RU2708728C1 - Method for manufacturing of grates of burning trolleys with increased operating life - Google Patents

Abstract

FIELD: metallurgy.SUBSTANCE: invention relates to the field of metallurgy, namely to production of grates of burning trolleys from cast heat-resistant steel. Steel is poured, containing, in wt. %: carbon 0.30–0.40, silicon 0.50–2.00, manganese 0.30–0.80, sulphur not more than 0.030, phosphorus not more 0.035, nickel 11.00–13.00, chromium 22.00–26.00, iron and admixtures - balance, to produce grates. Performing their thermal treatment by quenching in air at temperature of 1100 °C for 80 minutes and tempering at temperature 900 °C for 150 minutes with subsequent cooling in furnace or on air. As impurities, steel contains up to 0.07 wt. % of aluminium and cobalt in amount of up to 0.50 wt. %.EFFECT: raised (minimum) service life of grate bars from steel at operation in conditions of aggressive atmospheres with content of nitrogen and sulphur oxides at constant heat changes at temperature range of 20–1100 °C and with possibility of maintenance of short-term heating up to 1320 °C.3 cl, 7 dwg, 6 tbl

Description

1. Operating conditions and characteristics of the applicable grate

Grid-irons are made of two-phase chromonickel steel grade 40X24N12SL which according to GOST 977-88 is corrosion-resistant, heat-resistant and heat-resistant at temperatures up to 1000 ° C (GOST 977-88 Steel castings. General specifications. - Instead of GOST 977-75, GOST 2176-76; Introduced from 01.01.90. - Moscow: Publishing house of standards, 2004. - 33 C), the chemical composition of steel is presented in table 1.

The total length of the grate is 330 ± 2 mm, the width with the mounting protrusion of 40 ± 0.4 mm. On the grate there are cast overhangs - tides of 4 mm on the sides of the grate, which form, when assembling the grate, technological gaps for gas leakage through a layer about 8 mm wide. An increase in the width of the grate gap beyond the permissible norm leads to a bed spill from burnt pellets into the vacuum chambers.

The grate is removed from service (rejected from the kiln carriage) according to visual technological signs, such as: reducing the width of the grate from 40 ± 0.4 mm to 35 mm, changing the geometric dimensions and violating the integrity of the grate surface (warping, fusion, cracking or kinks, corrosion destruction).

The operation of the grate of the roasting trolleys takes place in an aggressive high-temperature atmosphere. The temperature range of the working space of the roasting machine is 20-1350 ° C, while the temperature of the grates in the firing zone can reach 850-1310 ° C, which depends on the temperature regime of the firing machine, the maximum temperature in the firing zone and the type of pellet to be fired (A. Butkarev , Butkarev A.P., Borodin A.A., Zhilin S.N., Malyavin B.Ya. Operating experience of upgraded firing machines of the Lebedinsky GOK and ways to improve them. - Steel. 2005. No. 3. P. 7-10) .

Today, the tendency to increase the productivity or quality of the product of the roasting machine is the main one, which in the operating mode is expressed by the same parameters. Thus, by increasing the height of the layer of the roasting machine, it is possible to achieve not only an increase in productivity, but also an increase in quality. Increasing the productivity of roasting machines adversely affects the operational life of grates, podsolonnikovyh beams and roasting trolleys. Based on the analysis of the operating modes of the roasting machine at different modes of annual output, we can conclude: with an increase in the productivity of the roasting machine, the heating temperature and the cooling rate of the roasting machine equipment (grates, roasting trolleys, etc.) increase. So, with an increase in productivity of a Lurgi roasting machine with an area of 480 m ^{2, the} temperature at the border of the grate - bed of burnt pellets in the burning zone increases, and in the drying zone it has lower values at almost the same speed of the grate (A. Butkarev ., Butkarev A.P., Zinchuk B.A., Shevchenko A.A., Dvornichenko I.F., Posokhov A.V. Development of technological solutions to increase the productivity of the Lurgi roasting machine. - Steel. 2007. No. 6. S. 2-6).

2. Analysis of existing steel grades

The prospect of the possibility of replacing the steel grade of the grates of the kiln carts with another so that the replacement of the brand did not entail a sharp increase in the cost of the part was studied.

To the greatest extent, the cost of the grate is determined by the content of nickel and chromium, which are the most expensive materials currently used for the production of grate. At the same time, heat resistance is also determined by these elements. Thus, grades of steel close in concentration to these elements were selected.

The role of other elements, such as silicon or aluminum in heat-resistant steel is ambiguous. According to E. Goodremont (Goodremont E. Special steels, ml. M. Metallurgy, 1959, 952 pp.), Silicon and aluminum contribute to increasing the resistance of steel against sulfur at high temperatures. However, silicon displaces carbon to the grain boundaries, which causes an increased redistribution of the carbide phase during long exposures. Therefore, their concentration was taken into account.

2.1. The alloys used in similar domestic industries were studied. The results of the analysis of the use of grate in domestic production are shown in table 2.

Kola MMC processes copper-nickel concentrates, the firing temperature of which does not exceed 900 ° C, which leads to a low nickel content and an increased carbon content. A number of researchers have found (Herbert N. Uhlig, R. Wimston Revie. Corrosion and Corrosion Control: An Introduction to Corrosion Science and Engineering, Fourth Edition.- P. 479) that in a sulfur-containing atmosphere at temperatures above 315 ° C, nickel-containing alloys undergo MCC. The paper proposes to change the chemical composition of parts to increase the stability of parts under such conditions in the direction of increasing the chromium content and reducing the nickel content. In addition to the lack of the possibility of developing intergranular corrosion under such an operating mode, this solution improves the casting properties of liquid steel and improves the quality of the grate structure. But for operation at temperatures above 900 ° C, this steel grade cannot be used.

Sokolovsko-Sarbaisky, Olenegorsky, Lebedinsky and Kostomuksha GOKs (enterprises No. 7-10 of Table 2) process the concentrate with a high sulfur content, as a result, cast iron grates that do not contain nickel were used. The main negative factor of which is low heat resistance in comparison with the grid-irons of the brand 40X24N12SL.

Enterprises under No. 1-5 of table 2 use steel grade 40X24N12SL or its foreign equivalent. The use of imported grates is associated with low resistance of domestic in the modern thermal regime of the roasting machine. It is worth noting that imported grates made of analog steel grade 40X24N12SL (mainly the difference between which is only in the content of Mo) have a much higher cost compared to domestic ones.

Thus, there are no domestic analogues of steel grade 40X24N12SL suitable for replacement.

2.2. The steels used in foreign industries were studied. Based on open data, a table of foreign analogues of steel 40Kh24N12SL was built. The selection is made according to the applicable standards (national and international) and is presented in table 3. The type of standard and steel grade are indicated in each column of column 2.

³ Steel additionally contains W≤5.0% and Nb≤1.5%.

⁴ Steel additionally contains N≤0.2%.

⁵ Steel additionally contains N≤0.2%.

