Powder Metallurgical High Speed Steel Research Articles

Cutting edge preparation of micro end mills by PVD-etching technology

Micromilling tools face significant challenges in achieving cutting edge preparation by mechanical processes due to their small size. To address these limitations, a new approach for cutting edge preparation of micro end mills by physical vapor deposition (PVD) etching technology is presented. By adapting the etching strategy, which is conventionally used as pre-treatment process for PVD technology to clean and condition the surface of cemented carbide substrate, cutting edges of micromilling tools were successfully modified. The high-energy process variation by Advanced Arc-Enhanced Glow Discharge (AEGD) offers the possibility of achieving high material removal rates on the tool surfaces and thereby altering the cutting edge geometry. Fundamental investigations on material removal as well as on the effects on surface topography, sub-surface properties, and coating adhesion of a subsequently applied PVD coating are performed. To influence the topography and the cutting edge geometry, the preparation time tp and bias voltage UB were selected. Subsequently, the machining performance of the modified tools is evaluated in a micromilling process of the hardened powder-metallurgical high-speed steel AISI M3:2 with a hardness of 62 ± 1 HRC. For this purpose, micro end mills of cemented carbide with a diameter of D = 1 mm were conditioned, achieving asymmetric geometric properties of cutting edges with varying average cutting edges rounding of S¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\overline S}$$\\end{document}= 5.8 … 14.8 µm and a form-factor of K = 2.0 … 2.7. Based on the application tests a positive influence on the process forces was found. Thus, an efficient approach to cutting edge preparation can be identified by specifically designing ion etching in the PVD coating process.

Mechanical Response and Microstructure Characteristics of Powder Metallurgical High-Speed Steel (ASP 60) Impacted at -195 °C and 800 °C

Mechanical Response and Microstructure Characteristics of Powder Metallurgical High-Speed Steel (ASP 60) Impacted at -195 °C and 800 °C

Experimental analysis on the influence of the tool micro geometry on the wear behavior in gear hobbing

Gear hobbing is a well-established manufacturing process for cylindrical spur gears. The cutting edge of a hobbing tool is, among others, characterized by the cutting edge radius and the form-factor K. The magnitude of these parameters is ideally chosen based on the machining conditions given by the workpiece and cutting material and the cutting parameters as well as the gear and tool geometry. However, the influence of the cutting edge geometry on tool life and wear behavior is hardly known, which complicates an optimized tool design. Furthermore, the preparation process regarding the coating thickness distribution on the wear behavior is equally relevant. Therefore, the objective was to identify the influence of the cutting edge radius, the form-factor K, and the preparation process on the wear behavior of gear hobbing tools made of powder metallurgical high-speed steel (PM-HSS). Fly-cutting trials were performed as an analogy process for gear hobbing in order to study the wear behavior and identify the respective tool lives. The trials indicated that the form-factor K influences the wear behavior, while a variation of the cutting edge radius did not have a significant effect. A homogenous coating thickness could extend the tool life significantly.

Comparison of methods for characterising the steel cleanness in powder metallurgical high-speed steels

ABSTRACT Powder metallurgical (PM) steels are characterised by a very high cleanness level. Single mesoscopic inclusions can nevertheless induce material failure under the extreme exposed stresses in the final product. Extensive knowledge about the cleanness of these steels is therefore essential. Various methods for inclusion analyses are available, providing different information about the non-metallic inclusion population present in the steel matrix. Several state-of-the-art methods of inclusion analysis are compared, considering morphological parameters and chemical composition of the detected inclusions as well as the required time effort and statistics behind the specific method. Tests were carried out with a standard PM steel HS6-5-3C. Combined with chemical extraction, automated SEM/EDS measurements enable a clear description of the microscopic cleanness level. Statistical analyses using the extreme value theory allowed the prediction of the maximum inclusion size in the investigated samples.

