Planar Spatial Arrangement of Precipitates in 0.3C-30Ni 18Cr-0.9Ti Austenitic Cast Steel Aged at 600 °C for 10 h

A method to determine the basic parameters describing planar two-dimensional (2D) spatial arrangements of precipitates is presented. It is based on the observation that the total number of particles (precipitates), the total area, and total perimeter length of features examined in the microstructure at each step of dilation/erosion transformation uniquely depend on their initial 2D geometry (size, shape, and mutual spatial arrangements). These parameters have a clear physical interpretation such as the distribution of distances between precipitate boundaries that are measured along the shortest pathway between them. It is also argued that the network of the shortest pathway represents the dendrite and sub-grain boundaries. On this basis, the width distributions of the dendrites and sub-grains were determined.     

Microstructure,Chemical- and Phase Composition of Sanicro 25 Austenitic Steel After Oxidation in Steam at 700 °C

Microstructural investigation of Sanicro 25 austenitic steel after steam oxidation at 700 °C for up to 5000 h was conducted using advanced analytical- and high-resolution electron microscopy methods. The study revealed the presence of Cr₂O₃ plates at the scale surface. Underneath the scale was a chromium-depletion zone that evolved with the oxidation process. In the area that was not affected by oxidation, numerous M₂₃C₆, ε-Cu precipitates and some W-rich Laves phase were formed.     

Steam Oxidation Resistance of Advanced Steels and Ni-Based Alloys at 700 °C for 1000 h

The aim of the work was to investigate corrosion resistance of highly alloyed steels and Ni-based alloys in a steam atmosphere for 1000 h at 700 °C. In these steam oxidation experiments, two solid solution strengthened alloys; Haynes^® 230^®, 617 alloy, two gamma-prime (γ′) strengthened alloys; 263 and Haynes^® 282^® and three Cr+Ni- rich stainless steels: 309S, 310S and HR3C austenitic steels were exposed. The study showed that the materials exposed commonly developed thin oxide scales; in Ni-based alloys, these consisted of mainly MnCr₂O₄ spinels and Cr₂O₃, with the exception of 617 alloy where NiCr₂O₄ spinels and Cr₂O₃ were found. In Fe-based alloys, Cr₂O₃, MnCr₂O₄ spinels, Fe,Mn(SiO)₄, and finally Fe₃O₄ developed. No evaporation of chromia has been found within 1000 h test period. Furthermore, the development of TiO₂ was not observed into a large extent in Haynes^® 282^® and 263 alloy, in contrast to the study performed at 800 °C under the same steam environment conditions.     

The Effect of Microstructural Evolution on Hardening Behavior of 2205 Stainless Steel in Long-Term Aging at 500 °C

The effect of microstructural evolution on hardening behavior of 2205 stainless steel in long-term aging at 500 °C was studied by optical microscope, scanning electron microscope, and transmission electron microscope. The results showed that the hardness of ferrite phase in matrix steadily increased with the aging time at the first stage of 4 months, presented a peak of hardness at about 5 months, and showed a downward trend for the aging time from 6 to 8 months, while the hardness of the austenitic phase remained constant. Analysis showed that the iron-rich α phase and the Cr-rich α′ phase generated by spinodal decomposition, Cr₂N precipitations, and Fe₂Mn (R-phase) were the main reasons for the generation of peak in hardness of ferrite phase. Further studies showed that some dislocation structure (changing with the aging time) in δ-ferrite of matrix is related to the microstructural evolution.     

Oxidation Behavior of Graded NiCrAlYRe Coatings at 900, 1000 and 1100 °C

A graded NiCrAlYRe coating was prepared by combining arc ion plating (AIP) with chemical vapor deposition (CVD) aluminizing. Quasi-isothermal oxidation tests of the graded NiCrAlYRe coating and the conventional NiCrAlYRe coating were performed in air at 900, 1000 and 1100 °C for up to 1000, 1000 and 200 h, respectively. The results showed that the graded NiCrAlYRe coating exhibited better long time oxidation resistance than the conventional NiCrAlRe coating. This favorable oxidation behavior was attributed to the rapid formation of a protective α-Al₂O₃ scale and a sufficient Al reservoir. The structures and morphologies of oxide scales varied under different oxidation conditions. θ-Al₂O₃ was observed on both coatings during oxidation at 900 °C, however, the graded coating showed more favorable conditions for θ-Al₂O₃ to grow than the conventional coating. For the graded coating, phase transformation from θ-Al₂O₃ to α-Al₂O₃ resulted in a sharp decrease in the parabolic rate constant k_p between 900 and 1,000 °C.     

