Formation Of Z-phase Research Articles

Design of a new 11Cr martensitic steel and evaluation of its long-term creep rupture strengths

A new grade of 11 wt% Cr steel was designed in which no nitrogen was added to minimise the driving force of the Z-phase formation and hence to maximise the stability of the fine MX precipitate particles. But, 6 wt% Co was added to completely suppress the δ-ferrite formation in the matrix. The creep data were measured in the range of 650–725 °C at different stresses and rationalised on the basis of a new creep model, which revealed that the effect of stress on creep was equivalent to that of temperature. The model parameters so determined was used to predict the 100,000 h creep rupture strengths. The results showed that these long-term creep rupture strengths were higher than those for the P92 Steel, which is a 9 wt% Cr and the strongest steel grade among commercial 9–12 wt% Cr steels at present. The reliability of the long-term creep rupture strength predictions for the new steel is also analysed.

Modified Z-phase formation in a 12% Cr tempered martensite ferritic steel during long-term creep

The formation of modified Z-phase in a 12Cr1MoV (German grade: X20) tempered martensite ferritic (TMF) steel subjected to interrupted long-term creep testing at 550°C and 120 MPa was investigated. Quantitative volumetric measurements collected from thin-foil and extraction replica samples showed that modified Z-phase precipitated in both the uniformly-elongated gauge (fv: 0.23 ± 0.02 %) and thread regions (fv: 0.06 ± 0.01 %) of the sample that ruptured after 139 kh. The formation of modified Z-phase was accompanied by a progressive dissolution of MX precipitates, which decreased from (fv: 0.16 ± 0.02%) for the initial state to (fv: 0.03 ± 0.01%) in the uniformly-elongated gauge section of the sample tested to failure. The interparticle spacing of the creep strengthening MX particles increased from (λ3D: 0.55 ± 0.05 μm) in the initial state to (λ3D: 1.01 ± 0.10 μm) for the uniformly-elongated gauge section of the ruptured sample, while the thread region had an interparticle spacing of (λ3D: 0.60 ± 0.05 μm). The locally deformed fracture region had an increased phase fraction of modified Z-phase (fv: 0.40 ± 0.20%), which implies that localised creep-strain strongly promotes the formation of modified Z-phase.

Thermodynamic Modelling and Microstructural Study of Z-Phase Formation in a Ta-Alloyed Martensitic Steel.

A thermokinetic computational framework for precipitate transformation simulations in Ta-containing martensitic Z-steels was developed, including Calphad thermodynamics, diffusion mobility data from the literature, and a kinetic parameter setup that considered precipitation sites, interfacial energies and dislocation density evolution. The thermodynamics of Ta-containing subsystems were assessed by atomic solubility data and enthalpies from the literature as well as from the experimental dissolution temperature of Ta-based Z-phase CrTaN obtained from differential scanning calorimetry. Accompanied by a comprehensive transmission electron microscopy analysis of the microstructure, thermokinetic precipitation simulations with a wide-ranging and well-documented set of input parameters were carried out in MatCalc for one sample alloy. A special focus was placed on modelling the transformation of MX into the Z-phase, which was driven by Cr diffusion. The simulation results showed excellent agreement with experimental data in regard to size, number density and chemical composition of the precipitates, showing the usability of the developed thermokinetic simulation framework.

Effect of creep temperature on Z-phase formation in heat-resistant 9% Cr–3% Co martensitic steel

