Microstructural Evolution In AISI Research Articles

Variation of Mechanical Characteristics and Microstructural Evolution in AISI 316 Austenitic Stainless Steel Subjected to Long-Term Thermal Aging at Elevated Temperature*

The objective of this study was to investigate the mechanical degradation of AISI 316 austenite stainless steel as light-water reactor pressure vessel materials in the primary system of nuclear power plants. The influences of long-term aging on the microstructural and mechanical characteristics were studied. The long-term aging tests were interrupted at various stages to obtain different levels of degraded specimens. The test specimens were heat-treated and exposed to an acceleration heat-treatment at 600°C for up to 10,000 hours. AISI 316 steel showed polygonalshaped grains with many annealing twins, and these tended to become gradually more circularshaped grains during long-term aging. In the initial material degradation, twins were distributed uniformly within most grains, but they all recovered and disappeared after a prolonged 10,000-hour aging time. Delta ferrite along austenitic grain boundaries transformed into sigma phases and Cr23C6 precipitates during long-term aging, and the area fraction on the grain boundary increased. The peak strength appeared at a 100-hour aging time and then decreased up to 1000 hours. With further aging time, the strength increased to a higher level than the initial state. However, the elongation and toughness decreased continuously, demonstrating the material embrittlement during long-term thermal aging.

Mechanical behavior and high-resolution EBSD investigation of the microstructural evolution in AISI 321 stainless steel under dynamic loading condition

The impact response of three regions (top, mid and center) across the thickness of AISI 321 austenitic stainless steel plate at high strain rates (>6000s−1) was studied using the split Hopkinson pressure bar system. The result shows that texture and stored energy heterogeneity across plate thickness influenced the mechanical responses of the investigated steel in these regions. Microstructural evaluation using high-resolution electron backscattered diffraction (HR-EBSD) analysis showed that strengthening in AISI 321 steel originates from the evolution of strain-induced martensite and formation of nano-carbides in addition to plastic deformation by mechanical twinning and slip. This resulted in a desirable combination of high strength and good ductility (approx. 2000MPa at 0.42 true strain). Phase transformation, dynamic recrystallization and formation of nano-carbides were confirmed within the adiabatic shear band (ASB) region. The average dynamic recrystallized (DRX) grain size in the shear band region is 0.28µm in comparison to grain size of 15µm outside the shear bands. The nano-sized grain inside the shear bands is proposed to form by rotational dynamic recrystallization. A comparative study of the alloy's behavior under dynamic and quasi-static compression shows that the stability of austenite is higher at high strain rates and lower at a low strain rate. The strength in the dynamically impacted specimen is compromised as a result of the suppressed evolution of strain-induced martensite and mechanical twin. Martensitic transformation under both loading conditions follows the FCC ɣ-austenite→BCC ά-martensite kinetic path and both phases obey the Kurdjumov-Sachs' {(111)ɣ||(110)ά and <−101>ɣ||<1–11>ά} orientation relationship.

Warm multiaxial forging of AISI 1016 steel

In this work, the microstructural evolution in AISI 1016 steel processed by using warm multiaxial forging technique is studied. With increase in multiaxial forging strain, a finer substructure evolved. Structural evolution in pearlite phase is addressed in detail considering the strain paths and strain rate. Pearlitic cementite fragmented into ultrafine particles of about 100–300 nm size. Warm multiaxial forging process also dispersed the ultrafine cementite particles into the ferrite matrix. Based on the grain boundary characterization and textural evolution, mechanism of ferrite grain refinement is explained. Up to six strain steps, crystallographic slip is the dominant mode of deformation and grain subdivision and recovery is the mechanism for ferrite grain refinement. At nine strain steps, dominant deformation mechanism appears to be grain boundary sliding and random grain rotation. After nine strain steps, initial grains of average 17 μm size reduced to submicron sized grains with the fraction of high angle grain boundaries exceeding 0.7. Double-n behavior is observed during tensile testing of some multiaxially forged steels. Tensile strength and hardness values of multiaxially forged steel increased by more than 100% after eighteen warm multiaxial forging strain steps, whereas ductility reduced by only about 30%.

Microstructural evolution in AISI 304 stainless steel during directional solidification and subsequent solid-state transformation

The microstructural evolution during solidification and subsequent solid-state transformation of AISI 304 stainless steel is studied by directional solidification and quenching methods in this paper. The phase transformation sequence in the steel directionally solidified is that the precipitation of primary ferritic dendrites, ferrite–austenite eutectic reaction, and the direct formation of austenite occur in sequence during the directional solidification of the austenitic stainless steel. At the eutectic reaction stage, plenty of columnar and cellular eutectic colonies composed of the coupled growth of lamellar ferrite and austenite take place. During the course of subsequent solid-state transformation, austenite grows into ferrite gradually, resulting in the disappearing of eutectic colonies and thinning of primary ferritic dendrites, and dendrite cores of primary ferrite are retained as the final skeletal ferrite in the final microstructure.

The Influence of Large Boride Particles on Microstructural Evolution in AISI 304 Steel.

Microstructural evolution in AISI 304 stainless steel and in a similar steel containing 1% boron during hot torsion was studied by means of quantitative optical microscopy and transmission electron microscopy. Particular attention was focused on the influence of the large volume fraction of hard boride particles on the dynamic recrystallization paramenters and the characteristics of the dislocation substructure. Dynamic recrystallization in the austenitic matrix was accelerated significantly in the presence of boride particles. This was accounted for by the superposition of both local strain accumulations around the curved particle interfaces and higher overall strain energy stored in the austenite. Hot workability, however, remained extremely poor probably because of the high strain rate used in this study. Changes of the dislocation substructure in the austenitic matrix during hot deformation were found to be similar in both steels and were interpreted according to a model that has been so far used predominantly for cold deformation. The study of the substructure of the boride particles showed that they remained almost undeformed during the deformation process.