4.8 · 8.8 · 10.9 · 12.9 — How to read steel bolt property classes
First number is tensile strength, second is the yield ratio. Legacy 4T/8T/10T marks, the steel behind each class (carbon, boron, alloy), and what to watch for.
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 → minimum tensile strength 800 N/mm² (number × 100)
- second .8 → yield strength is 80% of tensile → 640 N/mm²
One mark carries both tensile and yield. The standard is ISO 898-1 (KS B 0233), which covers carbon steel and alloy steel fasteners together. Stainless (A2-50 etc.) uses a separate system.
Class table
| Class | Min. tensile | Min. yield | Heat treatment | Hardness (HV) | Typical use |
|---|---|---|---|---|---|
| 4.8 | 420 N/mm² | 340 N/mm² | None (as cold-headed) | 130–220 | Cross-recessed machine screws, flange bolts, general assembly |
| 6.8 | 600 | 480 | None | 190–250 | Medium-strength assembly |
| 8.8 | 800 | 640 | Quenched & tempered | 250–320 | Structural joints, machine frames |
| 10.9 | 1,040 | 940 | Quenched & tempered | 320–380 | High-tension joints, socket head cap screws |
| 12.9 | 1,220 | 1,100 | Quenched & tempered (alloy steel) | 385–435 | Highest-strength socket screws, dies and equipment |
Per ISO 898-1:2013 (KS B 0233). 8.8 figures are for M16 and below; above M16, 830/660 N/mm². Hardness is Vickers.
The difference between 4.8 and 8.8 is heat treatment
4.8 and 6.8 are low-carbon steel as cold-headed (forged and thread-rolled). The strength comes from work hardening, there is no heat-treatment step, so cost is low and lead time short. Our carbon steel machine screws and flange bolts are in this group.
From 8.8 up, parts are quenched and tempered, and the material changes to a steel that responds to heat treatment — boron steel (51B20 etc.) or chromium-molybdenum alloy steel (SCM435). The standard allows boron-treated carbon steel or alloy steel for 8.8 and 10.9, and alloy steel only for 12.9. So although people say "carbon steel bolt", the actual material of a high-strength class is often an alloy steel. With the extra process and inspection it is a different product even at the same size: a stock 4.8 bolt cannot be "made a bit stronger" into an 8.8.
4T · 8T · 10T · 12T — the old JIS marks
The T marks still common on shop floors come from JIS B 1051 before 1985. The number is tensile strength in kgf/mm² divided by ten — 4T = 40 kgf/mm² ≈ 400 N/mm². The yield ratio is not in the mark; it was tabulated separately in the old standard.
| Old mark | Current class | Note |
|---|---|---|
| 4T | 4.6 / 4.8 | Same tensile; cold-headed small screws are normally supplied as 4.8 |
| 6T | 6.8 | |
| 8T | 8.8 | Q&T |
| 10T | 10.9 | Q&T |
| 12T | 12.9 | Q&T |
Marks like "4.8T" mix the two systems. For drawings, test reports and purchase documents we recommend the current class notation (4.8 etc.), which has a standard behind it. A request in T is matched per the table; supplying 4.8 against a 4T request meets it — same tensile, higher yield.
Choosing a class
- 4.8 — sheet metal, covers, electronics assembly, anywhere large clamp load is not needed. All our carbon steel machine screws and flange bolts are 4.8.
- 8.8 — structural joints and vibrating machine frames. From 8.8 up, the class only means something if the bolt is tightened to specified torque.
- 10.9 · 12.9 — socket head cap screws where high clamp load is required. Torque control is essential, and the mating thread (nut or tapped hole) must be strong enough — a 12.9 bolt in a weak thread fails the thread first.
Tightening torque reference: Tightening, loosening and breaking torque.
12.9 and plating — hydrogen embrittlement
Electroplating (zinc, nickel) a hard bolt such as 10.9 or 12.9 can introduce hydrogen that later causes sudden fracture (hydrogen embrittlement). High-strength bolts are therefore baked after plating (hydrogen relief), or given a non-electrolytic finish such as black oxide instead. We bake all plated 10.9 and 12.9 parts after plating.
12.9 is also weak in condensing or corrosive environments. For refrigeration, outdoor or wash-down use, a lower class in stainless usually lasts longer — see A2-50 · A2-70 · A2-80 for stainless property classes.
How classes appear in our catalog
| Product family | Material | Property class |
|---|---|---|
| Cross-recessed machine screws · tapping screws · flange bolts (carbon steel) | Carbon steel (10A) | 4.8 |
| Hex socket head cap screws · SEMS (high strength) | SCM435 · 51B20 etc. (heat-treatable steel) | 8.8 / 10.9 / 12.9 (Q&T) |
| All stainless items | A2 (STS304Cu) | A2-50 in stock — A2-70/80 made to order |
The class is shown in the "Property Class" row of every product page.
FAQ
What does the 8 in 4.8 mean?
That yield strength is 80% of tensile strength. Class 4.8 has minimum tensile 420 N/mm² and minimum yield 340 N/mm² (nominal 400 × 0.8 = 320). First number = tensile ÷ 100, second = yield/tensile ratio, per ISO 898-1 (KS B 0233).
Are 4T and 4.8 the same?
Nearly, but they are different systems. 4T is the old JIS mark and states only tensile strength, 40 kgf/mm² (≈400 N/mm²). 4.8 is the ISO class and fixes both tensile 400 and a 0.8 yield ratio. A 4T request is normally met with 4.8 — same tensile, higher yield. 8T≈8.8, 10T≈10.9, 12T≈12.9.
I need 8.8 but only 4.8 is in stock — can it substitute?
No. 4.8 and 8.8 differ by heat treatment and are separate products. 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 baking (hydrogen relief) after plating is required. We bake all plated 10.9 and 12.9 parts. In corrosive service, stainless is usually the better choice over plating.