Wear-Resistant Coatings Using Tungsten Carbide
Effective Protection for
Demanding Applications
ULRICH MORKRAMER
RICHARD-FRANZ MOLL
MANFRED OECHSLE
SULZER METCO
12
SULZER TECHNICAL REVIEW 3+4/2005
Most oil deposits are found between 900 and
5000 meters (3000 to 16,000 feet) below the
surface of the earth. Thus, the aggregates used
when drilling for oil are subjected to extreme loads;
they have to penetrate through layers of earth and
rock with completely different characteristics. Wearprotective coatings made from Sulzer Metco Woka
materials that contain tungsten carbide protect the
drilling equipment and extend the service life of the
tools. Sulzer Metco Woka is a German subsidiary of
Sulzer Metco.
4154
Tungsten is a gray-white
metal with a very high melting point and high tensile strength.
Its carbon compounds, tungsten
carbide WC and W2C, are almost
as hard as diamond. They are particularly suitable for the manufacture of tools that are exposed to
high loads, such as mining and
drilling equipment (Fig. 1). The
service life of drilling tools made
from tungsten carbide exceeds that
of steel drill bits by several times.
2 Sulzer Metco Woka delivers tungsten carbide that is tailor-made to
the needs of the customers. Specialized knowledge about the behavior of
the various carbide types makes it
possible to systematically manufacture surfaces with specific properties.
Crushed cast tungsten carbide
Spherical cast tungsten carbide
Macrocrystalline tungsten carbide
Cemented tungsten carbide pellets
Material Properties Combined
Pure tungsten carbide (WC) is
brittle, and is therefore mainly
used in sintered cemented carbides (CC). In the production of
cemented carbides, the extremely
hard tungsten carbide grains are
combined with a binder matrix of
ductile metal by sintering—baking
together of granular materials
under heat and sometimes pressure. According to the desired
hardness and strength of the cemented carbides, the grains of the
added tungsten carbide vary in
shape and size—from 0.2 µm to
around 20 µm. From a chemical
point of view, macrocrystalline
tungsten carbide is not different
from the WC used in cemented
carbides. It has grain sizes of up to
200 µm and yet unchanged excellent thermal stability, making this
material, which is also monocrystalline, particularly suitable for
coating processes by welding.
Another modification is cast tungsten carbide (WC-W2C), which
is used crushed—with sharp
edges—or in a spherical shape.
The crushed cast tungsten carbide
(CTC) is very hard and impactresistant. By contrast, due to its
production method, spherical cast
tungsten carbide (CTC-S), demonstrates significantly greater hardness. Cemented tungsten carbide
pellets (WC-6Co) are less hard, but
are considerably more ductile than
spherical cast tungsten carbide.
All these carbide types are used in
hard facing, where the proportion
and type of the various carbides
has a decisive influence on the
overall properties of the applied
facings (Fig. 2). It takes technical
know-how and experience in the
correct combination of the tungsten carbide powder and the
200 µm
1 Equipment used for oil extraction is exposed to strong
and diverse wear attacks. Here (left), a classification
screen for the processing of oil-bearing sand in Canada,
which has been coated by means of a PTA procedure.
PTA hard facings, which are produced with additives from
Sulzer Metco Woka, extend the service life of these tools.
The enlargement shows a typical PTA powder mixture of
tungsten carbide and matrix material.
choice of the matrix material to be
able to fulfill the specific requirements and customer specifications
in a targeted manner.
Thicker Layers
Tungsten carbide overlay coatings
can be applied by high-velocity
oxygen fuel spraying (HVOF) or
by welding. In general, the thinner
HVOF facings, which are between
10 µm and 1000 µm thick, are used
for the coating of valves and pistons and for the protection of
pump impellers. The extremely
abrasive conditions in mining and
oil drilling demand thicker coatings and better bonding than are
possible with spray processes. For
example, the plasma-transferred
arc (PTA) process using multilayer
Protection against Different Types of Wear
In order to effectively use a
coating to protect the surface
of a tool against wear, it is necessary to know what types of
wear the tool might be subjected to. Solid body abrasion (A),
erosion through fluid attack (B),
and impact or cavitation (C) are
A) Abrasion
possible types of wear. In most
cases, however, the wear mechanisms are extremely complex.
The decisive factor for a successful coating is a good match
in terms of the type, quantity,
form, and size distribution of
the carbide used.
B) Erosion
C) Impact and
cavitation
SULZER TECHNICAL REVIEW 3+4/2005
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Cathode
Cathode holder
Ignition
Plasma gas
Cooling
3 In the PTA procedure, the base material
that is to be coated and the matrix material for
the coating are melted in a controlled manner.
The hard components of the coating—
for example, tungsten carbide grains—thereby
retain their structure.
Carrier gas
Current
source
Protective gas
Direction of welding
technique can produce facings
with thicknesses of up to 10 mm
and more. PTA is an advanced
coating procedure in which the
powder is applied to the base material in a controlled protective gas
atmosphere (Fig. 3). The transferred arc melts the base material
in a controlled manner and thereby produces an extraordinarily
strong metallurgical bond between the coating and the base
material. Over and above the PTA
procedure, all the classical
processes for deposition welding
are used for the production of
wear protection coatings.
Specific Coating
Drill bits, such as those used in
deep drilling, are exposed to a
wide variety of wear attacks. Two
of the greatest loads that this kind
of drilling tool must withstand are
impact with pieces of rock at the
4 The requirements
for the surface affect
the composition of
the welding additives.
For the illustrated
roller bit, coatings
are required with
both high impact
stability and high
erosion resistance.
14
teeth of the drill bit and erosion
through the fine-grain outbreak
flushed upwards past the drill
head (Fig. 4). The hardness and
ductility of the coating can be
specifically adjusted by selecting
the correct tungsten carbide.
Where high abrasion resistance is
required, a hard facing can be applied; a suitably matched carbidematrix combination will be chosen
for locations where ductility is
essential. The characterization of
such hard facings takes place
according to standardized test
procedures, for example, through
the “G65” test of the international standardization organization ASTM International.
For welded coatings, Sulzer Metco
Woka offers an additive containing
tungsten carbide in the form of
filled tubular rods, wires, or flexible welding rods. These can
then be melted by oxy-acetylene
processes or by arc welding, and
applied to the surface. The PTA
procedure, on the other hand,
makes use of materials in powder
form that are mostly a mixture of
different tungsten carbide and matrix alloys. The complete product
range fulfills the relevant industrial standards, DIN EN ISO 9001
and AS 9100.
Increased Service Life
Better wear characteristics bring
substantial advantages in operation. Inventory can be reduced as a
result of extended service life. The
extremely high costs resulting from
long downtimes and tool changes
during deep drilling can thereby be
minimized. Sulzer Metco Woka’s
highly wear-resistant coating
materials containing tungsten carbide therefore lead to considerable
savings for customers in the oil and
gas industry.
Contact
500 µm
Coating with higher
impact stability
SULZER TECHNICAL REVIEW 3+4/2005
500 µm
Coated roller bit
Coating with higher
erosion resistance
Sulzer Metco Woka GmbH
Manfred Oechsle
Im Vorwerk 25
36456 Barchfeld
Germany
Phone +49 (0)36961 861 13
Fax +49 (0)36961 861 33
manfred.oechsle@sulzer.com