From the analysis of the data in table 3, we can draw the following conclusions:

- the amount of silicon (column 4) in all steels is in the range of 0.5-2.5%, which is close to the values of steel 40Kh24N12SL, it is obvious that silicon acts as a deoxidizer and an element that increases heat resistance,

- the amount of manganese (column 5) in all steels in the range up to 2.5%, in steel grade 40Kh24N12SL 0.3-0.8%, which is also close, manganese is present in steel as an element necessary to remove sulfur and part of oxygen at smelting, manganese is an austenite-forming element, which favorably affects the structure of steel,

- most steels have restrictions on the carbon content on both sides (column 3), which is obviously done for the reproducibility of the properties of the steel after heat treatment, in BS309C3040, MSZ40CrNiSi25-12, 40X24N12SL steels there is no lower limit on the carbon content, which can be considered a disadvantage, provided that upon receipt of parts from such steels, a heat treatment operation is provided, and the restriction precisely on the upper limit of the carbon content is explained by the need to increase the amount of austenite in the steel structure (carbon is the strongest Austenitic element)

- limiting the maximum content of sulfur and phosphorus (columns 6 and 7) in all steels (including 40X24H12SL) can be considered sufficient to exclude the possibility of defect formation due to the content of these elements in steel,

- the amount of nickel in all steels is in a fairly narrow range, from 9 to 15% (column 9), while the higher the chromium content, the proportionally higher the nickel content, it is obvious that nickel is introduced into the steel as an austenite-forming element that compensates for the ferrite-forming property of nickel . In steel 40Kh24N12SL 11-12% of nickel, which coincides with foreign analogues, which have similar chromium content,

- the amount of chromium in all steels varies from 22 to 30% (column 8), in steel 40Kh24N12SL 22-26% chromium, it is well known that chromium is the main element that determines heat resistance, and its concentration determines the temperature range of the possible operation of the part, from analysis it follows that foreign analogues operating at similar temperatures (up to 1350 ° C) have similar chromium content,

- in most steels molybdenum is present in fairly small amounts (column 10), mainly up to 0.5%, the main reason for the appearance of molybdenum can be considered its residues in scrap during steel smelting, but molybdenum can also be an element that increases heat resistance, and some sources, reduce the tendency of steels to MKC, due to the influence on the diffusion mobility of carbon atoms.

Thus, foreign analogues have a very close, and in some cases almost identical to the 40Kh24N12SL steel chemical composition, with some main exceptions - the presence of a small amount of molybdenum and a limitation of the lower limit of the amount of carbon.

The use of foreign analogues in domestic companies is widespread (OAO Stoilensky GOK, OAO OEMK, etc.), despite their large cost. This is due, to a greater extent, to a higher service life of steel, in relation to the domestic counterpart, made from steel 40Kh24N12SL. However, significant differences in chemical composition, in addition to those described above, were not found as a result of the analysis.

2.3. Also, work was done to search in the scientific literature for analogues or modifications of the grade 40Kh24N12SL suitable for replacement according to previous signs.

The known composition of austenitic steel (Pat. 1607428 RF Foundry steel. Rabinovich A.V., Tarasyev M.I., Martynov S.L. et al. 1989), intended for the manufacture of products operating in aggressive environments with cyclic temperature changes of 1000 - 1100 ° C. The purpose of the invention is to increase the heat resistance, heat resistance, heat resistance at temperatures up to 1100 ° C and fluidity. Laboratory studies have shown that the use of the proposed steel, according to the authors, will increase the operational resource of grate by 1.5 times. Steel composition,% mass: 0.25 ÷ 0.50 C; 23.0 ÷ 28.0 Cr; 10.0 ÷ 14.0 Ni; 1.0 ÷ 2.0 Si; 0.12 ÷ 0.23 N; 0.5 ÷ 1.2 Mn; 0.001 ÷ 0.005 V; 0.2 ÷ 0.5 Mo; 0.06 ÷ 4.0 Al; 0.005 ÷ 0.2 Zr; 0.001 ÷ 0.2 Ca; 0.005 ÷ 0.1 REM; iron is the rest. This steel, like steel of the same composition, not containing zirconium, was tested for heat resistance under ordinary conditions, and for corrosion resistance when exposed to a sulfur-containing medium was not tested. This steel is obviously high in sulfur. The disadvantage of steel is the presence of boron and a high nitrogen content. Steel is obtained by remelting waste, i.e. no oxidative period. Therefore, in their own waste (sprues, rejects, splashes), these elements will accumulate. There are a lot of modifying elements in steel, which complicates the smelting technology, and the presence of such a high aluminum content for a very low free oxygen medium is also debatable. The aluminum content is in a very wide range, which is characteristic of an alloying element with its high content in steel. Steel containing 0.06 and 4% aluminum will differ in properties very much, much more than in similar cases for manganese, silicon, molybdenum. The advantage of steel is the limitation of carbon content, which will allow to achieve a certain reproducibility of the results of heat treatment (when applied). Steel has a higher cost compared to the base with controversial performance.

Known steel for thermal units of the following composition,% mass: 0.20 ÷ 0.45 C; up to 23.0 ÷ 26.0 Cr; 11.0 ÷ 13.0 Ni; up to 1.5 Si; 0.4 ÷ 0.8 Nb; up to 0.8 Mn; 0.25 ÷ 0.45 V; up to 0.030 S; up to 0.030 R. The ratio of N _in / V = 1.6 ÷ 3.2. The introduction of niobium and vanadium into the base steel within the specified limits should increase its yield strength (Application No. 20011112240/02 of 04.23.2001 Heat-resistant steel. Kolchanov VN, Palavin RN, Pokolyanin IP). Strong enough carbide-forming and nitride-forming elements are introduced into steel. The absence of a lower limit on the content of silicon and manganese is incorrect, since these elements, when introduced together into steel, are effective deoxidizers. A very low silicon content in the steel (less than 0.4%) will lead to increased fumes of chromium during smelting. The advantage of steel is also the limitation of carbon content. Steel also has a higher cost compared to the base with controversial performance.

Known heat-resistant steel, which included gadolinium, to improve the mechanical and anti-radiation properties (Application No. 2007 145150/02 of the Russian Federation, 04.12.2007 (Pat. No. 2362830 of the Russian Federation) Heat-resistant steel. Kurunik Yu.K., Fedorov VA, Roschupkin V.P., Koritov K.S.). The composition of the steel is the following,% mass: 0.2 ÷ 0.6 C; 0.8 ÷ 1.5 Si; 0.4 ÷ 1.0 Mn; 22.0 ÷ 25.0 Cr; 12.0 ÷ 15.0 Ni; 0.10 ÷ 0.25 Ga. Steel has a higher content of nickel, sulfur and phosphorus compared to their content in base steel. Such a carbon content with such an amount of chromium will not allow to reproduce the properties of heat treatment, and therefore, the occurrence of the MCC. In addition, gadolinium is a rare and expensive element that will increase the cost of the part compared to the base.

