How to read the grade
A metric property class has two numbers separated by a dot, for example 8.8:
- First number × 100 = nominal tensile strength in MPa (N/mm²). 8 → 800 MPa.
- Second number ÷ 10 = ratio of yield strength to tensile strength. .8 → yield is 80% of tensile, so about 640 MPa.
So a 10.9 bolt has a nominal tensile strength of 1000 MPa and yields at about 900 MPa. The minimum values guaranteed by the standard are slightly higher, as the table shows.
Full property data for steel bolts (ISO 898-1 / IS 1367 Part 3)
| Property | 4.6 | 4.8 | 5.6 | 5.8 | 6.8 | 8.8 (≤M16) | 8.8 (>M16) | 9.8 (≤M16) | 10.9 | 12.9 |
|---|---|---|---|---|---|---|---|---|---|---|
| Tensile strength Rm min (MPa) | 400 | 420 | 500 | 520 | 600 | 800 | 830 | 900 | 1040 | 1220 |
| Yield ReL / 0.2% proof Rp0.2 min (MPa) | 240 | 340 | 300 | 420 | 480 | 640 | 660 | 720 | 940 | 1100 |
| Proof stress Sp (MPa) | 225 | 310 | 280 | 380 | 440 | 580 | 600 | 650 | 830 | 970 |
| Vickers hardness HV | 120–220 | 130–220 | 155–220 | 160–220 | 190–250 | 250–320 | 255–335 | 290–360 | 320–380 | 385–435 |
| Rockwell hardness | HRB 67–95 | HRB 71–95 | HRB 79–95 | HRB 82–95 | HRB 89–99 | HRC 22–32 | HRC 23–34 | HRC 28–37 | HRC 32–39 | HRC 39–44 |
| Elongation after fracture A min (machined test piece) | 22% | – | 20% | – | – | 12% | 12% | 10% | 9% | 8% |
| Impact KV at −20 °C, min (when required) | – | – | 27 J | – | – | 27 J | 27 J | 27 J | 27 J | by agreement |
| Heat treatment | none | none (cold work) | none | none (cold work) | none (cold work) | Q&T, temper ≥425 °C | Q&T, temper ≥425 °C | Q&T, temper ≥425 °C | Q&T, temper ≥425 °C | Q&T, temper ≥425 °C |
Classes 4.8, 5.8 and 6.8 are tested on full-size fasteners, so elongation is given in the standard as a full-size value. 9.8 applies to sizes up to M16 only. Always check the edition of the standard on your drawing.
Proof loads and breaking loads by size
The proof load is the load a bolt must carry without permanent stretch; the minimum breaking load is the load it must reach before it fails. These are the numbers to use when comparing bolts or checking a test report.
| Size | Stress area As (mm²) | 8.8 proof load (kN) | 8.8 min breaking load (kN) | 10.9 proof load (kN) | 10.9 min breaking load (kN) | 12.9 proof load (kN) | 12.9 min breaking load (kN) |
|---|---|---|---|---|---|---|---|
| M5 | 14.2 | 8.2 | 11.4 | 11.8 | 14.8 | 13.8 | 17.3 |
| M6 | 20.1 | 11.7 | 16.1 | 16.7 | 20.9 | 19.5 | 24.5 |
| M8 | 36.6 | 21.2 | 29.3 | 30.4 | 38.1 | 35.5 | 44.7 |
| M10 | 58 | 33.6 | 46.4 | 48.1 | 60.3 | 56.3 | 70.8 |
| M12 | 84.3 | 48.9 | 67.4 | 70.0 | 87.7 | 81.8 | 102.8 |
| M14 | 115 | 66.7 | 92.0 | 95.5 | 119.6 | 111.5 | 140.3 |
| M16 | 157 | 91.1 | 125.6 | 130.3 | 163.3 | 152.3 | 191.5 |
| M18 | 192 | 115.2 | 159.4 | 159.4 | 199.7 | 186.2 | 234.2 |
| M20 | 245 | 147.0 | 203.3 | 203.3 | 254.8 | 237.7 | 298.9 |
| M22 | 303 | 181.8 | 251.5 | 251.5 | 315.1 | 293.9 | 369.7 |
| M24 | 353 | 211.8 | 293.0 | 293.0 | 367.1 | 342.4 | 430.7 |
| M27 | 459 | 275.4 | 381.0 | 381.0 | 477.4 | 445.2 | 560.0 |
| M30 | 561 | 336.6 | 465.6 | 465.6 | 583.4 | 544.2 | 684.4 |
| M33 | 694 | 416.4 | 576.0 | 576.0 | 721.8 | 673.2 | 846.7 |
| M36 | 817 | 490.2 | 678.1 | 678.1 | 849.7 | 792.5 | 996.7 |
Coarse thread. Loads = stress area × proof stress Sp (or × minimum tensile strength), rounded to 0.1 kN; ISO 898-1 tables round to three figures. 8.8 uses 580 / 800 MPa up to M16 and 600 / 830 MPa above M16.
