Microstructure Of High-speed Steel Research Articles

Study on the Influence of Silicon and Aluminum on the Microstructure and Properties of Low Cobalt M42 High-Speed Steel

M42 HSS (W2Mo9Cr4VCo8 ) is widely used as a high cobalt super-hard high-speed steel (HSS) for various cutting tool materials, but high cobalt HSS does not bring good economic benefits. In this paper, the effects of Si and Al on the organization and mechanical properties of low cobalt M42 HSS were investigated. The microstructure of HSS was analyzed by OM, SEM, EDS, XRD, etc. Rockwell hardness test, impact toughness test and flexural strength test were conducted. The results of the study showed that only increasing the Si content in M42 HSS containing 4.0% Co did not significantly improve the material organization and properties, and caused a sharp deterioration in impact toughness. After adding 1.0% Si and 1.0% Al to M42 HSS containing 4.0% Co, Si and Al can partially replace Co, resulting in a more uniform and diffuse carbide distribution. The hardness of the material is increased to 68.3 HRC and the impact toughness is increased to 13.8 Jꞏcm-2, which has better overall mechanical properties and can meet the processing and cutting requirements under actual working conditions.

Influence of Co on the microstructure and mechanical properties of M42 high-speed steel

M42 high-speed steel (W2Mo9Cr4VCo8) is a kind of high cobalt super-hard high-speed steel (HSS), which is extensively used in various cutting tool materials, but because of the high price of cobalt, high cobalt HSS does not bring good economic benefits. In this thesis, the consequences of various cobalt contents (0.0%, 2.0%, 4.0%, 6.0%, 8.0%) on the organization and mechanical properties of M42 HSS have been studied. The microstructure of HSS was analyzed by OM, SEM, and EDS, and the Rockwell hardness test, impact toughness test, and flexural strength test were conducted. The results show that cobalt helps to increase the nucleation rate of eutectic carbide precipitation and hinder its growth during tempering of HSS and refine the size of eutectic carbide, and the tempering hardness can be maintained above 68 HRC when the cobalt content is 4%-8%; however, cobalt also significantly reduces the impact toughness of HSS, and each 2% increase in cobalt impact toughness decreases about 4 J·cm−2. From the point of view of cobalt savings, cobalt content of 4%, high-speed steel tempering hardness, impact toughness, and other mechanical properties can be at a higher level.

Effect of Sintering Temperature and Iron Addition on Properties and Microstructure of High Speed Steel Based Materials Produced by Spark Plasma Sintering Method

Attempts were made to describe the effect of the sintering temperature and pure iron powder addition on the properties of HSS-based materials produced by the spark plasma sintering method (SPS). After sintering, their density, hardness, flexural strength, and tribological properties were determined. The sintered materials were also subjected to microstructural analysis to determine the phenomena occurring at the particle contact boundaries during sintering. On the basis of analysis of the obtained results, it was found that the mechanical properties and microstructure were mainly influenced by the sintering temperature, which was selected in relation to the previously tested steel M3/2, adjusted upwards due to its chemical composition. The use of the temperature of 1050 °C allows materials to be obtained with a density close to the theoretical density (97%), characterized by a high hardness of about 360 HB. The addition of iron slightly reduces the hardness and also increases the flexural strength to 577 MPa. There was no diffusion of the alloying elements from the steel to the iron due to the short time of exposure to the sintering temperature.

Influence of increased carbon content on the processability of high-speed steel HS6-5-3-8 by laser powder bed fusion

