Coarse Carbide Particles Research Articles

Role of the heterogeneity in microstructure on the mechanical performance of the Autogenous Gas Tungsten Arc (GTA) welded dissimilar joint of F/M P91 and SS304L steel

The microstructure and mechanical behavior of the Gas Tungsten Arc (GTA) welded joint of ferritic/martensitic (F/M) P91 and austenitic SS304L steel was investigated in as-welded and post-weld heat treatment (PWHT) conditions. The microstructural studies included the particle size and its distribution, grain size, and percentage area fraction of the precipitates (%AFP) in different zones of the weldments. In as-welded condition, weld metal showed untempered martensitic microstructure devoid of carbide precipitates. After the PWHT, weld metal was characterized by tempered martensitic microstructure along with the decoration of the coarse carbide particles at PAGBs and fine carbonitride particles inside the matrix region. The mechanical behavior of the dissimilar weld was analyzed in as-welded and PWHT conditions by performing tensile testing, Charpy impact testing, and microhardness testing. A significant variation in microstructure and mechanical properties was obtained across the dissimilar welded joint (DWJ) for as-welded, which was minimized after the PWHT. The minimum%AFP in the coarse grain heat-affected zone was measured due to the formation of the precipitate-free soft δ ferrite patches, which also showed a negligible response to PWHT. The poor Charpy impact toughness (CIT) of 0.8 ± 1.5 J and high tensile strength of 864 ± 12.5 MPa were measured for the as-welded joint. The combination of high strength and low CIT was obtained mainly due to the untempered martensitic microstructure in the weld fusion zone. The DWJ after the PWHT showed a significant increase in Charpy impact toughness (CIT), and it was measured 70 ± 4 J. Improved mechanical behavior with a good combination of ultimate tensile strength and CIT were obtained after the PWHT.

Microstructure, Substructure and Mechanical Properties of 9CrNB Steel after Tempering

This paper deals with the analysis of microstructure and substructure of 9CrNB steel, after normalization at temperature of 1070 °C and tempering at 790 °C / 240 min. The tube was second time tempered at the following temperatures and holding times: 760 °C / 30 min (A1), 760 °C / 120 min (C1), 800 °C / 30 min (G1) and 800 °C / 120 min (I1). Microstructure after tempering consists of tempered martensite and bainite with lath morphology, while inhomogeneous redistribution of precipitates is visible. Substructure analysis of state A1 and I1 show, that a relatively large number of irregular, rod-shaped and oval carbide particles, often arranged in clusters, were precipitated at the primary original austenite grain boundaries. In case of state A1, the average size of these carbide particles is 300 nm and in case of state I1 the average size is 350 nm. A relatively large number of rod-shaped and oval shaped particles were found at the interface of the tempered martensite and bainite mainly in the form of clusters and also inside the tempered bainite with higher particle distribution. In the case of the state A1, they reached an average size of 150 nm. In some regions of substructure of the state I1, the fine carbide particles with an average size of 200 nm and coarse carbide particles with an average size of 400 nm were presented within the areas of tempered bainite. Particles were identified by EDX analysis and by selection electron diffraction. The mechanical properties after tempering were evaluated and compared with properties of P91 and P92 steel.

Effect of Cr and W contents on Charpy impact properties of Fe–Cr–W oxide dispersion strengthened steels

In this study, the effects of Cr and W contents on the microstructure and Charpy impact properties of Fe–Cr–W oxide dispersion strengthened (ODS) steels were investigated. ODS steels were fabricated by mechanical alloying followed by hot-rolling and tempering, and Charpy impact tests were conducted at −40 °C in both longitudinal-transverse (LT) and longitudinal-short transverse (LS) orientations. The high contents of Cr and W induced a significant <110> fiber texture in the elongated ferritic/martensitic matrix, which was associated with the high stability of ferrite at the hot-rolling temperature. This anisotropic microstructure caused the texture-induced delamination perpendicular to the notch base for the LT orientation and parallel to the notch base for the LS orientation. Such delamination in the LT specimens increased the tendency for ductile fracture, thereby resulting in the enhanced fracture propagation energy in a Charpy impact test. For the LS specimens, however, the Charpy impact energy exhibited enormous scatter depending on the formation of secondary delamination and the degree of plastic bending of the un-notched ligament after primary fracture. When the contents of Cr and W were relatively low, the equiaxed and fine grain structure without preferred orientation was obtained by the dynamic recrystallization of high-temperature austenite during hot-rolling. With this microstructure, the steel offered a high resistance to crack initiation, but the crack was propagated abruptly along the grain boundary decorated with coarse carbide particles.

