3. Results
3.1. Microstructure and hardness
The characterization of microstructure has been done with
optical microscopy after etching with 5 vol.% alcoholic nitric
acid. Typical microstructures of the welded deposits are shown
in Fig. 3. Alloy A is mainly martensitic with some islands of
austenite. Fine primary Niobium carbides are well distributed
throughout the micro-section. The hardness of the martensite is
about 800 HV0.1. The content of Nb carbides is given in Table 3
to 7% at a size of <3 min a spattered shape. Alloy B consists of
primary Fe/Cr carbides with a micro-hardness of roughly 1600
HV0.1 in a ledeburitic matrix. The content of Fe/Cr carbides is
listed in Table 3 to 57.1% at a size of 30–200 m. The chemistry
of the Fe/Cr carbides is reported for hypereutectic FeCrC HV0.1 are close to previous investigations from Fischer [12] and
Buytoz [13]. Besides small and evenly distributed primary Nb
carbides (light grey in image B of Fig. 3) at a volume content
of approximately 5% can be detected, which are supposed be of
major importance for increasing the resistance against erosion
and abrasion due to their high hardness. Alloy C has solidified
in hypoeutectic -dendrites with about 920 HV0.1, which are
embedded in an eutectic matrix of about 1000 HV0.1 (see Fig. 3).
A closed net or skeleton of brittle Fe/Cr carbides at a volume
content of 48.3% (see Table 3) is clearly surrounding the primary
dendrites which can be seen in detail in the SEM image of Fig. 4.
This appearance comes close to a solidification of the finally
solidifying eutectic melt described as N-type [12]. Alloy D is
built up of Fe carbo-borides in columnar structure with a hardness
of about 1500 HV0.1 in a hard eutectic matrix of about 1000
HV0.1 (see Fig. 3). The distribution of hard phases in this alloy is
quite uniform. Volume content and size of the Fe carbo-borides
are listed in Table 3 to 18.5% and 20–80 m, respectively. The
complex alloy E which contains an amount of Boron similar
to alloy D, but a much higher level of other elements like W,
Mo, Nb and Cr and shows a dense and uniform distribution
of very hard complex carbides and carbo-borides (see Fig. 3)
with hardness values between 1200 and 1900 HV0.1. Type and
content of the HP were determined to Fe/Cr carbo-borides at
a volume content of 52% at a size of 10–100m, Nb carbides
and Mo/W carbo-borides at a volume content of approximately
5% in blocky shape (see Fig. 5). In Ref. [14] hard phases of
a very similar alloy are described as M23(BC)6 and M7(CB)3
carbo-borides phases in a matrix exhibiting high fracture toughness
up to 73.3MPam1/2 due to an effective distribution of fine
carbide and boride phases in ductile dendrites/cells. The synthetic
multiphase alloy F shows the original fused and crushed
tungsten carbides (2500–2700 HV0.1) which are extensively
dissolved in the Fe-based matrix and lead to well distributed reprecipitated
carbides with a decreased hardness of 1200–1600
HV0.1. Content and size of the synthetically added tungsten car-bides is determined to 38.7% and 65–250 m, respectively. The
matrix has a hardness between 800 and 1100 HV0.1. Higher
welding amperage increases the rate of tungsten carbide dissolution.
Overall the tungsten carbides are irregularly distributed.
There is a higher density of original carbides close to the fusion
line, whereas at the surface only rests of carbides are visible.
3.2. Dry-sand rubber-wheel tests (3-body abrasion)
Wear tests with a standardASTMG65 dry-sand rubber-wheel
tester according to procedure A were carried out to simulate
3-body abrasion similar to practical applications. Quantitative
wear analysis was done by volume loss, and the results are given
in Fig. 6. It can be seen, that there is a correlation of abrasive
wear and hardness. In general, better performance against 3-
body abrasion can be obtained by increasing material hardness
[15]. High abrasive wear can be observed for alloy A which is in
good agreement with the relative low hardness (see Fig. 6). The
lowest abrasive wear resistance of the hardfacing alloys investigated
was observed for alloy C. It can be seen, that alloys A
and C are given in a high wear level, whereas the other materials
investigated are situated in a low level of abrasive wear.The influence of coarse primary precipitations on abrasive wear
resistance can be seen for alloys C and D. These alloys exhibit
similar macroscopic hardness, however wear behaviour differs
by a factor of about 5 (see Fig. 6). Very good wear resistance can
be obtained for alloy F (synthetic multiphase alloy with tungsten
carbides), where the macro-hardness is given to about 750
HV5. Alloy E, showing highest hardness, performed best wear
behaviour in dry-sand rubber-wheel testing.
3.3. Impeller-tumbler tests at low and high energy
The impeller-tumbler wear rates obtained for the six different
Fe-based hardfacing alloys tested under lowimpact loading (low
energy level) are given in Fig. 7. There, for the softest material,
alloy A, highest wear rate is observed. The better performance
of alloy C compared to alloy A can be explained by the higher
hardness (see Fig. 7). Best wear resistance can be seen for alloys
D and E, which is in good agreement with their high hardness.
The impeller-tumbler wear rates obtained for high impact
loading (high energy level) are given in Fig. 8. Very good correlation
of wear rate and material hardness can be seen. For alloy
F, the synthetic
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