M2 High Speed Steel Research Articles

Microstructure and properties of M2 high-speed steel cast by the gravity and vacuum investment casting

In this work the microstructure and properties of M2 high-speed steel cast into ceramic moulds using gravity and vacuum casting and subject to heat treatment were studied. The microstructure of castings after solidification and after heat treatment was subjected to detailed characterisation. The surface roughness and content of gases and solid non-metallic impurities of the samples were also evaluated together with mechanical tests for hardness, red hardness, impact toughness and bending strength. The results established that investment casting under vacuum enhanced the cleanliness of the metal and improved the microstructure and properties. The relationship between the microstructure and the properties is discussed. Some advantage of the vacuum cast high-speed steel with respect to the red hardness was attributed to the stronger secondary hardening of the steel during tempering. The refinement of the eutectic carbides and colonies as well as the formation of the more discontinuous and thinner interdendritic network of eutectic carbides and the metal cleanliness were the determining factors which provided enhanced toughness and strength properties of the vacuum cast high-speed steel compared with that of the gravity cast one.

Sintering behavior of NbC based cemented carbides bonded with M2 high speed steel

Densification behavior of NbC-12 wt.% M2 high speed steel (HSS) cemented carbide was studied using a thermo-mechanical analyzer. To be able to survey the complete sintering process, model-free and model-based kinetic analysis were combined. According to this approach, the sintering process was divided into two main parts. The first part is associated with high shrinkage and according to the model-free analysis, three steps can be identified in this part. In the second part, the presence of solution-reprecipitation can be identified according to the developed models for liquid phase sintering. By dividing the sintering behavior into two main parts, it is possible to overcome the effect of limited NbC solubility in Fe during the heating process. This effect can be observed in the densification curves as a slight expansion at the end of the first sintering part. It is assumed that poor wetting performance at low temperatures results in the outflow of the liquid phase to the surface of the compacts which results in a fake densification. It was evident that wetting performance improves significantly at elevated temperatures (above 1310 °C) inducing a re-infiltration of liquid drops on the surface inside the inner pores and the expansion in the shrinkage curve.

Metallographic analysis of M2 high speed steel granules

The article is devoted to the metallographic analysis of the M2 high-speed steel granules. The study is based on the investigation of the microstructure of the M2 high-speed steel granules obtained by melt atomization. It is demonstrated that granules of similar size can harden both by chemically separating and chemically non-separating mechanism. These last ones have supersaturated solid solution structure of the liquid melt composition, a dispersed dendritic-cellular structure and an increased microhardness HV = 10267±201 MPa.

Reprint of “Enhancement of toughness and wear resistance by CrN/CrCN multilayered coatings for wood processing”

The coatings applied onto woodworking tools must possess higher hardness, adhesion, and fracture toughness. The CrN/CrCN multilayered coatings are deposited on M2 high-speed steel (HSS) and Si (100) by cathodic arc evaporation with different C2H2 flow rates and characterized systematically. The addition of carbon to the CrN coatings can enhance the toughness of coatings and the H/E ratio. The H/E and H3/E2 ratios reach 0.085 and 0.176, respectively. The maximum nanohardness of the coatings is 24 GPa and the smallest friction coefficient is 0.48 together with the higher adhesion between the coating and substrate (HF1). Larger C2H2 flow rate can produce porous and loose microstructure with poor mechanical properties. The lifetime of the CrN/CrCN-coated blades made of high speed steel and cemented carbide materials is increased by about 33% and 7% respectively compared to that of CrN-coated blades, and by 170% and 110% compared to that of uncoated blades.

Contact Interaction Peculiarities at the Boundary of Layers of Structural Steel–High-Speed Steel Hot-Forged Powder Bimetal