Development and Properties of Cast TiAl Matrix &lt;i&gt;In Situ&lt;/i&gt; Composites Reinforced with Carbide Particles

The In Situ composites with microstructurally different types of intermetallic matrix such as nearly γ (TiAl) (composite A), multiphase with high amount of lamellar α2(Ti3Al) + γ (TiAl) regions (composite B) and fully lamellar α2 + γ (composite C) were prepared by centrifugal casting and consecutive heat treatments of Ti-44.5Al-8Nb-0.8Mo-3.6C-0.1B, Ti-37Al-7Nb-0.8Mo-5.9C-0.1B and Ti-46.4Al-5Nb-1C-0.2B (at.%) alloys, respectively. The centrifugal casting results in a uniform distribution of coarse primary carbide particles in the as-cast samples. Hot isostatic pressing (HIP) and heat treatments have no effect on the Vickers hardness of the in-situ composite B but lead to a significant softening of the in-situ composites A and C. The in-situ composite C with a coarse-grained fully lamellar matrix shows a higher flow stress at 1000 °C and improved creep resistance at 800 °C compared to those of the in-situ composites A and B.

The Effect of Thermal Treatment on Microstructure and Corrosion of Nickel Base Alloy 690 (UNS N0 6690)

Abstract Alloy 690 in the thermally treated (TT) condition is an advanced steam generator tubing material. The effect of thermal ageing treatments on the intergranular corrosion (IGC) behavior of the alloy was probed. Isothermal ageing was carried out at 600, 700, and 800°C with varying ageing time. Morphology of the carbides was studied as a function of thermal treatment temperature and duration. The carbides morphology was observed to affect the IGC resistance. Grain boundaries devoid of carbides (solutionized condition) or with coarse carbide particles (formed at 800°C) enhanced IGC resistance, whereas dendritic fine carbides formed at lower ageing temperature yield inferior IGC resistance.

Cracking of a Tool Steel Guide Roller in a Bar Mill

Failure analysis of a guide roller used in a bar mill of an integrated steel plant has been presented. The guide rollers are positioned at the entry of rolling stands and act as guide in the multi-grooved pass for aligning the hot bars and hold the twisted oval bar while it enters a round pass. The hot bar is at a temperature of ≈1100°C, and the rollers are water cooled. The component is subjected to cyclic thermal stress depending on the mill operating conditions. The rollers are cracking longitudinally leading to failure. The investigation consists of visual inspection, chemical analysis, fractography, characterization of microstructures using optical and scanning electron microscopes (SEM), energy dispersive spectroscopy (EDS) analysis, and measurement of micro-hardness. The chemical analysis indicates the material as AISI D2 grade of cold work tool steel. Visual observation of the failed component shows multiple longitudinal cracks on the roller surface associated with a dark circular band of oxidation. The fracture surface shows a dark oxidized area propagating from the roller surface followed by flat bright appearance indicating final brittle fracture. Fractography of the dark surface from where the crack initiates shows fatigue striations. Microstructural examination under optical and SEM shows a network of coarse carbide particles at the grain boundary as well as uniformly distributed fine carbide precipitates within the martensite matrix. Multiple cracks are observed to initiate and propagate from the surface through the clusters of grain boundary carbides. EDS analysis and elemental mapping suggest the carbides to be chromium carbides. SEM shows micro-cracking of carbide particles associated with the crack. Clustered distribution of carbides deteriorates toughness and initiates cracking at the roller surface subjected to thermal cycling because of their differential thermal expansion coefficient leading to the failure of the component.