Roles of Nb on the oxidation characteristics and oxide scale evolution of Fe-25Cr-35Ni-2.5Al-XNb alloys at 1000°C and 1100°C

The oxidation characteristics of Fe-25Cr-35Ni-2.5Al-_XNb (0, 0.6, and 1.2 wt%) alumina-forming austenitic alloys at 1000°C and 1100°C in air were investigated. Results show that Nb has an important effect on the high-temperature oxidation resistance. A bilayer oxide scale with a Cr₂O₃-rich outer layer and Al₂O₃-rich internal layer forms on the surface of the Nb-free alloy and exhibits a poor oxidation resistance at 1000°C and 1100°C. With Nb addition, both the 0.6Nb-addition and 1.2Nb-addition alloys exhibit better oxidation resistance at 1000°C. Because of the third element effect, Nb addition reduces the critical Al content and forms a single external protective Al₂O₃ scale, which greatly improves the oxidation resistance. After oxidation at 1100°C, niobium oxides (mainly Nb₂O₅) are formed on the surface of the 1.2Nb-addition alloy and destroy the integrity of the Al₂O₃ scale, which causes the formation of Cr-rich oxide nodules and eventually develops to be a loose bilayer oxide scale with NiCr₂O₄, Cr₂O₃, and Fe₂O₃ outer layers and Al₂O₃ inner layer.     

Effect of V–Ti Addition on Microstructure Evolution and Mechanical Properties of Hot-Rolled Transformation-Induced Plasticity Steel

In order to reveal the effect of V–Ti addition on the microstructure evolution and the mechanical properties of hot-rolled transformation-induced plasticity(TRIP) steel, two steels with 0.072 V–0.051 Ti steel(Bear-V–Ti steel) and 0.001 V–0.001 Ti steel(Free-V–Ti steel) were designed, respectively, and the comparison analyses were carried out by performing thermodynamic calculation and an experiment. With the thermodynamic calculation, the critical annealing temperature of a large fraction of retained austenite(～51%) obtained via solute enrichment was determined, and an optimized quenching at 650 °C and tempering at 200 °C adopted on the as-hot-rolled steel. The results show that the V–Ti TRIP steel displays more optimum mechanical stability during the tensile deformation, since the fraction and the mechanical stability of retained austenite are improved and the microstructure is also ultrarefined by V–Ti alloy precipitation. The yield strength of Bear-V–Ti steel increases from 650 to 800 MPa, and the ductility reaches 37%, showing that the comprehensive mechanical properties are greatly improved.     

Improvement of Oxidation Resistance of γ-TiAl at 900 and 1000 °C Through Hot-dip Aluminizing

Hot-dip aluminizing method and subsequent interdiffusion treatment were used to develop a TiAl₃ coating on Ti–45Al–2Cr–2Nb–0.15B (at.%) alloy. A two-phase coating consisting of an outer pure Al layer and an inner TiAl₃ layer formed on the alloy after the hot-dip and then a single phase TiAl₃ coating was obtained by using interdiffusion treatment. Oxidation of the TiAl₃ coating was conducted at 900 and 1000 °C. Both the interrupt oxidation and the isothermal oxidation tests indicated that the coating provided high protectiveness for the alloy. The coating was stable for at least 300 h during the interrupt oxidation at 900 and 1000 °C, and it was stable for at least 500 h at 1000 °C and 1000 h at 900 °C during the isothermal oxidation. The oxidation behavior of the coating was discussed in detail.     

Microstructure and Properties of SAE 2205 Stainless Steel After Salt Bath Nitrocarburizing at 450 °C

Nitrocarburizing of the type SAE 2205 duplex stainless steel was conducted at 450 °C, using a type of salt bath chemical surface treatment, and the microstructure and properties of the nitrided surface were systematically researched. Experimental results revealed that a modified layer transformed on the surface of samples with the thickness ranging from 3 to 28 μm changed with the treatment time. After 2205 duplex stainless steel was subjected to salt bath nitriding at 450 °C for time less than 8 h, the preexisting ferrite zone in the surface transformed into austenite by active nitrogen diffusion. The main phase of the nitrided layer was the expanded austenite. When the treatment time was extended to 16 h, the preexisting ferrite zone in the expanded austenite was decomposed and transformed partially into ε-nitride precipitate. When the treatment time extended to 40 h, the preexisting ferrite zone in the expanded austenite was transformed into ε-nitride and CrN precipitate. Further, a large amount of nitride precipitated from preexisting austenite zone. The nitrided layer depth thickness changed intensively with the increasing nitriding time. The growth of the nitride layer takes place mainly by nitrogen diffusion according to the expected parabolic rate law. The salt bath nitriding can effectively improve the surface hardness. The maximum values measured from the treated surface are observed to be approximately 1400 HV_0.1 after 8 h, which is about 3.5 times as hard as the untreated material (396 HV_0.1). Low-temperature nitriding can improve the erosion/corrosion resistance. After nitriding for 4 h, the sample has the best corrosion resistance.     

