What Steel Goes Into a Grade 8.8, 10.9 or 12.9 Bolt

By Suneer Kapoor, Managing Partner, S R Forgings  ·  3 September 2026

A property class is not a material. ISO 898-1 tells a manufacturer what the finished bolt must achieve — tensile strength, yield, hardness, minimum tempering temperature — and leaves the choice of steel to whoever is making it. Two bolts marked 8.8 can pass the same tests and be made from steels that cost very different money and behave very differently in service.

This is where most quiet substitution in the fastener trade happens. What follows is how the choice is actually made on a shop floor, and where each steel stops working.

What the standard actually requires

For property class 8.8 and 10.9, ISO 898-1 requires carbon steel with additives or alloy steel, quenched and tempered, with a minimum tempering temperature of 425 °C. For 12.9 it requires alloy steel and a minimum of 380 °C.

That tempering rule matters more than it looks. It is not about hardness on the day of testing — it is about the bolt keeping its properties afterwards, through whatever the part still has to go through. Hold on to it; it decides one of the most important choices further down this page.

Beyond that, the standard names no steels. Everything else is a manufacturer's decision, and it is governed by one property: hardenability.

The hardenability ladder

Hardenability is not hardness. It is how deep into the section the steel will harden when quenched. A steel with poor hardenability can reach the right surface hardness and still have a soft core — a bolt that passes a hardness check on the head and fails a proof load test.

Each rung on this ladder buys hardenability, and each rung costs more:

A note on the first two, because the trade uses them loosely. EN8 covers 0.36 to 0.44% carbon. EN8D is bought at 0.40 to 0.45% with sulphur and phosphorus each held to 0.040% maximum. The "D" is doing two jobs — it sits at the top of the carbon band and it is cleaner steel. For a part that gets cold forged and then quenched, the residuals matter as much as the carbon.

C45Cr, which several Indian rolling mills supply, is not a local improvisation. It corresponds to the 46Cr2 family in EN 10083 — a chromium steel that has long occupied the gap between plain C45 and 41Cr4, because that gap is a real one.

The constraint is often length, not diameter

Most material guidance says: bigger diameters need alloy steel. True, but incomplete, and the incomplete half causes more rejections.

Plain carbon steel can only reach hardness with a water quench, because water pulls heat out fast enough to beat the transformation. Water is a severe quenchant. On a short part that is manageable. On a long, slender part it is not: the vapour blanket around the bolt forms and collapses unevenly along its length, one side transforms before the other, and the bolt bows.

In practice the threshold arrives somewhere around 200 mm. Below it, water quenching a carbon steel bolt is routine. Above it, bolts come out of the tank bent, and every one of them has to be straightened and re-tempered — assuming the distortion is recoverable at all.

A steel with more hardenability reaches the same hardness from an oil quench. Oil cools more slowly and far more evenly, and the distortion largely disappears.

So the ladder reads differently once you have run the furnace. Good primary-mill carbon bar can reach around 640 MPa as rolled and passes 8.8 without difficulty in shorter lengths. It is not strength that stops it at length — it is the quench it forces you to use. Climbing to a chromium steel is often bought not for through-hardening at all, but for the ability to quench in oil.

Boron steel: cheap hardenability, with one trap

Boron steels — 10B21, 10B25, 19MnB4 and their higher-carbon variants — do something remarkable. A few parts per million of boron dramatically increases hardenability at low carbon content. That means a steel soft enough to cold forge beautifully, which still hardens properly. For high-volume cold-formed fasteners it is the reason the industry can make grade 8.8 at the price it does.

The honest disadvantages:

Boron has to be protected. It only works as free boron. If it combines with nitrogen it does nothing at all, so the steel must be made with enough titanium to fix the nitrogen first. This makes boron steel unusually dependent on steelmaking control — the same nominal grade from two mills is not the same steel.

It is tempered low. To reach 8.8 or 10.9 hardness, boron steel is often tempered at a lower temperature than a chromium steel would need. And now the tempering rule from the top of this page comes due.

