High tensile bolts are high strength bolts used to protect parts that are subjected to high stress, vibration and harsh operating environments. These bolts are available in various property classes (8.8, 10.9 and 12.9) each with different tensile strength, yield strength and hardness levels to meet specific fastening requirements. They have mechanical properties that vary according to the grade of steel, the process of their production and the heat treatment. This guide shows grades of high tensile bolts, material specifications, mechanical properties and industrial applications. It also comprises bolt strength tables, proof load, standards that apply, and difference between mild steel and high tensile bolts. The data is valuable to anyone who needs to select appropriate bolts in structures, machines, automobiles, and other structural joints.
What Are High Tensile Bolts
High tensile bolts are fasteners of ISO property class 8.8, 10.9 or 12.9, made from medium carbon or alloy steel that is quenched and tempered to reach a minimum tensile strength of 800 MPa or more. They are chosen where a joint carries heavy or repeated load and the bolt has to stay small. The term HT bolt is shorthand used on drawings, bills of materials and in the trade. Grade 8.8 is usually the entry point, because it is the first property class in the standard that is routinely quenched and tempered and that roughly doubles the strength of ordinary mild steel. From there, 10.9 and 12.9 step up again. Most high tensile bolts are made as hex bolts or heavy hex bolts, with socket head screws making up the 12.9 end of the range. The steels behind them are carbon and alloy steels, covered in our alloy steel bolts range.
High Tensile Bolt Grades 8.8, 10.9 and 12.9
This is the high tensile bolt strength chart most buyers need. The figures are the minimum values in ISO 898-1, which means a conforming bolt will meet or exceed them. For the complete range from grade 4.6 upward, including stainless and high temperature grades, see our bolt tensile strength chart.
| Grade | Tensile strength (MPa) | Yield strength (MPa) | Proof stress (MPa) | Hardness (HRC) | Typical use |
| 8.8 | 800 | 640 | 580 | 22 to 32 | Machinery, steel structures, general engineering |
| 10.9 | 1040 | 940 | 830 | 32 to 39 | Automotive, heavy machinery, high load joints |
| 12.9 | 1220 | 1100 | 970 | 39 to 44 | Tooling, engines, socket head screws |
Size matters for 8.8. Up to and including M16, the minimum figures are 800 MPa tensile, 640 MPa yield and 580 MPa proof stress. Above M16, they rise to 830, 660 and 600 MPa. The 8.8 row above shows the smaller size values. Always check the size against the standard before you design to the limit.
Higher grades give you strength, but they also give you less ductility. A grade 12.9 bolt is stronger than a 10.9, but it is also harder, so it tolerates misalignment, side loading and hydrogen exposure less well. We cover that trade off in the coatings section below. For a closer look at the top grade, read our guide to grade 12.9 bolts.
How to Read a High Tensile Bolt Grade
An HT bolt grade is stamped on the head, and it tells you everything about strength in two numbers.
- The first number multiplied by 100 gives the nominal tensile strength in MPa. Grade 10.9 is nominally 1,000 MPa, and the standard sets the guaranteed minimum a little higher, at 1040 MPa.
- The second number divided by 10 gives the ratio of yield to tensile strength. In grade 10.9, the 9 means yield is 90 percent of tensile.
- Multiplying the two and then by 10 gives the nominal yield. For 8.8 that is 8 times 8 times 10, or 640 MPa, which matches the table.
A bolt with no marking on the head cannot be assumed to be high tensile. Where traceability matters, ask for a mill test certificate to EN 10204 3.1, which ties the bolt to its heat and test results.
High Tensile Bolt Material and Heat Treatment
The high tensile bolt material is carbon or alloy steel, hardened by heat treatment. It is not stainless steel and it is not a nickel alloy, which is a common misunderstanding. The strength comes from quenching the bolt from a high temperature to harden it, then tempering it to bring back toughness. If your application needs corrosion resistance instead, see our stainless steel fastener range and our guide to stainless steel fasteners.
| Grade | Steel type | Typical examples | Heat treatment |
| 8.8 | Medium carbon steel, or low carbon boron steel | C45, 35B2 | Quenched and tempered |
| 10.9 | Boron steel, or alloy steel | 35B2, 34Cr4, 42CrMo4 | Quenched and tempered |
| 12.9 | Alloy steel | 42CrMo4, 34CrNiMo6 | Quenched and tempered |
ISO 898-1 sets limits on carbon, phosphorus and sulphur and a minimum tempering temperature for each class, but it does not fix one chemistry. Two 8.8 bolts from different mills can be made from different steels and still conform. If a drawing needs a specific steel, it has to be stated separately from the grade marking.