Microstructure Evolution in a P911 Steel under Creep Conditions

Microstructure evolution in a P911 heat resistant steel was examined under conditions of aging and creep at a temperature of 600°C and an applied stress of 200 MPa. The tempered martensite lath structure (TMLS) evolved after heat treatment consisted of prior austenite grains (PAG), packets, blocks and laths. The mean transverse lath size and the interior dislocation density were about 345 nm and 3.5 × 1014 m-2, respectively. Various second phase particles precipitated upon tempering. Fine MX carbonitrides were homogeneously distributed throughout the tempered martensite laths, while relatively coarse M23C6 carbide particles were located on high-and low-angle boundaries. Upon creep test, precipitation of Laves phases was found. The stability of TMLS during creep is discussed in detail.

Effect of Coarse Carbide Particle on SR embrittlement in the HAZ of 21/4Cr-1Mo steel

The authors pointed out that growth of the carbide particle is related to SR embrittlement in the HAZ of 2 1/4Cr-1Mo steel in a previous study. Suitable image processing was employed to evaluate the shape of a large number of carbide particles in a SEM image. An image-dividing technique was needed in for the difference of tone in each area of a SEM image. Image processing included image dividing, brightness and contrast, as well as a contraction and smoothing operation. The shape of the particle changed from the sphere type to the needle or polygon type because of the progress of SR treatment. The change of aspect ratio of the particle in the cross-section has a good correlation with the increment of SR treatment time. However, the aspect ratio of the particles on the fracture surface did not correspond with that of the cross-section in the long time range. This was confirmed by SEM observation on a pair of fracture surfaces, where coarse carbide particles were broken into pieces. Coarse carbide particle may accelerate SR embrittlement by acting as a path for crack propagation and a crack initiation site for brittle fracture. The type of carbide particles was investigated by X-ray diffraction. M2C (Mo2C type), M7C3 (Cr7C3 type) and M23C6 (Cr23C6 type) carbides were identified in the time range of SR embrittlement. The coarse carbide observed when SR embrittlement occurred significantly was confirmed as M23C6 type by TEM observation. The calculation results based on the Eshelby theory show that the change of shape of the carbide particle can affect crack propagation of brittle fracture.

Martensite/particle interactions and the shape memory effect in an Fe–Mn–Si-based alloy

The effect of carbide precipitates with a size range of 30–300 nm on the austenite to martensite transformation has been studied. Such particles are known to enhance shape memory, and it was the aim of this work to clarify how the particles exert a favourable effect on shape memory. Differential scanning calorimetry revealed that the presence of particles increases the amount of thermally induced martensite. X-ray diffraction showed that the presence of particles increases the amount of stress-induced martensite also. Surface-relief produced on a pre-polished surface by bending deformation showed that the particle-containing samples exhibited a more complex and highly tilted surface-relief indicative of the formation of a larger volume fraction of martensite. The reversion characteristics of the particle-containing and solution-treated samples were similar: both showed co-reversion of multiple variants of martensite within the same volume of microstructure. However, a higher volume fraction of martensite reverted for the particle-containing sample on recovery annealing. The increased density of nucleation sites for martensite formation and a higher volume fraction of stress-induced martensite for a given strain are therefore considered to be the main contributions of relatively coarse carbide particles to the improvement of shape memory performance.

The influence of fast neutron irradiation and irradiation temperature on the tensile properties of wrought LCAC and TZM molybdenum

The effects of irradiation temperature on embrittlement are evaluated by the irradiation of wrought low carbon arc cast (LCAC) and TZM molybdenum in the High Flux Isotope Reactor at 294–1100 °C to neutron doses between 0.6 and 13.1 dpa Mo. Irradiation at 300 °C is shown to result in elevation of the ductile to brittle transition temperature (DBTT) from a pre-irradiated value of −100 to −50 °C to a post-irradiated value of 800 °C for both alloys with an increase in fracture stress. Irradiation at 560 °C also resulted in an increase in fracture stress, but the post-irradiated DBTT for LCAC (300 °C) was much lower than TZM (700 °C). Irradiation of both LCAC and TZM between 935 °C and 1100 °C resulted in little radiation hardening and a −50 °C DBTT for both alloys. The finer grain size and absence of coarse carbide particles may explain the slightly improved embrittlement resistance of LCAC compared to TZM for 600 °C irradiations.

