Fluid-bed quenching of tool steels is a process in which significant stresses are created that cause deformation and sometimes cracks, especially in tools of complex configuration.
Fluid bed quenching is a heat treatment in suspension of fine solid particles (quartz, sand, carborundum, graphite, etc.) in a stream of air or other gas; It is characterized by an exceptionally high heat transfer coefficient [400-500 W / (m2 • K)], since heat from the coolant is transferred to the product under conditions of intensive mixing of particles that are in direct contact with the surface of the product. Heating in a fluidized bed provides a high uniformity of temperature distribution in the volume of the product and is used for heat treatment of products with a simple configuration and a small section, for which the heating time depends only on the heat transfer coefficient;
Recently, a new cooling medium – fluidized bed [1-3], which has a high coefficient (heat exchange (200–1000 kcal / m2 · h · hail and more) and allows for infinitely variable control of the rate of heating and cooling) is used for heat treatment. of the advantages of the fluidized bed is the smaller deformation of parts during quenching than during quenching in other media [1-3]. However, there is no information on quenching in a fluidized bed of tools.
The purpose of this work was to test the fluidized bed as a quenching medium for tools made of steels R18 and X12M.
The scheme of the fluidized bed is shown in Fig. 1. As the material of the fluidized bed used electrocorundum No. 12 with a particle size of 0.12-0.16 mm. The regime of optimal fluidization of the layer: air speed of 0.22 m / s, air flow 350 l / min. The maximum coefficient of heat transfer layer at this fluidization regime was 405.5 kcal / m2 h · deg. To cool the fluidized bed, the walls of the bath have a jacket in which cold water circulates.
Image1. Fluidized bed circuit:
1 – fluidized bed; 2 – mesh basket; 3 – air distribution plate.
The cooling capacity of the layer in comparison with water, oil and molten saltpeter (at 300 ° C) was investigated on a cylindrical specimen of steel H18N9G1 (img. 2). The temperature in the center and on the surface of the sample was measured with a chromel-alumel thermocouple. On the side surface of the specimen, the thermocouple hot junction was stained.
Image2.Sample for research.
The temperature was recorded with an EPP-09-3M electronic potentiometer with a chart movement speed of 3600 mm / h. The sample was heated in a G-30 furnace with a protective atmosphere up to 1000 ° C.
From the sample cooling curves in various media (Img. 3), it can be seen that the intensity of cooling in a fluidized bed approaches the intensity of oil cooling.
Image3. Sample center cooling curves:
1 – in the water; 2 – in transformer oil; 3 – in nitrate (300 ° C); 4 – in a fluidized bed; 5 – in the air.
Temperature differences over the cross section of products that cause internal stresses that affect the deformation of the product, the formation of cracks, is a significant factor in the cooling of metals and alloys. Analysis of temperature differences across the cross section of a sample cooled in different media (Fig. 4) showed that when using a fluidized bed at all cooling temperatures, the difference is significantly less than when cooled in cold liquid media.
Image4. Temperature difference across the sample section:
a – depending on the temperature of the sample center; b – depending on the sample cooling time; 1 – cooling in water; 2 – in oil; 3 – in nitrate (300 ° C); 4 – in a fluidized bed; 5 – in the air.
The maximum temperature difference during cooling in liquid media is observed in the region of high temperatures, which is especially dangerous for products of complex configuration.
Thus, internal stresses during quenching in a fluidized bed should be less than when quenched in liquid media.
The quenching in a fluidized bed of steel P18 and X12M was carried out on samples with a diameter of 8 and a length of 150 mm. For comparison, samples (types of fluidized beds) of these steels were also quenched in air, in oil, and in molten saltpeter at 300 ° C. The samples were heated in lechi G-30 with a protective atmosphere. Hardness, microstructure and deformation were determined after quenching and after tempering. For the deformation of the samples took deflection in the middle part, which was measured by the indicator when installing the samples in the centers. The results of measurements of hardness and deformation are given in table. 1. From the data presented it can be seen that the hardness of the samples quenched in a fluidized bed does not differ from the hardness of the samples quenched in other media.
|Temperature in ° C||Cooling medium||HRC||Deformation in mm||Temperature in ° C||HRC|
|Р18||1270||Air||61-61,5||0,18-0,28||560 (three times)||61-62,5|
|Saltpeter at 300 ° C||59,5-61||0,60-0,70||62-63,0|
|Saltpeter at 300 ° C||45,5-47||0,05-0,12||61-61,5|
|Saltpeter at 300 ° C||62,5-63||0,08-0,15||62,5-63,5|
The maximum deformation of the specimens from R18 was observed during quenching in nitrate at 300 ° C, which is apparently due to the redistribution of carbon in austenite during its isothermal transformation into martensite. The samples, hardened in a fluidized bed and in air, have less deformation than after quenching in oil. Samples of steel H12M, hardened with 1120 and 1000 ° C, also have less deformation after cooling in a fluidized bed and in air than after quenching in oil.
After tempering the hardened samples, the deformation in all cases did not change. In the microstructure of quenched and tempered samples there was no difference.
Heating in a fluidized bed samples from steel R18, which were tested for red resistance after heating at 600, 625, 650 and 675 ° C for 4 hours. The test results are given in Table. 2, show that the red resistance of samples quenched in a fluidized bed, oil, and nitrate is the same, while samples quenched in air are lower.
|Cooling medium||Hardness after heat treatment HRC||HRC after heating for 4 hours at a temperature in ° C|
|Saltpeter at 300 ° C||62..63||60..61||58..59||57..58||46..48|
The hardenability tests for steels R18 and X12M were carried out on samples with a cross section of 25 × 25 mm and a length of 150 mm according to the Gudtsov method . Samples were calcined to their full length when quenched in oil and in a fluidized bed.
Testing quenching in a fluidized bed simultaneously with quenching in oil and saltpeter of various types of tools made of steel R18, P9, R18K5F2, X12M and X12F1 (drills, broach, cutters, taps, cutters, etc.) showed that the deformation of the tool hardened in the boiling layer , significantly less than after quenching in other media with the same hardness.
Instruments for quenching were heated in a G-30 furnace with a protective atmosphere and in a chlorine-barium bath. A fluid-hardened tool had a clean surface. When using a chlorine-barium bath, corundum particles sticking to the instrument surface were observed, which did not significantly affect the cooling process. The layer of particles with a thickness of 0.2-0.3 mm was easily removed by rubbing or sandblasting after washing the instrument from salt.
The operation of the tool, hardened in ordinary media and in a fluidized bed, showed the same performance.
1. The cooling capacity of the fluidized bed is close to the cooling capacity of the oil.
2. The temperature difference over the section of parts cooled in a fluidized bed and their deformation is much less than the use of ordinary liquid media.
3. The fluidized bed can be successfully used as a quenching medium for high-alloy tool steels.
V.P. Kurbatov, V.I. Muraviev
ISSN 0026-0819. “Metallurgy and heat treatment of metals”, No. 2. 1970
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4. Reference “Metal science and heat treatment.” M .: Metallurgizdat, 1956.