Large Primary Carbides Research Articles

Dissolution of Primary Carbides and Formation and Healing of Kirkendall Voids in Bearing Steel under Pulsed Electric Current

The alloying elements in 8Cr4Mo4V bearing steel tend to form large primary carbides with carbon, which not only causes element segregation but also becomes the primary source of fatigue crack initiation, thereby decreasing the material's usability and machinability. Owing to the excellent thermal stability of primary carbides, traditional homogenization annealing requires high temperatures, which is both time‐ and energy‐intensive. Excessively high heat treatment temperatures can also result in “overburning” of the sample. Herein, primary carbides are rapidly dissolved at low temperatures using pulsed electric current treatment. The additional free energy introduced by the pulsed electric current lowers the thermodynamic barrier for carbide dissolution. During the second‐phase dissolution process, the unbalanced diffusion of elements may cause the formation of Kirkendall voids. Due to the difference in electrical conductivity between the voids and the matrix, the pulsed electric current generates thermal compressive stress on the voids, promoting rapid atom migration to these voids and accelerating their healing. This pulse‐current treatment technology offers a novel method for the rapid dissolution of carbides in alloys at low temperatures and for the rapid healing of related voids.

Refinement of Large Primary Carbides by Adjusting Annealing Parameters in 45Cr9Si3 Valve Steel

Refinement of Large Primary Carbides by Adjusting Annealing Parameters in 45Cr9Si3 Valve Steel

Surface crack analysis of 8Cr4Mo4V steel based on Crystal/Phase microstructure model by grinding

Metal polycrystalline materials tend to be homogeneous and isotropic at the macroscopic level, which leads it difficult to visually characterize the sprouting and expansion of cracks by traditional grinding simulation. In this study, the Johnson–Cook constitutive model was used for the finite element method (FEM) based on grinding characteristics with a high strain rate and large plastic deformation. The models are focused on the actual microstructure distribution, one of them contains grains and grain boundaries, and another contains phase organization (martensite and acicular martensite) and carbides. The study pinpoints the changes in microstructure under different grinding parameters during the grinding process and the influences on the formation of microscopic cracks. Ultimately, it was found that the shedding of large primary carbide is a critical factor for inducing surface cracks. The uncoordinated deformation between brittle carbides with the other phases is the main cause of carbide shedding. Therefore, this article visualized shows the influence of grinding parameters on surface cracks from the perspective of microstructure modeling, which is of great significance in guiding practical processing.

Study on modification of primary carbides and improvement of properties in H13 steel by CeO2 nanoparticles addition

The large primary carbides in H13 steel significantly affect its service life. In this study, pre-dispersion technology is used to disperse CeO2 nanoparticles, which are then introduced into H13 steel. The effects of CeO2 nanoparticles on the inclusions, primary carbides, microstructures, and properties of H13 steel are investigated. Ce-containing inclusions with size of 0.5–2 μm are generated and distributed uniformly in H13 steel owing to the reduction of CeO2 by Al and C. Both the dendrite structure and primary carbides in H13 steel are refined by CeO2 nanoparticles. The secondary dendrite arm spacing decreases from 13.51 ± 2.34 to 6.90 ± 1.23 μm. The size and number of primary carbides decrease from 1 to 5 to 1–3 μm, and from 1125 to 725/mm2, respectively. This is attributable to the disregistry of Ce-containing inclusions and primary carbides/austenite. The segregation of alloy elements in H13 steel is calculated using the Clyne–Kurz model. The properties of H13 steel are improved via the addition of CeO2; in particular, its tensile strength, total elongation rate, and hardness are 726.38 MPa, 23.43%, and 243.49, respectively, which corresponds to increases by approximately 5.58%, 8.27%, and 10.11%, respectively. The results can facilitate improvements to the performance of H13 steel and the development of rare earth steel.