Austempering of PM HSS ASP2030 for improved fracture toughness

This study presents the fracture toughness improvement of the powder metallurgical high speed steel ASP2030 by replacing the martensitic matrix with a bainitic one. For this purpose, the fracture toughness of the steel is compared in both the austempered and marquenched states. The heat treatment of the samples was carried out by austenitizing in the range of 1150–1185 °C followed by either austempering at 235 °C or marquenching processes. Triple tempering at 560 °C for 2 h was also performed on all specimens. Fracture toughness measurements were performed on circumferentially notched tensile specimens. The microstructural features of the specimens were investigated using X-ray diffraction, optical and electron microscopy, and electron backscatter analysis. The results showed that the fracture toughness is sensitive to small changes in austenitizing temperature at both heat treatment conditions. By increasing the austenitizing temperature from 1150 to 1170 °C, the fracture toughness increases and then decreases with a further increase in temperature to 1185 °C. The results suggest that the changes in fracture toughness are due to the simultaneous effects of the volume fraction of undissolved carbides and the lattice microstrain values at different austenitizing temperatures. It can be concluded that the fracture toughness increases when the martensitic matrix is replaced by a bainitic structure. This improvement is related to finer bainitic laths width and also to the larger spacing between the particles of the undissolved carbides.

Influence of residual stresses in hard tool coatings on the cutting performance

During the hard machining of powder metallurgical high-speed steel, finely dispersed carbides in the steel expose the tools to both thermal and mechanical load. This can influence the cutting performance and cause damage to the tools such as premature abrasive and adhesive wear. Thin hard coatings like TiAlN deposited by physical vapor deposition are widely used in order to improve the tool performance. High power pulsed magnetron sputtering (HPPMS) results in technical benefits, such as a more homogeneous coating thickness distribution on the tools compared to direct current magnetron sputtering (dcMS). The advantages of HPPMS can be combined with the high deposition rates of dcMS leading to a higher economic efficiency conducting a dcMS/HPPMS hybrid process. Adding silicon to the coating system TiAlCrN results in TiAlCrSiN leading to a nanocomposite coating architecture with improved mechanical properties. The influence of the residual stresses on the mechanical properties and on the roughing performance of nanocomposite coatings is of high interest and was therefore investigated in the present study. Four different TiAlCrSiN hybrid coatings deposited with four different substrate bias potentials were examined for this purpose. The residual stresses and the mechanical properties including the resistance against crack formation as well as the compound properties of the coatings on cemented carbide were investigated. Finally, the roughing performance of the coated cemented carbide tools were tested by milling the powder metallurgical high-speed steel HS6-5-3C. For the coatings investigated, it can be concluded that a compressive residual stress state of approx. -2 GPa ≤ σ ≤ −3 GPa leads to the highest resistance against crack formation, and thus to the best cutting performance.

Effect of heat treatment on microstructure and impact toughness of a Tungsten-Molybdenum powder metallurgical high-speed steel

Effect of quenching, tempering and deep cryogenic treatment (DCT) on microstructure and impact toughness of a new tungsten-molybdenum high-speed steel produced by powder metallurgy was investigated. The obtained results infer that the microstructure is mainly composed of martensite, M 6 C and VC. The impact toughness decreases with the increasing of austenitizing temperature, but when the austenitizing temperature increases to 1180 °C, the impact toughness changes little. DCT after quenching improves impact toughness slightly, but DCT followed tempering has little effect on impact toughness. The fracture micromechanism can be identified as that crack initiates at the interface between M 6 C and martensite caused by the inconsistency deformation, M 6 C carbides exhibit broken or cleavage fracture and VC carbides decohere at the interface. In addition, the variation of impact toughness value has been explained by the characteristics of martensite, the fracture ductility, compressive strain of martensite lattice and grain refinement are beneficial to impact toughness. • The type of carbides and interface relationship was obtained. • Evolution of microstructure during heat treatment was studied. • The role of carbides in impact toughness was investigated. • The variation of impact toughness value has been explained by the characteristics of martensite.

