Coarsening Of M23C6 Research Articles

Evolution of Microstructural and Mechanical Properties of Alloy 617B During Service on a Key-Component Test Platform at 700 °C.

The evolution of the microstructural and mechanical properties of alloy 617B during long-term service on a key-component test platform at 700 °C was systematically investigated. The precipitation behavior and size changes of the M23C6 and γ' phases were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that carbide M23C6 precipitated in the form of discontinuous particles, plates, or needles at grain boundaries and within grains, while the γ' phase had a spherical shape and was distributed in a dispersed manner. With prolonged service time, both the M23C6 and γ' phases gradually coarsened. After 24,000 h of service, the yield strength, tensile strength, and Brinell hardness of alloy 617B significantly increased; however, the impact toughness decreased, accompanied by intergranular embrittlement. The increase in precipitate volume fraction and its contribution to the strength of the alloy were evaluated by a precipitation strengthening model. The coarsening of M23C6 was identified as the main cause of embrittlement. The findings of this study provide important experimental data and theoretical support for the stability of 617B alloys under long-term high-temperature service conditions.

Microstructure evolution and deformation mechanisms of service-exposed P91 steel via interrupted uniaxial creep tests at 660 °C

Creep degradation behaviour of service-exposed P91 steel is evaluated during interrupted creep tests at 660 °C and 80 MPa using a number of material characterization techniques including transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), energy dispersive spectroscopy (EDS) and optical microscopy (OM) to identify the microstructural evolution and the associated deformation mechanisms. Microhardness has also been measured in order to evaluate the softening mechanism. Under the creep conditions examined, microstructural degradation is found to be governed by the disappearance of the lath sub-structure, lath widening and recrystallization, as well as dislocation density reduction, coarsening of M23C6 and creep cavitation while MX and Laves phases are stable. Hardness evolution, extrapolated from hardness data obtained from uniaxial creep tests, is used to characterize the softening of the material. On this basis, hardness decrease is justified in term of the aforementioned microstructural changes. Implications of the findings for specific in-service life management in thermal plant piping systems are addressed.

Effect of Boron on Microstructures and Low-Temperature Impact Toughness of Medium-Carbon CrMo Alloy Steels with Different Quenching Temperatures

The effect of boron (B) on the microstructures and low-temperature impact toughness of medium-carbon CrMo steel quenched at 870~1050 °C and tempered at 600 °C was studied via Charpy impact testing and microstructure characterizations. The results showed that with an increasing B content from 0 to 50 ppm, the low-temperature impact toughness deteriorated significantly at quenching temperatures (Tq) lower than 950 °C but increased at a higher Tq of 1050 °C. Undissolved M2B particles remained and coarsened during the holding process due to the low Tq, decreasing the critical stress required for crack initiation and deteriorating the impact toughness accordingly. However, this detrimental effect of B could be mitigated by a higher Tq, and the favorable influences on the impact toughness improvement could be attributed to (1) the finer M2B particles formed during quenching effectively pinning the austenite grain boundaries (GBs), leading to a finer block size and a high density of high-angle grain boundaries, which reduced the critical stress for crack initiation; and (2) the fact that the coarsening of M23C6 on the GBs during tempering was slightly suppressed by the segregated B, eventually increasing the energy required for crack propagation. However, the degree of the favorable effect due to B was still lower than the negative effect of a high Tq.

Synergistic effect of well/over-tempered hierarchical martensitic structure and carbides (M23C6 and MX) on austenite growth in 9Cr steel

To clarify the synergistic effect of well/over-tempered hierarchical martensitic structure and carbides (M23C6 and MX) on austenite growth, the austenite growth during step-heat austenitization for a selected 9Cr steel was investigated by dilatometry, multi-scale characterization and cellular automaton simulation. The austenite grain inherits the geometrical characteristics of the hierarchical martensite substructure to a significant extent through covariant and free growth. In particular, the initially heterogeneous grains with non-uniform martensitic substructure can easily produce mixed and heterogeneous grains. Step isothermal heating under AC1 can promote the evolution of martensitic structure to recrystallized ferrite and recovered equiaxed substructure, associated with coarsening of M23C6 and precipitation of MC. Apart from the recrystallized ferrite promoting the growth of equiaxed austenite grains, the recovered approximate equiaxed substructure decorated with coarse M23C6 and a large amount of MX can promote the subsequent development of fine spherical austenite grains by reducing the abnormal austenite growth and increasing the resistance to interfacial migration. The synergistic effect of the well/over tempered martensitic hierarchical structure and the carbides (M23C6, MX) is critical to the final austenite grain distribution. A window of critical temperature values for step heating austenitization to generate homogeneous austenite grain growth was established by exploiting the inheritance of the equiaxed substructure and reasonably reducing abnormal growth.