Aluminum inoculated overlay microstructure for enhanced scratch resistance

Large M7C3 carbides were refined by inoculation with small levels (0, 0.1, 0.5, and 1 wt%) of aluminum in chromium carbide overlay. Multiple cracks were observed in the large primary M7C3 carbides in 0% aluminum sample, while less cracking was observed for refined M7C3 carbides, under similar scratch tests. The refinement mechanism is believed to involve the heterogeneous nucleation of carbides on Al2O3 that has formed prior to M7C3 nucleation. Aluminum is shown to shorten the growth time of primary M7C3 carbides by raising the eutectic temperature. The overlay is slightly softer in the 1% aluminum sample due to a decrease the amount of martensite and an increase of retained austenite in the overlay matrix. A new approach is introduced to refine the primary M7C3 carbides by using a small amount of Al addition without producing new hard phases.

Effect of laser pulse power on solidification cracking susceptibility in surface processing of a high carbon high chromium tool steel

High carbon, high chromium tool steels are deep hardening steels widely used for cold forming dies. A 400 W Nd:YAG laser was employed to melt the surface of the AISI D2 tool steel with 5 and 6 ms single pulses of laser at 4–10 J energy levels. The microstructural study showed that complete dissolution of large primary carbides occurred during melting, but the cooling rate was too fast to form any primary carbide. Increasing the laser energy (peak power) led to a coarser dendritic microstructure and a reduction in solidification cracking intensity. The analyses indicate that the coarser dendritic structure has promoted the backfilling effect. However, despite having a coarser dendritic structure, the material laser processed with a preheat of 300 °C showed the most severe crack intensity due to the provision of sufficient time in cooling for the formation of a network of brittle carbides in the inter-dendritic regions. The HAZ was found to be sound although reaching a hardness of 750 HV. • A fine columnar dendritic microstructure was formed by laser remelting, which became coarser with increased laser peak power. • Facilitation of the backfilling effect in higher laser peak powers reduced the intensity of solidification cracks.. • The formation of the inter-dendritic carbide phase in the preheated sample increased the solidification cracking intensity.

High chrome cast iron for mineral processing and conveying equipment parts

The main reasons for the formation of microand macrocracks and other defects in waterjet castings, which significantly reduce their service life, are analyzed. It has been established that the introduction of ferroalloys together with the charge to the bottom of the arc furnace bath leads to a significant segregation of elements along the height (volume) of the melt in the bath furnace, and large primary chromium carbides remain in the casting structure. They, as a rule, form eutectics with austenite in the form of large conglomerates, which contribute to the appearance of microcracks in castings during their mechanical and thermal processing. It is shown that alloying, modification and homogenizing exposure of the melt for 5 min at 1550 °C and subsequent refining have a significant effect on the structure and morphology of the carbide phase and structural components of the metal base of high-chromium cast iron and purify it from harmful impurities and non-metallic inclusions. In addition, the amount of primary carbides is reduced, the formation of austenitic-carbide eutectic conglomerates is excluded, and the dispersion of the carbide phase and the structure of high-chromium cast iron are increased. Based on the ICH300Kh28NMWFTP cast iron created by JSC «NPO «TSNIITMASH», an effective export technology for the manufacture of waterjet castings has been developed, which makes it possible

Effect of Ti Content on the Behavior of Primary Carbides in H13 Ingots

The Ti element plays a role in pinning grain boundaries but also has a good binding ability to C and N, forming large primary carbides. Therefore, the effect of Ti content on primary carbides’ behavior in H13 ingots was comprehensively studied. A non-aqueous electrolysis method was used to determine the three-dimensional (3D) characteristics of primary carbides. We found a great difference between the two-dimensional (2D) and the three-dimensional characteristics of primary carbides. When performing 2D analyses, the density of the primary carbides appeared high, while their size was small. The actual characteristics of primary carbides can be obtained only by 3D observation. The primary carbide showed a typical dendritic structure, whose center consisted of Ti–V-rich carbide wrapped by V-rich carbide. As the Ti content increased, the size of the primary carbide increased from 24.9 μm to 41.3 μm, and the number density increases from 25.6 per/mm2 to 43.9 per/mm2. The Ti4C2S2 phase precipitated first, then changed into Ti–V-rich carbide, and finally further partly transformed into V-rich carbide. The addition of elemental Ti promoted the precipitation and transformation of primary carbides, resulting in an increase of the number density and size.

