Effect of alloying on interfacial energy of precipitation/matrix in high-chromium martensitic steels

The effect of cobalt, tungsten, and boron on interfacial energy of precipitate/ferritic matrix in the 9% Cr martensitic steels on the base of creep tests at 650 °C under different applied stresses ranging from 80 to 220 MPa was investigated. An interfacial energy of M₂₃C₆ carbides, the Laves phase particles, and MX carbonitrides was estimated by comparison of theoretical curves obtained by Prisma software for the model steels for the exposure time of 2 × 10⁴ h with experimental data measured by TEM in the gage sections of crept specimens. Addition of 3 wt% Co to Co-free 9Cr2W steel led to about 1.7 times increase in the interfacial energy of M₂₃C₆ carbides and MX carbonitrides, whereas Co did not effect on the interfacial energy of the Laves phase. Increasing W from 1.5 to 3 wt% in the Co-containing steels led to increase in the interfacial energy of the Laves phase up to 0.78 J m^?2 under long-term exposure, whereas it did not effect on the interfacial energy of M₂₃C₆ carbides and MX carbonitrides. In the steel with increased B up to 0.012 wt% and decreased N to 0.007 wt%, a strong decrease in the interfacial energy of M₂₃C₆ carbides to 0.12 J m^?2 occurred. Change in the interfacial energy of the precipitates was analyzed in comparison with coarsening rate constant.     

9%Cr heat resistant steels: Alloy design, microstructure evolution and creep response at 650 °C

In this work 9%Cr alloys were designed supported by computational thermodynamic methods. Two sets of alloys were produced: 9%Cr alloys with 0.1%C and 0.05%C and 9%Cr alloys containing ∼0.03% Ti with 0.1%C and 0.05%C (always wt%). Microstructure investigations showed good agreement with the predicted phases of the thermodynamic modeling. The volume fraction of precipitated M₂₃C₆ carbides is directly related to the carbon content of the alloys. For Ti-containing alloys the precipitation of nano-sized Ti-rich MX carbonitrides was observed. The microstructure evolution (sub-grain and particle size) during creep at 650 °C/100 MPa was investigated by STEM-HAADF. The sub-grain size evolution and the coarsening of precipitates (MX carbonitrides, M₂₃C₆ and Laves phase) were more pronounced for Ti-containing alloys. 9Cr alloys without Ti and with low carbon content presented the highest creep strength of all investigated alloys.     

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.     

Interpretation of creep behaviour of a 9Cr–Mo–Nb–V–N (T91) steel using threshold stress concept

Abstract

The creep behaviour and the microstructural evolution of a 9Cr–Mo–Nb–V (T91) steel were extensively evaluated by means of short term constant load creep tests and TEM analysis. Statistical analysis of the microstructural data revealed that the precipitated phases M₂₃ C₆ (where M is a metal, mainly Cr or Fe) and MX (where M is Nb or V, and X is C and/or N) were subject to coarsening during creep exposure. The coarsening law and its dependence on applied stress were identified, and the model was used to predict the magnitude of the Orowan stress at the time corresponding to the minimum creep rate. The minimum creep rate dependence on applied stress at 873 K was described by incorporating the threshold stress concept in a power law with stress exponent n = 5. In the resulting phenomenological model, the strengthening effect of the dispersed phases was thus expressed by a threshold stress proportional to the Orowan stress.     

Contribution of coarsening of MX carbonitrides to creep strength degradation in high chromium ferritic steel

Abstract

The coarsening process of MX carbonitrides during creep and its effect on creep behaviour have been investigated for P92 steel (9Cr-0.5Mo-1.8W-VNb). At 1023 K after long term aging, creep rupture strength and tertiary stage creep rate rapidly decrease and increase, respectively. The stress exponent n of minimum creep rate decreases with increasing temperature, and the value of n at 1023 K was evaluated to be 5.7. MX carbonitrides coarsen during creep and the coarsening process corresponds to the early stage of Ostwald ripening. Creep deformation accelerates the coarsening of the MX carbonitrides. At 1023 K, the coherent strain between the MX carbonitrides and matrix reduces to a negligibly small value at times longer than 550 h, leading to a decrease in the resistance to dislocation motion. This contributes to the rapid increase of creep rate in the tertiary stage and the abrupt decrease of creep rupture strength in the long term.     

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.     

