Heterogeneous Creep Deformations and Correlation to Microstructures in Fe-30Cr-3Al Alloys Strengthened by an Fe<Subscript>2</Subscript>Nb Laves Phase

A new Fe-Cr-Al (FCA) alloy system has been developed with good oxidation resistance and creep strength at high temperature. The alloy system is a candidate for use in future fossil-fueled power plants. The creep strength of these alloys at 973 K (700 °C) was found to be comparable with traditional 9 pct Cr ferritic–martensitic steels. A few FCA alloys with general composition of Fe-30Cr-3Al-.2Si-xNb (x = 0, 1, or 2) with a ferrite matrix and Fe₂Nb-type Laves precipitates were prepared. The detailed microstructural characterization of samples, before and after creep rupture testing, indicated precipitation of the Laves phase within the matrix, Laves phase at the grain boundaries, and a 0.5 to 1.5 μm wide precipitate-free zone (PFZ) parallel to all the grain boundaries. In these alloys, the areal fraction of grain boundary Laves phase and the width of the PFZ controlled the cavitation nucleation and eventual grain boundary ductile failure. A phenomenological model was used to compare the creep strain rates controlled by the effects of the particles on the dislocations within the grain and at grain boundaries. (The research sponsored by US-DOE, Office of Fossil Energy, the Crosscutting Research Program).     

The effect of tungsten on dislocation recovery and precipitation behavior of low-activation martensitic 9Cr steels

The effect of W on dislocation recovery and precipitation behavior was investigated for martensitic 9Cr-(0,l,2,4)W-0.1C (wt pct) steels after quenching, tempering, and subsequent prolonged aging. The steels were low induced-radioactivation martensitic steels for fusion reactor structures, intended as a possible replacement for conventional (7 to 12)Cr-Mo steels. During tempering after quenching, homogeneous precipitation of fine W₂C occurred in martensite, causing secondary hardening between 673 and 823 K. The softening above the secondary hardening temperature shifted to higher temperatures with increasing W concentration, which was correlated with the decrease in self-diffusion rates with increasing W concentration. Carbides M₂₃C₆ and M₇C₃ were precipitated in the 9Cr steel without W after high-temperature tempering at 1023 K. With increasing W concentration, M7C3 was replaced by M₂₃C₆, and M₆C formed in addition to M₂₃C₆. During subsequent aging at temperatures between 823 and 973 K after tempering, the recovery of dislocations, the agglomeration of carbides, and the growth of martensite lath subgrains occurred. Intermetallic Fe₂W Laves also precipitated in the δ-ferrite grains of the 9Cr-4W steel. The effect of W on dislocation recovery and precipitation behavior is discussed in detail.     

Precipitation of Fe2W laves phase and modeling of its direct influence on the strength of a 12Cr-2W steel

Precipitation of Fe₂W Laves phase in a 12Cr-2W power plant steel is investigated by using transmission electron microscopy (TEM). Fe₂W Laves phase is found to be coherent with the matrix and has a stacking fault structure. The influence of formation of Fe₂W Laves phase on the yield strength of the steel is quantitatively evaluated. The modeling result indicates that the strengthening effect from the formation of Laves phase particles is diminished by the loss of solid solution strengthening of alloying elements, and, as a result, the strength of the steel remains similar. The effect of clustering and coarsening of Laves precipitates on the strength of the steel is also studied. It is showed that clustering and coarsening decreases the strengthening effect of Laves phase. The limitation of the modeling approach currently adopted is also discussed.     

Hardness Variation in P92 Heat‐Resistant Steel based on Microstructural Evolution during Creep

In order to optimize the strength of P92 heat‐resistant steel, the variation in hardness and microstructural evolution during creep were investigated. The results show that before crept for 1429 h at 873 K, the coarsening of M₂₃C₆ is the main factor to decrease the hardness. When the creeping time prolongs from 1429 to 6063 h, the increase of hardness is mostly attributed to the precipitation of a large amount of Laves phase. Thereafter, the coarsening of Laves phase leads to the decline in hardness. The precipitation hardening resulting from MX drops distinctly in spite of the slight growth during creep. It is important to control the growth of MX, decrease the coarsening velocity of M₂₃C₆ and keep Laves phase to be fine.     

