CC BY
97
UDC 631.31.022:620.178.001.5:62-404.9
INVESTIGATIONOF HARDNESS OF TILLAGE TOOLS BEING HARDENED BY CARBO-VIBRO-ARC METHOD WITH PASTE APPLICATION
Kolomeichenko A.V., Doctor of Technical Sciences Titov N.V., Candidate of Technical Sciences Orel State Agrarian University, Orel City, Russia E-mail: [email protected]
ABSTRACT
The article presents the description of innovative method of hardening of tillage tools, operated in abrasive medium that allows increasing their operating life considerably. It is performed by carbo-vibro-arc surfacing of working bodies using composite metal ceramic pastes with simultaneous thermodiffusion hardening of basic metal of the working body. With application of computerized microhardness tester CMT-1 according to Vickers method the working bodies hardening being hardened by pastes containing aluminum oxide, silicon dioxide, boron carbide, ammonium nitrate is defined. Analysis of the data obtained proved that with the increase of percentage content of ceramic components in paste from 10% to 30% microhardness of the surfaced coating also increases regardless the used ceramic component. At the same time the maximum average value of microhardness of the surfaced coating (75 HRC) were showed by the working bodies being surfaced with the paste containing 30% of boron carbide B4C. Pastes containing 30% of aluminum oxide Al2O3 and silicone dioxide SiO2, were characterized by lower values of coatings microhardness - 69 HRC and 67 HRC correspondingly. With application of pastes containing ammonium nitrate NH4NO3, the obtained coatings have the minimum hardness 62 HRC. The optimal content and concentration of paste components that provide increase of wear resistance of the hardened working bodies on the average by 1,8...2,0 times are determined according to the results the carried out tests.
KEY WORDS
Carbo-vibro-arc hardening, metal ceramic material, working element, paste, thermodiffusion hardening, ceramic components, nitrogenous components, graphite electrode.
Working bodies of national and international tillage machines (shares, chisels, landsides, shellboards, blades and pointed and chisel-shaped hoes, smooth and toothed disks, etc.) are operated in the conditions of direct impact of abrasive particles. As the result they are strongly worn out, their cutting surfaces grow blunt, considerable changes in form, profile and working dimensions. For example, mean life of plow shares of Russian production is 5.20 ha depending on soil types and their structure, mean life of plow shellboards is 20.100 ha, of land-sides is 20.60 ha, of cultivators pointed hoes and sowing machines is 10.40 ha, of harrow disks and stubble plows is 8.20 ha [1-3, 16]. Utilization of worn working bodies results in reduction of the field works quality; agrotechnical terms violation, increase in machine fault time and growth of costs on soil processing and greases, reduction of amount of the obtained market products.
Modern innovative method carbo-vibro-arc hardening (CVAH) with application of graphite electrode and composite metal ceramic pastes allows the considerable increase of tillage tools wear resistance being operated in abrasive medium. The method subject-matter is in the following. At first, paste is applied on the working element cutting edge, which is dried to hardening. Then, electric arc between the surface with the applied paste under hardening and graphite electrode of the device strikes. When it burns, surfacing of metal ceramic coating obtained from paste components as well as thermodiffusion saturation of working bodies metal with alloy elements which are included into the paste composition and with carbon at the expense of its diffusion that results from graphite electrode sublimation takes place [4-8]. Application of metal ceramic materials in the paste form at carbo-vibro-arc
hardening (CVAH) is determined by their high resistance to abrasive and mechano-corrosive wear.
