Influence of Rare Earth Elements on Microstructure and Mechanical Properties of Cast High-Speed Steel Rolls

The influence of rare earth （RE） elements on the solidification process and eutectic transformation and mechanical properties of the high-V type cast, high-speed steel roll was studied. Test materials with different RE additions were prepared on a horizontal centrifugal casting machine. The solidification process, eutectic structure transformation, carbide morphology, and the elements present, were all investigated by means of differential scanning calorimetry （DSC） and scanning electron microscopy energy dispersive spectrometry （SEM-EDS）. The energy produced by crack initiation and crack extension was analyzed using a digital impact test machine. It was found that rare earth elements increased the tensile strength of the steel by inducing crystallization of earlier eutectic γ-Fe during the solidification process, which in turn increased the solidification temperature and thinned the dendritic grains. Rare earth elements with large atomic radius changed the lattice parameters of the MC carbide by forming rare earth carbides. This had the effect of dispersing longpole M C carbides to provide carbide grains, thereby, reducing the formation of the gross carbide and making more V available, to increase the secondary hardening process and improve the hardness level. The presence of rare earth elements in the steel raised the impact toughness by changing the mechanism of MC carbide formation, thereby increasing the crack initiation energy.

Precipitation and decomposition behaviors of carbides in AISI M2 high-speed steel with nitrogen and mischmetal

The behaviors of the precipitation and decomposition of carbides in AISI M2 high-speed steel modified by nitrogen and mischmetal were investigated using DSC, XRD, SEM and TEM. The as-cast microstructure of the experimental steel consists of dendrites of iron matrix, networks of eutectic carbides and secondary carbides. The average distance between networks is about 34 μm. The carbides mainly include M_2C, M(C,N) and M_6C, and their relative contents are 58.5%, 30.3% and 11.2%, respectively. The average spacing between the M_2C fibers is 1.5 μm. The decomposition of M_2C occurs from 897.2 to 1221.5 ℃(heating rate of 200 ℃/h). Some precipitated carbide particles occur in the M_2C matrix after holding for 15 min at 1100 ℃. With increasing holding time, the carbide fibers neck down more and more obviously until they are broken down. The spectral peaks of M_2C almost disappear after holding for 60 min. The spectral peaks of M_6C gradually strengthen with the holding time, and the relative content of M_6C increases to 79.8% after holding for 60 min. After holding for 180 min, the carbide fibers disappear, and the decomposition products consist of fine carbide particles(about 300 nm) and short rod-like carbides(about 3.5 μm).

PREDICTION OF FLOW STRESS OF HIGH-SPEED STEEL DURING HOT DEFORMATION BY USING BP ARTIFICIAL NEURAL NETWORK

The hot deformation behavior of TI (18W-4Cr-1V) high-speed steel was investigated by means of continuous compression tests performed on Gleeble 1500 thermomechan- ical simulator in a wide range of tempemtures (950℃-1150℃) with strain rotes of 0.001s-1-10s-1 and true strains of 0-0. 7. The flow stress at the above hot defor- mation conditions is predicted by using BP artificial neural network. The architecture of network includes there are three input parameters:strain rate,temperature T and true strain , and just one output parameter, the flow stress ,2 hidden layers are adopted, the first hidden layer includes 9 neurons and second 10 negroes. It has been verified that BP artificial neural network with 3-9-10-1 architecture can predict flow stress of high-speed steel during hot deformation very well. Compared with the prediction method of flow stress by using Zaped-Holloman parumeter and hyperbolic sine stress function, the prediction method by using BP artificial neurul network has higher efficiency and accuracy.

