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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 . 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 350С 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 . It is known  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).
"Development of low temperature nitriding of steel"