Carbide-matrix Interface Research Articles

Influence of prior austenite grain size on martensite-austenite islands morphology and decomposition characteristics during two-step tempering in Mn-Ni-Mo steel

Martensite-austenite (M/A) islands are typical in the quenched microstructure of Mn-Ni-Mo steel, frequently utilized for fabricating reactor pressure vessels (RPV). These islands can decompose into harmful long rod-shaped Fe3C carbides during tempering above 600°C, quantifying the negative effect of M/A island decomposition on mechanical properties and indicating a potential issue in RPV fabrication. To address this issue, the present study investigates a two-step tempering process on three different prior austenite grain size (PAGS) specimens (5 µm, 47 µm, and 119 µm). The effect of PAGS on M/A island decomposition characteristics during sub-600°C tempering and their impact on precipitated carbide morphology is investigated. A preliminary pre-tempering treatment at 200°C, 350°C, and 500°C for 2 hours was followed by a 3-hour final tempering step at 650°C. The findings revealed unique temperature-dependent decomposition characteristics of M/A islands. For example, at 200°C, partial M/A island decomposition took place, resulting in long rod-shaped carbides during final tempering. Meanwhile, the 350°C pre-tempering specimen revealed aggregated carbide precipitates of varying sizes in former M/A island locations. In contrast, the 500°C pre-tempering specimen showed coarse globular carbides and the absence of carbide precipitate clustering. This result can be attributed to the decrease in surface energy associated with the carbide-matrix interface as size increases. After tempering, all specimens produced three types of precipitates: rod-shaped Fe3C carbides, rhombus-shaped Mn7C3, and globular-shaped Mn23C6 carbides. The formation of long rod-shaped carbides during M/A island decomposition reduces the critical cleavage stress for micro-crack initiation, jeopardizing steel mechanical properties. Aggregated carbides cause the simultaneous formation of numerous micro-voids in close proximity, which eventually merge to form a large crack, resulting in material failure. When compared to other pre-tempered and directly tempered specimens, pre-tempering at 500°C produced the most favorable microstructure in terms of impact properties.

Crack initiation mechanism of M50 bearing steel under high cycle fatigue

The fatigue properties of M50 bearing steel under high cycle fatigue were studied by rotating bending fatigue test at room temperature. The fatigue cracks of M50 bearing steel were primarily nucleated at MgO-Al2O3 inclusions and MC/M2C primary carbides. The voids were initiated through inclusion debonding from the matrix and primary carbide cracking under cyclic loading. Small cracks were initiated on the void along the direction perpendicular to the tensile stress. Small cracks propagated at a speed of 8.81 × 10-10 m/cycle with hysteresis in the fine granular area (FGA). When the stress intensity factor range for the subsurface defect was lower than the stress intensity factor range threshold for M50 steel, the initiation stage of fatigue crack was composed of crack nucleation caused by defects and fatigue process in FGA. The fatigue cracks propagated at the fish-eye area (FiE) when the stress intensity factor range at the FGA front exceeded ΔKth. There was a stress concentration at the inclusion-matrix interface or the primary carbide-matrix interface during a long-term cyclic loading which resulted in the nucleation of fatigue cracks. A fatigue crack nucleation model was established based on the true stress at the defect, size of the defect, and number of cycles. A fatigue life model was constructed based on fatigue crack nucleation and fatigue crack propagation processes.

Effect of Carbide Precipitation on “Precipitation Plasticity” and Residual Stress during Tempering

Materials with residual stress can experience irreversible plastic strains as carbides precipitate during the tempering process, thereby relaxing the residual stress. In previous studies, this plastic behavior is named “precipitation plasticity.” Although precipitation plasticity plays a key role in the relaxation of the residual stress, the micromechanism involved is not yet elucidated. Here in this study, experiments are performed on low‐carbon steel to study precipitation plasticity and the residual stress during tempering. The results show that precipitation plasticity is mainly produced by the generation of interfacial vacancies at the carbide–matrix interface and the diffusion and evolution of these vacancies under temperature and stress fields. Two precipitation plasticity regimes are observed, namely, Precipitation Plasticity I, which is associated with vacancy diffusion, and Precipitation Plasticity II, which is associated with dislocation proliferation. Precipitation Plasticity I is found to be controlled by the volume mismatch and carbide size and is the main cause of residual stress relaxation. The finer and more dispersed the carbide precipitates are, the more significant the precipitation plasticity is. Precipitation Plasticity II causes a slight increase in the residual stress, which, however, is considerably smaller than the stress relaxation corresponding to Precipitation Plasticity I.

