---
title: "Property Class 12.9 Steel Heavy Hex Structural Bolts"
description: "Property Class 12.9 Steel Heavy Hex Structural Bolts (ISO 898-1 property class 12.9): requires plating or black-oxide protection."
url: https://engineeringhardware.com/guide/material/grade-12-9-steel-heavy-hex-structural-bolts/
source: Engineering Hardware & Fasteners
---

# Property Class 12.9 Steel Heavy Hex Structural Bolts

Bolts · ASTM F3125 Grade A325

Mechanical properties, corrosion behaviour and the full dimensional range for this grade.

Property Class 12.9 Steel Heavy Hex Structural Bolts are supplied to ISO 898-1 property class 12.9. Corrosion behaviour is requires plating or black-oxide protection, the material is magnetic, and the usable service range is commonly limited to approx. +300 °C; confirm against the coating and the load. In practice this grade is the highest common strength class, for high-load machine assembly.

- **Grade:** ISO 898-1 property class 12.9
- **Corrosion resistance:** Requires plating or black-oxide protection
- **Magnetic response:** Magnetic
- **Service temperature:** commonly limited to approx. +300 °C; confirm against the coating and the load

Property values are typical for the grade. Certified mechanical test data is available on request for production orders.

![Property Class 12.9 Steel Heavy Hex Structural Bolts reference image](https://engineeringhardware.com/wp-content/uploads/2026/09/Heavy-Hex-Structural-Bolts.webp)

Property Class 12.9 Steel Heavy Hex Structural Bolts are supplied to ASTM F3125 Grade A325. The grade provides requires plating or black-oxide protection and is magnetic; the practical service range is commonly limited to approx. +300 °C; confirm against the coating and the load. Property data, grade comparison and the full size table follow.

## Why Choose Property Class 12.9 Steel for Heavy Hex Structural Bolts

The highest common strength class, for high-load machine assembly. For heavy hex structural bolts, this matters when requires plating or black-oxide protection is the primary requirement.

## Mechanical & Physical Properties

| Grade designation | ISO 898-1 property class 12.9 |
| --- | --- |
| Magnetic response | Magnetic |
| Service temperature | commonly limited to approx. +300 °C; confirm against the coating and the load |
| Corrosion characteristic | Requires plating or black-oxide protection |

## Corrosion Behavior in Service

Carbon and alloy steel heavy hex structural bolts require a coating for corrosion protection. Zinc plating (with or without passivation) is the most common; hot-dip galvanizing is used for outdoor structural service; black-oxide is mainly a tarnish-prevention finish for tool-room use.

## Galvanic & Galling Considerations

For heavy hex structural bolts in steel, the dominant compatibility issue is hydrogen embrittlement at high property classes (10.9 and above) when electroplated. Plating must be followed by an appropriate baking treatment, or a non-electroplated finish (mechanical zinc, zinc flake) should be substituted.

## Sizes of Heavy Hex Structural Bolts Available in Property Class 12.9 Steel

| Size | Threads per inch | Width across flats (in) | Head height (in) | Thread length (in) | Minimum tensile (kN) | Mating nut | Reference standard |
| --- | --- | --- | --- | --- | --- | --- | --- |
| 1/2" | 13 | 7/8 | 5/16 | 1.00 | 91 | A563 DH | ASTM F3125 |
| 5/8" | 11 | 1-1/16 | 25/64 | 1.25 | 145 | A563 DH | ASTM F3125 |
| 3/4" | 10 | 1-1/4 | 15/32 | 1.375 | 215 | A563 DH | ASTM F3125 |
| 7/8" | 9 | 1-7/16 | 35/64 | 1.50 | 297 | A563 DH | ASTM F3125 |
| 1" | 8 | 1-5/8 | 39/64 | 1.75 | 389 | A563 DH | ASTM F3125 |
| 1-1/8" | 7 | 1-13/16 | 11/16 | 2.00 | 432 | A563 DH | ASTM F3125 |

Dimensions are independent of the material: every size above is available in Property Class 12.9 Steel.

Custom manufacturing · Made to drawing

## Need this bolt in a grade that is not listed?

Grade 12 9 Steel is one of the grades the catalog describes. Thread size and pitch, length, property class, drive form, head geometry and head marking are all specifiable — the standard part is where it starts, not where it stops. Made by cold heading or hot forging.

