Novel developments in surface engineering and protective coverings

PLASMA HARDENING

Novel developments in surface engineering and protective coverings

Stanislav PETROV, Alexander SAAKOV, "TOPAS" Research and
Production Enterprise of Kiev city

Working delays caused by equipment breakdown constitute some 80% of the
total idle time, thus raising running costs all the way up to balance-sheet value of the
equipment. Hence, under conditions of progressing market manufacturing, dependability and
durability issues become extremely urgent. This leads to dynamic development in surface
engineering via gas-thermal spraying of protective coats. Recently, the process of surface
hardening with plasma has been successfully progressing on.

The method of spraying thermal coatings uses plasma of hydrocarbon
combustion products. Plasma ensures high-temperature physical and chemical bonding of
sprayed material’s particles with the product’s surface and with each other.

Plasmotrones are utilized to apply such surface coatings.

In the upcoming millenium, to win in the ever-intensifying competition,
numerous metal manufacturers and consumers will pay critical attention to reliability and
durability of commodities, which are both determined by surface properties of components
and units. In attempt to increase product quality and expand the market, manufacturers
will continue concentrating on key targets. Utilization of novel materials, together with
surface engineering, will turn into the leading factors for increase in product value and
for stepping up the output. It will have to take less time for new products to reach their
markets. Alongside to intensification of production, the tendency for higher environmental
safety will be retained.

Gas-thermal spray enables applying coatings with a wide thickness range
made of various materials, including composite ones, without limitations on size or
dimensions of product being coated. As well, this is the way to effectively regenerate
worn components.

At the moment, various utilized gas-thermal coatings successfully protect
against different types of deterioration, oxidation, and corrosion, serve as thermal
barriers, as well as perform other unique functions. The thermal spray technology is
relatively safe for the environment, while the appropriate automated spray equipment can
be installed into production lines.

In Western Europe, the currently functioning market for gas-thermal
coatings embraces numerous industrial sectors.

Yet, at present, utilization of this method is confined by the following:

• rather high expenses required to implement the method in the
  corresponding manufacturing processes;

• deficiencies of marketed equipment and spray materials.

Only when manufacturers commence producing equipment and materials
adequately to market demands, thus offering novel materials and new equipment qualities,
the industry for gas-thermal coatings will actually expand and grow.

In Western Europe, many people associate further progress in gas-thermal
spraying business with new developments currently under way, which can supplant the
existing technologies and ensure manufacturing of coatings with new properties.

In the early 90s, the Gas Institute of the Ukraine’s National Academy of
Sciences, jointly with E.A. Paton Institute for Electric Welding, designed and
industrially adjusted new "KIEV-7S" apparatus for efficient supersonic plasma
spraying with power of 160 KW. The large number of carried out comparative tests proved
that this apparatus provides the best quality of gas-thermal coverings and that it is
economical, reliable, and reasonably priced for general consumers. However, this advanced
development has not found its implementation in our country yet. To implement this
concept, there have been designed series of supersonic plasmotrones and equipment systems
to spray coatings in plasma of hydrocarbon combustion products with air. Power of the
system amounts to 10-40-80-160 KW. The plasmotrones utilize a completely novel concept for
forming of stretched weakly underexpanded supersonic plasma jets. By depressing turbulence
in the boundary layer, there has been ensured lower scattering of the jet’s power. The
supersonic plasma jet generated has approximately 30% higher accelerating and heating
aptitudes. With the particles’ speed of 600 meters per second, all the refractory
materials become fused.

The main stages in apparatuses improvement predominantly relate to design
and implementation of new approaches, while, in the recent years, upgrade stages also
comprise rigging with measuring and numeric control facilities. "TOPAS" Research
and Production Enterprise has designed new technologies and mastered output of numerous
novel types of plasma equipment, such as:

• equipment for plasma spraying, based on unified block-modules
  “TOPAS-40”, “TOPAS-80”, and “TOPAS-180”;

• “USZM-2M” equipment for supersonic electric-arc metalization;

• equipment for plasma surface hardening of wheelsets (“UVPZ-2M” plant
  for rapid-action surface hardening and semi-automated hardening line for wheelsets).

By expanding our knowledge in reactions taking place in plasma of
combustion products, we have managed to develop and implement new effective technologies
recently.

By its essence, the supersonic plasma spray process is the evolutionary
descendant of the water stabilized plasma spray system, known today as WSP 500, and is the
analogue to the hypersonic plasma jet coating system, known as Plazjet-II-200 and
Plazjet-II-250.