Tool wear in dry gear hobbing of 20MnCr5 case-hardening steel, 42CrMo4 tempered steel and EN-GJS-700-2 cast iron

The demand for higher power densities in gear applications leads to the use of high-strength materials. This poses a challenge for manufacturing technologies with regard to the machinability of these materials. Even though gear hobbing is one of the most common technologies for gear machining, only limited investigations on the machinability of high-strength materials have been carried out. In this paper, the machinability of different workpiece materials by gear hobbing under dry cutting conditions is investigated. The tool life and wear behavior of powder metallurgical high-speed steel PM-HSS S390 and sintered tungsten carbide – cobalt WC-Co K30 tools with an AlCrN coating were analyzed. 20MnCr5 case-hardening steel, 42CrMo4 tempered steel and EN-GJS-700-2 cast iron were machined using the fly-cutting trial as an analogy process for gear hobbing. To identify a respective target tool life of LT,10 = 10 m, the cutting speed was varied while the feed rate was defined based on the respective tool concept and cutting substrate. The experiments showed a good machinability of the soft state material 20MnCr5 with PM-HSS S390 tools up to a cutting speed of vc = 350 m/min. A machining of the high-strength materials 42CrMo4 and EN-GJS-700-2 resulted in high tool wear or catastrophic tool failure. Even at low cutting speeds of vc = 50 m/min, the target tool life LT,10 could not be reached. When using WC-Co K30 tools, tool lives of LT > 6 m and cutting speeds up to vc = 400 m/min were feasible when machining 42CrMo4 tempered steel. For the machining of EN-GJS-700-2 cast iron, high abrasive tool wear was observed, which led to short tool lives of LT < 4 m at cutting speeds above vc ≥ 200 m/min. By reducing the cutting speed to vc = 100 m/min, a tool life of LT > 15 m could be achieved.

Verification of electric steel punching simulation results using microhardness

One of the most dominant manufacturing methods in the production of electromechanical devices from sheet metal is punching. In punching, the material undergoes plastic deformation and finally fracture. Punching of an electrical steel sheet causes plastic deformation on the edges of the part, which affects the magnetic properties of the material, i.e., increases iron losses in the material, which in turn has a negative effect on the performance of the electromagnetic devices in the final product. Therefore, punching-induced iron losses decrease the energy efficiency of the device. FEM simulations of punching have shown significantly increased plastic deformation on the workpiece edges with increasing tool wear. In order to identify the critical tool wear, after which the iron losses have increased beyond acceptable limits, the simulation results must be verified with experimental methods. The acceptable limits are pushed further in the standards by the International Electrotechnical Commission (IEC). The new standard (IEC TS 60034-30-2:2016) has much stricter limits regarding the energy efficiency of electromechanical machines, with an IE5 class efficiency that exceeds the previous IE4 class (IEC 60034-30-1:2014) requirements by 30%. The simulations are done using Scientific Forming Technologies Corporation Deform, a finite element software for material processing simulations. The electrical steel used is M400-50A, and the tool material is Vanadis 23, a powder-based high-speed steel. Vanadis 23 is a high alloyed powder metallurgical high-speed steel with a high abrasive wear resistance and a high compressive strength. It is suitable for cold work processing like punching. In the existing literature, FEM simulations and experimental methods have been incorporated for investigating the edge deformation properties of sheared surfaces, but there is a research gap in verifying the simulation results with the experimental methods. In this paper, FEM simulation of the punching process is verified using an electrical steel sheet from real production environment and measuring the deformation of the edges using microhardness measurements. The simulations show high plastic deformation 50 μm into the workpiece edge, a result that is shown to be in good agreement with the experimental results.

