Deformation Behavior and Evolution of Microstructure and Texture During Hot Compression of AISI 304LN Stainless Steel

Deformation behavior of hot-rolled AISI 304 LN austenitic stainless steel was studied by hot axisymmetric compression tests at 1173 K, 1273 K, and 1373 K (900 °C, 1000 °C, and 1100 °C) at strain rates of 0.01, 0.1, and 1 s^?1. The flow curves were examined to understand the deformation characteristics. The influence of Zener–Holloman parameter was analyzed using appropriate constitutive models. The activation energy for deformation was found to be 473 kJ/mol. Quantitative microstructural analysis was carried out using Electron backscattered diffraction. Compression at 1173 K (900 °C) at all true strain rates gave rise to partially dynamic recrystallized microstructure with strong α-fiber texture. The deformation texture is characterized by the formation of Brass component, and partial dynamic recrystallization (DRX) led to the development of Goss, S, and ube components. Necklace structure of small equiaxed recrystallized grains could be observed surrounding the large, elongated deformed grains. Compressions at 1273 K and 1373 K (1000 °C and 1100 °C) resulted in fully recrystallized microstructure consisting of mostly Σ3 and Σ9 coincidence site lattice high-angle boundaries. Compression at 1273 K (1000 °C) leads to the formation of low-intensity diffused α-fiber. DRX was confirmed by the presence of Goss, S, Cube, and rotated Cube components. Compression performed at 1373 K (1100 °C) resulted in nearly random texture with traces of α-fiber and prominent Cube/rotated Cube components. The microstructures of the 1173 K (900 °C)-compressed samples were partitioned using grain size and misorientation criteria to quantify DRX.     

Dynamic Recrystallization and Precipitation in 13Cr Super-Martensitic Stainless Steels

The influence of precipitation on the kinetics of static and dynamic recrystallization (DRX) was investigated in AISI 403 and 403Nb martensitic stainless steels. Hot compression tests were performed in the temperature range of 1073 K to 1473 K (800 °C to 1200 °C) and strain rates of 0.001 and 0.1 s^?1 to study DRX and precipitation behaviors. In parallel, stress relaxation tests were conducted with pre-strains of 0.1, 0.15, 0.2, and 0.25, a strain rate of 0.1 s^?1, and in the 1073 K to 1473 K (800 °C to 1200 °C) temperature range to study the kinetics of precipitation and recrystallization. Samples of hot compression and stress relaxation tests were quenched and the evolution of the microstructure was examined using optical and scanning electron microscopy. The results indicated that DRX interacts with dynamic precipitation (DP) over the temperature range of 1173 K to 1273 K (900 °C to 1000 °C). Hot compression testing results, confirmed by EBSD analysis, indicated that partial DRX occurs before precipitation in 403Nb, at 1073 K (800 °C). By contrast, no DRX was observed in 403 steel. At higher temperatures, i.e., over 1273 K (1000 °C), DRX preceded DP in both steels. Increasing the strain rate raised the temperature range of interaction between DRX and DP up to 1373 K (1100 °C). Strain-induced precipitation (SIP) was observed over the entire range of investigated test temperatures. Static recrystallization (SRX) took place predominantly in the temperature range of 1173 K to 1373 K (900 °C to 1100 °C), at which SIP significantly delayed the SRX finishing time. The results are analyzed in the framework of the classical nucleation theory and the underlying mechanisms are identified.     

