Volume 77, Issue 4

While maraging steels are excellent candidates for additive manufacturing of high-strength materials, most of them provide little to no corrosion resistance. In this study, the surface of additively manufactured maraging steel is augmented with diffusion-based chromizing. The diffusion of chromium into the surface results in the formation of α-Cr, σ-phase and austenite depending on the chromium content at the respective distance from the surface. The bulk consists of a ductile bcc martensite hardened by precipitation of intermetallics during a low temperature ageing treatment. The σ-case provides a very high hardness, while the austenite phase is rather soft and unaffected by the ageing treatment. The chromizing treatment significantly reduced the corrosion rate of the maraging steel up to a factor of 14. Decomposition of the σ-case into a uniform case of austenite (austenitic stainless steel) further improved the corrosion resistance almost 24 times. This showed the strong potential of the chromizing treatment on a highstrength material to obtain a combination of their beneficial properties.

In this work, the possibility of manufacturing complex-shaped components from a carbon-martensitic hardenable cold-work steel (1.2379; X153CrMoV12; D2) is investigated. For this purpose, cube-shaped samples with an edge length of 10 mm were produced using the fused-filament fabrication process, which were post-compacted after solvent debinding by supersolidus liquid-phase sintering. Using the knowledge of liquid phase volume content as a function of temperature, supersolidus liquid phase sintering experiments were performed. The microstructure formation process was characterized by electron microscopy and X-ray diffraction. The microstructure and hardness of the processed samples were compared in the heat-treated condition with the properties of the same steel 1.2379 (X153CrMoV12; D2) in the as-cast, deformed and heat-treated condition. The results demonstrate effective post-densificationc close to theoretical density of cold-work tool steel samples fabricated by fused-filamet fabrication using supersolidus liquid-phase sintering at 1280 °C. The defect-free microstructure in the heat-treated state is characterized by a martensitic matrix and eutectic Cr-rich M 7 C 3 and small amounts of V-rich MC carbides. The hardness of the annealed Supersolidus liquid phase sintering samples are 681 ± 5 HV10, which is above the level of the reference material 1.2379 (629 ± 7 HV10) in the as-cast, formed and heat-treated condition.

In a previous publication, the influence of various process steps during the case-hardening process on the resulting material properties of two distinct batches of ultra-clean gear steels were investigated. The steel batches were case-hardened in the as-delivered condition. However, in the industrial practice it is common that pre-heat treatments are done, such as annealing to a ferritic-pearlitic microstructure, or quenching and tempering, as well as forging as a process if for example larger gears are needed. It is known that the whole heat treatment process route can affect the grain size negatively. Therefore, this publication presents investigations into the effects of preheat treatments on the grain size in the core region of ultra-clean gear steels after case-hardening. The aim is to receive a deeper understanding on how the whole process route influences the grain size in the core region of ultra-clean gear steels.

The impact of plasma nitriding on the microstructure and the hardness of a recently developed 4 wt.-% medium manganese steel are presented. In contrast to standard quench and tempering steels, the investigated material achieves its martensitic microstructure by air-cooling from the forging heat, which enables the reduction of the carbon footprint of the forged components. The influence of nitriding on this grade of steel has not been investigated so far, but fundamental differences in comparison to standard nitriding steels are expected due to the increased manganese concentration. To address this issue, nitriding treatments with different temperatures (350 °C, 580 °C and 650 °C) have been performed, followed by examinations of the microstructure, the phase composition, the obtained hardness profiles and the tensile properties of the bulk material after nitriding, accompanied by thermodynamic equilibrium calculations. It is demonstrated that after nitriding at 580 °C similar hardness profiles like standard nitriding steels are achieved, with a shorter process as austenitization and hardening were omitted, reaching a hardness of approximately 950 HV0.1. Furthermore, it was demonstrated that austenite can be stabilized by manganese and nitrogen partitioning to room temperature during nitriding in the intercritical phase region.