Dynamic Precipitation of Laves Phase in FeCrAl Alloy with Zr Addition

FeCrAl alloy is one of potential candidates for accident-tolerant-fuel (ATF)-cladding materials due to its excellent oxidation and corrosion resistance at accident temperature, combining good mechanical properties at service temperature. Alloying strategy is an important way for improving comprehensive properties of FeCrAl alloy through the precipitation of fine Laves phase. Zr alloying can stabilize the Laves phase due to its lower diffusion coefficient and solubility in body-centered-cubic ferrite matrix. Herein, it is found that Zr addition changes the dynamic precipitation features of Laves phase in FeCrAl alloy during high-temperature deformation, from only one type of Fe₂M (M = Nb, Mo, Ta) Laves phase to Fe₂Zr combining Fe₂M-type Laves phase. The Fe₂Zr-type Laves phase precipitates dynamically first, and the interface precipitates between which with ferrite matrix creates more nucleation sites for subsequent precipitation of Fe₂M Laves phase. The results can be possibly applied for alloy design and microstructure tailoring in series of FeCrAl alloys used for ATF cladding in the near future.     

Creep Rupture Strength and Microstructure of Low C-10Cr-2Mo Heat-Resisting Steels with V and Nb

The new ferritic heat-resisting steels of 0.05C-10Cr-2Mo-0.10V-0.05Nb (Cb) composition with high creep rupture strength and good ductility have already been reported. The optimum amounts of V and Nb that can be added to the 0.05C-10Cr-2Mo steels and their effects on the creep rupture strength and microstructure of the steels have been studied in this experiment. The optimum amounts of V and Nb are about 0.10 pct V and 0.05 pct Nb at 600 °C for 10,000 h, but shift to 0.18 pct V and 0.05 pct Nb at 650 °C. Nb-bearing steels are preferred to other grades on the short-time side, because NbC precipitation during initial tempering stages delays recovery of martensite. On the long-time side, however, V-bearing steels have higher creep rupture strength. By adding V to the steels, electron microscopic examination reveals a stable microstructure, retardation during creep of the softening of tempered martensite, fine and uniform distribution of precipitates, and promotion of the precipitation of Fe₂Mo.     

Evaluation of structural stability and creep resistance of 9-12% Cr steels

During creep exposure of modified chromium steels lowering of solid solution strengthening due to precipitation of Laves phase as well as coarsening of all precipitates causes degradation of creep resistance. Two distinct domains of the stress dependence of creep rate and time to rupture have been observed in precipitation strengthened modified chromium steels. The stress characterizing the transition between these domains was found to be closely related to the Orowan stress. This stress consists in these steels of the contribution from large particles on subgrain boundaries (mainly M₂₃C₆ and during the limited time also Laves phase) and from small precipitates (Nb(C,N) and VN) inside subgrains. This has to be considered when measuring the interparticle spacing and calculating Orowan stress. Larson-Miller parametric equation is used to elucidate the necessity of long-term creep testing. By means of two heats of CrMoVNbN steel it is shown that reliable extrapolation of creep properties is possible only in a stress and temperature domain in which only one creep creep rupture mechanism operate. In the high stress domain Larson-Miller constant C_LM is well above 30 while in the low stress domain this constant does not exceed 25. When the extrapolation is based mainly on short-term creep tests, the C_LM constant is close to that valid in the high stress domain and therefore it overestimates long-term creep strength.     