Comparison between EDM and grinding machining on fatigue behaviour of AISI D2 tool steel

Non-conventional machining processes, such as the Electrical Discharging Machining (EDM) offers the possibility to machine very hard materials with great potential to generate complex geometries. However, the EDM process involves significant microstructural changes in the as machined surfaces, which are known to have a significant impact on fatigue behaviour of mechanical parts. This study aims at investigating the fatigue behaviour of the AISI D2 tool steel subjected to EDM machining. Both smooth and notched specimens are tested aiming at assessing the fatigue notch sensitivity of this material and results compared with conventional grinding techniques in terms of fatigue resistance and residuals stresses. The AISI D2 steel shows a microstructure composed of large dendritic primary carbides detrimental to the fatigue resistance. Also, the EDM process is responsible for the generation of detrimental tensile residual stresses that justifies the lower fatigue resistance when compared with the ground material that exhibits beneficial compressive residual stresses. The fatigue test data of the notched specimens machined by EDM process show a high fatigue notch sensitivity of the AISI D2 tool steel. The heterogeneous structure of the material, showing large undissolved carbides, leads to a notch fatigue reduction factor that is higher to the apparent theoretical elastic stress concentration factor.

Comprehensive investigation of the microstructure-property relationship of differently manufactured Co–Cr–C alloys at room and elevated temperature

The purpose of this study was to investigate the influence of the microstructure on sliding wear and hardness of four different Co–Cr–C alloys at room and elevated temperature. Different microstructures were produced by applying three different processes. The hardness, hot hardness and wear loss at room temperature of these alloys correlate strongly with the carbide volume content. In sliding wear tests against an Al2O3 ball, abrasive wear occurs at room temperature. The size or geometric arrangement of the carbides or metal matrix plays a minor role at room temperature. At 600 °C the wear behaviour changes due to the softening matrix. In alloys with small free matrix path lengths, the highest wear rates occur due to micro-fatigue and micro-cracking. In hypoeutectic alloys with a high free matrix path length, the carbides lose their effectiveness due to the lack of support by the matrix. In these alloys, wear is dominated by the properties of the matrix. A hypereutectic casting alloy with large primary carbides shows the best wear results, as the carbides support themselves due to their size and retain their wear-reducing effect.

Study of M50NiL steel under carburizing and nitriding duplex treatment

To better understand the behavior of M50NiL steel under carburizing and nitriding duplex treatment, different carburizing temperatures (ranging from 890 to 980 °C) were used for the carburizing pretreatment. The surface microstructures were characterized by X-ray diffraction (XRD), laser scanning confocal microscopy, microhardness testing, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicate that the proportions of γ′ and large submicron precipitates increase with increasing carburizing temperature, which corresponds to an increase in large primary carbides. The increase in the number/size of large primary carbides promotes the diffusion of nitrogen. The resulting changes in the nanometric precipitates after nitriding lead to different levels of solid solution by nitrogen (reaching a maximum at a 920 °C carburizing pretreatment). The growth mechanism of the submicron precipitates is proposed. The effect of various precipitates and the diffusion behavior of C and solid solution of N on the hardening behavior are discussed.