Effect of Steam Flow Rate and Sample Orientation on Steam Oxidation of Ferritic and Austenitic Steels at 650 and 700 °C

Steam oxidation of heat exchanger tubing is of growing interest as increasing the efficiencies of conventional pulverised fuel fired power plants requires higher steam temperatures and pressures. These new, more severe steam conditions result in faster steam oxidation reactions, which can significantly reduce the lifetime of boiler components. This paper reports some results from an investigation into the impact of steam flow rates and sample orientation on the steam oxidation of surface ground superheater tube materials. The results show that an increased steam flow rate not only causes faster oxidation rates but also a change in oxide scale morphology. In case of T23, it triggers formation of micro-layered inner oxide, whereas for T92 it promotes the formation of an outer haematite layer. For austenitic steels, the faster steam flow increases the formation of initially protective oxide scales, but also accelerates the growth of oxide nodules with prolonged exposure times.     

High Temperature Corrosion Behavior of Cold Spray Ni-20Cr Coating on Boiler Steel in Molten Salt Environment at 900 °C

Cold spray is an emerging technology that produces high density metallic coatings with low oxide contents and high thermal conductivity, which makes them ideal for high temperature corrosion resistance. In the current investigation, Ni-20Cr alloy powder was deposited on SA 516 boiler steel (0.19C-1.07Mn-0.020S-0.25P-0.010Si-balance Fe) by cold spray process. Oxidation kinetics was established for the uncoated and cold spray Ni-20Cr coated boiler steel in an aggressive environment of Na₂SO₄-60%V₂O₅ at 900 °C for 50 cycles by the weight change technique. X-ray diffraction, FE-SEM/EDAX, and x-ray mapping techniques were used to analyze the oxidation products. Uncoated steel suffered corrosion in the form of intense spalling and peeling of its oxide scale, which may be due to the formation of unprotective Fe₂O₃ oxides. The Ni-20Cr coating was successful in reducing the weight gain of the steel by 87.2% which may be due to the formation of oxides of nickel and chromium.     

Effect of Partial Replacement of Si with Al on the Microstructures and Mechanical Properties of 1000 MPa TRIP Steels

Two newly synthesized C-Mn-Si-Mo-Nb transformation-induced plasticity (TRIP) steels with and without Al addition were designed in order to achieve significant improvements in the mechanical properties. The effect of substitution of Si by Al on tensile properties and the microstructure of cold-rolled C-Mn-Si TRIP steel was investigated under different heat treatments. It was shown that a complex ultrafine microstructure composed of different phases was formed and two types of morphology for ferrite were detected (equiaxial and polygonal). The distribution of alloying elements was observed by using electron probe microanalysis. It was clear that C was concentrated in the retained austenite (RA) and small M/A (austenite/martensite) islands. The Al addition facilitated the formation of polygonal ferrite and increased the stability of the RA. The strain-hardening behavior was studied in detail. All the investigated specimens showed a very high strain-hardening exponent (instantaneous n) but their strain dependence was different. For the C-Mn-Si-Mo-Nb TRIP steel, the maximum n value was achieved when the strain was only about 0.04, while the n value of the Al substituted TRIP steel increased gradually until strains in the range of 0.07-0.10 were reached and the maximum value was achieved. As a result, the elongations of the steel with Al addition increased considerably without obvious deterioration of strength. It was the first time to find microtwinned martensite located between ferrite and bainitic ferrite after tensile deformation in the low alloy TRIP steel with Al.     

Synergistic Effect of NaCl and Water Vapor on the Corrosion of 1Cr-11Ni-2W-2Mo-V Steel at 500-700°C

The corrosion behavior of 1Cr-11Ni-2W-2Mo-Vsteel was studied in the presence of a NaCl deposit andwater vapor at 500-700°C. Results indicated thatcorrosion was not obvious without an NaCl deposit, while corrosion was accelerated remarkably by thesimultaneous presence of NaCl and water vapor at anytemperature. Optical microscopy (OM), X-ray diffraction(XRD), scanning electron microscopy (SEM), andelectron-probe microanalysis (EPMA) were used to analyze thecorrosion products. A mechanism of the synergisticeffect of water vapor and NaCl on corrosion of the steelis proposed.     

Effect of Addition of 4 % Al on the High Temperature Oxidation and Nitridation of a 20Cr–25Ni Austenitic Stainless Steel

In high temperature applications, the alumina forming austenites (AFA) have recently gained more focus. These utilise the advantageous effect of Al on oxidation resistance, and also have good mechanical properties. Two experimental alloys [20Cr–25Ni–1Mn–0.5Si–Fe (wt.%)] were prepared. To one of the alloys 3.77 wt.% Al was added. The alloys were studied in air and air/water at 700 °C and 1,000 °C, in a sulphidising/chlorinating environment at 700 °C and in a nitriding atmosphere at 1,000 °C. The time of exposure was 100 h, except for one 1,000 h exposure in air/water. At 700 °C in air and air/water, the AFA displayed lower mass gain than the reference material. After exposure in the sulphidising-chlorinating environment, the material displayed a surface alumina layer with some spallation. In air or air/water at 1,000 °C, internal aluminium nitride and alumina formation occurred, appreciably reducing the sound metal thickness. The nitridation was enhanced in the nitriding environment.     