Effect of melting rate on microsegregation and primary MC carbides in M2 high-speed steel during electroslag remelting

Large-size primary MC carbides can significantly reduce the performance of M2 high-speed steel.To better control the morphology and size of primary MC carbides,the effect of melting rate on microsegregation and primary MC carbides of M2 steel during electroslag remelting was investigated.When the melting rate is decreased from 2 kg·min^(-1) to 0.8 kg·min^(-1),the columnar dendrites are gradually coarsened,and the extent of segregation of Mo and V is alleviated,while the segregation of Cr becomes severe.At 2 kg·min^(-1),the number of primary MC carbides per unit area with the sizes in the range of 2 μm to 6 μm accounts for about 75% of all MC carbides,while the carbides are mainly concentrated on the size larger than 8 μm at 0.8 kg·min^(-1).Thermodynamic calculations based on the Clyne-Kurz (simplified to C-K) model shows that MC carbide can be precipitated in the final solidification stage and a smaller secondary dendrite arm spacing caused by higher melting rate (2 kg·min^(-1) in this experiment) facilitates the refinement of primary MC carbides.

Microstructure simulation of rapidly solidified ASP30 high-speed steel particles by gas atomization

In this study, the microstructure evolution of rapidly solidified ASP30 high-speed steel particles was predicted using a simulation method based on the cellular automaton-finite element （CAFE） model. The dendritic growth kinetics, in view of the characteristics of ASP30 steel, were calculated and combined with macro heat transfer calculations by user-defined functions （UDFs） to simulate the microstructure of gas-atomized particles. The relationship among particle diameter, undercooling, and the convection heat transfer coefficient was also inves- tigated to provide cooling conditions for simulations. The simulated results indicated that a columnar grain microstructure was observed in small particles, whereas an equiaxed microstructure was observed in large particles. In addition, the morphologies and microstructures of gas-atomized ASP30 steel particles were also investigated experimentally using scanning electron microscopy （SEM）. The experimental re- suits showed that four major types ofmicrostructures were formed： dendritic, equiaxed, mixed, and multi-droplet microstructures. The simu- lated results and the available experimental data are in good agreement.

Microstructure,Properties and Crack Suppression Mechanism of High-speed Steel Fabricated by Selective Laser Melting at Different Process Parameters

To enrich material types applied to additive manufacturing and enlarge application scope of additive manufacturing in conformal cooling tools,M2 high-speed steel specimens were fabricated by selective laser melting(SLM).Effects of SLM parameters on the microstructure and mechanical properties of M2 high-speed steel were investigated.The results showed that substrate temperature and energy density had significant influence on the densification process of materials and defects control.Models to evaluate the effect of substrate temperature and energy density on hardness were studied.The optimized process parameters,laser power,scan speed,scan distance,and substrate temperature,for fabricated M2 are 220 W,960 mm/s,0.06 mm,and 200℃,respectively.Based on this,the hardness and tensile strength reached 60 HRC and 1000 MPa,respectively.Interlaminar crack formation and suppression mechanism and the relationship between temperature gradient and thermal stress were illustrated.The inhibition effect of substrate temperature on the cracks generated by residual stresses was also explained.AM showed great application potential in the field of special conformal cooling cutting tool preparation.

Effects of electrode configuration on electroslag remelting process of M2 high-speed steel ingot

The electrode configuration determines the thermophysical field during the electroslag remelting(ESR) process and affects the final microstructure of the ingot. In this work, ingot with a diameter of 400 mm was prepared with two electrode configuration modes of single power ESR process, namely one electrode(OE) and two series-connected electrodes(TSCE). Finite element simulation was employed to calculate the electromagnetic field, flow field and temperature field of the ESR system. The results show that the temperature of the slag pool and the metal pool of the TSCE process is lower and more uniform than that of the OE process.The calculated temperature distribution of the ingot could be indirectly verified from the shape of the metal pool by the experiment. The experimental results show that the depth of the metal pool in the OE ingot is about 160 mm, while the depth of the TSCE ingot is nearly 40 mm shallower than that of the OE ingot. Microstructural comparisons indicate that coarse eutectic carbides are formed in the center of the OE ingot, whereas more even eutectic carbides appear in the center of the TSCE ingot. In general, compared with the OE process, the TSCE process is preferred to remelt high speed steel ingots.