Effects of Small Amounts of B, Nb and Ti Additions on Nucleation and Growth Processes of Intermediate Transformation Products in Low Carbon 3% Mn Steels.

The formation mechanism of intermediate transformation products and the effects of B as well as carbide forming elements such as Cr, Nb and Ti on the isothermal transformation characteristics of low carbon 3 wt% Mn steels have been investigated and the following results have been obtained.(1) Widmanstatten ferrite and bainite exibit both the diffusional and the displacive aspects in their formation processes.(2) The addition of the above described elements markedly retards the formation of primary ferrite, Widmanstatten ferrite and bainite in the specimens directly quenched to isothermal transformation temperatures from 1200°C, while the precipitation of coarse carbide particles in austenite accelerates the fololowing transformations.(3) Ferrite nucleation on coarse carbide particles can be explained in terms of the decrease in the activation energy for nucleation by the lattice correspondence at the interface.(4) The coarse carbide particles precipitated at austenite grain boundaries are related to the ferrite enclosing these particles with Kurdjumov-Sachs relationship.

Structures and mechanical properties of rapidly solidified deposits of an Fe-1.50wt.%C-10.7wt.%Cr alloy by low pressure plasma spraying

Powder of an Fe-1.50wt.%C-10.7wt.%Cr alloy was low pressure plasma sprayed onto substrates which were cooled to varying extents. When the maximum deposit temperature was below 773 K during spraying, the deposit was mainly composed of retained austenite supersaturated with the alloying elements. In a deposit which reached the maximum temperature of about 973 K, a large portion of the retained austenite decomposed to ferrite and fine carbide particles. Although the hardness of this deposit was high, its tensile strength, measured on tensile test pieces machined from the deposit which was separated from the substrate, was low because of weak adhesion at the boundaries between the flattened particles composing the deposit. In the case of maximum deposit temperature 1235–1357 K, the deposit consisted of the stable phases of ferrite and somewhat coarse carbide particles. The adhesion between the flattened particle was strong and the strength of the deposit was remarkably improved. This strengthening of adhesion was caused by enhanced diffusion of elements across the boundaries between the flattened particles when the sprayed droplets impinged onto the already-formed high temperature deposit.

Behavior of nucleation and growth of carbide layers on alloyed carbide particles in substrates in salt bath carbide coating

In salt bath immersion carbide coating, the effect of alloyed carbides in substrates is significant from the scientific and industrial points of view. This is because carbide layers grow by chemical bonding between carbon atoms in substrates and carbide forming elements in the baths, and by continuous supply of carbon atoms from substrates. It is concluded, from the use of substrates with very coarse carbide particles, that carbide growth behavior is highly dependent on combinations of species of the coating carbides and carbide particles in substrates. This can be explained in terms of the free energy for carbide formation of both coating carbide elements and substrate carbide elements.

The microstructural evolution, crystallography, and thermal processing of ultrahigh carbon Fe-1.85 pct C melt-spun ribbon

