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Bolt Breaking Torque Reference Tables | Stainless Steel A2/A4 and Carbon Steel Grade 8.8 to 12.9

Introduction

Breaking torque (also called failure torque or proof torque) is the minimum torque at which a bolt or screw will fracture under torsional loading. It is a critical parameter for fastener selection, installation tooling specification, and quality control testing.

This reference page provides breaking torque data for two major fastener categories:

  • Austenitic stainless steel bolts and screws — Grade 50, 70, and 80 per GB/T 3098.6-2000
  • Carbon steel bolts — Grade 8.8, 9.8, 10.9, and 12.9 per GB/T 3098.13

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Austenitic Stainless Steel Bolt & Screw Breaking Torque

Reference standard: GB/T 3098.6-2000 | Applicable to austenitic stainless steel fasteners (A1, A2, A4 series)

Thread Size Grade 50 (N·m) Grade 70 (N·m) Grade 80 (N·m)
M1.6 0.15 0.20 0.24
M2 0.30 0.40 0.48
M2.5 0.60 0.90 0.96
M3 1.1 1.6 1.8
M4 2.7 3.8 4.3
M5 5.5 7.8 8.8
M6 9.3 13 15
M8 23 32 37
M10 46 65 74
M12 80 110 130
M16 210 290 330

Grade reference for stainless steel fasteners:

  • A2-50 / A4-50: Cold-worked or lightly worked; minimum tensile strength 500 MPa
  • A2-70 / A4-70: Standard grade; minimum tensile strength 700 MPa (most common)
  • A2-80 / A4-80: High-strength grade; minimum tensile strength 800 MPa

For material details: SUS304 (A2) | SUS316 (A4)

Carbon Steel Bolt Breaking Torque (Grade 8.8 / 9.8 / 10.9 / 12.9)

Reference standard: GB/T 3098.13 | Applicable to 6g, 6f, 6e thread tolerance class bolts

Thread Size Pitch (mm) Grade 8.8 (N·m) Grade 9.8 (N·m) Grade 10.9 (N·m) Grade 12.9 (N·m)
M1 0.25 0.033 0.036 0.040 0.045
M1.2 0.25 0.075 0.082 0.092 0.10
M1.4 0.30 0.12 0.13 0.14 0.16
M1.6 0.35 0.16 0.18 0.20 0.22
M2 0.40 0.37 0.40 0.45 0.54
M2.5 0.45 0.82 0.90 1.0 1.1
M3 0.50 1.5 1.7 1.9 2.1
M3.5 0.60 2.4 2.7 3.0 3.3
M4 0.70 3.6 3.9 4.4 4.9
M5 0.80 7.6 8.3 9.3 10
M6 1.00 13 14 16 17
M7 1.00 23 25 28 31
M8 1.25 33 36 40 44
M8×1 1.00 38 42 46 52
M10 1.50 66 72 81 90
M10×1 1.00 84 92 102 114
M10×1.25 1.25 75 82 91 102

Note: Minimum breaking torque values apply to 6g, 6f, and 6e thread tolerance class bolts.

Key Notes on Breaking Torque

  • Breaking torque ≠ installation torque: Breaking torque is the torque at which the fastener fractures. Installation (tightening) torque is typically 60–80% of proof load torque, which is well below breaking torque. Never use breaking torque values as installation torque targets.
  • Fine-pitch threads have higher breaking torque: For the same nominal diameter, fine-pitch variants (e.g., M8×1 vs M8×1.25) have higher breaking torque due to larger stress area.
  • Grade progression: For carbon steel, breaking torque increases with grade: 8.8 < 9.8 < 10.9 < 12.9. For stainless steel: Grade 50 < 70 < 80.
  • Material selection for high-grade fasteners: Grade 10.9 and 12.9 bolts require alloy steel (e.g., 40Cr, 35CrMo, 42CrMo). Grade 8.8 and 9.8 can use boron steel such as 10B21.

Stainless vs Carbon Steel Breaking Torque Comparison (M6, M8, M10)

Size SS Grade 70 (A2-70/A4-70) Carbon 8.8 Carbon 10.9 Carbon 12.9
M6 13 N·m 13 N·m 16 N·m 17 N·m
M8 32 N·m 33 N·m 40 N·m 44 N·m
M10 65 N·m 66 N·m 81 N·m 90 N·m

A2-70/A4-70 stainless steel fasteners have breaking torque comparable to carbon steel Grade 8.8, confirming their equivalence in torsional strength at the same grade level.

Related Guides

Standards reference: GB/T 3098.6-2000 (stainless steel fasteners) | GB/T 3098.13 (carbon steel bolts)

Content authorized for publication by FULLERKREG.

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