The Effect of Intensity Pulsed Ion Beam Irradiation on Wear Resistance of High-speed Steel

The structural and phase transformations occurring in the near-surface layers of pre-quenched W6Mo5Cr4V2 high-speed steel (HSS) subjected to intensity pulsed ion beam (IPIB) melting have been investigated. The effect of IPIB irradiation on wear resistance of the HSS has also been studied. The IPIB consists mainly of Cn+(30%)^0 H+(70%), with a high beam current density of 80A/cm2, acceleration voltage of 250kV, pulse duration of 70 ns. Samples were bombarded with 1, 3, 5 pulses respectively. It has been revealed that after IPIB irradiation the initial martensite in the near-surface layer of HSS changed into austenite and produced residual stresses by using electron microscopy and X-ray diffraction. Redistribution and interlace of dislocations in the irradiated samples were generated under the impact of shock wave. With increasing pulse times gradual liquid-phase dissolution of M6C carbide particles occurs in the near-surface layer and produces nanocrystalline MC. This process results in the decrease of martensite crystal (a-phase) and increase of austenite (y-phase) content and the dispersed carbide. Wear resistance of the HSS is improved by a factor of 2, which is explained by the formation of metastable phases such nanocrystal and residual stresses and the redistribution and interlace of dislocations.

Experimental research on electromagnetic continuous casting high-speed steel composite roll

A high speed steel composite roll billet was fabricated, which is regular in shape, smooth in surface, slight in trace, compact in internal structure, free of slag inclusion, shrinkage cavity, cracks and other flaws, and good in macro quality of junction surface using a vertical continuous casting machine. The interface zone microstructure of bimetallic in billet of high speed steel composite roll was analyzed by metallurgical microscope(OM), X-ray diffractmeter(XRD), scanning electron microscopy(SEM) and energy-dispersive X-ray analysis(EDS). The results indicate that the microstructure of roll billet is composed of chilled solidified layer, dendrite zone, interfacial zone of bimetal and core material zone. The microstructure of outer shell material is composed of martensite + bainite + residual austenite + some small labyrinth-shape, small-short lath-shape, or dollop-shape eutectic carbides. The microstructure of core material is slice-shape pearlite and a little ferrite along boundary of cells. The interface region microstructure of bimetallic composite roll consists of diffusion region, chilled solidified layer and columnar grain region.

On microstructure and performance of tempered high-boron high-speed steel roll

Influences of the tempering temperature on the microstructure, mechanical property and wear resistance of High-Boron High Speed Steel （HBHSS） roll materials were investigated by means of optical microscopy, scanning electron microscopy （SEM）, X-ray diffraction, hardness measurement, impact tester, tensile tester and pin abrasion tester. The results show that the as-cast structure of HBHSS consists of a great amount of martensite and M2（B,C） and a few retained austenites and M23（B,C）6. After solution treated at 1,050℃ and followed by oil cooling, the amount of M23（B,C）6 carbo-borides in quenched HBHSS increases obviously and the macrohardness of the quenched HBHSS is 66 HRC, which is very close to the 65.8 HRC of as-cast HBHSS. On the whole, the hardness of HBHSS alloy shows a trend of slight decrease with increasing tempering temperature when tempered below 500 ℃. While when above 500 ℃, the hardness increases slightly as the tempering temperature increases and reaches a peak at 525 ℃ and then decreases obviously. The impact toughness of HBHSS has a tendency to increase as the tempering temperature increases. Tempering can improve the tensile strength and elongation of HBHSS, but a higher tempering temperature causes a slight decrease in both tensile strength and elongation. Excellent wear resistance can be obtained by tempering at 500 to 550 ℃.