Effect of Heat Treatment Schedules on Creep Performance of Ni-Based Superalloy Mar-M247 at 871 °C and 250 Mpa

The effects of heat treatment (H1 and H2) on the creep behavior and microstructures of Mar-M247 at 871 °C/250 MPa are studied. The results show that the as-cast microstructure is composed of eutectics, γ phase, γ’ phase, MC and M23C6 carbides, while new M6C appears in heat-treated microstructures, indicating the transformation of carbides after heat treatments. The Mar-M247 is excellent, with over 1500 h of creep life, and H1 is 48% higher than H2. The addition of post-brazing and diffusion heat treatment in H2 is detrimental to creep resistance; the two steps promoted the transformation of MC into M23C6 in advance. The increase and coarsening of M23C6 would consume more γ-phase-forming elements, weakening the solution strengthening at grain boundaries. As a result, the resistance of the grain boundary and γ/γ’ interface to dislocation motion is significantly reduced, leading to the cracks’ initiation and propagation along the grain boundaries.

An enhancement in mechanical properties of grade 91 steel on microalloying with boron and nitrogen

ABSTRACT Effects of microalloying the grade 91 steel with boron along with control on nitrogen content (P91BN) on its microstructure, tensile and creep properties have been investigated and compared with those of the standard grade 91 steel (P91). The P91BN steel possesses finer martensite lath structure and M23C6 precipitates in comparison with the P91 steel. Tensile strengths of the P91BN steel were comparable with the P91 steel up to 823 K. The P91BN steel has possessed higher tensile strengths than the P91 steel at higher temperatures (above 823 K). Microalloying the steel with boron decreases the creep deformation rate with significant increase in creep rupture life. Finer lath width, higher number density of fine M23C6 precipitates in pre-creep condition along with sluggish coarsening of M23C6 precipitates resulting in better stabilisation of martensite structural features in the P91 steel on boron micro-alloying, leads to the significant improvement in high temperature tensile and creep strengths.

Microstructural evolution of Super304H stainless steel during high-temperature creep rupture

This study is aimed to explore the microstructural evolution of Super304H stainless steel during long-term creep. Creep-rupture specimens of this steel are analyzed via microhardness measurement, metallographic structure observation, X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and energy-dispersive spectroscopy. The results show that the matrix structure of the steel was stable during high-temperature stress-rupture test without grain growth. Twin boundaries in austenite gradually disappeared with the prolongation of time, which process was accelerated at higher temperatures. The steel hardness sharply increased in the initial stage of high-temperature stress-rupture test due to the second-phase precipitation, and then gradually dropped with time, due to the coarsening of precipitates. The Cu-rich phase and secondary Nb(C,N) with the size less than 50 nm, and M23C6 were the main precipitated phases in the high-temperature stress-rupture test. This allowed the steel under study to retain a high stress-rupture limit due to the precipitation-strengthening effect. The microstructure aging resulted in the formation of M23C6 chain at the grain boundary and coarsening of M23C6 and primary Nb(C,N) in grains. With the coarsening of precipitated phases, hardness and stress-rupture limit of Super304H stainless steel decreased.

Effects of Isothermal Aging on Microstructure and Mechanical Property of Low-Carbon RAFM Steel

In order to investigate the microstructure and mechanical property evolution of low-carbon reduced activation ferritic/martensitic (RAFM) steel during isothermal aging, the normalized and tempered specimens were aged at 600 °C for 500, 1000, and 3000 h, respectively. The microstructural evolution with aging time was analyzed, including the precipitation and growth of M23C6 and MX-type carbides as well as the formation of Laves phase. The results indicate that the coarsening of M23C6 is more obvious than that of MX with increase in aging time. During the long-term thermal exposure, the Fe2W Laves phase precipitates adjacent to M23C6 along the prior austenite grain boundaries and packet boundaries. Lower carbon content can delay the precipitation of Laves phase compared to the steel containing higher carbon. In addition, the Laves phase precipitated along boundaries can provide the precipitation strengthening, slightly increasing the tensile strength of low-carbon RAFM steel after aging for 3000 h.

