Influence of Microstructure on the Machinability of Prehardened Mould Steels

Mould steels are mainly employed in moulding of plastic products, and also some metals withlow melting temperature such as magnesium alloys. The ever-growing usage of plastics in theindustry, especially the automotive industry, and in everyday life, has increased the utilizationof mould steels. Of all properties of these steels, machinability has the highest practical andeconomic importance as it covers over 60% of the cost of the mould making process. Due tothe more practical efficiency and shorter lead time, the use of the mould steels in prehardenedcondition has become more popular over traditional rough machining in the soft annealedstate followed by post-roughening hardening and final machining. The present study isconcerned with the influence of microstructure on the machinability of prehardened mouldsteels at a hardness level of around 40 HRC. The studied materials were some popularcommercial mould steels and the experimental methods were chosen with consideration toindustrial practices and demands. Also, the comparison of machinability and other propertieswas conducted at identical hardness, with the help of proper heat treatments and materialbatches. In addition to machinability, the mechanical properties of the studied materials wereexamined as well. Screening of 10 different plastic mould steels in machinability tests, twomilling and two drilling, showed a considerable difference when ranking the machinability ofthe materials. The precipitation hardened steels exhibited a higher drillability than thequenched and tempered steels. The machinability of Uddeholm Impax HH (a modified AISIP20) and Uddeholm Nimax, hardened to a similar hardness with and without heat treatment,respectively, was compared with respect to tool life, cutting force and cutting temperature.Uddeholm Nimax showed a better machinability, with a longer tool life and lower cuttingforce although its cutting temperature was a little higher, beside superior mechanicalproperties. Furthermore, the effect of some microstructural features, e.g. martensite packetsize, carbide morphology and retained austenite, on the machinability was examined inUddeholm Impax HH. The results showed an improved machinability with longer tool lifeand a somewhat lower cutting force in the steel variant with smaller martensite packet size. Inaddition, retained austenite in the microstructure of a bainitic (austempered) variantdeteriorated its machinability. Moreover, a variant which contained spheriodized carbidesexhibited superior machinability over other heat treated variants with more plate-likecarbides