Influence of Temperature and Holding Time on the Interaction of V,Al, and N in Microalloyed Forging Steels

A medium-carbon vanadium microalloyed steel (38MnSiVS5) with three different aluminum levels (0.006, 0.020, and 0.03 wt pct) was used to examine the interaction of vanadium, aluminum, and nitrogen during the heating and cooling cycle for forging. The thermal cycle was simulated using a Gleeble^® 1500. Hold times varied from 5 to 45 minutes and temperature varied from 1323 K to 1523 K (1050 °C to 1250 °C). Thermal simulation specimens and as-received material were characterized by quantitative metallography, hardness, and chemical analysis of electrolytically extracted precipitates. The hardness was observed to be relatively constant for all aluminum levels after all thermal simulations at and above 1423 K (1150 °C). Hardness, pearlite fraction, and austenite grain size decreased with increasing aluminum content at the two lowest temperatures examined, which were 1323 K and 1373 K (1050 °C and 1100 °C). The amount of vanadium precipitated in the lowest aluminum steel was very consistent, approximately 70 pct, for the thermal simulations. The amount of precipitated vanadium decreased with increasing amount of aluminum nitride for the 0.03 wt pct Al level.     

Hot Deformation Behavior of As-Cast 2101 Grade Lean Duplex Stainless Steel and the Associated Changes in Microstructure and Crystallographic Texture

The hot deformation behavior of 2101 grade lean duplex stainless steel (DSS, containing ~5 wt pct Mn, ~0.2 wt pct N, and ~1.4 wt pct Ni) and associated microstructural changes within δ-ferrite and austenite (γ) phases were investigated by hot-compression testing in a GLEEBLE 3500 simulator over a range of deformation temperatures, T _def [1073 K to 1373 K (800 °C to 1100 °C)], and applied strains, ε (0.25 to 0.80), at a constant true strain rate of 1/s. The microstructural softening inside γ was dictated by discontinuous dynamic recrystallization (DDRX) at a higher T _def [1273 K to 1373 K (1000 °C to 1100 °C)], while the same was dictated by continuous dynamic recrystallization (CDRX) at a lower T _def (1173 K (900 °C)]. Dynamic recovery (DRV) and CDRX dominated the softening inside δ-ferrite at T _def ≥ 1173 K (900 °C). The dynamic recrystallization (DRX) inside δ and γ could not take place upon deformation at 1073 K (800 °C). The average flow stress level increased 2 to 3 times as the T _def dropped from 1273 to 1173 K (1000 °C to 900 °C) and finally to 1073 K (800 °C). The average microhardness values taken from δ-ferrite and γ regions of the deformed samples showed a different trend. At T _def of 1373 K (1100 °C), microhardness decreased with the increase in strain, while at T _def of 1173 K (900 °C), microhardness increased with the increase in strain. The microstructural changes and hardness variation within individual phases of hot-deformed samples are explained in view of the chemical composition of the steel and deformation parameters (T _def and ε).

     

Dynamic Recrystallization Behavior of Medium Carbon Cr-Ni-Mo-Nb Steel during Hot Deformation

Hot compression deformation behaviors of medium carbon Cr-Ni-Mo-Nb steel were investigated at deformation temperatures ranging from 1223 to 1423 Kand strain rates of 0.1,1and 5s^-1.Dynamic recovery（DRV）and dynamic recrystallization（DRX）were observed during the hot compression deformation.For all of the samples,DRX occurred at deformation temperatures above 1323 Kat different strain rates,while below 1223 K,no DRX was observed.The activation energy of the tested steel was determined as 386.06kJ/mol.The ratio of critical stress to peak stress and the ratio of critical strain to peak strain were 0.835 and 0.37,respectively.Kinetic equations interpreting the DRX behavior of the tested steel were proposed,and the corresponding parameters including the volume fraction and the average grain size were determined.Moreover,the microstructures induced under different deformation conditions were analyzed.     

Oxidation of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Scale-Forming MAX Phases in Turbine Environments

