Influence of precipitated phases on properties during prolonged aging in TP347H steel at 700 °C

The aging of austenitic stainless steel TP347H (18% Cr-12% Ni-1% Nb) was performed at 700 °C for 500, 800, 1500, 2500 and 3650 h. Microstructure, precipitates and mechanical properties were examined on aged materials to analyze the impact of microstructure on mechanical properties. These tests showed that the main precipitate of the TP347 specimen was Nb(C,N) while M₂₃C₆ carbides precipitated at the aging time of 500 h, with the coarsening of M₂₃C₆ and MX phases during prolonged aging. The fine and dispersive Nb(C,N) particle precipitation up to 1500 h aging is a benefit for hardness and creep resistance. After aging for 3650 h, σ phase precipitated. Meanwhile, coarsening of Cr₂₃C₆ and Nb(C,N) led to creep cavity and brittle intergranular fracture. No clear change in tensile properties at room temperature during aging were observed. A distinct decline in creep properties was caused by an average diameter increase and precipitation of σ phase and bulky Cr₂₃C₆.     

Precipitation in V bearing microalloyed steel containing low concentrations of Ti and Nb

Abstract

The paper describes the precipitation behaviour in a thermomechanically processed V bearing microalloyed steel containing small additions of Ti and Nb (0·007–0·008 wt-%) using analytical transmission electron microscopy. An intriguing aspect is the significant precipitation of titanium and niobium at these low concentrations, contributing to strength. A high density of multimicroalloyed precipitates of (V, Nb, Ti)(C, N) are observed instead of simple TiN, TiC, and NbC precipitates. They are characterised as cuboidal (45–70 nm), spherical (20–45 nm), irregular (20–45 nm), and fine (10–20 nm). Estimation of solubility products of carbides and nitrides of V, Nb, and Ti implies that the precipitation of titanium occurs primarily in austenite. Interphase precipitation of niobium occurs during austenite to ferrite transformation, while complete precipitation of vanadium takes place in the austenite–ferrite region close to completion of transformation. Substoichiometric concentrations of Ti and Nb, the presence of nitrogen, and the mutual extensive solubility of microalloying carbonitrides explains the formation of core shell (triplex/duplex) precipitates with highly stable nitrides ((Ti, Nb, V)N) in the core and carbides ((Ti, Nb, V)C) in the shell. The qualitative stochiometric ratios of triplex and duplex carbonitrides were Ti_0·53Nb_0·35V_0·12 and Ti_0·6V_0·4, Nb_0·51V_0·49 and Ti_0·64Nb_0·36. Extensive precipitation of fine carbides on dislocation substructures, and sub-boundaries occurred. They were generally characterised as vanadium carbide precipitates with ordered cubic L1₂ structure and exhibited a Baker–Nutting orientation relationship with the ferrite matrix. M₄C₃ types of carbides were also observed similar to the steel, having high concentrations of Ti and Nb.     

Effect of Nb on long-term creep life of JIS SUS304HTB and JIS SUS347HTB steels – heat-to-heat variation and life assessment of stainless steels

Heat-to-heat variation in creep life has been investigated for the 9 heats of JIS SUS 304HTB (18Cr–8Ni steel) and also for the 9 heats of JIS SUS 347HTB (18Cr–12Ni–Nb steel) in the NIMS Creep Data Sheets, mainly taking the effect of Nb into account. The heat-to-heat variation in creep life of 304HTB is mainly caused by the variation in precipitation hardening due to fine NbC carbides at short times, while it is mainly caused by the variation in available nitrogen concentration, defined as the concentration of nitrogen free from AlN and TiN, at long times. The heat-to-heat variation in creep life of 347HTB is mainly explained by the variation of boron concentration, 3–27 ppm, but not by the variation of solution temperature, Nb/C atomic ratio and phosphorus concentration. Boron reduces the coarsening rate of fine M₂₃C₆ carbides along grain boundaries, which enhances the grain boundary precipitation hardening.     

