Metastable Austenitic Steel Structure and Mechanical Properties Evolution in the Process of Cold Radial Forging

Area of Science:

• Materials Science
• Metallurgy
• Mechanical Engineering

Background:

• Metastable austenitic stainless steels are crucial engineering materials.
• Understanding structure evolution under severe plastic deformation is key to optimizing properties.
• Cold radial forging (CRF) is an effective severe plastic deformation technique.

Purpose of the Study:

• To investigate the influence of structure formation on the mechanical properties of 321 metastable austenitic stainless steel during CRF.
• To correlate the stages of structure evolution with the kinetics of strain-induced martensitic transformation.
• To analyze the impact of varying degrees of true strain on tensile and impact properties.

Main Methods:

• Austenitization of 321 stainless steel.
• Cold radial forging (CRF) at room temperature with true strain values of 0.26, 0.56, 1.00, 1.71, and 2.14.
• Microstructural analysis to identify structure formation stages (lamellar austenite, trapezoidal, equiaxial).
• Mechanical testing including hardness, tensile strength, yield strength, elongation to fracture, and impact strength (KCT, KCV).

Main Results:

• Structure formation during CRF occurs in three stages: lamellar austenite, trapezoidal, and equiaxial grain structures.
• Strain-induced α'-martensitic transformation kinetics are linked to these structural evolution stages.
• Hardness, ultimate tensile strength, and yield strength consistently increase across all stages.
• Elongation to fracture significantly decreases in the first stage and remains constant thereafter.
• Impact strength of fatigue-cracked specimens (KCT) drops sharply initially, then rises, while V-notch impact strength (KCV) continuously declines with increasing deformation.

Conclusions:

• CRF induces distinct microstructural transformations in 321 stainless steel.
• Mechanical properties respond predictably to these structural changes, with trade-offs in ductility and toughness.
• The study provides insights into tailoring the properties of 321 steel through controlled deformation processing.