Mechanical Properties, Microstructures and Defects of Tool Steels Fabricated by Additive Manufacturing

Licentiate thesis, 2021

Additive Manufacturing (AM) has been gaining significant interest in the manufacturing of metallic materials in the recent years. From a techno-commercial standpoint, tooling is one of the areas with low production volumes but very high demands on part performance and complexity in design, all of which are characteristic for metal AM. In this thesis study, the focus has been on AM of tool steel intended for hot work applications. Tool steels manufactured through laser beam powder bed fusion (LB-PBF) and directed energy deposition (DED) are studied.
A modified H13 hot work tool steel was produced by means of laser beam powder bed fusion (LB-PBF). The effect of two post processing routes, direct tempering (DT) of as-built part and conventional austenitizing followed by tempering (QT), was evaluated with respect to their impact on microstructure and mechanical properties in terms of hardness, tensile properties and impact toughness. The typical microstructure observed in DT condition retained the melt pool boundaries and cellular structure from the as-printed state as well as tempered martensite. A more uniform microstructure including tempered martensite with carbides possibly along lath boundaries was obtained in the QT condition. While comparable hardness and tensile properties were obtained in these two conditions, QT sample exhibited significantly higher impact toughness compared to DT sample due to its higher work hardening ability and strain rate sensitivity originating from the varied microstructure.
For hot-work applications, resistance of the steel to softening at high temperatures is critical and determines the life of the tool. After long term exposure at elevated temperatures, the response of modified H13 after DT and QT treatments was evaluated by means of hardness measurements and microstructural observation. Thermal softening resistance was strongly influenced by the post-AM treatment and the conditions of subsequent exposure at high temperatures. As expected, lesser softening was observed for 550 ℃ exposure than that for 600 ℃. The decrease of hardness was more severe for QT samples. It is hypothesized that finer grain size of ferrite, less coarsened carbides and the cellular structure being preserved contribute to DT samples being more resistant to thermal softening. Evolution of carbides was analyzed by a combination of experiment and simulation using JMatPro software.
For AM tool steels having high hardness, high strength and poor ductility, defects constitute a particularly serious concern. Uddeholm Vanadis 4 Extra (V4E) cold work steel was deposited on a hot work tool steel with a modified H13 composition (Uddeholm Dievar) by DED with varying number of layers. In the as-built state, defects including pores and cracks were found in the deposited zone. The number of both kinds of defects increased with the building height. Three types of pores were identified: large irregular ones, keyhole and shrinkage pores. Thin Si oxide film (~20 nm) was detected on the internal surface of the larger irregular pores by means of EDX and AES, below which a layer enriched in alloying elements and C was detected. The formation of this type of pore was supposed to be associated with the elemental segregation on the pore surface and insufficient heat input. Solidification cracking was observed as well, especially in the 3rd and 4th layers. Two factors are considered to contribute to higher cracking susceptibility: a) large temperature range for the solidification of V4E steel, simulated by ThermoClac software; b) change in microstructure with the building height from cellular to columnar dendrite.