HPPMS TiAlCrSiN - Influence of substrate bias and pulse frequency on cutting performance

The machining of powder metallurgical high-speed steel leads to thermal and mechanical loads which cause damages such as abrasive and adhesive wear as well as oxidation of the tool. Due to the high hardness and thermal resistance of hard coatings, their application on cutting tools is the state of the art. In order to improve the tool lifetime and the performance of the cutting process and to fulfill the increasing requirements in machining, the complex coating TiAlCrSiN was developed. The embedding of the nanocrystalline grains in an amorphous Si3N4 matrix is expected to result in advantageous properties in comparison to the coating TiAlCrN. The thermal resistance and elastic-plastic properties, for example, are enhanced. The coating deposition was conducted in an industrial coating unit using a hybrid process consisting of direct current Magnetron Sputtering and High Power Pulsed Magnetron Sputtering (dcMS/HPPMS). By using this hybrid process, the advantages of HPPMS, such as a dense morphology and high indentation hardness, are combined with the higher deposition rate of dcMS. In this work, the influence of the substrate bias and the pulse frequency of the HPPMS cathodes during the hybrid process are observed. The coating properties are only slightly affected by variations in the process parameters. Transmission electron microscopy investigations reveal the nanocrystalline microstructure of the coating. The compound properties between the different coatings and the indexable inserts made of tungsten carbide were improved by reducing the substrate bias. Furthermore, the coatings are tested by milling the powder metallurgically produced high-speed steel 1.3345. The milling tool coated with an increased frequency and a reduced substrate bias shows the highest tool lifetime during the cutting tests.

Wear behavior and thermal stability of HPPMS (Al,Ti,Cr,Si)ON, (Al,Ti,Cr,Si)N and (Ti,Al,Cr,Si)N coatings for cutting tools

During the hard machining of powder metallurgical high-speed steel, finely dispersed carbides in the steel expose tools to both thermal and mechanical load. PVD hard coatings are used to protect the tools from wear, oxidation and diffusion processes. With regard to mechanical and thermal properties, (Ti,Al,Cr,Si)N hard coatings are advantageous compared to (Ti,Al)N due to their nanocomposite coating architecture. In addition to monolithic (Ti,Al,Cr,Si)N, a bilayer coating with a (Ti,Al,Cr,Si)ON top layer is deposited in order to investigate the influence of oxygen on the interaction with the workpiece during machining. The coatings were deposited in an industrial scale coating unit using a hybrid technology consisting of direct current and high power pulse magnetron sputtering (dcMS/HPPMS). The influence of the oxygen/nitrogen and the aluminum/titanium ratio on the coating as well as compound properties of indexable inserts made from cemented carbide were investigated. Furthermore, the oxidation and the phase stability were investigated. Finally, the coating systems were examined in cutting tests during which powder metallurgical high-speed steel was milled using 6 mm cemented carbide milling tools. The coatings show a fine crystalline morphology with a cubic crystal structure and a smooth surface. For oxynitride coatings, both hardness and resistance against plastic deformation show increased values compared to the nitride coatings. The additional oxygen might lead to a more brittle deformation behavior. The coating with an increased titanium/aluminum ratio shows, on the one hand, the best compound properties. On the other hand, its oxidation and phase stability are lower compared to the coatings with a lower ratio. In contrast to titanium, aluminum forms protective oxide layers which increase thermal resistance. In the cutting tests, the nitride coating shows a slightly higher tool life compared to the oxynitride coating. The tool life of the nitride coating with an increased titanium/aluminum ratio is significantly increased compared to the other coatings.

Microstructural Evolution, Behavior of Precipitates, and Mechanical Properties of Powder Metallurgical High-Speed Steel S390 During Tempering

The evolution of the microstructure and mechanical properties of a powder metallurgical (PM) high-speed steel (HSS) were investigated using a series of tempering times. A near Baker–Nutting (B–N) orientation relationship (OR) between the MC carbide and martensitic matrix was detected, while no OR between the M6C carbide and matrix was observed. The average grain size of the investigated alloys was approximately 4 to 6 μm, which was slightly larger than the thermodynamic calculated grain size of 2 μm. The results indicated that the Rockwell hardness, compressive strength, and in-situ nano-hardness of martensite decreased as the tempering times increased. After quadruple tempering, the fracture strain, compressive strength, and flexural strength declined. Triple tempering contributed to the superior combination of compressive strength, fracture strain, Rockwell hardness, and flexural strength of 3245 MPa, 30.6 pct, HRC 62.2, and 2094 MPa, respectively. The investigated alloys demonstrated strong susceptibility to cracks. The results of this study could provide useful information for the application of this HSS.

