Geometric dynamic recrystallization of austenitic stainless steel through linear plane-strain machining

ABSTRACT

Type 316L austenitic stainless steel was severely plastically deformed at room temperature using linear plane-strain machining in a single pass that imparted shear strains up to 2.2 at strain rates up to 2 × 10³ s⁻¹. The resulting microstructures exhibited significant grain size refinement and improved mechanical strength where geometric dynamic recrystallization was identified as the primary microstructural recrystallization mechanism active at high strain rates. This mechanism is rarely observed in low to medium stacking fault energy materials. The critical stress required for twin initiation is raised by the combined effects of refined grain size and the increase in stacking fault energy due to the adiabatic heating of the chip, thus permitting geometric dynamic recrystallization. The suppression of martensite formation was observed and is correlated to the significant adiabatic heating and mechanical stabilisation of the austenitic stainless steel. A gradient of the amount of strain induced martensite formed from the surface towards the interior of the chip. As the strain rate is increased from 4 × 10² s⁻¹–2 × 10³ s⁻¹, a grain morphology change was observed from a population of grains with a high fraction of irregular shaped grains to one dominated by elongated grain shapes with a microstructure characterised by an enhanced density of intragranular sub-cell structure, serrated grain boundaries, and no observable twins. As strain rates were increased, the combination of reduction in strain induced martensite and non-uniform intragranular strain led to grain softening where a Hall-Petch relationship was observed with a negative strengthening coefficient of −0.08 MPa m^1/2.

KEYWORDS:

• Nanocrystalline
• 316L stainless steel
• severe plastic deformation
• microstructure
• mechanical properties
• dynamic recrystallization

Acknowledgements

The Nuclear Regulatory Commission (NRC-38-09-935), the National Science Foundation (NSF CMMI #1635926) and the Materials Micro-Characterization Laboratory, Department of Mechanical Engineering and Materials Science, of the Swanson School of Engineering at the University of Pittsburgh are acknowledged for their financial and infrastructure support.

Disclosure statement

No potential conflict of interest was reported by the author(s).

ORCID

Yaakov Idell http://orcid.org/0000-0002-4557-8653

Additional information

Funding

This work was supported by Division of Civil, Mechanical and Manufacturing Innovation: [Grant Number 1635926]; U.S. Nuclear Regulatory Commission: [Grant Number NRC-38-09-935].