For the manufacture of chains in clinker furnaces, steel of the following composition is proposed,% of mass: 0.25 ÷ 0.45 C; 1.5 ÷ 2.5 Si; 6.0 ÷ 8.0 Mn; 25.0 ÷ 27.0 Cr; 12.5 ÷ 14.5 Ni; 0.3-0.6 Al; 0.25-0.40 Nv; 0.15 ÷ 0.30 Cu; 0.15 ÷ 0.30 N; 0.008 ÷ 0.015 V; 0.15 ÷ 0.25 V; 0.005 ÷ 0.015 Ca. The use of steel of the proposed composition allows to increase its heat resistance to 1150 ° C, mechanical properties in the cast state and wear resistance (Patent 2124067 of the Russian Federation. Heat-resistant steel. Bibikov A.M., Kervyakov A.M., Simonov. BI BI 1998, No. 36). Increasing the last two properties of the grate is not required. The steel has a high content of manganese, which reduces its heat resistance. This necessitated an increase in the content of nickel and chromium. The high content of silicon, aluminum, niobium, as well as vanadium contributes to an increase in abrasion resistance and can cause cracking during heat exchanges under the conditions of operation of grates. The cost of this steel is significantly higher than the cost of serial steel 40Kh24N12SL used for chains.

For the manufacture of cast equipment for thermal units, the compositions of nitrided heat-resistant steels are known (Table 4) (Perevyazko AT, Smirnov V.M., Cherevko P.V., Rezinkina G.P. Heat-resistant casting steels economically alloyed with nickel. Foundry, 1991 , No. 5, p. 10).

The first two steels (table 4) have a very low chromium content, which at a low nickel content will not provide casting resistance at temperatures above 1000 ° C. The last two steels contain a high content of silicon, which increases the scale resistance. But the combined presence of such amounts of silicon and nitrogen in the steel composition will strengthen the steel and reduce its heat resistance, especially when the nickel content is reduced (steel from 8 ÷ 11% Ni). Positive results for these steels were obtained with prolonged heating and rare heat exchanges, which is due to the heat treatment conditions. The disadvantages of steels are high sulfur and phosphorus, as well as low carbon content. The latter reduces the casting properties of steels and the quality of the cast structure.

In the work (Lunev V.V., Ivakhnenko E.I., Ferkov V.A., Mzhakova V.I. Improving the durability of cast parts of mining and metallurgical equipment. Foundry, 1992, No. 10, p. 15-16) the research results according to the definition of wear resistance and heat resistance of known steels and developed 100X28NSTL used for the manufacture of replaceable parts of sinter machines (overfill shelves, threshold stops and others). The content of the elements of the proposed steel was as follows,% mass: 0.89 C; 28.10 Cr; 1.30 Si; 0.90 Ni; 0.88 Mn; 0.25 Ti; 0.008 S; 0.020 P. A very high carbon content, an increase in the chromium content of 2% relative to steel 40Kh24N12SL, as well as the presence of titanium indicate that the main task in the development of new steel was to increase the wear resistance at elevated temperatures. Tests of steels for heat resistance (exposure of samples for 100 hours at 1000 ° C) gave the following results, g / (m ² ⋅ h): 35X18H24S2L - 0.0054; 35Х23Н7СЛ - 0.0028; 40Х24Н12СЛ - 0.0057; 100X28NSTL - 0.0021. The presented results do not reflect the true picture, since steels with a higher content of chromium and nickel after exposure at a temperature of 1000 ° C in a normal atmosphere cannot have a mass loss greater than steels with a low content of these elements. It should be noted that the maximum firing temperature in the sintering machine is 1200 ° C and the above machine parts are not operated in this zone. For parts experiencing increased abrasive loads, chromium, silicon, titanium, and carbon were additionally introduced into the steel. The absence of nickel will lead not only to a decrease in heat resistance, but also to a tendency of steel to. When the temperature in the working zone is more than 1200 ° C, steel 100X28NSTL cannot be operated.

⁶ All steels contained up to 0.030% sulfur and up to 0.035% phosphorus

In the work (Kolokoltsev V.M., Adishchev V.V., Naumenko V.D. et al. Increasing the service life of castings made of heat-resistant steel by modification. Foundry, 1991, No. 5, pp. 19-20) investigated the effect of modifiers on heat resistance at 900 ° С for 100 hours of steel 40Kh24N12SL of average composition. It was found that under these conditions boron and titanium increase heat resistance, and cerium and calcium are reduced. A new steel has been developed for castings “core” weighing from 500 to 3200 kg, which are used for pulling pipe bends at temperatures up to 1000 ° C. Studies of new steel 35Kh22N10STRL (steel composition before modification,% mass: 0.28 C; 0.85 Si; 0.48 Mn; 22.0 Cr; 8.1 Ni; 0.028 S; 0.03 P) showed the best properties by compared with the properties of steel 40Kh24N12SL, which is due to the introduction of modifiers (0.15 ÷ 0.25%) Ti + (0.005 ÷ 0.10%) V at a metal temperature of 1515 ÷ 1530 ° C into the ladle when melting. A positive result was achieved by thinning primary carbides. However, when the pouring temperature is reduced to 1490 ÷ 1510 ° C (in the case of grate), it is also possible to obtain a finer primary structure without the introduction of modifiers. The increase in the service life of the cores may be associated with an increase in the hardening of their surface during broaching branches due to changes in the properties of new steel.

The data of the work (Shcheglovitov L.A., Katsaran T.K., Zelepukin P.A. and others. Modified steel alloys with rare-earth metals. Foundry, 1971, No. 3, pp. 19-20) showed that the increase in the operational life of the grate sintering machines were obtained as a result of the modification of 30Kh24N12SL RZM steel with their residual content in the metal from 0.025 to 0.035% (the best option). The difference in the results can be explained by different test conditions: in the first case, the atmosphere in the laboratory furnace is oxidizing, and in the second, slightly oxidizing (industrial gas furnace).

In the work (L. Kolombier, I. Gokhman, Stainless and heat-resistant steels. Translated from French M .: Metallurgizdat, 1958, p. 479), it was noted that the service life of a high-alloy alloy with the addition of 0.2% of cerium group metals doubled Compared to the life of ordinary nickel-chrome wire.

Known heat-resistant steel 30Kh23AN6STYUL (Krivonos V.N., Timofeev P.V., Kozlov L.Ya. New heat-resistant steel for operation at high temperatures. / V Res. Lauch. Technical conference. "Improving the technical level and improving technological processes of casting production) ", Dnepropetrovsk, DMETI, t.2, 1990, pp. 32-33), the full chemical composition of which is not fully provided and the application for the invention was not filed. Steel contains nitrogen in an amount of 0.12 ÷ 0.15% (based on the replacement of 1% nickel with 0.020 ÷ 0.025% nitrogen). It is treated with AHM (Ca, Ba) and microalloying additives (Ti, Al, B) are introduced. The heat resistance of the proposed steel after aging in a muffle furnace at 1100 ° C was higher than that of steel 40Kh24N12SL. Obviously, this is due to the high content of aluminum, which forms oxidizing conditions with very refractory alumina under oxidizing conditions. The disadvantages of the proposed steel should include its very high temperature of the release of 1630 ÷ 1650 ° C, which implies a large waste of ligature. In addition, steel simultaneously contains both nitrogen and titanium, which already form strong titanium nitride in liquid steel at a mass ratio of 1: 2. Thus, all nitrogen must be bound to nitrides. Meanwhile, the authors claim that in their steel the amount of ferrite is only 5 ÷ 10%, and in 40Kh24N12SL steel it reaches 40%, although in the first steel there is half as much nickel and nitrogen is removed from the solution (both austenite-forming elements). Base steel does not contain strong nitride-forming elements and the content of dissolved nitrogen stabilizing the austenitic phase in electric steel is not lower than 0.020%, and most often it is in the range of 0.025 ÷ 0.050%. The proposed steel was smelted in an acid furnace, which leads to a steel content of more than 0.030% sulfur. The content in the chromium steel at the lower limit will be less than 23%, which will negatively affect its heat resistance with increasing temperature to 1250 ° C.

