EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Thread rolling processes are most used to manufacture external threaded screws, as plastic deformation for external threading significantly increases productivity, saves materials, and enhances the reliability and durability of the products. Except for the premature in-service failure, fatigue failure is the most considerable factor which limits the lifetime of the rolling dies. Often changing of the rolling dies during manufacturing causes increasing in rolling die cost and decreasing in productivity.
Following the failures analysis on failed rolling dies from the factory, and the simulation results for EPB screw threading rolling process using Forge software, the most frequent fatigue failures are located at the crests of the rolling dies. With help of the Dang Van multiaxial fatigue limit criterion, localization of fatigue damage is predicted. M2 high-speed tool steel is typical material to fabricate rolling dies for cold forging. Fatigue strength for industrial heat-treated M2 tool steel is estimated at about 800 MPa. Concerning rolling contact, increase the hardness of the material or/and generate compressive residual stress generated in the material can improve the fatigue life. Gas nitriding is a surface thermal chemical treatment method. During gas nitriding, nitrogen atoms diffuse into the steel surface which acts as a catalyst for the decomposition of ammonia, nitrides precipitate companying with different stages of phase transformation, these two phenomena offer the material gradients of hardness and residual stress. Therefore, the fatigue life of rolling dies could be increased significantly with a fit gas-nitriding process.