The Z-phase (CrVN) precipitation in a 9% Cr–3% Co martensitic steel during creep at 923 K and 948 K has been investigated with aim to establish the effect of temperature on the nucleation mechanism of these particles and their coarsening behavior. Ostwald ripening of VX carbonitrides strongly affects these processes. An increase in creep temperature significantly accelerates the transformation of the nanoscale MX carbonitrides into the Z-phase particles causing the Z-phase nucleation in a shorter time. Two different mechanisms of the Z-phase precipitation have been observed. Firstly, for creep tests at 923 K and 948 K with creep rupture time longer than 2000 h, the formation of the stable Z-phase particles with a tetragonal lattice occurs through in-situ transformation of a cubic lattice of the MX carbonitrides leading to a continuous flux of Cr atoms from the ferritic matrix into these particles. The coarsening Z-phase occurs at the expense of dissolution of VX carbonitrides. Secondly, for creep test at 948 K with duration of 773 h, the strain-induced metastable Z-phase with the cubic lattice nucleates on the V-rich MX/ferrite interfaces with following transformation into the stable Z-phase with the tetragonal lattice under creep testing with longer duration. Concurrently, extensive coarsening Cr-rich VX carbonitrides occurs independently. As a result, coarsening Z-phase leads to insignificant dissolution of VX carbonitrides. The creep strength breakdown appearance is not related to the formation and/or coarsening of the Z-phase at both temperatures.

The Effect of Creep and Long Annealing Conditions on the Formation of the Z-Phase Particles

The influence of the temperature, applied stress, degree of creep deformation, and aging time on the formation of Z-phase (CrVN) particles in 9% Cr 10Kh9K3V2MFBR martensitic steel at temperatures of 650 and 675°С upon long aging is studied in this work. The particles with a chemical composition determined by energy dispersive spectrometry as 50% (Cr + Fe) and 50% (V + Nb) are detected and identified as particles of the Z‑phase. An increase in the creep test time at both temperatures led to the growth of Z-phase particles. An increase in the temperature provokes earlier nucleation of small Z-phase particles with a change in the mechanism of their nucleation. An enlargement of particles is observed upon localization of plastic deformation. Long-term annealing at temperatures of 650 and 675°C does not lead to the formation of Z-phase particles. Plastic deformation has a crucial effect on the formation of Z-phase particles in steels containing 9% Cr. The temperature, time, applied stress, and strain localization are aggravating factors that cause the growth of this phase.

Peculiarities of xonotlite synthesis from the raw materials with different SiO2 activities

In this work, the peculiarities of xonotlite hydrothermal synthesis at 200 °C from lime and SiO2 materials with different pozzolanic activities (AP) were investigated: calcinated opoka (Ap = 170.1 mgCaO kg−1), granite sawing waste (Ap = 52.2 mgCaO g−1) and reagent SiO2·nH2O (Ap = 336.8 mgCaO kg−1). By XRD, DSC, TG, SEM, FT-IR methods have been shown that the formation of crystalline calcium silicate hydrates and the sequence of the intermediate phases existence are influenced not only by SiO2 component activity, but by other factors too. The use of the most active raw meal with SiO2·nH2O results in a very rapid formation of z-phase, C–S–H(I) and gyrolite, which hardly recrystallize into thermodynamically stable mineral—xonotlite. The impurities in the starting materials may promote the formation of some other compounds and retard the synthesis of stoichiometric ones: high content of Al-containing minerals in granite sawing waste (15.41% of Al2O3) predetermines that 1.13 nm tobermorite even after 72 h of hydrothermal curing did not recrystallize into xonotlite. Regardless of its average activity, calcinated opoka is an excellent material for the synthesis of crystalline calcium silicate hydrates. Amorphous SiO2 from opoka begins to react first, followed by tridymite and cristobalite. 1.13 nm tobermorite and xonotlite are formed at the beginning of the hydrothermal synthesis (4 h), and this greatly reduces the probability of the existence of amorphous phases.