Microstructural stability during creep of Mo- or W-bearing 12Cr steels

The evolution and stability of particulate phases during creep of molybdenum- or tungsten-bearing 12Cr steels have been investigated in considerable depth. The important finding is that the performance of Laves-phase precipitation in the molybdenum-bearing alloy is significantly different from that in the tungsten-bearing alloy. It is generally believed that such differences in kinetics will influence creep behavior. Data on Laves-phase precipitation kinetics as a function of time and temperature were quantified using the Wert-Zener equation in conjunction with the proprietary Thermo-Calc software, to determine equilibrium solute concentrations in these complex steels. The progressive depletion of Mo and W from the matrix as the particles of Laves phase evolve has been quantitatively modeled using experimental data obtained on both steels over a range of times and temperatures. The Isothermal coarsening rates of M₂₃C₆ and MX carbide particles were measured and found to occur at a constant volume fraction, in accordance with Ostwald ripening kinetics, with no significant differences in rates found between the two steels. The coarsening rates of M₂₃C₆ particles, found on subgrain boundaries, were consistent with a third-power dependence on particle radius, with an activation energy similar to that of volume diffusion. The smaller MX particles, which lay on subgrain-interior dislocation lines, were better explained by dislocation pipe diffusion, with a fifth-power dependence on particle radius and an activation energy approximately half that of volume diffusion.     

Structure and properties of high-temperature austenitic steels for superheater tubes

The structure and properties of high-temperature austenitic steels intended for superheater tubes are analyzed. Widely used Kh18N10T (AISI 304) and Kh16N13M3 (AISI 316) steels are found not to ensure a stable austenitic structure and stable properties during long-term thermal holding under stresses. The hardening of austenitic steels by fine particles of vanadium and niobium carbides and nitrides and γ′-phase and Fe₂W and Fe₂Mo Laves phase intermetallics is considered. The role of Cr₂₃C₆ chromium carbides, the σ phase, and coarse precipitates of an M ₃B₂ phase and a boron-containing eutectic in decreasing the time to failure and the stress-rupture strength of austenitic steels is established. The mechanism of increasing the stress-rupture strength of steels by boron additions is described. The chemical compositions, mechanical properties, stress-rupture strength, and creep characteristics of Russian and foreign austenitic steels used or designed for superheater tubes intended for operation under stress conditions at temperatures above 600°C are presented. The conditions are found for increasing the strength, plasticity, and thermodeformation stability of austenite in steels intended for superheater tubes operating at 700°C under high stresses for a long time.     

Effect of carbon concentration on precipitation behavior of M<Subscript>23</Subscript>C<Subscript>6</Subscript> carbides and MX carbonitrides in martensitic 9Cr steel during heat treatment

The distributions and precipitated amounts of M₂₃C₆ carbides and MX-type carbonitrides with decreasing carbon content from 0.16 to 0.002 mass pct in 9Cr-3W steel, which is used as a heat-resistant steel, has been investigated. The microstructures of the steels are observed to be martensite. Distributions of precipitates differ greatly among the steels depending on carbon concentration. In the steels containing carbon at levels above 0.05 pct, M₂₃C₆ carbides precipitate along boundaries and fine MX carbonitrides precipitate mainly in the matrix after tempering. In 0.002 pct C steel, there are no M₂₃C₆ carbide precipitates, and instead, fine MX with sizes of 2 to 20 nm precipitate densely along boundaries. In 0.02 pct C steel, a small amount of M₂₃C₆ carbides precipitate, but the sizes are quite large and the main precipitates along boundaries are MX, as with 0.002 pct C steel. A combination of the removal of any carbide whose size is much larger than that of MX-type nitrides, and the fine distributions of MX-type nitrides along boundaries, is significantly effective for the stabilization of a variety of boundaries in the martensitic 9Cr steel.     

The Computational Design of W and Co-Containing Creep-Resistant Steels with Barely Coarsening Laves Phase and M23C6 as the Strengthening Precipitates

Generally, Laves phase and M₂₃C₆ are regarded as undesirable phases in creep-resistant steels due to their very high-coarsening rates and the resulting depletion of beneficial alloying elements from the matrix. In this study, a computational alloy design approach is presented to develop martensitic steels strengthened by Laves phase and/or M₂₃C₆, for which the coarsening rates are tailored such that they are at least one order of magnitude lower than those in existing alloys. Their volume fractions are optimized by tuning the chemical composition in parallel. The composition domain covering 10 alloying elements at realistic levels is searched by a genetic algorithm to explore the full potential of simultaneous maximization of the volume fraction and minimization of the precipitates coarsening rate. The calculations show that Co and W can drastically reduce the coarsening rate of Laves and M₂₃C₆ and yield high-volume fractions of precipitates. Mo on the other hand was shown to have a minimal effect on coarsening. The strengthening effects of Laves phase and M₂₃C₆ in the newly designed alloys are compared to existing counterparts, showing substantially higher precipitation-strengthening contributions especially after a long service time. New alloys were designed in which both Laves phase and M₂₃C₆ precipitates act as strengthening precipitates. Successfully combining MX and M₂₃C₆ was found to be impossible.     