Microstructure and Modeling of the High‐Temperature Deformation Behavior of Carbide‐Hardened Superalloys

Thermal barrier coatings (TBC) have the potential to improve considerably the efficiency of stationary gas or aircraft turbines by increasing the operating temperature. This report describes the use of creep experiments and microstructure investigations in order to predict the deformation behavior of uncoated and TBC‐coated superalloys NiCr22Co12Mo9 and CoCr22Ni22W14. The results of mechanical tests and transmission electron microscopy investigations have been used as input data into two models in order to describe the hot deformation behavior. The deterioration in the creep properties of the superalloy NiCr22Co12Mo9 as a result of coating was due to the degraded state of the M₂₃C₆ precipitates in the substrate metal, as well as to the weakening of the solid solution‐ and precipitation‐hardening mechanisms responsible for the creep strength of the material, in the region of the boundary surface.     

Review of Z phase precipitation in 9–12 wt-%Cr steels

For high temperature applications, 9–12?wt-%Cr steels in fossil fired power plants rely upon precipitate strengthening from (V,Nb)N MX nitrides for long term creep strength. During prolonged exposure at service temperature, another nitride precipitates: Cr(V,Nb)N Z phase. The Z phases lowly replace MX, eventually causing a breakdown in creep strength. The present paper reviews the Z phase and its behaviour in 9–12?wt-%Cr steels including thermodynamic modelling, crystal structure, nucleation process and precipitation rate as a function of chemical composition. The influence of Z phase precipitation upon long term creep strength is assessed from several different 9–12wt-%Cr steel grades and alloy design philosophies.     

The effect of Ca and RE elements on the precipitation kinetics of Mg17Al12 phase during artificial aging of magnesium alloy AZ91

The effect of simultaneous alloying with Ca and rare earth (RE) elements on the age hardening kinetics of AZ91 was studied through the fitting of the Johnson-Mehl-Avrami (JMA) equation. The results showed that the addition of both Ca and RE elements not only suppress discontinuous precipitation of the Mg₁₇Al₁₂ phase during the age hardening process, but also decrease the alloy hardness. Fitting the JMA equation to the experimental data indicated that the phase transformation during age hardening of an alloy variant containing both Ca and RE (at 170 °C and 190 °C) and standard AZ91 (at 170 °C) takes place by the nucleation of precipitates on dislocations. In contrast, the precipitation during age hardening of AZ91 at 190 °C occurs via nucleation at grain boundaries. Although it was observed that the creep strength of age hardened specimens are lower than that of the as cast specimens, but age hardening treatment has lower deleterious influence on the creep resistance of the alloy containing Ca and RE in comparison with conventional AZ91. This may be ascribed to the decreased precipitation rate resulting from the addition of both Ca and RE elements.     

Microstructural evolution during creep of a hot extruded 2D70Al-alloy

Microstructural evolution during creep of a hot extruded Al–Cu–Mg–Fe–Ni (2D70) Al-alloy was investigated in this study using transmission electron microscopy (TEM). The samples for creep test were carried out two-stage homogenization, followed by extruding. The creep ultimate strength dropped and the temperature increased gradually from 312 to 117 MPa and from 423 to 513 K, respectively. The microstructural observation for the crept samples showed that the S′ phase coarsened with increased creep temperature and the aging precipitates transformed from S″ phase to S′ phase during creep process. Meanwhile, excess solute atoms in supersaturated solid solution dynamically precipitated to further form finer S′ phase and S″ phase, which pinned the dislocations and impeded the dislocation movements. Large amount of dislocations piled up around the micron-scale Al₉FeNi phase, and a lot of dislocation walls were generated along 〈220〉 orientation. S phase accumulates around these defects. The interaction between dislocations and precipitates was beneficial for the improved performances at elevated temperature.     

Microstructural Evolution of 2.25Cr-1.6W-V-Nb Heat Resistant Steel during Creep

2.25Cr-i.6W-V-Nb developed in Japan, is a low alloy heat resistant steel with good comprehensive properties. Influence of long term creep at elevated temperature on the structure of 2.25Cr-I.6W-V-Nb steel was studied in this paper, and the micromechanism of creep strength degradation was elucidated, too. Both TEM observation and thermodynamic calculation reveal that during creep the transformation occurs from M7C3 and M23C6 to M6C, which can be cavity nucleation sites. Besides, creep at 600℃ also leads to the decrease of dislocation density, the coarsening and coalescence of M23C6, the nucleation of cavities and development of cracks. The strength decrease of 2.25Cr-1.6W-V-Nb steel after long term creep is related to the decrease of dislocation hardening,precipitation hardening,solution hardening,the nucleation of cavities and development of cracks.     