STEM investigations of γ-TiAl produced by additive manufacturing after isothermal oxidation

Short-term isothermal oxidation of TiAl 48-2-2 alloy produced by Electron Beam (EBM) was investigated at 750, 800, 850 and 900 °C with special focus on the oxidation kinetics and metal-scale interface evolution. It was shown that at the lowest oxidizing temperature a passivating and thin mixture of alumina and titania is formed with subsurface Z-phase formation. At higher temperatures a porous and non-protective oxide scales formed with subsequent nitridation at the metal-scale interface. Moreover, long-term isothermal oxidation at 750 °C up to 3000 h in 500 h intervals was performed and related to the initial band microstructure of the alloy resulting from EBM process. Internal oxidation was observed on lamellar α2 / γ colonies while α-alumina mixed with TiN and Ti2AlN was found to form on equiaxed γ-TiAl regions. Analytical Scanning Electron Microscopy (STEM) aided with Energy Dispersive Spectroscopy (EDS) and Electron Energy Loss Spectroscopy (EELS) were used for investigations of the oxide scales as well as nitride layers formation at different temperatures.

Investigation of Ta-MX/Z-Phase and Laves Phase as Precipitation Hardening Particles in a 12 Pct Cr Heat-Resistant Steel

A 12 pct Cr martensitic/ferritic steel was designed and produced to study Laves and Z-phase as precipitation hardening particles under creep conditions (650 °C). According to thermodynamic calculations, W and Cu additions were selected to ensure the precipitation of Laves after tempering. It is known that Z-phase formation does not follow the classical nucleation theory. Indeed, MX particles are transformed into Z-phase by Cr diffusion from the matrix to the precipitate. Therefore, to promote fast Z-phase formation, Ta, Co, and N additions were used to produce Ta-MX, which will be transformed into Z-phase. The main result achieved was the precipitation of Laves after tempering, with a particle size of 196 nm. As regards to Z-phase, the transformation of Ta-MX into Z-phase after tempering was confirmed by the formation of hybrid nanoparticles of 30 nm. Although W and Ta have a low diffusion in the martensitic/ferritic matrix, characterization of the precipitates after isothermal aging revealed that Laves and Z-phase have fast growth kinetics, reaching 400 and 143 nm, respectively, at 8760 hours. Consequently, creep test at 650 °C showed premature failures after few thousand hours. Therefore, it is expected that future research in the field of martensitic/ferritic steels will focus on the growth and coarsening behavior of Laves and Z-phase.

Strain-induced Z-phase formation in a 9% Cr-3% Co martensitic steel during creep at elevated temperature

The size and distribution of Cr(V,Nb)N (Z-phase) particles in a 9Cr-3Co-2W-0.6Mo-0.1Ni-0.2V-0.06Nb-0.1C-0.05N-0.005B steel subjected to creep rupture test during 11,151 h at 650 °C under an applied stress of 100 MPa were studied. The replacement of V-rich (V,Nb)(C,N) carbonitrides by Z-phase was accelerated by plastic flow as suggested by a comparative analysis of these particles in different portions of crept specimen, namely, the grip section, the portion of uniform elongation, and the necked portions. Coarse “hybrid” Z-phase particles evolved throughout the crept specimen by an in situ transformation mechanism. The strain-induced Z-phase nucleated on the V-rich (V,Nb)(C,N)/ferrite interfaces, leading to the formation of numerous Z-phase particles with dimensions less than 50 nm in the necked portion.

Mechanism for Z-phase formation in 11CrMoVNbN martensitic heat-resistant steel

The mechanism for the formation of Z-phase was investigated for samples of an 11CrMoVNbN steel aged at 593°C for up to 50,000h. X-ray diffraction indicates that Z-phase appears after 5000h of aging, and its amount gradually increases up to 50,000h aging at the expense of mainly Cr2N. Transmission electron microscopy shows that Z-phase nucleates in the V-enriched rim region of Cr2N precipitates, which forms due to the diffusion of V into the precipitates from the matrix. Nucleated Z-phase tends to grow relatively rapidly compared with the preexisting precipitates such as Nb(C,N) and M23C6. Z-phase seems to consume Cr2N by the nucleation and growth mechanism rather than the transformation mechanism. The main difference in the aged samples is that the Nb content of Z-phase is lower and that Z-phase does not replace Cr2N completely, compared to the results of the crept samples.