By using laser powder bed fusion (LPBF) in the tooling industry, tools with complex geometries and integrated functions, such as adapted cooling channels, can be manufactured with high potential in terms of product lifetimes or cycle times. When processing tool steels with a carbon content > 0.6 wt.-% by means of LPBF, hard and brittle phases are formed as well as soft retained austenite and carbides. Combined with high internal stresses in the part this often leads to crack formation. Using base plate preheating up to 500 °C can enable the production of crack free components, however, to increase the processability of high-speed steel, varying the alloy composition by stabilizing austenite could lead to reduced residual stresses. In this study, we investigate the processability and microstructure of high-speed steel HS6-5-3-8 reference and HS6-5-3-8 with increased carbon content by rapid alloy development (RAD) in the LPBF process. The manufactured specimens are analysed based on part density, hardness, microstructure, and homogeneity of the microstructure. Light-optical and electron microscopic methods (i.e. SEM, EDS, EBSD) are used for microstructure investigations. We found that an increased carbon content in the alloy leads to an increased carbide volume fraction and a fully austenitic matrix. However, contrary to the hypothesized beneficial impact of austenite stabilization on crack formation, we observe a worsening of cracking with C content. Based on these results, we discuss which microstructural features are the deciding factors for cold cracking in LPBF of high-carbon steels. Based on the results, a relationship between alloy composition of high-speed steel and their processability is discussed and the application of RAD by means of LPBF is assessed.

Effect of tempering temperature on microstructure and properties of aluminium‐bearing high boron high speed steel

AbstractHeat treatment is of great significance to the performance improvement of high speed steel. Via heat treatment, the microstructure of high speed steel can be improved, thus greatly improving the material performance. The effect of tempering temperature on the microstructure of aluminium‐bearing high boron high speed steel (AB‐HSS) was investigated by optical microscope (OM), scanning electron microscope (SEM) and x‐ray diffraction (XRD). The hardness and wear resistance of the alloy at different tempering temperatures were tested by Rockwell hardness tester, micro‐hardness tester and wear tester. The experimental results indicate that the tempering microstructure of aluminium‐bearing high boron high speed steel consists of α‐Fe, M2B and a few of M23(C, B)6. Tempering temperature could greatly affect the wear resistance of materials. With the increase of tempering temperature, the wear resistance of aluminium‐bearing high boron high speed steel firstly increase and then decrease. The alloy tempered at 450 °C has the best wear resistance and minimum wear weight loss. This study provides a reference for the formulation of heat treatment process of aluminium‐bearing high boron high speed steel.

Influence of Heat Treatment on Content of the Carbide Phases in the Microstructure of High-Speed Steel

Abstract This article presents the results of investigations of the effect of heat treatment temperature on the content of the carbide phase of HS3-1-2 and HS6-5-2 low-alloy high-speed steel. Analysis of the phase composition of carbides is carried out using the diffraction method. It is determined that with increasing austenitising temperature, the intensification of dissolution of M6C carbide increases. As a result, an increase in the grain size of the austenite and the amount of retained austenite causes a significant reduction in the hardness of hardened steel HS3-1-2 to be observed. The results of diffraction investigations showed that M7C3 carbides containing mainly Cr and Fe carbides and M6C carbides containing mainly Mo and W carbides are dissolved during austenitisation. During austenitisation of HS3-1-2 steel, the silicon is transferred from the matrix to carbides, thus replacing carbide-forming elements. An increase in a degree of tempering leads to intensification of carbide separation and this process reduce the grindability of tested steels.

A study of the debinding and sintering behavior of T42 high-speed steel produced by powder injection molding (PIM)

Tool steels have a large range of applications including hot and cold working of metals and injection molding of plastic or light alloys. High-speed steel (HSS) is specifically used for cutting tools and for components subjected to extreme wear conditions, because of its high strength, wear resistance, hardness, toughness, and fatigue resistance. The microstructure of HSS can be described as a metallic matrix iron composite containing a dispersion of hard and wear-resistant carbides. Specimens were manufactured experimentally from T42 powder (50–80 vol%) and mixed binder (20–50 vol%) by powder injection molding. The binders (green parts) were debound in n-hexane solution at 60 °C for 8 h and thermally debound in a mixed N2–H2 gas atmosphere for 8 h. Specimens were sintered under high vacuum (10−5 Torr) at different temperatures. When sintering was performed at 1,260 °C, the specimen sintered under high vacuum had the highest hardness (550 Hv). The carbides were smaller (1 μm) and well distributed. Grain size was 10 μm. When sintering was performed under high vacuum at temperatures above 1,280 °C the carbides changed to eutectic carbide located at grain boundaries. Grain growth was observed. Specimens sintered at other pressures had lower density and hardness than those sintered under high vacuum.