Prediction of long-term creep behaviour of Grade 91 steel at 873 K in the framework of microstructure-based creep damage mechanics approach

A detailed analysis has been performed for the prediction of long-term creep behaviour of tempered martensitic Grade 91 steel at 873 K using the microstructure-based creep damage mechanics approach. Necessary modifications have been made into the original kinetic creep law proposed by Dyson and McLean in order to account for the influence of microstructural damages arising from the coarsening of M23C6 and conversion of useful MX precipitates into deleterious Z-phase on creep behaviour of the steel. An exponential rate relationship has been introduced for the evolution of number density of MX precipitates with time. It has been shown that the developed model adequately predicts the experimental long-term creep strain–time as well as creep rate-time data. The role of Z-phase on long-term creep behaviour of Grade 91 steel has also been discussed.

The effect of phase parameter variation on hardness of P91 components after service exposures at 530–550 °C

As a significant property index required by the related ASME and ASTM codes, hardness values of some P91 heat resistant steel components have been evidently reduced after a relatively short term (tens of thousands of hours) of service exposures in quite a number of thermo-power generation sets, leading to a worldwide concern in the related fields. Through the application of SEM-EDS+MPST technology and data-fittings, functions are established to quantitatively reveal the weighted relationships of total amount of carbon in matrix (PC-matrx) and hardness value (HV0.1) with volume fractions and equivalent diameters of M23C6 carbides and Laves phase (VM23C6, VLavesdM23C6 and dLaves) for the P91 components after service exposures at 530–550 °C and 3.53–17.4 MPa for 57,000–63,000 h, respectively. An excessive increase in VM23C6 and dM23C6 resulted in matrix softening with carbon-depletion of martensite and formation of ferrite (abnormal microstructure) due to an insufficient heat treatment after uneven thermoforming while a relatively large increase/decrease in VLaves/dLaves of Laves phase (normal microstructure) could effectively inhibit further precipitation and coarsening of M23C6 and hence keep the matrix relatively strong in the service exposure duration investigated. The relevant causes of forming the abnormal microstructures and the precipitate coarsening mechanism associated with Cr content in M23C6 carbides (CCr-M23C6) and Fe/Cr ratio in Laves phase (CFe-Laves/CCr-Laves) were analyzed. The elemental exchanges of Cr, Mo and Si between phases and the interactions between precipitates and boundaries were also discussed. The quantitative relationship and the related mechanism can be used as reference for microstructure and property degradation assessments of service exposed P91 components.

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.

Effect of thermal ageing on microstructure, tensile and impact properties of reduced activated ferritic-martensitic steel

Effects of thermal ageing on tensile and impact properties of India-specific Reduced Activated Ferritic-Martensitic (INRAFM) steel (9Cr-1.4W-0.06Ta-0.22V) have been studied by correlating with microstructure. The tempered martensitic steel received in the normalized and tempered (N&T) condition was thermally aged at 873K for 5000h. Thermal ageing was found to decrease hardness and tensile strength and the decrease was accompanied with increase in ductility. The steel displayed plateaus/humps in the temperature variation of yield stress and ultimate tensile strength and dip in ductility over the range 473–673K, which is considered due to the occurrence of dynamic strain ageing (DSA). The prevalence of DSA activity was found more on thermal ageing. Increase in ductile to brittle transition temperature (DBTT) and decrease in upper shelf energy (USE) of the steel were observed on thermal ageing. Detailed microscopic investigation of the steel revealed coarsening of martensitic sub-boundaries and M23C6 carbides along with the precipitation of Laves phase on thermal ageing. Coarsening of microstructural features on thermal ageing is considered to influence the reduction in strength and USE of the steel; whereas coarsening of M23C6 and precipitation of Laves phase is considered to cause the increase in DBTT. SEM fractographic study of the impact specimens revealed three distinct zones as shear lip, fibrous dimple and cleavage in an ordered away from the notch tip. With decrease in test temperature, the widths of the shear lip and fibrous dimple zones deceased whereas width of the cleavage zone increased progressively. Thermal ageing increased the widths of the shear lip and fibrous dimple zones and decreased the cleavage zone in the N&T steel at relatively higher temperatures but at lower test temperatures cleavage zone became more predominant. TEM study of the specimens extracted from the impact fractured samples at the vicinity of fracture surface, revealed substantial activity of dislocations manifested as dislocation debris in N&T steel even for impact testing at −80°C against individual dislocations for the aged steel, implying enhancement of DBTT on thermal ageing.

