Influence of Microstructure on the Machinability of Prehardened Mould Steels
Doktorsavhandling, 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.
Prehardened mould steels
Milling
Drilling
Turning
Tool life
Machinability