Growth of austenitic grain in steel during induction heating. In hardened and tempered steel with a martensitic structure, the grain boundaries of austenite obtained by heating are retained . These boundaries in hardened steel can be identified by special etching methods . Austenite grain affects the strength and toughness of martensitic steel. In addition, the austenitic grain size significantly affects the hardenability of the material. Thus, the regulation of the grain size is a prerequisite for obtaining products with desired properties.
At present, at a number of factories, the gears of an average module are quenched with induction heating  – . In this case, it is necessary to have a fine grain to reduce hardenability (in order to obtain a viscous core) and to increase strength .
In the present work, the influence of induction heating modes and steel properties on the growth kinetics of austenitic grain was investigated. Austenitic grain was detected by etching hardened samples in a saturated aqueous solution of picric acid with the addition of 1% salts of synthetic fatty alcohols .
The method of induction heating allows to obtain austenite with a very fine initial grain, which is obtained when the α + γ transformation is completed . The initial grain in the pre-eutectoid steel is formed at the time of the disappearance of the ferrite, when austenitic grains converge inside the ferritic areas. Probably, the size of the ferritic areas determine the size of the initial austenite grain .
We investigated the dependence of the size of the initial austenite grain on the size of the ferritic areas in the initial structure during induction heating. It turned out that for steel with 0.5% C, when the average size of the ferritic area in the initial structure varies from 5 to 11 microns, the average diameter of the initial austenite grain increases from 6 to 11 microns, which corresponds to a change on the GOST scale from 12 to 10 points. Of great importance for induction heating is the dependence of the size of the initial grain on the heating rate for quenching.
The dependence of the initial austenite grain on the heating rate was studied on samples of steel 40. The samples were heated for quenching (in the area of phase transformations) at a speed of 8, 200 and 1000 degrees / sec (heating conditions up to 300 degrees / sec are used under production conditions). For comparison, we determined the size of the initial austenite grain for the same steel with fast (2 deg / sec) and slow (0.03 deg / sec) furnace heating. Rapid furnace heating was carried out in a furnace heated to 950 ° C. Samples with a thickness of 1.2 mm were quenched from temperatures close to point Ac3.
For different heating rates, the grain size was determined on samples quenched from the lowest temperature at which ferrite was absent in the structure, that is, the initial austenite grain size was established. The results are shown in Table. one.
|Heating method||Heating rate in degrees / sec||Temperature in °||Average area of austenitic grain in microns|
In the case of induction heating, the average size of the initial austenitic grain formed at heating rates of 8, 200, 100 degrees / s and temperatures of 830, 870, and 900 ° C, respectively, turned out to be the same. Thus, the value of the initial grain is almost independent of the rate of induction heating (with an increase in the heating rate from 8 to 1000 degrees / second).
With rapid furnace heating (2 deg / sec) in the region of phase transformations, the value of the initial grain is close to the value of the initial grain during induction heating. Slow furnace heating results in larger austenite grains, which is associated with a prolonged (about 1 h) sample being in the temperature range of phase transformations.
The study showed that the initial austenitic grain with induction heating for normalized carbon steel with a carbon content of 0.35-0.65% corresponds to a score of 10 or higher. In fig. 1 shows the grain growth curves of steel with an increase in the induction heating temperature at various heating rates.
Image1. The actual grain of austenite steel 40, depending on the temperature and speed of induction heating:
1 – 8 deg / s; 2 – 200 degrees / second; 3 – 1000 degrees / second.
If the size of the initial grain during induction heating does not depend on the heating rate, then the further growth of the already formed grain substantially depends on the heating rate. This is due to the combined effects of time and temperature. At the same heating temperature, a lower heating rate leads to a large grain size, which is associated with a significant grain growth rate in the initial period of isothermal aging .
Thus, with induction heating, a significant growth of austenitic grain is possible if the processing technology requires a sufficiently long exposure at temperatures above Ac3. In order to avoid obtaining coarse grains during induction heating, hereditarily fine-grained steels are used .
The kinetics of growth of austenitic grain during induction heating of hereditarily fine-grained steels has specific features.
The chemical composition of the studied melts of fine-grained steel, differing in the size of stable austenitic grain, is given in table. 2. The change in the size of austenitic grain for three melts of fine-grained steel during induction heating at a speed of 3 deg / s is shown in img. 2
|№ heat||The size of austenitic grain, stable when furnace heating, in microns||Chemical composition in%|
Melting differ in the nature of the temperature dependence of the grain size. In curves 1 and 2, three stages of change in the grain size with increasing temperature can be noted. At the first stage, the formed initial grain increases to a certain stable value. In those melts of fine-grained steel, in which the size of a stable grain coincides with the initial, the first stage is absent (curve 3, img. 2).
Image2. The actual grain of austenite is hereditarily fine-grained steels, depending on the temperature of induction heating (heating rate 3 deg / s):
1 – melting 1; 2 – melting 2; 3 – melting 3 (table 2).
Grain growth from the initial size to sustainable is a feature of the grain growth curve of fine-grained steels with temperature during induction heating. Under normal modes of furnace heating of fine-grained steel, stable grain is formed and the first stage, grain growth is not observed. At the second stage, the grain does not change with increasing temperature. The size of a stable grain during induction heating is different for different melts of fine-grained steel and coincides with the size of a stable grain during furnace heating (see Table 2). At the third stage, further growth of grain is observed. With an increase in the rate of induction heating of fine-grained steels, the beginning and end of the second stage of the grain occurs at higher temperatures.
