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27SHS METALLURGY OF THE HARD ALLOY ON THE BASIS OF TUNGSTEN CARBIDE WITH THE NICKEL BOND
S. L. Silyakov*", V. I. Yukhvid", N. Yu. Khomenko", T. I. Ignat'eva", and N. V. Sachkova"
aMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, Moscow, 142432 Russia *e-mail: ssl@ism.ac.ru
DOI: 10.24411/9999-0014A-2019-10165
It is the most expedient to choose surfacing materials based on carbide-tungsten for protection and restitution of mechanisms details and junctions for the purpose of their resistance to all types of intensive wear under operating conditions. A relit is most often applicable as a reinforcing component phase of the surfacing carbide-tungsten materials. Often the cast relit is used in combination with cobalt and nickel bonds [1-4]. And in hard-facing alloys mainly cobalt is used as a cementing phase. However, its high cost forces to replace it with other metals, first in properties, particularly nickel. Comparative tests for two alloys WC-C and WC-Ni show advantage of the first in durability, hardness and wear resistances in comparison with the alloy with a nickel bond [5, 6]. However, in some cases manufacture of at mechanism details or while coating by means of flame spraying or high-velocity oxygen fuel spraying (HVOF), etc WC-Ni alloys are out of the competition. In this case mechanisms details are capable to resist to collateral influence of wear and corrosion.
Powder metallurgy remains the main traditional way of receiving surfacing materials based on carbide-tungsten [5]. As a result, this technique provides synthesis of tungsten carbide (WC), relit (WC-W2C) and alloys with a nickel bond on their basis. Due to the need for new approaches to receiving carbide-tungsten coverings, various ways of synthesis of carbide-tungsten surfacing materials on a nickel basis are offered [6-10].
In the present article we demonstrate the results on synthesis of a cast carbide-tungsten material by self-propagating high-temperature synthesis with a 15% nickel bond with use of the exothermic mixture of a thermite type.
The initial composition and the optimal ratios between the components of the exothermic mixture of a thermite type WO3:Al:Ca:C=0.705:0.090:0.164:0.041 are received during the research work [11]. This ratio provides synthesis of the cast carbide-tungsten material with the content of the fixed carbon 2.1 wt % at the minimum impurity content. For further researches the initial composition of the exothermic mixture WO3/Al/Ca/C is complemented with a mixture NiO:Al=0.806:0.194 which after combustion becomes a nickel source.
As components of the initial high-exothermic mixtures of a thermite type we use pure-grade powders of the tungsten oxide (VI) and nikel (II), aluminum powder (ASD-1 brand) calcium granules (CAS 7440-70-2, 99.1% activity) and graphite (GMZ, grain size 90/63 ^m). Before mixing, these components are dried. The synthesis is carried out in a constant pressure bomb. In all experiments, the exothermic mixture is burned in graphite glasses with 20 mm in internal diameter and 60 mm in height. The exothermic mixtures weighing 30 g are poured out into the glasses at filling density of 1.98-2.00 g/cm3. The ignition is conducted using a tungsten helix. The mixtures combustion is carried out in a technical nitrogen atmosphere at initial gas pressure of 5.0 MPa.
In the experiments the relative weight loss (^1) and the relative yield of the metal-ceramic phase in the ingot (^2) are fixed and calculated by formulas: = [(m1 - m2)/m1] x 100%;
iSHS 2019
Moscow, Russia
"2 = m3/mi, where mi is a weight of the initial mixture, m2 is a weight of the combustion products, m3 is a weight of the ingot of the metal-ceramic carbide-tungsten material.
During the experiments video filming is carried out, at the same time the concentration limit and the burning rate (u) are determined. The burning rate is calculated as an average burning rate between four basic points by a formula: u = H/ti, where Hi is height of the exothermic mixture layer, ti is time of combustion of mixture layer.
The synthesis products are explored by methods of classical chemical and X-ray analysis. The microstructure of the cast materials is studied using Zeiss Ultra plus- a high-resolution field-emission scanning electron microscope. The calculation of the adiabatic combustion temperatures is carried out using the "Thermo" program.
The experimental research of the process of the exothermic mixture combustion [a(WO3/Al/Ca/C) + (100 - a)(NiO/Al)] shows that the combustion temperatures realized during synthesis are sufficient for synthesis of cast combustion products. It is recorded that at the mixture combustion WO3/Al/Ca/NiO/C the synthesis process proceeds at any a ratio. With increasing share (NiO/Al) in the initial mixture the values of the burning rate (u) and the relative yield of the metal-ceramic phase in the ingot (^2) increase on the initial part, and then the curves come for saturation, Fig. 1. Simultaneously the relative weight loss (ni) monotonically decreases.
U, cm/S 1I1.I2*. «*■
rjl " -"
U
■--■--1 L --------- 1i
0 20 40 60 80 100
Fig. i. Influence of the mixture share (NiO/Al) in the initial exothermic mixture [a(WO3/Al/Ca/C) + (100 - a)(NiO/Al)] on the burning rate (u), the relative weight loss at combustion of the initial mixture (ni) and the relative yield of the metal-ceramic phase in the ingot ("2). Po = 5 MPa.
The study of the chemical composition of the cast metal-ceramic products in a range 5 < a < 25 wt % of NiO/Al content in the mixture WO3/Al/Ca/NiO/C reveals a linear increase in nickel content in the synthesized carbide-tungsten ingot with a increase, Fig. 2. Increase in a share of NiO/Al provides increase in nickel content in cast products from 5 to 24 wt %. At the same time, we observe a negative linear increase of the impurity aluminum in the cast ingot with the general carbon drop in the synthesized ingots. To achieve the research goal the mixture 85(WO3/Al/Ca/C) + 15(NiO/Al) is chosen with the fixed carbon content of 2.3 wt %. The ingot is formed on the basis of phases: W2C, W, Ni2W4C, Ni3W9C4.
