Grade 10.9 Blue-White Zinc Hex Bolt DIN 933 | ISO 4017 | Full-Thread Hexagon Bolt with Zinc Electroplating & Blue-White Passivation

2026-07-10

The Grade 10.9 Blue-White Zinc DIN 933 hex bolt delivers high-strength clamping capability (tensile strength ≥ 1,040 MPa) with a cost-effective, RoHS-compliant decorative-and-functional surface finish. It is the fastener of choice for automotive engine assemblies, heavy machinery, wind turbine connections, and structural steelwork where compact, high-clamping-force joints are required.

Grade 10.9 DIN 933 Hex Bolt with Blue-White Zinc Electroplating Finish

Part 1 — Grade 10.9: High-Strength Fastener Class

Grade 10.9 is the second-highest standardised property class for metric fasteners under ISO 898-1, sitting between the widely used Grade 8.8 and the ultra-high-strength Grade 12.9. It offers a compelling balance of high strength, acceptable ductility, and predictable fatigue performance, making it the default specification for critical bolted joints in automotive, energy, and heavy machinery sectors.

Property Class Decoding

First digit (10): Indicates 1/100 of the nominal minimum tensile strength in MPa → minimum tensile strength = 1,000 MPa (standardised at 1,040 MPa).
Second digit (9): Indicates the ratio of minimum yield strength to minimum tensile strength × 10 → yield ratio = 0.90, meaning minimum yield strength = 940 MPa.

Mechanical Properties

Property	Value	Standard	Test Method
Tensile Strength (min.)	1,040 MPa	ISO 898-1	Tensile test
Yield Strength (min.)	940 MPa	ISO 898-1	Tensile test
Proof Load Stress	830 MPa	ISO 898-1	Proof load
Yield Ratio	0.90	ISO 898-1	Calculation
Hardness (Rockwell)	32–39 HRC	ISO 6508	Rockwell test
Hardness (Vickers)	320–380 HV	ISO 6507	Vickers test
Elongation at Break (min.)	9%	ISO 898-1	Tensile test
Impact Toughness (min.)	27 J @ -20 °C	ISO 898-1	Charpy test

Grade 10.9 fasteners are manufactured from alloy steels — typically 40Cr, 35CrMo, 42CrMo, or SCM435 — that undergo quenching and tempering (Q&T) heat treatment. The higher carbon and alloy content (Cr, Mo) provides the hardenability necessary to achieve the required 32–39 HRC core hardness throughout the cross-section, even in larger diameters (up to M36).

Key Application Fields

Automotive: Engine mounting bolts, connecting rod bolts, flywheel fasteners, suspension arm pivot bolts, steering linkage.
Wind energy: Tower section flanges, nacelle structural connections, pitch bearing bolts.
Heavy machinery: Excavator track bolts, bulldozer frame connections, crane slew ring fasteners.
Steel construction: Pre-loaded friction-grip connections (BS EN 14399-3 HR system), bridge bearing assemblies.

Part 2 — DIN 933: Full-Thread Hexagon Bolt Standard

DIN 933 specifies fully threaded hexagon-head bolts with metric coarse pitch threads from M1.6 to M52. Although technically superseded by ISO 4017, DIN 933 remains the most frequently referenced bolt standard in European procurement documents, technical drawings, and bills of materials worldwide.

Key Dimensional Features

Full-thread shank: Threading extends along the entire length, maximising clamp length flexibility and simplifying inventory — one part replaces multiple grip-length-specific partially threaded bolts.
Hexagon head: Width across flats (s) per DIN EN ISO 272. Nominal head height (k) provides sufficient bearing area for standard washer assemblies and socket engagement.
Metric coarse pitch: Per ISO 261/ISO 724. Coarse threads are more tolerant of surface damage, less prone to galling, and faster to assemble than fine-pitch alternatives.
Tolerance grade: Product grade A for sizes ≤ M24 and length ≤ 10d or ≤ 150 mm; product grade B for larger sizes (ISO 4759-1).

DIN 933 Grade 10.9 Quick Reference

Common standard DIN 933 sizes for Grade 10.9 production (M6–M24). Tensile loads calculated at the minimum tensile stress area × 1,040 MPa.

Size	Pitch (mm)	s (mm)	k (mm)	Min. Tensile Load (kN)	Length Range
M6	1.00	10	4.0	15.8	10–90 mm
M8	1.25	13	5.3	29.1	16–150 mm
M10	1.50	17	6.4	46.5	20–200 mm
M12	1.75	19	7.5	67.3	20–300 mm
M16	2.00	24	10.0	122	25–300 mm
M20	2.50	30	12.5	191	30–400 mm

s = nominal width across flats, k = nominal head height. Tensile load per ISO 898-1 stress area × 1,040 MPa.

