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 10 1 -10 2 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 .

Hot Deformation Behaviour of Additively Manufactured 18Ni-300 Maraging Steel

“…According to the calculations performed in this research article, the deformation activation energy of the additively fabricated maraging steel 18-Ni300 is 379 kJ mol −1 . It is apparent that the calculated here activation energy value for AM 18Ni300 steel is similar to that of conventionally manufactured M300 grade (391 kJ/mol) [ 19 ], M350 grade (371 kJ/mol) [ 20 ] and is much lower than that of CF250 grade (458.8 kJ/mol) [ 27 ] in magnitude. Further, the activation energy value for AM 18Ni300 value is much larger than the values for self-diffusion in γ-iron ( Q = 280 kJ mol −1 ), implying that dynamic recovery and dynamic recrystallization are the dominant mechanisms instead of diffusion during hot deformation [ 28 ].…”