High Tensile Bolt Specifications and Standards
High tensile bolt specifications come from several standards at once, because one standard sets the strength and another sets the dimensions. Knowing which does what avoids a lot of confusion on purchase orders.
| Standard | What it covers | Note |
| ISO 898-1 | Mechanical properties of bolts, screws and studs, classes 4.6 to 12.9 | The source of every strength figure on this page |
| ISO 898-2 | Mechanical properties of nuts, classes 04 to 12 | Used to match nuts to bolts |
| ISO 4014 and ISO 4017 | Hex head bolts, partly threaded and fully threaded | Metric dimensions |
| DIN 931 and DIN 933 | Hex head bolts, partly threaded and fully threaded | Older German standards still quoted widely, see our DIN 933 page |
| IS 1364 | Indian standard for hexagon head bolts | Dimensions and product grades |
| ASTM F3125 | Structural bolts, which now includes Grades A325 and A490 | Inch series, not an ISO property class |
| SAE J429 | Grade 5 and Grade 8 inch bolts | Roughly comparable to 8.8 and 10.9 |
Treat the comparisons as approximate. ASTM A325 and SAE Grade 5 sit near 8.8 in tensile strength, and ASTM A490 and SAE Grade 8 sit near 10.9, but the standards test and size their bolts differently and none of them is a drop in replacement without checking the drawing. Our ASTM A325 bolts and structural bolts pages go further, and our ISO standards section lists the dimensional references. For bolting in high temperature pressure service, the relevant standard is different again, as explained in our guide to ASTM A193 bolt grades.
Difference Between Mild Steel and High Tensile Bolts
The difference between mild steel and high tensile bolts is strength, and strength comes from heat treatment. When people say mild steel bolts they usually mean grades 4.6 and 4.8, made from low carbon steel with no hardening step. High tensile bolts start at 8.8 and are quenched and tempered.
Mild Steel Bolts Compared with High Tensile Bolts
| Feature | Mild steel, grade 4.6 | High tensile, grades 8.8 to 12.9 |
| Minimum tensile strength | 400 MPa | 800 to 1220 MPa |
| Minimum yield strength | 240 MPa | 640 to 1100 MPa |
| Steel | Low or medium carbon steel | Medium carbon, boron or alloy steel |
| Heat treatment | None | Quenched and tempered |
| Hardness | About 120 to 220 HV | 22 to 44 HRC |
| Typical use | Light duty, non critical fixings, fencing, timber work | Structures, machinery, vehicles, high load joints |
| Relative cost | Lowest | Higher, rising with grade |
High tensile is not automatically better. A mild steel bolt is cheaper, more ductile and perfectly adequate where the load is light and the joint is not safety critical. It also bends and stretches visibly before it fails, which some designers value. Choose high tensile when the joint needs the strength or when you want a smaller bolt to carry the same load. Ourcarbon steel bolts range covers the lower grades.
Proof Load of High Tensile Bolts by Size
Proof load is the tensile force a bolt can carry without any permanent stretch, and it is the number to design against. You work it out as the tensile stress area multiplied by the proof stress. For an M16 bolt in grade 8.8, that is 157 square mm multiplied by 580 MPa, which gives 91.1 kN.
| Size | Stress area (mm2) | Grade 8.8 (kN) | Grade 10.9 (kN) | Grade 12.9 (kN) |
| M8 | 36.6 | 21.2 | 30.4 | 35.5 |
| M10 | 58.0 | 33.6 | 48.1 | 56.3 |
| M12 | 84.3 | 48.9 | 70.0 | 81.8 |
| M16 | 157.0 | 91.1 | 130.3 | 152.3 |
| M20 | 245.0 | 147.0 | 203.3 | 237.7 |
| M24 | 353.0 | 211.8 | 293.0 | 342.4 |
| M30 | 561.0 | 336.6 | 465.6 | 544.2 |
| M36 | 817.0 | 490.2 | 678.1 | 792.5 |
Proof stress for 8.8 is 580 MPa up to M16 and 600 MPa above, which is why the step from M16 to M20 is slightly larger than the area alone would suggest. These are limits, not working loads, and a design factor applies on top. For bolt weights to go with these loads, use our bolt weight chart, and for full tensile load figures across more sizes see our bolt tensile strength chart.
Where High Tensile Bolts Are Used
The grade you choose sets where a high tensile bolt belongs. Grade 8.8 is the general purpose choice, 10.9 suits heavier or more critical joints, and 12.9 is used where space is tight and the load is high.
| Grade | Typical applications | Why this grade |
| 8.8 | Machinery frames, steel structures, general engineering, trailers and agricultural equipment | Strong enough for most joints at the lowest cost and with the best ductility of the three |
| 10.9 | Automotive and truck assemblies, heavy machinery, high load structural joints | Higher clamp load from a smaller bolt |
| 12.9 | Tooling, engines, dies and socket head cap screws | Highest strength where the joint is compact; needs care over hydrogen and alignment |
Coatings and Hydrogen Embrittlement
High tensile bolts are carbon or alloy steel, so they rust unless protected. The finish you choose matters more as the grade goes up, because harder steel is more sensitive to hydrogen.
