The processing of plastics, particularly reinforced composites, necessitates the use of corrosion-and wear-resistant materials for tools that come into contact with the polymer. For such applications, plastic mold steels were developed that offer not only a good wear resistance due to the presence of carbides in a martensitic matrix, but also good corrosion resistance provided primarily by a sufficient amount of dissolved chromium. The common processing route for these high-alloyed materials is the hot isostatic pressing (HIP) of gas-atomized powders (PM-HIP). In this context, sintering plays an insignificant role, except for the processing of metalmatrix composites (MMCs). The development of novel wear-and corrosion-resistant MMCs based on plastic mold steels requires knowledge of the sintering behavior of prealloyed powders of such tool steels. It is well known that alloyed powders can be processed by supersolidus liquidphase sintering (SLPS), a method leading to almost full densification and to microstructures without significant coarsening effects. In this work, two different gas-atomized powders of plastic mold steels were investigated by computational thermodynamics, thermal analysis, sintering experiments, and microstructural characterization. The results show that both powders can be sintered to almost full density (1 to 3 pct porosity) by SLPS in a vacuum or a nitrogen atmosphere. Experimental findings on the densification behavior, nitrogen uptake, and carbide volume fractions are in good agreement with calculations performed by computational thermodynamics.
Applications in plastics processing bear increased requirements for the used materials, especially with respect to their corrosion and wear resistance. For this reason, special powder metallurgical tools steels were developed that fulfil these demands. The common processing route for their production is hot isostatic pressing (HIP) of pre-alloyed powders which is followed by hot working if semi-finished parts are to be produced. As an alternative to HIP, super solidus liquid phase sintering (SLPS) permits the consolidation of pre-alloyed tool steel powders to near net-shape parts. It can be performed in different sintering atmospheres. In this work, the plastic mould steel X190CrVMo20-4 was processed by SLPS in vacuum as well as under nitrogen atmosphere. The resulting materials were analysed with respect to their microstructure, tempering behaviour and corrosion resistance in 0.5 molar sulphuric acid in dependence of the heat treatment. As a reference, the HIPed and the HIPed and worked state were also investigated. The results show that different heat treatments alter the ranking of the sintered and the HIPed state with respect to corrosion resistance. As expected, a high tempering for maximum secondary hardness causes a significant loss of corrosion resistance. The experimental findings were supported by thermodynamic calculations based on slight alterations in chemical composition that result from the different manufacturing processes.
In the present study, compositionally-graded structures of AISI 316L and CoCrMo alloy are manufactured by powder-based laser-beam directed energy deposition (DED-LB). Through a process-integrated adjustment of powder flow, in situ alloying of the two materials becomes feasible. Thus, a sharp and a smooth transition with a mixture of both alloys can be realized. In order to investigate the phase formation during in situ alloying, a simulation approach considering equilibrium calculations is employed. The findings reveal that a precise compositional as well as functional gradation of the two alloys is possible. Thereby, the chemical composition can be directly correlated with the specimen hardness. Moreover, phases, which are identified by equilibrium calculations, can also be observed experimentally using scanning electron microscopy (SEM) and energy-dispersive X-ray-spectroscopy (EDS). Electron backscatter diffraction (EBSD) reveals epitaxial grain growth across the sharp transition region with a pronounced <001>-texture, while the smooth transition acts as nucleus for the growth of new grains with <101>-orientation. In light of envisaged applications in the biomedical sector, the present investigation demonstrates the high potential of an AISI 316L/CoCrMo alloy material combination.
For testing the corrosion resistance of metallic materials, the measurement of current density potential curves in acid environments e. g. sulphuric acid is a widely used method. These curves are usually evaluated by means of the current density in the passive area, the maximum current density in the active peak and the width of the passive area. Stainless cold work tool steels however feature a more complex behavior which pushes this approach to a limit. Thus, a more detailed evaluation is necessary to understand the current density potential curves of such multiphase-materials. In this paper the relation of the curve progression and corrosion phenomena on the microscopic scale were investigated in order to contribute to a better understanding of experimental data.
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