Modeling of Grain Size Distributions during Single Hit Deformation of a Nb-Containing Steel

A homogenized (to remove segregation effects from continuous casting) 0.046 wt pct Nb-microalloyed steel was used to examine the validity of the Dutta–Sellars equation in describing the degree of recrystallization, grain size distributions, and the influence of precipitation at this Nb level for deformation between 1248 and 1348 K (975 and 1075 °C). Using the Dutta–Sellars equations with the mode grain size overestimated the degree of recrystallization for deformation at temperatures above 1283 K (1010 °C), while modeling the behavior for individual grain size classes gave better agreement for the degree of recrystallization and also predicted the resulting grain size distributions. Within the conditions studied (reaustenitization at 1498 K (1225 °C) and single hit thermomechanical simulation with 0.3 strain, 10-second hold), there was no effect of strain-induced precipitation on the extent of recrystallization detected.     

Mechanical Behaviors of Ultrafine-Grained 301 Austenitic Stainless Steel Produced by Equal-Channel Angular Pressing

The technique of equal-channel angular pressing (ECAP) was used to refine the microstructure of an AISI 301 austenitic stainless steel (SS). An ultrafine-grained (UFG) microstructure consisting mainly of austenite and a few martensite was achieved in 301 steel after ECAP processing for four passes at 523 K (250 °C). By submitting the as-ECAP rods to annealing treatment in the temperature range from 853 K to 893 K (580 °C to 620 °C) for 60 minutes, fully austenitic microstructures with grain sizes of 210 to 310 nm were obtained. The uniaxial tensile tests indicated that UFG 301 austenitic SS had an excellent combination of high yield strength (>1.0 GPa) and high elongation-to-fracture (>30 pct). The tensile stress–strain curves exhibited distinct yielding peak followed by obvious Lüders deformation. Measurements showed that Lüders elongation increased with an increase in strength as well as a decrease in grain size. The microstructural changes in ultrafine austenite grains during tensile deformation were tracked by X-ray diffraction and transmission electron microscope. It was found that the strain-induced phase transformation from austenite to martensite took place soon after plastic deformation. The transformation rate with strain and the maximum strain-induced martensite were promoted significantly by ultrafine austenite grains. The enhanced martensitic transformation provided extra strain-hardening ability to sustain the propagation of Lüders bands and large uniform plastic deformation. During tensile deformation, the Lüders bands and martensitic transformation interacted with each other and made great contribution to the excellent mechanical properties in UFG austenitic SS.     

High-Temperature (1023 K to 1273 K [750 °C to 1000 °C]) Plastic Deformation Behavior of B-Modified Ti-6Al-4V Alloys: Temperature and Strain Rate Effects

A minor addition of B to the Ti-6Al-4V alloy, by ~0.1 wt pct, reduces its as-cast prior β grain size by an order of magnitude, whereas higher B content leads to the presence of in situ formed TiB needles in significant amounts. An experimental investigation into the role played by these microstructural modifications on the high-temperature deformation behavior of Ti-6Al-4V-xB alloys, with x varying between 0 wt pct and 0.55 wt pct, was conducted. Uniaxial compression tests were performed in the temperature range of 1023 K to 1273 K (750 °C to 1000 °C) and in the strain rate range of 10^–3 to 10⁺¹ s^–1. True stress–true strain responses of all alloys exhibit flow softening at lower strain rates and oscillations at higher strain rates. The flow softening is aided by the occurrence of dynamic recrystallization through lath globularization in high temperature (1173 K to 1273 K [900 °C to 1000 °C]) and a lower strain rate (10^–2 to 10^–3 s^–1) regime. The grain size refinement with the B addition to Ti64, despite being marked, had no significant effect on this. Oscillations in the flow curve at a higher strain rate (10⁰ to 10⁺¹ s^–1), however, are associated with microstructural instabilities such as bending of laths, breaking of lath boundaries, generation of cavities, and breakage of TiB needles. The presence of TiB needles affected the instability regime. Microstructural evidence suggests that the matrix cavitation is aided by the easy fracture of TiB needles.     

