PEŁNY TEKST/FULL TEXT

ADAM PIASECKI, DARIUSZ BARTKOWSKI, ANDRZEJ MŁYNARCZAK
Study of the surface layers of 18G2A steel after
plasma surfacing with WC and Fe-Cr powders
INTRODUCTION
Surfacing technology allows for production of surface layers on
any materials, on products of any chemical and phase
composition, and any shape. Rods or powders made of ceramics,
metals, cermets or plastics may be used as surfacing materials.
This method is used to produce surface layers with special
properties, such as wear and corrosion resistance, refractoriness
and creep resistance [1÷7].
Plasma surfacing method consists of melting additional
material (in the form of bulk or powder) along with substrate in
plasma arc at temperatures of about 15 000÷20 000°C. Additional
material and melted metalic substrate create a weld overlay in
which substrate participation can reach tens of percent.
Mechanical finishing is not necessary because weld overlays are
homogeneous, and their faces are smooth [2, 5].
High hardness (WC – 2240 HV, B4C – 2800 HV), wear
resistance as well as corrosion and oxidation resistance are the
major advantages of carbides. Another important advantage is the
high strength to density ratio as well as the high melting point
(above 2000°C) [1, 5, 7].
The aim of this study was to determine microstructure,
thickness, hardness and chemical composition of the surface layers
produced on 18G2A steel by plasma surfacing.
The plasma surfacing of 18G2A steel was carried out by using
EWM Microplazma 50 device. Argon shielding gas, whereas, as
a mixture of argon (75%) and helium (25%) was used as plasma gas.
The Neophot 2 light microscope, PMT-3 hardness tester and
Vega TESCAN 5135 scanning electron microscope were used for
material characterization.
EXPERIMENTAL DETAILS
The surface layers were produced using WC and Fe-Cr powders.
The four different powder mixtures were used, and their
percentage compositions are shown in Table 1. The rectangular
specimens (width of 30 mm, length of 50 mm and height of 10
mm) were used for the study. The tungsten carbide used in the
study was in the form of irregular crystals with sharp edges, and
their size ranged from 1.5 µm to 4 µm (Fig. 1). The ironchromium powder is characterized by spheroidal shape and size
ranging from 1.5 µm to 15 µm (Fig. 2). The chemical composition
of Fe-Cr powder is shown in Table 2.
Fig. 1. SEM image of tungsten carbide powder
Rys. 1. Obraz mikroskopowy proszku węglika wolframu
Table 1. Composition of powder mixtures in weight percentages, wt %
Tabela 1. Skład procentowy mieszanin proszkowych, % mas.
No. of mixtures
1
2
3
4
WC
Fe-Cr
10
90
20
80
30
70
40
60
Carbides
Table 2. Chemical composition of Fe-Cr powder
Tabela 2. Skład chemiczny proszku Fe-Cr
Element
Line
wt %
at. %
Cr
Fe
Kα1
Kα1
87.90
12.
88.64
11.36
Dr inż. Adam Piasecki ([email protected]), mgr inż. Dariusz
Bartkowski, dr hab. inż. Andrzej Młynarczak, prof. nzw. – Instytut Inżynierii
Materiałowej, Politechnika Poznańska
Fig. 2. SEM image of iron-chromium powder
Rys. 2. Obraz mikroskopowy proszku żelazo-chromu
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RESULTS AND DISCUSSION
The study showed that layers produced in plasma surfacing
process were characterized by surface waviness. The thickness of
produced weld overlay layers was about 3 mm. The produced weld
overlays are characterized by multi-phase dendritic structure. An
arrangement of dendrites indicates the direction of heat flow (Fig.
4).
In the substrate material, directly under the weld overlay, grain
growth was observed. It is due to the high temperature realized in
plasma surfacing process. However, with the higher participation
of WC the biggest grain growth in the substrate under weld
overlay was observed. It is associated with the longer duration of
the plasma surfacing process, because this process was conducted
until homogeneous weld pool was obtained. In addition, weld
overlay with a larger percentage of WC was characterized by the
higher porosity (Fig. 4b). The lowest porosity and the smallest
grain size were observed in weld overlay with the lowest share of
tungsten carbide (Fig. 4a).
At the dendrites boundary, the presence of eutectics with
carbides and solid solution of chromium and tungsten in iron was
detected. Microstructure observations in secondary electron
contrast using scanning electron microscope as well as EDS
analysis, showed that in weld overlay of 10% WC + 90% Fe-Cr at
grain boundaries of dendrites, chromium carbides occur in the
form of dark inclusions. In the remaining weld overlays with
a greater amount of WC, a small share of dark inclusions in the
eutectic was observed. Additionally, inclusions in the form of
bright grains of various sizes were observed. Probably these
inclusions are complex-carbides (W, Cr, Fe)C. It should be noted
that fine-grained eutectics was obtained close to the substrate in
weld overlay of 40% WC + 60% Fe-Cr (Fig. 6).