The Role of Microstructure in Creep Strength of 9-12%Cr Steels

Tempered martensite lath structure (TMLS) plays a vital role in creep resistance of high chromium martensitic steels. Under creep conditions the TMLS could be stabilized by three agents: (i) a dispersion of boundary M23C6 carbides and Laves phase; (ii) a dispersion of M(C,N) carbonitrides, which are homogeneously distributed within ferritic matrix; (iii) substitutional alloying element within ferritic matrix. The boundary particles exert a large Zener drag force which effectively hinders migration of low-and high-angle boundaries. A dispersion of M(C,N) carbonitrides both within ferritic matrix and lath boundaries provides the pinning of mobile dislocations. This process is responsible for reliving long-range elastic stress field originated from lath boundaries. In addition, M(C,N) carbonitrides provide high threshold stress. Substitutional elements as W and Mo effectively slowing down diffusion in ferritic matrix retard climb of lattice dislocation that also prevents the aforementioned knitting reaction. The suppression of knitting reaction between lattice dislocation and low-angle boundaries prevents their transformation to subboundaries by concurrent operation of all three agent types. Depletion of W and Mo from solid solution leads to the occurrence of static recovery and precipitation of Laves phase at boundaries under long-term aging. This process is responsible for creep strength breakdown. The strain-induced formation of Z-phase at the expense of V-rich M(C,N) carbonitrides highly facilitates this process. However, slow strain-induced coarsening of M23C6 carbides and M(C,N) carbonitrides provides the suppression of the knitting reaction between mobile lattice dislocations and intrinsic dislocations of lath boundaries and replacement of TMLS by subgrain structure. Ostwald ripening of boundary M23C6 carbides and Laves phase leads to rapid creep rate increase with strain in tertiary creep and premature rupture owing to the formation of subgrain structure replaced TMLS and further subgrain growth.

Effect of Tungsten on a Dispersion of M(C,N) Carbonitrides in 9 % Cr Steels Under Creep Conditions

Two 9Cr–3Co–0.5Mo–VNb steels with 2 and 3 wt% W subjected to normalizing at 1050 °C and subsequent tempering at 750 °C for 3 h were crept at 650 °C under applied stresses ranging from 100 to 220 MPa. Both steels exhibit the creep strength breakdown at a rupture time of ~2000 h. The two-phase separation of M(C,N) carbonitrides to Nb- and V-rich particles occurs during tempering. No additional strain-induced coarsening of M(C,N) carbonitrides takes place under creep conditions. Long-term ageing leads to coarsening of both types of carbonitrides in the 9Cr–3Co–2W–VNb steel. 1 wt% W additives completely cease coarsening of Nb-rich M(C,N) particles at rupture times ≤2000 h and hinder their coarsening at higher times. Dimensions of V-rich M(C,N) particles in the 9Cr–3Co–3W–VNb steel are less than in the 9Cr–3Co–2W–VNb one. Moreover, in the 9Cr–3Co–2W–VNb steel the transformation of V-rich M(C,N) carbonitrides into Z-phase (CrVN) starts to occur at a rupture time of ~5000 h; particles of Z-phase with an average dimension of ~50 nm appear. At a rupture time of ~11,000 h, Z-phase particles with an average size of ~270 nm completely replace nanoscale V-rich carbonitrides. The increase in W content to 3 wt% slows down this process. Only separated particles of Z-phase with an average size of ~50 nm were found in the 9Cr–3Co–3W–VNb steel at a rupture time of ~16,000 h. No consuming of Nb-rich M(C,N) particles for the formation of Z-phase were found in both steels.

Effect of Nb addition on Z-phase formation and creep strength in high-Cr martensitic heat-resistant steels

The effect of Nb addition on the creep strength of high-Cr martensitic heat-resistant steel was investigated at temperatures ranging from 839 to 894K. The short-term creep properties were improved by Nb addition due to the enhanced MX precipitation. However, Nb addition deteriorated the steel's long-term stability resulting in an even shorter creep-rupture life than the original alloy. Onset of the accelerated degradation in Nb-containing steel was attributed to the gradual formation of Z-phase at the expense of M2N precipitates during creep exposure.

