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DonNTU & nbsp; Portal Masters

Abstract on the topic "Investigation of ways to protect the heating elements of thermal furnaces from gas corrosion"

Content

Introduction

Heat-resistant alloys are alloys based on nickel, cobalt, nickel with iron, possessing high thermal strength in tense conditions of operation and the environment. In addition to heat resistance, such alloys have other high properties. Nickel-based heat-resistant alloys have the greatest use as high-temperature alloys. This is due to the fact that they combine the following advantages: high heat resistance, scaling resistance, processability and resistance to gas corrosion. Nickel-based refractory alloys can simultaneously contain a large number of alloying elements, each of which affects certain properties. Industry requires constantly new inventions and improvements, aviation and space technology are not lagging behind and also require improvements: materials, structures. With increasing temperature of operation of the material, the importance of doping increases. Such materials include high-temperature alloys.

1. ANALYTICAL REVIEW OF METHODS OF MANUFACTURE AND QUALITY OF HEAT-RESISTANT ALLOYS

1.1 General characteristics of high-temperature alloys

Currently, nickel-based alloys are the most common refractory materials and are widely used for the manufacture of parts operating at high temperatures. These temperatures reach 0.8Tpl. The widespread practical use of nickel-based alloys has led to intensive studies of their structure and properties.

Nickel high-temperature alloys are characterized by the following structures: alloy matrix ( γ-phase) - representing a nickel-based solid solution with an FCC lattice, usually with a high content of nickel-soluble elements: cobalt, chromium, molybdenum and tungsten; hardening - is the Ni3(Al, Ti) intermetallic γ phase with an ordered fcc lattice. It is formed during the crystallization of the alloy (primary γ-phase), as well as when dispersed is separated from the supersaturated solid solution of the matrix. The conjugacy of γ- and γ'-phase grids and the proximity of their periods (the mismatch of the lattice periods is less than 0.1%) creates the possibility of the formation of interphase boundaries with low surface energy. This leads to high dimensional stability of the γ'-phase [5].

Modern nickel-based superalloys have a very complex composition: they contain up to 12 basic alloying elements and a large number of admixtures, the content of which should be controlled in the alloy. Nickel forms solid solutions with many elements, which makes it possible to achieve high heat resistance of alloys based on it. Many of the alloying elements soluble in nickel or nichrome are effective hardeners and increase the creep resistance of alloys: chromium, cobalt, molybdenum, tungsten, vanadium, rhenium. The total mass of the main alloying components can reach 40% [3].

The main hardener is the γ`-phase, which is an A3B intermetallic compound with an Ni3Al-based fcc lattice, in which, besides Al, elements such as Ti, Hf, Nb, Ta, and also in minor quantities W, V, Co, Ru, Mo, Cr and Re. The phase gratings γ and γ' are similar in type and close in size. The place "A" is occupied by relatively electronegative elements: Ni, Co, and "B" is more electropositive: Al, Ti, Ta, Nb, etc. The volume fraction of the hardener phase is about 60-70% [4], and reaches 90%, for example, in the VKNA alloy [3].

Thus, we can conclude that Ni-based superalloys are widely used in industries, hardening in such alloys, is carried out through the formation of intermetallic compounds ( γ'-phase), such as Ni3 (Al, Ti). Such alloys operate at sufficiently high T equal to 0.8 × Tm.

1.2 Ways to protect the heating elements of a thermal furnace

Pure metals tend to be corrosion resistant, but require additional protection measures when operating in environments with high aggressiveness.

There are many ways to increase the protection of heaters against gas corrosion, one of which is heat-resistant alloying. The alloying element should form an oxide with high electrical resistance. High ohmic resistance (low electrical conductivity) is one of the main conditions for the formation of the protective properties of the film, since the movement of ions in the oxide layer is impeded.

The energy of formation of the oxide of the alloying element should be greater than the energy of formation of the oxide of the base metal, i.e. This condition ensures the durability of the oxide of the alloying component in the presence of the base metal. The oxide component of the additive is more stable than the oxide of the base metal. If this condition is not met, then the oxide of the alloying element will be reduced by the base metal.

The alloying component and the base metal must form a solid solution, then under this condition it is possible to provide a continuous film of oxide of the alloying component over the entire surface of the alloy.

Heat resistance can be influenced by influencing the surface or surface layers of the alloy, by methods such as:

Spray wire or powder melted and directed by a stream of compressed air. Non-metallic coatings can also be used as sprayed ones: heat-resistant enamels, metal ceramics and refractory compounds. The disadvantages are not strong adhesion of the coating to the surface and the porosity of the applied coating, for their solution annealing is necessary [12].

Thermal diffusion saturation. The surface layer is saturated with aluminum, chromium, and silicon in the form of: powder mixtures (for individual production), melts (for mass production). The depth of the layer depends on the temperature and time of saturation. With multi-component saturation, the order of saturation plays an important role: a) joint, b) sequential.

Thus, from the subsection it follows that there are many ways to increase the durability of products made of nichrome, the most relevant and promising are those that contribute to the formation of spinels or those methods that also minimize the possibility of scale formation and metal destruction.

1.3 Conclusions

1) Ni-based refractory alloys are widely used in industries, hardening in such alloys is carried out through the formation of intermetallic compounds ( γ'-phase), such as Ni3(Al, Ti). Such alloys operate at sufficiently high temperatures of 0.8 × Tm.

