APPLICATION OF PROTECTIVE ENVIRONMENTS AND HEATING UNDER THERMAL TREATMENT - Official site Termolit Plus

APPLICATION OF PROTECTIVE ENVIRONMENTS AND HEATING UNDER THERMAL TREATMENT

During heat treatment, various protective media are widely used. These include: protective coating (chalk, on the basis of clay and liquid glass), cast iron chips, graphite, spent carburizer, getters with high affinity for oxygen, used in devices with sand shutter or in sealed containers [1]. However, the use of protective coatings does not always provide reliable protection of parts against oxidation and decarburization, and not all coatings are technological.

In the literature there are data on the use of protective coating based on glass powder [2]. Such coatings reliably protect parts from oxidation and decarburization, but have technological disadvantages (it is difficult to remove them from the surface of the parts after heat treatment).

The purpose of this work was to develop a technologic coating and to study the effect of various media on decarburization, carburization and oxidation of steels, copper and brass.

Protective coating. It was necessary to choose a coating to protect the threads of parts made of structural steels, heat treatment of which is impossible in salt baths [3].

Experienced various compositions of coatings based on liquid glass and glass powder with various additives. The glass powder was prepared from window glass in a ball mill with a granulation of not more than 0.316 mm. Chamotte clay, chalk, kaolin, talc, oxides of aluminum, temple and titanium were used as additives. Glass-based formulations were diluted in water to a pasty state.

Preliminary tests were carried out on plates of steel U9A with a thickness of 0.3 mm. A brush was applied to the degreased surface, after which the samples were dried at 60-100 ° C, heated in a chamber furnace to 900 ° C, 30 minutes and cooled in water. It was established that liquid glass with various additives decarburizes the plates from steel U9A to contain a carbon content of 0.19-0.44%, and glass powder with the same additives up to 0.30-0.82% (Img. 1).

The change in carbon content in the tape of steel U9A, depending on the type of coating

Image1. The change in carbon content in the tape of steel U9A depending on the type of coating:
a – coating based on liquid glass; b – based on glass powder; additives: 1 – chamotte clay; 2 – alumina; 3 – chromium oxide; 4 – titanium oxide; 5 – talc; 6 – chalk; 7 – kaolin.

From img. 1 shows that the best coating consists of 50-55% glass powder and 50-45% chamotte clay. The disadvantage of this coating is its fragility after drying. Coating with talc had satisfactory strength. Talc was added to the coating consisting of 50% glass powder + 50% chamotte clay. Positive results were obtained with the addition of 10% talc. The coating on the samples was durable.

In img. Images 2 and 3 show the test results for various smears.

The change in carbon content in the tape of steel U9A depending on the exposure time in a protective environment

Image2. The change in carbon content in the tape of steel U9A depending on the exposure time in a protective environment:
1 – plastering of 40% chamotte clay + 50% glass powder + 10% talc; 2 – 40% talc + 20% kaolin + 40% liquid glass; 3 – 20% glass powder + 80% liquid glass; 4 – chalk coating; 5 – without coating.

Weight loss per unit area for samples of steel 30HGSA depending on the type of plastering and holding time at 900 ° C

Image3. Weight loss per unit area for samples of steel 30HGSA depending on the type of plastering and holding time at 900 ° C:
1 – plastering of 40% chamotte clay + 50% glass powder + 10% talc; 2 – 40% talc + 20% kaolin + 40% liquid glass; 3 – 20% glass powder + 80% liquid glass; 4 – chalk coating; 5 – without coating.

Depending on the type of coating, the depth of the decarburized layer changed:

Type of coating The depth of the decarburized layer in mm
40% chamotte clay + 50% glass powder + 10% talcum powder 0,02
40% talcum powder + 20% kaolin + 40% liquid glass 0,12
20% glass powder + 80% liquid glass 0,18
Chalk plaster 0,29
No coating 0,42

It can be seen that the best properties are obtained with a plaster containing 40% chamotte clay, 50% glass powder and 10% talc. Tests of its technological properties showed that it cracks and flies away when quenched in water, oil, alkali and saltpeter; the surface of the parts is clean without scale. Removing the coating from the internal holes is carried out by immersing the parts in alkali at 370-400 ° C for 3-10 minutes, followed by cooling in water.

The protective environment used when heated in a fixture with a sand shutter. The effect of iron shavings, graphite, sand, fireclay, chromium oxide, talc and copper chips on decarburization and carburization of steels U9A, P18, as well as oxidation of copper M2 and brass L62 when heated in a sand-gate device (Img. 4) was investigated. .

Sand Shutter Sketch

Image4. Sand Shutter Sketch:
1 – cover; 2 – sand shutter; 3 – outer wall; 4 – carburizer (charcoal); 5 – inner wall; 6 – details.

Samples were loaded inside fixtures with protective medium. To protect copper and brass parts, only charcoal was used, which was piled up between the walls of the fixture. Thus, in an adaptation with a sand shutter, an atmosphere of products of interaction of air with a protective environment and samples in it was created.

Heat treatment of the samples was carried out according to the mode: heating 840 ° С from 0.5 to 3 h, cooling in water. The test results are shown in the table.