Hydrogen embrittlement happens when hydrogen atoms enter the steel during acid cleaning or electroplating and later cause a sudden brittle crack under sustained load. The risk rises with hardness, which is why it is a real concern for 10.9 and especially 12.9, and a minor one for 8.8. Processes that avoid hydrogen, or a bake after plating, are the usual controls.
| Finish | Typical use | Point to watch |
| Zinc plating | Indoor and mild outdoor use | Hydrogen risk on 10.9 and above, so baking or a different process is advised |
| Hot dip galvanising | Outdoor structures and exposed steelwork | Thread allowances and process controls apply, and it is generally avoided on 12.9 |
| Zinc flake coating | Automotive and high strength fasteners | Applied without electroplating, so it avoids the hydrogen risk |
| Black oxide and oil | Indoor machinery | Light protection only |
| Phosphate and oil | Machinery and assembly work | Light protection, helps lubrication |
Where corrosion is the main concern, the better answer may be a different material altogether. Our comparison of galvanised bolts against stainless steel explains when the extra cost of stainless is worth paying.
Choosing the Right High Tensile Bolt
Five questions settle most selections.
- What load does the joint carry? Work out the required proof load and pick the smallest grade and size that exceeds it with your design factor.
- What is the environment? Damp, outdoor or chemical exposure points to a coating, or to stainless steel instead.
- What is the temperature? Standard HT bolts suit ordinary service. High or low temperature bolting needs a specific grade, so check the specification.
- What kind of joint is it? Vibration, side loading and shock favour a tougher grade such as 8.8 or 10.9 over 12.9.
- What does the drawing call out? If it names a standard such as ISO 4014 or ASTM F3125, supply to that standard and not to an approximate equivalent.
Pair the bolt with a nut of at least the same property class: a class 8 nut for an 8.8 bolt, class 10 for 10.9 and class 12 for 12.9. Our heavy hex nuts are available to match. A softer nut on a hard bolt strips before the bolt reaches its rated load.
Frequently Asked Questions
What are high tensile bolts?
High tensile bolts are fasteners commonly available in property classes 8.8, 10.9 and 12.9, made from quenched and tempered carbon or alloy steel. Their minimum tensile strength starts at 800 MPa, compared with 400 MPa for property class 4.6.
Which grades are high tensile?
Grades 8.8, 10.9 and 12.9 are standard high tensile classes under ISO 898-1. Grade 8.8 is a common starting point for high-strength metric bolts. Property classes 14.9 and 12.8 are not standard classes specified in ISO 898-1.
What is the difference between 8.8, 10.9 and 12.9?
The minimum tensile strengths are 800 MPa, 1,040 MPa and 1,220 MPa, respectively. Higher classes provide greater strength but generally lower ductility and increased sensitivity to hydrogen embrittlement.
What does HT bolt grade mean?
HT stands for high tensile. The HT bolt grade refers to the property class marked on the bolt head, such as 8.8 or 10.9. The first number indicates nominal tensile strength in hundreds of MPa, while the second represents the ratio of nominal yield strength to nominal tensile strength.
What is the difference between mild steel and high tensile bolts?
Property class 4.6 bolts have a minimum tensile strength of 400 MPa, while classes 8.8 and above provide higher strength. High tensile bolts typically undergo controlled heat treatment to achieve their specified mechanical properties.
What material are high tensile bolts made from?
High tensile bolts are made from medium carbon steel, boron steel or alloy steel, depending on the grade. Grade 8.8 often uses medium carbon steel, while grades 10.9 and 12.9 use boron or alloy steels. These materials are hardened by quenching and tempering.
Are high tensile bolts stainless?
No. Property classes 8.8, 10.9 and 12.9 under ISO 898-1 refer to carbon and alloy steel fasteners. Stainless steel bolts are marked A2 or A4 and follow a different standard, ISO 3506.
Can high tensile bolts be galvanised?
Yes, but galvanising requires careful process control because high-strength fasteners can be susceptible to hydrogen embrittlement. Grade 12.9 fasteners require particular caution. Confirm coating suitability and any required hydrogen-relief treatment with the fastener manufacturer and applicable specification.
Is grade 12.9 stronger than 10.9?
Yes. Grade 12.9 has a minimum tensile strength of 1,220 MPa, compared with 1,040 MPa for grade 10.9. It is also stronger and generally less ductile, making it more sensitive to hydrogen embrittlement, misalignment and side loading. Choose grade 12.9 when the additional strength is necessary for the application.
What nut goes with a 10.9 bolt?
Use a compatible property class 10 nut under ISO 898-2, such as a class 10 hex nut or heavy hex nut where applicable. A nut with insufficient strength may strip before the bolt reaches its intended load capacity.