Carbide changes and sulphur segregation in erMa steel during creep

An investigation was carried out into microstructural changes at the grain boundary of interrupted creep-tested and long-term heated 2sCr-1Mo steel by means of transmission electron microscopy with energy dispersive X-ray spectrometry. Grain boundary precipitates alternated their types of M₇C3 carbides with M₆C carbides, and were spheroidized during creep or long-term heating. These carbide changes and spheroidization of precipitates were found to be accelerated by creep stress. Sulphur migrated to segregate at the interface between M₆ C and the matrix, while this phenomenon was not observed at the Ms3 carbide or precipitate-free grain boundary. Consequently it was considered that the intergranular creep damage in 2sCr-Mo steel was caused by a reduction in interfacial energy due to the sulphur segregation at the interface between the matrix and M₆C carbides.     

The effect of creep deformation on the stability of intergranular carbide dispersions in an austenitic stainless steel

The stability of intergranular TiC in a 20% Cr-30% Ni, Ti stabilized stainless steel and the transformation of TiC to M₂₃C₆, has been investigated as a function of creep deformation over a wide range of stresses at 800° C. It was found that diffusion creep does not make a significant contribution to the general ageing process or to the transformation of TiC to M₂₃C₆. However, dislocation creep strongly accelerates this transformation and increases the general rate of coarsening of intergranular carbides. It is concluded that this acceleration occurs through the combined action of an increase in the number of available nucleation sites (extrinsic grain boundary dislocations) and dislocation enhanced diffusion.     

An improvement in creep strength of thermo-mechanical treated modified 9Cr-1Mo steel weld joint

Modified 9Cr-1Mo steel weld joints generally experience the type IV premature failure in the intercritical region (ICR) of HAZ under long term creep exposure at high temperature. Possibility of improving the resistance of this joint to type IV cracking through thermo-mechanical treatment (TMT) of the steel has been explored. Weld joints have been fabricated from the TMT and conventional normalized and tempered (NT) steels using electron beam (EB) welding process. Creep tests have been carried out on NT and TMT steels joint at 923 K (650°C) and 110–100 MPa applied stress. Creep rupture life of the TMT weld joint was significantly higher than the NT steel weld joint. Significant variations of microstructural constituents such as M₂₃C₆ precipitate; lath structure and hardness across the joint have been examined in both the joints. The coarser M₂₃C₆ precipitate and lath, and subgrain formation in the ICR resulted in the soft zone formation and was predominant in the ICR of NT steel joint. The enhanced MX precipitation through TMT processing and reduction in coarsening of M₂₃C₆ precipitate under thermal cycle resulted in improved creep rupture strength of TMT steel weld joint.     

Precipitates change of ferritic‐martensitic 11 % chromium steel under short‐term creep

A ferritic‐martensitic (FM) 11 % chromium steel with final heat treatment was subjected to a short‐term creep test at a stress of 150 MPa and 600 °C for 1100 h in order to study the change of precipitates in the steel during the creep test. Except for Nb‐rich metall carbides (MC, M₂₃C₆) and Laves phases, Fe‐W‐Cr‐rich M₆C (based on Fe₃W₃C) carbides forming during the creep test were also identified in the crept steel by electron diffraction and x‐ray diffraction in combination with energy dispersive x‐ray analysis of extraction carbon replicas. The identified M₆C carbides have a fcc crystal structure, a metallic element composition of approximately 44Fe, 32 W, and 20Cr in atomic %, and large sizes ranging from 100 nm to 300 nm in diameter. The M₆C carbides are a dominant phase in the crept steel. M₆X precipitates are generally not easy to form during high temperature creep, even if it is a long‐term creep, in ferritic‐martensitic 9–12 % chromium steels with a final heat treatment. The present work provides the evidence for the M₆C carbides forming during short‐term creep in ferritic‐martensitic high chromium steels. The formation of the M₆C carbides was discussed.     

