A stainless steel hex head bolts are a threaded fastener with a six-sided (hexagonal) external head and a partially or fully threaded shank. Understanding each geometric element is essential for specifying the correct component.
The six key dimensions are: nominal diameter (d), nominal length (L) measured from the underside of the head to the tip, wrench width across flats (s), head height (k), thread pitch (p), and thread engagement length (b). Specifying all six unambiguously eliminates ordering errors and ensures interchangeability.
Material selection is the single most consequential decision when specifying a stainless steel hex bolt. The dominant grades are AISI 304 (A2) and AISI 316 (A4), but specialty alloys exist for extreme conditions.
Composition: 18% chromium, 8% nickel (the classic "18-8" formula). This austenitic grade offers excellent general-purpose corrosion resistance, is non-magnetic after cold working, and is the most economical stainless option. It performs well in freshwater, mild chemicals, food-contact surfaces, and atmospheric environments with moderate humidity.
Composition: 16–18% chromium, 10–14% nickel, 2–3% molybdenum. The addition of molybdenum significantly boosts resistance to chloride pitting and crevice corrosion — the primary degradation mechanism in seawater, road-salt, and chlorinated process streams. SS316 hex head bolts are the standard choice for marine deck hardware, offshore platforms, swimming pool equipment, and coastal civil structures.
SS410 (martensitic): Heat-treatable for higher hardness; used where wear resistance matters more than corrosion immunity. Duplex 2205: Offers roughly twice the yield strength of 316 with excellent chloride resistance — increasingly specified in subsea and chemical-plant piping flanges. Grade 904L: High-alloy austenitic with superior resistance to sulfuric acid, used in pharmaceutical reactors.
Hex bolt geometry and tolerances are governed by three dominant standards families. Specifying the standard in your purchase order is non-negotiable for ensuring dimensional interchangeability.
ISO 4014 defines normal hexagon bolts (property class ≥ 5.6) while ISO 4016 covers coarser-tolerance commercial bolts. Thread geometry follows ISO 261 (coarse pitch) or ISO 262 (fine pitch). This is the most widely adopted standard in Europe, Asia, and international procurement.
DIN 931 specifies partially threaded hex bolts; DIN 933 covers fully threaded variants (studs excepted). Although Germany officially adopted ISO standards, DIN drawings remain the engineering reference in many industries. DIN 931 is technically superseded by ISO 4014, but the two are dimensionally near-identical and the terminology persists widely in supplier catalogs.
Governs hex cap screws and heavy hex bolts in Unified National thread series (UNC/UNF). Common in North American and some Southeast Asian markets. Key distinction: ANSI bolts are typically specified as "hex cap screws" when fully threaded and "hex bolts" when partially threaded — a subtle but contractually important difference.
All stainless steel bolt, nut, screw, and washer products Internal supplied by Tuyue are manufactured in compliance with DIN, ISO, and ANSI standards. Each batch undergoes dimensional checks, tensile testing, and surface inspection before shipment.
For stainless steel fasteners, strength classification uses the ISO 3506 system — distinct from the familiar 8.8 / 10.9 classes used for carbon steel. The format is [Steel Group][Strength Class].
| Class | Steel Grade | Min Tensile Str (MPa) | 0.2% Proof Stress (MPa) | Elongation (%) | Hardness (HV) |
|---|---|---|---|---|---|
| A2-50 | 304 | 500 | 210 | 0.6×d | max 220 |
| A2-70 | 304 | 700 | 450 | 0.4×d | max 220 |
| A2-80 | 304 (strain-hardened) | 800 | 640 | 0.3×d | max 220 |
| A4-50 | 316 | 500 | 210 | 0.6×d | max 220 |
| A4-70 | 316 | 700 | 450 | 0.4×d | max 220 |
| A4-80 | 316 (strain-hardened) | 800 | 640 | 0.3×d | max 220 |
Source: ISO 3506-1:2020 — Fasteners — Mechanical properties of corrosion-resistant stainless steel fasteners.
The A4-70 class represents the workhorse specification: 700 MPa tensile strength combined with the superior corrosion immunity of 316 steel. For structural applications subject to dynamic loading (bridges, crane beams), A4-80 is preferred due to its higher proof stress, which reduces the risk of preload relaxation under vibration.