Ferrite Grain Size Distributions in Ultra-Fine-Grained High-Strength Low-Alloy Steel After Controlled Thermomechanical Deformation

Plane-strain compression testing was carried out above, around, and below the A _r3 temperature with the deformation temperature, T _def, varying between 1323 K and 973 K (1050 °C and 700 °C), using Gleeble 3500, to develop uniform distribution of ultra-fine ferrite (UFF) grains. Prior austenite (γ) grain structure, developed after soaking at 1473 K (1200 °C), was mixed in nature, comprising both coarse- and fine-γ-grain sizes. Applying heavy deformation in a single pass, just above the austenite-to-ferrite (α) transformation temperature (A _r3), and cooling to room temperature resulted in the formation of UFF grain sizes (average α-grain size ~2 to 3 μm), with the largest grain sizes extending up to ~10 to 12 μm. Water quenching just after deformation prevented the coarsening of UFF grains and restricted the largest grain sizes to under 6 μm. Although the ferrite grain structures appeared homogeneous in slowly cooled samples (cooling rate (CR) 1 K/s), careful observation revealed the presence of alternate bands of coarse- (5 to 10 μm) and fine-α grains (<1 to 3 μm). The final α-grain size distributions were explained in view of the starting γ-grain size variation, dynamic recrystallization (DRX) of γ, dynamic strain-induced γ-to-α transformation (DSIT), and DRX of α and grain growth during slow cooling. Electron backscattered diffraction analysis (EBSD) revealed the presence of a large fraction (70 to 80 pct) of high-angle boundaries, having misorientation ≥15 deg. Compared to the use of the single, heavy deformation pass, the application of a number of lighter passes between A _e3 and A _r3 temperatures is more suitable in industrial rolling conditions, and also has the potential of developing UFF grains with high-angle boundaries.     

Selective Oxidation and Reactive Wetting during Galvanizing of a CMnAl TRIP-Assisted Steel

A transformation induced plasticity (TRIP)-assisted steel with 0.2 pct C, 1.5 pct Mn, and 1.5 pct Al was successfully galvanized using a thermal cycle previously shown to produce an excellent combination of strength and ductility. The steel surface chemistry and oxide morphology were determined as a function of process atmosphere oxygen partial pressure. For the 220 K (–53 °C) dew point (dp) + 20 pct H₂ atmosphere, the oxide morphology was a mixture of films and nodules. For the 243 K (–30 °C) dp + 5 pct H₂ atmosphere, nodules of MnO were found primarily at grain boundaries. For the 278 K (+5 °C) dp + 5 pct H₂ atmosphere, nodules of metallic Fe were found on the surface as a result of alloy element internal oxidation. The steel surface chemistry and oxide morphology were then related to the reactive wetting behavior during continuous hot dip galvanizing. Good wetting was obtained using the two lower oxygen partial pressure process atmospheres [220 K dp and 243 K dp (–53 °C dp and –30 °C dp)]. An increase in the number of bare spots was observed when using the higher oxygen partial pressure process atmosphere (+5 °C dp) due to the increased thickness of localized oxide films.     

Microstructures and Mechanical Properties of As-Cast and Hot-Rolled Mg-8.43Li-0.353Ymm (Y-riched mischmetch) Alloy

Mg-8.43Li-0.353Ymm (Y-riched mischmetch) alloy was prepared using the metal model casting method and then hot-rolled at 573 K (300 °C) to total reduction of 81 pct, and microstructures and mechanical properties were evaluated. Results show that the rolling procedure introduces great dynamic recrystallization, and β-Li grains in the rolled alloy present an ultra-low average size of 2.18 μm. The ultimate tensile strength and yield strength of the rolled alloy are 158–167 MPa and 124–130 MPa, respectively, 42–63 pct higher than that of the as-cast alloy. The elongation of the rolled alloy is improved to 31–40 pct, as about four to five times that of the as-cast alloy, which is ascribed to the great grain refinement of the β-Li phase.     

