Hard-alloy instrument with carbide coating has a number of technical and economic advantages, the most important of which are reduced consumption of expensive and scarce tungsten-containing materials and shortened nomenclature of hard alloys required for

INSTRUMENT TO BECOME MORE DURABLE

INSTRUMENT TO BECOME MORE DURABLE

Vladimir LOSKUTOV, doctor of sciences, Viktor KHIZHNYAK,
doctor of sciences, Marina BOBINA, candidate of sciences, the department of metal research
and thermal treatment of the National Technical University “KPI”

Hard-alloy instrument with carbide coating has a number
of technical and economic advantages, the most important of which are reduced consumption
of expensive and scarce tungsten-containing materials and shortened nomenclature of hard
alloys required for all types of metal-making.

Ukraine’s development as an independent industrial
country is impossible without working out and introduction of new technological processes
with high competitive capacity on the global market.

Technical progress in many industries depends, first of
all, on improvement of reliability and durability of machinery parts and gears and
instrument. Not only does such an improvement make the machinery’s service longer but it
also curtails metal consumption, saves financial means required for manufacturing of spare
parts, reduces equipment’s revamping and repair downtime, increases labor productivity,
and provides economical utilization of energy and labor resources.

Possible ways to increase durability of gears and
instrument are to upgrade their construction, use new steels and alloys for their
manufacturing, or create and practically introduce various types of protective coatings
based on carbides, borides, nitrides, and oxides. Comparative analysis of the three
indicated directions of advancement (in particular, their improvement potentials and
expected economic efficiency) has yielded the conclusion that development and introduction
of new protective coatings is the most promising one.

Among the well-known coatings, those based on carbides of
transition metals have the greatest potential. Apart from high hardness and strength
parameters, they possess a complex of other valuable properties, such as high durability,
corrosion and cavitation resistance, score resistance, heat resistance, etc.

In developed countries, 80% of all polyhedral throw-away
hard-alloy mechanically fastened instrument tips (PHATs) are coated before they are sold
to consumers.

Currently, PVD and CVD coating technologies are used in the
worldwide practice. They increase the operational capacity of hard-alloy and steel cutting
tools by 1.5-2 times. However, they also have a number of disadvantages. For example,
owing to insufficient adhesion of the protective coating to the hard-alloy surface, the
former is bound to shell off from the coated surface. PVD and CVD technologies require a
large input of chemical agents and employ complex, energy-consuming, and expensive
equipment.

The technological process of carbide coating has exhibited
of the indicated disadvantages. Carbidization is performed in closed reaction space under
reduced pressure (102 Pa). Titanium-, zirconium-, vanadium-, niobium-, and chromium-based
carbide coatings are applied under the temperature of 900-11000С during 1-6 hours. The
carbide-coating unit is simple in construction and does not require high-skilled personnel
to operate it. The aggregate cost of the main component modules of the coating unit does
not exceed $300.

The main difference of these coatings from the widespread
ones is that they combine such parameters as strong adhesion to the surface, high
microhardness (16.5 to 40.0 hPa), and a wide range of micro-brittleness indicators (0,20?
10-3 to 0,63? 10-3 conventional units). The combination of these parameters several times
increases hard-alloy instrument’s wear resistance.

The most important parameter for all hard alloys used in
manufacturing of cutting instrument is their durability for cutting of various alloys.

Geometric shape of a cutting instrument is also very
important for its durability, in particular, it concerns the rounding radius of the main
and auxiliary blade edges. During the instrument’s operation, its coating gets into
especially extreme conditions. If the coating’s thickness approaches the blade edge’s
rounding radius, the coating is very likely to fail. In this case, tangency stress may
scour off the coating at the front and back planes of the instrument. A coating, the
thickness of which is 2-6 times less than the rounding radius (for hard alloys, this
parameter is within 10-20 mcm), perform most efficiently.

Performance of carbide-coated hard alloys indicated a
positive effect of carbide coatings, which improved the instrument’s durability in all
instances. During selection of the optimal thickness of a coating, it is necessary to take
into account the durability (resistance) variation coefficient.

The research into dependence of durability of hard-alloy
instrument on thickness of carbide coating yielded the optimal figures for thickness of
carbide coatings on VK8 and T15K6 hard alloys (Table 1).

Table 2 exhibits the results of research into durability of
hard-alloy instrument with multi-component carbide coatings.

Durability of polyhedral hard-alloy tips (PHATs) with
applied carbide coating turned out to be 2.5 to 27.4 times higher than that of the
original, non-coated hard-alloy tips. In most instances, hard alloys coated with
titanium-based complex carbides indicated the highest durability.

Independently of the type of carbide coating applied,
during turning of high-carbon and alloyed steels, the wear time of carbide-coated PHATs
was always significantly longer than that of any standard PHAT. However, despite the
general outstanding durability of all carbide-coated PHATs, those with vanadium-,
niobium-, zirconium-, and chromium-based carbide coatings indicated a more intensive wear
in the initial stage of turning of steel 20, as compared with non-coated tips.

It is not reasonable to use hard-alloy instrument tips with
heat-resistant coatings for working up alloys with high content of alloying elements,
owing to the resulting jump of adhesive deterioration effect. For example, during turning
of titanium-based VT6 alloy, durability of PHATs with single-component coatings based on
titanium carbide turned out significantly poorer than that of standard non-coated
instrument tips. Notwithstanding high hardness of the carbide layer, which resists
abrasion, and high chemical inertness of titanium carbide, intensive diffusive reaction
between the surface material and the coating was observed. Particles of the alloy stick to
the blade edge of the tip and detach, thus removing particles of the coating material and
later – those of the hard-alloy surface. The resulting effect, apart from a general
shortening of the instrument’s durability, is a worsening of the machined surface’s
smoothness. The conducted experiments indicated that during machining of VT6 alloy,
multi-component coatings based on zirconium and chromium exhibited the best performance as
regards durability.

The effect of protective coatings on cutting properties of
PHATs is mainly connected with such mechanical parameters as hardness, strength, and heat
resistance of the coating material, which usually alter with the rise of the temperature.
The temperature directly in the cutting area may reach such high figures as 1073-1273 K.
As the temperature rises, hardness and strength of carbide coatings and those of the
coated hard-alloy surface material go down. At the same time, the dense oxide film, which
is formed on the hard-alloy surface during the cutting process, makes a positive effect on
the instrument’s performance.

Proceeding from the conducted experiments with coated
PHATs, it is possible to conclude that the wear process does not alter during cutting with
carbide-coated instrument tips, as compared with non-coated tips. It is noteworthy that in
some instances, the speed of cutting, at which the wear is minimal for PHATs with
multi-component carbide coatings, is 1.5-2.1 times higher than the corresponding
minimal-wear speed of non-coated hard-alloy instrument tips.

Notwithstanding a higher cost of carbide-coated hard-alloy
instrument, additional expenses on its chemical and thermal treatment quickly pay off,
owing to the economy of the instrument’s parts and a higher productive capacity, which
is reached through the instrument’s improved cutting performance and a shorter time
required for replacement of the failed instrument. The most noteworthy technical and
economic advantages of hard-alloy carbide-coated instrument are reduced consumption of
expensive and scarce tungsten-containing materials and shortened nomenclature of hard
alloys required for all types of metal-making.

This technology was successfully implemented at Production
Association named after Korolyov, Machine-Tool Factory, and Aircraft Factory named after
Antonov, the city of Kiev.

Table 1. Parameters of carbide coatings on hard
alloys

Table 2. Dependence of durability of coated
hard-alloy instrument on various coating types during turning