Resistance to Fatigue Fracture of the Core of Cemented Parts under Various Heat Treatment Conditions
Abstract Generated abstract
This study examines how core microstructure affects the impact fatigue resistance of carburized 20Kh steel parts when fatigue cracks originate in regions without a carburized layer. Cylindrical specimens were heat treated to produce structures representative of different quenching and tempering regimes, then tested under one-sided bending impact fatigue in smooth and notched conditions. The results indicate that structurally free ferrite markedly reduces fatigue resistance: at comparable hardness, homogeneous structures showed a conditional fatigue limit about three times higher than structures containing free ferrite. Although the detrimental effect was somewhat reduced in notched specimens, the findings support specifying heat treatments for impact-cyclically loaded carburized parts that exclude structurally free ferrite from the core.
Full Text
RESISTANCE TO FATIGUE FAILURE OF THE CORE OF CARBURIZED PARTS UNDER VARIOUS HEAT-TREATMENT CONDITIONS
V. V. Morozova, G. V. Toporov
(Presented by the scientific seminar of the Department of Applied Mechanics)
In the heat treatment of parts made of carburizing steels, quenching is most often carried out from temperatures at which complete recrystallization of the core does not occur. This is dictated by the desire to avoid overheating of the carburized layer. Much less frequently, double quenching with an intermediate temper is used, in which complete recrystallization of both the carburized layer and the core takes place. In this case, in the structure of the carburized layer, excess carbides are arranged in the form of separate, dispersed fine inclusions, and the core has no structurally free ferrite [1].
When specifying a strengthening technology for parts subjected to cyclic impact loading, certain regions of the parts that experience the highest stresses are protected from carburizing, or the carburized layer is removed by machining before quenching. The basis for this is the very widespread view that the carburized layer has poorer resistance to cyclic impact loads than the core.
Leaving aside the question of the validity of this proposition, in the present work we set out to determine the influence of the core structure on the resistance to fatigue failure of carburized parts in which the fatigue crack originates in regions free of the carburized layer.
The study was carried out on 20Kh steel, which, as is known, is widely used for parts subjected to carburizing. The specimens had a cylindrical shape, with a diameter of 8 mm and a length of 80 mm.
In order to obtain structures corresponding to the structures of the cores of carburized products after various types of quenching, the specimens were subjected to the heat-treatment variants indicated in Table 1.
Fatigue tests were carried out on a testing machine designed by A. I. Lampsi and V. N. Polyubin according to a one-sided bending scheme.
The test results are presented in Fig. 1 in the form of limited-fatigue curves in the coordinates: impact energy—logarithm of the number of impacts to failure.
The tests showed that the presence of structurally free ferrite has a very unfavorable effect on fatigue resistance. In ...
Table 1
| Variant no. | Type of heat treatment | Vickers hardness | Structure |
|---|---|---|---|
| 1 | Quenching in oil from 750–760°, tempering at 200° | 315 | Tempered martensite + ferrite |
| 2 | Quenching in oil from 750–760°, tempering at 500° | 250 | Sorbite + ferrite |
| 3 | Quenching in oil from 850–860°, tempering at 460° | 310 | Troostite |
| 4 | Quenching in oil from 850–860°, tempering at 590° | 255 | Sorbite |
Fig. 1. Impact endurance curves of smooth specimens. 1–4 — specimens subjected to the corresponding heat-treatment variants (Table 1).
Fig. 2. Impact endurance curves of notched specimens. 1, 2 — specimens subjected to the corresponding heat-treatment variants (Table 1).
at the same hardness, specimens with a homogeneous structure have a conditional fatigue limit, expressed in unit-impact energy, three times higher than specimens in whose structure structurally free ferrite is present.
It is known [2] that the process of impact-fatigue failure is influenced by the presence of a notch in the specimens, which acts as a stress concentrator. We carried out impact-fatigue tests also on notched specimens treated according to variants 1 and 3. The diameter of the specimens at the notch was the same—8 mm. The notch was V-shaped; the notch depth was 0.25 mm, and the radius of curvature at the bottom of the notch was 0.1 mm.
The test results are given in Fig. 2. It is evident from the curves that, in notched specimens, the influence of the longevity of structurally free ferrite is somewhat less than in smooth specimens, but still sufficiently significant to indicate that it is inadmissible in the structure of parts subjected to impact-cyclic loading.
The investigations carried out show that, for carburized parts in which individual areas do not have a carburized layer and which experience impact cyclic loading, it is necessary to specify a subsequent heat-treatment technology that excludes the presence of structurally free ferrite in the core.
REFERENCES
- L. A. Shmykov. Handbook for the Heat Treater. Mashgiz, Moscow, 1961, p. 111.
- S. V. Tolkachnik. Izvestiya AN SSSR, OTN, No. 5, 1958.