Large-size Carbides Research Articles

Combination effects of laser heating and water cooling on the repair performance of Hastelloy C-276 by underwater laser direct metal deposition

The Ni-based Hastelloy C-276 was repaired by in-air and underwater laser direct metal deposition (in-air DMD and UDMD) with four gradient cooling rates (in-air DMD, in-air DMD with interlayer cooling, UDMD and UDMD with each layer re-covered by water). The microstructure and mechanical properties of repaired samples were systematically characterized with respect to the cooling rates. All samples presented good metallurgical bonding and columnar dendrites with epitaxial growth. The underwater environment had a more pronounced acceleration effect on the cooling rate compared to interlayer cooling. The rapid cooling induced intense thermal cycling, resulting in increased dislocation density and grain refinement with primary dendrite arm spacing (PDAS) diminishing from 5.30 to 3.21 μm. Accelerated cooling rate contributed to precipitation of massive M6C carbides. Carbides coarsened (sizes from 257 to 354 nm) and exhibited a reduced content as cooling rate slowed down due to intrinsic heat treatment (IHT). The residual water film in UDMD repair promoted oxide formation. Samples with increased cooling rates exhibited superior mechanical performance. The improved microhardness, tensile and impact properties were primarily attributed to the enhanced fine grain strengthening and Orowan strengthening facilitated via reduced grain size, increased dislocation density and carbide content. Quantities of large-size carbides were detrimental to tensile and impact properties as displayed in the in-air DMD sample with interlayer cooling (carbide size of 341 μm). The study could serve as a reference for in-situ restoration in deep-water environments.

Effect of Alloying and Microalloying Elements on Carbides of High-Speed Steel: An Overview

In high-speed steel, carbides are essential phase constituents, which have a direct impact on engineering performance and qualities of high-speed steel. The formation, morphology, and distribution of carbides are dictated by alloying elements. In this paper, various types of carbides in high-speed steel are presented. The effects of different alloying elements such as C, W, Mo, Cr, and V on the formation of carbides in high-speed steel are discussed. Research progresses on carbide improvement by microalloying elements such as N, B, Mg, and rare earth (RE) elements are reviewed. It is reported that Cr promotes the precipitation of M2C, N enhances the formation of fibrous M2C, Mg effectively shatters the large-size carbide grid, Nb refines granular carbide MC, and rare earth elements encourage the formation of M6C, resulting in irregular M2C lamellae. The incorporation of microalloying elements improves the distribution and size of carbides and also refines the solidification structure of high-speed steel.

Effects of V/Cr on the nucleation and transformation of Mo-containing carbides on Fe matrix in M50 bearing steel

The Mo-containing carbides in M50 bearing steel have an important effect on fatigue and wear resistance. It is significant to study the nucleation mechanism of Mo-rich carbides for controlling the type of carbides and improving the properties of steel. In this study, the α-Fe (100)/MoC (100) interface and the α-Fe (100)/Mo2C (100) interface with different structures are studied based on density functional theory. The results show that the interface with Fe–C covalent bond has the highest stability. The study of the electronic structure of the interface shows that the main component bonding of the interface is covalent and metallic, the electron orbital contribution consists mainly of the d orbitals of Fe and Mo atoms and the s and p orbitals of C atoms. In addition, the effects of V and Cr elements on the precipitation of Mo-rich carbides were studied. The results show that interface stability is improved with the increase of V element concentrations. With the addition of Cr atoms, the precipitation of Mo-rich carbides is inhibited. Under the effect of V element segregation, the spherical VC gradually nucleates at the Fe/Mo2C interface, and the large-size carbides are refined.