On the basis of microscopic observation and chemical
composition research it can be concluded that the weld overlay
with participation of 10% WC consisted of solutions of Cr and W
in iron. In weld overlay with participation of 20% WC in the
additive material smaller share of iron at the expense of tungsten
content was observed. The amount of chromium was at a similar
level as in the weld overlay with participation of 10% WC and was
amounted as approximately 20%. During the microscopic
examination, in weld overlays with participation of 30 and 40%
WC the presence of clear bright precipitates were found. On the
basis of EDS research one can suppose that these precipitates are
the complex carbides (W, Fe, Cr)C. Therefore, that the weld
overlays with participation of 30% WC and 40% WC contained
more tungsten, in these overlays the carbides were formed. In the
weld overlay with 40% WC the greater precipitates were found.
The smallest Fe participations were found in this weld overlay on
a linear distribution of elements concentrations, and the largest
participation of Cr (in addition to tungsten).
The hardness of weld overlay produced using the method of
plasma surfacing is within the range of 520 HV0.2 to 1470 HV0.2
(Fig. 3). The specimen of weld overlay with a content of 10% WC
+ 90% Fe-Cr was characterized by the lowest hardness, where the
maximum value reached 866 HV0.2. The specimen of weld
overlay with a content of 40% WC + 60% Fe-Cr had highest
hardness (1467 HV0.2). The substrate hardnesses of all the
specimens are similar and their values oscillate about 200 HV0.2.
Irregular hardness profile in the cross-section of 30% WC and
40% WC weld overlays reflects their multiphase construction. The
higher hardness values were comparable to the carbides hardness.
The higher hardness values of matrix results from the highter solid
solution strengthening by tungsten as well as and fine-grain
eutectic consisting of carbides and solid solution of tungsten and
chromium in iron. Furthermore, in the weld overlay with 30% WC
and 40% WC presence of grains of high chromium content of
about 30 at. % was revealed.
The study of chemical composition of all specimens was
carried out along the line of about 300 microns in length and the
data was taken from the boundary area between weld overlay
layers and substrates (Fig. 6÷10). Also the EDS point analysis was
performed (Fig. 5).
The obtained results showed that in weld overlay layers with
the largest amount of Fe-Cr (90% and 80%), the largest amount of
iron in weld overlays was observed.
Fig. 3. Microhardness profiles in weld overlay layers and in substrate
of 18G2A steel
Rys. 3. Profile mikrotwardości w warstwach napawanych i podłożu stali
18G2A
Fig. 4. Microstructure of specimens after plasma surfacing with powders containing: a) 10%WC + 90%Fe-Cr, b) 20% WC + 80%Fe-Cr
Rys. 4. Mikrostruktura próbek napawanych plazmowo proszkiem o zawartości: a) 10% WC + 90% Fe-Cr, b) 20% WC + 80% Fe-Cr
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Fig. 5. SEM image and results of quantitative EDS X-ray microanalysis in weld overlay of 40% WC + 60% Fe-Cr
Rys. 5. Obraz mikroskopowy i wyniki ilościowej mikroanalizy rentgenowskiej EDS w napoinie 40% WC + 60% Fe-Cr
Fig. 6. Microstructure and selection of areas of linear X-ray microanalysis: a) 10% WC + 90% Fe-Cr, b) 20% WC + 80% Fe-Cr, c) 30% WC +
70% Fe-Cr, d) 40% WC + 60% Fe-Cr
Rys. 6. Mikrostruktura i miejsca wykonania mikroanalizy rentgenowskiej wzdłuż linii: a) 10% WC + 90% Fe-Cr, b) 20% WC + 80% Fe-Cr,
c) 30% WC + 70% Fe-Cr, d) 40% WC + 60% Fe-Cr
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Fig. 7. Profile of element concentrations in weld overlay (10% WC +
90% Fe-Cr) and in substrate
Rys. 7. Profile stężenia pierwiastków w napoinie (10%WC + 90%Fe-Cr)
i podłożu
Fig. 9. Profile of element concentrations in weld overlay (30% WC +
70% Fe-Cr) and in substrate
Rys. 9. Profile stężenia pierwiastków w napoinie (30%WC + 70%Fe-Cr)
i podłożu
Fig. 8. Profile of element concentrations in weld overlay (20% WC +
80% Fe-Cr) and in substrate
Rys. 8. Profile stężenia pierwiastków w napoinie (20%WC + 80%Fe-
Fig. 10. Profile of element concentrations in weld overlay (40% WC +
60% Fe-Cr) and in substrate
Rys. 10. Profile stężenia pierwiastków w napoinie (40%WC + 60%Fe-Cr)
i podłożu
Cr) i podłożu
CONCLUSIONS
REFERENCES
 Weld overlays produced by plasma surfacing method with
powders WC and Fe-Cr were characterized by good surface
layer condition without any cracks.
 The highest porosity was found for the weld overlay with
20 wt % WC.
 Geometrical structure of the weld overlays is characterized by
surface waviness resulting from the process of layer production.
 Produced weld overlays showed dendritic arrangement of
grains. At boundaries of dendrite eutectics carbides made of
solid solution of chromium, tungsten and carbon in iron were
present.
 On the entire thickness of the weld overlays, the non-uniform
hardness distribution was found. This is due to the multiphase
structure.
 The highest hardness (1470 HV0.2) of the weld overlays was
obtained using powder with 40% WC, and 60% Fe-Cr.
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