EFFECT OF THE DECARBURIZED LAYER ON THE RESISTANCE OF STEEL 45 SPECIMENS UNDER REPEATED IMPACT

G. V. Toporov, G. Ya. Smokotin

Some parts of impact-action machines and tools have surfaces which, after heat treatment, are not subjected to mechanical machining (pick points of pneumatic hammers, drill bits, teeth for cutting machines and coal combines, etc.). As a result of heating during heat treatment, the surface layers of these parts may become decarburized. Under practical conditions, especially with repeated heating, the depth of the decarburized layer may be quite considerable, reaching 0.5–0.8 mm and more.

The adverse effect of surface decarburization on the wear resistance and strength of parts, especially cutting tools, is known in some cases [1, 2]. Ya. B. Fridman [3–5] believes that, for parts operating primarily under static and single-impact loads, the intentional creation of a decarburized (soft) surface layer is accompanied by an increase in the strength of the parts. S. V. Serensen [6] established that a decarburized layer increases the resistance to fracture under static fatigue of steel (bending with rotation of the specimen), if the number of cycles is small.

In work [7], the influence of a decarburized layer (depth about 0.15 mm) on the endurance of specimens of normalized Steel 45 under repeated impact was studied. Specimens similar in shape to those for static-fatigue testing, with a diameter in the working section of 8 mm, were used. The results of the work made it possible to conclude that the effectiveness of decarburization is manifested to a greater extent when operating at a small number of impacts with high single-impact energies. Thus, at an impact energy of 13.5 kg·cm, the decarburized and non-decarburized specimens withstood, respectively, 58 and 46 thousand impacts (Fig. 1).

The literature contains no data for a comparative evaluation of the influence of a decarburized layer on the impact-cyclic resistance of specimens made of steel 45 (and other steels) after quenching and tempering. The principal cause of failure of many parts of impact machines is fracture due to impact fatigue [8—10]. Therefore, naturally, it becomes necessary to study the influence of decarburization on the tendency toward impact-fatigue fracture of specimens heat-treated to high hardness.

Fig. 1. Endurance of specimens of normalized steel 45 under repeated alternating impact:
1—specimens decarburized; 2—nondecarburized [7].

In the present work, experimental data are presented that were obtained in tests of steel 45 specimens with and without a notch, both nondecarburized and decarburized to various depths.

Test procedure

All specimens were made of steel 45 of the following chemical composition: 0.45% C; 0.18% Si; 0.50% Mn. To determine impact toughness, standard specimens measuring \(10 \times 10 \times 55\) mm with a Mesnager notch, 2 mm deep and with a notch-root radius of 1 mm, were prepared. The notch of the specimens of one series was made according to the standard, i.e., after machining and all heat treatments. The specimens of the second series were notched before decarburization and subsequent heat treatment. The impact toughness was determined on a GZIP pendulum impact tester.

For tests under repeated impact, cylindrical specimens 10 mm in diameter with and without a notch were made. The circumferential notch, 0.15 mm deep, with an angle of \(120^\circ\) and a radi...

with a notch-root radius of 0.1 mm was made on a thread-grinding machine after machining and all heat treatments. Some of the specimens were subjected to decarburization with the notch present.

Decarburization of specimens, both for determining impact toughness and for impact-fatigue tests with and without a notch, was carried out simultaneously in a muffle electric furnace at a temperature of 900°. Different depths of the decarburized layer of the specimens were obtained by holding them at the specified temperature for 2, 5, and 10 h. The decarburized layer thus obtained was, respectively, 0.15, 0.25, and 0.55 mm. After decarburization the specimens were cooled in air.

Next, the decarburized and non-decarburized specimens were subjected to hardening and tempering. Hardening was carried out from 820° in water. Tempering was carried out at temperatures of 200, 250, 300, and 400° with a holding time of 30 min. At each given temperature 14–18 differently treated specimens were tempered: 1) with different depths of the decarburized layer, 2) without a decarburized layer (normal), 3) notched and smooth. After hardening and tempering, the normal specimens without a notch were ground to the final size. Specimens decarburized to different depths, with and without a notch, were tested without grinding, i.e., with a black surface. Scale was removed manually with emery paper.

Tests of specimens under repeated impact were carried out by the method described earlier [9, 10], on an impact-fatigue machine designed by Prof. A. I. Lampsi and Eng. V. N. Polyubin. The energies of a single impact were: for smooth specimens, 191.2 kg·cm (40)¹; for notched specimens, 47 kg·cm (18)¹. This method is not intended to determine impact fatigue limits. The characteristic of resistance is taken to be the number of impacts to fracture of the specimens. Statistical processing of the test results shows that the scatter of the data lies within ±7–11%.

Test results and their discussion

The results of tests of notched specimens for impact toughness and of smooth specimens for resistance under repeated impact are given in the table. The hardness numbers in this table refer to non-decarburized specimens. Whereas the impact-toughness values of normal specimens in the tempering-temperature range 200–300° remain practically the same, the number of repeated impacts changes sharply. Specimens hardened and tempered at 200–250° were not fractured by the application of 600 thousand impacts. Specimens after tempering at a temperature of 300° fractured in re-

¹ G. Ya. Smokotin and V. I. Kopytov, On calculating impact energy, Izvestiya vuzov MVO SSSR, Mining Journal, No. 4, 1959.