The influence of long-term thermal exposure on microstructural stabilization and mechanical properties in 9Cr-0.5Mo-1.8 W-VNb heat-resistant steel

A series of long-term thermal aging experiments on P92 steel were carried out at 650°C from 200h up to 1.1×104h. Microstructural evolution of the samples during long-term aging exposure was investigated using OM observations, SEM examinations and TEM investigations. Additionally, variations of mechanical properties, including tensile properties, Charpy impact toughness and Vickers micro-hardness were also exhibited. Experimental results indicate that martensitic lath structure, M23C6 carbides as well as MX carboniterides all exhibit a high stability during short-term aging exposure, whereas Laves phase grows rapidly in this period. After the long-term aging treatment, the recovery of martensitic lath structure occurs after an incubation period, which is resulted from the continuous growth and coarsening of M23C6 and Laves phase. Based on the correlations between the microstructural evolution and the variations of mechanical properties during long-term aging exposure, the degradation mechanism of P92 steel was discussed.

Type IV cracking behaviour of modified 9Cr-1Mo steel weld joints

Creep deformation and rupture behaviour of modified 9Cr–1Mo steel weld joints fabricated by single-pass activated TIG (A-TIG) and shielded metal arc welding (SMAW) processes have been investigated at 923 K over a stress range of 50–110 MPa after post-weld heat treatment. The formation of coarse M23C6 precipitates and recovery of martensite lath structure into subgrain in the ICHAZ of the weld joints resulted in hardness trough. The weld joints exhibited significantly lower creep rupture lives than the base metal at lower applied stresses. Creep rupture location of the weld joints was found to occur in the ICHAZ. The reduced ductile mode of failure was observed in the low stress level in comparison with high stress level. An extensive localised creep deformation, coarsening of M23C6 precipitates in the ICHAZ with creep exposure led to the premature Type IV failure of the joints. The coarsening of M23C6 precipitates was extensive in the mid-section of the ICHAZ than the sub-surface of the joints, and was more predominant in the SMAW joint. While A-TIG weld joint exhibited reduced creep cavitation and coarsening of M23C6 precipitates due to lower deformation constraints by adjacent regions in the ICHAZ. Hence, A-TIG weld joint exhibited higher creep rupture life than the SMAW joint.

Failure transition mechanism in creep rupture of modified casting 9Cr-1.5Mo-1Co welded joint

Creep rupture behavior and microstructure evolution of modified casting 9Cr-1.5Mo-1Co steel welded joint were systematically investigated in this paper. Based on a series of creep rupture tests at 620°C under 110–170MPa, a curve of stress versus rupture time was achieved to evaluate the service life of welded joint. The results indicated that creep rupture is considered to occur in over tempered zone (OTZ) under relatively high stress above 130MPa while failure transferred to weld metal (WM) under lower stress below 130MPa. The ruptured specimens showed less ductility and necking with the decrease of stress. Microstructure characterization revealed OTZ with less grain boundaries than WM and relatively lower hardness than BM was more possible to deform in short-term creep rupture accompanied by the coarsening of M23C6. Besides, formation of Laves phase and deterioration of martensite lath structure were detected during long-term creep exposure. Mo segregation near M23C6 provided the condition for Laves phase to form adjacent to M23C6. The difference of plastic deformation resistance in each zones of welded joint and nucleation and coarsening of Laves phase in WM are considered as the main factors to result in the failure transition.

Effect of microstructure evolution on strength and impact toughness of G18CrMo2-6 heat-resistant steel during tempering

G18CrMo2-6 steel is one kind of low alloy CrMo steel with ferrite/bainite phase constituent. It is widely adopted in the large casting parts of pressure vessels. This paper aims at investigating evolution of G18CrMo2-6 steel microstructure, tensile strength and impact toughness during the tempering (680°C). The tempered microstructure characterization shows that the tempering time mainly affects the precipitation of three kinds of carbides, naming MC, M3C and M23C6. Both tensile strength and impact toughness do not obey the monotonic evolution with the tempering time but the complex changes. The shift of mechanical properties results from the interaction among matrix phases and the evolution of three different kinds of carbides. The softening of matrix due to the dislocation recovery and the decrease of the amout of carbon and other alloy elements in matrix lattice due to the precipitation of M23C6 cause the decreasing tensile strength for the whole tempering except for that an increase during the short-term tempering controlled by the precipitation of MC and the spheroidization and refinement of M3C carbide. On the other hand, both the softening of matrix and the spheroidization and refinement of M3C carbide are responsible for the increasing impact toughness. However, the precipitation and coarsening of M23C6 at ferrite grain boundaries during long-term tempering results in the sharp deterioration of impact toughness. It attributes to the larger grain boundary carbides result in the lower critical fracture stress of a carbide-ferrite interface.