It should be noted that the reasons for the stabilization of grain in hereditarily fine-grained steels are not well understood. In , , it was stated that obstacles to grain growth arise directly in the process of the α → γ transformation. The hypothesis of “barriers”  is becoming more widespread, according to which in normalized hereditarily fine-grained steel there are already secretions limiting the growth of crystallites when heated. The growth of grain from initial to steady confirms the hypothesis of “barriers” and cannot be explained by the theory of the formation of obstacles to the growth of grain in the process of α → γ transformation.
From consideration of the growth kinetics of austenitic grains of fine-grained steels during induction heating, it can be concluded that the use of such steels makes it possible to obtain fine grains at different temperatures and heating rates. Under temperature-time heating conditions, under which the first and second stages of grain growth are observed, small austenite crystallites are obtained (8–11th point).
The effect of grain size on hardenability. In fig. 3 for one melting of fine-grained steel, the dependence of the grain size and the depth of the hardened layer on the temperature of induction heating at a speed of 3 degrees / sec.
Image3. The actual austenitic grain and the hardenability of hereditarily fine-grained steel depending on the temperature of induction heating (smelting 2, Table 2); heating rate of 3 degrees / sec):
The nature of change in hardenability of hereditarily fine-grained steel with increasing temperature of induction heating is similar to the change in grain size: hardenability increases with temperature as the grain grows from initial to stable, stabilizes in the temperature range of stable grain and continues to increase with temperature in the third stage.
A change in the grain size may, to a greater or lesser degree, affect the hardenability depending on the heat. Figure 4 shows the dependence of hardenability on the specific surface of austenitic grains for three melts of fine-grained steel.
Image4. The ideal critical diameter depending on the specific surface area of austenite grains of hereditarily fine-grained steels:
1 – melting 4; 2 – melting 1; 3 – melting 2 (table 2).
The hardenability was determined by the hardness distribution over the section of a hardened cylinder with a diameter of 15 mm and was converted to the ideal critical diameter.
With a grain size of 7-11 points, a linear relationship is observed between the ideal critical diameter and the specific surface area of austenitic grains. With decreasing grain size, hardenability decreases. The angle of inclination of a straight line characterizes the effect of a unit specific surface area on hardenability for a given steel smelting. Each fusion is characterized by its own angle of inclination of this straight line. This, apparently, is associated with different dispersion and the amount of precipitates, as well as differences in the chemical composition of individual melts of steel.
The effect of grain size on the static bending strength of hardened steel. The effect of grain size on static flexural strength was investigated on 55PP steel . The test was subjected to plate size 3 × 12 × 80 mm Samples were heated to different temperatures (heating rate of 3 degrees / s) and cooled with a water shower. In all cases, the small thickness of the samples ensured through quenching. After this treatment, the samples were released at 150 ° C. The test results are shown in Table. 3
From tab. 3 shows that an increase in the grain area by 2 times leads to a drop in strength by 30%. These results correspond to the data obtained in  for tool steels.
|Heating temperature for quenching, ° C (heating rate 3 deg / s)||Austenitic Grain Size||σизг, кг/мм2|
1. The minimum grain size during induction heating is determined by the initial austenitic grain, the value of which depends on the dispersity of the initial structure.
2. The speed of induction heating at 8-1000 degrees / sec does not affect the size of the initial grain.
Further growth of austenite crystallites with increasing temperature significantly depends on the heating rate: low speeds and high temperatures of induction heating can lead to significant grain growth.
3. The use of hereditarily fine-grained steels makes it possible to expand the range of temperatures and rates of induction heating, at which fine austenitic grain can be obtained.
The decrease in the actual austenitic grain from 8 to 12 points leads to a decrease in hardenability and a significant increase in the strength of steel having a martensitic structure. At the same time, fine austenitic grains and the corresponding properties of hardened steel can be obtained at both low and high heating rates.
4. The use of fine-grained steels and the use of optimal modes of induction heating is a significant reserve for increasing the strength of machine parts.
V.D. ZELENOVA, G. A. OSTROVSKY, K. 3. SHEPELIAKOVSKY
ISSN 0026-0819. “Metallurgy and heat treatment of metals”, № 6. 1963
1. Arkharov V.I. Crystallography of hardened steel. Metallurgizdat, 1951.
2. Geller Yu. A. Instrumental steels. Metallurgizdat, 1961.
3. Shepelyakovsky K. 3. Surface hardening during induction heating of heavily loaded machine parts from
steel reduced hardenability. TsITEIN edition, No. M-61-458 / 17, 1961.
4. Natanzon E. I. “Automotive Industry”, 1962, No. 8.
5. Shepelyakovsky K. 3. “Automotive industry”, 1962, № 10.
6. Shepelyakovsky K. 3. “MITOM”, 1962, No. 2.
7. Kidin I. N. Heat treatment of steel during induction heating. Metallurgizdat, 1950.
8. Grossman, M. A. Fundamentals of Heat Treatment, GNTI, 1946.
9. Hanemann H., Schra d e r A. Atlas Metaillograplhicus, 1936.
10. Kamenetskaya, DS, Piletskaya, IB, Problems of Metal Science and Metal Physics. Issue 4. Metallurgizdat, 1955.
11. Dorn I., Harder O. „TASM”, 1938, v. 26
12. Woodjine, V., Gaurra11, A., Journal of the Iron and Steel Institute, 1960, v. 196.
13. Abramov, V.P., “Plant Laboratory,” 1962, No. 5.
This article was taken from this resource.