Технические науки/Металлургия
Zhiguts Yu.Yu., Legeta
Ya.P., Prjadko
S.I.
Uzhgorod
National University, Ukraine
COMBINED TREATMENT
WITH THE HELP OF LASER SURFACE HARDENING AND SELF-PROPAGATING HIGT-TEMPERATURE
SYNTHESIS
The
laser surface hardening (LSH) of
metals was discovered in 1965. It has won strong positions in technology of
metals [1]. Nowadays in the whole world hundreds of patents have been awarded
to branch inventions including those dealing with combination of LSH with SHS (self-propagating high-temperature synthesis). One of them [2]
is dedicated to combining of LSH
(Laser Surface Hardening) with SHS
(self-propagation g high-temperature synthesis) [1]. Formerly SHS was combined with other technologies
of surface hardening of components [1-3].
The
impotent problem within the LSH is
the decreasing of the losses of beam energy because of its reflection by the
surface of metal under machining. In the given investigation, as well as in the
invention [1], the mixture of powders Ti (65%),
carbon in black state (18%) and Fe
(14% by mass) were used in the role of light-absorbing paint. The mixture was
damped by solution of 2 % latex in gasoline, and then it was put on the surface
of stalls of mark 10 and 20 and was dried in an open air, forming the layer 80,
200 or 500 mkm thick. Thermochemical calculations showed that in such a mixture
practically all Ti interacts, thanks to non oxygen combustion, with carbon,
forming the carbide TiC. The seer
plus of carbon and very small account of Ti alloy the iron forming liquid steel
of condition, which under fast cooling turns into troostite in layers of 80 mkm
thick.
In
typical microstructure of metal in cross-cut of harded layer got under density
og power 17 W·m-2, diameter of „spot” − 0,4 mm, the
speed of scanning 12 min/s and expense of argon (for the defense of Ti from air oxidation) − 0,5 l/s
is shown. The thickness of alloy is ~500mkm. This layer consists of ~50%
particles TiC and ~50%(by volume) of
metal link-instrumental carbon steel of type “У8”.
The
investigations made have proved that the microhardness of carbides TiC is
higher than the hardness of steel almost 10 times. Thus, in the given work we
managed to organize the SHS process
in comparatively thin layer thanks to using of LSH technology simultaneously for solving of two tasks: for
heating, flashing and carbonating of an iron; for flashing Ti particles and its „combustion” in carbon with forming of carbides TiC.
This
reaction is strongly exothermic and is accompanied by great decreasing of Hibbs
free energy:
ΔGº=-183,0246+0,01008T kJ·mol-1, T=298−1155 K;
ΔGº=-186,9709+0,01325T kJ·mol-1, T=1155−2000 K.
The
adiabatic temperature of non oxygen comeestion of equiatomic mixture Ti-C equals to 3200 K. The real temperature of combustion of selected mixture 68% (% in mass particles) is more than 1850 K that provides the formation of
hard-liquid dross (TiC-melding) with
the large interval liquids solid us. The formation of dross instead of
one-phase alloy influences positively on the quality of surface of hardened
layer after its full growing hard and cooling as well as on supporting of this
layer even on inclined planes.
It is
important to note that in the mentioned non oxygen combustion none of non
metallic phase and its including is formed. Welding of hardened layer with
basic metal is obtained automatically „metallurgic ally”, excluding the
necessity of soldering or other methods of connecting one alloy (e.g.
instrumental) with other (e.g. with the basis of cutting tool).
The
substitution of a part of iron powder by the powder of carbon ferrochrome (e.g.
12%Fe+2%FeCr instead of 14%Fe
in the formulae of SHS mixture) allows to get layers of carbidosteel with the
link not in the shape of steel “У8” but from alloyed
steel “X12” which after fast
cooling of these layers thanks to accelerated drain of heat to cold metal of
the basis gets austenite-martensite-carbide structure. In the process of work
of the instrument such metal link additionally grows hard thanks to
pre-transforming of austenite into martensite and getting older of the later
one. The hard of such a carbidosteel reaches HV1400 (14000MPa).
The
substitution of a part of iron in the SHS-mixture
by ferrochrome increases greatly corrosion resistance of carbidosteel and
decreases its oxidizing wear in the process of its exploitation. The
substitution of carbon in SHS-mixtures
by the powder is also long-range. The same effect is obtained also with the
substitution in another field of hot machining of metals namely the using of SHS-reactions for inmoulding process
(modification within of the form) in casting manufacturing.
The
substitution of carbon in SHS-mixtures
by the powder of boron is also perspective. In such a case it is possible to
reach the liquidus-solidus interval to 1500 K,
that in other technologies it is practically impossible to meet. Thus, while
the above mentioned method on the one hand high refractory diborides TiB2 and CrB2 (with high
hardness) are formed and, on the other hand, very easily melted complex
eutectics are formed.
Conclusions: 1. Combination of LSH and SHS in one operation allows to solve the whole complex of technical
problems connected with producing of materials with high hardness like
carbidosteels and hard alloys on metal surface. 2. Evolution of inner chemical
heat in SHS-mixtures allows to
decrease the power of laser radiation. 3. New complex technological process
allows to build up wearied surfaces of parts of machines and devices to the
high of 0,5 mm.
References: 1. Жуков А.А., Жигуц Ю.Ю.,
Шилина Е.П. Комбинированная поверхностная обработка лазерным поверхностным
упрочнением и самораспространяющимся высокотемпературным синтезом// Изв. ВУЗов.
Черная металлургия. М. 1998. №5. − С. 60−63.
2. Жуков А.О., Жигуц Ю.Ю., Шиліна Е.П. Комбінована обробка за
допомогою лазерного по-верхневого зміцнення і саморозповсюджувального
високотемпературного синтезу// Науковий вісник УжНу. Серія Фізика. № 10.
Ужгород. 2001. − С. 31–34.
3. Жигуц Ю.Ю. Сплави,
синтезовані металотермією і СВС-процесами. Ужгород: Ґражда, 2008. − 276
с.