High strength and ductile low density austenitic FeMnAlC steels: Simplex and alloys strengthened by nanoscale ordered carbides

Abstract

We introduce the alloy design concepts of high performance austenitic FeMnAlC steels, namely, Simplex and alloys strengthened by nanoscale ordered κ-carbides. Simplex steels are characterised by an outstanding strain hardening capacity at room temperature. This is attributed to the multiple stage strain hardening behaviour associated to dislocation substructure refinement and subsequent activation of deformation twinning, which leads to a steadily increase of the strain hardening. Al additions higher that 5 wt-% promote the precipitation of nanoscale L′1₂ ordered precipitates (so called κ-carbides) resulting in high strength (yield stress ～1·0 GPa) and ductile (elongation to fracture ～30%) steels. Novel insights into dislocation–particle interactions in a Fe–30·5Mn–8·0Al–1·2C (wt-%) steel strengthened by nanoscale κ-carbides are discussed.     

Thermal stability and mechanical properties of nanocrystalline Fe–Ni–Zr alloys prepared by mechanical alloying

The thermal stability of nanostructured Fe_100?x?y Ni _x Zr _y alloys with Zr additions up to 4 at.% was investigated. This expands upon our previous results for Fe–Ni base alloys that were limited to 1 at.% Zr addition. Emphasis was placed on understanding the effects of composition and microstructural evolution on grain growth and mechanical properties after annealing at temperatures near and above the bcc-to-fcc transformation. Results reveal that microstructural stability can be lost due to the bcc-to-fcc transformation (occurring at 700 °C) by the sudden appearance of abnormally grown fcc grains. However, it was determined that grain growth can be suppressed kinetically at higher temperatures for high Zr content alloys due to the precipitation of intermetallic compounds. Eventually, at higher temperatures and regardless of composition, the retention of nanocrystallinity was lost, leaving behind fine micron grains filled with nanoscale intermetallic precipitates. Despite the increase in grain size, the in situ formed precipitates were found to induce an Orowan hardening effect rivaling that predicted by Hall–Petch hardening for the smallest grain sizes. The transition from grain size strengthening to precipitation strengthening is reported for these alloys. The large grain size and high precipitation hardening result in a material that exhibits high strength and significant plastic straining capacity.     

High temperature deformation behavior of permanent casting AZ91 alloy with and without Sb addition

The elevated temperature deformation behavior of permanent cast magnesium alloy AZ91 with and without Sb addition has been investigated using slow strain rate (5.0 × 10^–4s^–1) elevated temperature tensile and constant load creep testing at 150°C and 50 MPa. The alloy with 0.4 wt% Sb showed a higher elevated temperature tensile strength and creep resistance due to the formation of thermal stable Mg₃Sb₂ precipitates and a smaller microstructure as well as the suppressing of the discontinuous precipitation. Plastic deformation of AZ91 based alloys is determined by motion of dislocation in basal plane and non-basal slip systems. The dislocation motion in a slip system is influenced by temperature, precipitates and other lattice defects. Dislocations jog, grain boundaries and/or precipitates are considered as obstacles for moving dislocations. The deformation twinning were founded in the creep process by TEM. Cross slip of dislocations was taken into account as the main softening mechanism for permanent cast AZ91 alloy during elevated temperature deformation process.     

Effect of dissolution and precipitation of Nb on the formation of acicular ferrite/bainite ferrite in low-carbon HSLA steels

Effect of dissolution and precipitation of Nb on the phase transformation during cooling was investigated. It is firstly recognized that either the formation of acicular ferrite or the separation of bainite ferrite could be adjusted by the preparation of the steel specimens with different amounts of solute Nb and Nb-precipitates in austenite (isothermally holding at 850 °C for different durations). An increase in isothermal duration at 850 °C would spawn more Nb(CN) precipitates, leading to a microstructural evolution from bainite ferrite to acicular ferrite/bainite ferrite dual phase, and eventually to acicular ferrite in the final microstructure. This could be explained by the solution of Nb in the austenite, due to the solute dragging effect of Nb, can decrease the A_r3 temperature and promote the formation of bainite ferrite, while the precipitation of NbC can increase the A_r3 temperature and promote the formation of acicular ferrite by increasing the nucleation sites of acicular ferrite. Thus, the properties of acicular ferrite/bainite ferrite dual phase steel can generally be improved by appropriately controlling the state of Nb (Nb(CN) as precipitates and Nb in solution) in the austenite before cooling, which provides a new approach to the modification of acicular ferrite/bainite ferrite ratio.     

