High strength double ended bolts refer to specialised fasteners of grade 8.8 and above (8.8, 10.9, 12.9) with threads at both ends and a smooth shank in the middle. They rely on matching nuts at both ends to achieve a clamped connection between two components. As they are required to withstand heavy loads, alternating vibrations and high-pressure loads, they undergo specialised quenching and tempering heat treatment and are widely used in critical areas such as steel structures, pressure vessels, construction machinery and pipe flanges.

• Grade 8.8: The first digit, 8, denotes a minimum tensile strength of ≥800 MPa; the second digit, 8, denotes a yield-to-tensile strength ratio of 0.8, resulting in a minimum yield strength of 800 × 0.8 = 640 MPa. The raw material is typically high-quality 45# medium-carbon steel, which undergoes quenching and high-temperature tempering. This is the most cost-effective general-purpose high-strength grade.
• Grade 10.9: Tensile strength ≥ 1040 MPa, yield-to-tensile ratio 0.9, with a yield strength of up to 936 MPa. The base material is typically 40Cr chromium alloy steel, offering excellent comprehensive tensile, shear and fatigue resistance. It is commonly used for medium- and high-pressure flanges and heavy-duty equipment.
• Grade 12.9: The top-tier grade in the high-strength series, with a tensile strength of ≥1220 MPa, a yield-to-tensile ratio of 0.9, and a yield strength of 1098 MPa. It utilises high-quality alloy steels such as 35CrMo and is suitable for harsh operating conditions involving high temperatures, high pressure and severe vibration.

Grade matching: Grade 8.8 bolts must be used with Grade 8 nuts, and Grade 10.9 bolts with Grade 10 nuts. Mixing different grades is strictly prohibited, as this may lead to fastening failure;
Installation guidelines: For heavy-duty flange connections, a torque wrench must be used to tighten the bolts evenly in stages. Do not tighten one side forcefully to prevent stress fractures in the studs; In vibrating conditions, spring washers or lock nuts may be fitted;
Environmental Selection: For dry indoor environments, black oxide or electro-galvanised finishes may be selected; for damp outdoor environments, hot-dip galvanised finishes should be chosen; in coastal or chemical corrosive environments, Darco or 304/316 stainless steel materials are recommended.

High strength double ended bolts are commonly used for removable connections in heavy-load and vibration-prone applications, such as pressure vessels, pipe flanges, steel structures, construction machinery, locomotives and high-pressure equipment