Effects of mischmetal addition on phase transformation and as-cast microstructure characteristics of M2 high-speed steel

The influence of mischmetal （Ce-La） addition on phase transformation and as-cast microstructure characteristics of M2 high-speed steel （HSS） was investigated using Thermo-Calc software, differential scanning calorimetry, X-ray diffractometry and scanning electron microscopy with energy dispersive spectrometry. The results showed that the measured phase transition points of M2 HSS were broadly consistent with the theoretical results. After mischmetal addition, the liquidus peak temperature, the peak temperature of the eutectic precipitation of M6C and MC were all increased, especially for the M6C which was affected significantly and increased about 31 °C. The contents of Mo and V in the eutectic carbide decreased and that of Fe increased, while in the matrix, the Mo, V and Cr contents all increased slightly. Furthermore, the microstructure of as-cast dendrite and ledeburite were refined, the total eutectic carbide content decreased and distributed into a discontinuous network, the lamellar spacing of M2C was reduced and the lamellae became thinner.

Effect of N and Zr on as-cast microstructure and properties after annealing of a high-speed steel

Effects of N and Zr on the as-cast microstructure and properties after annealing of high-speed steel （HSS） were investigated by using electronic probe micro-analysis, Rockwell hardness test, X-ray diffractometry and differential scanning calorimetry with combination of microstructure analysis. The results indicate that the addition of N and Zr will refine the eutectic structures and enhance the stability of carbides which are mainly MC, M2C and M7C3. The coarse dendritic structures decrease significantly and most of the carbides are distributed in the microstructure uniformly. Moreover, a kind of Zr-Si compound which only exists in VC is discovered, and this new phase is speculated to be related with the spheroidization of VC. The annealing process is set up to 6 different time periods which are 1, 3, 6, 10, 15 and 20 h, respectively. In different annealing processes at 750 ℃ which is lower than austenitizing temperature, the addition of N and Zr makes the decrease of hardness more obvious and restrains the precipitation of secondary carbides with the extension of time. Moreover, when the annealing time reaches 20 h, some clusters appear in the matrix of the two samples, and the density of clusters in HSS1 is lower, but the matrix of HSS1 contains more C and alloying elements which indicate more carbides precipitate.

Evolving Mechanism of Eutectic Carbide in As-cast AISI M2 High-speed Steel at Elevated Temperature

The evolution in type, size and shape of carbides in as-cast American Iron and Steel Institute （AISI） M2 high-speed steel before and after annealing were investigated. The micromechanism which was responsible for those changes was also analyzed and discussed. At the initial stage of reheating, metastable M2C-type carbide decomposed continuously. M6C-type carbide nucleated at the interface of M2C/γ firstly and grow from surface to center. Then MC-type carbide nucleated at both surface of M6C/M6C and inner of M6C. With the increasing decomposition of the metastable M2C-type carbide, the rod-shaped construction of eutectic carbide began neck- ing, fracturing and spheroidizing gradually. Held enough time or reheated at higher temperature, particle-shaped product aggregated and grew up apparently, while secondary carbide precipitated in cell and grew up less sig- nificantly than the former. Based on the above microstructural observation, the thermodynamic mechanism for decomposition of M2C carbide, for spheroidization of products, and for the growth of particles were analyzed. The rate equations of carbides evolution were derived, too. It shows that the evolving rate is controlled by diffusion coefficients of alloy atoms, morphology of eutectic carbides and heating temperature.

An innovative method for calibrating local cooling rate in electroslag remelting of M42 high-speed steel

The determination of the local cooling rate has a great significance in optimizing the parameters of electroslag remelting(ESR)and improving the quality of the ingots.An innovative method was proposed for calibrating the local cooling rate of M42 high-speed steel(HSS)in the ESR process.After resolidification at different cooling rates under high-temperature laser confocal microscopy,the carbide network spacing of the specimen was observed using a scanning electron microscope.A functional relationship between the cooling rate and average carbide network spacing was established.The average local cooling rate of the solidification process of the M42 HSS ingot was calibrated.The results show that the higher the cool-ing rate,the smaller the network spacing of the carbides.For the steel ingot with a diameter of 360 mm,the average local cooling rate was 0.562℃/s at the surface,0.057℃/s at the position of 0.25D(where D is the diameter of the ingot),and 0.046℃/s at the center of the ingot.