In the work (Word AR Newer Alloys Wriden the Scope for Castings in Heat Treatment Furnaces. "HeatTreat. Metals", 1997, 4, No. 4, p. 89-94), it was shown that castings made of steel containing (% wt.) 0 , 50 C; 23 Cr; 6.5 Ni; 1,5 Si work satisfactorily at temperatures up to 1000 ° C in a reducing atmosphere. Thus, this steel grade is not suitable for roasting machines.

GOST 2176-76 provides for a number of ferritic steels, heat-resistant up to 1150 ° C, for example, 15X25TL, 75X28L, 185X34L. This group of steels has high abrasion resistance and brittleness, which is associated with intensive grain growth and carbide formation. With high thermal cycling (30 minutes of heating with a sharp temperature drop and 30 minutes of cooling, the first five minutes with air blowing), the data became susceptible to cracking. The absence of nickel lowers the heat resistance of steel. Therefore, to work in the conditions of the existing roasting machine, the use of ferritic grade steels is not advisable.

As a result of the analysis of existing steel grades described above, it was found that it is impossible to replace 40Kh24N12SL steel for the production of grates of roasting machines not with one of the existing domestic grades, since steel was selected in the optimal way according to the heat resistance criterion. Based on what there is a need to either find ways to improve the existing steel grade, but based on foreign experience, the chemical composition of steel does not need significant changes. To further search for ways to increase the operational resource, it is necessary to conduct a statistical analysis of the reasons for the failure of domestic grates from steel 40X24N12SL.

3. Determination of the reasons for the low operational resource of domestic grates of steel 40X24N12SL

To determine the causes of a sharp decrease in the operational life of the grate of the firing trolleys, a macro- and microanalysis of the grate was carried out at different periods of operation (including new and rejected ones). This study, conducted by the authors, revealed the main reason for the failure of domestic grid-irons - MKK (95.5% failure), the study is described in detail in the source (Ivanov, D.I. Mechanism of high-temperature corrosion of heat-resistant chromium-nickel steel / D.I. Ivanov, A.A. Kozhukhov, L.F. Urazova // News of higher educational institutions. Ferrous metallurgy. - 2016. - Volume 59. - No. 3. - P. 180-184).

The studies were carried out on an OK-306 roasting machine. Macroanalysis included an assessment of the state of the surface of the parts at different periods of operation (depth of corrosion, degree of destruction of the part at different surfaces).

The result of macroanalysis was the following conclusions: all grates after 6 months of operation are not subject to uniform corrosion damage. Corrosion is most prevalent at the “upper” surface (Fig. 2a), as evidenced by the data (Fig. 2b), which shows the “upper” zone of the middle part of the grate removed from the beam after 12 months of operation. The term “upper” zone or surface refers to the surface of the grate, facing the burners during operation, in other words, the surface that first comes into contact with the downward flow of coolant and it is this surface that contacts the fired material.

In order to determine the mechanism of the occurrence of corrosion damage, a study was made of the grate microstructure on an image analyzer consisting of a MEIJITECHNOIM 7200 metallographic microscope, a television camera, and a computer with an image analysis program THIXOMETPRO. Grids with a life of up to 8 months were investigated.

For the etching of thin sections, the Marble solution was used, which is used for austenitic and austenitic-ferritic steels, nickel and iron-nickel alloys. The composition of the Marble solution: 20 g of copper sulfate, 100 cm ^{3 of} hydrochloric acid, 100 cm ^{3 of} ethyl alcohol.

As a result of the study of grates with a life of 6 months (Fig. 3), it was established that large carbides 1 in the metal matrix were abundantly precipitated in the “upper zone” (the surface of the grate opposite to the direction of the coolant supply (facing the burners)), mainly grains in the interdendritic areas, corrosion 2 spread along the boundaries of large carbide precipitates, which caused peeling of parts of part 3. Moreover, the density of carbide precipitates in the “upper zone” drops sharply at a depth of more than 500 μm surface. The nature of the spread of corrosion suggests that it is intergranular corrosion (MCC). In other planes of the grate, MCC was not observed.

Corrosion occurs according to the MCC mechanism described by the authors (Gulyaev A.P., Gulyaev A.A. Metallurgy. Textbook for universities, 7th edition. - Moscow: Metallurgy, 2011. - 643 pp.) For chromium-nickel stainless steels of austenitic-ferritic class quenching (with supersaturated austenite) with a carbon content of more than 0.005%. The authors conclude that the MCC occurs at the grain boundaries with the formation of a carbide phase (M ₂₃ C ₆ ) with the participation of all carbon in the metal matrix, and chromium only from the border areas, thus, zones with a low chromium content appear (less than 12 %) at the grain boundary. The latter is due to the large difference in the diffusion rates of carbon and chromium. Also in favor of this version is the rounded nature of the end of the voids, during operation, the metal matrix was de-charred at the grain boundaries, and they underwent corrosion damage (Fig. 2).

The results of the study allow us to conclude that carbide formation occurs from the very beginning of the operation of grid-irons, and the pronounced development of intergranular corrosion of a part begins from the 3rd month of operation, after the formation of a continuous carbide network at the dendrite boundaries in the surface zone. The process of carbide formation occurs in the entire volume of the grate, while the process proceeds with the highest intensity in the “upper zone” of the part, which can be explained by a large temperature gradient over the section of the part during operation, due to the low thermal conductivity of the steel.

Thus, the MCC in 40Kh24N12SL steel develops as a continuous rather wide carbide network forms at grain boundaries. The larger the primary austenite grain, the longer the continuous precipitation of carbides and the more actively the MSC will develop. The primary austenite grain, as well as the degree of dendritic segregation, are determined by the casting conditions (dendrite size). Moreover, the carbide structure (especially in the surface zones) is predominantly formed during operation. Increasing the life of the grate can be achieved by:

- reducing the size of dendrites and, as a result, the size of austenitic grains and the degree of development of dendritic segregation, this can be achieved by applying casting on vibrating stands;

- stabilization of the structure and control of carbide precipitation (creating favorable conditions for the isolation of small globular carbides), which can be achieved as a result of heat treatment, and the intensity of the carbide formation process can also be reduced by reducing the carbon content.

