Home / Materials
Grades on the print
Fastener materials, property classes and grades
The grade on a fastener print fixes the steel, the heat treatment, the safe coatings and the tests the lot must pass. These tables give the minimum values the standards set, so a print can be checked before it goes out for quotes.
Values are minimums from the named standard for the stated size range. Always design to the standard and edition your customer requires.
Carbon and alloy steel, ISO 898-1
The first number times 100 is the nominal tensile strength in MPa. The second number times 10 is the ratio of yield to tensile in percent. So 8.8 is 800 MPa nominal with yield at 80 percent of it. The table shows the minimums, which for most classes sit above the nominal value.
Swipe the table sideways to see all columns.
| Class | Min tensile Rm | Min yield or 0.2% stress | Proof stress Sp | Hardness | How it gets there |
|---|---|---|---|---|---|
| 4.6 | 400 MPa | 240 MPa | 225 MPa | 120 to 220 HV | Low carbon steel, no heat treatment |
| 4.8 | 420 MPa | 340 MPa | 310 MPa | 130 to 220 HV | Low carbon steel, strength from cold heading |
| 5.8 | 520 MPa | 420 MPa | 380 MPa | 160 to 220 HV | Low or medium carbon steel, cold worked |
| 8.8 (d up to 16 mm) | 800 MPa | 640 MPa | 580 MPa | 250 to 320 HV | Medium carbon or boron steel, quenched and tempered |
| 8.8 (d over 16 mm) | 830 MPa | 660 MPa | 600 MPa | 255 to 335 HV | As above |
| 10.9 | 1,040 MPa | 940 MPa | 830 MPa | 320 to 380 HV | Boron or alloy steel, quenched and tempered |
| 12.9 | 1,220 MPa | 1,100 MPa | 970 MPa | 385 to 435 HV | Alloy steel, quenched and tempered under tight control |
Hex and hexalobular head bolts from M5 up carry the class on the head. Overview of the standard: ISO 898. What each class means for joint design: 8.8, 10.9, 12.9, A2 and A4 explained.
Which wire goes into which class
The print names the class. The plant chooses the wire that reaches it at your diameter. These are typical choices, not requirements, and the material certificate shows what was actually used.
Cold heading wire: 10B21, SCM435 and SWCH covers the trade-offs.
- Class 4.8 and 5.8Low carbon cold heading wire such as 1018 or SWCH18A. No heat treatment, so the wire grade and the heading reduction set the strength.
- Class 8.8Medium carbon or boron steel such as 1035 or 10B21, quenched and tempered. Boron gives hardenability with good headability.
- Class 10.9Boron or chromium alloy steel such as 10B33 or SCM435. Larger diameters need more alloy to harden through the section.
- Class 12.9Chromium-molybdenum alloy steel such as SCM435 or 4140. Tight furnace control, because hardness and decarburization limits are narrow.
Stainless steel, ISO 3506
The letter and digit give the steel group, the number after the dash the strength class. A2 is the 304 family, A4 the 316 family with molybdenum for chloride resistance. Strength comes from cold work, so classes 70 and 80 are typically limited to moderate diameters, and large or machined parts fall back to class 50.
| Grade and class | Steel family | Min tensile Rm | Min 0.2% stress | Where it fits |
|---|---|---|---|---|
| A2-50 | 304 type, soft | 500 MPa | 210 MPa | Machined or hot formed parts, larger diameters |
| A2-70 | 304 type, cold worked | 700 MPa | 450 MPa | General corrosion resistant bolts and screws |
| A4-70 | 316 type, cold worked | 700 MPa | 450 MPa | Chlorides, marine air, food and chemical plant |
| A4-80 | 316 type, more cold work | 800 MPa | 600 MPa | Higher clamp load in the same environments |
Austenitic stainless threads gall under load. Specify a lubricant, a coating or mixed grades on bolt and nut where assembly torque is high. Martensitic and precipitation hardening grades are covered in 17-4 PH, 410 and 416 fasteners.
Inch series and pressure bolting
North American prints and pressure equipment use their own grade systems. They are not one-to-one swaps for metric classes, so quote to the standard on the print.