Characterisation of nano-scale precipitates in BOR60 irradiated T91 steel using atom probe tomography

Atom probe tomography has characterised the microstructural changes in T91 steel after BOR60 reactor irradiation at five temperatures between 376 °C and 524 °C to doses between 14.6 dpa and 35.1 dpa. Irradiation-induced precipitation and segregation to carbide/matrix interface induced by neutron irradiation has been characterised. Atom probe tomography characterisation shows that Mn, Ni, Si -rich (MNS-rich) clusters form in T91 steel irradiated in BOR60 reactor at temperatures between 376 °C and 415 °C, which is not observed at higher temperatures 460 °C and 524 °C. The number density, volume fraction and composition of MNS-rich clusters have been characterised. Ni, Mn, Si and P is found to segregate at carbide matrix interface after irradiation at lower temperature and only P segregation is observed at 524 °C.

Hydrogen embrittlement of the coarse grain heat affected zone of a quenched and tempered 42CrMo4 steel

The coarse grain heat affected zone (CG-HAZ) of welds produced in a quenched and tempered 42CrMo4 steel was simulated by means of a laboratory heat treatment consisting in austenitizing at 1200 °C for 20 min, oil quenching and finally applying a post weld heat treatment at 700 °C for 2 h (similar to the tempering treatment previously applied to the base steel). A tempered martensite microstructure with a prior austenite grain size of 150 μm and a hardness of 230 HV, similar to the aforementioned CG-HAZ weld region, was produced. The effect of the prior austenite grain size on the hydrogen embrittlement (HE) behaviour of the steel was studied comparing this coarse-grained microstructure with that of the fine-grained base steel, with a prior austenite grain size of 20 μm.The specimens used in this study were charged with hydrogen gas in a reactor at 19.5 MPa and 450 °C for 21 h. Cylindrical specimens were used to determine hydrogen uptake and hydrogen desorption behaviour. Smooth and notched tensile specimens tested under different displacement rates were also used to evaluate HE.Embrittlement indexes, EI, were generally quite low in the case of hydrogen pre-charged tensile tests performed on smooth tensile specimens. However, very significant embrittlement indexes were obtained with notched tensile specimens. It was observed that these indexes always increase as the applied displacement rate decreases. Moreover, hydrogen embrittlement indexes also increase with increasing prior austenite grain size. In fact, the embrittlement index related to the reduction in area, EI(RA), reached values of over 20% and 50% for the fine and coarse grain size steels, respectively, when tested under the lowest displacement rates (0.002 mm/min).A comprehensive fractographic analysis was performed and the main operative failure micromechanisms due to the presence of internal hydrogen were determined at different test displacement rates. While microvoids coalescence (MVC) was found to be the typical ductile failure micromechanism in the absence of hydrogen in the two steels, brittle decohesion mechanisms (carbide-matrix interface decohesion, CMD, and martensitic lath interface decohesion, MLD) were observed under internal hydrogen. Intergranular fracture (IG) was also found to be operative in the case of the coarse-grained steel tested under the lowest displacement rate, in which hydrogen accumulation in the process zone ahead of the notch tip is maximal.

The importance of steel chemistry and thermal history on the sensitization behavior in austenitic stainless steels: Experimental and modeling assessment

The sensitization of austenitic stainless steels is dependent on various factors such as chemical composition, heat treatment temperature and time. To study these effects, the degree of sensitization in five austenitic stainless steel compositions that were subjected to isothermal heat treatments in the temperature range 550–820 °C has been determined using double loop electrochemical potentiokinetic reactivation testing. The nucleation and growth of grain boundary M23C6 carbides, that are responsible for sensitization, has been modelled with the help of the precipitation and diffusion modules in Thermo-Calc, assuming local multicomponent equilibrium, flux balance at the carbide-matrix interface, to quantitatively predict the Cr depletion. Based on the Cr depletion characteristics, a depletion parameter has been established that can predict sensitization in austenitic stainless steels and predict the effects of individual alloying elements.