Effect of Aging Isothermal Time on the Microstructure and Room-Temperature Impact Toughness of Fe–24.8Mn–7.3Al–1.2C Austenitic Steel with κ-Carbides Precipitation

The microstructure and impact toughness of the as-cast Fe–24.8Mn–7.3Al–1.2C austenitic steel after solution treatment and subsequent aging treatment were investigated in the present work. Research on the κ-carbides precipitation behavior was carried out by transmission electron microscope. The results show that nano-sized coherent κ-carbides were obtained in the as-solutionized steel after aging treatment, which produced precipitation hardening. After being aging treated at 550 °C for 1 h, the steel with regular hexagonal grain structure exhibited a good combination of yield strength (~?574 MPa) and room-temperature impact toughness (~?168 J). In the present steel, the typical cube-on-cube orientation relationship between austenite and κ-carbides was observed. However, due to the long aging isothermal time and high C content, the coarse intergranular κ′-carbide was formed and grew along the austenite grain boundary, which caused this orientation relationship to be destroyed and a dramatical increase of the coherency strain energy at grain boundary. Furthermore, serious embrittlement of grain boundaries caused that cleavage cracks trend to propagate along the grain boundaries. Accordingly, the room-temperature impact toughness decreased sharply. After aging isothermal time prolonging to 13 h, the Charpy V-notch impact toughness was only ~?5 J and fracture mode turned to fully brittle fracture accompanied with flat facets, shear cracks and well-developed secondary crack.     

Cyclic Oxidation Behaviour of 1Cr–0.5Mo (T11) Boiler Tube Steel and Its Weldments in Air at 900 °C

Oxidation behaviour of weldments at elevated temperature has become an object of scientific investigation. Weldments were prepared using shielded metal arc welding and tungsten inert gas processes to weld together 1Cr–0.5Mo (T11) boiler tube steels. This paper reports the oxidation behaviour of welded and unwelded 1Cr–0.5Mo (T11) boiler tube steel specimens after exposure to air at 900 °C under cyclic condition. The thermogravimetric technique was used to establish kinetics of oxidation. X-ray diffraction and scanning electron microscopy/energy-dispersive analysis techniques were used to analyse the oxidation products. The unwelded steel showed a higher oxidation rate (in terms of weight gain) than that of welded steels.     

Effect of Zn Additions on the Oxidation of Cu–2 at.% Al and Cu–4 at.% Al Alloys in 1 atm O2 at 800 °C

Four ternary Cu–Zn–Al alloys containing 5 or 10 at.% Zn and 2 or 4 at.% Al plus an alloy containing 2 at.% Al and 15 at.% Zn have been oxidized at 800 °C in 1 atm O₂, and their behavior has been compared with that of the corresponding binary Cu–Zn and Cu–Al alloys. For the alloy containing 4 at.% Al, which is already able to form external alumina scales, the addition of Zn is only effective in reducing the mass gain during the fast, initial-oxidation stage. Conversely, the addition of 15 at.% Zn to Cu–2Al is able to prevent the formation of external scales containing mixtures of the Cu and Al oxides, resulting in the formation of external alumina scales after an initial stage of faster rate, producing a limited third-element effect. Finally, the addition of Al to both Cu–5Zn and Cu–10Zn is able to prevent the internal oxidation of Zn, producing a kind of reversed third-element effect. Possible mechanisms for these effects are examined on the basis of general treatments concerning the scaling behavior of ternary alloys.     

Corrosion Behavior of Bare,Cr3C2-25%(NiCr), and Cr3C2-25%(NiCr)+0.4%CeO2-Coated Superni 600 Under Molten Salt at 900 °C

Cr₃C₂-25(NiCr) and Cr₃C₂-25(NiCr)+0.4%CeO₂ coatings were deposited on nickel-based superalloy Superni 600 by Detonation-gun technique. Studies were conducted on bare and coated alloys in molten salt environment (Na₂SO₄-25%NaCl) at 900 °C under cyclic condition. Characterization of the corrosion product was done using field emission scanning electron microscopy/energy dispersive spectroscopy and x-ray diffraction techniques. The bare Superni 600 shows penetration of corrosion beneath the metal layer thereby indicating internal oxidation. The coating of Cr₃C₂-25(NiCr) with 0.4%CeO₂ leads to the formation of adherent scale.