Mechanical Behavior and Microstructural Change of a High Nitrogen CrMn Austenitic Stainless Steel during Hot Deformation

The hot deformation behavior of a high nitrogen CrMn austenitic stainless steel in the temperature range 1173 to 1473 K (900 to 1200 °C) and strain rate range 0.01 to 10 s⁻¹ was investigated using optical microscopy, stress-strain curve analysis, processing maps, etc. The results showed that the work hardening rate and flow stress decreased with increasing deformation temperature and decreasing strain rate in 18Mn18Cr0.5N steel. The dynamic recrystallization (DRX) grain size decreased with increasing Z value; however, deformation heating has an effect on the DRX grain size under high strain rate conditions. In the processing maps, flow instability was observed at higher strain rate regions (1 to 10 s⁻¹) and manifested as flow localization near the grain boundary. Early in the deformation, the flow instability region was at higher temperatures, and then the extent of this unstable region decreased with increasing strain and was restricted to lower temperatures. The hot deformation equation as well as the quantitative dependence of the critical stress for DRX and DRX grain size on Z value was obtained.     

Modeling the Flow Curve Characteristics of 410 Martensitic Stainless Steel Under Hot Working Condition

The hot deformation behavior of AISI 410 martensitic stainless steel was investigated by conducting hot compression tests between 1173 K (900 °C) and 1423 K (1150 °C) and between strain rates of 0.001 s⁻¹ to 1 s⁻¹. The hyperbolic sine function described the relation well between flow stress at a given strain and the Zener–Hollomon parameter (Z). The variation of flow stress with deformation temperature gave the average value of apparent activation energy as 448 kJ/mol. The strain and stress corresponding to two important points associated with flow curve (i.e., peak strain and the onset of steady-state flow) were related to the Z parameter using power-law equations. A model also was proposed based on the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation to estimate the fractional softening of dynamic recrystallization at any given strain. This model can be used readily for the prediction of flow stress. The values of n and k, material constants in the JMAK equation, were determined for the studied material. The strains regarding the peak and the onset of steady-state flow were formulated in term of applied strain rate and the constants of the JMAK equation. A good agreement was found between the predicted strains and those obtained by the experimental work.     

Initial-stage Sintering Kinetics of Nanocrystalline Tungsten

Initial-stage sintering kinetics of nanocrystalline tungsten has been studied in the temperature range of 1273–1473 K (1000–1200 °C). Nanocrystalline tungsten sinters initially through a grain boundary diffusion mechanism. The calculated activation energy was 388 ± 11 kJ/mol at low temperatures (1273–1373 K (1000–1100 °C)) and 409 ± 7 kJ/mol at high temperatures (1373–1473 K (1100–1200 °C)), which are close to the experimentally measured activation energy for grain boundary diffusion (385 kJ/mol).     

A comparison of fracture behavior of low alloy steel with different sizes of carbide particles

The fracture behaviors of low alloy steels with similar grain sizes but different sizes of carbide particles were investigated using precracked and notched specimens. The results indicate that in precracked specimens (COD), steel with coarser carbide particles has a lower toughness than steel with finer carbide particles over a temperature range from –196 °C to – 90 °C. However, in notched specimens (four-point bending (4PB) and Charpy V), these two steels shows similar toughness at low temperature where specimens are fractured by cleavage without fibrous cracking. In the transition temperature range, the steel with coarser carbide particles conversely shows a little higher toughness due to the longer extension length of the fibrous crack. This phenomenon indicates that in precracked specimens, the second-phase particles play a leading role in cleavage fracture, while in notched specimens, the grain size dominates the fracture behavior.     

Effects of Annealing Temperature on Microstructure and Tensile Properties in Ferritic Lightweight Steels

An investigation was conducted into the effects of annealing temperature on microstructure and tensile properties of ferritic lightweight steels. Two steels were fabricated by varying the C content, and were annealed at 573 K to 1173 K (300 °C to 900 °C) for 1 hour. According to the microstructural analysis results, κ-carbides were formed at about 973 K (700 °C), which was confirmed by equilibrium phase diagrams calculated from a THERMO-CALC program. In the steel containing low carbon content, needle-shaped κ-carbides were homogeneously dispersed in the ferrite matrix, whereas bulky band-shaped martensites were distributed in the steel containing high carbon content. In the 973 K (700 °C)-annealed specimen of the steel containing high carbon content, deformation bands were formed throughout the specimen, while fine carbides were sufficiently deformed inside the deformation bands, thereby resulting in the greatest level of strength and ductility. These results indicated that the appropriate annealing treatment of steel containing high carbon content was useful for the improvement of both strength and ductility over steel containing low carbon content.     