Chemical composition and structural identification of primary carbides in as-cast H13 steel

The aim of this study was to characterize the primary carbides formed in as-cast H13 steel. The composition, morphology, type, and size of primary carbides in as-cast H13 steel were investigated by optical microscope (OM), field emission scanning electron microscopy (FE-SEM), electron back-scattered diffraction (EBSD), and X-ray diffraction (XRD) analysis. The number of primary carbides was investigated by ASPEX automated inclusion analysis system. The results indicated that primary carbides in as-cast H13 steel are mainly composed of Cr, Mo, V, and Ti, and there exist four kinds of primary carbides in the interdendritic zones of H13 steel, which are stripy Mo–Cr-rich M2C, eutectic Mo–Cr-rich M2C, V-rich MC, and V-rich MC with Ti and N. Thermodynamic calculation indicated that M2C precipitates in liquid phase at solid fractions larger than 0.99, while MC precipitates in liquid phase at solid fractions larger than 0.96. Statistical results indicated that the number of M2C is much greater than the number of other kinds of primary carbides. Most primary carbides are blocky, with lengths of no more than 10 μm and a length/width ratio of no more than 3. The large primary carbides in as-cast H13 steel are mainly M2C.

Evolution Behavior of M23C6 Carbides Under Different Hot Deformation Conditions in Alloy 602 CA

Alloy 602 CA is a M23C6 carbides strengthened Nickel-based superalloy. In this alloy, M23C6 is different from that found in other superalloys because of its high volume fraction, wide size distribution and distinct precipitation locations. To investigate M23C6 evolution behaviors during hot deformation in Alloy 602 CA, a series of tests are carried out in this study. The test temperature ranges from 900 to 1100 °C and strain rate ranges from 0.001 to 0.1 s−1, with a true strain of 0.7. It is found that hot deformation of the material decreases the distance between carbides which leads merging happening to change the morphology of primary M23C6. Secondary carbides mainly come from the dissolution of large primary carbides and precipitate along grain boundaries and twin boundaries which impede dynamic recrystallization (DRX) from happening. Volume fractions of M23C6 decrease from 900 to 1100 °C. At 900 °C and 1100 °C, the precipitation equilibrium is reached in the preheating period before hot deformation and deformation only change the size distribution of carbides. 1000 °C is a transition temperature where the volume fraction decreases as strain rate increases. The temperature rise initiated by adiabatic heating has a great influence on this precipitation behavior.

Microstructure and Wear Performance of Cobalt-Containing Iron-Based Slag-Free Self-shielded Flux-Cored Wire

A new type of cobalt-containing iron-based slag-free self-shielded flux-cored wire was developed for hardfacing. The influence of cobalt additions on the microstructure and wear performance of the hardfacing was investigated. The cobalt-containing iron-based hardfacing alloy has a typical hypereutectic microstructure consisting of primary M7(C, B)3 carbide, and a eutectic of M3(C, B) carbide and austenite (part of which subsequently transformed into martensite). Cobalt promoted the formation of austenite and changed the morphology of the primary carbide and eutectic colonies. The morphology of primary carbide became fragmented with increasing cobalt due to the possible impeding effect of cobalt to the movement of the M7(C, B)3/liquid interface. Different structures of the eutectic carbides were observed—the chrysanthemum structure for 1.42 wt.% cobalt, the skeleton form for 2.88 wt.% cobalt, the radial stripe 4.37 wt.% cobalt and the discrete stripe for 5.75 wt.% cobalt. Wear loss of the hardfacing alloy containing 2.88 wt.% cobalt was the smallest among all the alloys, owing to the large primary large carbide and skeleton-shaped eutectic carbides, the proper amount of austenite phase separating the M7(C, B)3 and M3(C, B) carbides, the solid solution strengthening by cobalt in the matrix, and the high hardness of the alloy. Moreover, the wear resistance has a tendency to decrease with the increase in velocity as well as load.

Precipitation Behavior of Large Primary Carbides in Cast H13 Steel

The large primary carbides precipitated during solidification can significantly reduce the performance of H13 steel. To better inhibit the formation of primary carbides, it is necessary to obtain the precipitation mechanism of primary carbides in H13 steel. First, an electronic probe microanalyzer (EPMA) is used to analyze the microsegregation of alloying elements in the last‐to‐solidify region. The enrichment of elemental V, Mo, Cr, and C in the last‐to‐solidify region is the main reason for the formation of primary carbides. The number density, size, and morphology of the carbides in a two‐dimensional (2D) observation are obtained through an automatic inclusion analysis system. A non‐aqueous electrolysis method is used to show the three‐dimensional (3D) morphology of the primary carbides. There is a large difference between the 2D and 3D primary carbide observation. Finally, the precipitation mechanism and process of carbides during solidification are calculated with the Thermo‐Calc thermodynamic software, and the calculated results are in keeping with the experimental observation.