Effect of Zr on microstructure and hot cracking susceptibility of ZGH451 superalloy fabricated by direct energy deposition

Four groups of newly developed Ni-based superalloy samples with different Zr contents were manufactured by Direct energy deposition (DED). This study focuses on the influence of Zr element concentration on microstructure and cracking formation mechanism in DED-processed Ni-based superalloy ZGH451. Although Zr content is low, it strongly affects the formation of the precipitated phase and hot tearing tendency. Measurement of the crack area showed that the crack density value increases with the Zr concentration. According to thermodynamic simulation results and microstructures observed, Zr microsegregation in the interdendritic regions and considerable amounts of Zr is present in oxides or carbides. Zr contributes to forming large-size carbides mainly due to zirconia providing nucleation sites and the dissolution of Zr. Simultaneously, the solidification has been retarded due to the continuous accumulation of large Zr atoms, which increases the γ-γ′ eutectic fraction. Moreover, Zr addition could effectively decrease the solidus temperature of ZGH451 alloys, leading to the critical liquid film at the end of solidification. Based on these results, staying below 0.05 wt% of Zr content to reduce the risk of hot tearing in additive manufacturing superalloy was proposed.

Study on microstructure and properties of laser-clad Fe-based (Ti, V)C composite coatings

An in-situ autogenous ceramic phase reinforced laser cladding containing vanadium (V) and titanium (Ti) has been developed for improving the surface properties of rolls in the field of iron and steel metallurgy. In this study, the effect of the V and Ti content on the microstructure, hardness, shear property, and wear behavior in high temperatures of the laser cladding was evaluated. The results showed that with the increase of the V and Ti content, TiC and VC precipitated with a growing volume in the grain and grain boundaries, and their shapes gradually changed from circle to polygon. In addition, the grain size was smaller and smaller while the microhardness was higher and higher. The precipitation of TiC and VC ceramic phase has a bad effect on the shear properties of the cladding layer due to the weak bonding strength between large-size carbide and matrix. The cladding layer with 5 % content of V and Ti showed better wear-resisting properties which are >21 times and twice higher than that without V and Ti at room temperature and 600 °C, respectively. Indicating that the size of the ceramic phase has the most important effect on the wear property because the ceramic particles with larger sizes were easy to dislodge and cause scratches to the specimen.

Effects of Simulated PWHT on the Microstructure and Mechanical Properties of 2.25Cr1Mo0.25V Steel for a Hydrogenation Reactor

The effect of post-welding heat treatment (PWHT) on the microstructure and mechanical properties of large-thickness 2.25Cr1Mo0.25V steel was investigated through simulated post-welding heat treatment (SPWHT). The results showed that an increase in the SPWHT time decreased the toughness, hardness, and strength of the steel. After Min.SPWHT, the high-temperature tensile strength decreased more significantly, and the damage of Min.SPWHT to the high-temperature tensile strength reached approximately 80% of the Max.SPWHT. The microstructure of the tested steel before and after SPWHT consisted of granular bainite and lath bainite. After SPWHT, intergranular carbides were precipitated as coarsened carbides, carbide clusters, and chains of carbides; alloy element segregation occurred, and the segregation of Mo was the most serious, followed by Cr, and V. The precipitation behavior of the carbides and the increase in the effective grain size caused by the widening of the bainite–ferrite lath worked together and resulted in the decline of the impact toughness; the reduction in the solid solution and precipitation strengthening effects were the main factors in the strength reduction of the tested steel. In the high-temperature tensile tests, defects first appeared around the coarse carbides and carbide clusters. Controlling the size of the intergranular large-size carbides and the degree of cluster precipitation in the NT state structure may be a means of obtaining higher strength of the base metal subjected to PWHT.

Effect of magnesium treatment on microstructure and property of H13 die steel during EAF-LF-VD-CC steelmaking process

Due to the advantages of Mg on inclusions and carbides in H13 steel, Mg wire was added into industrial ladle after VD process to replace traditional Ca treatment in this paper, which is of great significance for industrial production of H13 steel. This paper focuses on the effects of Mg treatment and traditional Ca treatment on the inclusions, carbides, microstructure and mechanical properties in H13 steel. The results show that feeding Mg wire during EAF→ LF→ VD → CC steelmaking process can ensure the submerged nozzle not being blocked under the condition of continuous casting 15 heats of steel ladle, which means feeding Ca wire can be replaced with Mg wire from the aspect of steelmaking production. The primary carbides in Mg treatment steel are mainly precipitated with MgO or MgAl2O4 as the cores and small in size, while the primary carbides in Ca treatment steel are almost large-size carbides without core. The samples taken from the intermediate ladle show that the primary carbide types are all V-rich, Mo-rich and V–Mo-rich composite, and the size of primary carbides in Mg treatment steel is much smaller than that in Ca treatment steel. Adding Mg is helpful to improve the equiaxed crystal rate of continuous casting ingot. The annealed hot rolling samples show that Mg treatment could interrupt the M23C6 (M = Cr, Fe) chain carbides distributed along the grain boundary. The impact test shows that the dimple fracture surface become small and deep after adding Mg, which improves impact toughness significantly.

