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Section 3. Materials Science
Shakirov Shukhrat Musayevich, Phd., of a Tashkent State technical University Norkhodjaev Fayzulla Ramazonovich, Professor, of a Tashkent State technical University E-mail: [email protected]
SCALE RESISTANCE OF POROUS PERMEABLE MATERIALS BASED ON IRON AFTER THE THERMO-DIFFUSION SATURATION WITH SIMULTANEOUSLY CHROMIUM AND ALUMINUM
Abstract: This article presents the results of a study of the hardness of porous permeable materials based on iron powder that have been thermally diffusion saturated simultaneously with chromium, aluminum and silicon in hermetically sealed containers, in two versions: - thermal diffusion saturation after sintering of compacted samples; and - thermal diffusion saturation simultaneously with sintering of compacted samples; scale resistance of the corresponding samples, two methods were used: - measurements of the electrical resistivity of the material the sample in intensive filtering process and - microstructural analysis, the obtained results of the analysis are the most resistant to scale, the sample has passed the thermodiffusion saturation with sintering.
Keywords: powder metallurgy, thermo-diffusion saturation, porous permeable materials, scale resistance, chromium and aluminum, oxidation, sintering.
At present, the demand for porous permeable mate- The increase in the scale resistance is achieved by
rials is rapidly increasing, as they have been used prac- alloying the iron base mainly with chromium and alu-
tically in all fields of technology as filters for cleaning minum or silicon, that is, elements in a solid solution
gas-air fluids, various aggressive liquids and solutions and forming protective films of (Cr, Fe)2O3, (Al, Fe)2O3
from small abrasive particles [1-5]. In a number of oxides during heating [6]. Doping of porous permeable
industries, filtration of different media can be carried materials based on iron powder simultaneously with
out at high temperatures. When porous permeable ma- chromium and aluminum can be carried out by thermal
terials are used in high temperature conditions, their diffusion saturation from the gas phase [7]. Thermodif-
scale resistance is considered as a important property. fusion saturation of powder materials from the gas phase
Scale resistance is the ability of a material to resist the can be carried out in two cases, after sintering from the oxidizing action of a heated medium. The process of oxi- formed powders in the article or during sintering. In both dation of the pore surface is influenced by the chemical cases, the saturation of the volume of the porous matecomposition of the material of particles and oxide films, rial from the gas phase simultaneously with chromium the temperature, the composition of the filtered gas (air), and aluminum can be assessed by testing the appropriate the duration of the oxidation process, and the structural samples for scale resistance.
characteristics of the porous material. For this purpose, samples of porous materials were
The initial stage of oxidation is a purely chemical made in the form of a bush with dimensions: the outer
process. But the further course of oxidation is already diameter of 40 mm, the inside diameter of 34 mm and the
a complex process, consisting not only in the chemical height of 50 mm., From the powder of the mark n^Bl of
combination of oxygen and metal, but also in diffusion of iron with particle sizes of 200-250 ^m. The powder was
oxygen and metal atoms through a multiphase oxidized pressed at a pressure of 300 MPa in die molds. The ther-
layer [5]. As a result, the material is destroyed. modiffusion saturation of the samples prepared simulta-
neously with chromium and aluminum was carried out in two ways: - after sintering the compacted samples and together with the sintering of the compacted samples.
A saturating medium consisting of a mixture of 40% Cr + 15% Al + 40% Al2O3 + 5% NH4Cl powders was prepared for the thermodiffusion saturation of the porous samples simultaneously with chromium and aluminum. Saturation of porous samples simultaneously with chromium and aluminum was carried out at a heating temperature of 1150 °C for 3 hours in a hermetically sealed container. In Fig. 1 shows scheme of a container with a hermetically sealed cover, porous samples and a saturating medium disposed therein.
