The Metallurgical Aspects of Machinability in Prehardened Mould Steels

Licentiate thesis, 2009

High economical efficiency obtained by utilizing prehardened mould steels in plastic mould making has increased the usage of these types of steels to a great extent. Generally, prehardened mould steels are applied for large moulds and moulds for moderate production runs, and one of the advantages of using this type of steel is that no heat treatment is necessary for putting the mould in use. Today, around 80 percent of the mould steels are produced in prehardened condition which is typically ~40 HRC. This hardness limit is required for providing both high hardness and machinability which are of the most important demands upon mould steels. In addition to these properties, a superior polishability, nitriding stability, weldability toughness, corrosion resistance and thermal conductivity are wanted by either moulders or mould makers. Among these properties, machinability is extremely important because of its direct influence on cost of the mould and consequently the plastic part. The machining expenses accounts for more than 60 percent of the total cost of a mould. Hence, improving the machinability in prehardened mould steels has been the concern of tool steel suppliers. In this thesis work, the focus is laid on metallurgical aspects of machinability of prehardened mould steels. Therefore, the machinability of different types of prehardened mould steels is investigated with respect to milling and drilling operations which are the most frequently used machining operations in mould production. In order to understand the metallurgical reason for machinability differences, detailed studies are performed on two steel grades, Uddeholm Nimax and Impax HH. Thorough microstructure characterization is complemented by examining the machinability of the materials in terms of cutting temperature, cutting force and tool life. The materials have shown very different properties in different machining operations and their machinability varies greatly depending on the cutting condition and machining operation type. Comparing the two deeply studied steel grades, Uddeholm Nimax with bainitic structure is benefitted with higher machinability and possesses superior impact toughness and ductility which are important properties for plastic mould steels. Uddeholm Nimax has also the advantage of not needing costly quenching and tempering processes for its production.