4. Development of measures to increase the operational resource of grate

4.1. Correction of the chemical composition of steel 40X24N12SL

As it was established earlier, today replacing steel 40X24N12SL with an analogue will not bring a significant increase in the operational resource, but at the same time it will increase the cost of the cost of the part. This is primarily due to the fact that the low service life of 40Kh24N12SL steel is associated not with the chemical composition of the steel, but with the cast structure, as was established in the above studies. Thus, in order to increase the operational life of the grates of domestic roasting trolleys to an acceptable one taking into account modern operating conditions (warm and gas-blast mode of the roasting machine), it is necessary to make adjustments to the technology for producing grates of roasting trolleys in the direction of obtaining an air-conditioned cast structure.

To eliminate the main reason for rejection of grates (95.5% of rejects) - destruction during operation according to the MKK mechanism, it is necessary to heat treat the steel to obtain a conditioned cast structure. The main criteria for the selection of heat treatment (TO) steel will be:

- the precipitation of carbides after casting not only at the grain boundaries but in the grain body in a favorable rounded shape, which will increase the resistance of the MCC material and the development of intergranular cracks, as well as reduce the degree of uncontrolled carbide precipitation during operation,

- the presence of a sufficient distance between the precipitation of carbides in the metal matrix, which will eliminate their coagulation during operation, this principle is reflected in the work (Kolokoltsev V.M., Goltsov A.S., Stolyarov A.M. Choice of a new composition of heat-resistant cast iron. Foundry. 2013. No. 6. P. 2-7) - the criterion of scale resistance for heat-resistant steels is maximum when the distance between carbide precipitates is about 15 microns,

- reproducibility of results - all grates made of steel 40X24N12SL, after maintenance should have the most similar structure.

In order for the heat treatment mode to be effective regardless of the chemical composition of different melts (reproducible), it is necessary that the carbide-forming elements (carbon and chromium) are within limited limits. If the amount of chromium is sufficiently limited according to GOST 977-88: “Steel castings. General technical conditions ”, the amount of carbon is limited only by the upper limit: not more than 0.4%. Such a wide range of possible carbon content will not allow heat treatment of grates of different heats in one mode without sacrificing quality. As a result, it is proposed to limit the amount of carbon, without going beyond the requirements of GOST 977-88, in the range: 0.30 ÷ 0.40%. Taking into account the error stipulated by GOST of ± 0.02, the amount of carbon in the finished grate should be in the range 0.28 ÷ 0.42%.

The choice of such a range of carbon content is due to the advisability of reducing the ferritic phase in the grate. At this carbon content, the fraction of the ferritic phase in the grate will not exceed 7% (estimate using the structural diagram of stainless steels (Zakharov BP, Termist BP Zakharov. - Sverdlovsk, Moscow: Mashgiz [Ural-Siberian Branch], 1961 . - 313 s)).

It should be noted that an increase in the carbon content in the liquid melt reduces the activity of chromium, which will reduce its fumes (and, accordingly, cost), the calculation of the activity of chromium at different concentrations of carbon is given below.

To reduce the waste of expensive alloying elements during steel melting, it is also proposed to increase the maximum silicon content by 0.5%. According to GOST 977-88, silicon in steel 40Kh24N12SL should be in the range of 0.5 ÷ 1.5%, it is proposed to make the silicon concentration in the range of 0.5-2.0%, the calculation of the activity of chromium at different concentrations of silicon is given below. Such a solution will make it possible to optimize the technology of steel smelting 40X24H12SL to a greater extent, if the manufacturer wishes, to reduce the waste of expensive elements, and therefore the cost.

It should be noted, based on the data in table 3, most of the world analogues have a limit on the silicon content of not more than 2.0%. Thus, the proposed solution does not contradict the general global trend.

To determine the effect of the amount of carbon and silicon on the carbon monoxide in a liquid melt, we calculated the activity of chromium at a temperature of 1500 ° C for steel grade 40Kh24N12SL with different contents of carbon and silicon, the silicon content was increased above the grade from 1.5 to 2%.

The calculation was carried out using a technique based on taking into account the first-order interaction parameters. The choice of a methodology for calculating activity through first-order interaction parameters is based on high concentrations of alloying elements in the liquid iron (Ivanov D.I. Mechanism of high-temperature corrosion of heat-resistant chromium-nickel steel / D.I. Ivanov, A.A. Kozhukhov, L.F. Urazova // Izvestiya higher educational institutions. Ferrous metallurgy. - 2016. - Volume 59. - No. 3. - S. 180-184). Calculation through the mole fractions of the components will give less accurate results.

To calculate the activity of chromium, the carbon content was taken in the range of 0.1-0.4% with a step of 0.1%, which is within the grade composition of steel 40Kh24N12SL and the silicon content in the range of 0.5-2.0% with a step of 0, 5%, which is outside the grade composition of steel 40Kh24N12SL (silicon in steel is limited to 0.5-1.5%). The calculation data are presented in table 5.

According to the calculation of Table 6, graphical dependences of the effect of carbon and silicon on the coefficient of activity of chromium in a liquid melt at a temperature of 1500 ° C (Figure 4) and the effect of the chemical composition of steel on the activity of chromium in the melt (Figure 5) were constructed.

The graphical dependencies (Figures 4 and 5) show that the coefficient of activity of chromium as well as activity in a liquid melt with respect to the 1% standard state decreases with increasing concentrations of both silicon from 0.5 to 2.0% and carbon from 0 , 1 to 0.4% linearly. Moreover, an increase in carbon content reduces the activity of chromium much more than an increase in silicon content.

As a result of the foregoing, we can conclude: to reduce the fumes of an expensive alloying element - chromium during the melting of 40Kh24N12SL steel, it is necessary to introduce the maximum amount of carbon provided by the grade steel composition - 0.4% ± 0.02%, and up to 2% silicon, which higher than the grade composition of steel by 0.5% of the upper limit. It should be noted that a high carbon content will increase the amount of austenite in steel, which is a positive factor for performance, as described in Chapter 4, and a 0.5% increase in silicon will have a minimal effect on the austenitic-ferritic balance but will increase heat resistance and wear resistance.

5.2. Development of a heat treatment regime for grate after casting from steel 40X24N12SL with adjusted chemical composition

To increase the resistance of MCC to grates of domestic production from austenitic-ferritic steel 40X24N12SL according to GOST 977-88 and STP 26.260.484-2005, heat treatment is provided: quenching at a temperature of 1100 ° C for 60 minutes and cooling in air, in water or oil. We can conclude that the provided heat treatment regime of the grate is not enough to prevent the occurrence of the MCC.