The main problem in the production of bimetals (BMs) is the necessity to ensure adhesive interaction at the contact boundary of the layers, preventing their delamination during the operation. Hot forging of porous preforms (HFPP) offers the possibility of fabricating high-density powder BMs with a minimal amount of pores both in the bulk of the layer material and at the boundary between the layers, which promotes an increase in the adhesion strength. When fabricating hot-forged powder BMs, it is possible to mix materials of charges of a working layer and a substrate, which can lead to uncontrollable interface “spread.” In this work, to fabricate porous BM performs of the structural steel–high-speed steel type, the previously proposed method foreseeing the preliminary prepressing of the powder of the hard-to-deform material is used. In order to determine mechanical properties and perform a structural analysis, bilayer cylindrical samples 20 mm in diameter and 30 mm in length are prepared. The BM base material is steel PK40 and that of the working layer is atomised high-speed steel M2 powder with satisfactory compressibility characteristics. Porous preforms of BM samples are pressed in a specially designed mold using a hydraulic press, enabling one to perform two-side pressing of bilayer powder moldings with the specified distribution of density and strength of layers. Cold-pressed BM preforms were sintered in a protective atmosphere and then subjected to hot repressing using a laboratory drop hammer. Some preforms are examined as sintered. In addition, hot repressing of cold-pressed green preforms is performed. The satisfactory process strength of the working layer material is observed at its porosity of 34% 45% and delaminates at Pwl < 34%. It is established that the maximal layer bonding strength and BM thermal-shock resistance is provided by the application of a flowsheet foreseeing the preliminary sintering of cold-pressed preforms and subsequent hot forging. The optimal repressing pressure of the working layer is 145 MPa.

Influence of Tempering Time on the Microstructure and Mechanical Properties of AISI M42 High-Speed Steel

AISI M42 high-speed steel is prone to fracture as a result of its brittle martensitic microstructure together with abundant carbides located at the grain boundaries. In this study, a series of property tests including hardness, impact toughness, and wear loss were performed to study the effect of tempering conditions on the mechanical properties of AISI M42 high-speed steel over holding time ranging from 1 to 20 hours. The effects of the tempering time on the characteristics and growth of carbides were also investigated. The results indicated that carbides in the experimental steels were obviously coarsened when the tempering time exceeded 4 hours. The dimension of the carbides increased, while the volume fraction decreased with the increasing tempering time, and the grain sizes were significantly augmented due to the reducing of small carbides. Moreover, the dislocation density decreased with the increasing tempering time, which led to the reducing of the yield stress of high-speed steel. An appropriate holding time (4 hours) resulted in fine-scale secondary carbides and a smaller grain size, which efficiently improved the impact toughness and wear resistance simultaneously. Nevertheless, a prolonged tempering time (> 4 hours) promoted the coarsening and coalescence of carbides, which were detrimental to the impact toughness and wear resistance. Consequently, the formation of fine-scale secondary carbides is the major influential factor to improve both the wear resistance and impact toughness.

Contact interaction peculiarities at the boundary of layers of «structural steel–high-speed steel» hot-forged powder bimetal

The main problem in the production of bimetals (BMs) is the need to ensure adhesive interaction at the contact boundary of layers to prevent their peeling during operation. Hot forging of porous preforms (HFPP) provides the possibility of obtaining high-density powder BMs with a minimum amount of pores both in the volume of the layer material and at the layer interface to increase adhesion strength. Production of hot-forged powder BMs may involve mixing of working layer and substrate charge materials, which can lead to uncontrolled interface «blurring». This study uses the previously proposed method for pre-pressing of hard-to-deform material powder to produce «structural steel – high-speed steel» porous BM preforms. Two-layer cylindrical ∅20×30 mm samples were obtained in order to determine mechanical properties and conduct structural analysis. The BM base material was PK40 steel, and the working layer was atomized powder of M2 high-speed steel featuring satisfactory compressibility properties. The porous preforms of BM samples were pressed in a specially designed mold at a hydraulic press enabling two-sided pressing of two-layer powder moldings with predetermined distribution of layer densities and strengths. Cold-pressed BM preforms were sintered in protective environment, and then subjected to hot repressing using a laboratory drop hammer. Some preforms were examined as sintered. In addition, hot repressing of cold-pressed green preforms was performed. Satisfactory process strength of the working layer material is observed at its porosity (Pwl) in the range from 34 to 45 %. When Pwl&gt; 45 %, powder is not molded, and at Pwl&lt; 34 % the working layer delaminates. The maximum layer bonding strength and thermal shock resistance of BM provides the use of a flow route that involves preliminary sintering of cold-pressed preforms and subsequent hot forging. The optimum pressure of working layer pre-pressing is 145 MPa.