Evolution of White Layer during Wire-Cutting and Comparison of Several Methods to Improve Surface Integrity for Fineblanking Tools

Powder metallurgical high speed steel (such as S390) has superior mechanical properties and been used as fineblanking tools. The electrical discharge machining has been widely used for cutting fine blanking tools which are made of especially hard tool steels. Whereas, its thermal nature causes great concerns regarding surface integrity, which matters a lot to tool life. In the present study, the evolution of surface integrity of the S390 with multi-cutting is comprehensively compared. The result shows that the surface roughness, white layer thickness and surface residual stress decrease with the increase of cutting pass. Additionally, the effectiveness to remove white layer on HSS S390 by manual and towed polishing and electrolytic polishing are compared. At last, a device of abrasive water jet polishing is designated to remove the white layer resulted from wire-cutting.

Evolution of the microstructure and mechanical properties of powder metallurgical high-speed steel S390 after heat treatment

The evolution of microstructure and mechanical properties of commercial powder metallurgical high-speed steel S390 steel after different heat treatments were studied. The results show that the austenitic transformation of the alloy starts at 1110 K (837 °C), whereas the martensite transformation starts at 624 K (351 °C). The density of hot isostatic pressed alloy is around 99.2 vol%, and the average pore size is less than 1 μm. There are two types of metal carbides (MC): vanadium-rich MC-type and tungsten-rich M6C-type. After proper heat treatment, the alloys exhibit a high Rockwell-C hardness of 64.4 and compressive strength of 3870 MPa, as well as an ultra-large fracture strain of 22.7% under compression, which results from the changes in the size and distribution of the carbides during heat treatment. In addition, the experimental results concerning the dissolution of vanadium carbide varying with temperature are similar to the results obtained by thermodynamic analysis. Finally, for the experimental iron alloy, deep cryogenic treatment during the triple tempering treatment process has little effect on the compressive strength and room temperature hardness.

Surface Modification of Broaches from the Powder High Speed Steels Applied to Processing of Aviation Products from Heat-Resistant Alloys

This paper considers an approach to improve durability of broaching tools made of high alloyed powder metallurgical high-speed steels by complex treatment which includes ion nitriding and following deposition of wear-resistant coating (Nb, Ti,Al)N using two-stage vacuum-arc discharge. Broaching tools complex hardening conditions which provide minimal wear rate when machining heat-resistant nickel alloy Inconel 718 used for turbine discs are defined.

Increased thermal stability of Ti1 − xAlxN/TiN multilayer coatings through high temperature sputter deposition on powder-metallurgical high-speed steels

Although spinodal decomposition of metastable cubic Ti1−xAlxN-based coatings and the underlying mechanisms are widely understood, investigations on the influence of high deposition temperatures are lacking. It is thus the aim of this work to elucidate structure, properties, thermal stability and wear performance of Ti1−xAlxN/TiN multilayer coatings sputter deposited on powder-metallurgical high-speed steels at substrate temperatures between 375 and 575°C. At higher substrate temperatures, sharper column boundaries and sharper transition zones in the multilayer arrangement yield increased hardness in the as-deposited state, while the detrimental formation of wurtzite AlN during vacuum annealing is retarded by 50°C according to Rietveld refinement of X-ray diffractograms. Tribological tests at room temperature and up to 650°C corroborate the high potential of increased coating temperatures, while demonstrating the crucial importance of using a substrate material with adequate hot hardness. Cutting tests with coated high-speed steel end mills verify the high temperature deposition approach showing a tool life increase of ~40%.