High temperature oxidation of alumina-forming MAX phases, Ti₂AlC and Cr₂AlC, were examined under turbine engine environments and coating configurations. Thermogravimetric furnace tests of Ti₂AlC showed a rapid initial transient due to non-protective TiO₂ growth. Subsequent well-behaved cubic kinetics for alumina scale growth were shown from 1273 K to 1673 K (1000 °C to 1400 °C). These possessed an activation energy of 335 kJ/mol, consistent with estimates of grain boundary diffusivity of oxygen (~375 kJ/mol). The durability of Ti₂AlC under combustion conditions was demonstrated by high pressure burner rig testing at 1373 K to 1573 K (1100 °C to 1300 °C). Here good stability and cubic kinetics also applied, but produced lower weight gains due to volatile TiO(OH)₂ formation in water vapor combustion gas. Excellent thermal stability was also shown for yttria-stabilized zirconia thermal barrier coatings deposited on Ti₂AlC substrates in 2500-hour furnace tests at 1373 K to 1573 K (1100 °C to 1300 °C). These sustained a record 35 µm of scale as compared to 7 μm observed at failure for typical superalloy systems. In contrast, scale and TBC spallation became prevalent on Cr₂AlC substrates above 1423 K (1150 °C). Cr₂AlC diffusion couples with superalloys exhibited good long-term mechanical/oxidative stability at 1073 K (800 °C), as would be needed for corrosion-resistant coatings. However, diffusion zones containing a NiAl-Cr₇C₃ matrix with MC and M₃B₂ particulates were commonly formed and became extensive at 1423 K (1150 °C).     

A Comparative Study on the Hot Working Behavior of Inconel 718 and ALLVAC 718 Plus

Hot compression tests were performed on Inconel 718 and ALLVAC 718 PLUS (718+) at temperatures and strain rates in ranges of 1223 K to 1373 K (950 °C to 1100 °C) and 0.001–1 s⁻¹, respectively. Discontinuous yield behavior was observed in the flow curves of both alloys. For both alloys, the drop in stress at the yield point (yield drop) was maximized at 0.01 to 1 s⁻¹. The alloy 718+ showed larger yield drop than 718 over the studied deformation conditions. The different yield behaviors were attributed to the various chemical compositions. The peak strain for both alloys increased in temperature range of 1223 K to 1273 K (950 to 1000 °C) and strain rates of 0.01 to 1 s⁻¹. This uncommon behavior was ascribed to the change in the mechanism of microstructural evolution from continuous to discontinuous dynamic recrystallization (DRX). The kinetics of DRX was described by the Avrami equation and the exponent was determined at different deformation conditions. The Avrami exponent increased in the middle values of Zener–Hollomon (Z) parameters, i.e., 29.3 < lnZ < 32.9 for 718 and 31.4 < lnZ < 34.5 for 718+. The unusual variation of the Avrami exponent was attributed to the change in the mechanism of DRX.

     

Strain Effect on the �á� Dissolution at High Temperatures of a Nickel-Based Single Crystal Superalloy

The kinetics of the ?á? phase dissolution have been studied at 1473 K and 1523 K (1200 °C and 1250 °C) for CMSX-4^® alloy. Interrupted creep tests at 1323 K (1050 °C) and pure thermal aging at 1373 K (1100 °C) have been used to vary the initial ?á? morphology. Subsequent dissolution studies were conducted with or without an applied load. Differences in ?á? dissolution kinetics were observed between the dendrite cores and the interdendritic regions, resulting from the chemical microsegregations remaining after the standard heat treatments. It is shown that the initial ?á? morphology, the relaxation of the coherency stresses and the accumulated plastic strain are critical parameters controlling the dissolution kinetics. An increase of the accumulated plastic strain leads to an increase of the dissolution kinetics, whatever the location in the dendritic structure. In addition, once a given accumulated creep strain is reached prior to the dissolution tests (i.e., a given dislocations density), the dissolution kinetics are the same at 1473 K and 1523 K (1200 °C and 1250 °C). A modified equation of the recently developed Polystar model is proposed to incorporate the role of the accumulated plastic strain on the ?á? dissolution kinetics and to achieve a better predictability of the creep deformation under non-isothermal loading paths.     