Effect of deep tempering on microstructure and hardness of carburized M50NiL steel

Through the deep medium-temperature tempering treatment, microstructure observation and hardness test of the carburized M50NiL bearing steel, the influence of the temperature and times of deep tempering on the microstructure and hardness of the steel were studied. The results show that the deep medium-temperature tempering is beneficial to the dissolution of large-size carbides in the carburized layer of the steel at the initial state, uniform distribution of carbides, and the precipitation of granular carbides from lath martensite. During deep tempering treatment, some granular carbides are connected to form rod-like carbides. With the increase of the number of deep tempering, the lath martensite in the steel is decomposed and the size is obviously refined, and more carbides in the core area are precipitated, whose distribution is more dispersed uniform. The deep tempering at 460 °C and 480 °C can slightly increase the surface hardness of bearing steel, but five times of medium temperature tempering at 500 °C makes the hardness of the steel slightly decreased. There exists the lowest point of hardness in the subsurface layer of the initial-stated M50NiL steel, which is however eliminated after 5-time deep tempering at 460 °C and 500 °C, and this is related to the elimination of stripe-like carbides after the two deep tempering processes.

Carbide Evolution in High-Carbon Martensitic Stainless Cutlery Steels during Austenitizing

The carbide phase is critical in high-carbon martensitic stainless steels (HCMSS) used for cutlery. In this study, carbide evolution during the austenitizing of different HCMSS (5Cr15MoV, 7Cr17MoV, 9Cr14MoV, and 9Cr18MoV) and its effect on the microstructure and mechanical properties are investigated. The statistical analysis of carbides suggests that the dissolution of small-sized secondary carbides ( 0.5 μm). Due to the influence of the elemental diffusion distance, the interfacial energy, and directional dissolution, small-sized carbides are more soluble than large-size carbides, which can be determined by the change of the ratio of Cr/Fe (wt.%) of carbides.

Effect of Partial Solution Treatment Temperature on Microstructure and Tensile Properties of 440C Martensitic Stainless Steel

The 440C martensitic stainless steel is considered to be among the hardest steels, owing to its high carbon content. Careful heat treatment of this material introduces multiple carbide particles, which can alter microstructure and mechanical properties. This study focused on the effect of austenitisation temperature on the microstructure and tensile properties of 440C steel. Austenitisation was performed on the austenite + carbide region, because 440C steel lacks a single-phase region. The steel was austenitised at two different temperatures; namely, 1160 °C and 950 °C, and subjected to oil quenching. The as-quenched samples showed a typical lath martensite structure with retained austenite phase. The treatments at 1160 °C and 950 °C promoted the formation of M7C3 and M23C6 carbides, respectively. The austenite grains in the sample treated at 1160 °C showed a higher growth rate than those in the sample treated at 950 °C. The sample treated at 1160 °C showed low-fraction and a large-size carbide phase. The Zener pinning force decreased, thereby increasing the austenite grain growth in the sample treated at 1160 °C. The hardness and 0.2% proof stress of the sample treated at 950 °C were higher than those of the sample treated at 1160 °C, owing to the higher martensite content in the former. The strength–ductility balance of the sample treated at 950 °C was higher than that of the sample treated at 1160 °C. The decreased austenitisation temperature resulted in improved mechanical properties of the steel. Therefore, the austenitisation temperature alters the microstructure and mechanical properties of 440C steel.