Micromechanical behavior of a fine-grained China low activation martensitic (CLAM) steel

Micromechanical behavior of a fine-grained China Low Activation Martensitic (CLAM) steel under nanoindentation was studied in this work. The grain size of the as-prepared 0.1Ti-CLAM steel is ∼5 μm and the average diameter of the spherical precipitates is ∼5 nm. Both elastic modulus and hardness decrease with increasing contact depth of the nanoindenter, following an exponential decreasing function. The abnormally large contact depths should be resulted from defect concentration under the indenter. The effect of nanosized precipitates on hardness is responsible for the pop-ins occurring in the load-depth curves, corresponding to the blockage of nanosized precipitates to the dislocation movement. Nanosized VC and M₂₃C₆precipitates with the volume fractions of 0.32% and 1.21% can be identified, respectively. Different strengthening mechanisms originated from the two types of nanosized precipitates. The blockage of dislocations by VC particles leads to an Orowan strengthening whilst dislocations could cut through theM₂₃C₆particles because of the large size of the particles. The strengthening effects originated from the VC and M₂₃C₆ precipitates lead to the strength increase of ∼448 MPa and ∼254 MPa, respectively.     

Creep behavior of INCOLOY alloy 617

The microstructural features of INCOLOY alloy 617 in the solution annealed condition and after long-term creep tests at 700 and 800 °C were characterized and correlated with hardness and creep strength. Major precipitates included (Cr,Mo,Fe)₂₃C₆ carbides and the δ-Ni₃Mo phase. M₆C and MC carbides were also detected within the austenitic grains. However, minor precipitates particularly γ′-Ni₃(Al,Ti) was found to play an important role. At different exposure temperatures, the microstructural features of the Ni–22Cr–12Co–9Mo alloy changed compared with the as-received condition. The presence of discontinuously precipitated (Cr,Mo,Fe)₂₃C₆ carbides and their coarsening until the formation of an intergranular film morphology could be responsible both for a reduction in rupture strength and for enhanced intergranular embrittlement. The fraction and morphology of the γ′-phase, precipitated during exposure to high temperature, also changed after 700 or 800 °C exposure. At the latter test temperature, a lower volume fraction of coarsened and more cubic γ′ precipitates were observed. These microstructural modifications, together with the presence of the δ-phase, detected only in specimens exposed to 700 °C, were clearly responsible for the substantially good creep response observed at 700 °C, compared with that found at 800 °C.     

High temperature creep behaviour of an Ni-Cr-W-B alloy

The high-temperature creep behaviour of a solid-solution strengthened Ni-Cr-W-B alloy was studied, with emphasis on microstructural parameters. Creep strength was determined from tests conducted at 925°C/40 MPa. Various techniques of analytical electron microscopy were used to characterize the microstructure and microchemical composition. A number of microstructural parameters which promote creep strength, including (1) pinning of grain boundaries by tungsten-rich M₆C carbide, (2) relatively low stacking-fault energy, and (3) boron segregation to M₂₃C₆ carbide, were identified. However, their beneficial effects were suppressed by the initial presence of discontinuously precipitated M₂₃C₆ carbide at grain boundaries which accelerated intergranular cracking. Suppression of the discontinuous grainboundary reaction and a significant improvement in creep strength could be achieved by a proper heat treatment which appeared to induce a sufficiently high defect density promoting intragranular carbide precipitation. Competition between intergranular and intragranular precipitation was found to be influenced by an external stress. Strengthening by intragranular carbide precipitates appeared to occur by an attractive interaction with dislocations. Dislocations bowing out at subboundaries, cross-slip, motion of jogged screw dislocations and generation of dislocations at high-angle grain boundaries appeared to operate simultaneously as strain-producing mechanisms during steady-state creep.     

Study of nucleation,growth and coarsening of precipitates in a novel 9%Cr heat resistant steel: Experimental and modeling

Nucleation, growth and coarsening of three different precipitates (NbC, M₂₃C₆ and V(C,N)) in a novel 9%Cr heat resistant steel designed by the authors were investigated. The microstructure evolution after tempering (780 °C/2 h) and after creep (650 °C/100 MPa) was characterized using transmission electron microscopy in the scanning mode (STEM). Thermodynamic and kinetic modeling was carried out using the softwares Thermo-Calc, DICTRA and TC-PRISMA. The Thermo-Calc software predicted formation of NbC, V(C,N) and M₂₃C₆ carbides at the tempering temperature of 780 °C. STEM investigations revealed that M₂₃C₆ precipitated on prior austenite grain boundaries and lath or block boundaries whereas NbC and V(C,N) were located within sub-grains. Simulations by TC-PRISMA showed that M₂₃C₆, NbC and V(C,N) particles nucleation begins as soon as the tempering treatment starts and it is completed in a very short time, reaching the equilibrium volume fraction after 40 s for M₂₃C₆, 100 s for NbC and 80 s for V(C,N). Best agreement between simulations and experimental investigations was found for low interfacial energy values of 0.1 J m⁻². Both STEM measurements as well as DICTRA simulations indicate very low coarsening rate for both kind of precipitates. Creep tests up to 4000–5000 h suggest that this special combination of NbC, V(C,N) and M₂₃C₆ may provide increased pinning of dislocations reducing boundary migration therefore enhancing creep strength.     