Minimum order from 2,000 pieces; samples within 7 days of drawing approval.

[Custom bolt manufacturing](https://engineeringhardware.com/custom/bolts/)[Send a drawing](https://engineeringhardware.com/custom/bolts/#rfq)

## Technical Drawing

Dimensioned line drawing of 1/2" Heavy Hex Structural Bolts to ASTM F3125 Grade A325. Side view and top view, drawn to the dimensions in the table below and redrawn for any size selected.

- **Threads per inch:** 13
- **Width across flats (in):** 7/8
- **Head height (in):** 5/16
- **Thread length (in):** 1.00
- **Minimum tensile (kN):** 91
- **Mating nut:** A563 DH

## Dimensions & Specifications (ASTM F3125 Grade A325)

| Size | Threads per inch | Width across flats (in) | Head height (in) | Thread length (in) | Minimum tensile (kN) | Mating nut | Reference standard |
| --- | --- | --- | --- | --- | --- | --- | --- |
| 1/2" | 13 | 7/8 | 5/16 | 1.00 | 91 | A563 DH | ASTM F3125 |
| 5/8" | 11 | 1-1/16 | 25/64 | 1.25 | 145 | A563 DH | ASTM F3125 |
| 3/4" | 10 | 1-1/4 | 15/32 | 1.375 | 215 | A563 DH | ASTM F3125 |
| 7/8" | 9 | 1-7/16 | 35/64 | 1.50 | 297 | A563 DH | ASTM F3125 |
| 1" | 8 | 1-5/8 | 39/64 | 1.75 | 389 | A563 DH | ASTM F3125 |
| 1-1/8" | 7 | 1-13/16 | 11/16 | 2.00 | 432 | A563 DH | ASTM F3125 |

Product catalog

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[View all Structural & Heavy Hex Bolts →](https://engineeringhardware.com/fastener-products/bolts/structural-heavy-hex-bolts/) [Full product catalog](https://engineeringhardware.com/products/)

## Materials & Grades

Select a material class to see the grades we supply it in. Each designation is listed with its ISO/EN, AISI/SAE, JIS and GB equivalents.

Stainless Steel38 Carbon Steel66 Alloy Steel41 Bearing Steel5 Spring Steel25 Aluminum and Aluminum Alloys21 Brass17 Bronze11 Copper8 Titanium and Titanium Alloys18 Nickel and Nickel Alloys22 Iron-Nickel and High-Temperature Alloys10 Tool Steel and Specialty Hardened Steel9 Structural and Construction Steel9

### Stainless Steel

#### Austenitic Stainless Steel

- A1 AISI 303 · SAE 303 · SUS303 · EN 1.4305 · X8CrNiS18-9
- A2 AISI 304 · SAE 304 · SUS304 · EN 1.4301 · X5CrNi18-10 · 06Cr19Ni10
- 304L AISI 304L · SUS304L · EN 1.4307 · X2CrNi18-9 · 022Cr19Ni10
- 302 AISI 302 · SUS302 · EN 1.4310
- 305 AISI 305 · SUS305 · EN 1.4303
- A4 AISI 316 · SAE 316 · SUS316 · EN 1.4401 · X5CrNiMo17-12-2 · 06Cr17Ni12Mo2
- 316L AISI 316L · SUS316L · EN 1.4404 · X2CrNiMo17-12-2 · 022Cr17Ni12Mo2
- 321 AISI 321 · SUS321 · EN 1.4541 · X6CrNiTi18-10
- 310 / 310S AISI 310 · AISI 310S · SUS310S · EN 1.4845
- 302HQ AISI 302HQ
- A3
- A5
- 347 AISI 347 · SUS347 · EN 1.4550 · X6CrNiNb18-10
- 309 / 309S AISI 309 · AISI 309S · SUS309S

#### Martensitic Stainless Steel

- AISI 410 SUS410 · EN 1.4006 · X12Cr13 · 12Cr13
- AISI 420 SUS420J1 · SUS420J2 · EN 1.4021 · EN 1.4028 · 20Cr13 · 30Cr13
- AISI 431 SUS431 · EN 1.4057 · X17CrNi16-2
- C1
- C3
- C4
- AISI 440A
- AISI 440B
- AISI 440C SUS440C · EN 1.4125