The rule that follows: coating decides material

A hot-dip galvanising bath runs at around 450 °C. A bolt tempered below that temperature gets de-tempered in the kettle. The hardness that passed inspection walks out in the zinc. The bolt arrives on site marked 10.9 and is not 10.9 any more, and nothing about its appearance says so.

This is precisely what the 425 °C minimum tempering temperature in ISO 898-1 is there to prevent.

Zinc flake is different: it cures at roughly 240 to 320 °C, comfortably below the tempering temperature, so nothing changes. Black bolts see no thermal exposure at all.

Which gives a clean practical rule that we follow:

• Black or zinc flake — boron steel is a sound choice for 8.8 and 10.9
• Hot-dip galvanised — use a steel tempered above the bath temperature: 41Cr4 or 42CrMo4

Anyone selling hot-dip galvanised high tensile bolts made from low-tempered boron steel is selling a bolt whose certificate was true before it was dipped.

Mill quality is a variable, not a constant

Two bars to the same specification are not the same bar.

Primary steel from an integrated mill is made from iron ore through a controlled route, with known chemistry and consistent cleanliness. Re-rolled bar is made from scrap, and its chemistry carries whatever the scrap carried. It can meet a specification on paper and still behave differently in heat treatment, because residual elements and inclusion content are not on the test certificate.

For large sections the difference becomes decisive. Centre soundness — whether the middle of a big bar is fully consolidated or carries porosity from casting — determines whether a 36 or 42 mm bolt hardens through. Mills using ladle refining and hot-top casting produce bar that performs at those sizes; mills that do not, produce bar that will not, whatever the chemistry says.

This is why the same grade of steel qualifies for 10.9 at 30 to 42 mm from one supplier and not from another. It is a real effect, it is invisible on a test certificate, and it is learned by running the material rather than by reading about it.

Some Indian rolling mills have responded to this well. Mills including Arora Steel and Skyway supply C45Cr and EN8DCr as an alternative to plain carbon bar, giving fastener makers a usable rung between plain carbon steel and full alloy steel without going straight to 41Cr4.

How to read a test certificate against the grade

If you buy fasteners, the material test certificate tells you more than most buyers take from it:

• Look at the chemistry, not just the mechanical results. A 10.9 bolt at 30 mm made from plain carbon steel should raise a question, whatever the hardness figures say.
• Check chromium. Its presence and level tells you which rung of the ladder you are on.
• For anything hot-dip galvanised, ask the tempering temperature. It should be above the galvanising bath temperature. This is a fair question and a manufacturer who cannot answer it has told you something.
• Ask where the bar came from. Primary or re-rolled is a legitimate question on a structural order.
• On large diameters, ask about core hardness, not just surface hardness. That is where poor hardenability hides.

What we use, and why

At S R Forgings the choices are deliberately one rung above what the market minimum allows:

• Grade 8.8 — 41Cr4 (EN18) across the whole size range. Much of the trade makes 8.8 in EN8D or C45Cr because it is cheaper and it passes. Using a chromium steel throughout means an oil quench at every size, less distortion on long bolts, and a genuinely through-hardened core rather than a hard skin over a soft centre.
• Grade 10.9 — 41Cr4 up to 27 mm, 42CrMo4 (4140) above it. Above 27 mm the molybdenum earns its cost in through-hardening.
• Grade 12.9 and ASTM A193 B7 — 42CrMo4.
• Boron steel — at M16 and M20, for black and zinc flake work only. Never under hot-dip galvanising, for the reason set out above.

None of this is visible in a finished bolt. Two bolts marked 8.8 look identical, and both will pass the tests. The difference shows up in a long bolt that arrives straight, in a large diameter that is hard all the way through, and in a galvanised bolt that is still the grade it says it is after the kettle.

If you have a specification and want to know what steel it actually needs — or you have a certificate you would like read properly — send it across.