Hydrogen Embrittlement of a 1500-MPa Tensile Strength Level Steel with an Ultrafine Elongated Grain Structure

A deformation of a tempered martensitic structure (i.e., tempforming) at 773 K (500 °C) was applied to a 0.6 pct C-2 pct Si-1 pct Cr steel. The hydrogen embrittlement (HE) property of the tempformed (TF) steel was investigated by a slow strain rate test (SSRT) and an accelerated atmospheric corrosion test (AACT). Hydrogen content within the samples after SSRT and AACT was measured by thermal desorption spectrometry (TDS). The tempforming at 773 K (500 °C) using multipass caliber rolling with an accumulative are reduction of 76 pct resulted in the evolution of an ultrafine elongated grain (UFEG) structure with a strong 〈110〉//rolling direction (RD) fiber deformation texture and a dispersion of spheroidized cementite particles. The SSRT of the pre-hydrogen-charged notched specimens and the AACT demonstrated that the TF sample had superior potential for HE resistance to the conventional quenched and tempered (QT) sample at a tensile strength of 1500 MPa. The TDS analysis also indicated that the hydrogen might be mainly trapped by reversible trapping sites such as grain boundaries and dislocations in the TF sample, and the hydrogen trapping states of the TF sample were similar to those of the QT sample. The QT sample exhibited hydrogen-induced intergranular fracture along the boundaries of coarse prior-austenite grains. In contrast, the hydrogen-induced cracking occurred in association with the UFEG structure in the TF sample, leading to the higher HE resistance of the TF sample.     

Microstructure and Mechanical Properties in Hot-Rolled Extra High-Yield-Strength Steel Plates for Offshore Structure and Shipbuilding

Key parameters for a thermomechanically controlled processing and accelerated cooling process (TMCP-AcC) were determined for integrated mass production to produce extra high-yield-strength microalloyed low carbon SiMnCrNiCu steel plates for offshore structure and bulk shipbuilding. Confocal scanning microscopy was used to make in-situ observations on the austenite grain growth during reheating. A Gleeble 3800 thermomechanical simulator was employed to investigate the flow stress behavior, static recrystallization (SRX) of austenite, and decomposition behavior of the TMCP conditioned austenite during continuous cooling. The Kocks–Mecking model was employed to describe the constitutive behavior, while the Johnson–Mehl–Avrami–Kolmogorov (JMAK) approach was used to predict the SRX kinetics. The effects of hot rolling schedule and AcC on microstructure and properties were investigated by test-scale rolling trials. The bridging between the laboratory observations and the process parameter determination to optimize the mass production was made by integrated industrial production trials on a set of a 5-m heavy plate mill equipped with an accelerated cooling system. Successful production of 60- and 50-mm-thick plates with yield strength in excess of 460 MPa and excellent toughness at low temperature (213 K (–60 °C)) in the parent metal and the simulated coarse-grained heat affected zone (CGHAZ) provides a useful integrated database for developing advanced high-strength steel plates via TMCP-AcC.     

Microstructural Evolution and Flow Analysis during Hot Working of a Fe-Ni-Cr Superaustenitic Stainless Steel

Hot compression tests were conducted in a temperature range of 1173 K to 1323 K (900 °C to 1050 °C) and strain rates of 0.001 seconds⁻¹ to 1 second⁻¹ to investigate the hot deformation behavior of the austenitic stainless steel type 1.4563. The results showed that hot deformation at low temperatures, i.e., 1173 K to 1223 K (900 °C to 950°C), and at low and medium strain rates, i.e., 0.001 seconds⁻¹ to 0.1 seconds⁻¹, results in the dynamic formation of worm-like precipitates on existing grain boundaries. This in turn led to the restriction or even inhibition of dynamic recrystallization. However, at higher temperatures and strain rates when either the time frame for dynamic precipitation was too short or the driving force was low, dynamic recrystallization occurred readily. Furthermore, at low strain rates and high temperatures, there was no sign of particles, but the interactions between solute atoms and mobile dislocations made the flow curves serrated. The strain rate sensitivity was determined and found to change from 0.1 to 0.16 for a temperature increase from 1173 K to 1323 K (900 °C to 1050 °C). The variations of mean flow stress with strain rate and temperature were analyzed. The calculated apparent activation energy for the material was approximately 406 kJ/mol. The hyperbolic sine function correlated the Zener-Hollomon parameter and flow stress successfully at intermediate stress levels. However, at low levels of flow stress a power-law equation and at high stresses an exponential equation well fitted the experimental data.     