#### Ferritic Stainless Steel

- AISI 430 SUS430 · EN 1.4016 · X6Cr17 · 10Cr17
- F1
- AISI 434
- AISI 444

#### Precipitation-Hardening Stainless Steel

- 17-4 PH AISI 630 · UNS S17400 · SUS630 · EN 1.4542 · X5CrNiCuNb16-4
- 15-5 PH UNS S15500
- 17-7 PH UNS S17700

#### Duplex and Super Duplex Stainless Steel

- Duplex 2205 UNS S31803 · UNS S32205 · EN 1.4462
- Super Duplex 2507 UNS S32750 · EN 1.4410
- D2
- D4
- D6
- D8
- Lean Duplex 2101 UNS S32101
- Super Duplex S32760 UNS S32760 · EN 1.4501

### Carbon Steel

#### Low-Carbon Steel

- SAE 1006
- SAE 1008
- SAE 1010
- SAE 1018
- SAE 1020
- Q195
- Q235
- SAE 1012
- SAE 1015
- Q215
- ML08
- ML08Al
- ML10
- ML15
- ML20
- SWRCH6A
- SWRCH8A
- SWRCH10A
- SWRCH12A
- SWRCH15A
- SWRCH18A

#### Medium-Carbon Steel

- SAE 1035
- SAE 1045
- S45C
- C45
- SAE 1030
- SAE 1040
- SAE 1050
- 30
- 35
- 40
- 45
- 50
- S35C
- S40C
- S50C
- C35
- C40

#### High-Carbon Steel

- SAE 1060
- SAE 1070
- 65Mn
- SAE 1065
- SAE 1075
- SAE 1080
- SAE 1085
- SAE 1090
- SAE 1095
- 60
- 65
- 70
- 75
- 80
- 85

#### Cold-Heading Steel

- SWRCH22A
- 10B21
- SWRCH22K
- SWRCH25K
- SWRCH30K
- SWRCH35K
- SWRCH40K
- ML20Mn
- ML25
- ML30
- ML35
- ML40
- 15B25

### Alloy Steel

#### Chromium Steel

- 40Cr
- 45Cr
- AISI 5140
- SAE 5140
- AISI 5120
- SAE 5120
- SCr420
- SCr440

#### Chromium-Molybdenum Steel

- 35CrMo
- 42CrMo
- AISI 4140
- SCM435
- SCM440
- 42CrMo4
- 30CrMo
- AISI 4130
- SAE 4130
- SAE 4140
- AISI 4142
- AISI 4145
- SCM415
- SCM420
- SCM445
- EN 1.7220
- EN 1.7225
- 25CrMo4
- 34CrMo4

#### Nickel-Chromium-Molybdenum Steel

- AISI 4340
- SAE 4340
- 40CrNiMo
- SNCM439
- EN 1.6511
- 36CrNiMo4

#### Boron Alloy Steel

- 10B22
- 10B21
- 15B21
- 15B25
- 20MnTiB
- 20Mn2B
- 35VB
- 40MnB

### Bearing Steel

#### Bearing Steel

- GCr15
- AISI 52100 SAE 52100 · UNS G52986
- SUJ2
- 100Cr6 EN 1.3505
- GCr15SiMn

### Spring Steel

#### Carbon Spring Steel

- SAE 1065
- SAE 1070
- SAE 1075
- SAE 1080
- SAE 1095
- 65
- 70
- 75
- 80
- 65Mn

#### Silicon-Manganese Spring Steel

- 60Si2Mn
- 60Si2MnA
- SUP6
- SUP7

#### Chromium-Vanadium Spring Steel

- 50CrVA
- SAE 6150
- AISI 6150
- SUP10

#### Stainless Spring Steel

- AISI 301
- SUS301
- EN 1.4310
- AISI 302
- AISI 304
- AISI 316
- 17-7 PH

### Aluminum and Aluminum Alloys

#### Commercially Pure Aluminum

- 1050
- 1060
- 1070
- 1100

#### 2xxx Series Aluminum

- 2024
- 2011
- 2014
- 2017

#### 5xxx Series Aluminum

- 5052
- 5083
- 5754

#### 6xxx Series Aluminum

- 6061
- 6061-T6
- 6063
- 6082
- 6082-T6

#### 7xxx Series Aluminum

- 7075
- 7075-T6
- 7005
- 7050
- 7075-T73

### Brass

#### Free-Cutting Brass

- C36000
- H62
- HPb59-1
- C3601
- C3602
- C3604
- CuZn36Pb3
- CuZn39Pb3
- EN CW614N
- EN CW617N
- H65

