3.3.3. Boron steel
The boron steel, being the softest of the materials tested in this
study, showed the highest wear rates at all temperatures.Fig. 13
(a) shows a heavily deformed surface with the presence of
microploughing and microcutting as the main wear mechanisms
after being tested at 20°C. Similarly to Toolox
33
, a reduction in the
wear rate as well as an increase in wear energy was observed as
the temperature was increased from 20 to 100°C. A possible
explanation for this behaviour is the development of a tribolayer
consisting of silica particles embedded in the plastically deformed
surface (Fig. 13(b)). This behaviour has also been reported by
Petrica et al.[27]where an increase in wear energy was obtained
in the case of hardfaced coated materials.
Even though the hardness of the boron steel is around 1.7 times
lower than that of Toolox
33
within the temperature range from
100 to 300°C, the wear rates of both materials are quite similar at
these temperatures.
A possible explanation for this behaviour is the work hardenability of the boron steel. It is shown inFig. 13(b) that the worn
surface of the boron steel was plastically deformed during the
wear process, resulting in a strain hardened layer. The presence of
a layer with silica particles and oxidised wear debris on top of the
Fig. 11.SEM micrograph of Toolox
44
tested at 800°C showing the presence of a
tribolayer on the top surface.
Recrystallized grains
Fractured silica particle
Fig. 12.SEM micrographs of Toolox
33
after testing at (a) 20°C, (b) 600°C and (c) 800°C.
S. Hernandez et al. / Wear 338-339 (2015) 27–35 33
hardened layer further protected the bulk material and thus
decreased the wear rate.
Aseriesofmicro-hardnessmeasurementswerecarriedout
within the plastically deformed layer of a boron steel sample tested
at 100°C, resulting in a mean value of 237 HV(20 g) i.e. an increment
of31%withrespecttothehardnessatthattemperature.Inawork
carried out by Sundström et al.[24] it was shown that non-martensitic steels work-harden to a higher extent than the martensitic
steels during impact/abrasion wear tests. They concluded that the
worn surface hardness is a more important parameter than the
hardness of the surface prior to wear.
At higher temperatures (from 400 to 800°C), an increase in wear
rate mainly due to large fragmented particles causing microcutting
was observed (Fig. 14(a)). In this range of temperatures, the work
hardening phenomenon is cancelled out and outweighed by the
temperature induced recovery process [19]. Fig. 14(b) shows the
presence of recrystallized ferrite grains at the surface of the sample
exposed to 600°C. During deformation, the dislocations generated
increase the freeenergy and the number of possible sites for new
grains to nucleate, accelerating the diffusional-phase transformations
[28]. This is a well-known phenomenon called dynamic recrystallization[19].
The occurrence of recrystallization in steels has proved to be
detrimental in terms of wear resistance. In a work carried out by
Berns et al.[29], the materials containing precipitates or carbides
within the microstructure showed the best wear resistance at elevated temperatures. They attributed this behaviour to the effectiveness of precipitates or carbides to prevent or significantly delay the
movement of dislocations, which is necessary for recrystallization to
occur. In agreement with this, during this study both tool steels
showed lower wear rates compared to the boron steel, especially
above 600°C, since they were able to hinder the movement of dislocations due to their carbide content.