws follow ISO 2702, stainless tapping screws ISO 3506-4, and in Korea KS B 0251 and similar standards, which specify surface and core hardness, torsional strength and so on. That is why our catalogue shows no property class for tapping screws.
References
- ISO 898-1:2013 Fasteners — Mechanical properties of fasteners made of carbon steel and alloy steel — Part 1: Bolts, screws and studs with specified property classes — Table 2 permitted materials, Table 3 nominal and minimum tensile, Rp0.2, Rpf (not test values for 4.8 and 6.8), proof stress, hardness
- ISO 2702:2022 Heat-treated steel tapping screws — Mechanical properties / KS B 0251 — tapping screws are specified by hardness, torsional strength etc., not property classes
- JIS B 1051:2014 — Japanese equivalent; permitted materials by class, delayed-fracture caution for 12.9
- KS B 0233:2000 (based on ISO 898-1:1999) · KS B ISO 898-1 — related Korean standards, possibly a different edition from ISO 898-1:2013; 4.8 Vickers max 220, at the end 250 (Table 3 note 6)
- ISO 4042:2022 Fasteners — Electroplated coating systems — hydrogen embrittlement measures (baking) for high-strength plated parts
Last revised 2026-10-12