Optimization of Hot Workability and Control of Microstructure in 18Ni (M250 Grade) Maraging Steel Using Processing Maps

Abstract

This article examines the hot working characteristics of M250 grade maraging steel by performing isothermal compression tests varying the temperatures and strain rates. A processing map has been developed, indicating microstructurally “stable” and “unstable” regions during hot working. At strain rates (ϵ, ˙, , , , , ) above 0.1 s−1 and temperatures (T) below 1,000°C, stress–strain curves show strain hardening behavior, whereas steady-state behavior is shown at strain rates below 0.01 s−1 and temperatures above 1,050°C. Above 1,150°C, flow softening is observed at 0.001 and 0.01 s−1. In the processing map, two distinct microstructurally stable domains are observed: one is at T = 1,150–1,200°C and ϵ, ˙, , , , , = 0.001–0.01 s−1, and another is at T = 1,000–1,150°C and ϵ, ˙, , , , , = 0.001–0.1 s−1. Based on microstructural observations, electron backscattered diffraction results, and the high efficiency of power dissipation, dynamic recrystallization is the softening mechanism. A constitutive relation useful in computer modeling is developed, using the generated flow stress data.