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What steel is used for large-scale 10.9 high-strength bolts with a steel structure above M82?

Apr 23, 2018

This involves a problem of composition and hardenability. Currently used 42CrMo, B7, 40CrNiMoA steels, the current technology can meet the hardness and strength requirements, but for other properties such as elongation after breaking, impact toughness is difficult to meet the requirements. 34CrNi3Mo steel is generally used, and 34CrNi3Mo steel is alloy steel for large-section quenching and tempering. Its chemical composition is 0.30% to 0.40% C, 0.17% to 0.37% Si, 50% to 0.80% Mn, 0.70% to 1.10% Cr, and 0.25. % - 0.40% Mo, 2.75% - 3.25% Ni.

The critical diameter of oil quench can reach φ60 ~ 80mm, hardenability advantages, other steel can not be compared. The critical temperature of 34CrNi3Mo steel is Ac1=725°C, Ac3=790°C. The quenching temperature is selected from 860 to 880°C aqueous solution cooling and 880 to 900°C rapid quenching oil cooling. For the 10.9 grade bolt tempering temperature, generally 560 to 590°C. Tempering water cooling.

High-strength bolts require that the core of the threaded section obtain a martensitic structure requirement of approximately 90% before being "quenched" and tempered.

In order to ensure the hardenability, for the diameter greater than φ50mm, quenching medium selection has the greatest impact, when the oil quenching can not meet the mechanical performance requirements, you must use water quenching, the principle is not to quench the crack under the premise of using aqueous solutions. Because water quenching has more advantages, such as deeper hardened layers, better mechanical properties, and lower production costs.

If we do heat treatment, we will not be able to put the ingredients in the first place. Steels with carbon content ≥ 0.34% to 0.38% can be used. Stripe segregation should be controlled below 1.5. Control of non-metallic inclusions, such as sulfides, aluminas, and spherical oxides, should not be greater than 1.5%. Level 1.5 (including coarse and fine lines).

If the impact toughness is not enough, consider adding a 580 ~ 600 °C and then tempering process, and then quickly cooled.

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