PRECIPITATION HARDENING IN MONEL K500, 17-4 PH STAINLESS STEEL AND 350 GRADE MARAGING STEEL

摘要

The strengthening mechanisms in three different types of alloys: Monel K500, Maraging Steel of 350 Grade and 17-4 PH Stainless Steel (17-4 PH SS) have been examined. Monel K500 is a nickel base super alloy with an austenitic matrix, and 17-4 PH SS and maraging steel have a martensitic matrix. In Monel K500 significant hardening is associated with the precipitation of gamma'. In the under-aged condition particle shearing is the pronounced mode of dislocation-precipitate interaction. In the peak-aged condition both particle shearing and dislocation looping around the precipitates are the strengthening mechanisms. The principal mode of strengthening in this alloy in the over-aged condition is looping of dislocations around the precipitates. In 17-4 PH stainless steel the strengthening contribution of soft copper-rich precipitates accounts for the observed strength increment in the peak-aged condition. The modulus-hardening which causes an attractive interaction between dislocation and soft precipitate particles is associated with the precipitate shearing process during the plastic flow of the material. Such a flow mechanism does not cause increased work-hardening of the material with precipitation hardening. The ductility of the material does not show a significant drop even in the peak-aged condition. The strengthening precipitates in the 350-Grade Maraging Steel are Ni_3(Ti,Mo) and Fe_2Mo types of intermetallic phases. In the peak-aged condition, the deformation of the material is associated with the shearing of precipitates by dislocations and with extremely low work-hardening. Overageing causes an increase in the work-hardening, suggesting the occurrence of looping of precipitates by dislocations.