Grain Boundary Engineering of a Low Stacking Fault Energy Ni-based Superalloy

The effects of thermo-mechanical processing parameters on the resulting microstructure of an experimental Nickel-based superalloy containing 24 wt pct Co were investigated. Hot compression tests were performed at temperatures ranging from 1293 K to 1373 K (1020 to 1100 °C) and strain rates ranging from 0.0005 to 0.1/s. The mechanically deformed samples were also subject to annealing treatments at sub-solvus 1388 K (1115 °C) and super-solvus 1413 K (1140 °C) temperatures. This investigation sought to quantify and subsequently understand the behavior and evolution of both the grain boundary structure and length fraction of Σ3 twin boundaries in the low stacking fault energy superalloy. Over the range of deformation parameters investigated, the corresponding deformation mechanism map revealed that dynamic recrystallization or dynamic recovery was dominant. These conditions largely promoted post-deformation grain refinement and the formation of annealing twins following annealing. Samples deformed at strain rates of 0.0005 and 0.001/s at 1333 K and 1373 K (1060 °C and 1100 °C) exhibited extensive grain boundary sliding/rotation associated with superplastic flow. Upon annealing, deformation conditions that resulted predominately in superplastic flow were found to provide negligible enhancement of twin boundaries and produced little to no post-deformation grain refinement.     

Hot Deformation Behavior and Microstructural Evolution of a Nickel-Free Austenitic Steel with High Nitrogen Content

In current study, the effect of microstructure on hot ductility of nickel-free austenitic high nitrogen steel DIN EN 1.4452 was investigated. Phase transformations and precipitation were modeled as well as experimentally determined via microstructural evaluation. Hot tensile and compression tests were used to simulate the hot deformation behavior at temperatures between 1173 K and 1573 K (900 °C and 1300 °C). Hot tensile test determined the high-temperature properties. The effect of temperature on cracking sensibility during hot deformation was investigated using hot compression test. The results showed that better hot ductility is observed at temperatures ranging from 1423 K to 1523 K (1150 °C to 1250 °C). The increase of hot ductility depends on grain refinement due to dynamic recrystallization at this temperature range.     

Effect of Strain Rate on Hot Ductility Behavior of a High Nitrogen Cr-Mn Austenitic Steel

18Mn18Cr0.6N steel specimens were tensile tested between 1173 K and 1473 K (900 °C and 1200 °C) at 9 strain rates ranging from 0.001 to 10 s^?1. The tensile strained microstructures were analyzed through electron backscatter diffraction analysis. The strain rate was found to affect hot ductility by influencing the strain distribution, the extent of dynamic recrystallization and the resulting grain size, and dynamic recovery. The crack nucleation sites were primarily located at grain boundaries and were not influenced by the strain rate. At 1473 K (1200 °C), a higher strain rate was beneficial for grain refinement and preventing hot cracking; however, dynamic recovery appreciably occurred at 0.001 s^?1 and induced transgranular crack propagation. At 1373 K (1100 °C), a high extent of dynamic recrystallization and fine new grains at medium strain rates led to good hot ductility. The strain gradient from the interior of the grain to the grain boundary increased with decreasing strain rate at 1173 K and 1273 K (900 °C and 1000 °C), which promoted hot cracking. Grain boundary sliding accompanied grain rotation and did not contribute to hot cracking.     

Microstructure,Mechanical and Abrasive Wear Behavior of 8.0 wt pct Cr White Iron Subjected to Continuous and Cyclic Annealing Treatment

Continuous annealing treatment (austenitization for 4 hours followed by furnace cooling) and cyclic annealing treatment (four cycles of austenitization, each of 0.66 hours duration followed by forced air cooling) of 8.0 wt pct Cr white iron samples are undertaken at 1173 K, 1223 K, 1273 K, 1323 K, and 1373 K (900 °C, 950 °C, 1000 °C, 1050 °C, and 1100 °C) as steps of destabilizing the as-cast structure. Continuous annealing results in precipitation of secondary carbides on a matrix containing mainly pearlite, while cyclic annealing treatment causes similar precipitation of secondary carbides on a matrix containing martensite plus retained austenite. On continuous annealing, the hardness falls below the as-cast value (HV 556), while after cyclic annealing treatment there is about 70 pct increase in hardness, i.e., up to HV 960. Decrease in hardness with increasing annealing temperature is quite common after both heat treatments. The as-cast notched impact toughness (4.0 J) is nearly doubled by increasing to 7.0 J after both continuous and cyclic annealing treatment at 1173 K and 1223 K (900 °C and 950 °C). Cyclic annealing treatment gives rise to a maximum notched impact toughness of 10.0 J at 1373 K (1100 °C). Abrasive wear resistance after continuous annealing treatment degrades exhibiting wear loss greater than that of the as-cast alloy. In contrast, samples with cyclic annealing treatment show reasonably good wear resistance, thereby superseding the wear performance of Ni-Hard IV.

