Stainless Steel Hex Bolts & Nuts Set (A2-70 / A4-80)

What Are Stainless Steel Hex Bolts and Nuts?

Stainless steel hex bolts and nuts are threaded mechanical fasteners used to clamp metal, plastic, or composite parts into a removable assembly. The hexagonal head allows high-torque tightening with standard open-end or socket wrenches, while the matching nut provides a secure, reusable joint. A2-70 and A4-80 denote austenitic stainless steel grades classified under ISO 3506, with minimum tensile strengths of 700 MPa and 800 MPa respectively. These fasteners are standard in outdoor structures, marine hardware, industrial machinery, electrical enclosures, and automotive assemblies where both mechanical strength and corrosion resistance are required.

Specifications to Confirm Before Requesting a Quote

Before you ask a Chinese fastener supplier for pricing, lock down the following parameters in writing:

• Material grade: A2-304 for indoor or sheltered applications; A4-316 for marine, coastal, swimming pool, or chemical plant exposure.
• Size and thread: Metric sizes from M4 to M24, lengths from 10 mm to 200 mm; specify thread tolerance 6g for bolts and 6H for nuts per ISO 4017 / ISO 4032 unless your assembly needs a precision fit.
• Head type: Hex head per DIN 933 (full thread) / DIN 931 (partial thread), or socket head per ISO 4762.
• Tensile class: 700 MPa for A2-70 or 800 MPa for A4-80.
• Surface finish: Passivated bright finish; no plating for stainless grades.
• Certifications: ISO 3506, RoHS compliance, and a mill test report (MTR) tied to the heat number.

For assemblies bound for the EU market, also confirm the RoHS declaration and any required material certificates are current. Industrial buyers in the industrial IoT sector frequently need A4-80 hardware for outdoor cabinet, gateway, and sensor-mount installations.

A2 vs A4 Grade Selection and Material Traceability

A2 (304 stainless) is suitable for most indoor and sheltered outdoor applications. A4 (316 stainless) contains 2.0–3.0% molybdenum, which gives it significantly better resistance to chloride corrosion — salt water, coastal environments, swimming pools, and chemical plants. Substituting A2 for A4 in marine applications leads to crevice corrosion within 6–18 months. The visual appearance is identical; the only way to distinguish grades reliably is XRF analysis or a mill test report (MTR). When sourcing stainless fasteners, always require MTR documentation.

MTR traceability is the baseline quality requirement for industrial and construction buyers: the MTR documents the heat number, chemical composition (Cr, Ni, Mo percentages), mechanical properties, and certifying standards. Reputable Chinese fastener manufacturers in the Jiaxing and Ningbo production clusters can provide MTRs as standard. Factories that cannot provide MTRs are either producing low-grade material or buying from unknown sources — avoid them for any application requiring material certification. A factory audit should include MTR verification as a standard checkpoint.

Dimensional Inspection and Packaging for Resellers

ISO 4017 tolerance class 6g/6H is the standard commercial tolerance for hex bolts and nuts — this is what buyers expect unless specifying fine threads. Check thread engagement on a sample bolt/nut pair: the nut should run down freely by hand but with no wobble. Galling (thread seizure) during assembly is a common complaint with stainless fasteners — verify that the factory passivates the threads after machining (passivation forms a chromium oxide layer that reduces galling).

For reseller kit packaging, verify the sorted assortment accuracy. A 240-piece kit should have exactly the specified count per size — short packing (270-piece boxes with only 240 fasteners) generates returns and reputation damage. Specify quality inspection with 100% count verification on a sample of 3–5 kits per production run. Bulk packaging in 1 kg sealed bags with clear labeling (grade, standard, size) reduces handling errors downstream. Buyers consolidating a fastener and assembly order often pair these bolts with nylon cable ties from the same supplier to cut freight and inspection overhead. For a systematic approach to evaluating fastener suppliers, see our factory audit checklist.

Thread Standards and Head Styles

Hex head bolts per DIN 933 (full thread) and DIN 931 (partial thread) are the most common for general mechanical assemblies. DIN 931 leaves an unthreaded shank that carries shear loads better, while DIN 933 threads run to the head and are preferred for fully threaded clamping. Socket head cap screws per ISO 4762 are used where external wrench clearance is limited and a clean, low-profile head is required. For the same strength class, socket heads typically cost 15–25% more than hex heads. When ordering, specify the standard number explicitly; Chinese suppliers often quote “hex bolts” generically and may ship whichever head style they have in stock.

Salt Spray and Corrosion Testing

For marine, coastal, or road-salt environments, specify a salt spray test on top of the material grade. A4-316 bolts should survive 1,000 hours of neutral salt spray (NSS) per ASTM B117 or ISO 9227 with only minor surface staining and no red rust. A2-304 typically survives 500–720 hours in the same test. Ask for the test report with photos of the samples after exposure, not just a pass/fail statement. If your product warranty depends on corrosion performance, make salt spray testing a routine incoming-goods check.

Common Quality Red Flags

Watch for these warning signs when vetting suppliers:

• Grade substitution: 201 or 410 stainless sold as 304, or 304 sold as 316. The price gap is real; if a quote is 20–30% below market, verify with XRF.
• Missing or generic MTRs: A one-page MTR without a heat number or without matching chemistry is not valid traceability.
• No passivation line: Ask for the passivation process and bath records; dark or stained threads are a sign of skipped steps.
• Wrong tolerance class: Bolts marked 6g but gauging loose may be stamped from off-spec stock.

Typical Use Case: Outdoor Electrical Enclosure Distributor

A European distributor of outdoor electrical enclosures needs M8×25 and M12×40 A4-80 hex bolts with full traceability for a 50,000-unit project. The bolts are exposed to coastal air and must survive a 1,000-hour salt-spray test. The buyer works with a Jiaxing sourcing agent who pre-qualifies mills, pulls MTRs per heat, arranges third-party tensile testing, and inspects thread tolerance before shipment.

Recommended Next Steps

Before committing to a 1,000 kg order, request a current MTR for the exact heat lot and specify thread tolerance class 6g/6H in writing. For first-time suppliers, book a factory audit that includes alloy verification and passivation-line inspection. If your shipment is headed to the EU or US, run the tariff and landed-cost numbers through our tariff calculator before signing the purchase order.