Ni, % wt.
AI.C, % wt.
30
20
10
Ni
L T^AI
5.0
10.0
15.0
20.0
25.0
a
Fig. 2. Influence of the mixture share (NiO/Al) in the initial exothermic mixture [a(WO3/Al/Ca/C) + (100 - a)(NiO/Al)] on the chemical composition of the metal-ceramic ingot. Po = 5MPa.
The most universal method of increasing of the fixed carbon content in the carbide-tungsten ingot is introduction of its excess content into the initial exothermic mixture of a thermite type 85(WO3/Al/Ca/C) + 15(NiO/Al). In the experiments this approach provides an effective increase in the fixed carbon content to 4.2 wt %, Fig. 3. However, due to partial participation
of the carbon (graphite) in a recovery stage along with aluminum and calcium the content of
Fig. 3. Influence of the excess carbon content (AC) in the initialmixtureWO3/Al/Ca/C/NiO on the chemical composition of the metal-ceramic ingot. ACo = WO3:Al:Ca:C:NiO = 0.599:0.106:0.139:0.035:0.121. Po = 5.0 MPa.
Graphite introduction considerably influences a synthesis process speed. With increase in carbon share in the initial exothermic mixture of a thermite type the burning rate and the relative weight loss at combustion of the initial mixture (n0 fall, Fig. 4. The concentration limit of combustion is recorded at nine fold increase of the graphite content in the initial mixture.
U, cm/S r|,,ri2%wt
40.0 30.0 20.0 10.0 AC
Fig. 4. Influence of the excess carbon content (AC) in the initial mixtureWO3/Al/Ca/C/NiO on the burning rate (u), the relative weight loss at combustion of the initial mixture (n0 and the relative yield of the metal-ceramic phase in the ingot (^2). ACo = WO3:Al:Ca:C:NiO = 0.599:0.106:0.139:0.035:0.121. P0 = 5.0 MPa.
At AC = 9.0 the exothermic mixture ignites, and the further process of combustion fades. The area of synthesis of the cast carbide-tungsten products at introduction of excess carbon is limited by double excess. At AC > 2.0 only sintered cermet materials are synthesized. The analysis of influence of excess carbon content (AC) in the initial mixture on combustion characteristics (n1, n2) and chemical composition of the synthesized cast metal-ceramic carbide-tungsten materials allows to choose the composition with AC = 1.8 which is perspective for further researches, Fig. 3. Unfortunately, the cast material contains impurity aluminum (2.5 wt %) and is formed on the basis of four phases WC, W2C, Ni3W10C3,4, AlNi3.
Impurity aluminium reduction in the cast carbide-tungsten material is carried out by its share decrease in the initial mixture compositionWO3/Al/Ca/C/NiO, Table 1.
Table 1. Influence of aluminum deficiency in the initial mixture WO3:NiO:Al:Ca:C = 0.579:0.113:0.106:0.135:0.067 on the chemical composition of the cast carbide-tungsten materials and the synthesis parameters._
AAl -2.5% -5% -7.5% -10% -15%
Al share in the initial mixture 10.04 9.81 9.57 9.34 8.87
WOs/NiO/Al/Ca/C, % wt.
m 11.37 12.46 9.52 7.36 6.05
46.9 46.76 47.32 47.45 45.32
Al content in the ingot, %wt. 1.28 0.58 0.53 0.47 0.46
C content in the ingot, %wt. 3.21 3.48 3.95 3.42 3.24
Ni content in the ingot, %wt. 15.80 18.60 17.50 16.20 16.00
W content in the ingot, %wt. the rest the rest the rest the rest the rest
the impurity aluminum also increases.
AI.C. %sec
< >_
v" i □ : __[
1 1.2 1.4 1.6 1.8 20 22 24
- n2.
___u k
ii
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0
ÏSHS2019
Moscow, Russia
Thus, aluminium deficiency of 7.5 wt % in the initial exothermic mixture brings to impurity aluminium decrease to 0.53%, in the ingot at the satisfactory values of the relative weight loss at combustion of the initial mixture (ni) and relative yield of the cast carbide-tungsten material in the ingot (n2). In the ingot we also find the impurity content of calcium and free carbon which is according to the chemical analysis is Cfr = 0.219 wt %, Ca = 0.1 wt %. The synthesized material is formed on the basis of three main phases: WC, W2C, N13W10C3.4, preferential of which is the phase WC.
The detailed analysis of a microstructure of the synthesized material is given in Fig. 5. The microstructure of the cross section at increase X1000 is presented as large WC grains of rectangular and triangular forms about 100 |im in size. The fine grains of W2C of 1-2 |im in size are grouped in the area of 20-50 |im and located in a bond of Ni-W-C. The element composition in weight percentage is presented in the table in Fig. 5. The excessive carbon content is caused by imperfection of a cleaning technique of the micro section sample from diamond paste after grinding.
№ C Al Ni W
1 9.35 0.49 0.09 90.07
2 5.6 0.94 14.87 78.58
3 2.58 6.32 74.54 16.55
Fig. 5. Microstructure and EDS data of the cast carbide-tungsten material with a nickel bond.
This work is financially supported by the Russian Foundation for Basic Research (project
no. 17-08-00903).
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