Heat-to-heat variation of creep strength and long-term stability of microstructure in Grade 91 steels

Heat-to-heat variation of long-term creep strength and microstructural changes was investigated in ASME Gr. 91 steels (heat name: MGC heat and MgC heat) by focusing on the effects of minor elements. The heat-to-heat variation of creep strength was not observed at 500°C and 550°C. At 600°C and 650°C, there was no significant difference in the creep strength up to 10,000h in the two heats. However, at 600°C, the creep strength of the MGC heat was clearly lower than that of the MgC heat at around 100,000h. The effect of Ni content (MGC heat: 0.28mass%, MgC heat: 0.04mass%) in the range of the specification (Ni ≤0.40mass%) on microstructural changes was examined. There was no significant difference in the dislocation structure after creep rupture in the two heats. No effect of Ni content was observed on the coarsening of M23C6 particles during the creep. In the MGC heat, a significant number of modified Z-phase particles were formed, consuming a large number of MX particles during the creep. On the other hand, the number of modified Z-phase particles of the MgC heat was lower after the creep rupture than with the MGC heat. An increase in the Ni content promotes modified Z-phase formation and eliminates MX particles during the creep, indicating that the deformation resistance can decrease after a long time in steel with high Ni content even in the range of the specification.

Effect of Creep Exposure on Microstructure and Mechanical Properties of Modified 9Cr-1Mo Steel

The prolonged exposure at elevated temperatures under stress significantly alters the microstructure of steels resulting in degradation of the mechanical properties. Therefore, the assessment of structural integrity of high temperature components needs the evaluation of microstructure and mechanical properties. In this investigation, modified 9Cr-1Mo steel has been subjected to creep tests at 923K and at stress levels of 100, 120 and 140MPa The creep tests have been interrupted at different periods to assess the change in the microstructure and mechanical properties. While extensive TEM studies were carried out to determine the variation on defect structure and precipitation behaviour of the steel, hardness and ball-indentation testing were employed to evaluate the mechanical properties of creep exposed samples. Microstructural analysis using electron microscopy indicates that distinct morphological modifications in the martensitic lath structure, area fraction of carbides and formation of embrittling phases, which associate well with the decrease in the strength parameters as determined by small specimen test results. While the sudden drop in the strength of the steel has been correlated to the formation of z-phase, the continuous decrease in the strength as a function of time fraction of creep has been attributed to dislocation-free subgrain formation and extensive coarsening of carbides. Present paper would discuss the correlation of these structural variations with the mechanical properties.

The influence of Zn containing components on the synthesis of Z-phase

The influence of Zn containing compounds on the formation of Z-phase during hydrothermal treatment at 200?C temperature was investigated. The molar ratio of primary mixtures was CaO/(SiO2 + ZnO) = 0.55 and ZnO/(SiO2 + ZnO) = 0; 0.025 (it corresponds 2.27 % ZnO in the mixture). Compounds containing Zn2+ ions were used: soluble in water Zn(NO3)2?6H2O and insoluble in water 5ZnO?2CO2?4H2O. It was determined that the formation of Z-phase increases approximately twice due to influence of Zn2+ ions from the mixture. The high crystalline compound formed after 4 hours, while in the mixtures without additives - after 8 h of hydrothermal synthesis at 200?C temperature. Moreover, Zn2+ ions penetrate into Z-phase crystal lattice by exchanging Ca2+ ions. It was observed that insoluble 5ZnO?2CO2?4H2O additive has no essential influence on the begining of Z-phase formation, but it promotes Z-phase recrystallization into gyrolite. The products were characterized using X-ray powder diffraction, simultaneous thermal analysis, FT-IR spectra and scanning electron microscopy.

