A4-70 Hex Head Bolt

About A4-70 Hex Head Bolt

An A4-70 hex head bolt is a high-strength, marine-grade metric fastener manufactured from 316-grade (18/10/2) austenitic stainless steel that has been cold-worked to achieve a minimum tensile strength of 700 MPa (N/mm). The "A4" designation signifies its premium, acid-proof chemical compositionalloyed with molybdenum to withstand highly corrosive chloride and marine environmentswhile the "70" denotes its high-strength performance tier. Combining the superior corrosion resistance of 316 stainless steel with a robust six-sided external drive head, this heavy-duty fastener allows for secure, high-torque tightening in aggressive structural, industrial, and offshore setups.

Technical Specifications

• Material Composition: Grade 316 Stainless Steel (typically 16-18% Chromium, 10-14% Nickel, and 2-3% Molybdenum).
  
  

• Mechanical Strength Class: Class 70 (cold-worked).

• Minimum Tensile Strength: 700 MPa / 700 N/mm (comparable to Grade 8.8 carbon steel bolts).

• 0.2% Proof Stress: 450 N/mm.
  
  

• Magnetic Profile: Virtually non-magnetic under normal conditions; can exhibit faint residual magnetism after severe cold-working.

• Standard Conformity: Fully compliant with ISO 3506-1 standards, matching dimensional guidelines like DIN 933 (fully threaded) or DIN 931 (partially threaded shank).

• Thread Availability: Coarse, fine, and extra-fine metric thread options.

Key Features & Product Benefits

• Ultimate Pitting Resistance: Molybdenum content prevents pitting, crevice corrosion, and stress cracking caused by seawater, brine, and aggressive chemical washes.

• High Structural Integrity: Delivers up to 40% more tensile strength than basic A4-50 bolts, making it perfect for load-bearing assemblies.

• Stable Torquing: The classic external hex design prevents cam-out or tool slippage, distributing torque evenly during high-tension structural pre-loading.

• Thermal Endurance: Performs exceptionally well in extreme cold (cryogenic applications) without turning brittle and resists oxidation at elevated temperatures.

Primary Applications

• Offshore & Marine Operations: Critical structural assemblies on oil rigs, shipbuilding hulls, docks, and coastal defence barriers exposed to constant sea spray.

• Chemical Processing & Refining: Securing pressure vessels, industrial pipes, mixing vats, and acid-transfer pipelines where chemical corrosion is severe.

• Desalination & Water Treatment: Fastening submerged components, pumps, gates, and frameworks inside water purification facilities.

• Heavy Engineering & Cleanrooms: Demanded by pharmaceutical labs, subsea robotics, high-end architecture, and medical manufacturing equipment requiring high strength and perfect sanitization.