Matching nuts and their properties
| Bolt class | Nut class (ISO 898-2 / IS 1367 Part 6) | Nut steel and treatment | Notes |
|---|---|---|---|
| 4.6, 4.8, 5.6, 5.8 | 5 (or higher) | Low-carbon steel, as formed | Hex nuts style 1 |
| 8.8 | 8 | Carbon steel; quenched and tempered for larger sizes | Standard high-tensile nut |
| 10.9 | 10 | Carbon or alloy steel, quenched and tempered | Required for 10.9 joints |
| 12.9 | 12 | Alloy steel, quenched and tempered | Use with 12.9 only where specified |
Raw material and heat treatment by grade
| Class | Typical raw material | Heat treatment | Typical uses |
|---|---|---|---|
| 4.6 | Low-carbon steel (C10C–C15C, SWRCH10A–15A, 1010–1018) | None needed; sometimes annealed after forming for ductility | General light fixing, foundation bolts, mild steel work |
| 4.8 / 5.8 | Low-carbon cold-heading steel | None; strength comes from cold work | Machine screws, light hardware |
| 5.6 | Low to medium carbon steel (0.13–0.55% C) | Usually none | General engineering |
| 8.8 | Boron steel (10B21, 20MnB4, 23MnB4) for small sizes; medium-carbon (C35, C45, 35K/45K) for larger | Quench and temper, temper at 425 °C minimum | Machinery, structures, automotive: the everyday high-tensile bolt |
| 10.9 | Boron or Cr-boron steel (32CrB4, 35B2); Cr-Mo (SCM435, 34CrMo4) for larger sizes | Quench and temper, temper at 425 °C minimum | Heavy machinery, automotive, structural friction-grip joints |
| 12.9 | Cr-Mo alloy steel (SCM435/440, 34CrMo4, 42CrMo4 / 4140) | Quench and temper, temper at 425 °C minimum | Socket cap screws, dies, tooling, high-load joints |
For the full chemistry limits see fastener raw materials, and for temperatures see the heat treatment temperature chart.
Matching nuts
A nut must be at least as strong as the bolt, or its thread strips before the bolt reaches full load. Under ISO 898-2, use a class 8 nut with an 8.8 bolt, class 10 with 10.9 and class 12 with 12.9. A higher-class nut can always be used on a lower-class bolt. For ASTM flange studs, B7 studs are paired with A194 2H heavy hex nuts.
Inch grades: SAE and ASTM equivalents
American bolts use grades instead of property classes. The closest metric matches are approximate, not exact replacements:
| Inch grade | Head marking | Min tensile | Closest metric |
|---|---|---|---|
| SAE J429 Grade 2 | No marks | 60–74 ksi (about 415–510 MPa) | 4.6 |
| SAE J429 Grade 5 / ASTM A325 (F3125) | 3 radial lines / "A325" | 120 ksi (about 830 MPa) | 8.8 |
| SAE J429 Grade 8 / ASTM A490 (F3125) | 6 radial lines / "A490" | 150 ksi (about 1035 MPa) | 10.9 |
| ASTM A193 B7 studs | "B7" | 125 ksi (about 860 MPa) up to 2½" | close to 8.8, for pressure service |
Stainless steel grades
Stainless bolts are marked with the steel group and strength class to ISO 3506: A2-70 (304 type, 700 MPa), A4-70 (316 type, 700 MPa) and A4-80 (316 type, 800 MPa). They are not heat treated; their strength comes from cold working. They are not a direct swap for 8.8 in a highly loaded joint, because their yield and fatigue behaviour differ.
Grades in Indian standards and Indian trade terms
India uses the same property class system. IS 1367 (Part 3) covers bolts, screws and studs, and IS 1367 (Part 6) covers nuts, aligned with ISO 898-1 and ISO 898-2. So an 8.8 bolt to an Indian standard is the same class as an ISO 8.8.
| What people say in the market | What it usually means |
|---|---|
| MS bolt (mild steel) | Class 4.6, low-carbon steel, not hardened |
| HT bolt (high tensile) | Usually 8.8; sometimes 10.9. Always confirm the class on the head |
| Black bolt | Product grade C (wider tolerances), IS 1363 / ISO 4016, often hot forged, plain finish |
| Bright bolt | Product grade A or B (close tolerances), IS 1364 / ISO 4014 and ISO 4017 |
| Allen bolt | Socket head cap screw, DIN 912 / ISO 4762, usually 12.9 |
| HSFG bolt | High-strength friction-grip structural bolt to IS 3757, classes 8.8S and 10.9S, with IS 6623 nuts and IS 6649 hardened washers |
| SS 304 / SS 316 bolt | Stainless A2 / A4, normally class 70, to IS 1367 (Part 14) / ISO 3506 |
For bridges, railways, power plants and steel buildings, Indian drawings often call for IS 3757 HSFG bolts. These are supplied as matched sets (bolt, nut and washers from the same lot) and tightened to a controlled preload, so do not mix them with ordinary 8.8 or 10.9 hardware.
Figures are typical values for guidance. For design, inspection and purchase, always follow the current edition of the standard on your drawing.