Hot Deformation Behavior and Dynamic Recrystallization of a Medium Carbon Cr–Mo Steel for Class 16.8 Bolts

This study investigates the hot workability of medium carbon Cr–Mo steels for manufacturing ultra-high strength bolts. The main purpose of this research is to establish a constitutive equation and a kinetic model of dynamic recrystallization of the material for industrial applications. Hot torsion tests were conducted at temperatures of 1173, 1273, and 1373 K and strain rates of 0.1, 0.5, 1.0, and 2.0/s. Strain rate sensitivities and work-hardening coefficients of the material were calculated at various strain rates and temperatures using the Fields–Backofen equation. Results indicated that the material’s strain rate sensitivities and work-hardening coefficients generally increased with temperature. Stress–strain curves showed a general variation depending on the strain rates and the deformation temperatures, with higher strain rates and lower temperatures resulting in increased flow stresses. However, peak stresses were not clearly observed due to the presence of Nb in the material. A constitutive equation for flow stresses was derived, with a thermal activation energy for deformation of 238.57 kJ/mol. Zener–Hollomon parameter was used to analyze the relationship between peak strains and critical strains, finding that the critical strains being about 0.17 times the peak strains. The volume fractions of dynamic recrystallization were estimated using an Avrami form of the kinetic equation, and the Avrami constants \(k\) and \(m\) were 0.84 and 6.89, respectively. The kinetic model of dynamic recrystallization that we developed can be applied to arbitrary deformation conditions, enabling the optimization of the hot rolling process for this material.

Graphic Abstract