Sintering of AISI M<sub>2</sub> High Speed Steel with the Addition of NbC

The influence of adding 6 wt% (NbC) niobium carbide on the sintering temperature and microstructure of high speed steel - AISI M2(0.87% C, 5.00% Mo, 6.00% W, 4,00% Cr, 2.00% V and Fe bal.) powder was studied. The starting material was obtained by vacuum melting followed by atomization in water. The samples were axially cold compacted in a cylindrical matrix and then vacuum sintered at 1250 and 1350 °C. Dilatometry and differential scanning calorimetry measurements indicated an increase in sintering temperature with addition of niobium to the AISI M2steel. Optical and scanning electron microscope observations revealed a microstructure with uniformly distributed niobium carbides.

A Study on the Microstructure of AISI M2 High Speed Steel Manufactured by Continuous Casting

Continuous casting has been widely applied in the production of steels and other metals. However, it has been rarely used in producing high speed steels, which are still manufactured by the conventional method of mould-casting. Thus, little is known about the microstructure of high speed steels made by the continuous casting technology. In the present work, AISI M2 steel is produced by horizontal continuous casting and the difference of solidification microstructure of ingots by different casting technologies has been examined. The results show that the networks of M2C eutectic carbides are greatly refined in the ingot by continuous casting compared to that by mould casting. Meanwhile, the morphology of M2C eutectic carbides changes from the plate-like type to the fibrous one, due to the increasing cooling rates. Compared with the plate-like M2C, the fibrous M2C in continuous casting ingots is less stable and decomposes faster at high temperatures, spheroidizing obviously after heating and refining dimensions of carbides.

Effect of Carbon on Frictional Wear Behaviours of High Vanadium High Speed Steel under Dry Sliding Condition

The high vanadium high-speed steel (HVHSS) with about 9wt% vanadium and different carbon contents were prepared using casting process. The effects of carbon on wear properties of HVHSS were studied using pin-on-ring tester, and the failure behaviors were investigated via SEM. Results show the optimal wear resistance is obtained when HVHSS possesses moderate carbon content (2.58wt.%). The cause is that the matrix microstructure of moderate carbon HVHSS is mainly low-carbon lath martensite with good toughness and high hardness, and it can effectively resist micro-cutting and figure wear at the same time, so the role of high-hardness vanadium carbides (VC) can be played enough because of the strong support of matrix. If carbon content is too low, the wear failure of HVHSS is mainly caused by severe micro-cutting and adhesive wear on contact surface because the matrix microstructure of high speed steel is ferrite with very low hardness, which leads to poor wear resistance. While, the matrix microstructure is mainly composed of high carbon martensite with poor toughness when carbon content is too high, therefore, it possesses very poor resistance to cycle fatigue and thermal fatigue, resulting in decrease of wear resistance.

Diffusion and Secondary Carbide Precipitation in High-Speed Steels

Besides primary carbides, so-called secondary carbides are usually observed in high-speed steel (HSS) microstructure, which precipitate from the oversaturated solid solution upon its cooling. Some of the secondary carbides precipitate from an oversaturated austenite at cooling during solidification or quenching, in both cases, in the form of the relatively big particles in the bulk of a matrix. Considerably finer secondary carbides precipitate in martensitic condition during tempering inducing secondary hardening in HSSs. In general, secondary carbides of both types have obvious effect on the fracture mode of HSSs. The precipitation of secondary carbides in HSSs in solidified and heat-treated conditions has been investigated. The microstructures of HSSs have been examined using light and electron microscopy (both scanning and transmission), X-ray diffraction analysis, and energy dispersive X-ray spectrometry. Results indicate that diffusion of both carbon and alloying elements plays an important role in the secondary carbide precipitation in HSSs.