The primary advantage of the process lies in plasma properties.
Chemically-active plasma of combustion products has easily controlled oxidation-reduction
potential, as well as high heating and accelerating aptitude of sprayed powder particles.
Besides, this plasma is cheap and abundant. It builds up lengthy high-enthalpy jet with
well-satiated ranges of temperatures and velocities. These very qualities facilitate
effective heating and acceleration of all the powder particles regardless of their flying
trajectory. Thus, it allows spraying all the powders, which do not decompose when heated,
with equal efficiency. Equipment for supersonic plasma spray is rather easy to operate,
user-friendly, and reliable. This equipment ensures stability and reproducibility of all
the parameters during the regulated time needed to switch cathode and anode. Equipment can
be used for every possible type of efficient plasma spraying of high-quality coatings. The
equipment is especially expedient to use in the areas lacking sufficient quantities of
inert gases (as compared to the conventional plasma equipment) or oxygen (as compared to
the HVOF). This equipment features ability to technologically use the effect of
plasmochemical reactions between chemically-active carbonic plasma and sprayed material.

This machinery is able to stably operate in a wide range of parameters and
can easily be made completely automatic. All this ensures process flexibility at spraying
various powders. The equipment is easy to operate when obtaining coats with varying
composition, porosity, thickness etc.

Positive results have been yielded when applying iron-based amorphous
coatings. This process ensures the highest degree of coating amorphousness as compared to
knocking spray, traditional plasma, or the HVOF sprayings. Besides, the process provides
for 1.5-2-time higher corrosion-resistance and durability and may well compete with
galvanic chromium-plating (see table 1).

Table 1. Performance attributes of galvanic chromium-plating and
supersonic air-gas plasma spray of amorphous coatings

Aluminum, zinc-aluminum, and zinc thermal coatings are among the typical
corrosion-resistant coverings to protect constructions and structures made of ferrous
metals. These coatings are rivals to varnish-and-paint coats, galvanic coats obtained by
dipping into melts, as well as to glass-enamel, bituminous, bituminous-rubber, polymer,
and epoxy coverings.

One the basis of data collected from pipelines’ operations, there have been
compared dependability of steel structures covered with bitumen, bituminous-rubber
materials, and other metalized coats. The comparison compilation proved that aluminum
metalized coating is the most appropriate for structures, which are subject to
simultaneous effect of temperature and moisture, as well as soil corrosion (such as
hot-water systems or oil & gas pipelines).

It should be mentioned that aluminum coatings have indicated good
performance when sprayed over welds as well. In this case, such coatings protect against
corrosion, as well as provide the weld with increased resistance to initiation of
corrosion-fatigue cracks, including those originated in fusion line at repeated static
loading in neutral environment. Besides, aluminum coating protects areas with partially
peeling coverings.

The method of spraying anticorrosive coatings by utilizing plasma of
combustion products at transonic speeds (high-energy plasma spray) is described with large
weight coefficient of 1 kg of sprayed material per 1 kW of power; density of particles in
plasma equaling 20 kg/m3; high productivity amounting to 50 sq. m of surface covered per
hour. Besides, the method allows for new technological opportunities, such as obtaining
aluminoceramic and amorphous coats, and ensures a substantial improvement in performance
attributes of sprayed coatings.

New compound aluminoceramic coating has been obtained by spraying particles
of aluminum and ceramics, fused in plasma flux, on the prepared surface. If the spray mode
is chosen correctly, all the particles of aluminum powder completely fuse and are
accelerated to the speed of 300-500 m/s, whereas surface of ceramic particles becomes fire
polished and the particles reach the speed of 200-400 m/s depending on material’s density.
Fractional compositions of aluminum and ceramic powders, as well as parameters of powder
injection into plasma flux, are selected with aim to reduce disintegration of flying
melted particles to the lowest and to intensify collision and coagulation of aluminum
particles with ceramic ones during the flight. All this facilitates better fusion of
heterogeneous materials and provokes initiations of chemical bonds, intermetallic
compounds etc. When hitting the surface, particles become deformed, implant into surface
irregularities, and make up cohesive points. Compared to aluminum, ceramic particles have
higher heat energy content, thus forming areas of micro-fusion with intermetallic
compounds in the coating. Areas surrounding ceramic particles have higher cohesion and
density. This leads to formation of a dense composite aluminum coat (with a 0.5%
porosity), well-bound with the surface, with uniformly distributed particles of ceramics
and intermetallic compounds. Such a coating enjoys increased corrosion resistance and high
mechanical performance. Due to presence of hard ceramic particles embedded in soft
aluminum matrix, mechanical durability and endurance of the coating increase a number of
times.

Special "TOPAS-80" plasma equipment is used to spray
aluminoceramic coats on tubes, while "TOPAS-40" plant equipped with hand
plasmotrone is used to apply coatings on joints.

The carried out tests on aluminoceramic coatings have shown that under
effect of major service deterioration causes (such as temperature, temperature and
moisture jointly, aggressive mediums, electric potentials) the tested coating does not
change its initial properties and serves as protection.

The coating features over than twofold improvement of corrosion resistance
as compared to aluminum coatings manufactured by electric-arc metalization or flame
spraying. Taking into account a tenfold increase in wear resistance of such coating in
comparison with conventional gas-thermal aluminum coatings, one should also expect a
considerable rise in reliability and life span of aluminoceramic coated pipelines.