Figure 1. Container: scheme of the location of samples and the saturating medium in the container; 1 - container body; 2 - cover; 3 - saturating powder medium; 4 - porous samples; 5 -rubber; 6 - thermocouple; 7 - the valve for gas removal; 8 - radiator
The peculiarities of the structure of porous permeable materials do not allow the methods used to assess the scale resistance of non-porous materials to be used to assess the scale resistance of a porous material. The difficulty in determining the degree of oxidation of porous permeable materials is that in the pores of the porous body there remains the product of the interaction between the material and the medium. Attempting to remove oxidation products, blowing air or other methods from the entire branched surface of the porous material can lead to distortion of the true picture of the oxidative destruction of the porous material.
To assess the scale resistance of porous permeable materials, the measurement of the specific electrical resistivity of a porous material is most suitable [8]. This
method is based on measuring the specific electrical resistivity of a porous sample during a filtration process. In (Fig. 2) shows the scheme of the stand for testing cylindrical porous samples for scale resistance. The samples were tested in a mode of filtration of a mixture of air with water vapor heated to 600 °C. The hot air flow rate during the test was 0.25 m3/h.
Figure 2. Scheme of the stand for scale resistance testing: 1 - body; 2 - cover; 3 - porous sample; 4 -conductor; 5 - insulator; 6 - clamp; 7 - outputs for measuring the electrical resistance
The results of the scale resistance tests are shown in (Fig. 3) in the form of a diagram of the change in the specific electrical resistance of porous permeable samples, depending on the duration of running time.
650
a 600
S 550
<v
s 500
■a
i—i -a 450
"fl
u 13 4UU1
1
<D 350
300
1 j
I
10 20
30 40
rime, hour
50 60 70
Figure 3. Diagram of the change in the specific electrical resistance of porous samples depending on the duration of running time: 1 - unsaturated sample; 2 - saturated after sintering; 3 -saturated simultaneously with sintering
Section 3. Materials Science
SCALE RESISTANCE OF POROUS PERMEABLE MATERIALS BASED ON IRON AFTER THE THERMO-DIFFUSION SATURATION.,
As can be seen, in a diagram (Figures 3 and 3) under identical conditions of thermodiffusion saturation of pre-sintered and not sintered samples, the most resistant to oxidizing actions of heated air with water vapor turned out to be a porous sample in which the thermal diffusion saturation with chromium and aluminum was carried out simultaneously with sintering.
The relatively low scale resistance of a saturated after sintering sample is most likely due to the difficulty of penetrating the saturating gas into the central parts of the porous body. To clarify this, microstructure analyzes of the samples were carried out. In (Fig. 4) shows a micrograph of porous samples obtained on a microscope MIM 7 at 200 times magnification.
a) b)
Figure 4. Photograph of the microstructure of the samples: a - saturation after sintering; b - saturation with sintering (x 200)
Microstructural analysis established that the thermodiffusion saturation of the pre-sintered sample occurs due to the passage of the saturating gas through the porous channels of the porous body (Fig. 4, a). In the initial stages of the process, the deposition of elements from the gas phase occurs in places with increased curvature of the surface of the walls of the pore channels. Because of the difference in the rates of entry of elements from the gas phase and their diffusion into the depth of walls of the channel, the pore diameter narrows. As a consequence, the access of the saturating gas into depth of the porous body is complicated. In connection with this, there is an inhomogeneous saturation of the thoroughbred body with elements of the gas phase.
The thermodiffusion saturation of unsintered samples occurs due to the passage of the saturating gas, along the surface of the compressed particles penetrating into the contact areas between the particles. In this case, the deposition of elements from the gas phase occurs mainly in contact areas between the particles. Due to the high rate of diffusion of the elements of the gas phase into the particle of the powder, there is no accumulation of elements that prevent the gas from entering the central parts of the porous body (Fig. 4, b). In connection with these, the porous body is uniformly saturated throughout the volume with chromium and aluminum, which ensures the best scale resistance of the samples.
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