Thermoplastic processing and debinding behavior of NbC-M2 high speed steel cemented carbide

The aim of this study is to evaluate the thermoplastic processing on a new generation of hard metals based on NbC bonded with high speed steel. The conventional technique in producing cemented carbide parts is dry pressing with paraffin binder and post production. To overcome shape restriction in pressing and post production, thermoplastic shaping processes can be used. For this purpose, a previously known binder system for ceramic feedstocks containing paraffin wax and ethylene vinyl acetate was selected and used for the first time with cemented carbides. The stated organic binders were mixed with NbC-12 wt.% M2 high speed steel powder and feedstocks with solid loading content from 50 to 60 vol.% were obtained. The rheological behavior of the feedstocks as well as the critical solid loading content were assessed using a capillary rheometer. A model fitting approach was used for the first time in hardmetal feedstocks to estimate the critical solid loading. Experimental values were fitted to a model from Chong (1971) which is based on bimodally disperse powder mixtures and a critical powder loading of 70 vol.% could be calculated. However, based on the abrasion problems during mixing at high filler content, feedstock with 57.5 vol.% powder loading had the optimum processability characteristics, and was therefore selected for further shaping processes. Thermo gravimetric analysis on green samples was performed after shaping in order to develop an optimum thermal debinding profile. The thermal debinding program was adjusted with special attention on decomposition onset, maximum mass loss rate and decomposition offset temperatures. Initial trials using decomposition offset temperature as holding steps, resulted in fragmented samples after debinding. The final designed heating profile resulted in a consistent mass loss rate according to thermo gravimetric analysis which limited crack formation during debinding. Debinding under a forming gas (95N2-5H2) atmosphere using optimized heating cycles resulted in sound sintered bodies only when green parts were embedded in a powder bed during debinding.

Heat Treatment of Sintered Valve Seat Inserts

The characterization of sintered valve seat inserts (VSIs) after being subjected to different heat treatment operations has been carried out. The VSIs were obtained from three different alloys by mixing iron powder with AISI M3:2, AISI M2 high-speed steels, and AISI D2 tool steel. After sintering, the VSI were quenched in air followed by double tempering at seven different temperatures. The cooling rate during air quenching was measured by means of a thermocouple type k attached to a data acquisition system. The characterization of the mechanical and physical properties of the VSIs was achieved by measuring relative density, apparent hardness and crush radial strength. The resulting microstructures for the sintered parts were interpreted using the isothermal and continuous cooling transformation diagrams for similar alloys. The VSI obtained with AISI M3:2 and AISI M2 high-speed steels after air quenching and double tempering at 600 oC showed the best results in terms of apparent hardness and crush radial strength.

A Novel Method for Improving Cast Structure of M42 High Speed Steel by Pressurized Metallurgy Technology

A Novel Method for Improving Cast Structure of M42 High Speed Steel by Pressurized Metallurgy Technology

Effect of cooling rate on the transformation characteristics and precipitation behaviour of carbides in AISI M42 high-speed steel

ABSTRACT The influence of cooling rate during austenitising process on the transformation characteristics and precipitation behaviour of carbides in AISI M42 high-speed steel was investigated through thermodynamic calculation, dilatometry, microstructural analysis, and steel inclusion analysis system. Results show that under low cooling rate (<1 K s−1), the carbides in the steel are coarse, growing along the grain boundaries, and forming a network distribution. Under high cooling rate (≥1 K s−1), the morphology of carbides turns into block shape, and martensitic microstructure has been achieved. The volume fraction of the carbides decreases with the increase of cooling rate, meanwhile, the size distribution of the carbides is meliorated. However, when the cooling rate further increases from 5 to 20 K s−1, the precipitated carbides becomes non-uniform with the increasing cooling rate. Consequently, an appropriate cooling rate of heat treatment can be selected to optimise the distribution of carbides.

Enhancement of toughness and wear resistance by CrN/CrCN multilayered coatings for wood processing

The coatings applied onto woodworking tools must possess higher hardness, adhesion, and fracture toughness. The CrN/CrCN multilayered coatings are deposited on M2 high-speed steel (HSS) and Si (100) by cathodic arc evaporation with different C2H2 flow rates and characterized systematically. The addition of carbon to the CrN coatings can enhance the toughness of coatings and the H/E ratio. The H/E and H3/E2 ratios reach 0.085 and 0.176, respectively. The maximum nanohardness of the coatings is 24 GPa and the smallest friction coefficient is 0.48 together with the higher adhesion between the coating and substrate (HF1). Larger C2H2 flow rate can produce porous and loose microstructure with poor mechanical properties. The lifetime of the CrN/CrCN-coated blades made of high speed steel and cemented carbide materials is increased by about 33% and 7% respectively compared to that of CrN-coated blades, and by 170% and 110% compared to that of uncoated blades.

