The present work describes a new methodology designed to characterize the microstructures of tool steels containing carbide hard phases, with the focus set on their abrasive wear resistance. A series of algorithms were designed and implemented in MATLABÒ to (i) recognize each of the features of interest, (ii) measure relevant quantities and (iii) characterize each of the phases and the alloy in function of attributes usually neglected in wear description applications: size distribution, shape and contiguity of the hard phases. The new framework incorporates new parameters to describe each one of these attributes, as observed in SEM micrographs. All three aforementioned stages contain novel contributions that can be potentially beneficial to the field of materials design in general and to the field of alloy design for severely abrasive environments in particular. Models of known geometry and micrographs of different powder metallurgy steels were analyzed, and the obtained results were compared with the obtained by the linear intercept method. The relation between the new parameters and the ones available in the scientific literature is also discussed.
Introduction: A full three-dimensional (3D) microstructure characterization that captures the essential features of a given material is oftentimes desirable for determining critical mechanisms of deformation and failure and for conducting computational modeling to predict the material’s behavior under complex thermo-mechanical loading conditions. However, acquiring 3D microstructure representations is costly and time-consuming, whereas 2D surface maps taken from orthogonal perspectives can be readily produced by standard microscopic procedures. We present a robust and comprehensive approach for such 3D microstructure reconstructions based on three electron backscatter diffraction (EBSD) maps from orthogonal surfaces of two-phase materials.Methods: It is demonstrated that processing surface maps by spatial correlation functions combined with principal component analysis (PCA) results in a small set of unique descriptors that serve as a representative fingerprint of the 2D maps. In this way, the differences between surface maps of the real microstructure and virtual surface maps of a reconstructed 3D microstructure can be quantified and iteratively minimized by optimizing the 3D reconstruction.Results: To demonstrate the applicability of the method, the microstructure of a metastable austenitic steel in the two-phase region, where austenite and deformation-induced martensite coexist at room temperature, was characterized and reconstructed. After convergence, the synthetic 3D microstructure accurately describes the experimental system in terms of physical parameters such as volume fractions and phase shapes.Discussion: The resulting 3D microstructures represent the real microstructure in terms of their characteristic features such that multiple realizations of statistically equivalent microstructures can be generated easily. Thus, the presented approach ensures that the 3D reconstructed sample and the associated 2D surface maps are statistically equivalent.
The severe sliding abrasion of single-phase metallic materials is a complex issue with a gaining importance in industrial applications. Different materials with different lattice structures react distinctly to stresses, as the material reaction to wear of counter and base body is mainly determined by the deformation behavior of the base body. For this reason, fcc materials in particular are investigated in this work because, as shown in previous studies, they exhibit better hot wear behavior than bcc materials. In particular, three austenitic steels are investigated, with pure Ni as well as Ni20Cr also being studied as benchmark materials. This allows correlations to be worked out between the hot wear of the material and their microstructural parameters. For this reason, wear tests are carried out, which are analyzed on the basis of the wear characteristics and scratch marks using Electron Backscatter Diffraction. X-ray experiments at elevated temperatures were also carried out to determine the microstructural parameters. It was found that the stacking fault energy, which influences the strain hardening potential, governs the hot wear behavior at elevated temperatures. These correlations can be underlined by analysis of the wear affected cross section, where the investigated materials have shown clear differences.
The influence of short‐time heat treatment on the widely used and commercially available ledeburitic cold‐work tool steel 1.2379 (X153CrMoV12; AISI D2) is examined herein. Starting from a soft annealed initial condition, the influence of different austenitizing temperatures and holding times on the metastable microstructural states after heat treatment/hardening is investigated. The experimental implementation of the heat treatment is used in a quenching dilatometer, and a microstructural simulation model is built using these results. As validation of the model, on the one hand, the martensite start temperature (Ms) is used, measured experimentally by dilatometry. Additionally, the carbide content and distribution, as determined by quantitative image analysis, are compared with the simulated data and used as an indicator of the model accuracy. Through the developed simulation model, arbitrary heat treatment‐induced metastable microstructural states can be calculated. As a possible application of this model, the live‐adaption of the industrial heat treatment process in dependence on the batch chemical composition is discussed.