The corrosion immunity of stainless steel stems from a thin, self-renewing chromium oxide (Cr₂O₃) passivation layer — typically 1–5 nm thick — that forms spontaneously in the presence of oxygen. This passive film is thermodynamically stable across a wide pH range and reforms within microseconds if mechanically disrupted.
When stainless bolts contact dissimilar metals (e.g., aluminum alloy frames, carbon steel flanges), a galvanic cell forms. Stainless steel is cathodic relative to most structural metals, meaning the base metal preferentially corrodes. Mitigation strategies include: isolating washers (nylon or PTFE), applying anti-seize compounds, and — in particularly aggressive environments — specifying coated alternative fasteners such as those in Tuyue's coated tapping screw range Internal.
Austenitic stainless grades (304, 316) are inherently more resistant to hydrogen embrittlement than high-strength carbon steel fasteners. However, at property classes approaching A4-80 (640 MPa proof stress), caution is warranted in electroplating post-treatment and cathodic protection systems, where nascent hydrogen uptake can occur.
The table below lists key nominal dimensions for standard metric hex head bolts per ISO 4014. Tolerances on pitch diameter follow 6g tolerance class.
| Nom. Diameter (d) | Pitch — Coarse (mm) | Width Across Flats (s mm) | Head Height (k mm) | Min Thread Length ≤125mm (b mm) |
|---|---|---|---|---|
| M6 | 1.0 | 10 | 4.0 | 18 |
| M8 | 1.25 | 13 | 5.3 | 22 |
| M10 | 1.5 | 16 / 17* | 6.4 | 26 |
| M12 | 1.75 | 18 / 19* | 7.5 | 30 |
| M16 | 2.0 | 24 | 10.0 | 38 |
| M20 | 2.5 | 30 | 12.5 | 46 |
| M24 | 3.0 | 36 | 15.0 | 54 |
| M30 | 3.5 | 46 | 18.7 | 66 |
| M36 | 4.0 | 55 | 22.5 | 78 |
* ISO 4014 defines both narrow and wide across-flat variants for M10 and M12. Confirm with supplier. Full tolerance tables in ISO 4014:2011 Annex A.
Tuyue stocks standard and custom sizes across the full metric range. For specialty dimensions or fine-pitch requirements, enquire via the Contact page Internal. Related complementary fasteners — hex nuts Internal and hex socket (Allen) bolts DIN912 Internal — are available from the same product family.
Achieving the correct clamp force (preload) is the primary objective of bolt tightening. Under-torquing allows joint slip and fatigue failure; over-torquing causes thread stripping or bolt fracture. The standard torque-preload relationship is:
T = K · F · d
Where T = applied torque (N·m), K = nut factor (typically 0.12–0.20 depending on lubrication), F = desired preload (N), and d = nominal bolt diameter (m). For dry stainless-on-stainless joints, K ≈ 0.17–0.22. Applying anti-seize lubricant drops K to 0.13–0.16, necessitating a torque reduction to avoid over-stressing.
| Size | Class A4-70 — Dry (N·m) | Class A4-70 — Lubricated (N·m) | Estimated Preload (kN) |
|---|---|---|---|
| M8 | 22 | 17 | 17.5 |
| M10 | 44 | 34 | 28.1 |
| M12 | 76 | 59 | 40.6 |
| M16 | 185 | 145 | 74.1 |
| M20 | 363 | 284 | 117 |
| M24 | 627 | 490 | 169 |
Values approximated for A4-70 at 70% proof-load preload target. Always verify with a certified torque-tension calculation for safety-critical joints. Reference: VDI 2230 Part 1.
Thread engagement: Minimum engaged thread length should be at least 1.0× nominal diameter for steel nuts and 1.5× for aluminum or soft-alloy tapped holes. Shorter engagement risks thread stripping before bolt yield — a dangerous failure mode with no warning.
Washers: Use flat washers (DIN 125 / ISO 7089) to distribute bearing stress and prevent surface damage. Spring washers (DIN 127) or serrated lock washers (DIN 6797) help maintain preload under vibration. Tuyue offers a full range of stainless steel washers Internal designed to pair with hex head bolt assemblies.
Re-torquing: For gasket-sealed flanges and elastomeric joints, preload relaxation of 10–20% within the first 24 hours is normal due to embedment and creep. Schedule a re-torque check within one working day of initial installation.