Influence of the Nitrogen Content on the Carbide Transformation of AISI M42 High-Speed Steels during Annealing

Attempts were made to elucidate the effect of nitrogen on primary eutectic carbides in as-cast and annealed AISI M42 high-speed steel. Particular emphasis was placed on the transformation of carbides during forging and annealing in steels with different nitrogen concentrations and the influence of final carbides on the impact toughness of the steel. Microstructural observation, electrolytic extraction method, X-ray diffraction analysis, automated inclusion analysis (INCASteel), and impact toughness measurement combined with fractographic observation were conducted on the specimens. Primary M2C carbides were found to be dominant precipitates in the as-cast ingot, with a certain amount of V(C,N). Nitrogen addition promoted the formation of fibrous M2C, whereas lamellar M2C predominated in M42 steel with a low nitrogen concentration (w[N]% = 0.006). Fibrous carbides M2C tend to decompose into more stable carbides M6C and MC during forging and annealing compared to lamellar M2C. Nitrogen alloying only affected the morphologies and dimensions of carbides, but did not change the types of carbides. These improvements in the dimensions and fractions of carbides naturally increased the impact toughness of annealed steel. Hence, it was suggested that the addition of nitrogen to AISI M42 high-speed steel was required to achieve homogeneous distribution of carbides and sufficient impact toughness.

Investigation of high temperature wear resistance of TiCrAlCN/TiAlN multilayer coatings over M2 steel

In this study, TiCrAlCN/TiAlN multilayer coatings were deposited on M2 high speed steel substrates by the Closed-Field Unbalanced Magnetron Sputtering system. The chemical composition, microstructure, morphology, mechanical and high temperature wear resistance properties of the coatings were characterized, analyzed and compared to the substrate. The high temperature wear tests were carried out under a load of 2 N at the lap (wear test distance) of 50 m and in dry sliding condition at Room Temperature (RT), 150, 300, 450, and 600 °C on atmospheric conditions. It has been found that the TiCrAlCN/TiAlN multilayer coatings have a higher wear resistance than the M2 substrate. The stable friction behavior and low friction tendency was determined at 600 °C. When the test temperature increased, the wear rates decreased. Narrow and smooth wear tracks and also the lowest wear rate were obtained at 600 °C.

Tribological Behaviour of Ti:Ta-DLC Films Under Different Tribo-Test Conditions

Diamond-like carbon (DLC) films are suitable applicants for cutting tools due to their high hardness, low friction coefficient and wear rate. Doping metals in DLC films have been improved its tribological properties. In this study, titanium and tantalum doped hydrogenated DLC films were deposited by closed-field unbalanced magnetron sputtering system onto M2 high speed steels in Ar/N2/C2H2 atmosphere. The friction and wear properties of Ti:Ta-DLC film were investigated under different tribo-test conditions including in atmospheric pressure, distilled water, commercial oil and Ar atmosphere. The coated specimens were characterized by SEM and X-ray diffraction techniques. The bonding state of C-C (sp3) and C=C (sp2) were obtained with XPS. The tribological properties of Ti:Ta-DLC were investigated with pin-on-disc wear test. Hardness measurements performed by micro-indentation. Our results suggest that Ti:Ta-doped DLC film shows very dense columnar microstructure, high hardness (38.2 GPa) with low CoF (µ≈0.02) and high wear resistance (0.5E-6 mm3/Nm).

Laser additive manufacturing of M2 high-speed steel

ABSTRACTM2 high-speed steel samples were fabricated by laser additive manufacturing and tempered at different times at a temperature of 560°C. The microstructures of deposited samples were characterised by fine equiaxial grains, dendrites and inter-dendritic network-shape eutectic carbides and were composed of supersaturated martensite, retained austenite and M2C-type carbides. The content of retained austenite gradually decreased with increasing tempering times. Meanwhile, the micro-hardness of deposited samples was 688 ± 10 HV, while the first, second and third tempering times were 833 ± 13 Hv, 710 ± 6 Hv and 740 ± 7 Hv, respectively (standard deviations).Wear resistances of all samples showed an adhesive wear mechanism, and M2 HSS without tempering had a lower friction coefficient with an average of 0.52. M2 HSS after tempering twice at 560°C/2 h had a larger wear volume loss than others.