Effect of Tungsten on Long-Term Microstructural Evolution and Impression Creep Behavior of 9Cr Reduced Activation Ferritic/Martensitic Steel

The present study describes the changes in the creep properties associated with microstructural evolution during thermal exposures to near service temperatures in indigenously developed reduced activation ferritic-martensitic steels with varying tungsten (1 and 1.4 wt pct W) contents. The creep behavior has been studied employing impression creep (IC) test, and the changes in impression creep behavior with tungsten content have been correlated with the observed microstructures. The results of IC test showed that an increase in 0.4 pct W decreases the creep rate to nearly half the value. Creep strength of 1.4 pct W steel showed an increase in steels aged for short durations which decreased as aging time increased. The microstructural changes include coarsening of precipitates, reduction in dislocation density, changes in microchemistry, and formation of new phases. The formation of various phases and their volume fractions have been predicted using the JMatPro software for the two steels and validated by experimental methods. Detailed transmission electron microscopy analysis shows coarsening of precipitates and formation of a discontinuous network of Laves phase in 1.4 W steel aged for 10,000 hours at 823 K (550 °C) which is in agreement with the JMatPro simulation results.

     

Effect of Compositional Changes of Laves Phase Precipitate on Grain Boundary Embrittlement in Long-Term Annealed 9 Pct Cr Ferritic Steel

In 9 to 12 pct chromium steels, the high-temperature mechanical properties are known to be strongly dependent on the formation and coarsening of Laves phase precipitates at boundaries. During high-temperature deformation, the Laves phase precipitate coarsening to over a critical size has been considered to trigger cavity formation at the precipitate-matrix interfaces. This coarsening, accompanied by the diffusion of W, Mo, and Cr, should change the mechanical properties and chemical composition of both Laves phase precipitates and the matrix. In this study, we aimed to clarify the effects of compositional changes of Laves phase precipitates on cavity formation during coarsening. The values of the Fe/Cr and W/Mo ratios in Laves phase precipitates were shown to induce different levels of strain energy in the vicinity of the Laves phase precipitate, consequently promoting the formation of cavities. Therefore, the compositional change of Laves phase precipitates was found to play a critical role in the grain boundary embrittlement of high Cr steel at elevated temperature.

     

Investigation of precipitation behavior in a weld deposit of 11Cr-2W ferritic steel

This article is aimed at investigating the difference in precipitation behavior in the fine-grained heat-affected zone (FGHAZ), coarse-grained heat-affected zone (CGHAZ), and base metal for the welded joint of high Cr ferritic heat-resistant steel (11Cr-0.4Mo-2W-1Cu-V-Nb, normalized 1323 K×1 h and tempered 1033 K×1 h). Simulated HAZ (SHAZ) specimens were used, whose thermal cycles were controlled to be the same as those in the actual welded joint with peak temperatures of 1523 and 1173 K to represent CGHAZ and FGHAZ, respectively. Based on scanning electron microscopy (SEM) observation, it looks that the precipitates in FGHAZ specimens (1173 K) were fewer and larger than those in CGHAZ (1523 K) specimens and base metal specimens. This phenomenon implied that the growth and coarsening of precipitates in FGHAZ may play a role in the deterioration of creep properties and type IV cracking, which was observed in previous creep tests. X-ray diffraction analysis for the electrolytic extraction showed that the types of precipitates are the same for the 1173 K specimens and base metal specimens, including M₂₃C₆, MX, Laves phase, and μ phase. Further, the elemental analysis of the extraction showed that the mass percentages of Cr, W, and Mo in the precipitates to specimen mass were higher in the FGHAZ specimen than those in the base metal specimen, especially during the period between 600 and 2464 hours. Finally, a two-dimensional (2-D) model was proposed to simulate the precipitation behavior of the Laves phase.     

