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Views: 0 Author: Site Editor Publish Time: 2026-08-29 Origin: Site
Property | 8.8 | 10.9 | 12.9 |
|---|---|---|---|
Tensile strength (min) | 800 MPa | 1000 MPa | 1200 MPa |
Yield strength (min) | 640 MPa | 900 MPa | 1080 MPa |
Hardness (HV) | 250–320 | 320–380 | 385–435 |
Typical material | Medium-carbon steel, quenched & tempered | Alloy steel, quenched & tempered | Alloy steel, quenched & tempered |
Common use | General construction, structural steel, machinery | Vehicles, engines, heavy equipment, high-load joints | Racing, aerospace-grade fixtures, precision tooling |
Vibration resistance | Good | Good | Poor (brittle) |
Hydrogen embrittlement risk | Low | Moderate | High — avoid zinc plating |
Cost | 1× | ~1.5–2× | ~2–3× |
Head marking | "8.8" | "10.9" | "12.9" |
The two digits encode strength in megapascals:
First digit × 100 = minimum tensile strength. 8.8 → 800 MPa; 10.9 → 1000 MPa; 12.9 → 1200 MPa.
First digit ÷ 10 × second digit = yield strength ratio. 8.8 → 0.8 × 800 = 640 MPa yield; 10.9 → 0.9 × 1000 = 900 MPa; 12.9 → 0.9 × 1200 = 1080 MPa.
So "8.8" reads as: 800 MPa tensile, 80% yield ratio. This system is defined by ISO 898-1 for metric bolts, and applies to DIN and GB/T fasteners too.
Match the original spec — never replace an 8.8 with a lower grade. Upgrading (8.8 → 10.9) is allowed only if the mating nut and washer are also upgraded and the joint design accepts the higher preload.
Think in yield strength, not just tensile — the design limit for a bolt is yield (where it permanently stretches). 10.9 gives ~40% more yield capacity than 8.8 at similar size.
Vibration & fatigue — for high-cycle or vibrating joints, higher grade is not automatically better: 12.9 is brittle and can fail suddenly. Use 10.9 + a lock nut for most dynamic joints.
Environmental caution — hydrogen embrittlement — 12.9 bolts are highly sensitive to hydrogen, and zinc electroplating can cause delayed brittle failure. If you must plate 12.9, use special low-hydrogen processes (and many industries forbid plating 12.9 entirely). For corrosive environments, prefer 10.9 with hot-dip galvanizing or stainless 316 (e.g. A4-80 ≈ 800 MPa) instead of plated 12.9.
Match nuts and washers — property class 8 nuts with 8.8 bolts, class 10 with 10.9, class 12 with 12.9. The nut's class number is stamped on it; mismatching creates a weak link.
12.9 is the strongest common metric grade, but its high hardness makes it brittle: it tolerates misalignment, shock loads and hydrogen poorly. Common industries (automotive, construction) largely standardize on 10.9 as the practical maximum. Use 12.9 only for: precision clamping in tooling/fixtures, racing components, or designs that explicitly call for it — and never electroplate it.
Q1: Is a 10.9 bolt stronger than an 8.8? Yes — 1000 MPa vs 800 MPa tensile strength (25% stronger in tension, ~40% stronger at yield).
Q2: What does the marking "8.8" on a bolt head mean? 800 MPa minimum tensile strength with a 0.8 yield-to-tensile ratio (yield ≈ 640 MPa).
Q3: Can I replace an 8.8 bolt with a 10.9? Yes, if the nut/washer match and the joint tolerates the higher preload; never downgrade.
Q4: Why is 12.9 sometimes avoided? Brittleness and hydrogen embrittlement risk — it can fail without warning, especially when zinc-plated.
Q5: What grade bolts for outdoor use? Consider 10.9 hot-dip galvanized, or stainless steel 304 (A2-70) / 316 (A4-80) for wet or marine environments.
Q6: Do stainless steel bolts have grades like 8.8? Stainless uses a different system: A2-70 / A4-80 (material + tensile strength, per ISO 3506). A4-80 ≈ 800 MPa — comparable to 8.8 in strength, far better in corrosion resistance.
Ningbo Weifeng Fastener Co., Ltd. manufactures and exports metric hex bolts in grades 4.8, 8.8, 10.9 and 12.9, ISO 4014/4017, DIN 931/933 and GB/T 5782/5783, with zinc plating, hot-dip galvanizing, Dacromet or stainless steel (304/316) — browse our hex bolt range or send us your grade and size for a quote within 24 hours.
Sources: ISO 898-1 (mechanical properties of carbon steel bolts), ISO 3506 (stainless fasteners), DIN 267, GB/T 3098.1, ANSI/ASME B18.2.1 (inch grades: SAE J429 Grade 5/8).