Interfacial Oxides Evolution of High-Speed Steel Joints by Hot-Compression Bonding

The interfacial oxidation behavior of Cr_(4)Mo_(4) V high-speed steel(HSS)joints undergoing hot-compression bonding was investigated by using optical microscopy(OM),scanning electron microscopy(SEM),and transmission electron microscopy(TEM).In the heating and holding processes,dispersed rod-like and granularδ-Al_(2)O_(3) oxides were formed at the interface and in the matrix near the interface due to the selective oxidation and internal oxidation of Al,while irregular Si-Al-O compounds and spheroidal SiO_(2) particles were formed at the interface.After the post-holding treatment,SiO_(2) oxides and Si-Al-O compounds were dissolved into the matrix,andδ-Al_(2)O_(3) oxides were transformed into nanoscaleα-Al_(2)O_(3) particles,which did not deteriorate the mechanical properties of the joints.The formation and migration of newly-formed grain boundaries by plastic deformation and post-holding treatment were the main mechanism for interface healing.The tensile test results showed that the strength of the healed joints was comparable to that of the base material,and the in-situ tensile observations proved that the fracture was initiated at the grain boundary of the matrix rather than at the interface.The clarification of interfacial oxides and microstructure is essential for the application of hot-compression bonding of HSSs.

Uniformly Dispersed Carbide Reinforcements in the Medium-Entropy High-Speed Steel Coatings by Wide-Band Laser Cladding

A medium-entropy high-speed steel(ME-HSS)coating with the 76 at.%of Fe and multiple alloying elements was prepared by the wide-band laser cladding.Compared with the commercial W6 Mo5 Cr4 V2(M2)HSS coating which contains a large number of network lamellar M2 C-type carbides along the grain boundaries,the presented ME-HSS coating has a high quantity of fi ner and more uniformly dispersed M C-type carbides;on the other hand,the coating has less retained austenite and much lower brittleness as well as similar secondary hardening eff ect and tempering hardness.

Effect of Fe content on type and distribution of carbides in medium-entropy high-speed steels

The effect of iron content on the type and distribution of carbides in the vacuum arc melted Fex(AlCoCrCuNiV)_(88.05-x)Mo_(5)W_(6)-C_(0.95)(x=69,76,83 wt%,respectively) medium-entropy high-speed steels(ME-HSSs) was studied.The homogeneous distribution of granular MC carbides(M refers to the carbides forming transition metal elements,such as W,Mo and V),both at the grain boundary and in the martensitic matrix,was obtained in the Fe_(76)(AlCoCrCuNiV)_(12.05)Mo_(5)W_(6)-C_(0.95)ME-HSS,after quenching at 1210 ℃ followed by triple tempering at 530 ℃.A maximum hardness of about 841 HV_(0.5) was achieved,even higher than the maximum hardness of the commercially available M_(2)(W_(6)Mo_(5)Cr_(4)V_(2)) HSS,826 HV_(0.5).The medium-entropy effect was shown to be beneficial to transform the solidified primary carbide network into finely dispersed granular MC.However,when the iron content was further reduced to 69 wt%,the enhanced entropy effect promoted the dispersion of carbides but at the same time led to a decrease in the carbide content,so that the maximum hardness(816 HV_(0.5)) was slightly lower than the maximum hardness of M2-HSS.The results provide a novel alloy system together with a simple heat treatment method to obtain hard HSSs,and more importantly to eliminate the primary carbide network which is harmful to the toughness of traditional HSS.