The purpose of the qualification work is to perform a comparative study of changes in the structure of austenite during high-temperature heating of 10G2FB continuous-cast steel and this steel after controlled rolling.

2) There are many ways to increase the durability of products made of nichrome, the most relevant and promising are those that contribute to the formation of spinels or those methods that also minimize the possibility of scale formation and metal destruction.

2. MATERIALS AND RESEARCH TECHNIQUE

2.1 Material, methods, equipment for research.

Electric heaters of thermal furnaces, in particular nichrome, are capable of being destroyed during operation. There may be local burnout or burnout of the entire heater. When the heater is oxidized, the film of oxides on it gradually thickens, and the cross section of the living metal decreases. Subsequently, the resistance of the heater gradually increases, and the power released in it decreases. When this reduction in power becomes significant (about 10-15%), the heater has to be replaced with a new one, its service life ends.

The causes of failure can be such: during the formation of the “accordion” shape from tape heaters, microcracks form at the bend points; in consequence of chemical interaction during work with materials of the lining of an electric furnace, or with its atmosphere; inclusions of oxide films and other weakened areas in which an increase in resistance is observed.

Therefore, the aim of the thesis is to study ways to protect electric heaters of thermal furnaces from gas corrosion by applying heat-resistant coatings.

The use of such coatings reduces the likelihood of accidental nichrome heaters, as well as improve the performance of furnaces and reduce the likelihood of electric shock.

2.2 Method of processing experimental data

To study the structure, grain, microhardness; estimates of the thickness and structure of the oxide layer of the failed nichrome heaters from the factory, it is necessary to make transverse or longitudinal sections.

For the manufacture of thin sections of the available were selected: heater number 1 - spring heater; heater No. 4 - belt heater.

Samples 1, 2 were laid in the clamp, and samples 3, 4, 5, 6 in cold welding. The samples in the frozen cold weld and the samples in the clamp were ground either on the grinding wheel, in order to obtain a more even surface of the ground section. Then the sections were processed using emery skins, dispersibility: 40, 80, 160, 240 and 0. Before each transition to emery paper of a smaller grain, the section was turned by 90 '. After the samples were processed on a polishing wheel using chromium oxide, washed with water, dried thoroughly and degreased before etching. The sections were etched in aqua regia, the time was chosen during the experiment and was about 5 minutes each.

The microstructure was studied on an MIM-7 microscope. Increase x600 times. After studying the microstructure on the manufactured thin sections, microhardness was measured on a PMT-3 microhardness tester. Load 50 g (0.5 N ).

2.3 Study of the structure and properties of production heaters.

On the basis of the results of the inspection of the appearance and information that the plant managed to obtain about the heaters, a register of samples was made

Porosity is observed in the heater structure, at the edges of the pore cluster. On the structure 1.1 shown in Figure 2.2, it is clear that the exfoliation of the oxidized layer has occurred. The thickness of the oxide film on all heaters ranges from 40 ÷ 140 m + 50 ÷ 100 m oxide inclusions deep into the metal.

Table 2.1 shows the microhardness of the heaters under study from the DONETSKGORMASH plant.

Table 2.1 Heaters microhardness, load 50g (0.5 N)

Sample No. Microhardness heater Microhardness dross
1 2861 ± 100 -
2 2365 ± 80 3170 ± 110
3 1426 ± 25 1933 ± 50
4 1272 ± 30 1495 ± 35
5 1330 ± 30 1737 ± 40
6 1300 ± 30 1831 ± 50

The structure of the oxide film has cracks, and therefore the microhardness of the oxides is low. The first sample failed to measure the microhardness of the oxide layer, as it was either absent or exfoliated.

Thus, the state of the oxide layer was evaluated and it was found that the oxides exfoliate from the sample surface and have many cracks, which led to a decrease in the microhardness index.

2.4 Analysis of macrostructure and properties after typical heat treatment regimes

For the experiment at temperatures of 800 and 900°C, 4 samples were taken for each temperature: 1 - clean without coating; 2 - rubbed Al; 3 - in a mixture: calcined borax + calcined kaolin + ?2? (in proportions 9: 2: 2); 4 - in a mixture of calcined kaolin and water.

Calculation of the corrosion index based on the mass change coefficient (K_m+ + or K_m-) of samples subjected to gas corrosion at 800°C:

1) K_m+ = (1.45925-1.38565) / (0.3625 × 1) = 0.203 ( g / ( m2×h ))

2) K_m+ = (1.7057-1.63875) / (0.315 × 1) = 0.2125 ( g / ( m2×h ))

3) K_m- = (1.7932-1.6628) / (0.3632 × 1) = 0.359 ( g / ( m2×h ))

4) K_m+ = (1.5115-1.46525) / (0.3106 × 1) = 0.149 ( g / ( m2×h ))

CONCLUSIONS

It follows from the work that there are many ways to increase the durability of products made of nichrome, the most relevant and promising are those that contribute to the formation of spinels or those methods that also minimize the possibility of scale formation and metal destruction.

Macro-structural analysis of the surface of the samples after conducting gas corrosion tests has established that a large amount of surface area has local adhesion of the protective coating, this fact made a significant contribution to the difficulty of correctly assessing the change in the mass indicators of gas corrosion on the samples after removing the coatings, therefore, their calculation was carried out taking into account the mass of the sample with coating before the test and after.

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