Alexander S. Chaus

Development of high-speed steels for cast metal-cutting tools

As-cast high-speed steels heat-treated have completely much lower impact toughness than that of the steels of a similar chemical composition but undergone hot working – rolling or forging. That is attributed to the influence of eutectic carbides, which especially being coarse, provide easily brittle intergrain fracture sites under low stress intensity factor levels. This is especially real for cast cutting tools. In order to exhibit good all-round performance the impact toughness enhancement of as-cast high-speed steels is obligatorily needed. In this connection it is expedient to turn from high-speed steels of conventional ledeburitic origin to high-speed steels of hypereutectoid and ferritic-carbidic ones with considerably lower carbide heterogeneity resulting in enhanced impact toughness. In the present work special features of the structure, phase composition and properties of such high-speed steels designed for cast tool are studied. In order to substitute tungsten by chromium in as-cast high-speed steel a special alloying system has also been developed.

Effect of Austenitising Temperature on Structural Changes in Modified High-Speed Steel of AISI M2 Type

The transformation of the solidification microstructure and the phase changes in AISI M2 grade high-speed steel modified with powder addition of TiB2 have been studied focusing on the effect of austenitising temperatures. In order to investigate kinetics of both the microstructure and phase transformations in eutectic carbides, primarily M2C carbide decomposition, upon heat treatments with respect to diffusion processes, different techniques of electron scanning microscopy, X-ray diffraction analysis and energy dispersive X-ray spectrometry have been used.

Effects of Austenitizing Temperatures on Primary Structure Transformation in Cast High-Speed Steel of M2 Type Inoculated with Powder Additions of W and TiB 2

The influence of heat treatment on the microstructure of AISI M2 type high-speed steels modified with powder additions of W and TiB2 was studied. The primary focus was on the effects of austenitizing temperatures on the prior austenite grain size and size distribution, volume fraction, and morphology of the eutectic carbides M2C, M6C, and MC, as well as the character of the secondary precipitation of carbides in the high-speed steels studied. It was shown that austenitizing significantly affects the changes in the chemical composition, morphology, size, and character of distribution of the eutectic carbides in the high-speed steels studied, as well as the volume fraction of retained austenite in the matrix and the prior austenite grain size. These changes in carbides seemed to be induced by a diffusion-induced redistribution of alloying elements between the steel matrix and the carbide phases due to their mutual interaction at high temperatures, which is accompanied by carbide decomposition, coagulation and dissolution and, as a consequence, saturation of the solid solution by alloying elements.

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.

Diffusion and Transformation of Eutectic Carbides in High-Speed Steels during Heat Treatment

The structure and phase composition of high-speed steels of different grades after casting, annealing, and subsequent final heat treatment (quenching and tempering) have been studied focusing on diffusion processes. In order to investigate kinetics of both the structure and phase transformations in eutectic carbides upon heat treatments, different techniques of optical microscopy, electron microscopy (both scanning and transmission ones), X-ray diffraction, and energy dispersive X-ray analysis have been used.

Diffusion in MC Carbides in High-Speed Steels during High-Temperature Treatments

The microstructure and chemical composition of MC carbides in the modified variants of AISI M2 type high-speed steel after casting, annealing, carburising and subsequent final heat treatment (quenching and tempering) have been studied by focusing on diffusion processes in these carbides and their stability during high-temperature treatments. Significant changes in both the microstructure and chemical composition of the MC carbides as well as in their volume fraction shown to occur in the high-speed steels studied were attributed to decomposition, coagulation and dissolution of these carbides during heat treatments caused by strong diffusion of alloying elements and carbon. It was also shown that boron, in general, strongly suppressed the diffusion of carbon during carburising.

Diffusion and Structural Changes in High-Speed Steels Alloyed with Boron

The structure and phase composition of high-speed steels of different grades additionally alloyed with boron after casting, annealing, and subsequent final heat treatment (quenching and tempering) have been studied focusing on diffusion processes. In order to investigate kinetics of both the structure and phase transformations in eutectic carbides upon heat treatments, different techniques of optical microscopy, electron microscopy (both scanning and transmission ones), X-ray diffraction, and energy dispersive X-ray analysis have been used.

Diffusion Induced Changes in Eutectic Carbides in Wrought M2 High-Speed Steel at Austenitising

The influence of heat treatments on the microstructure of AISI M2 type high-speed steel obtained using conventional metallurgy has been studied. The primary focus was on the effects of austenitisation temperatures on the behaviour of the initial eutectic carbides during austenitisation. In order to investigate kinetics of both the microstructure and phase transformations in eutectic carbides upon heat treatments with respect to diffusion processes, different techniques of electron scanning microscopy and energy dispersive X-ray spectrometry have been used. The effect of the austenitisation temperature on the primary austenite grain size, martensite microstructure and volume fraction, size distribution, and morphology of the primary carbides of eutectic origin as well as their behaviour during heat treatment in the wrought M2 high-speed steel was thoroughly investigated.

Effect of Austenitising Temperature on Microstructural Changes in High-Speed Steel of M2 Type Inoculated with Addition of WC Powder

The transformation of the solidification microstructure in AISI M2 grade high-speed steel inoculated with powder addition of WC have been studied focusing on the effect of austenitisation. In order to investigate the kinetics of both the microstructure and phase transformations in eutectic carbides, primarily M2C carbide decomposition, upon heat treatments with respect to diffusion processes, electron scanning microscopy, XRD analysis and EDS have been used.

Microstructure and Properties of CBN Diffusion Coating on High-Speed Steel

CBN diffusion coating on the ball nose end mills made of AISI-M35 high speed steel (HSS) has been produced thermo-chemically. The microstructure and component depth profiles of the CBN diffusion layer have been studied by scanning electron microscopy and energy dispersive X-ray spectrometry. The results on laboratory cutting tests of ball nose end mills made of AISI-M35 HSS with and without complex CBN diffusion coating are also introduced in the paper. The relationship between wear kinetics and tool life has been established. It was shown that under the used cutting conditions the tool life of the mills with the coating was a factor of 1.6 higher than that of the mills without the coating. The higher tribological stability of the coating in cutting process was supported by metallographic observations of the worn surfaces using scanning electron microscopy.