Based on the fact that carbides dissolve at temperatures: 723 ° С - (CrFe) ₇ C ₃ and 1100 ° С - (CrFe) ₂₃ C ₆ (Elagina O.Yu. Technological methods to increase the wear resistance of machine parts: textbook / O. Y. Elagina.- M .: Logos, 2009.- 488 pp .: ill. - (New University Library) - ISBN 978-5-98704-450-6) - it is not advisable to heat steel when hardening below 1100 ° С. But at the same time, the solubility of carbon in austenite is much higher than in ferrite. Therefore, if austenite is converted to ferrite (γ → α) in the steel structure during quenching, a large amount of carbon will be converted to chromium carbides. At temperatures above 1100 ° С, the steel structure passes the γ-solvus line and falls into the region of existence of the two-phase austenite-ferrite structure (α + γ). At the same time, in the temperature range 1100-1200 ° С, the amount of ferrite (α) in steel will increase, and at a temperature above 1200 ° С it will be at all maximum, because the full γ → α transformation will occur (analysis was performed using a diagram of pseudobinary cross sections of the ternary system: Fe-Cr-Ni, with an iron content of 60%). Thus, overheating of steel 40Kh24N12SL above 1100 ° C will lead to a γ → α transition, which will increase the tendency of steel to MKC. After analyzing the hardening temperature, we can conclude that it is optimal, thus, the high tendency of grate bars to MKC is connected precisely with the insufficient exposure time of steel at this temperature and with the insufficiency of the selected heat treatment mode.

The exposure time when choosing a heat treatment mode for high alloy steels and alloys is set depending on the duration of diffusion processes, and the standard exposure time depending on the thickness of the product (as in the sources (Shubadeeva LI, Revyakina OK, Makarchuk T.V., Gurvich L.Ya .The effect of heatings on corrosion resistance of 12Kh18N10T stainless steel // Protection of Metals and Physical Chemistry of Surface. 1996. V.2 - P. 119) and STP 26.260.484-2005) will be unacceptable. Based on these considerations, it can also be concluded that for 40Kh24N12SL steel, the holding time should be selected based on the content of carbide-forming elements, i.e. in order for the heat treatment mode to be effective regardless of the chemical composition of different melts (to be reproducible), it is necessary that the carbide-forming elements (carbon and chromium) are within rather limited limits. If the amount of chromium is sufficiently limited according to GOST 977-88, then the amount of carbon is limited only by the upper limit: not more than 0.4%. Such a wide range of possible carbon content will not allow heat treatment of grates of different heats in one mode without sacrificing quality, in addition, such a range of carbon content can lead to sharp changes in the ratio of austenite and ferrite in steel. As a result, it is proposed to limit the amount of carbon, without going beyond the requirements of GOST 977-88, in the range: 0.30-0.40%.

The choice of such a range of carbon content is due to the advisability of reducing the ferritic phase in the grate. At this carbon content, the proportion of the ferritic phase in the grate will not exceed 7% (assessment using a structural diagram of stainless steels not subjected to heat treatment (Gulyaev A.P., Metallurgy: textbook for university students / A.P. Gulyaev. - 5- ed., revised. - M.: Metallurgy, 1977. - 647 c)).

To exclude the possibility of MCC flowing, it is proposed to stabilize the grate structure to a state that excludes intensive uncontrolled carbide formation during operation (the metal structure approaches the “equilibrium” state at operating temperatures). To do this, it is necessary to conduct heat treatment of the part after casting:

Hardening - so that large primary (eutectic) carbides partially dissolve in the metal structure, leaving point sites that can serve as centers for carbide precipitation during subsequent tempering and operation, and the amount of austenite (γ) in the steel structure is maximum. To reduce the tendency to MCC, carbide precipitates remaining in the metal structure after quenching should have relatively small residual sizes of primary precipitates (with a diameter or width of up to 5 μm), the nature of the precipitates in the form of globules and the distance between large precipitates is not less than 15 μm (Kolokoltsev V. M., Goltsov A.S., Stolyarov A.M. Choice of a new composition of heat-resistant cast iron. Foundry. 2013. No. 6. P. 2-7).

Vacation - in order to activate the process of growth, coagulation and the creation of new centers for the precipitation of carbide phases, as a result of tempering, it is necessary to create a structure in which the possibility of the formation of continuous precipitates of carbides, which are formed along the boundaries of dendrites and grains during operation, is excluded. Thus, in the structure after tempering, it is necessary to have a large number of new small carbide precipitates at a sufficient distance from each other, which will serve as centers for the precipitation of a new carbide phase during operation, while large carbides should have a more favorable shape - globules.

To find the optimal heat treatment parameters for the grates of the firing trolleys made of 40Kh24N12SL steel, comparative metallographic studies of the grates made according to standard technology were carried out before operation (Figure 6 a, b) and after 3 and 8 months of operation (Figure 6 c, d), as well as cast cubes made to determine the optimal parameters of the experimental regime of heat treatment with dimensions of 40 × 40 × 40 mm from steel 40Kh24N12SL with a carbon content in the range of 0.30-0.40% after quenching (Figure 6, e) and after quenching and tempering (Figure 6, e).

Metallographic studies of the microstructure were carried out using an image analyzer consisting of a MEIJI TECHNO IM7200 metallographic microscope, a television camera and a computer with the THIXOMET PRO image analysis program. The aim of the research was: to evaluate the size and location of carbide precipitates after each stage of the proposed technology for producing grates, and to compare the results of micro-research of experimental grates and grates after operation.

As a result of the analysis of the microstructure of grates made using standard technology (quenching after casting), it was found that they have a tendency to develop MCC due to the unfavorable cast structure of the part preserved after quenching, which has a carbide mesh up to 2 μm thick ( Figure 6, a) and large carbide precipitates in the interdendritic areas up to 25 μm thick (Figure 6, b). In the grate structure after quenching by standard technology, primary (eutectic) carbides and carbides are observed which are precipitated from the solid solution after casting. In this case, the solid solution remains in a state far from equilibrium. In addition, it is obvious that the structure of the part along the cross section is not uniform, the number of carbides and their sizes increase from the periphery to the center of the casting. This is explained by the general theory of crystallization - the middle part of the casting hardens later than the periphery, as a result of which, due to dendritic segregation, sections with the content of alloying and impurity elements significantly exceeding their average content are formed in the central zone. It is worth noting that after casting there are no carbides precipitated inside the grains in the grates (Figure 6, a, b).

Microanalysis of grates after 3 months of operation showed that carbide formation intensively takes place in the grates during operation (Figure 6, c), abundant precipitation and growth of existing carbide precipitates occurs along the boundaries of dendrites and grains, and the distances between these precipitates are very small ( between carbides with a dimatra of more than 1 μm, the distances are in the range of 0.5-4.0 μm). At such small distances between carbide deposits, the development of MCC during further operation is inevitable. It should be noted that according to the available data (Figure 6, c), it is clear that some carbides began to coagulate and form large carbides with a large length of the boundaries, and inside the grains there is a massive release of very small carbides (Figure 6, c), which is explained by the relatively low diffusion mobility of chromium atoms, large grain size, long time necessary for the formation of a stable nucleus of a new phase inside the grain. It is obvious that the MCC process has been actively proceeding in the structure of grate bars since the start of operation.

The local precipitation of the carbide phase in the interdendritic regions and at the grain boundaries is explained by the general laws of crystallization - the boundaries are rich in crystallization centers, and the activation energy of the process at the boundary is less than in the depth of the grain. The presence at the boundaries of carbide residues, with a sufficient release density, from the previous state of the metal, does not require energy for the formation of the nucleus, therefore, the process of carbide precipitation proceeds mainly through the growth and coagulation of these residues.