#### Lead-Free Brass

- C69300
- C27450
- CW510L
- CW511L

#### Naval Brass

- C46400
- CuZn39Sn1

### Bronze

#### Silicon Bronze

- C65500
- C65100
- C65600

#### Phosphor Bronze

- C51000
- C52100
- CuSn6
- CuSn8

#### Aluminum Bronze

- C95400
- C95500
- C63000
- CuAl10Ni5Fe4

### Copper

#### Copper

- C11000
- T2
- C10100
- C10200
- C12200
- T1
- TU1
- TU2

### Titanium and Titanium Alloys

#### Commercially Pure Titanium

- Grade 2
- Grade 1
- Grade 3
- Grade 4
- TA1
- TA2
- TA3

#### Alpha-Beta Titanium Alloys

- Grade 5
- Ti-6Al-4V
- TC4
- UNS R56400

#### Low-Interstitial Grade

- Grade 23
- Ti-6Al-4V ELI
- UNS R56407

#### Other Titanium Fastener Alloys

- Ti-6Al-6V-2Sn
- Ti-6Al-2Sn-4Zr-2Mo
- Beta-C titanium
- Ti-10V-2Fe-3Al

### Nickel and Nickel Alloys

#### Commercially Pure Nickel

- Nickel 200
- UNS N02200
- Nickel 201
- UNS N02201

#### Inconel Alloys

- Inconel 625 UNS N06625
- Inconel 718 UNS N07718
- Inconel 600 UNS N06600
- Inconel 601 UNS N06601
- Inconel 686 UNS N06686
- Inconel X-750 UNS N07750

#### Monel Alloys

- Monel 400 UNS N04400
- Monel K-500 UNS N05500

#### Hastelloy Alloys

- Hastelloy C-276 UNS N10276
- Hastelloy C-22 UNS N06022
- Hastelloy B-2 UNS N10665
- Hastelloy B-3 UNS N10675
- Hastelloy X UNS N06002

#### Other Nickel Alloys

- Alloy 20 UNS N08020
- Alloy 625
- Alloy 718
- Alloy 825 UNS N08825
- Alloy 925 UNS N09925

### Iron-Nickel and High-Temperature Alloys

#### A286

- A-286
- UNS S66286
- EN 1.4980

#### Invar

- Invar 36
- Alloy 36
- UNS K93600
- FeNi36

#### Alloy 42

- Alloy 42
- UNS K94100
- FeNi42

### Tool Steel and Specialty Hardened Steel

#### Tool Steel and Specialty Hardened Steel

- AISI D2
- AISI H13
- SKD11
- Cr12MoV
- AISI O1
- AISI A2
- AISI S7
- SKD61
- 9CrSi

### Structural and Construction Steel

#### Structural and Construction Steel

- ASTM A36
- Q235B
- Q355B
- S355JR
- Q235C
- Q355C
- S235JR
- S275JR
- S355J2

## Applications & Industries

Heavy Hex Structural Bolts are bought for a small number of well-defined jobs. These are the ones we ship them for. Stocked from 1/2" to 1-1/8" (screw size).

### Industries that specify Heavy Hex Structural Bolts

- **Bridges & heavy civils:** Slip-critical connections with hardened washers and a specified tensioning method.
- **Crane & lifting structures:** Bolted connections in gantries, masts and runway beams.
- **Structural steelwork:** Beam-to-column, splice and bracing connections, tensioned and inspected on site.
- **Automotive & transport:** Chassis, driveline and body fixings that see vibration and are assembled to a controlled preload.
- **Steel fabrication:** Bolted connections in frames, enclosures, walkways and weldments, on the bench and on site.
- **Energy & infrastructure:** Tower, pipework, switchgear and plant fixings specified for long service life and inspectable joints.