Carbide behaviour during high temperature low cycle fatigue in a cobalt-base superalloy

The carbide behaviour of a directionally solidified cobalt-base superalloy has been investigated after low cycle fatigue at 900°C. During fatigue, primary carbides, M₇C₃ and MC, decomposed sluggishly and a great amount of secondary carbide, chromium-rich M₂₃C₆ precipitated. The inhomogeneous distribution of M₂₃C₆ brought about a different dislocation substructure. In the vicinity of the primary carbides, densely-distributed fine M₂₃C₆ pinned up dislocations effectively, resulting in a uniform distribution of dislocations, while in the interior of grains, since precipitates were coarse and scarce, dislocations were arranged in a planar array and piled up in the front of the precipitates. M₂₃C₆ also acted as an obstacle deflecting fatigue crack. Primary carbides on the surface of specimens were oxidizied preferentially, causing a precipitate depletion around them. The oxidized primary carbides were crack initiation sites. The primary carbides hindered fatigue crack propagation, causing the formation of shear steps.     

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.     

Precipitation in 9Cr–1Mo steel after creep deformation

The carbides present after creep testing a 9Cr–1Mo steel at 566 °C over a range of stress levels giving rupture times of up to 7300 h have been characterized and identified using a transmission electron microscopy, energy-dispersive X-ray spectroscopy and electron diffraction. The initial carbide precipitates present were M₇C_3, (NbV)C and VC and it was determined that M₆C carbide precipitates were present in all specimens after elevated temperature exposure for greater than approximately 1700 h. No precipitation of M₂₃C₆ was detected. The evolutionary sequence from the initially present carbides during high temperature exposure involved the formation of the stable M₆C carbide directly, without the intermediate formation of M₂₃C₆, as is reported to occur in other Cr–Mo steels.     

基于微观组织演化的P91钢长时蠕变寿命预测

通过对P91耐热钢在高温长时蠕变过程中微观组织演化行为的综合考察,探讨了影响其长时蠕变寿命的主要因素,其中包括强化相(M_(23)C_6、MX)与析出相(Laves、Z相)的粗化现象以及和位错间的交互作用等。在此基础上,通过对蠕变幂率本构方程中耦合相应内应力参量,并结合Monkman-Grant方程,从微观组织演化的角度建立了P91耐热钢长时蠕变寿命预测模型。最后利用该模型对873K(600℃)时的P91耐热钢的相关蠕变寿命进行了预测,结果显示其计算数值与实验数据吻合较好,从而进一步表明基于微观组织演化的预测模型在P91耐热钢长时蠕变寿命的研究中具有重要意义。     

Nano-sized precipitates in 11 % Cr ferritic-martensitic steel after thermomechanical treatment

9 %–12 % Cr ferritic/martensitic steels with a good long-term creep strength at temperatures up to 650 °C and higher are being developed in order to increase steam temperature of coal-fired power plants.Thermomechanical treatment can effectively enhance the mechanical properties of high-Cr ferritic/martensitic steels mainly due to plenty of nano-sized precipitates produced by thermomechanical treatment. Nano-sized precipitates in an 11 % Cr ferritic/martensitic steel produced by a thermomechanical treatment, including warm rolling at 650 °C plus tempering at 650 °C for 1 h, were investigated by transmission electron microscopy. The average size of precipitates in the steel after the thermomechanical treatment was determined to be about 30 nm in diameter, which is only one-third of the average size of precipitates in the steel with the normalized and tempered condition. A large number of Cr-rich precipitates having an average diameter of about 25 nm in the steel produced by the thermomechanical treatment were identified as Cr-rich M₂C carbide with a hexagonal crystal structure, rather than M₂₃C₆ or MX phase. The plenty of nano-sized Cr-rich M₂C carbides were dominant phase in the steel after the thermomechanical treatment. The reason why prior precipitate phase formed in the steel during the thermomechanical treatment was Cr-rich M₂C carbide is also discussed.