Effects of Topography and Modified Layer by Plasma-Shot Treatment on High-Speed Steel

In this study,plasma shot(PS)treatment was applied to high-speed steel(HSS)surfaces using a titanium carbide electrode to confirm the effect of discharge current(Ip)on the formation of a single dimple and analyze a modified layer.The roughness of modified surfaces increased when Ip increased,and energy-dispersive X-ray spectrometry showed an increase in titanium atom density whenIp and electrode consumption volume(Ve)increased.A friction test confirmed that the modified surface's friction was reduced by discharge dimples under low-load conditions.Vickers hardness test confirmed that the hardness of the modified surface was-300-600 HV higher than that of an untreated HSS surface.Moreover,it increased with an increase inIp.However,application of PS treatment to the edge of surfaces on the workpiece caused shape deterioration.The deterioration size of the edge of the modified layer increased when Ip increased.To solve this issue,we propose a novel method named position-adjusted PS(PA-PS)treatment.PA-PS treatment is used to adjust the end of the electrode in the order of tens of micrometers from the edge of the workpiece to avoid the deterioration of the edge form.Under Ip=21 A,PA-PS formed a modified layer without deteriorating the edge shape of the workpiece,thus confirming the PS characteristics applied to HSS surfaces.Moreover,PA-PS treatment solved the shape deterioration of the edge on modified surfaces via PS treatment.

Numerical Study on Reduction in Aerodynamic Drag and Noise of High-Speed Pantograph

Reducing the aerodynamic drag and noise levels of high-speed pantographs is important for promoting environmentally friendly,energy efficient and rapid advances in train technology.Using computational fluid dynamics theory and the K-FWH acoustic equation,a numerical simulation is conducted to investigate the aerodynamic characteristics of high-speed pantographs.A component optimization method is proposed as a possible solution to the problemof aerodynamic drag and noise in high-speed pantographs.The results of the study indicate that the panhead,base and insulator are the main contributors to aerodynamic drag and noise in high-speed pantographs.Therefore,a gradual optimization process is implemented to improve the most significant components that cause aerodynamic drag and noise.By optimizing the cross-sectional shape of the strips and insulators,the drag and noise caused by airflow separation and vortex shedding can be reduced.The aerodynamic drag of insulator with circular cross section and strips with rectangular cross section is the largest.Ellipsifying insulators and optimizing the chamfer angle and height of the windward surface of the strips can improve the aerodynamic performance of the pantograph.In addition,the streamlined fairing attached to the base can eliminate the complex flow and shield the radiated noise.In contrast to the original pantograph design,the improved pantograph shows a 21.1%reduction in aerodynamic drag and a 1.65 dBA reduction in aerodynamic noise.

High-speed penetration of ogive-nose projectiles into thick concrete targets:Tests and a projectile nose evolution model

The majority of the projectiles used in the hypersonic penetration study are solid flat-nosed cylindrical projectiles with a diameter of less than 20 mm.This study aims to fill the gap in the experimental and analytical study of the evolution of the nose shape of larger hollow projectiles under hypersonic penetration.In the hypersonic penetration test,eight ogive-nose AerMet100 steel projectiles with a diameter of 40 mm were launched to hit concrete targets with impact velocities that ranged from 1351 to 1877 m/s.Severe erosion of the projectiles was observed during high-speed penetration of heterogeneous targets,and apparent localized mushrooming occurred in the front nose of recovered projectiles.By examining the damage to projectiles,a linear relationship was found between the relative length reduction rate and the initial kinetic energy of projectiles in different penetration tests.Furthermore,microscopic analysis revealed the forming mechanism of the localized mushrooming phenomenon for eroding penetration,i.e.,material spall erosion abrasion mechanism,material flow and redistribution abrasion mechanism and localized radial upsetting deformation mechanism.Finally,a model of highspeed penetration that included erosion was established on the basis of a model of the evolution of the projectile nose that considers radial upsetting;the model was validated by test data from the literature and the present study.Depending upon the impact velocity,v0,the projectile nose may behave as undistorted,radially distorted or hemispherical.Due to the effects of abrasion of the projectile and enhancement of radial upsetting on the duration and amplitude of the secondary rising segment in the pulse shape of projectile deceleration,the predicted DOP had an upper limit.