Quenched and tempered (Q&T) steels for fasteners: 42CrMo4, 30CrNiMo8 and related grades

Quenched and tempered steels are steels with 0.2 to 0.6% carbon and alloying elements such as chromium, molybdenum and nickel, which achieve high strength combined with good toughness through quenching and tempering, i.e. hardening followed by tempering. They are the material basis for almost all high-strength bolts. They are standardised in EN ISO 683-2; for fasteners at elevated temperatures, EN 10269 also applies. In enquiries to RS Randack, 42CrMo4 is by far the most frequent material.

The most important grades at RS Randack

• 42CrMo4 (1.7225), also 42CrMoS4 (1.7227): The standard for 10.9 and 12.9 up to medium diameters, equivalent to the American grade B7. Readily forged, machined and rolled.
• 25CrMo4 (1.7218): Lower carbon content, for 8.8 and 10.9 in smaller sizes, tougher than 42CrMo4.
• 30CrNiMo8 (1.6580) and 34CrNiMo6 (1.6582): Nickel-alloyed Q&T steels with high through-hardenability for large diameters, 12.9 and low temperatures; they cool slowly in charcoal after forging.
• C45 and other non-alloy steels: for 5.6 to 8.8 and turned parts without quenching and tempering.

Why the diameter determines the steel

During hardening, the steel must cool quickly enough right through to the core, otherwise a softer microstructure remains there. The larger the diameter, the higher the alloy content must be: 42CrMo4 can be quenched and tempered through to the core up to about 100 mm, nickel-alloyed steels considerably further. This is why, for large bolts in 10.9, RS Randack often specifies 30CrNiMo8, even if the drawing only states the property class.

What you should specify

Material number or property class with size, service temperature, impact test requirement and type of certificate. If you specify both, RS Randack checks before quoting whether the material achieves the class at that diameter.