Influence of Microstructure on the Machinability of Prehardened Mould Steels

Doctoral thesis, 2011

Mould steels are mainly employed in moulding of plastic products, and also some metals with low melting temperature such as magnesium alloys. The ever-growing usage of plastics in the industry, especially the automotive industry, and in everyday life, has increased the utilization of mould steels. Of all properties of these steels, machinability has the highest practical and economic importance as it covers over 60% of the cost of the mould making process. Due to the more practical efficiency and shorter lead time, the use of the mould steels in prehardened condition has become more popular over traditional rough machining in the soft annealed state followed by post-roughening hardening and final machining. The present study is concerned with the influence of microstructure on the machinability of prehardened mould steels at a hardness level of around 40 HRC. The studied materials were some popular commercial mould steels and the experimental methods were chosen with consideration to industrial practices and demands. Also, the comparison of machinability and other properties was conducted at identical hardness, with the help of proper heat treatments and material batches. In addition to machinability, the mechanical properties of the studied materials were examined as well. Screening of 10 different plastic mould steels in machinability tests, two milling and two drilling, showed a considerable difference when ranking the machinability of the materials. The precipitation hardened steels exhibited a higher drillability than the quenched and tempered steels. The machinability of Uddeholm Impax HH (a modified AISI P20) 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 cutting force although its cutting temperature was a little higher, beside superior mechanical properties. Furthermore, the effect of some microstructural features, e.g. martensite packet size, carbide morphology and retained austenite, on the machinability was examined in Uddeholm Impax HH. The results showed an improved machinability with longer tool life and a somewhat lower cutting force in the steel variant with smaller martensite packet size. In addition, retained austenite in the microstructure of a bainitic (austempered) variant deteriorated its machinability. Moreover, a variant which contained spheriodized carbides exhibited superior machinability over other heat treated variants with more plate-like carbides.