     

Removal of Sulfur from CaF2 Containing Desulfurization Slag Exhausted from Secondary Steelmaking Process by Oxidation

The oxidation behavior of sulfur in desulfurization slag generated from the secondary steelmaking process with air has been investigated in the temperature range of 973?K to 1373?K (700?°C to 1100?°C). Although a high removal rate of sulfur is not achieved at temperatures lower than 1273?K (1000?°C) because of the formation of CaSO₄, most of the sulfur is rapidly removed from slag as SO₂ gas in the 1273?K to 1373?K (700?°C to 1100?°C) range. This finding indicates that the desulfurization slag generated from the secondary steelmaking process can be reused as a desulfurized flux through air oxidation, making it possible to reduce significantly the amount of desulfurization slag for disposal.     

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).     

Thermodynamic Modeling of the Al-Ti-V Ternary System

The sub-binary systems Al-Ti, Ti-V, and Al-V are reviewed and adopted from the previous assessments, the thermodynamic analysis of the Al-Ti-V ternary system is performed by the CALPHAD approach, and a set of self-consistent thermodynamic parameters of the ternary system are obtained. Furthermore, the isothermal sections of this system at 1073 K, 1173 K, 1273 K, 1373 K, and 1473 K (800 °C, 900 °C, 1000 °C, 1100 °C, and 1200 °C) and the ternary invariant equilibria are calculated and compared with the corresponding experimental data, and all are in good agreement with most of the experimental results. Thus, the optimized thermodynamic parameters in this study may provide more accurate guidance to develop the new alloys involving it.     

Microstructure Evolution During Hot Deformation of a Micro-Alloyed Steel

In the present investigation, hot deformation by uniaxial compression of a microalloyed steel has been carried out, using a deformation dilatometer, after homogenization at 1200 °C for 20 min up to strains of 0.4, 0.8 and 1.2 at different temperatures of 900, 1000 and 1100 °C, at a constant strain rate of 2 s^?1 followed by water quenching. In all the deformation conditions, initiation of dynamic recrystallization (DRX) is observed, however, stress peaks are not observed in the specimens deformed at 900 and 1000 °C. The specimens deformed at 900 °C showed a combination of acicular ferrite (AF) and bainite (B) microstructure. There is an increase in the acicular ferrite fraction with increase in strain at all these deformation temperatures. At high deformation temperature of 1100 °C, coarsening of DRXed grains is observed. This is attributed to the common limitations involved in fast quenching of the DRXed microstructure, which leads to increase in grain size by metadynamic recrystallization (MDRX). The strain free prior austenite grains promote the formation of large fraction of both bainite and martensite in the transformed microstructures during cooling. The length and width of bainitic ferrite laths also increases with increase in deformation temperature from 900 to 1100 °C and decrease in deformation strain.     

High-Temperature Creep Degradation of the AM1/NiAlPt/EBPVD YSZ System

The failure mechanisms of a NiAlPt/electron beam physical vapor deposition yttria-stabilized-zirconia thermal barrier coating system deposited on the AM1 single crystalline substrate have been investigated under pure creep conditions in the temperature range from 1273 K to 1373 K (1000 °C to 1100 °C) and for durations up to 1000 hours. Doubly tapered specimens were used allowing for the analysis of different stress states and different accumulated viscoplastic strains for a given creep condition. Under such experiments, two kinds of damage mechanisms were observed. Under low applied stress conditions (i.e., long creep tests), microcracking is localized in the vicinity of the thermally grown oxide (TGO). Under high applied stress conditions, an unconventional failure mechanism at the substrate/bond coat interface is observed because of large creep strains and fast creep deformation, hence leading to a limited TGO growth. This unconventional failure mechanism is observed although the interfacial bond coat/top coat TGO thickening is accelerated by the mechanical applied stress beyond a given stress threshold.     