After 8 months of operation, a carbide network is formed in the grates along the grain boundaries (Figure 6, d), which is accompanied by the dehumidification of the matrix sections adjacent to the grain boundaries and, therefore, the material becomes especially prone to MKC. After 8 months of operation, larger carbide precipitates are clearly visible in the metal structure, compared with data obtained after 3 months of operation. The development of the IWC is already in the final stage - the grate after 8 months of operation is actively failing due to the IWC.

The results of the study of grates made according to standard technology (quenching after casting at 1100 ° C for 60 minutes) before and after operation confirm the conclusion that standard heat treatment technology is not enough to exclude the possibility of MCC flowing.

To exclude the possibility of MCC flowing in 40Kh24N12SL steel, the optimal heat treatment parameters were determined experimentally.

For hardening, as indicated earlier, the temperature was optimally selected (1100 ° C), therefore, only the holding time was selected during hardening. Three cast samples, made of steel 40Kh24N12SL with a carbon content limit of 0.30-0.40%, were aged for 70, 80 and 90 minutes. At a holding time of 70 minutes, the results of microanalysis were similar to the data in Figure 6, b. Large carbide precipitates just started to dissolve and the distance between the residues was 2-4 microns. It is worth noting that no carbide mesh was detected. After exposure at 80 and 90 minutes, the results were almost identical: the primary carbides were not completely dissolved in the metal matrix, the remaining carbide precipitates had medium sizes (width up to 5 μm) and a small length along the grain boundaries, while the distances between large precipitates (residues from primary carbides) ranged from 2 to 5 μm (Figure 6, e). Thus, the optimal exposure time for hardening at a temperature of 1100 ° C is 80 min (Figure 7).

It must be remembered that even optimal hardening does not lead to a “equilibrium” state of the steel structure, but on the contrary, the amount of excess carbon that can be released during subsequent operation in chromium carbides only increases. Given the nature of the operation of the grate (the natural presence of local overheating in the upper zone of the part) and the high diffusion mobility of carbon atoms - all this sharply increases the tendency of quenched grate to MCC precisely in the overheating zones. Thus: after hardening, it is necessary to stabilize the steel structure over the entire cross section of the part (carbides should be evenly distributed over the entire structure of the part, which eliminates the possibility of uncontrolled abundant carbide formation in the areas of overheating of the part during operation).

To stabilize the structure of the steel after quenching, it is necessary to conduct a vacation. The tempering temperature was previously determined in the range of 900-910 ° C. It is known (Elagina O.Yu. Technological methods for increasing the wear resistance of machine parts: textbook / O.Yu. Elagina.- M .: Logos, 2009. - 488 pp., Ill. - (New University Library) - ISBN 978-5 -98704-450-6), that at temperatures above the critical temperature A ₃ diffusion sharply increases, the temperature A ₃ for steel 40Kh24N12SL is in the range of 850-880 ° C. It is worth noting that, at lower temperatures, the process of growth and coagulation of carbide phases does not actively develop (Kolesov, A.A. Ways to increase the wear resistance of austenitic chromium-nickel steel 12X18H9T / A.A. Kolesov, V.A. Ivanova // Sixty-ninth All-Russian Scientific -technical conference of students, undergraduates and graduate students of higher educational institutions with international participation. Yaroslavl: collection of materials conf. [Electronic resource]. - Yaroslavl: Publishing house of YAGTU, 2016, S. 834-837-1 electron, opt. disk ( CDROM). - ISBN 978-5-9914-0524-9). In addition, there is a risk of abundant release of a close-packed σ-phase, twisting the metal, which is released in the temperature range of 600-900 ° C.

The duration of the vacation, as for hardening, was determined experimentally. Diffusion processes inside a solid metal take a long time, and the lower the temperature of the metal, the longer it takes. Thus, tests were carried out on three cast specimens made of 40Kh24N12SL steel with a carbon content limitation of 0.30-0.40% after quenching according to the previously selected mode. The exposure time of the samples was 100, 150 and 200 minutes at a temperature of 900 ° C. After tempering the grates for 100 minutes, coarsening of residual carbides up to 8 μm wide was observed, while the nucleation of new nuclei, which could serve as centers for the precipitation of a new carbide phase during operation, was practically not observed. After exposure for 150 minutes, abundant precipitation of small carbides of small size (up to 1 μm) occurred in the interdendritic areas, as well as at the grain boundaries, uniformly throughout the grate structure. The carbides remaining after quenching began to round and grow (width up to 10 μm) (Figure 6, f). After exposure for 200 minutes, it was found that the processes of precipitation and growth of carbides proceeded extremely slowly. Primary large carbides had a width of up to 10 μm, i.e. practically did not increase in size. Almost no new carbide precipitates were observed, and small changes in the precipitation of a new carbide phase could be observed only at large increases in small carbides, i.e. with an exposure of 200 minutes, there is mainly a slight increase in existing carbide precipitates. Thus, the processes occurring in the metal structure during tempering for more than 150 minutes (after quenching) can also occur during operation without increasing the tendency of grates to MKK.

After tempering for 150 minutes, a large number of new centers were created in the steel structure for the precipitation of carbides at the boundaries of dendrites and grains over the entire structure of the part, and the intensity of this process was significantly reduced due to bringing the material to a more equilibrium state at temperatures close to operational. As a result of the tempering procedure, the intensity of volume redistribution in the grid-irons during their operation (carbide precipitation and α↔γ phase transition) is reduced. After tempering, the steel structure lacks large carbides and a carbide mesh. Thus, the tempering procedure made it possible to eliminate the tendency of grid-irons to MKK during operation. Based on the foregoing, the optimal tempering regime after quenching is holding at a temperature of 900 ° C for 150 minutes (Figure 7).

In the manufacture of grates from austenitic-ferritic steel 40Kh24N12SL, quenching is identical to that described, but the recommended holding time for quenching is indicated standard (depending on the thickness of the product) or not at all (GOST 977-88, STP 26.260.484-2005). In the work done, it was established that such an approach in the direction of combating MKK in the structure of steel 40Kh24N12SLN is unacceptable. At the same time, in the complex, quenching with tempering to stabilize the structure and prevent the occurrence of MCC did not find wide application. For metallurgical practice, the use of high-temperature tempering to combat the tendency to MCC is not common. In most cases, tempering is used to achieve certain mechanical characteristics (as stipulated in many GOSTs: 5582-75, 5949-61, 7350-77). The selected experimental tempering regime is similar to the stabilizing annealing of chromium-nickel steels after welding (Medovar B.I. Welding of heat-resistant austenitic steels and alloys. M .: Metallurgy, 1966. - 284 s), which is carried out to transfer carbon from chromium carbides to special niobium carbides and titanium, while the released chromium goes to increase the corrosion resistance of steel (usually the annealing temperature is 850-950 ° С), which differs somewhat from the transformations occurring in the structure of steel 40Kh24N12SL - the transition of excess carbon to chromium carbides and iron deza along the boundaries of dendrites and the subsequent "stabilization" of the structure.