The microstructural evolution, mechanisms of grain refinement, crystallography, and thermal processing of a rapidly solidified Fe-1.85 pct C alloy have been studied by transmission electron microscopy (TEM). Melt-spun ribbons quenched in liquid nitrogen consist of carbide-free highly twinned martensite plates between 0.5-and 2.0-µm long and 0.1-and 0.5 -µm thick, with approximately 40 pct retained austenite also present. Ribbons tempered at 600 °C for 10 seconds consist of ferrite of approximately the same grain size and both intragranular and intergranular cementite precipitates. The intragranular cementite particles are about 0.1 /um or less in size and exhibit a single variant of the Bagaryatskii orientation relationship with respect to a given ferrite grain; the intergranular particles are about 0.1 µm in thickness and can be as long as 0.5 µm due to growth and/or coalescence along ferrite grain boundaries. A heat-treatment cycle investigated with a view toward generating structures suited for superplastic consolidation of the rapidly solidified ribbons consists of quenching the ribbon in liquid nitrogen, tempering at 600 °C for 10 seconds, “upquenching” to 750 °C (austenitizing) for 10 seconds, and subsequently quenching again in liquid nitrogen. This treatment results in martensite grains highly misoriented with respect to one another and typically 0.5 µm or less in both length and thickness and cementite particles 0.4 µm or less in size. (Occasionally, longer martensite plates were observed; but they never exceeded 1 µm in length.) The microstructures produced here offer the potential for producing fine-grained ultrahigh carbon steels of very high strength without the brittleness associated with the formation of coarse carbide particles or the loss of strength due to graphite formation. This investigation has thus provided the basis for follow-on studies currently underway in ultrahigh carbon Fe-C-Cr and Fe-C-Cr-Si steels, with the intent of producing similar microstructures which will also exhibit enhanced high-temperature stability.

Short-fatigue-crack growth in a nickel-base superalloy at room and elevated temperature

An optical system, developed to monitor short-crack growth at elevated temperature, was used to study the fatigue behaviour of Waspaloy. Tests conducted at 19°C and 500°C revealed that the dominant mechanism of crack formation was slip band cracking. Crack formation was also associated with coarse carbide particles within the matrix. The dominant failure mechanism at 19°C and 500°C was one of mixed mode-I and -II fracture, with the mode-II shear displacements giving rise to non-closure induced by surface roughness. Oxide- and plasticity-induced non-closure processes made only a minor contribution to the overall growth process. Short-fatigue-crack growth measured at R=0.1 was faster at 500°C than at 19°C at an equivalent value of ΔK 1. This was attributed to a change in slip character from highly planar to one involving increasing amounts of cross slip at 19°C and 500°C, respectively.

Characterization of Precipitates in Vanadium and Titanium Microalloyed Steels.

Transmission electron microscopic observations were carried out on carbon extraction replicas prepared from two commercial V-microalloyed steels and one commercial Ti-microalloyed steel hot rolled to 10 mm diameter bars at different rolling conditions.The presence of about 0.13% Cu in V- or Ti-microalloyed steels led to the formation of copper sulphides preferentially rather than manganese sulphides although the steels containing more than 1% Mn. In Ti-microalloyed steel, globular titanium carbosulphides of the type Ti4C2S2 are observed in addition to copper sulphide.In V-steels, transmission electron microscopy revealed very fine precipitates in the pro-eutectoid ferrite (≅5 nm) which are identified as carbides or carbonitrides of the type M (C, N) (M=V and Cr, where V/Cr≅5). Some relatively coarse particles (≅0.1 μm) were also observed (with V/Cr>15) which are suggested to be V-nitrides or carbonitrides formed at relatively high temperatures in austenite.In Ti-steel, coarse titanium nitride particles (>5μm) were observed together with precipitates of the type MC (M=Ti and Cr) in the form of very fine precipitates (≅5 nm, Ti/Cr≅9) or relatively coarse carbide particles (≅0.1μm, Ti/Cr>30).

Heat treatment of high-speed steel with continuous laser

1. When preliminarily hardened high speed steel, tempered at 350–560°C, is treated by a continuous CO2-laser with energy density J=34±3 MJ/m2, a strengthened layer with maximal thickness and hardness forms. 2. Accelerated heating by laser beam to temperatures in the range between Ac3 and Tpl and practically instantaneous cooling to normal temperature at rates of more than 104°C/sec give rise to a highly disperse (in melting) and fine-grained structure recrystallized by precipitation hardening (in quenching in the solid state) and consisting of martensite, residual austenite (in increased amount), and carbides (in a small amount). The intense dissolution of ledeburitic carbides type M6C in the laser-hardened zone causes additional alloying of the solid solution, increased stability of the residual austenite, and super-sautration of the finely accular martensite. 3. The decomposition of residual austenite and the intense dispersion hardening in the process of tempering at 560–600°C 1 h increase the hardness of the laser-hardened layer of high speed steel R6M5 by 2–4 HRCe, and resistance to tempering by 40–50°C compared with conventional heat treatment. The absence of coarse carbide particles in the hardened layer reduced the probability of brittle failure by chipping in operation of the cutting tool.