### Typical applications

- slip-critical connections tensioned by turn-of-nut or DTI washer
- supplied as a matched assembly with a heavy hex nut and hardened washer
- bolting structural steel members where the shear plane falls on the plain shank
- joints where the property class is stated on the drawing
- clamping two machined faces to a calculated preload
- bolting brackets, motors and gearboxes to a frame

### Service conditions to specify against

- **Marine and coastal:** Available in A4 (316) stainless for chloride exposure — coastal structures, pool halls, water treatment and deck hardware, where A2 will eventually pit.
- **High load:** Supplied in ISO 898-1 property classes, so the joint can be designed to a stated proof load and tightened to a calculated torque rather than by feel.
- **Torque and preload:** The joint is designed around bolt tension, so lubrication, coating and the friction coefficient all change the torque figure — state them together.

### Where to use something else

- Not an alternative to an ordinary hex bolt on a machine: the short thread and large head suit plate connections, not tapped holes.
- Do not use a machine thread into unthreaded sheet or timber; use a tapping screw, rivet nut or insert made for that parent material.

[Full application guide: where Heavy Hex Structural Bolts are used, how to specify them, and the cases they are wrong for](https://engineeringhardware.com/applications/heavy-hex-structural-bolts/)

Inside the plant

## Manufactured in our own plant

This part is cold-formed, threaded, heat-treated and finished in-house. The stages below are where its class and finish are actually decided.

1. ![Wire rod feeding into a cold heading machine](https://engineeringhardware.com/wp-content/uploads/2026/08/09-wire-rod-feed-cold-heading-machine.webp) 01 **Cold heading** Wire rod is drawn to size and cold-formed into blanks. Forming rather than cutting keeps the grain flow continuous around the head, which is where a bolt is most likely to fail. Head dimensions and under-head radius checked at set-up and per shift.
2. ![Thread rolling machine producing bolt threads](https://engineeringhardware.com/wp-content/uploads/2026/08/03-thread-rolling-machine-bolt-production.webp) 02 **Thread rolling** Threads are rolled between hardened dies rather than cut. Rolled threads are stronger in fatigue than cut threads and hold pitch and flank angle far more consistently across a batch. Go / no-go ring gauges to ISO 965 tolerance class 6g.
3. ![Baskets of bolts staged for heat treatment](https://engineeringhardware.com/wp-content/uploads/2026/08/05-heat-treatment-baskets-black-bolts.webp) 03 **Heat treatment** Quench-and-temper brings carbon and alloy steel to the property class ordered — 8.8, 10.9 or 12.9 — with the furnace charge tracked so a finished lot can be traced back to it. Core hardness and tensile tested per lot to ISO 898-1.
4. ![Zinc plated bolts in a treatment basket](https://engineeringhardware.com/wp-content/uploads/2026/08/23-zinc-plated-bolts-treatment-basket.webp) 04 **Surface treatment** Zinc, zinc-nickel, hot-dip galvanizing, black oxide, phosphate and Dacromet, selected against the service environment rather than against the price list. Coating thickness and neutral salt spray hours to ISO 9227.

## Frequently Asked Questions

### Are Property Class 12.9 Steel Heavy Hex Structural Bolts magnetic?

Property Class 12.9 Steel Heavy Hex Structural Bolts are magnetic.

### What corrosion resistance do Property Class 12.9 Steel Heavy Hex Structural Bolts offer?

Property Class 12.9 Steel Heavy Hex Structural Bolts provide requires plating or black-oxide protection with a service range of commonly limited to approx. +300 °C; confirm against the coating and the load.

### When should I move from ISO 898-1 property class 12.9 to class 8.8 for Heavy Hex Structural Bolts?

Step up to class 8.8 when the joint preload approaches the proof load of the current class. The higher class adds preload at the same diameter, often saving size and weight.

### Is Property Class 12.9 Steel prone to hydrogen embrittlement?

Class 10.9 and above can be susceptible to hydrogen embrittlement when electroplated. Specify post-plating baking, or use mechanical-zinc / zinc-flake finishes which avoid the cathodic plating step.

### How do I request a quotation?

Use the technical drawing on this page to pick the exact configuration, then submit it with the quote form. We respond within one business day.

## Related Pages

[View the full Heavy Hex Structural Bolts page](https://engineeringhardware.com/fastener/heavy-hex-structural-bolts/)

### More Heavy Hex Structural Bolts Guides