Precipitation hardening in Inconel* 625

Abstract

The hardness of the nickel based superalloy Inconel 625, aged at 625, 700, and 760°C for different intervals of time ranging from 1 to 335 h, has been measured. Peak hardening is found to occur much earlier at 760°C than at 700°C. Also the peak hardness is higher at 700°C than at 760°C. The results have been discussed in terms of precipitation. Scanning electron microscopy revealed the presence of precipitates in specimens aged at 760°C for a longer time. Electron probe microanalysis results show these precipitates to be rich in Ni, Nb, and Mo indicating that these are γ″ precipitates of Ni₃ (Nb, Mo) type. Transmission electron microscopy confirmed that these are γ″ precipitates. It also suggests that nucleation takes place heterogeneously on dislocations and stacking faults. Longer aging causes somewhat uniform nucleation but still preferentially on the secondary defects. At 700°C γ′ precipitates have been observed in addition to γ″ precipitates. The orientation relationship between the precipitates and the matrix has also been determined.     

Effect of microalloying on aging of a Cu-bearing HSLA-100 (GPT) steel

Investigations were carried out on aging of a HSLA-100 steel containing Cu as the major alloying element and Nb, Ti and V as microalloying elements. The aging process after varying amounts of cold deformation was followed by hardness measurements and microstructural changes were studied using light and electron microscopy. Presence of Ti activates the formation of (Nb, Ti)C precipitates and completely suppresses the precipitation of Cu. Even a solution treatment at 1100°C is not sufficient to completely dissolve Nb and Ti in the matrix and undissolved (Nb, Ti)C precipitates were observed in oil quenched state. Strain induced aging at 400° C causes simultaneous coarsening of existing precipitates and nucleation of fresh carbides, which results in multi-stage hardening in this steel. Strong precipitate-dislocation interactions cause retardation in recrystallization of deformation structure leading to retention of high hardness levels even on prolonged aging     

Evolution of precipitated phases during prolonged tempering in a 9%Cr1%MoVNb ferritic-martensitic steel: Influence on creep performance

Ferritic-martensitic steels of the 9%Cr1%Mo type have been extensively used in power plant components, heat exchangers, piping and tubing, etc., due to an excellent combination of properties such as creep resistance, toughness and resistance to oxidation at high temperatures. In these steels the stabilizing role of MX carbonitrides (M = Nb, V; X = C, N) is one of the main factors responsible for the resistance under creep conditions. The control of precipitation and coarsening of MX phases during prolonged, high temperature tempering or post-weld heat treatment is then a key point to obtain the desirable microstructure and hence, to achieve high temperature resistance under service conditions.In the present contribution we report the evolution of the precipitated phases during heat treatment at 780 °C for increasing times in the range 40 min to 7 h for an ASTM A213 T91 steel. The Nb and V contents in solid solution were determined as a function of the time of treatment and maxima were observed for 5 and 5.66 h, respectively. Creep tests to rupture were also conducted at 600 °C - 190 MPa for as-treated specimens. A maximum creep rate was observed to occur in coincidence with the maximum values of Nb and V contents in solid solution. We suggest possible relationships between the observed second phase evolution and the creep resistance behavior.     

Formation of Re-containing Carbides in a Second Generation Directionally Solidi¯ed Ni Base Superalloy

The precipitates at grain boundary in a directionally solidified Ni base superalloy after heat treatment, aging at 975°C, and creep rupture test have been characterized. Besides the primary MC carbides and fine particles of μ phase, the Re-containing M₂₃C₆ was observed. The precipitation kinetics revealed that the formation of M₂₃C₆ was associated with the dissolution of μ phase and MC carbides. TEM image shows that the continuous precipitation of M₂₃C₆ particles effectively hinders the dislocation movement and strengthens the grain boundaries. The high strength of the alloy suggests that M₂₃C₆ carbides are beneficial to the properties although Re as an important matrix strengthening element was consumed.     