Formation of Intergranular M23C6 in Sensitized Type-347 Stainless Steel

Formation of intergranular M23C6 carbides and thereafter Cr-depletion zones in commercially available type-347 stainless steels were observed by optical microscopy, conventional transmission electron microscopy and analytical transmission electron microscopy. At the early stage of sensitization, only NbC carbides were observed both intragranularly and intergranularly. At the later stage, formation of intergranular M23C6 carbides and intergranular corrosion were found. A model is proposed to describe the formation kinetics of intergranular precipitation of NbC and of M23C6, as well as Cr-depletion zones, thermodynamically. Evolution mechanisms of intergranular precipitates were found in the order of 1) the formation of intergranular NbC, 2) the coarsening of NbC, 3) the formation of M23C6, then 4) the coarsening of M23C6 and formation of Cr-depletion zones.

Effect of long-term aging on microstructure and local behavior in the heat-affected zone of a Ni–Cr–Mo–V steel welded joint

Evolution of microstructure, micro-hardness and micro-tensile strength behavior was investigated in the heat-affected zone of a Ni–Cr–Mo–V steel welded joint after the artificial aging at 350°C for 3000h. After detailed characterization of microstructures in optical microscopy, scanning electron microscopy and transmission electron microscopy, it is revealed that the change of martensite–bainite constituent promotes more homogeneous microstructure distribution. The aging treatment facilitates redistribution of carbon and chromium elements along the welded joint, and the micro-hardness is increased slightly through the welds due to enrichment of carbon. The types of precipitates in the weldment mainly include M3C, MC, M2C and M23C6. The carbides in base metal, weld metal and coarse-grained heat-affected zone are prone to change from ellipsoidal to platelet form whereas more uniform spherical carbides are observed in the fine-grained zone. Precipitation and coarsening of M23C6 near the fusion line, and formation of MC and M2C, are responsible for the tensile strength decrease and its smooth distribution in the aged heat-affected zone. This implies that the thermal aging can relieve strength mismatch in the weldments.

Influence of thermal aging on microstructure and mechanical properties of CLAM steel

In order to investigate the influence of thermal aging on microstructure and mechanical properties of CLAM (China low activation martensitic) steel, a comparison study was made on the as-tempered and the aged steels. The tempered CLAM steels were subjected to aging treatment at 600°C for 1100h and 3000h, and at 650°C for 1100h, respectively. The changes of microstructure were characterized by both transmission electron microscope (TEM) and scanning electron microscope (SEM). The mechanical properties were evaluated by Charpy impact, tensile and Vickers hardness tests. The upper shelf energy (USE) of the thermal aged CLAM steel decreased with the extension of aging time, while the yield strength changed slightly. After long-term thermal aging, the MX type precipitates remained stable. The coarsening of M23C6 and the formation of Laves phase were confirmed by scanning/transmission electron microscopes. The Laves phase was the main factor leading to the increase of DBTT.

Coarsening behavior for M23C6 carbide in 12 %Cr-reduced activation ferrite/martensite steel: experimental study combined with DICTRA simulation

Based on the multi-component aspects of thermodynamics and diffusion, coarsening behavior of M23C6 (M = Cr, Fe, W) carbide at 650 °C in 12 %Cr-reduced activation ferrite/martensite steel has been investigated experimentally using scanning transmission electron microscopy, combined with DICTRA simulation. Both the experimental measurements as well as the simulations indicate that the interfacial energy of M23C6 carbide in this steel at 650 °C is probably 0.5 J m−2, and the coarsening rate of M23C6 carbide is very low. The influence of a change in Mn, V, and Ta content and temperature on the coarsening rate of M23C6 carbide is also investigated. The results show that the coarsening rate is increased by adding Mn and reduced by V and Ta addition, respectively, while an increase in the coarsening rate by an order of magnitude with increasing temperature per 50 °C between 600 and 750 °C. Precipitation of Laves (Fe2W) phase during aging has a negligible effect on the coarsening of M23C6.