High Tensile Bolts: Specifications, Grades, Strength & Uses

High tensile bolts are fasteners that are made from heat-treated carbon or alloy steel and are designed to resist pulling (tensile) forces significantly higher than a conventional bolt. Unifit Metalloys Inc., a leading high tensile bolts manufacturer and supplier in Mumbai, India, can provide high tensile bolts specifications that conform to both Indian (IS) and International (ASTM, DIN, ISO) standards with ISO 9001:2015 certified manufacturing. This guide addresses high tensile bolts specifications and grades, grade-wise tensile and yield strength, head markings, applications and choosing the right grade for your project. You will come across information regarding tensile and yield strength per grade, means of identification, applications in the industry, and guidance on choosing the appropriate grade. 

What Are High Tensile Bolts?

High tensile bolts are made from medium or high-carbon steel or alloy steel, quenched and tempered to enhance their resistance to pulling force. “High tensile” is not a reference to hardness or surface finish, but to the amount of pulling stress the high tensile bolts strength where it can withstand without breaking. A Grade 8.8 or 10.9 bolt is different from a mild-steel bolt because it is subjected to this heat-treatment process, which makes it the preferred choice wherever a joint is required to withstand heavy structural, dynamic or vibration loads. A bolt's high tensile strength rating depends on the extent of the tensile pulling stress it can bear before it fails.

High Tensile Bolts vs. Standard & Stainless Steel Bolts

High tensile bolts have a much greater load capacity per unit size, although stainless steel bolts perform better in corrosive environments; that's why, in a straight high tensile bolts vs stainless steel bolts comparison, high tensile grades are the more common choice for structural and heavy-engineering applications, typically coated for corrosion resistance.

High Tensile Bolt Specifications

High Tensile Bolt Grades & Tensile Strength Chart

The most critical decision in specifying a high tensile bolt is the grade. Below is a table of tensile and yield strength for the entire range of products we make:

How to Read High Tensile Bolt Head Markings

Each high tensile bolt has the grade and manufacturer's identification stamped on its head, for instance "8.8" or "10.9". The markings allow engineers and inspectors to be sure that the correct grade has been installed without a lab test. Metric grades (4.6 through 14.9) are identified by a pair of numbers separated by a decimal, representing tensile and yield strength ratios, and ASTM bolts (A325, A490) are identified by the standard designation, sometimes with radial lines. Wherever possible, check head markings before installing, particularly at critical structural joints.

Applications of High Tensile Bolts by Industry

We are as Unifit Metalloys are the most trusted manufacturer and supplier of high tensile bolts, which are widely used in Refineries, Oil & Gas, Power Generation, Pharmaceutical, and Pulp & Paper, industries, making them the biggest part of our fastener supply to these sectors. In addition to high tensile bolts uses, these bolts are installed on most construction and structural steelwork projects, all automotive assembly projects, most railway systems, wind turbine towers, heat exchanger systems and offshore oil drilling platforms, and anywhere else where joints are subjected to high static load, vibration and/or fatigue cycling.

How to Choose the Right High Tensile Bolt Grade

Choices for high tensile bolt grades should be made based on:

• The load type — static structural loads may be able to utilise 8.8; dynamic/impact loads are better suited to 10.9 or 12.9
• Vibration and fatigue exposure – higher grades have better resistance to loosening and fatigue cracking.
• Operating temperature — changes may be needed not only in the grade but in the material in very high temperatures or sub-zero temperatures.
• Corrosion environment — do not over-specify strength, but match the grade with an appropriate coating.
• A high price for getting a better grade - extra specification equates to extra expense - but no extra benefit in real-world terms.

Our engineering team is available to help customers correlate grade to application prior to ordering, which is not just to a specification sheet.

High Tensile Bolt Dimensions & Weight Chart

Bolt weight varies by diameter, length, and grade. For a complete size-wise and length-wise weight breakdown, refer to our dedicated ASTM A193 GR.B16 Bolts page, which is built to be used alongside this specification guide.

Surface Coatings & Finishes for High Tensile Bolts

• Thicker zinc layer for outdoor and structural steel exposure (Hot-dip galvanising (HDG))
• Black oxide — gives some level of protection from corrosion, but doesn't alter dimensions, typical on manufactured bolts.
• Phosphating is used to improve paint adhesion and as a base for other coating methods.
• Dacromet/Geomet is a high-performance coating for automotive and critical fasteners that will not cause hydrogen embrittlement.

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