Influence of heat treatment and aging on microstructure and mechanical properties of Mg‐1.8Zn‐0.7Si‐0.4Ca alloy

In order to optimize the aging treatment of Mg‐1.8Zn‐0.7Si‐0.4Ca alloy, different times and temperatures of solid solution and age hardening were applied to the alloy specimens. Microstructures and mechanical properties of the specimens were investigated using the optical microscopy, field emission scanning electron microscopy equipped with an energy dispersive x‐ray spectrometer, x‐ray diffraction, hardness, and shear punch tests. The lowest hardness and strength were achieved by solution treating of the alloy at 500 °C for 8 h, presenting the optimal condition for solution treatment of the alloy. The microstructural examinations revealed three different precipitates consisting of CaMgSi, Ca₂Mg₆Zn₃, and Mg₂Si in the solid solution specimens. It was found that the highest peak hardness and strength are obtained by aging the alloy at 150 °C for 16 h. This condition was confirmed by differential scanning calorimetry (DSC) tests performed on the solid solution and aged specimens.     

Precipitate design for creep strengthening of 9% Cr tempered martensitic steel for ultra-supercritical power plants

Abstract

It is crucial for the carbon concentration of 9% Cr steel to be reduced to a very low level, so as to promote the formation of MX nitrides rich in vanadium as very fine and thermally stable particles to enable prolonged periods of exposure at elevated temperatures and also to eliminate Cr-rich carbides M₂₃C₆. Sub-boundary hardening, which is inversely proportional to the width of laths and blocks, is shown to be the most important strengthening mechanism for creep and is enhanced by the fine dispersion of precipitates along boundaries. The suppression of particle coarsening during creep and the maintenance of a homogeneous distribution of M₂₃C₆ carbides near prior austenite grain boundaries, which precipitate during tempering and are less fine, are effective for preventing the long-term degradation of creep strength and for improving long-term creep strength. This can be achieved by the addition of boron. The steels considered in this paper exhibit higher creep strength at 650 °C than existing high-strength steels used for thick section boiler components.     

Creep damage and grain boundary precipitation in power plant metals

Abstract

There is clear evidence that creep damage in power plant steels is associated with grain boundary precipitates. These particles provide favourable nucleation sites for creep damage such as grain boundary cavities and microcracks. Monte Carlo based grain boundary precipitation kinetics is combined with continuum creep damage mechanics (CDM) to model both the microstructural evolution and creep behaviour in power plant metals. It is found that grain boundary precipitates, such as M₂₃C₆ in most Cr containing ferritic steels, are harmful to the creep properties of the material, in line with experimental observations. It is also found that to improve the creep behaviour of the material, means should be found either to increase the proportion of MX type particles, such as VN, or to decrease or remove the larger grain boundary precipitates, such as M₂₃C₆. Hafnium has been ion implanted into thin foils of a 9 wt-%Cr ferritic steel to study the effect of hafnium on the grain boundary precipitation kinetics. It is found that the implantation of hafnium to the steel completely prohibits the formation of the common grain boundary M₂₃C₆ particles. Instead, two new types of precipitates are formed. One is hafnium carbide, which is an MX type precipitate, and is very small in size and has a much higher volume fraction as compared with the volume fraction of VN in conventional power plant ferritic steels. The other is Cr- and V-rich nitride of formula M₂N. CDM modelling shows that implantation of hafnium can markedly improve the creep property of the material. In addition, the replacement of M₂₃C₆ with hafnium carbide increases the concentration of Cr in the matrix and is expected to improve the intergranular corrosion resistance of the material.     

Stress dependence of the creep behaviors and mechanisms of a third-generation Ni-based single crystal superalloy

Elevated temperature creep behaviors at 1100℃ over a wide stress regime of 120–174 MPa of a thirdgeneration Ni-based single crystal superalloy were studied. With a reduced stress from 174 to 120 MPa,the creep life increased by a factor of 10.5, from 87 h to 907 h, presenting a strong stress dependence.A splitting phenomenon of the close-(about 100 nm) and sparse-(above 120 nm) spaced dislocation networks became more obvious with increasing stress. Simultaneously, a_0010 superdislocations with low mobilities were frequently observed under a lower stress to pass through γ'precipitates by a combined slip and climb of two a_0110 superpartials or pure climb. However, a_0110 superdislocations with higher mobility were widely found under a higher stress, which directly sheared into γ'precipitates.Based on the calculated critical resolved shear stresses for various creep mechanisms, the favorable creep mechanism was systematically analyzed. Furthermore, combined with the microstructural evolutions during different creep stages, the dominant creep mechanism changed from the dislocation climbing to Orowan looping and precipitates shearing under a stress regime of 137–174 MPa, while the dislocation climbing mechanism was operative throughout the whole creep stage under a stress of 120 MPa, resulting a superior creep performance.