Hot Workability and Microstructure Control through the Analysis of Stress–Strain Curves during Hot Deformation of M350 Grade Maraging Steel

Abstract

The ultrahigh strength (2,400 MPa) 18Ni maraging steel (M350 grade) is widely used for critical structural applications, such as aircraft landing gears, in which the strength–toughness balance is the essential criterion for material selection. Microstructure control during thermomechanical processing is the key to obtain the desired mechanical properties on a repeatable basis in a manufacturing environment. This involves thorough understanding of the hot deformation behavior under a wide range of temperatures and strain rates to map the microstructural evolution as a function of process parameters to obtain defect-free products. Towards achieving this goal of optimization of hot workability with a view to control microstructure for M350 grade maraging steel, hot deformation processing maps have been developed and correlated to the microstructure evolved. Further, analysis of stress–strain curves was carried out to obtain fine prior austenite grain (PAG) size via discontinuous dynamic recrystallization, and the same was verified experimentally by the microstructures evolved through hot isothermal compression tests on cylindrical specimens subjected to different strain levels. A single peak DRX type σ − ε curve was selected for analyses. The theoretically determined critical strain value was verified experimentally for initiation of DRX (DRXI) and transition from DRX dominant region to grain growth dominant region (DRXT). Hot isothermal compression tests have been conducted at T = 950°C and ε˙=0.01 s−1 and obtained PAG size of 3.14 µm in the specimen deformed to theoretically determined optimum strain of 0.74, thereby validating the used models.

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

“…whereas in the temperature range of 1100–1200 °C and the strain rate range of 0.1–10 s −1 , DRX microstructures can be clearly distinguished. It is worth noting that similar behaviour was reported for M350 grade maraging steel [ 29 ].…”