S390 PMHSS Thread Rolling Dies And Study Of Deep Cryogenic Treatment

Thread-rolling dies plays important roles in the thread cold-rolling processing, and it also has been used in different fields such as machining, automobile and aerospace manufacturing. With the development of large mechanical manufacturing, the comprehensive mechanical properties need to be improved for threaded fasteners as the growth in industrial demand. By now, Titanium-alloy and super-alloy, which have the performance such as high strength and working-hardening tendency, have been applied in the processing. However, Thread rolling dies have to suffer large amount of extrusion and bending moment in the severe service condition, which would, S390 powder metallurgy highspeed steel(PMHSS) treated by hot isostatic pressing (HIP) is a kind of steel with high carbon and high strength, has been equipped with higher hardness and wear resistance, it has overcome the gregation of carbides during traditional casting and its homogeneity of structure and organization also has been improved. Besides, Deep cryogenic treatment(DCT) has been applied to developed the traditional heat treatment process, which can improve the comprehensive mechanical properties and optimize the microstructure of S390 PMHSS significantly.The numerical simulations on the rolling process of thread rolling dies have been carried out with MARC software and the quality and failure fracture of two kinds of thread rolling dies have been compared by failure analysis. The results show that the stress concentration occurs at the root of tooth during the process of thread screwing, which will lead to the cause and process of crack generation and propagation on the tooth root. When the crack occurs near the surface of dies, it will extends with low cycle fatigue and peel off the tooth, reducing the service life of dies. Based on the Fatigue, the area of embryonic regions and growth regions will impact the service life of thread rolling dies, which brings higher demand for resistance capability of unstable crack propagation. Machining scratch, gully, spalling pits of carbides being found in the surface of tooth, which would lead fatigue crack initiation.This paper studies the comprehensive mechanical properties of S390 PMHSS by Comparing the performance of three typical high thread rolling dies such as composition, microstructure, hardness, impact toughness, bending strength, and bending-strength. It turns out that S390 PMHSS has smaller grain size, better size and morphology of carbides. The hardness and bending-strength tests prove that the performances meet the demands of thread rolling dies.This paper also studies the influence of the sequence in DCT process and the methods of insulating (single-treatment and cycle-treatment) on S390 PMHSS. The results show that, adding the DCT procedure between quenching and tempering will optimize the shape of primary carbides and secondary precipitated carbides, and improve the mechanical properties observably. The cycle-treatment’s effect an efficient way to obtain refined grains. the best tempering temperature has been investigated after DCT, and the change rules of temperature also have been discussed which corresponds the hardening-front and the least wear-loss of material.