At the moment, there has been created industrial technology for application
of aluminoceramic coats on tubes, 100-1,200 mm in diameter. This technology is utilized at
enterprises with Moscow-based "MosEnergo" and "AKOR" of Ulyanovsk.

The examples of application of plasmatic spraying under this technology
include:

• catalyst to clean motor fumes;

• regeneration technology for crankshafts;

• wear-resistant coating in "plunger-cylinder" system;

• local heat treatment.

Novel catalytic neutralizer for comprehensive cleaning of exhausted motor
fumes has been designed with utilization of the plasma spray technology. The new
neutralizer has 10-20-time lower content of precious metals, at the same time maintaining
high degree of cleaned away noxious substances (removing 80-90% of CO, 60-65% of NOx, and
50-55% of CmHn). Metal-based compound composite ceramic coating with 5% porosity is used
as a reactive agent.

There has been developed industrial technology for manufacturing screen
catalyst to provide comprehensive purification of exhaust gases from CO, hydrocarbon
compounds, and nitrogen oxides. Screen catalyst is based on secondary bearer made of
ceramics.

Technology for regeneration of crankshafts has been introduced, thus,
increasing the lifetime of used powerful diesel engines.

Among the wide variety of technological methods for regeneration of worn
components, some 85% of all the crankshaft journals in all the engine types were renewed
with utilization of welding and build-up methods. The common deficiency of various
build-up methods is high heat effect of arc on the component, which melts the latter and
causes residual stress, deformations, and cracks. Subsequently, this leads to reduction in
fatigue resistance by 50-70% and in wear resistance as compared to brand-new components.
Considerable diversity of mechanical performance attributes is typical for built-up metal.
The aforementioned deficiencies stimulated development of gas-thermal spraying methods to
regenerate crankshafts. Presently, sufficient experience in component renovation by
gas-thermal coat spraying has been accumulated in the former USSR and abroad. However, as
long as the traditional approaches were used, the problem of regenerating crankshaft
journals of locomotive diesel engines remained unsettled. The main reason was inability to
achieve sufficiently high parameters of coats sprayed. Such major parameters include
cohesive strength of the coating with surface metal, which should not be less than 60-80
MPa, and porosity of coating’s composite structure with increased friction-resistance
within the range of 2-3%. It is known that porous coatings not only ensure positive effect
and boost wear-resistance, but prolong the time period till metal of crankshaft journals
and inserts becomes clamped after cessation of oil feed. Porous coating of a crankshaft
journal can take increased loads, at the same time maintaining its pattern and service
performance.

Properties of metalized coats, sprayed on crankshaft journals, depend on
composition of powder filling in wires and on spraying method. Shaft journals operate with
bearings, made of either leaded bronze or aluminum-tin alloys. In compliance with the
above-mentioned requirements, there has been created coating material, which ensures fair
pinion-technical performance of friction shaft-bearing pair (see table 2). New
“USZM-2M” equipment for supersonic electric-arc metalization enables solving this
problem on a completely new quality level.

Table 2. Performance attributes of coating material for
crankshaft-bearing systems

Nowadays, the Railways of Lithuania have a five-year experience in
operating over 50 diesel trains with crankshafts renewed by supersonic electric-arc
metalization. No technical faults have been registered up to date.

The new technology is being implemented at Velikiye Luki Locomotive Repair
Works. A tentative set of crankshafts has been regenerated and is now under service tests.

There has been created technology for gas-thermal plasma spray of boron
carbide-based coats, resistant to abrasive wear, on friction plunger-cylinder pairs.
Special nickel-clad boron carbide powder has been developed for coat application. This
powder enables obtaining high working surface hardness of cylinder sleeves, which, in its
turn, increases life span of mortar-pump as high as 5-6 times compared to metal sleeves.

The "TOPAS" technology for plasma surface hardening has new
opportunities for increasing contact-fatigue strength of metal. As a result, it improves
reliability of wheelsets equipping traction and rolling stock. Deterioration rate of
plasma-hardened wheel ridges is 2.5-3 times lower than that of serial-made crankshafts.
The developed technology for wheel hardening has two peculiar features. Firstly, it
provides local (in the area of highest wear) surface hardening of wheel ridge 2.5-3 mm
deep and 35 mm wide, increasing the hardness from 280 HB of the source material all the
way up to 450 HB. This ensures optimal rigidity balance of contacting surfaces of wheels
and rails. Secondly, the hardening technology changes the pattern of hardened wheel area
from ferrite-pearlite alloy with 30-40 micrometre source grains to a 50/50 compound of
fine-needled martensite with rosette troostite. This improves mechanical performance,
including a reduction in friction coefficient in the contact point of wheel ridge with
rail side surface, and increases crack-resistance of wheel material in the plasma hardened
area.