Scratch adhesion characteristics of PVD TiAlN deposited on high speed steel, cemented carbide and PCBN substrates

Modern tool materials, ranging from powder metallurgical high speed steel to super hard materials such as polycrystalline cubic boron nitride and diamond, are used as cutting tools in the metal cutting industry. In order to further improve the cutting performance, these tools are frequently coated by thin, hard PVD coatings such as TiN, TiAlN, AlCrO3, etc. In order to develop and design new PVD coatings it is important to characterize the mechanical properties of the coatings and understand the coating/substrate deformation mechanisms in a tribological contact, e.g. metal cutting. For example, it is important to be aware that the mechanical properties of the substrate (tool material) have a significant impact on the practical coating adhesion and the coating failure mechanisms.In the present study scratch testing has been used in order to evaluate to increase the understanding of the mechanical response and potential coating failure modes of cathodic arc evaporated TiAlN deposited on high speed steel, cemented carbide and polycrystalline cubic boron nitride. Post-test characterization of the scratched samples using optical profilometry, scanning electron microscopy and energy dispersive X-ray spectroscopy were performed and the cohesive and adhesive surface failure mechanisms are described and related to the substrate material properties. The results clearly show that, although all substrate materials can be regarded as hard, they result in completely different coating failure mechanisms at the normal load corresponding to substrate exposure. Also, coating failure resulting in substrate exposure does not necessarily correspond to interfacial cracking resulting in adhesive fracture along the coating–substrate interface.

Study of the Surface Integrity of Powder-Metallurgy High-Speed Steel (S390) Multi-Cut by Wire Electrical Discharge Machining

The surface integrity of powder metallurgical high speed steel (S390) cut by wire electrical discharge machining (WEDM) has a great influence on the its fatigue life. In this paper, a study focused on surface integrity including white layer, surface finish, retained austenite, carbide and residual stress changed with multi-cutting has been described. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectrograph (EDS) analysis were performed in the examination of the evolution of surface integrity. The experimental results reveal that the surface roughness, micro-cracks, white layer thickness and surface residual stress decrease with the increase of cutting pass, but the amount of retained austenite in the resolidified layer of the surface after the second cutting pass is the highest, compared with the other cutting passes. The content of carbides increases with the cutting pass and few carbides appear in the top section of the white layer of the first two cutting passes.

Surface density of growth defects in different PVD hard coatings prepared by sputtering

Growth defects are present in all PVD hard coatings. They have detrimental influence on their tribological properties (higher sticking of workpiece material, higher friction coefficient, worse corrosion resistance, higher gas permeation). In order to improve the tribological properties of PVD hard coatings it is important to minimize the concentration of growth defects. Conventional TiAlN single layer as well as AlTiN/TiN and TiAlN/CrN nanolayer coatings were deposited on cemented carbide, powder metallurgical high speed steel (ASP30) and cold work tool steel (D2) by magnetron sputtering in the CC800/7 and CC800/9 sinOx ML (CemeCon) deposition systems, respectively. The surface morphology of the coated substrates was examined by scanning electron microscope (FE-SEM) in combination with focused ion beam (FIB), and 3D stylus profilometer. By means of 3D-profilometry we performed several measurements and detailed analysis on a series of samples from the several hundred production batches. The influence of growth defects on GDOES (glow-discharge optical emission spectrometry) depth resolution and pitting corrosion was also studied.

Cutting performance of coated tools with various adhesion strength quantified by inclined impact tests

Coated tools with PVD-films of almost the same mechanical properties may perform differently. This effect can be attributed to the film adhesion. For highlighting this phenomenon, the adhesion of coatings with practically identical mechanical properties deposited on cemented carbide and powder metallurgical high speed steel inserts was quantified by FEM-based evaluations of inclined impact test and nano-indentation results. Moreover, the films’ mechanical properties were assessed by repetitive nano-impacts. Based on these data and on FEM calculations of the cutting process, the coated tools cutting performance was explained in all investigated material and interrupted cutting process cases.