The formation of the cutting tool microgeometry by pulsed laser ablation

Laser ablation is considered as an alternative to other methods that allow precise processing of various tool materials and work with both the base material and thin films of wear-resistant coatings. This article presents an investigation of the possibility of setting the microgeometry of the surface of cutting tool made of high-speed steel M2 and hard alloy WCCo3 using ablation with nanosecond infrared marking Nd:YVO4 laser. Dependencies of the width and depth of the resulting tracks on the specified laser power are obtained. It was revealed that their depth depends on a set of several specified factors: power, scanning speed, pulse frequency rate, and a number of passes. The modes with a radiation power of up to 70% are of interest for the treatment of the tool surface using a laser because they have a more predictable profile and more efficient energy distribution. The most uniform and smooth surface were obtained after laser treatment with the following parameters: power 60%, frequency 10 kHz, scanning speed 200 mm/s irrespective of the material being processed. The greater flexibility of the coating material for laser processing was observed. The depth of tracks on the coated (TiAl)N samples is larger than on the uncoated samples. The possibility to obtain a chamfer with the specified width and angle parameters by laser processing based on the obtained dependencies is demonstrated.

Improvement of discharge and microstructure of Cr-C-N coatings by electromagnetically enhanced magnetron sputtering

The Cr-C-N coatings were deposited on M2 high-speed steel (HSS) and Si (100) wafer using conventional magnetron sputtering (CMS) and electromagnetically enhanced magnetron sputtering (EMEMS) techniques. Different external magnetic field strength was generated by varying the electromagnetic coil current from 0 to 90 A. The substrate current density (J) was measured and the microstructure and hardness of the Cr-C-N coatings was investigated by field-emission scanning electron microscopy, X-ray diffraction, Raman spectroscopy, and micro-hardness tester. The results showed that the J increased by a factor of more than 2 with the assistance of a coil current of 90 A, indicating a significantly enhanced glow discharge. The Cr-C-N coatings showed dense columnar grain structure. All coatings exhibited a granular surface morphology with a decrease in the granular size as the coil current increased. The structural improvement was attributed to the enhanced ion bombardment from the increased ion flux generated by the EMEMS technique. By increasing the coil current above 30 A, Cr-C-N coatings exhibited smaller intensity ratio of the D-Raman peak and the G-Raman peak (ID/IG), which indicated higher sp3 contents in the coatings. The maximum microhardness (approximately HV 1560) of the coating was obtained at a coil current of 90 A.

Characterization and mechanical properties of TiN/TiAlN multilayer coatings with different modulation periods

The TiN/TiAlN nanomultilayer coatings with different modulation periods were deposited successfully on M2 high-speed steel by a pulsed bias arc ion plating system. The effect of modulation period on the microstructure and mechanical properties was investigated. With the increasing of deposition period, the Al contents in TiN/TiAlN multilayer coatings monotonically increased continuously a little from 0.135 to 0.142, whereas the Ti and N contents were nearly constant. The microstructure TiN/TiAlN multilayer coatings were crystallized with preferred orientation in (111) crystallographic planes. At modulation period of 164 nm, the nanohardness reached a maximum of 38.9 GPa. The Young’s modulus values were in the range of 418–591 GPa. The adhesion strength reached a maximum of 66.2 N at modulation period of 54 nm.

Sintering of AISI M2 Tool Steel Processed in High-Energy Planetary Mill

This work aimed to evaluate the effect of pre-sintering annealing heat treatments and sintering times in AISI M2 high-speed steel powders processed by high energy milling. Turning chips were obtained from an AISI M2 drill bit that was annealed during 2 hours at 900°C, under argon atmosphere, before machining. Subsequently, the chips were milled during 10 hours in a high energy planetary mill with a power ratio of 10:1, also under argon atmosphere. Half of the powder mass was annealed at 650oC during 30 minutes under argon atmosphere after milling. Three different samples were prepared, consisting of: non-annealed powder, annealed powder and a mixture 1:1 of annealed and non-annealed powders. All powders were compacted by uniaxial pressing before sintered. Compressibility curves were obtained for all samples. Sintering process was conducted at 1200°C during 1, 2 and 3 hours and samples were cooled inside the furnace. The annealed powder sample presented the best compactation behavior, due to its restored ductility, followed by the 1:1 mixture of annealed and non-annealed powders. The microstructure of sintered samples displayed a ferritic matrix surrounded by carbide networks at grain boundaries. Higher sintering times resulted in carbon impoverishing, leading to lower volume fractions of carbides and hence reducing its hardness. Non-annealed powders showed higher dependency of sintering time to reduce their porosity. The best results were obtained for the annealed powder with shorter sintering time, since it presented low volume fraction of porosities and smaller grain sizes.