Precipitates in normalized and tempered 10%Cr ferritic/martensitic steels

Precipitates in the two 10%Cr ferritic/martensitic steels in the normalized-and-tempered condition were investigated through transmission electron microscopy with an energy dispersive X-ray system. A large number of precipitates with large difference in size and block-like, or plate/rod-like, or sphere-like morphology have been observed in the two steels. Dominant precipitate phases in the two steels were Nb-rich carbide NbC and Cr-rich carbide M23C6. Z-phases have been found in the two steels. Two types of modified Z-phases, Cr(V,Nb)(C,N), with similar chemical composition and totally different crystal structures, body-centered tetragonal and NaCl-type cubic lattices, as well as original Z-phase, CrNb(C,N), were identified in the two steels. The modified and original Z-phases were observed to coexist with a clear orientation relationship within the same precipitate particles of the steel. The formation of Z-phase in the steels has also been discussed.

Z-PHASE SYNTHESIS USING REAGENTS AND INDUSTRIAL WASTE MATERIAL SILICA GEL

The process of synthesis of calcium silicate hydrate, called Z-phase, was examined under hydrothermal conditions (200 ºC under a saturated water steam pressure) using both pure CaO (98.7%) and amorphous SiO2·nH2O. The fineness of the starting mixture was found to have a crucial influence on the formation of the mentioned compound: the finer the SiO2·nH2O, the faster Z-phase is formed. For example, only traces of this calcium silicate hydrate were identified when the specific area of SiO2·nH2O was ~400 m2/kg, but when this parameter was increased to ~2 000 m2/kg, Z-phase was the dominant compound in the synthesis products. Also, attempts to synthesize Zphase from industrial material have been made. Silicagel, a waste of AlF3 production, was used as the main component because of its high amorphous SiO2 content (78.9%), adding a calculated amount of pure CaO (98.7%). The results have shown that the process of Z-phase formation in this case is analogous as compared with that of pure SiO2·nH2O with the specific area of about 2 000 m2/kg and that the mentioned compound is stable when the duration of the synthesis is 4–8 hours. Also, the transition of Z-phase into gyrolite – thermodynamically most stable phylosilicate compound group – occurs faster (after 24 hours instead of 48 when using pure SiO2·nH2O).DOI: http://dx.doi.org/10.5755/j01.ct.59.1.1524

Microstructural degradation of Gr.91 steel during creep under low stress

Microstructural changes during creep at 600 °C under 70 MPa were investigated in the case of interrupted Gr.91 steel samples by taking into account the dislocation structure and Z-phase formation. The creep life monotonically increased with a decrease in the applied stress at each temperature considered in the study. However, the long-term creep life was shorter than that determined from the short-term creep data at 600 °C and 650 °C, meaning premature failure. The subgrain size gradually increased during creep up to 70,000 h, after which rapid subgrain coarsening occurred. Preferential recovery of the subgrain structure occurred around the prior-austenite grain boundary (PAGB) after 50,000 h and 70,000 h. After creep rupture, subgrain recovery was observed over the entire area of each sample. Z-phase formation was clearly visible for 30,000 h after creep. The number density of the MX particles gradually decreased after 30,000 h because of Z-phase formation. After creep rupture, the number density of the MX particles was almost the same as that of the Z-phase particles. During creep, the V content of the Z-phase gradually increased but the Nb content decreased. Changes in the chemical composition of the Z-phase occurred after a longer time in Gr.91 steel than in 12Cr steel.

Investigations on coarsening of MX and M 23C 6 precipitates in 12% Cr creep resistant steels assisted by computational thermodynamics

Coarsening of MX and M 23C 6 precipitates in two 12% Cr high alloyed steels have been investigated at creep conditions of 650 °C, 100 MPa up to 10,000 h. The evolution of precipitates at different creep times was determined experimentally using scanning transmission electron microscopy (STEM). Phase equilibria were calculated with Thermocalc and simulations of coarsening of precipitates were carried out with Dictra. Both the STEM measurements as well as kinetic simulations indicate very low coarsening rates for MX and M 23C 6 precipitates. Dictra simulations showed a reduced influence of Laves phase and Z-phase formation on coarsening of M 23C 6.