Development of low temperature nitriding of steel

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					      DEVELOPMENT OF LOW-TEMPERATURE NITRIDING OF STEEL P6M5
              SUBJECTED TO PRELIMINARY THERMAL TREATMENT
         M.Yu. Smolyakova1, D.S. Vershinin1, Yu.R. Kolobov1, O.A. Druchinina2
      1
        Centre “Nanostructured materials and nanotechnologies” of Belgorod State
                                      University,
   2
     Joint Research Center for the Diagnostics and Investigations of Nanostructured
                                       Materials

       At present one of the key tasks of mechanical engineering progress is
improvement of quality instruments and producing of them in required quantity. This
task can be solved by the way of improvement of technical level of tool manufacture.
Expenses on logistics and producing of instruments in cost price of manufacture are
very high. It is caused not only by high cost of instrument but by small service life of
instrument, and additional expenses caused by rapid wearability of tools. Service life of
instrument that made of tool steel defines the cost of manufactured products. In
addition, with increasing of service life of instrument the requirement of instrument is
decreasing. This leads to deallocation of production capacities on manufacturing of such
instrument. In considerable degree the quality of instrument is determined by properties
of the surface layer.
       Nitriding is one of perspective methods of improvement of surface layer working
characteristics. Nitriding is widely used in different branches of manufacturing for
increasing of working time and reliability of many critical parts and tools. The strength,
hardness, wearability, scoring resistance, fatigue strength and corrosion resistance of steels
and alloys are increasing after nitriding. Proceeding from it the task of development of new
methods of nitriding of tool and constructional steels and alloys is actual.
      In the framework of current paper nitriding of high-speed steel P6M5 (chemical
composition (wt%) is as follows: C – 0,87; W – 6,07; Mo – 5,23; V – 1.96; Cr – 4,03
and Fe balance) subjected to preliminary standard thermal treatment (quenching with
following tempering) was done. Nitriding was performed on upgraded ion-plasma setup
ННВ–6.6-И1м type in plasma of non-selfsustained low-pressure arc discharge [1].
Process of nitriding was done in gaseous mixture nitrogen (60%)-argon (40%).
      For the purpose of determination of optimal parameters of nitriding of high-speed
steel P6M5 in plasma of non-selfsustained low-pressure arc discharge some series of
experiments were done. The first series of experiments were made at different
temperatures (250, 300, 350 and 400°C) during 120 minutes. It was found that optimal
nitriding temperature is temperature 350°С. Therefore on the next stage of experiments
the influence of treatment time on formation of modified surface at optimal temperature
was investigated. In this series of experiments the treatment time was 40, 120 and 240
minutes. The third series of experiments was done on samples made of high-speed steel
P6M5 in initial state (after annealing) at nitriding temperature 350 0С and different
nitriding times (40, 120 and 240 minutes).
       Estimations of thickness of near-surface nitrided layer and its structure were done
with usage of the optical microscope Olympus GX 71 and by measuring of
microhardness on cross sections of samples. Changes of surface morphology after
nitriding were investigated with usage of scanning electron microscope Quanta 600
FEG. Microhardness measuring was made by Vicker’s method at load on indenter
0.49N. These measurements were made as on the surface of nitrided samples so on
cross sections.
      As a result of performed experiments it was found that temperature of nitriding in
plasma of non-selfsustained low-pressure arc discharge has a considerable influence on
thickness of nitrided layer and surface microhardness of steel P6M5 subjected to
preliminary thermal treatment. According to carried out investigations the increase of
nitriding temperature leads to increasing of thickness of nitrided layer in ~3-4 times at
the same duration of process. But nitriding at 400°C leads to decreasing of
microhardness in bulk of material that can be attributed to beginning of tempering. In
consideration of obtained data it can be said that optimal nitriding temperature for this
type of steel in such type of discharge is temperature of 350°C.




   Fig.1. Profiles of microhardness distribution on
                                                      Fig.2. The view of steel P6M5
   cross-section of quenched steel P6M5 after
                                                      surface (quenching with following
   nitriding in plasma of non-selfsustained low-
                                                      tempering) after nitriding at
   pressure arc discharge at temperature 350°C
                                                      temperature     3500С,    scanning
   during: 1.1 – 40 minutes, 1.2 – 120 minutes,
                                                      electron microscopy, х100 000.
   1.3 – 240 minutes.

     Nitriding at optimal temperature regime and different times of process (40,120 and
240 minutes) was performed to determine regularities of influence of nitriding process
duration (at T = const) on thickness of nitrided layer. Results of microhardness
measurements on samples cross sections presented on Fig.1. It can be seen that
increasing of nitriding process duration at temperature 350С allows modifying near-
surface layer of treated material on greater thickness. The surface microhardness is
increasing with the increase of nitriding time up to 120 minutes. Such significant
microhardness increase is caused by formation of highly dispersed particles of V, W and
Mo nitrides coherent with matrix. Following increasing of process time up to 240
minutes leads to growth of nitride particles and formation of γ' and ε phases. As the
result the microhardness on the surface has decreased what agrees with literature data
[2].
     It is known [3] that steel P6M5 in initial state should be nitrided at temperature
~5000C, decreasing of nitriding temperature on 1500С will allow reducing of time and
power inputs. So experiments on nitriding of steel P6M5 in initial state were performed
at temperature 3500C. It was found that increasing nitriding time from 40 to 240 minutes
leads to increasing of surface microhardness in 4-5 times and increasing of thickness of
nitrided layer from 20 to 70 µm. It should be noted that optimal process time is 120
minutes.
      Thus, possibility of the low-temperature nitriding of the high-speed steel P6M5,
subjected to preliminary thermal treatment, in plasma of non-selfsustained low-pressure
arc discharge is showed. Consequently at execution of nitriding process at temperature
3500C it is possible to increase surface microhardness in 1.5-2 times and there is no
decreasing of microhardness in the bulk of material. Strengthening at nitriding of steel
P6M5 is caused by formation of solid solution of nitrogen in iron with highly dispersed
particles of special nitrides of active nitride-formative elements (W, Mo, V). Nitrided
layers, which characterize such structure, will allow providing high running abilities of
nitrided instrument made of high-speed steel.

Experiments were made with usage of analytical equipment of Joint Research Center for the
Diagnostics and Investigations of Nanostructured Materials. The work was partially supported by
grants RFBR 09-02-00857-a and FAP 02.523.11.3007, 02.552.11.7017.

                                      Literature
       1.    Vershinin D.S.,   Goncharenko I.M.,       Koval N.N.,       Koshkin K.A.,
Lopatin I.V., Shirinkina A.V. Nitriding of constructional and tool steels in plasma of
glow and arc discharges // Equipment and technologies of thermal treatment of steels
and alloys. Kharkov: SSC KhFTI, IPTs «Kontrast», 2006, с.209-213. (in Russian);
       2.    Ion chemical-thermal treatment of alloys / B.N. Arzamasov,
A.G. Bratukhin, Yu.S. Eliseev, T.A. Panaioti. – Moscow.: Izd-vo MSTU im.
N.E. Baumana, 1999. – 400с. (in Russian);
       3.    I.M. Pastukh Theory and practice of hydrogen-free nitriding in glow
discharge. – Kharkov: SSC KhFTI, 2006. – 364с. (in Russian).

				
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