Microstructure and Properties of Additively Manufactured Tool Steels for Hot Stamping

Doktorsavhandling, 2023

Hot work tool steels are commonly used to produce dies for hot stamping, where the steels are exposed to cyclic thermal and mechanical loads. There is a constant demand to improve the lifetime of the dies. Additive manufacturing (AM) provides new solutions for tool design. For example, laser beam powder bed fusion (L-PBF) can print die with complex cooling channels, which can improve cooling efficiency and extend mould life. Directed energy deposition (DED) can easily do a hard facing for the tool surface and refurbish a worn die. This thesis evaluated the microstructure and properties of hot stamping tool steel fabricated by both L-PBF and DED techniques. The softening resistance was also assessed at elevated temperatures.
Before addressing the properties of AM tool steels, a case study was performed on the worn surface of a hot stamping insert die. Galling was observed, which was a result of accumulated layers transferred from the steel workpieces to the die. Material softening of the die was detected in the sublayer of ~ 200 μm. It is the softening of the die material that promotes galling. Galling together with the spalling of the white layer are supposed to be the primary wear mechanisms for the tool.
A modified H13 (M-H13) hot work tool steel was fabricated by L-PBF. The effect of two types of post-processing, direct tempering from as-built condition (DT) and conventional quenching followed by tempering (QT), on the microstructure and mechanical properties was evaluated. The softening resistance at elevated temperatures was investigated. Its correlation with the microstructure was also focused on. The evolution of carbides was discussed based on the microanalysis results and the JMatPro simulation.
Three different types of tool steels, Vanadias 4 Extra (V4E), a high-boron steel (HBS) and a newly developed maraging steel (NMS), were cladded on a hot work tool steel by means of DED for hard-facing purpose. For all tool steels, a near-dense cladded zone was obtained except V4E. Defects, including pores and cracks, were found in the deposited zone of V4E, the number of which increased with the building height or number of layers deposited. The factors that contribute to the formation of pores and cracks were identified.
After being tempered, the cladded tool steels were exposed at high temperatures to assess the softening resistance in terms of hardness. The abrasive wear resistance of the tempered and softened tool steels manufactured by DED was also evaluated at room temperature. A comparison with conventional counterparts on softening resistance and wear resistance was made. The microstructural evolution as a function of temperature and time was characterized and the precipitates were identified. Numerical simulations were applied to NMS to analyze the coarsening behavior of the precipitate and its influence on the mechanical property. The wear mechanism was discussed, and the governing factors were proposed.