Marking Identification

Each Grade 10.9 DIN 933 bolt is head-marked with “10.9” and the manufacturer’s identification symbol as required by ISO 898-1. The blue-white zinc finish provides immediate visual identification and assists with quality control sorting.

Part 3 — Blue-White Zinc Electroplating

Blue-white zinc (Zn/An/Cn/T0 per ISO 2081, or Type II SC 1 per ASTM B633) is a zinc electroplated coating finished with a trivalent chromium (Cr-III) blue-white conversion layer. It combines moderate corrosion protection, an attractive bright decorative appearance, and low cost — making it one of the most popular surface treatments for high-strength fasteners used in semi-protected environments.

How Blue-White Zinc Plating Works

The process involves three sequential steps:

Electro-cleaning & activation: The bolt surface is degreased and acid-activated to ensure uniform zinc adhesion.
Zinc electrodeposition: A layer of pure zinc (8–12 µm) is electrolytically deposited onto the steel substrate in an acid or alkaline zinc bath.
Blue-white passivation: The freshly plated bolt is immersed in a trivalent chromium (Cr-III) passivation solution, forming a thin transparent-to-blue conversion film that seals the zinc and provides the characteristic bright blue-white appearance.
Hydrogen de-embrittlement bake: Grade 10.9 bolts are baked at 190–220 °C for 4–8 hours within 4 hours of plating (ISO 9588) to drive out diffusible hydrogen and prevent delayed brittle fracture.

Technical Specifications

Parameter	Specification	Standard	Remarks
Coating Type	Zinc electroplate + blue-white passivation	ISO 2081 / ASTM B633	Trivalent Cr(III)
Thickness	8–12 µm (Fe/Zn 8c)	ISO 2081	Preserves thread fit
Salt Spray (NSS)	≥ 96 h white rust, ≥ 240 h red rust	ISO 9227	Type II, SC 1
Appearance	Uniform blue-bright with iridescence	ISO 2081	Decorative + functional
Hydrogen De-embrittlement	Bake at 190–220 °C for 4–8 h within 4 h of plating	ISO 9588	Mandatory for Grade 10.9
RoHS / REACH	Compliant (Cr-VI free trivalent passivation)	2011/65/EU	Environmentally safe
Friction Coefficient	µ = 0.12–0.20 (as-plated)	ISO 16047	Reliable preload control

Salt spray resistance per ISO 9227 neutral salt spray (NSS). White rust = zinc corrosion products; red rust = steel substrate corrosion. Hydrogen de-embrittlement is mandatory for fasteners with hardness ≥ 320 HV per ISO 4042.

Advantages for Grade 10.9 Applications

Visual quality assurance: The bright, uniform blue-white finish makes surface defects (scratches, unplated areas, contamination) immediately visible during inspection.
Thread compatibility: At 8–12 µm, the coating is thin enough to preserve standard thread tolerances (6g/6h) without re-tapping — critical for high-strength bolted joints.
Controlled friction: The as-plated surface provides a consistent friction coefficient (µ ≈ 0.12–0.20), supporting reliable torque-based preload calculations.
Cost-effective: Zinc electroplating is the most economical corrosion protection method for fasteners, with significantly lower processing cost than Dacromet or hot-dip galvanising.
Environmental compliance: Trivalent (Cr-III) passivation meets RoHS and REACH requirements; unlike older hexavalent chromium (Cr-VI) processes, it poses no environmental or occupational health hazard.

Limitations & Best Practices

Blue-white zinc plating is intended for indoor and semi-protected outdoor environments (corrosivity categories C1–C2 per ISO 12944). For direct outdoor exposure, coastal environments (C3+), or applications requiring ≥ 480 h salt spray resistance, consider Dacromet or hot-dip galvanising. Always specify hydrogen de-embrittlement bake on purchase orders for Grade 10.9 zinc-plated fasteners.

About Fujirui Metal — Professional Fastener Manufacturer & Exporter

Fujirui Metal manufactures and exports precision fasteners including Grade 8.8 / 10.9 / 12.9 hex bolts, DIN 933 full-thread bolts, DIN 931 partial-thread bolts, stud bolts, and custom fasteners. Surface treatment options include blue-white zinc, yellow zinc, Dacromet, Ruspert, hot-dip galvanising, and mechanical zinc plating. All products are manufactured to ISO, DIN, and ASTM standards with full material test certificates (MTC) available.

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