Bolt Property Classes Explained: How to Read and Choose 8.8, 10.9 and 12.9 per GB/T 3098.1
Executive Summary
The "8.8", "10.9", "12.9" stamped on bolt heads are not arbitrary model numbers but the property-class codes defined by GB/T 3098.1-2010: the figure left of the point is one-hundredth of nominal tensile strength, and the figure right of the point is ten times the yield-to-tensile ratio. Reading this code correctly establishes a common strength language between drawing and shop floor. Against the national-standard tables, this article explains the real strength figures, material/heat-treatment requirements and identification marks for the 8.8, 10.9 and 12.9 classes.
Table of Contents
The Property-Class Code: What the Two Figures Mean
Per GB/T 3098.1-2010 "Mechanical properties of fasteners—Bolts, screws and studs", Chapter 5 "Designation system", the property-class code for bolts, screws and studs consists of two figures separated by a point: the one or two figures left of the point are 1/100 of the nominal tensile strength Rm,nom (in MPa); the figure right of the point is ten times the yield-to-tensile ratio—the ratio of the specified non-proportional extension 0.2% stress (Rp0.2) or lower yield strength to nominal tensile strength.
Take the standard's own example: when nominal tensile strength Rm,nom = 800 MPa and the yield-to-tensile ratio is 0.8, the property class is marked "8.8". That is, an 8.8-class bolt has nominal tensile strength 8×100 = 800 MPa, and nominal yield strength 800×0.8 = 640 MPa. By the same logic: 10.9 means nominal tensile strength 1000 MPa, ratio 0.9, nominal yield 900 MPa; 12.9 means nominal tensile strength 1200 MPa, ratio 0.9, nominal yield 1080 MPa.
| Code | Left Figure (Nominal Tensile) | Right Figure (Yield Ratio) | Derived Nominal Yield |
|---|---|---|---|
| 8.8 | 8×100 = 800 MPa | 0.8 | 800×0.8 = 640 MPa |
| 10.9 | 10×100 = 1000 MPa | 0.9 | 1000×0.9 = 900 MPa |
| 12.9 | 12×100 = 1200 MPa | 0.9 | 1200×0.9 = 1080 MPa |
Key Strength Figures of 8.8 / 10.9 / 12.9 Compared
The data below is taken directly from GB/T 3098.1-2010 Table 3 "Mechanical and physical properties of bolts, screws and studs". Note that tensile strength is split into nominal and minimum values; engineering acceptance judges against the minimum. Class 8.8 is further split by nominal diameter d≤16 mm vs d>16 mm, with slightly lower minimum requirements for smaller diameters.
| Property | 8.8 | 10.9 | 12.9 |
|---|---|---|---|
| Nominal tensile strength Rm,nom | 800 MPa | 1000 MPa | 1200 MPa |
| Minimum tensile strength Rm,min | 800 / 830 MPa | 1040 MPa | 1220 MPa |
| Nominal Rp0.2,nom | 640 MPa | 900 MPa | 1080 MPa |
| Minimum Rp0.2,min | 640 / 660 MPa | 940 MPa | 1100 MPa |
| Nominal proof stress Sp,nom | 580 MPa | 830 MPa | 970 MPa |
| Vickers hardness HV (min–max) | 250–320 | 320–380 | 385–435 |
| Elongation after fracture A,min (machined specimen) | 12% | 9% | 8% |
From 8.8 to 12.9, tensile strength rises about 50%, but elongation after fracture falls from 12% to 8%—the higher the class, the "more brittle" the material. This is exactly why high-strength bolted joints are especially sensitive to preload control, hydrogen embrittlement and stress corrosion. The "proof stress Sp" in the table is the stress at which the bolt produces no permanent deformation under a specified load, corresponding to the proof-load test (GB/T 3098.1 clause 9.6), and is an important basis for checking the upper limit of assembly preload.
Material and Heat Treatment: Why 12.9 Demands Caution
Per national-standard Table 2, classes 8.8 through 12.9 all require quenching and tempering: they may be made from alloyed carbon steel (e.g. boron, manganese or chromium) quenched and tempered, or from carbon/alloy steel quenched and tempered; the minimum tempering temperature is generally 425°C, with 12.9 (boron-steel route) at 380°C. For 8.8–12.9, maximum phosphorus (P) and sulfur (S) content are both set at 0.025% to ensure hardenability and toughness.
Pay special attention to the national-standard warning for 12.9: "When considering the use of 12.9, caution should be exercised." The reason is that 12.9 is high in strength and hardness (up to 435 HV), making it more sensitive to environment and process: the surface must have no white-phosphide aggregate layer detectable metallographically; besides surface treatment, the service environment may also cause stress-corrosion cracking; and if post-plating hydrogen removal is incomplete, hydrogen-embrittlement fracture is highly likely. So 12.9 is not "the higher the better", but should be used only where there is a clear high-strength need and surface treatment and assembly quality can be controlled.
Head and Packaging Marks: How to Identify on Site
Per national-standard clause 10.3, hex and hex-flange head bolts, socket-cap screws and similar fasteners of nominal diameter ≥5 mm must carry marks: one is the manufacturer's identification mark (factory mark/trademark), and the other is the property-class code (e.g. 8.8, 10.9, 12.9; the point may be omitted). Marks are usually recessed or raised on the head top; socket-cap screws may mark on the head side.
When a screw is too small or the head shape does not allow direct stamping, national-standard Table 20 gives clock-face mark symbols: using the manufacturer's mark or a dot at 12 o'clock as reference, the number of long ticks denotes the class—one or two long ticks denote different classes, and 12.9 is denoted by a single dot. In addition, packaging must carry the manufacturer's trademark, property-class code and production-batch mark (clause 10.5). On site, a paired "factory mark + class number" is a traceable conforming part; a class number without a factory mark is of questionable origin.
Selection Principle: Do Not Blindly Chase 12.9
Selection is not simply "the higher the better". Translating the national-standard indicators into engineering decisions:
- General steel structures, equipment bases, covers: 8.8 is fully sufficient, low cost, good toughness, insensitive to surface treatment—the most widely used class.
- Space-limited joints needing greater clamp force at a smaller size: e.g. engines, gearboxes, high-strength connections—choose 10.9, compensating small diameter with higher strength.
- Extremely limited space and high preload density: e.g. certain automotive chassis, robot joints—only then use 12.9, paired with hydrogen removal, lubrication and strict preload control.
- Alternating load and vibrational fatigue: while meeting strength, prefer a lower class with better ductility rather than piling on strength—a more ductile material has better fatigue margin.
Property class is a trade-off between strength and toughness, not "the higher the more advanced". Reading the code, comparing against national-standard Table 3, and then selecting in light of material heat treatment and surface-treatment capability is how you both ensure joint reliability and avoid unnecessary cost and brittle-fracture risk.
Conclusion: Remember Bolt Property Classes in One Table
GB/T 3098.1-2010 unifies bolt strength language with a two-part code of "1/100 tensile strength + 10× yield ratio": 8.8 = 800/640 MPa, 10.9 = 1000/900 MPa, 12.9 = 1200/1080 MPa; all three classes require quenching and tempering, and the head is stamped with factory mark and class number (d≥5 mm). When selecting, first set the class by clamp-force need, then assess ductility, surface treatment and hydrogen-embrittlement risk; 12.9 must be used with caution.
YF Zhichengjia, founded in Shenzhen in 2005, is a fastener manufacturer and can provide customization and selection support for 8.8/10.9/12.9-class bolts, screws and studs per GB/T 3098.1. For more thread-joining technical material, see the technical documentation library, or directly contact our technical engineers for a one-on-one selection evaluation.