The performance properties of chromium cast irons with different degree of eutectic

The influence of carbon and the type of forms on the wear resistance, hardness and strength of samples from chromium iron made of ИЧХ18ВМ cast iron was investigated. The analysis of the structural structure of the samples of cast iron ИЧХ18ВМ depending on the type of shape and amount of carbon was made. It is established that regardless of the type of forms with increasing carbon content the formation of large primary carbides is observed, and then the emergence of particularly large trigonal carbides in a hypereutectic iron was confirmed. It is shown that regardless of the type of forms the formation and growth of carbides are influenced by such factors as an increase in the carbon content and the rate of solidification. A comparison was made between the hardness, strength, and wear resistance of samples obtained at different heat exchange and solidification rates.

Control of Carbide Precipitation During Electroslag Remelting-Continuous Rapid Solidification of GCr15 Steel

The effect of the electroslag remelting-continuous rapid solidification (ESR-CRS) process on element segregation and carbide precipitation in GCr15 bearing steel was investigated. The results showed that a microstructure with fewer primary carbides and less segregation can be achieved via the ESR-CRS process. In the specimen subjected to ESR, the morphology of primary carbide changed from angular to lumplike. After the ESR-CRS process, the dimension of primary carbide and the mass fraction of Cr in primary carbide decreased. With the increase of cooling intensity during the ESR-CRS process, the microstructure of ESR ingot became more refined and uniform and the size of large primary carbides in ESR ingot gradually decreased. Suppressing the formation of grain boundary cementite and primary carbide during ESR of GCr15 steel is beneficial to inhibiting the presence of large secondary carbide formation after annealing. With the increase of cooling intensity, the mass fraction of carbides in ESR ingot decreased and the mass fraction of Cr in carbides increased, whereas the types of carbides did not change; all the M3C, M3C2, and M7C3 exist before and after ESR-CRS. Al2O3 inclusions promoted the formation of Ti(N,C) by serving as preferred heterogeneous nucleation sites, whereas the formation of Ti(N,C) was suppressed through the refinement of Al2O3 inclusions by increasing the cooling intensity.

MICROSTRUCTURE FEATURES OF CHROME-NICKEL COATING WELDED WITH FILLER WIRE PL AN-111 WITH A 50% OVERLAP

Purpose . The paper involves investigation of microstructure features of the coating welded with filler wire PL AN-111 with a 50% beads overlap. Methodology. Wear-resistant layer was formed by means of electric arc deposit welding using filler wire PL AN-111 on the plate from steel 09G2S. Deposit welding was conducted under the following parameters: welding current is of 650-750 A; arc voltage is of 30-34 V; welding speed is of 32 m/h. Microstructure was researched with application of optical microscopies “Neophot-21”, “Nikon Eclipse M200” and electron scanning microscopy JEOL JSM-6510 LV. Microhardness of structural constituents was measured with tester FM-300 (Future-Tech) under loading of 10-50 g. Findings . It is shown that the overlap of the beads leads to the formation of inhomogeneous microstructure in the cross section that varies by zones from free-carbide austenite to hypereutectic microstructure with primary chromium carbides. The analysis of the microhardness of the structural constituents in various coating areas was carried out. It was found that hardness of austenite, carbide eutectic and carbides M 7 C 3 varies in coatings in the range of 3 100-3 850 МPа, 4 100-6 800 МPа and 12 100-15 100 МPа, accordingly. Originality . Authors determined that Cr-Ni coating comprises substantially austenitic-carbide eutectic with different density and thickness of carbide fibers within eutectic colonies. Along the border “base/coating” a single-phase austenitic layer lies which turns into a layer with a hypoeutectic structure. In the heat affected zone from beads fusion austenite disintegration with the granular carbides formation was recorded. This leads to decreasing of matrix corrosion resistance due to chromium depletion. Above the zone of beads fusion, the coating has a hypereutectic structure with the presence of large primary chromium carbides. Practical value . It is shown that deposit welding with filler wire PL AN-111 with a 50% overlap of beads results in a coarsening of the structure due to formation of a hypereutectic structure comprising in the presence of large primary chromium carbides with lower microhardness.

