Bolts & stud bolts for flanges: grades 8.8, 10.9, A2 and A4
A flange joint is only as good as its bolting. Studs, nuts and washers must suit the pressure rating, temperature and medium – and they must be tightened correctly. Both are regularly underestimated.
Property classes for steel
For steel bolts the property class to EN ISO 898-1 indicates tensile strength and yield strength. The first number times 100 gives the tensile strength in N/mm², the second describes the ratio of yield to tensile strength.
| Class | Tensile strength | Temperature (guide) | Use |
|---|---|---|---|
| 5.6 | 500 N/mm² | up to approx. 250 °C | Low pressure, secondary connections |
| 8.8 | 800 N/mm² | up to approx. 300 °C | The standard in plant engineering |
| 10.9 | 1,000 N/mm² | up to approx. 300 °C | Higher loads, larger pressure ratings |
| A2-70 | 700 N/mm² | up to approx. 300 °C | Stainless, standard grade |
| A4-80 | 800 N/mm² | up to approx. 300 °C | Molybdenum-alloyed, chloride resistant |
Guide values. Above these temperatures the permissible load drops sharply – creep-resistant grades are then required.
Stainless: A2 or A4
A2 corresponds in material terms roughly to 1.4301 and is the standard stainless grade for general corrosive exposure. A4 corresponds to 1.4404 or 1.4401, is molybdenum-alloyed and therefore additionally resistant to chlorides – the right choice outdoors, near seawater and with chloride-bearing media.
Caution with mixed joints: stainless bolts combined with unalloyed steel can cause galvanic corrosion in the presence of moisture. And stainless bolts tend to gall when tightened – treat the threads with a suitable compound before assembly.
Creep-resistant grades for steam and boilers
Above roughly 300 °C, 8.8 and 10.9 are no longer sufficient. In boiler and power plant construction, creep-resistant bolt materials are used that maintain their preload under sustained loading.
| Material | Material no. | Temperature (guide) |
|---|---|---|
| 25CrMo4 | 1.7218 | up to approx. 500 °C |
| 21CrMoV5-7 | 1.7709 | up to approx. 550 °C |
| 40CrMoV4-6 | 1.7711 | up to approx. 550 °C |
The matching pipe materials: 16Mo3, 13CrMo4-5, 10CrMo9-10 and P91.
Studs instead of hex bolts
Flange joints frequently use studs – threaded rods with two nuts, often with a reduced shank to DIN 2510. Their advantages:
- more even tightening, because both sides are freely accessible
- easier to release during maintenance than through bolts
- the reduced shank absorbs thermal expansion and holds preload longer
Tightening correctly
Preload is decisive – too little and the joint leaks; too much and the gasket is crushed or the bolt overstretched.
- Crosswise and in at least three stages, roughly 30 %, 60 % and 100 % of the target torque.
- Lubricate threads and nut faces – unlubricated, much of the torque is lost to friction instead of producing preload.
- Retighten hot joints once operating temperature has been reached for the first time.
The number and size of bolts for your nominal size and rating is determined by the piping calculator. The matching gasket: choosing the right gasket.
Complete from one source
We supply the complete bolting to match flange and gasket – bolts or studs to EN ISO 4014 and DIN 2510, nuts and washers in matching property classes, hot-dip galvanised or in stainless on request.
Frequently Asked Questions
What does property class 8.8 mean?
The first number times 100 gives the tensile strength in N/mm², so 800 N/mm². The second number describes the ratio of yield to tensile strength. 8.8 is the standard in plant engineering.
What is the difference between A2 and A4?
A2 corresponds in material terms to about 1.4301 and covers general corrosive exposure. A4 corresponds to 1.4404 or 1.4401, is molybdenum-alloyed and additionally resistant to chlorides, for example outdoors and near seawater.
Which bolts for steam and boilers?
Above roughly 300 °C, 8.8 and 10.9 are no longer sufficient. Creep-resistant grades are then used, such as 25CrMo4 to about 500 °C and 21CrMoV5-7 and 40CrMoV4-6 to about 550 °C.
Why studs instead of hex bolts?
Studs can be tightened more evenly because both sides are accessible, are easier to release during maintenance, and the reduced shank to DIN 2510 holds preload longer under temperature cycling.
How is a flange joint tightened correctly?
Crosswise and in at least three stages, roughly 30, 60 and 100 percent of the target torque. Lubricate threads and nut faces, otherwise much of the torque is lost to friction. Retighten hot joints once operating temperature has been reached.
Do you need exactly these parts?
Grade, size, pressure rating and quantity are enough – you usually receive your quote the same day, with an inspection certificate 3.1 on request.
Request a quote