Plastic Flow Properties and Microstructural Evolution in an Ultrafine-Grained Al-Mg-Si Alloy at Elevated Temperatures

An AA6082 alloy was subjected to eight passes of equal channel angular pressing at 100 °C, resulting in an ultrafine grain size of 0.2 to 0.4 μm. The tensile deformation behavior of the material was studied over the temperature range of 100 °C to 350 °C and strain rate range of 10⁻⁴ to 10⁻¹ s⁻¹. The evolution of microstructure under tensile deformation was investigated by analyzing both the deformation relief on the specimen surface and the dislocation structure. While extensive microshear banding was found at the lower temperatures of 100 °C to 150 °C, deformation at higher temperatures was characterized by cooperative grain boundary sliding and the development of a bimodal microstructure. Dislocation glide was identified as the main deformation mechanism within coarse grains, whereas no dislocation activity was apparent in the ultrafine grains.     

Evolution of Microstructure and Texture During Hot Compression of a Ni-Fe-Cr Superalloy

Superalloys are being employed in more extreme conditions requiring higher strength, which requires producers to forge products to finer grain sizes with less grain size variability. To assess grain size, crystallographic texture, and substructure as a function of forging conditions, frictionless uniaxial compression testing characteristic of hot working was performed on INCOLOY 945 (Special Metals Corporation, Huntington, WV), which is a newly developed hybrid of alloys 718 and 925, over a range of temperatures and strain rates. The microstructure and texture were investigated comprehensively using light optical microscopy, electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI), and transmission electron microscopy (TEM) to provide detailed insight into microstructure evolution mechanisms. Dynamic recrystallization, nucleated by grain/twin boundary bulging with occasional subgrain rotation, was found to be a dominant mechanism for grain refinement in INCOLOY 945. At higher strain rates, static recrystallization occurred by grain boundary migration. During deformation, duplex slip along {111} planes occurred until a stable 〈110〉 fiber compression texture was established. Recrystallization textures were mostly random but shifted toward the compression texture with subsequent deformation. An exception occurred at 1423 K (1150 °C) and 0.001 seconds⁻¹, the condition with the largest fraction of recrystallized grains, where a 〈100〉 fiber texture developed, which may be indicative of preferential growth of specific grain orientations.     

The Coexistence of Two Different Pearlites,Lamellae of (Ferrite + M<Subscript>3</Subscript>C), and Lamellae of (Ferrite + M<Subscript>23</Subscript>C<Subscript>6</Subscript>) in a Mn-Al Steel

Two different pearlites after two separate eutectoid reactions were observed in an Fe-19.8 Mn-1.64 Al-1.03 C (wt pct) steel. The steel specimens were processed under solution heat treatment at 1373 K (1100 °C) and received isothermal holding at temperatures from 1073 K to 773 K (800 °C to 500 °C). The constituent phase of the steel is single austenite at temperatures between 1373 K and 1073 K (1100 °C and 800 °C). At temperatures below 1048 K (775 °C), M₃C and M₂₃C₆ carbides coprecipitate at the austenitic grain boundaries. Two different pearlites appear in the austenite matrix simultaneously at temperatures below 923 K (650 °C). One is lamellae of ferrite and M₃C carbide, and the other is lamellae of ferrite and M₂₃C₆ carbide. These two pearlites are product phases from two separate eutectoid reactions, i.e., austenite → ferrite + cementite and austenite → ferrite + M₂₃C₆. Therefore, the supersaturated austenite has decomposed into two different pearlites, separately.