Paste content includes steel matrix (surfacing powder), aluminum oxide Al2O3, silicone dioxide SiO2, boron carbide B4C, which are ceramic components, and also the substances containing nitrogen (ammonium nitrate NH4NO3, carbamide NH2CONH2 etc.), and cryolite Na3AlF6, improving stability and quality of arc burning [1, 4, 9-12]. Adhesive substance is 50% water solution of PVA glue. It was proved that the paste components considerably influence the hardness and wear resistance of the hardened surfaces. To determine paste composition we carried out the corresponding investigations. Pastes with above mentioned components in different proportions were prepared by mechanical mixing. Powder PG-10N-01 was used as matrix material. The content of cryolite Na3AlF6 in all analyzed pastes was accepted as 10% [4, 15]. The pastes were applied on the samples produced from steel 65G. This steel type selection was determined by the fact that it is used for the majority of working bodies and other machinery, working in the abrasive wear conditions. Thickness of applied paste layer was 2,5...3,0 mm. Then pastes were dried at temperature 90...95°C to hardening. Usually the setting time does not exceed 8.10 min.
Device VAGH-2 (vibro-arc-graphite hardening) that is developed and is produced in Federal State Budget Research Establishment of State Scientific Technological Institute of All Russian Research Technological University was used for CVAH (carbo-vibro-arc hardening) [10, 12, 14]. Sample hardening was done in the following modes: current intensity I=70...80 A, voltage U=60 V, vibration frequency of graphite electrode is 25 Hz. The surfaced coating thickness was on the average 0,8 mm.
To determine surfaced samples micro hardness sections were prepared in the following way. At first, the sample was cut on the high precision cutting machine «Minitom Struerus». Then it was located in automatic hydraulic press «CitoPress-1 Struerus», where it was pressed in resin. The ready-made disk with the sample was polished on abrasive machine «LaboPol-5 Struerus» to the high luster using the emulsion [4, 15].
Microhardness of the surfaced coating and hardened basic metal of the samples were defined in Nanocenter in Federal State Budget Research Establishment of State Scientific Technological Institute of All Russian Research Technological University using computerized microhardness tester CMT-1 according to Vickers method at loading F=1H and exposure time t=15 s. The number of changes was accepted to provide the required validity of the obtained results. The measurement of the obtained prints was done by means of video device, being connected to the personal computer. The statistical processing of the results was done by means of specialized software using statistical analysis of the picture according to the hardness measurements standards.
The investigation results on the definition of microhardness of hardening coatings being surfaced on the samples presented in Figures 1. 4.
Analysis of the data obtained proved that with the increase of percentage content of ceramic components in paste from 10% to 30% microhardness of the surfaced coating also increases regardless the used ceramic component. At the same time the maximum average value of microhardness (1508 HV, that corresponds to microhardness 75 HRC) were showed by the samples being surfaced with the paste containing 30% of boron carbide B4C (Figure 3, curve 1). The obtained coating hardness on the average by 1,5.1,7 times exceeds hardness of tempered steel 65r, from which the majority of series produced working bodies are made. It is connected with the following that boron carbide is one of the hardest materials and according to its hardness it gives place to diamond and borazon (boron nitride). Pastes containing 30% of aluminum oxide Al2O3 and silicone dioxide SiO2, were characterized by lower values of coatings microhardness - 991 HV (69 HRC) and 922 HV (67 HRC) correspondingly (Figures 1, 2). It can be explained that at surfacing as the result of arc high temperatures influence subsolution of Al2O3 and SiO2 with the corresponding coating hardness reduction takes place.
The samples being hardened with application of pastes containing ammonium nitrate NH4NO3, were characterized by the minimum values of coatings microhardness, that does not depend on the above mentioned component content. At the same time the maximum
average value of microhardness (763 HV, that corresponds to hardness 62 HRC) was obtained at paste content of 30% NH4N03 (Figure 4, curve 1).
Figure 1 - Changing of HV microhardness of the coating being surfaced in depth h depending on the content of aluminum oxide Al2O3: 1 - Al2O3 - 30%; 2 - Al2O3 - 20%; 3 - Al2O3 - 10%;
4 - Al203 - 35% in paste.
