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UDC 621.391.92.03
INNOVATIVE METHOD OF TILLAGE TOOL HARDENING Titov N.V., Candidate of Technical Sciences Kolomeichenko A.V., Doctor of Technical Sciences
Orel State Agrarian University, Orel City, Russia E-mail: ogau@mail.ru
Litovchenko N.N., Candidate of Technical Sciences
All-Russian Scientific Technological Institute of Repair and Exploitation of Machine and Tractor Fleet, Moscow, Russia E-mail: gosniti@list.ru
ABSTRACT
The article describes the method of hardening of tillage tools operating in the conditions of abrasive wear that provides to increase their resource considerably. The method consists in short-circuited arc surfacing of nanometal ceramic composite powder materials with simultaneous thermodiffusion hardening. Equipment, methods and research results of hardness, microstructure and wear resistance of the hardened working surfaces are presented. On their ground the optimal composition and concentration of material components, providing the best physical and mechanical properties and resource of the hardened tillage tools are defined.
KEY WORDS
Short-circuited arc surfacing; Nanometal ceramic composite material; Working organ; Thermodiffusion hardening; Abrasive wear; Graphite electrode.
In all countries of the world the investigations on obtaining highly effective metal ceramic hard alloys that are able to increase the resource of working organs of tillage tools into several times are being carried out. But the first results of foreign metal ceramic plates application for hardening of plow shares operating on sabulous clayey and loamy soils did not provide their wear resistance increase because of higher brittleness of plates [1-3, 5, 14].
In this connection nowadays the works on creation of new metal ceramic materials having high wear resistance and impact elasticity in the conditions of intensive abrasive wear at considerable static and dynamic loadings are continued. Now at the result of the complex of the carried out scientific investigations nanometal ceramic composite material (NMCM) allowing solving the set problem is elaborated. It consists of steel matrix (deposite powder), with incorporated aluminium oxides Al2O3 or silicone SiO2, boron carbide B4C, pounded to nanodimentional condition, and also alloying elements: boron, nitrogen and aluminium. Boron is included into powder composite content in the form of borax Na2B4O7, nitrogen is included into nitric acid sodium [2, 4-6]. Aluminium is used for ferrum deoxidation and formation ceramic phase by means of its conversion into aluminium oxides Al2O3 and at the following fusion into corundum. The latter in the mineralogical table by hardness occupies the 9th place it is very close to diamond hardness. Matrix is shockproof, consolidate and composite carcass NMCM. That is why steel filler powders with high hardness and abrasive resistance, for example nr-10H-01, nP-H70X17C3P4, nr-CP4 can be used reasonably as a matrix material.
To obtain wear resistant hardening coating from the suggested NMCM we developed the short-circuited arc surfacing method with application of graphite electrode. At its simultaneous usage together with surfacing thermodiffusion hardening of the working organ surface by means of boron, nitrogen and carbon (boronnitrocementation) occurs. At depositing carbon is emitted at the account of sublimation of graphite electrode. Arc vibration of electrode provides the obtaining stronger and denser coating for the sake of the working organ material fusion and mixture of ceramic and alloying components NMCM [2,4,7-8].
More than that at short-circuited arc surfacing usage not so strong heat embedding as some other surfacing types usage takes place. Before surfacing NMCM in the paste form is applied on the working organ surface.
Preliminary done investigations stated that every component of NMCM considerably influences on the effectiveness of the working organs hardening. In this connection the researches on the influence of the NMCM components on hardness, microstructure and wear resistance of the hardened surfaces were carried out. To carry out the researches several pastes (table 1) were prepared. The pastes were prepared by mechanical mixture of the components some of which were broken to nanosized condition in a centrifugal mill in advance. As a bounding material 20% water solution of sodium silicate solute Na2SiO3 was used. After preparation the paste was applied on a sample surface prepared from steel 65H The given material selection is provided that it is used for production of the majority of working organs of tillage tools. The thickness of the applied paste layer was approximately 2,5...3,0 mm. Then the paste was dried to hardening. At the temperature of 90...95°C hardening time does not exceed 8.10 min.
Table 1 - Chemical composition of metal ceramic pastes
№ paste Chemical composition of paste Mass ratio of components in paste, %
№1 №2 №3
Steel powder nr-10H-01 15 15 13
Borax Na2B4O7 25 15 10
Boron Carbide B4C 45 55 65
Nitric acid sodium NaNO3 4 4 4
Silicon oxide SiO2 5 5 5
Aluminium powder Al 6 6 3
№4 №5 -
Steel powder nr-10H-01 50 10
Boron Carbide B4C 50 70
2 Borax Na2B4O7 9 -
Cryolite Na3AlF 5
Aluminium powder Al 6
№6 - -
Steel powder nr-10H-01 20
Boron Carbide B4C 63
3 Borax Na2B4O7 12
Aluminium powder Al 5
Short-circuited arc surfacing of the samples was done on device Bflry-2, which is developed and produced in State Scientific Institution of State Scientific Technological Institute of Russian Agricultural Academy. The device includes inverter thyristor current source of type MASTER 162 for 200...250A, operating console and vibrator with graphite electrode of diameter 6.10 mm fixed in it [7, 11]. Surfacing was done at the burning of direct arc in the following modes: current rate I=70...80A, voltage U=60B, vibration frequency of graphite electrode - 100.110 vibrations per second.
