Research On Improving Fracture Toughness Of Corrosion Resistanced Ultrahigh-Strength Steel

In this paper, corrosion resistanced ultrahigh-strength steel SY alloy is investigated. Thermo-Simulation experiments and heat treatment experiments are carried out in laboratory. Some factors that affect the fracture toughness of steel SY are investigated. Some useful regulations are gained. The main work includes:1. We investigated the recrystallization of SY alloy on Gleeble-1500 Thermo-Mechanical Simulator and acquired the proximate recrystal temperature of tested alloy, which gives the basis to improve ductility and toughness through reducing grain size in forging process.2. By several different heat treatments, we studied the factors affecting fracture toughness, and some regulations between heat treatment and mechanical properties came out.3. By microanalysis, some factors and regulations that affect mechanical properties are acquired, which is beneficial to improve fracture toughness of SY alloy.The results show that:1. Under experimental conditions, dynamic recrystallization of SY alloy finishes at nearly 900℃,and grain size is grade YB12 when dynamic recrystallization happens. Static recrystallization finishes at nearly 700℃; and grain size grows up quickly with rising of holding temperature.2. Oil cooling when quenching can make fracture toughness improved from 121 MPa m^1/2 (air cooling) to 132 MPa m^1/2 but has no obvious effect on other mechanical properties.3. Deep-freezing improves fracture toughness of SY from 95 MPa m^1/2 to 127 MPa m^1/2 ; Hierarchical aging improve fracture toughness of SY slightlybut strength decreased, so hierarchical aging isn't beneficial to mechanical properties of SY.4.A short time pre-aging improves strength of SY and fracture toughness dosen' t change greatly.. The mechanism of improving should be investigated further. Various time of pre-aging has no obvious effect on fracture toughness of SY alloy.