Dynamic impact response and corrosion behavior of coarse- and ultrafine-grained AISI 321 austenitic stainless steel
Date
2019-08-23
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0003-3952-578X
Type
Thesis
Degree Level
Doctoral
Abstract
The excellent corrosion-resistance of metastable AISI 321 austenitic stainless steel makes it a choice material in the fabrication of nuclear and chemical plants, pressure vessels, automobile and aircraft components, etc. However, AISI 321 is characterized by low-yield strength and poor tribological properties that hinder its widespread application. Therefore, it is important to improve its yield-strength to expand its structural applications without compromising its excellent corrosion resistance.
In this study, the effect of grain refinement via cryo-rolling followed by annealing on the strength and corrosion resistance of AISI 321 austenitic stainless steel is investigated. The mechanical behavior of the as-received coarse-grain and refined alloy (fine-grain and ultrafine-grain) were investigated at high (dynamic impact) and low (quasi-static compression) strain rates using the split Hopkinson pressure bar and Instron R5500 mechanical testing machine, respectively. The corrosion resistance of coarse-grained (CG), fine-grained (FG), and ultrafine-grained (UFG) specimens were also investigated using electrochemical methods. Scanning and transmission electron microscopy (SEM, TEM), X-ray diffraction (XRD), and electron-backscattered diffraction (EBSD) were used for the microstructural and textural characterization of various specimens of the alloy before and after plastic deformation.
The optimum thermomechanical process conditions for developing UFG structure in the AISI 321 steel is cryo-rolling to 50 % reduction of plate thickness followed by process annealing at 1023 K (750 ℃) for 600 s (10 minutes). The hardness of the UFG steel specimens is determined to be ~195 % higher than that of the as-received (CG) AISI 321 steel. The developed UFG specimens have strong intensity of ζ-fibre ({110}<uvw>) texture, which is attributed to pseudo-texture memory effect in AISI 321 steel. The mechanism for pseudo-texture memory in AISI 321 steel is proposed.
The yield strength of the UFG AISI 321 steel is ~400 and ~200% higher than those of the CG specimens under both quasi-static and dynamic deformation conditions, respectively. Slip and twinning are the active deformation mechanisms in CG specimens. Both are highly suppressed in the UFG specimens due to spatial restriction effect. During plastic deformation, γ-FCC to martensite (αʹ-BCC) phase transformation occurred, which is more favored in the UFG specimens and at low strain rates. The co-existence of martensitic phase transformation paths with and without an intermediate phase (HCP ɛ-martensite) is confirmed in AISI 321 steel during plastic deformation under both quasi-static and dynamic loading conditions. Irrespective of grain size, Shoji-Nishiyama, Kurdjumov-Sachs and Burgers orientation relationships exist between the γ and ɛ, γ and αʹ, and ɛ and αʹ phases, respectively. Thus, the phase transformation sequence follows both FCC γ → BCC αʹ and FCC γ → HCP ɛ → BCC αʹ path. The stable end-orientation of the austenite phase in compression is [110]||CD texture while that of the martensitic phase is [100]||CD with spread towards [111]||CD texture.
Under dynamic impact load, UFG specimens exhibit lower critical strain and strain rate at which shear strain localization (adiabatic shear bands) occurs. EBSD analysis revealed the development of equiaxed ultrafine-grained structure (average grain sizes of ~0.17 μm in CG and ~0.14 μm in UFG specimens) inside transformed shear bands by rotational dynamic recrystallization mechanism. The five strengthening sources that contribute to strain hardening in AISI 321 steel are determined to be: (a) grain boundary strengthening, (b) deformation-induced martensite transformation, (c) deformation twinning acting as a barrier to dislocation motion (d) dislocation-dislocation interactions, and (e) dislocation interaction with titanium carbides. On the stability of the austenite phase in AISI 321 steel, EBSD analyses confirmed the evolution of both thermally- and deformation-induced martensite that is grain size and orientation-dependent. The results of corrosion studies show that the excellent corrosion resistance of AISI 321 steel is not compromised by strength enhancement through grain refinement. Although the presence of TiC particles in AISI 321 is not detrimental to its corrosion resistance, that of TiN particles is.
Description
Keywords
AISI 321 austenitic stainless steel, Thermomechanical processing, Ultrafine-grained structure, Slip, Twinning, Phase transformation, Split Hopkinson pressure bar, Dynamic shock loading, Quasi-static compression, Adiabatic shear band, Corrosion, Transmission electron microscope, Electron backscattered diffraction, Texture, Texture memory, Strain rate sensitivity, Activation volume
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Mechanical Engineering
Program
Mechanical Engineering