Transition of Crack from Type IV to Type II Resulting from Improved Utilization of Boron in the Modified 9Cr-1Mo Steel Weldment

The roles of boron and heat-treatment temperature in improving the type IV cracking resistance of modified 9Cr-1Mo steel weldment were studied. Two different heats of P91 steel, one without boron, designated as P91 and the other with controlled addition of boron with very low nitrogen, designated as P91B, were melted for the current study. The addition of Boron to modified 9Cr-1Mo steel has increased the resistance against softening in fine-grained heat-affected zones (FGHAZ) and intercritical heat-affected zones (ICHAZ) of the weldment. Creep rupture life of boron containing modified 9Cr-1Mo steel weldment, prepared from 1423?K (1150?°C) normalized base metal, was found to be much higher than that prepared from 1323?K (1050?°C) normalized base metal because of the stabilization of lath martensite by fine M₂₃C₆ precipitates. This finding is in contrast to the reduction in creep rupture life of P91 weldment prepared from 1423?K (1150?°C) normalized base metal compared with that of the weldment prepared from 1323?K (1050?°C) normalized base metal. The trace of failure path from the weld metal to ICHAZ in P91B weldment was indicative of type II failure in contrast to type IV failure outside the HAZ and base metal junction in P91 weldment, which suggested that boron strengthened the microstructure of the HAZ, whereby the utilization of boron at a higher normalizing temperature seemed to be significantly greater than that at the lower normalizing temperature.     

A Study on Microstructural Evolution and Dynamic Recrystallization During Isothermal Deformation of a Ti-Modified Austenitic Stainless Steel

Dynamic recrystallization (DRX) behavior in hot deformed (by uniaxial compression in a thermomechanical simulator in the temperatures range 1173 K to 1373 K [900 °C to 1100 °C]) Ti-modified austenitic stainless steel was studied using electron back scatter diffraction. Grain orientation spread with a “cut off” of 1 deg was a suitable criterion to partition dynamically recrystallized grains from the deformed matrix. The extent of DRX increased with strain and temperature, and a completely DRX microstructure with a fine grain size ~4 μm (considering twins as grain boundaries) was obtained in the sample deformed to a strain of 0.8 at 1373 K (1100 °C). The nucleation of new DRX grains occurred by the bulging of the parent grain boundary. The DRX grains were twinned, and a linear relationship was observed between the area fraction of DRX grains and the number fraction of Σ3 boundaries. The deviation from the ideal misorientation of Σ3 boundaries decreased with an increase in the fraction of Σ3 boundaries (as well as the area fraction of DRX) signifying that most Σ3 boundaries are newly nucleated during DRX. The generation of these Σ3 boundaries could account for the formation of annealing twins during DRX. The role of Σ3 twin boundaries on DRX is discussed.     

In Situ Measurement of the γ/γ′ Lattice Mismatch Evolution of a Nickel-Based Single-Crystal Superalloy During Non-isothermal Very High-Temperature Creep Experiments

The evolution of the ??/???? lattice mismatch of the AM1 single-crystal superalloy was measured during in situ non-isothermal very high-temperature creep tests under X-ray synchrotron radiation. The magnitude of the effective lattice mismatch in the 1273?K to 1323?K (1000?°C to 1050?°C) temperature range always increased after overheatings performed at temperatures lower than 1403?K (1130?°C). In contrast, a decrease of its magnitude was observed after overheatings at temperatures greater than 1453?K (1180?°C) due to massive dislocation recovery processes occurring at very high temperature.