Swipe the table sideways to see all columns.
| Standard and grade | Material | Min tensile | Min proof or yield | Typical use |
|---|---|---|---|---|
| SAE J429 Grade 2 | Low or medium carbon steel | 74 ksi (1/4 to 3/4 in) | Proof 55 ksi | Light duty inch bolts |
| SAE J429 Grade 5 | Medium carbon steel, quenched and tempered | 120 ksi (1/4 to 1 in) | Proof 85 ksi | Inch equivalent of class 8.8 |
| SAE J429 Grade 8 | Medium carbon alloy steel, quenched and tempered | 150 ksi | Proof 120 ksi | Inch equivalent of class 10.9 |
| ASTM A193 B7 | Cr-Mo alloy steel, quenched and tempered | 125 ksi (to 2.5 in) | Yield 105 ksi | Studs for flanges and pressure equipment |
| ASTM A193 B8 / B8M class 1 | 304 / 316 stainless, solution treated | 75 ksi | Yield 30 ksi | Corrosive or cryogenic bolting at low load |
| ASTM A320 L7 | Cr-Mo alloy steel, impact tested | 125 ksi (to 2.5 in) | Yield 105 ksi | Low temperature flange bolting |
| ASTM F1554 Gr 36 / 55 / 105 | Carbon or alloy steel | 58 / 75 / 125 ksi | Yield 36 / 55 / 105 ksi | Anchor bolts cast into concrete |
More on A193 B7, B16 and A320 L7 studs and SAE Grade 5 vs Grade 8.
Non-ferrous and specialty alloys
Brass, bronze, aluminum, titanium and nickel alloys are chosen for conductivity, weight, corrosion or temperature rather than strength per cost. ISO 8839 covers the mechanical properties of non-ferrous bolts and nuts. Most of these parts are machined, because the volumes are lower and several alloys do not cold head well.
Swipe the table sideways to see all columns.
| Material | Typical designations | Route | Use it for | Watch for |
|---|---|---|---|---|
| Free cutting brass | CuZn39Pb3, CW614N, C36000 | Machined, or headed in low lead grades | Electrical terminals, decorative and low load screws | Low strength. Lead content limits in some markets |
| Silicon bronze | C65100, C65500 | Headed or machined | Marine and timber fastening, electrical hardware | Cost per kilo, limited stock sizes |
| Aluminum | 2024-T4, 6061-T6, 7075-T73 | Headed (rivets, screws) or machined | Weight sensitive assemblies, aluminum structures | Galling and low clamp load. Anodize changes thread fit |
| Titanium | Grade 2 (UNS R50400), Grade 5 Ti-6Al-4V (UNS R56400) | Machined, sometimes warm headed | Aerospace style, motorsport, medical and seawater parts | Galling. Needs anti-seize or a coating on threads |
| Nickel alloys | Inconel 718 and 625, Monel 400 and K-500 | Machined or hot headed | Heat, seawater and chemical exposure | Expensive stock, slow machining, long lead times on bar |
Specific guides: titanium grade 5 bolts, Inconel and Monel fasteners and silicon bronze and brass bolts.
Match the nut to the bolt
A nut weaker than its bolt strips before the bolt yields. Specify the nut class with the bolt, and order them from the same maker when you can, so the coatings and thread allowances match.
- Class 8.8 boltISO 898-2 class 8 nut
- Class 10.9 boltISO 898-2 class 10 nut
- Class 12.9 boltISO 898-2 class 12 nut
- A2-70 or A4-70ISO 3506-2 nut of the same steel group and class
- ASTM A193 B7 studASTM A194 grade 2H heavy hex nut
Writing the grade on the RFQ
Name the standard, the class or grade and the size range it applies to: "ISO 898-1 class 10.9" or "ASTM A193 B7", not "high tensile". Add the tests and documents you need, and any hydrogen embrittlement requirement if the part is plated.
Grade also steers the route. Class 12.9 and titanium narrow the field of plants, and a stainless class 80 part above moderate diameters may need machining. How each step is made is on the processes page. Testing is covered in proof load, wedge tensile and hardness.
Material questions
Is SAE Grade 8 the same as class 10.9?
They are close but not the same. Grade 8 has a minimum tensile of 150 ksi (about 1,034 MPa), class 10.9 a minimum of 1,040 MPa, and the proof loads, hardness ranges and thread series differ. Specify the standard your joint was calculated to.
Why is A4-80 stainless not hardened like 12.9 steel?
Austenitic stainless steels cannot be hardened by quench and temper. Classes 70 and 80 reach their strength by cold working the wire, which is why the higher classes are only available up to moderate diameters.
Can I ask for a higher class than the joint needs?
You can, but it is not free. Class 12.9 costs more, needs tighter heat treatment and is more sensitive to hydrogen embrittlement from electroplating. Over-specifying the grade can make a joint less reliable, not more.
What documents prove the grade?
A material certificate for the wire or bar (often EN 10204 type 3.1), plus mechanical test results on the finished parts: tensile or wedge tensile, proof load and hardness per ISO 898-1 or the inch standard. Ask for them in the RFQ.