Environment Average,% The deviation of the carbon content from the source, in%
No backfill 0,84/0,82 -0,07/-0,09
Untold sand 0,72/0,66 -0,20/-0,26
Hardened sand 0,87/0,83 -0,01/-0,11
Undiluted graphite 0,92/0,72 -0,02/-0,20
Dried graphite 0,94/0,84 -0,00/-0,13
Cast iron chips (3.4% C) 0,80/0,52 -0,07/-0,44
Fireclay wet 0,16 -0,69
Chamotte calcined 0,69 -0,20
Chrome oxide, undiluted 0,61 -0,29
Dried chromium oxide 0,74 -0,14
Talc uncalcined 0,62 -0,25
Calcined Talc 0,61 -0,29
Copper chips 0,26 -0,63
90% sand + 10% graphite 0,52 -0,34
70% sand + 30% graphite 0,76 -0,09
50% sand + 50% graphite 0,77 -0,11
Note: Exposure at 840 ° С 3 h. In the second column, the first value corresponds to an exposure of 0.5 h, the second – 3 h. From the table it can be seen that isolation of samples from the external environment with a sand shutter, even without filling, eliminates decarburization. Thus, in hermetic furnaces, with a suitable ratio of the amount of metal and air in the furnace, protective atmospheres can be created. According to the data of [4], the oxygen of the air in such a furnace, interacting with the metal surface, creates an atmosphere containing 7% CO2, 14% CO, 79% N2.

 

The presence of moisture in all environments increases the degree of decarburization. The greatest decarburization occurs when samples are filled with wet fireclay.

In the absence of moisture, all media also decarburize steel in different ways. The most inert environment is calcined sand, the use of which in a sand-gate device is particularly suitable for dispersion-hardening and high-alloyed and titanium steels and alloys. At low shutter speeds, graphite and cast iron chips can cause carburization, and at high shutter speeds – decarburization. Oxidized iron shavings decarburize more than not oxidized.

A mixture of sand and graphite, as well as chamotte chips, cause slight decarburization in the absence of moisture.

The same corrosive environment as a wet chamotte crumb is copper chips, which is explained, apparently, by the presence of a large amount of oxygen in copper and a lower affinity for oxygen in comparison with iron.

In this work, we studied the decarburization of high-speed steel on samples processed by annealing of the instrument after docking: heating to 700 ° C for 3 hours, heating to 840 ° C for 1 hour, holding for 4 hours, cooling to 750 °, holding for 5 hours, slow cooling to 400 ° C, air cooling. Half of the samples were annealed in a sand shutter fixture, the rest in the air atmosphere of a chamber furnace. The test results are shown in Fig. 5. During annealing of the docked instrument in the atmosphere of an untight furnace, decarburization takes place by 2-2.5 mm per side, and when using the device with a sand gate, by 0.01-0.04 mm.

The change in the depth of the carbon content and microhardness depending on the heating medium

Image5. The change in the depth of the carbon content and microhardness depending on the heating medium:
1 – heating in the device with sand shutter; 2 – in the atmosphere of a chamber furnace.

The oxidation of copper and brass was investigated by annealing in a device with a sand gate in the atmosphere of the products of interaction of charcoal with air in it. Samples were made of 1.5 mm thick M2 copper sheet and L62 brass wire with a diameter of 6 mm. The samples were determined by the gain per unit area, depending on the temperature, the heating medium and the exposure time (Fig. 6). The weight gain per unit area increases significantly with an increase in the annealing temperature when heated in an air atmosphere, and the post remains unchanged during annealing in the fixture. After annealing in an air atmosphere of 15 min at 300–400 ° C, scale forms on the surface of copper and brass samples, and when annealed in a device at temperatures up to 600 ° C and exposure to 2.5 hours, no scale was detected, at 700 ° C faint color tint. Mechanical properties depending on the atmosphere did not change. It was found that annealing in the presence of aluminum in the device with (copper leads to a strong oxidation of copper.

Gain of samples of M2 copper and L62 brass per unit area, depending on the annealing temperature

Image6. The gain of samples of copper M2 and brass L62 per unit area, depending on the annealing temperature:
1 – M2, air heating; 2 – L62, heated in air; 3 – M2, heating in the fixture; 4 – L62, heating in the fixture.

Findings.

1. Liquid glass with the addition of talc, kaolin, chamotte, aluminum oxide, chromium and titanium is not an effective coating, especially for protecting steel from decarburization.
2. Glass powder – the best base for protective coating. To protect the steel from decarburization, the best protective properties and high processability are provided by a coating containing 40% chamotte clay, 50% glass powder and 10% talc.
3. Air nitrogen is a good protective environment against decarburization when using a sealed furnace and with appropriate selection of the charge of metal.
4. The products of interaction of charcoal and air, which are in adaptation with the sand gate, create a protective atmosphere against the oxidation of copper and copper alloys.

Bibliography:

V.I. MURAVYEV
ISSN 0026-0819. “Metallurgy and heat treatment of metals”, No. 12. 1968

1. Gavrilov P. D. “MITOM”, 1959 No. 2.
2. “Materials Protection”, 1963, v.2, No. 9; Materials in Design Engineering, 1963, v.57, No. 5.
3. Muravyov V.I., Tarnetsky B.A. “MITOM”, 1966, № 7.
4. Lenar E. “Industs Gas”, 1966, No. 9.
5. Kuboshevsky O., Hopkins B. Oxidation of metals and alloys. M., “Metallurgy”, 1966.