The effect of initial carbide morphology on abnormal grain growth in decarburized low carbon steel

The effect of fine or coarse carbide particles on abnormal grain growth in a low carbon steel subjected to small deformation was studied. Grain growth was induced in a decarburizing atmosphere at 1450 ‡F (788 ‡C) in order to study the effect of the initial carbide particle morphology on the subsequent motion of the grain boundaries at temperature. In this manner, the primary influence of second phase particles during straining could be evaluated. Transmission electron microscopy revealed that the large carbide particles exhibit a larger dislocation density area around them than the small carbide particles after straining up to ten percent. At higher strains, cell formation begins and the dislocation distribution is more uniform. Both coarse and fine carbide particle samples demonstrated three stage abnormal grain growth at strains below 10 pct. An incubation stage, which decreases with increasing strain, is followed by an abnormal grain growth stage, in which certain selected grains grow at the expense of other grains. Finally an equilibrium stage is reached in which the largest grain size is obtained for the lowest strain. At the lowest strains coarse carbide material has a longer incubation time and coarser Stage III grain size than the fine carbide alloy.

Creep-fracture initiation in 2¼%Cr-1%Mo steel

AbstractA detailed study has been made of grain-boundary damage in a 2¼%Cr-1%Mo steel after creep at 565°C. By subjecting the material to specific heat-treatments, a quantitative description of the resultant structures together with an assessment of creep damage has enabled creep cavitation to be correlated with certain microstructural features. Large numbers of cavities were distributed heterogeneously on prior austenite grain boundaries and an association with coarse carbide particles was evident. The cavity population was relatively insensitive to changes in the prior austenite grain size but was found to be inversely related to the width of precipitate-free zones (PFZ's) at the prior austenite grain boundaries. This result was linked with surface observations of zone shear and suggested that stress concentrations necessary for cavity nucleation were induced by local accommodation strains adjacent to the grain boundaries. On these lines, a theoretical nucleation model has been proposed and discussed in...

Comparison of the dispersion hardening of niobium alloys with carbides and nitrides

1. The improved strength characteristics of binary niobium-carbon and niobium-nitrogen alloys are mainly due to solid-solution hardening, which remains operative up to temperatures of 1000–1100°C. The carbide and nitride phases in these alloys are precipitated in the form of large irregular particles whose strengthening effect is negligible. 2. In multicomponent niobium alloys, dispersion hardening is achieved by the use of up to 3 vol.% of carbides and nitrides of the Group IVA metals. The composition, production technology, and methods of processing of these alloys are determined by the solubility of carbon and nitrogen, respectively, in the matrix phases. 3. At any given volume concentration of the hardening phases in these alloys, carbide hardening is more effective at temperatures below 1000–1100°C. At temperatures above 1100°C and also where improved processing characteristics are required, nitride hardening is preferable. 4. Carbide dispersion hardening is achieved by subjecting niobium alloys to special thermomechanical treatment, as a result of which very fine carbide particles are precipitated out of the supersaturated solid solution of the alloys. Nitride hardening is a result of the formation of a disperse two-phase structure in the course of solidification. 5. Certain processing operations (welding, heat treatment) raise the ductile-to-brittle transition temperature of dispersion-hardened niobium alloys. This effect is due to the presence of an excess nitrogen content in the solid solution in the case of nitride hardening and to the appearance in the structure (mainly at me grain boundaries) of coarse carbide particles in the case of carbide hardening.