Effect of thermal aging on embrittlement of Cr–Mo–V pressure vessel steel

Cr–Mo–V reactor pressure vessel steel (RPV) was subjected to accelerated thermal aging at 450 °C for durations up to 8400 h. This steel showed embrittlement with aging, as the impact energy and the hardness reduced monotonically with aging duration. These coincided with an increase in the transgranular (TG) brittle fracture, and a conspicuous absence of intergranular (IG) fracture. Further, absence of preferential attack at the prior austenite grain boundaries ruled out the possibility of impurity as a cause for the observed degradation. In addition, insignificant changes in grain size and misorientation indicated no major role of microstructural changes in the embrittlement. However, the coarsening of carbides and the corresponding depletion of the Cr & Mo at the carbide matrix interface region, supported by the reduced hardness, were shown to have resulted in the degradation. The carbides were identified to be predominantly of Cr-rich M7C3 along with M23C6 in the as-received low alloy steel. While M23C6 increased with aging duration, indicating a transformation of M7C3→M23C6, the primary reason for the embrittlement is attributed to the coarsening of the carbides.

Tensile Deformation Behaviors and Damage Mechanisms of SRR99 Superalloy Bicrystals with Different Grain Boundary Misorientations

To understand the intrinsic tensile influence mechanisms of grain boundary (GB) misorientations, SRR99 bicrystal superalloys with the different misorientation angles () of 4°, 10°, 16°, and 18° are prepared to carry out conventional and in situ tensile tests at room temperature and fracture feature analysis. With the increment of , tensile strength and elongation to fracture show successive decrement. The fracture features exhibit two types (4° and 10°–18°): (I) when equals to 4°, the cracks initiate and propagate along slip bands (SBs)‐matrix interface. (II) As ranges from 10° to 18°, the cracks initiate along the carbide–matrix interface at GBs. Interestingly, the main crack in 10° bicrystal propagates first along the GB then along the SB, but completely along the GB in 16°–18° bicrystals. The former one is absolutely controlled by SBs, and the latter is dominated by carbides and GBs. Finally, a new parameter, defined as the cracking threshold value (), is proposed to explain the tensile deformation behaviors and damage mechanisms. The new findings will beneficially provide research foundation for the further understanding fatigue damage mechanisms at high temperature.

High temperature tensile deformation of a directionally solidified nickel base superalloy: Role of micro constituents

High temperature tensile deformation behaviour of directionally solidified nickel base superalloy CM 247 DS was studied by conducting tensile tests in the temperature range 25–955°C and extensive electron microscopic examinations to understand the influence of different phases on tensile behaviour. Micro-constituents, especially carbides and γ−γ′ eutectics influence the deformation and fracture behaviour of alloy CM 247 DS. Cracking of carbide precipitates was extensively observed during tensile deformation at 25°C. However, tendency of carbide cracking decreased and more carbide-matrix interface de-bonding were observed with increase in test temperature.

Evolution of microstructure and mechanical properties of a 25Cr-20Ni heat resistant alloy after long-term service

This work investigated the effect of long-term service on the evolution of microstructure and properties of HR3C alloy. Its results show that large amount of discontinuous chromium carbide is accompanied by a chromium depletion area that is formed along the grain boundary. This depletion area promotes the crack nucleation and propagation at the carbide-matrix interface at the grain boundary, leading to a decrease in the alloy ductility at room temperature. However, this deteriorative effect is not obvious at high temperatures primarily because the fast drop of the grain strength. Therefore, the crack propagates easily across the grain during the alloy deformation at high temperatures.