To investigate hardness and microstructure the sample preparation was done in the following way. At first it was cut with manual cutting machine Labotom-3, and then it was placed into automatic hydraulic press Citopress, where it was pressed into resin. Readymade disc with the sample was polished to the full gloss on abrasive machine Labopol-5 using emulsion.
The hardness of the deposited layer and the hardened substrate of the sample were defined with computer microhardness tester KMT-1 (Figure 1, a) according to Vickers method at loading F=1H and exposure time t=15 c. At the same time 32 measurements were done: 16 - by the depth of the deposited layer, 16 - by no depth of the hardened substrate. Measurement of imprints was done with video device connected to a personal computer, using special software, with statistical processing and possible automated analysis of image data according to the standards of hardness measurement. Metallographic tests of the deposited NMCM samples were done with metallurgic microscope OLYMPUS GX51 (Figure 1,b).
Relative wear resistance of the hardened samples was defined with device ИМ-01 -construction of All-Russian Research Institute of Agricultural Engineering under the name of V.P. Goryachkin (JSCo «ARIAE»). Duration of the tests was 30 min. Every test was done with three replications. As abrasive material high silica sand sizing 0,16.0,32 mm was used. Average contact pressure in the friction zone was maintained 0,33 МПа. Wear rate was defined by sample mass decrease by means of its weighing on balance BJIP-200 with precision 0,1 mg before and after tests.
b)
Figure 1 - Equipment for tests: (a) computerized microhardness tester KMT-1; (b) metallurgic microscope OLYMPUS GX51
The results of the tests on the hardness definition of hardened samples are given in Table 2.
Table 2 - Average hardness values of hardened samples
№ sample Hardened substrate Deposited layer
HV HRC HV HRC
1 719 61 1193 87
2 485 49 872 66
3 481 48 1048 75
4 614 56 1097 78
5 402 40 1104 80
6 650 58 1412 95
The tests results show that the maximum hardness of the deposited layer and the hardened substrate are typical for samples № 1 u № 6. Analyzing the content of the pastes being used for their hardening (table 1) we can conclude that the leading role in obtaining the maximum hardness belongs to boron carbide B4C and boron included into borax Na2B4O7. At the result of paste melting also atomic nitrogen is formed. It together with carbon that is formed in the result of sublimation of graphite electrode at arc burning diffuses into hardened substrate forming hard solution which increases its hardness.
The resource of working organs being hardened with the short-circuited arc surfacing method and working in the abrasive wear conditions considerably depends not only on hardness, but on the condition of the border of deposit layer and substrate, that is from the effect of substrate material fusion and its mixing with paste fusion [5]. The most complete idea about the deposit layer and substrate structure is presented with microstructure analysis.
The carried out researches allow stating that the microstructure of the hardened samples consists of 3 zones in spite of the content of applied pastes (Fifures 2, 3).
Figure 2 - Microstructure of sample №1 at amplification of 5x (a), 10x (b), 25x (c), 50x (d). Zones: 1 -
substrate; 2 - transitional; 3 - basic hardened
Substrate (zone 1) has evident phase changes to boundary line due to diffusion of the elements incorporated into deposited paste and carbon content. The deposited layer consists of 2 zones - transitional and basic hardened. Transitional zone is alloy of melted surface layer of substrate and paste material. Basic hardened zone is the hardest and consists of steel matrix, holding the formed ferrous carbides FeC, Fe2C3, borides FeB and Fe2B and ceramic phases - ferrous spinel, boron carbide and corundum. Junction line of the deposited layer into the substrate is more obviously detected on the photographs of microstructure at small amplification.
Metallographic tests also showed that the structure of the deposited metal ceramic coating is heterophase and it is represented as space-distributed crystal phases, which form multiphased crystal solid body. Structure heterophase of coating is obvious at amplification 50x (Figures 2 and 3, d), where light phase is steel matrix, that keeps the formed carbides, borides and ceramic phases - dark light inclusions. At the result of phase transformations in the parent phase separate areas or crystals of new thermodynamically more stable phases appear. They grow, interact and as the result form heterophase structure [5, 12]. Different phases have different hardness. This is an explanation of some irregularity at hardness measurement by deepness of the deposited layer.
c) d)
Figure 3 - Microstructure of sample №6 at amplification of 5x (a), 10x (b), 25x (c), 50x (d). Zones: 1 - substrate; 2 - transitional; 3 - basic hardened
The results of the comparative tests on wear resistance of the hardened and unhardened samples are presented in Table 3.
Table 3 - Results of the comparative tests on wear resistance of the hardened samples
Sample number Sample mass Mass change Relative wear resistance
before test after test
standard 47,3400 47,3078 0,0322 1
1 41,5440 41,5350 0,0090 3,58
2 41,9614 41,9432 0,0182 1,76
3 51,5975 51,5823 0,0152 2,12
4 41,0270 41,0160 0,0110 2,93
5 42,5786 42,5647 0,0139 2,32
6 43,8791 43,8708 0,0083 3,88
Note: as a master-sample the sample from steel 65Г was taken, it endured hardening and abatement at modes being used at tillage tools production.
Analyzing the obtained data it is obvious that the maximum wear resistance possess samples № 1 and № 6. It is in 3,58 and 3,88 times correspondingly higher than hardened steel 65r possesses taken as transfer standard.
Thus, the results of the carried out tests allowed assuming that according to all the data (hardness, homogeneity of the structure, wear resistance) paste №3 used to harden sample №6 was the most optimal. Its usage must considerably allow increasing the resource of the tillage tools in the exploitation and correspondingly increase their reliability.
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