HV
1000
950 900 850 800 750 700 650 600
1
A / A
▲ A - --* f-A
♦ 2 ♦ -
t t ♦ f
3 I
m m-+-1-i i * ¥
* — m *-!- * i \ (- X
\ 4
0,1
0,2
0,3
0,4
0,5
0,6
0,7 o,8h,mm
Figure 2 - Changing of HV microhardness of the coating being surfaced in depth h depending on the content of silicone dioxide SiO2: 1 - SiO2 - 30%; 2 - SiO2 - 20%; 3 - SiO2 - 10%;
4 - SiO2 - 35% in paste
HV 1600
1500
1400
1300
1200
1100
1000
A * / 1 / A --*
+ A A
A 4 2 > . / ♦ -a
1-1 * *
v \ A X
■ ■ \ 4
------- \ 3 " " ~
0,1
0,2
0,3
0,4
0,5
0,6
0,7 0,8 h,mm
Figure 3 - Changing of HV microhardness of the coating being surfaced in depth h depending on the content of boron carbide B4C: 1 - B4C - 30%; 2 - B4C - 20%; 3 - B4C - 10%;
4 - B4C - 35% in paste
HV
800
700
650
600
550
0,1
0,2
A 1 / i A A A
k i > 2 ♦ ♦
X ♦ 4 ♦ t / ■ ■ -1---- ♦ ■
■ X 1- s 3 w-*
0,3
0,4
0,5
0,6
0,7 0,8 h,mm
Figure 4 - Changing of HV microhardness of the coating being surfaced in depth h depending on the content of ammonium nitrate NH4NO3: 1 - NH4NO3 - 30%; 2 - NH4NO3 - 20%;
3 - NH4NO3 - 10%; 4 - NH4NO3 - 35% in paste
The carried out investigations also showed that the increase in paste content more than 30% of ceramic and as well as nitrogenous components does not result in further larger increase of surfaced coating hardness. This is explained that in this case at CVAH (carbo-vibro-arc hardening) electric arc ignites and burns worse and it results in decrease of uniformity, quality and thickness of hardening coating.
The results of investigations on determining of base metal microhardness of samples after their hardening are presented in Figure 5.
HV
700
500
0,2 0,4 0,6 0,8 l 1,2 1,4 1,6 1,8 2 h,mm
Figure 5 - Changing of HV microhardness of base metal by depth h after hardening depending on the used in paste ceramic and nitrogenous components: 1 - B4C - 30%; 2 - Al2O3 - 30%;
3 - SiO2 - 30%; 4 - NH4NO3 - 30%
The investigations proved that the maximum average value of microhardness (645 HV, that corresponds to hardness 57 HRC) was detected in samples afyter their hardening by the paste containing ammonium nitrate NH4NO3 (Figure 5, curve 4). It is connected with the fact that at electric arc burning thermodissociation of ammonium nitrate with atomic nitrogen formation takes place. Atomic nitrogen in its turn diffuses into base metal and forms nitrides Fe4N, Fe3N, Si3N4, which are hardening components. Base metal microhardness of the samples after their hardening by other pastes turns out to be about the same - on the average 586 HV or 54 HRC. Difference of microhardness values was not more than 18.20 HV.
Thus, the described investigations proved that the paste containing 30% of boron carbide, 60% of matrix powder and 10% of cryolite is the most optimal paste for hardening of tillage tools, being operated in abrasive medium. The results of the comparative tests of the samples after hardening by such a paste and unhardened samples from tempered steel 65G showed that wear resistance after hardening increases by 1,8.2,0 times. That is why the application of paste of this content ought to increase tillage tools life considerably and so to increase their reliability.
On the ground of the complex of the carried out investigations we developed universal technology of hardening of tillage tools with application of metal and ceramic materials in the paste form. It can be used not only to increase new tillage tools life but also to recondition the worn ones with hardening. In this case before hardening one should realize reconditioning of working element by surfacing the compensating for wear element Nomenclature of tillage tools and also their net weight and configuration do not have any restrictions on hardening by this method. Hardening can be realized in steady-sate operating conditions at specialized enterprises, engaged in machines production as well as at small work-shops, and even in the field conditions if there is an electric energy source.
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