Cutting tools are commonly made of high-speed steel to simultaneously fulfill the requirements regarding hardness, strength, and toughness. The microstructure of high-speed steel alloys in as-cast condition is characterized by a microstructure consisting of a metal matrix and eutectic carbides. [1][2][3][4] The further manufacturing route of these materials comprises hot forming to break down the carbide network [1,2] and subsequent heat treatment. The behavior of the carbides during austenitization was examined in several publications. [3][4][5][6][7][8][9] Metastable carbides with the stoichiometry M 2 C (M¼ metal atoms, C ¼ carbon), which are present after casting, are transformed to carbides of the types MC and M 6 C with the transformation path of [3,[10][11][12] Dependent on the austenitization temperature and time and thus on the amount of dissolved carbides, different contents of retained austenite remain after quenching. [9] During typical subsequent tempering at temperatures where a secondary hardening effect occurs, retained austenite decomposes, and the precipitation of finely dispersed tempering carbides occurs. [13][14][15] Besides the conventional production route with casting and hot forming, alternative manufacturing processes like spray forming [16][17][18] or powder metallurgy (PM) with hot isostatic pressing are available. Several studies found that powder metallurgic high-speed steel exhibit higher toughness and better cutting performance compared to cast material. [19][20][21][22][23] In most cases, this is explained by the finer distribution, the isotropic properties, and the more regular shape of the eutectic carbides due to the rapid solidification of the powder particles during powder manufacturing. The carbides are affected in their type, [8] size, [5,24] and distribution [25] by the process variables during primary manufacturing and different heat treatments. Mishnaevsky et al. have shown that the fracture in tool steels typically starts in or along eutectic carbides. [26] It is, therefore, to be assumed that the morphology of the eutectic carbides must have an influence on the mechanical properties of high-speed steels.High-speed steel alloys typically exhibit a transformation gap in the isothermal time-temperature-transformation (TTT) diagram. The scientific consensus is that no transformation of the metastable austenite takes place in the temperature regime of the transformation gap. [27] Kešner et al. found that the size of carbides is affected by hardening in a salt bath under isothermal conditions at temperatures between 300 and 650 °C, which is in the temperature regime of the transformation gap of high-speed steels. [28] However, previous works have paid no attention to the effect of different heat treatment parameters on the shape of eutectic carbides. Our preliminary work shows that isothermal
Resource efficiency and circularity in the context of sustainability are rapidly gaining importance in the steel industry. One concept regarding circular economy is “repurposing”. In the context of this work, worn-out machine circular knives are used to produce new chisels for woodturning. The chisels can be extracted parallel or perpendicular to the rolling direction of the primary production process, resulting in an associated carbide orientation of the repurposed tool. The rolling direction, and therefore carbide alignment, will influence the wear resistance and the thermophysical properties, whereby the thermal conductivity will determine the temperatures at the tip of the chisel. Therefore, the thermal conductivity was investigated with the dynamic measurement method, where the specific heat capacity, density and thermal diffusivity of the extracted chisels and industrial reference chisels were measured separately. Moreover, the electrical resistivity was measured in order to calculate the electronic thermal conductivity according to the Wiedemann–Franz–Lorenz law. It was shown that all of these parameters exhibited different degrees of variability with rising temperature. In a detailed analysis, the thermal diffusivity could be identified as an essential parameter of thermal conductivity. By taking two conventional chisels with different chemical compositions and heat treatments into account, it can be seen that the microstructure determines the thermophysical properties. Considering the carbide direction, the chisels that were extracted parallel to the rolling direction showed differing thermophysical properties. Therefore, the carbide orientation is shown to play a significant role regarding the heat dissipation at the cutting edge, because differences, especially in the electronic thermal conductivity in the parallel and perpendicular extracted chisels, can be measured. In addition to the wear resistance factor, the thermal conductivity factor now also supports the removal of the repurposed chisels parallel to the rolling direction.
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