Optimization of Hot Workability and Control of Microstructure in CF250 Grade Cobalt-Free Maraging Steel: An Approach Using Processing Maps

Abstract

This paper presents the details on the development of a processing map from the isothermal compression test results of cobalt-free (CF250 grade) maraging steel over a wide range of temperature (900–1200 °C) and strain rate (0.001–100 s−1). Processing maps identifying ‘stable’ and ‘unstable’ regions during hot working were developed. Two safe processing windows with high efficiency of power dissipation and strain rate sensitivity can be noticed in the power dissipation and maps of strain rate sensitivity parameter at low strain rates (10−3 s−1). However, a stable higher strain rate processing window occurring in the temperature range of 1050–1150 °C and strain rate range of 101–102 can be used for increased productivity. Based on the high values of the efficiency of power dissipation, microstructural observations, and EBSD results, dynamic recrystallisation was found to be the softening mechanism occurring in this material. The measured flow stress data were used for the development of constitutive model for hot deformation behavior of this alloy. The activation energy for deformation of CF250 maraging steel was calculated to be 458.8 kJ mol−1.