The most similar to the experimental heat treatment mode was proposed by the authors (Shubadeeva LI, Revyakina OK, Makarchuk T.V., Gurvich L. Ya. The effect of heatings on corrosion resistance of 12Kh18N10T stainless steel // Protection of Metals and Physical Chemistry of Surface. 1996. V. 2 - P. 119): after hardening of 12X18H10T steel, it is proposed to carry out stabilizing tempering at a temperature of 650 ° C for 500 hours. This is explained by the same reasons: due to the structural features of hardened steel in the temperature range 500-550 ° C (the operating temperature of the grates is identical), a M ₂₃ C ₆ carbide network forms on the basis of Cr ₂₃ C ₆ chromium carbide along grain boundaries, whereas under the same conditions, carbides are uniformly distributed throughout the structure, which excludes the possibility of MCC. The main disadvantage of the proposed tempering regime is its high cost due to the long duration of the process, it is established that the proposed experimental tempering regime (shorter exposure time of 150 minutes, but at an elevated temperature of 900 ° C) is sufficient to exclude the possibility of MCC flowing in the structure of hardened steel during further operation.

5.3 Conclusions

In steel 40Kh24N12SL, it is necessary to introduce a lower limit on the carbon content (0.30%), thereby creating conditions for reproducibility of the heat treatment results, which will allow to preserve the unity of results in the manufacture of grates by different suppliers.

It is also necessary to increase the silicon content limit in steel 40Kh24N12SL from 1.5 to 2.0% in order to reduce the burning of expensive alloying elements during the melting of steel 40Kh24N12SL.

Grids after casting must be subjected to heat treatment: quenching at a temperature of 1100 ° C for 80 minutes and tempering at a temperature of 900 ° C for 150 minutes. Quenching allows you to dissolve the carbide mesh in the cast structure and grind large carbide precipitates to medium size. After tempering, a large number of small carbides (with a diameter of less than 1 μm) appear evenly distributed throughout the structure of the part, which was not observed in the hardened structure, which will serve as centers for the precipitation of the carbide phase during operation. At the same time, the process of carbide formation slows down due to the reduction of the structure to a more "equilibrium" state. Thus, the developed heat treatment regime excludes the possibility of the formation of continuous carbide precipitates at the grain boundaries in the grate structure; therefore, it excludes the possibility of the formation of MCC in the structure of 40Kh24N12SL steel.

6. The technical result of the work (invention)

The invention relates to metallurgy, in particular to heat-resistant chromium-nickel cast steels, intended for the manufacture of parts operating in aggressive atmospheres containing nitrogen and sulfur oxides with constant heat changes in the temperature range of 20-1100 ° C, with the ability to withstand short-term heating to 1320 ° C ( no more than 10 minutes).

The technical result of the invention is to increase the guaranteed (minimum) service life of steel from 6 to 12 months under operating conditions in aggressive atmospheres containing nitrogen and sulfur oxides with constant heat changes in the temperature range of 20-1100 ° C, with the ability to withstand short-term heating up to 1320 ° C (no more than 10 minutes).

This is achieved by the fact that the proposed steel after casting is subjected to air quenching (holding for 80 minutes at a temperature of 1100 ° C) followed by tempering (holding for 150 minutes at a temperature of 900 ° C) and contains a modified chemical composition for reproducible heat treatment results, namely, an additional (lower) limit on carbon content is 0.30%. In addition, to reduce the waste of expensive alloying elements - steel has an increased upper limit of the silicon content of 2.0%, with a ratio of components, weight. %:

Carbon 0.30-0.40;

Silicon 0.50-2.00;

Manganese 0.30-0.80;

Chrome 22.0-26.0;

Sulfur no more than 0,030;

Phosphorus no more than 0,035;

Nickel 11.00-13.00;

Iron is the rest.

7. Brief Description of the Drawings

Figure 1: new grates of the roasting trolley for the OK-306 roasting machine (3 pcs.), General view - to familiarize yourself with the configuration of the part.

Figure 2: grates after 6 months of operation: a - mounted on a roasting trolley, b - middle part of the grate removed from the beams, top view - general view of the occurrence of intergranular corrosion (MKK) in a part manufactured by standard technology (GOST 977-88), after 6 months of operation.

Figure 3: not etched thin section of the upper part of the grate after 6 months of operation: carbide precipitation in surface zone 1, corrosion spreading along them 2 and peeling sections 3 at the “upper” surface of the not etched thin section from the grate after 6 months of operation, magnification × 150 - Microscopic examination of grates exposed to MCC proving the course of MCC

Figure 4: the effect of carbon and silicon on the coefficient of activity of chromium in a liquid melt at a temperature of 1500 ° C - an increase in the carbon content decreases the coefficient of activity of chromium, respectively, its fumes during melting.

Figure 5: the effect of carbon and silicon on the activity of chromium in a liquid melt at a temperature of 1500 ° C - an increase in the silicon content reduces the activity of chromium, respectively, its fumes during melting.

Figure 6. Microstructure 1.5 mm from the “upper” surface of the middle part of the grate: a - after casting, a carbide mesh 1-2 microns thick along the boundaries of the primary austenitic grain, b - after casting, large primary carbide with sizes up to 25 microns, c - after 3 months of operation, the formation of new carbides, the growth of primary carbides and their coagulation, g - after 8 months of operation, the occurrence of MCC, d - after casting and quenching, large primary carbides and a carbide mesh are dissolved in the metal matrix, medium-sized carbides remain in the structure (up to 5 μm), e - after lit I, hardening and tempering, carbides of small sizes (up to 1 μm) appeared in the metal structure and medium-sized carbides (up to 10 μm) were slightly enlarged - a, b, c, d - evolution of the grate structure made according to standard technology (GOST 977- 88), during operation according to the IWC mechanism, a, e, d are the evolution of the grate structure, during production by the experimental technology, the difference between a and e is the difference between the standard and experimental structure in new grates.

Figure 7: heat treatment scheme of grates from steel 40X24N12SL - quenching and tempering on the graph of time and temperature.

Claims (3)

1. A method of manufacturing a grate of roasting trolleys of cast heat-resistant steel, comprising casting steel to obtain grate and heat treatment by quenching and tempering, while casting steel containing, in wt.%: Carbon, 0.30-0.40, silicon 0 , 50-2.00, manganese 0.30-0.80, sulfur not more than 0.030, phosphorus not more than 0.035, nickel 11.00-13.00, chromium 22.00-26.00, iron and impurities - the rest, quenching is carried out in air at a temperature of 1100 ° C for 80 minutes, and subsequent tempering is carried out at a temperature of 900 ° C for 150 minutes, followed by cooling in an oven or air heh. 2. The method according to p. 1, characterized in that the steel as an impurity contains aluminum in an amount up to 0.07 wt.%. 3. The method according to p. 1, characterized in that the steel as an impurity contains cobalt in an amount up to 0.50 wt.%.

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