Structure and elevated temperature properties of carbon-free ferritic alloys strengthened by a laves phase

A Laves phase, Fe₂Ta, was utilized to obtain good elevated temperature properties in a carbon-free iron alloy containing 1 at. pct Ta and 7 at. pct Cr. Room temperature embrittlement resulting from the precipitation of the Laves phase at grain boundaries was overcome by spheroidizing the precipitate. This was accomplished by thermally cycling the alloys through theα →γ transformation. The short-time yield strength of the alloys decreased very slowly with increase in test temperature up to 600°C, but above this temperature, the strength decreased rapidly. Results of constant load creep and stress rupture tests conducted at several temperatures and stresses indicated that the rupture and creep strengths of spheroidized 1 Ta−7 Cr alloy were higher than those of several commercial steels containing chromium and/or molybdenum carbides but lower than those of steels containing substantial amounts of tungsten and vanadium. When molybdenum was added to the base FeTa-Cr alloy, the rupture and creep strengths were considerably increased. Formerly with Lawrence Berkeley Laboratory.     

Creep and creep rate curves of a 10cr-30mn austenitic steel during carbide precipitation

The effect of carbide precipitation on creep and creep rate curves was investigated for 10Cr-30Mn austenitic steel containing 0.003 to 0.55 wt pct carbon. After solution annealing, the specimens were subjected to creep testing at 873 K for up to 30 Ms (8300 hours). In the low-carbon steels containing below 0.1 wt pct carbon, where carbide precipitation scarcely occurred, the decrease in creep rate with time in the transient creep region was described by log έ = A - (1/3) log t, where A is a constant depending on stress and carbon concentration. On the other hand, in the high-carbon steels containing above 0.2 wt pct carbon, where extensive precipitation of M₂₃C₆ occurred, the creep rate decreased significantly at long times above 3 to 5 ks (1 hour), deviating from the preceding equation for the low-carbon steels. The Johnson-Mehl equation with the time exponent n = 2/3 provided a reasonable approximation for the significant decrease in creep rate at long times. This resulted from a stress-induced precipitation of M₂₃C₆ on dislocation lines produced by creep deformation. The rate constant of the Johnson-Mehl equation depended on carbon concentration but not on stress levels examined.     

Creep Behavior and Degradation of Subgrain Structures Pinned by Nanoscale Precipitates in Strength-Enhanced 5 to 12 Pct Cr Ferritic Steels

Creep behavior and degradation of subgrain structures and precipitates of Gr. 122 type xCr-2W-0.4Mo-1Cu-VNb (x = 5, 7, 9, 10.5, and 12 pct) steels were evaluated during short-term and long-term static aging and creep with regard to the Cr content of steel. Creep rupture life increased from 5 to 12 pct Cr in the short-term creep region, whereas in the long-term creep region, it increased up to 9 pct Cr and then decreased with the addition of Cr from 9 to 12 pct. Behavior of creep rupture life was attributed to the size of elongated subgrains. In the short-term creep region, subgrain size decreased from 5 to 12 pct Cr, corresponding to the longer creep strength. However, in the long-term creep region after 10⁴ hours, subgrain size increased up to 9 pct Cr and then decreased from 9 to 12 pct, corresponding to the behavior of creep rupture life. M₂₃C₆ and MX precipitates had the highest number fraction among all of the precipitates present in the studied steels. Cr concentration dependence of spacing of M₂₃C₆ and MX precipitates exhibited a V-like shape during short-term as well as long-term aging at 923 K (650  °C), and the minimum spacing of precipitates belonged to 9 pct Cr steel, corresponding to the lowest recovery speed of subgrain structures. In the short-term creep region, subgrain coarsening during creep was controlled by strain and proceeded slower with the addition of Cr, whereas in long-term creep region, subgrain coarsening was controlled by the stability of precipitates rather than due to the creep plastic deformation and took place faster from 9 to 12 pct and 9 to 5 pct Cr. However, M₂₃C₆ precipitates played a more important role than MX precipitates in the control of subgrain coarsening, and there was a closer correlation between spacing of M₂₃C₆ precipitates and subgrain size during static aging and long-term creep region.