Effect of Solidification upon Features of Primary Carbides in Niobium-Titanium High-Speed Steels

Abstract Multi-stage solidification process is responsible for final microstructure of high-speed steels (HSSs). The typical HSSs grades contain large amount of eutectic carbides, mostly of M6C and M2C types, inhomogeneously distributed in a metal matrix. Application of niobium and titanium in alloying these steels results in increase of primary MC carbides precipitation temperature. In the investigated Nb-Ti HSSs characteristic carbon chains surrounding large MC particles were observed. These carbides have been analysed using scanning and transmission electron microscopy as well as high-temperature calorimetry. The results indicate that in the Nb-Ti HSSs an eutectic constituent has a form of M23C6 and M2C carbide chains around large MC type primary carbides of niobium and titanium. The carbide chains have contributed to good embedding of the MC particles in the matrix and consequently better service properties of the tools.

Effect of K/Na on microstructure of high-speed steel used for rolls

In the present work, the effect of K/Na on microstructure of high-speed steel (HSS) used for rolls was investigated utilizing Hi-scope video microscope (HSVM) and electron probe microanalyser (EPMA). As-cast microstructure of the alloy is mainly composed of pearlite matrix, M 7C 3, M 2C and MC eutectic carbides. The carbides are connected or placed next to each other to form a network along grain boundaries. After K/Na modification, the morphology, size and distribution of carbides change greatly. The carbide network tends to break, and all carbides are refined and distributed homogeneously in the matrix. The mechanism of K/Na modification on microstructure of the alloy is also discussed.

Cutting performance and microstructure of high speed steels: Contributions of matrix strengthening and undissolved carbides

While it is accepted that both the hot strength of the matrix and the amount of undissolved carbides are important for the cutting performance of high speed steels, the relative weights of their contributions are unknown. In this work, they are separately identified and a model is presented that provides a quantitative prediction of tool life (solely in uninterrupted cutting) on the basis of microstructural and compositional data over a wide range of alloy compositions and cutting speeds. The model seems to describe the individual contributions to tool life well enough to serve as a guide in alloy development. The model has been developed using 13 different steels, spanning the entire range of customary compositions. It is based on the following parameters: volume fractions and compositions of undissolved carbides; precipitates formed during tempering (secondary hardening) and during operation (tertiary precipitates); and, finally, residual solute in the matrix. Tool life is modeled as a linear combination of contributions from the undissolved carbides and from the precipitate population, including a contribution due to the action of Co, and with an additional term due to solute strengthening of the matrix. The weight factors are determined by multiple linear regression analysis. They reflect the relative importance of each contributing factor, and their dependence on cutting speed can be interpreted in terms of the change in operative wear mechanism with tool temperature.

Effects of silicon additions on the mechanical properties and microstructure of high speed steels

The effects of silicon additions up to 3.5 wt% on the mechanical properties and microstructure of high speed steels 6W3Mo2Cr4V, W3Mo2Cr4V and W9Mo3Cr4V have been investigated. In order to understand these effects further, a Fe16Mo0.9C alloy is also used. The results show that silicon additions can increase the temper hardness of steels Fe16Mo0.9C, 6W3Mo2Cr4V and W3Mo2Cr4V, but yield an opposite influence on the temper hardness in W9Mo3Cr4V steel. A critical tempering temperature exists for the bending strength of high speed steels containing silicon. If tempering is carried out at temperatures lower than the critical temperature, the bending strength of the high speed steels can be improved by the addition of silicon, otherwise their bending strength is decreased. Transmission electron microscopy reveals that silicon additions can obviously refine secondary hardening carbides and inhibit the formation of M 3C cementite at peak temperature. However, they are also found to accelerate both the depletion of martensite and the formation of coarse M 6C precipitates during tempering. The mechanism whereby silicon additions affect the secondary hardness of high speed steels is discussed in detail, and the types of high speed steel in which silicon additions can be used are suggested.