Characterization of (Nb,Ti,Mo)C Precipitates in an Ultrahigh Strength Martensitic Steel

A study on ultrahigh strength steel plate subjected to novel thermo-mechanical control process was presented. The mechanical properties examination showed that the investigated steel exhibited excellent combination of ultra-high strength (2200 MPa) and toughness (26 J). The microstructure of the experimental steel was observed by scanning electron microscope and transmission electron microscope. Desired martensitic lath with width of about 180–230 nm was obtained. Nanostructured carbide precipitates with sizes of 20–50 nm, which contained Nb, Ti and Mo, were observed in the lath martensitic microstructure, and confirmed to be MC-type carbides with B1 structure by means of selected area electron diffraction. The compositional characteristics revealed by energy dispersive X-ray spectrometer mapping implied that the carbide forming elements Nb, Ti and Mo distributed in the precipitates evenly. Three-dimensional atom probe tomography reconstruction further indicated that Mo incorporated into the precipitates without enrichment in the carbide-matrix interface and probably substituted for Nb and Ti to form the (Nb,Ti,Mo)C carbides.

Influence of dwell times on the thermomechanical fatigue behavior of a directionally solidified Ni-base superalloy

In-phase (IP) and out-of-phase (OP) thermomechanical fatigue tests with T=100–750°C and optional dwells of 20min at 750°C were carried out on directionally solidified Ni-base Alloy 247 LC DS. Introducing dwells reduced the lifetime for both phase angles to about one sixth. Specific damage mechanisms were internal carbide and carbide–matrix interface cracks in IP tests and crack propagation along {111}-microtwin planes in OP tests. Introducing dwells intensified both effects, thus contributing to the lifetime reduction. During dwells, the gauge length may exhibit transversal creep because of extensometer forces distorting the strain measurement.

A micromechanical image-based model for the featureless zone of a Fe–Ni dissimilar weld

This paper deals with the constitutive modelling of the ‘featureless’ region located on the Nickel side of a AISI8630/IN625 dissimilar weld interface. Fractography of failed weld interfaces show that cracks propagate in this carbides ()-rich region in the presence of hydrogen. In this paper, TEM images of the carbide-rich region are converted into a finite element mesh through an image-based mesh generation scheme. Simulations of the response of these structures show that in areas where the hydrogen content is high the matrix surrounding the carbides softens and plastic flow is localized. Moreover, the presence of hydrogen lowers the cohesive strength, giving rise to microcrack formation at the carbide-matrix interface. The amount of deformation then increases in a localized region adjacent to the region where (a) hydrogen content is high and (b) the carbide/matrix interface has debonded. As deformation proceeds the microcracks grow and link to form macrocracks, which generates the failure surface.

The effect of silicon on the nanoprecipitation of cementite

The current work presents a comprehensive study that aims at understanding the role of silicon on θ precipitation, as well as on the ε→θ carbide transition in tempered martensite. Cementite nucleation was modelled under paraequilibrium conditions in order to ensure the presence of silicon in the carbide, where both thermodynamic and misfit strain energies were calculated to evaluate the overall free energy change. The growth stage was investigated using in situ synchrotron radiation; three alloys containing 1.4–2.3wt.% silicon contents have been studied. Silicon appears to play a significant role in carbide growth. It was observed throughout tempering that cementite precipitation was slower in the higher silicon content alloy. Literature reports that cementite growth is accompanied by silicon partitioning, where the silicon content inside the carbide decreases as tempering progresses. Therefore it appears that the limiting factor of the growth kinetics is the rate at which silicon is rejected from the carbide; the silicon piles up at the carbide–matrix interface, acting as a barrier for further growth.

Irradiation-induced precipitation in a SUS316 stainless steel using three-dimensional atom probe

Precipitation and segregation were investigated in a compositionally modified 316 austenitic stainless steel, neutron-irradiated at 862K using a three-dimensional atom probe. In the solution-annealed specimen, Mo, Ti, Nb, C and P enrichment were observed in a silicide, with nominal composition Fe3Cr2Ni2Mo2Si2. In a Ti-rich carbide, nominaling Fe5Cr8Ni10Mo2Ti11Si2C6, enrichment of Mo, Si, O, and Nb was observed. Radiation-induced segregation (RIS) at the precipitate–matrix interface was also investigated at an atomic scale. RIS of Ni and P atoms, which are undersized in Fe, was also analyzed around the interface of the Ti-rich carbide and matrix. Results suggest that the carbide–matrix interface is a sink with an interstitial bias. In the cold-worked specimen, complex-precipitates consisting of silicide and carbide were formed.

