Investigation of Static Recrystallization Behavior of 9Ni Steels during Thermomechanical Processing

摘要

In order to study the workability and optimise the hot deformation processing parameters for 9Ni steels, the static recrystallization behavior in austenite region was investigated by isothermal double-hit compression on a Gleeble 3800 thermo-simulator. In this paper,two offset methods(2% offset method and 0. 2% offset method) were employed to measure the softening rate. The results showed that the soitemng rate measured by 0. 2% onset method commonly used by other authors was higher than by the 2% offset method, especially the inter-pass time was less than 60 s. The softening fration increased with increasing strain rate,strain of the first pass ,and decreasing the initial grain sise indicated that higher strain energy and less time for recovery had produced higher nucleation rate for the recrystalliaation of 9Ni steels. Relationships between initial grain size, strain in the first pass deformation, stain rate, and, the time for 50% re crystallization, t0.5 , were established by liner regressions. The active energy for static re crystallization was determined to be 194 kj/mol based on the values measured by using the 2% onset method. Ihe equation lor static recrystallization kinetics was established, in which the Avrami exponent was temperature dependant as n,=0. 0023 exp(6948/T) , and had great difference from those of plain carbon steels. Recrystallization fractions deformed at 900T: with different inter-pass times were measured by using the optical microscope and compared with predicted results. The calculated results based on the 2% offset method were in better agreement with the measured values than the 0.2% offset method, indicated that the 2% offset method could better exclude recovery effects when plastric deformation occurred.