Effect of nitrogen alloying on the microstructure and abrasive impact wear resistance of Fe-Cr-C-Ti-Nb hardfacing alloy

Abstract The effect of nitrogen additive on the microstructure and abrasive impact wear resistance of high chromium Fe-Cr-C-Ti-Nb hardfacing alloy, which developed by an open arc welding technique was investigated in this work. The phase structure and microstructures of the hardfacing alloys were studied by means of optical microscopy (OM), X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), energy dispersive X-ray spectrometry (EDS) and transmission electron microscope (TEM). Moreover, the hardness of carbides in the hardfacing alloy was measured by nanoindentation test, and abrasive impact wear resistance was determined. Besides, the influence of variables on the wear behavior was investigated. The results show that, the high chromium Fe-Cr-C-Ti-Nb hardfacing alloys with different nitrogen additive mainly consist of primary (Cr, Fe)7C3 carbides, eutectic matrix and (Ti, Nb)(C, N) precipitates. With increase of nitrogen content in high chromium Fe-Cr-C-Ti-Nb hardfacing alloy, the primary carbides can be refined obviously. Meanwhile, the hardness of (Cr, Fe)7C3 is increased slightly from 18.7 GPa to 21.3 GPa, and that of (Ti, Nb)(C, N) is from 27.0 GPa to 27.6 GPa. Moreover, the volume loss of hardfacing alloys is decreased from 72 mm3 to 39 mm3. During abrasive impact wear test, the large primary carbides in the hardfacing alloy without nitrogen additive are fractured and spalled from the matrix, its abrasive impact wear resistance is low. While, the refined (Cr, Fe)7C3 carbides in the hardfacing alloy with nitrogen additive can be bended during the wear test, which increase its abrasive impact wear resistance. The lattice disregistry between (Ti, Nb)(C, N) and (Cr, Fe)7C3 is 6.15%, which indicates that (Ti, Nb)(C, N) can serve as heterogeneous nucleation sites for the primary (Cr, Fe)7C3 carbides. So that, the large primary (Cr, Fe)7C3 are refined and the abrasive impact wear resistance is improved. In addition, it is found that the volume loss of HC2N is increased with the increase of hardness, size, velocity and impact angle of the abrasive particles.

Effect of Oxide Inclusions Modification During Electroslag Remelting on Primary Carbides and Toughness of a High-Carbon 17 mass% Cr Tool Steel

The effect of oxide inclusions modification during electroslag remelting (ESR) process on nitrides and primary carbides in as-cast and annealed 8Cr17MoV tool steel was investigated. Impact toughness testing and fractography were performed. Primary carbides M7C3 were found to be dominant precipitates in as-cast ingot, besides a very small fraction of M23C6-type carbides. Al2O3 and MgO·Al2O3 inclusions promoted the formation of (Ti,V)N and primary carbides through serving as preferred nucleation sites, whereas heterogeneous nucleation of (Ti,V)N and primary carbides was avoided by modifying Al2O3 and MgO·Al2O3 inclusions to calcium aluminates. Oxide inclusions modification only refined the dimensions and distribution of carbides, but did not change the types and compositions of carbides. The improvement in the dimensions and distribution of carbides increased significantly the impact toughness of annealed steel. Large secondary carbides precipitated near retained primary carbides in annealed steel, which lowered greatly the impact toughness of 8Cr17MoV steel. Suppressing the formation of large primary carbides in as-cast ingot was favorable to preventing large secondary carbides formation after annealing, which improved greatly the impact energies of annealed steel accordingly.