Comparison of Isolated Indentation and Grid Indentation Methods for HVOF Sprayed Cermets

This paper compares the results of two approaches of instrumented indentation for characterization of mechanical properties of HVOF coatings. Three types of HVOF sprayed coatings (Cr3C2-NiCr, WC-Co, (Ti, Mo)(C,N)-NiCo) were investigated by the means of isolated nanoindentation and grid indentation methods. The results of the isolated indentation revealed hardness and elastic modulus of the individual phases in very good agreement with the corresponding bulk material. The grid indentation method, based on statistical evaluation of a large number of indentations, was influenced by the carbide-matrix interface, which gave rise to a third peak apart from the two peaks corresponding to the carbides and metallic matrix. As a consequence, the bimodal Gaussian fit was insufficient and a trimodal fit had to be used. The results extracted from low load grid nanoindentations were quite close to the results of isolated indentations whereas higher load grid nanoindentation revealed overall properties of the coating.

Irradiation-induced changes of the atomic distributions around the interfaces of carbides in a nuclear reactor pressure vessel steel

Irradiation-induced changes of the atomic distributions of solute and impurity elements around carbides in a reactor pressure vessel steel of a Belgium nuclear power reactor were investigated by laser-assisted local electrode-type three-dimensional atom probe, before and after in-service irradiation of 12 years. Before irradiation, nano-scale Fe–Mn–Cr–Mo carbides were found to be intragranular. The atomic distributions of Mn, Cr and Mo inside the carbide indicate that their concentrations around the inner carbide–matrix interface were enhanced, while a clear segregation of P at the interface was observed. After irradiation, the Mn concentration in the carbide increased substantially. In addition, the enhancement of Mn, Cr and Mo concentrations around the interface and the segregation of P were markedly intensified.

Fatigue Behavior of High Speed Steel Roll Materials for Hot Rolling by Laser Impacting

The fatigue behavior of high speed steel (HSS) roll materials for hot rolling was researched under water-cooling conditions by laser impacting. The microstructure of HSS sample and the morphologies of fatigue samples were observed by scanning electron microscope. The phase structure was detected by XRD. The morphology of situ oxide scale was observed by optical microscope, and the expansion coefficient was measured by TGA. The experiment results indicate that the cracks come into being at the carbide-matrix interface, but there are no cracks in the matrix after many times of laser impacting treatment, for the situ sample taken from the fractured roll surface, big carbides are more sensitive to the fatigue, and peel off prior to small ones. The relevant fatigue mechanisms are also discussed.

In situ Neutron Diffraction Studies of Carbide-Matrix Interactions in HAYNES® 230® Nickel Based Superalloy

ABSTRACTHAYNES 230 is a solid solution strengthened, face centered cubic (FCC), nickelbased superalloy, with a small amount (1–5 vol. %) of semi-coherent FCC carbides. Neutron diffraction experiments were performed to study the interaction of the carbides with the matrix during tensile and compressive loading. The behavior of the elastic lattice strains during in situ loading clearly showed a tension-compression asymmetry. Although the volume percentage is small, the interaction between the carbides and the matrix had a significant effect on the load sharing. In compression, the carbides began load sharing at the macroscopic yield point, while the tension test suggests no load sharing. Debonding at the carbide-matrix interface is proposed to explain the lack of load sharing during the tensile loading based on the disparities observed between the experimental results and the finite element models.

Study on the cavitation erosion behavior of hardfacing alloys for nuclear power industry

The cavitation erosion behavior of wear resistant hardfacing alloys for nuclear power industry Co-base Stellite 6 and new Fe-base alloy was investigated using 20 kHz vibratory cavitation erosion test equipment. Although the crack was initiated at the carbide-matrix interfaces, the matrix hardened by strain-induced phase transformation from austenite to martensite effectively repressed the crack propagation. The cumulative weight loss of new Fe-base alloy was about 2.40 mg/cm 2 after exposing to cavitation for 150 h and the cavitation erosion characteristic was similar to that of Stellite 6 on the whole range. The improved cavitation erosion resistance of new Fe-base alloy was due to the matrix hardening by the strain-induced phase transformation and to the small volume fraction of the carbide-matrix interface acting as crack initiation sites.