Effect of large-size carbides on the anisotropy of mechanical properties in 11Cr-3Co-3W martensitic heat-resistant steel for turbine high temperature blades in ultra-supercritical power plants

9–12%Cr martensitic heat-resistant steels (MHRS) are the key materials for making ultra supercritical (USC) unites turbine blades. In this study, the anisotropy of mechanical properties in Cr-Co-W-type MHRS 1Cr11Co3W3NiMoVNbNB is discussed. The samples were prepared for impact and tensile tests. By utilizing characterization techniques, such as OM, FESEM, TEM, and EDS, steel sample fracture surfaces and the microvoids as well as the microstructure-anisotropy mechanical properties relationship were thoroughly investigated. It was pointed out that a strong anisotropy of mechanical properties was observed as much lower impact toughness and tensile plasticity in transverse direction in the steel sample after a heat treatment process. The anisotropy was mainly due to the large-size directionally distributed M23C6-type and M6C-type carbides. Large-size directionally distributed carbides lead to anisotropy of mechanical properties mainly by promoting the formation of microcracks and the propagation of cracks in transverse specimens. For transverse impact specimens, the propagation of microvoids is promoted to banded brittle cracks by large-size directionally distributed carbides. These banded brittle cracks become the pre-cracks of matrix and markedly decrease the toughness. For transverse tensile specimens, the propagation of microvoids is promoted to shallow chain-distributed dimples by large-size directionally distributed carbides, which markedly decrease the plasticity.

Influence of Inter-Pass Cooling on Microstructural Evolution and Plastic Deformation of Heavy EH47 Plates.

Herein, the influence of inter-pass cooling (IC) and conventional two-stage rolling (CTR), on microstructural evolution and plastic deformation behavior of ultra-heavy EH47 plates, is demonstrated. It is reported that the deformation amount and deformation rate, in every deformation pass during rough rolling, at 1/4- and 1/2-thickness of IC steel were higher than the CTR steel. The volume fraction of ferrite and acicular ferrite was 45% and 18%, at 1/4-thickness, and 35% and 50% at 1/2-thickness of IC steel, respectively, whereas the sum of both ferrite phases was smaller than 25% in the CTR steel. The austenite grain boundary area and high-angle grain boundary fraction in the IC steel were higher than the CTR steel. The high density of fine and shapeless pearlite has been observed in IC steel, whereas large-size carbides, with hexagonal structure, have been observed in CTR steel. Compared to the CTR steel, the density of precipitates was apparently lower in IC steel. Two kinds of Nb containing precipitates, such as (Ti, Nb)(C, N) and (Nb, Ti)C, were observed in the tested steels. Total ductility and uniform elongation of the IC steel were higher than the CTR steel. During the tensile process, the crack initiation energy and crack propagation energy of the IC steel were higher than the CTR steel. Moreover, the volume fraction of retained austenite (FCC) was reduced from 7.71% to 0.42% near the tensile fracture in IC steel at 1/4-thickness. In additon, the strain of synergetic plastic deformation of the IC steel was higher than the CTR steel. Meanwhile, compared to the CTR steel, the synergetic plastic deformation of the IC steel occurred at low stress after the yield point, which can be ascribed to the presence of fewer microcracks in the IC steel. Hence, a delayed fracture has been observed in the IC steel plate.

Selection of wall thickness of circular dies used in press tools for pressing large-size carbide parts

1. The strength of dies used for pressing carbide components will be adequate if the valuea=r2/r1=1.15. However, strength should not be the only criterion for design as this leads to poor quality of pressed components. Additional calculation of rigidity is essential. 2. The rigidity of dies used for pressing carbide components will be adequate if the valuea=r2/r1=1.3. It is preferable to design only the working zone of the die for rigidity and the remaining portion for strength, i.e., to use dies with variable wall thickness.

Research on Pitting Strength under Rolling Contact of White Cast Iron

A study on the pitting resistance of white cast iron containing 1.1-2.8%C, which was cast or forged, was carried out by performing experiments using an Amslar-type wear testing machine under sliding conditions with light oil as a lubricant.The results obtained are summarized as follows:(1) Pitting resistance of white cast iron which contains higher carbon (C>1.8%) is significantly improved by forging.(2) The most hardened layer generated by repeating contact stresses exists in 0.10∼0.20mm below the contact surface. The higher the contact stress is, the more the hardness increment of the hardened layer is.(3) It seems that the propagation of crack is strikingly affected by the shape of large-size carbide and the adhesive strength between carbide and matrix.