... as a result of 20 thousand impacts. Thus, even at a high hardness of the normal specimens (from \(H_{R_c}=52\) to \(H_{R_c}=47\)), their endurance decreases by more than 30 times.

The presence of a minimal (in our experiments, 0.15 mm) decarburized layer lowers the impact-toughness value of Steel 45. With an increase in the depth of the decarburized layer (to 0.55 mm), the impact toughness increases. A particularly significant increase in impact toughness was established for specimens quenched and tempered at 400° (more than twofold).

The impact-cyclic endurance of specimens in the presence of a decarburized layer of 0.15 mm decreases sharply (by more than 20 times) in comparison with non-decarburized specimens. However, specimens tempered at 300° showed a certain increase in endurance under repeated impact. This latter fact requires further verification.

Increasing the depth of the decarburized layer to 0.25 mm for all the tested specimens causes a further decrease in their endurance. However, a greater decrease in endurance is observed for specimens whose core hardness is higher. The endurance of specimens tempered at 400° remains low and practically constant, whereas for specimens tempered at 200–300° and with a layer depth of 0.55 mm, a certain increase in endurance is observed. Nevertheless, the endurance of all decarburized specimens remains at a level more than 30 times lower than the endurance of non-decarburized specimens.

The sharp decrease in the impact-cyclic endurance of smooth decarburized specimens in comparison with non-decarburized ones can apparently be explained by taking into account the combined influence of the soft layer and the quality of machining of the specimen surface.

The initiation of an impact-fatigue crack is facilitated by roughness

Fig. 2. Fractures of smooth specimens decarburized to a depth of:
a—0.15; b—0.55 mm. (Quenching from 820° in water, tempering at 200°). ×3.

Fig. 3. Number of blows to fracture of notched specimens as a function of the tempering temperature of Steel 45; specimens decarburized to a depth of:
1—0.15; 2—0.25; 3—0.55 mm; 4—not decarburized.

Vertical axis: number of blows to fracture, \(N \cdot 10^3\).
Horizontal axis: tempering temperature, °C.

toughness resulting from oxidation of the specimen surface. The shear nature of the propagation of fatigue cracks along slip planes also has favorable conditions in the presence of a decarburized (ferritic) layer.

Photographs of the fracture surfaces of specimens (Fig. 2) illustrate the stated point of view. These fractures were obtained in testing smooth decarburized specimens, quenched and tempered at 200°. The fractures are brittle. However, with increasing depth of the decarburized layer, the site of initiation and development of the fatigue crack increases (in the lower photograph).

The results of repeated-impact tests of notched specimens are presented graphically in Figs. 3 and 4. The endurance level of these specimens without a decarburized layer (Fig. 3) in the tempering-temperature range of 200–300° is higher than that of decarburized specimens. Both increasing the depth of the decarburized layer and raising the tempering temperature after quenching increase the endurance of notched specimens.

The notch eliminates the influence of the condition of the specimen surface (surface finish) on the process of fatigue-crack initiation. The crack initiates in the notch. As already indicated, the decarburized layer favors the development of the crack that has arisen. With increasing depth of the decarburized layer, the state of the surface in the notch is characterized by higher ductility in comparison with the core of the specimen. The low impact energy causes slow propagation of the fatigue crack, and fracture of a specimen with a deeper soft layer occurs later, i.e., the endurance increases.

Fig. 4. Number of impacts to fracture of notched specimens as a function of the depth of decarburization and the tempering temperature.

Figure axis labels: vertical — number of impacts to fracture, \(N \cdot 10^3\); horizontal — depth of the decarburized layer, mm. Curves are marked \(200^\circ\), \(250^\circ\), \(300^\circ\), and \(400^\circ\).

Figure 5 presents the fracture surfaces of decarburized notched specimens, quenched and tempered at 200°. If, at a decarburization depth of 0.15 mm, the specimen fractured in a brittle manner, then at a depth of 0.25 mm the number of impacts to frac-

increased almost threefold, and a noticeable gray zone of fatigue appeared. With a layer depth of 0.55 mm, the zone of fatigue-crack propagation amounts to about 15% of the specimen cross-sectional area, and the number of impacts is five times greater than the number of impacts before the appearance of a crack in specimens decarburized to a depth of 0.15 mm.

In our work [10] it was noted that the transition of steel from the plastic state to the brittle state is accompanied by an increase in the sensitivity of this steel to a notch. Analysis of the curves (Figs. 3 and 4) permits the conclusion that the presence of a decarburized layer in the notch reduces the notch sensitivity of specimens of steel 45, especially those treated to high hardness \((H_{R_c} = 56)\), without increasing their endurance as compared with that of non-decarburized specimens.

Fig. 5. Fractures of notched specimens decarburized to depths: \(a\)—0.15; \(b\)—0.25; \(c\)—0.55 mm. (Quenching from 820° in water, tempering at 200°). ×3.

The endurance of decarburized notched specimens will exceed the endurance of non-decarburized ones when the notch sensitivity of the steel is substantially weakened. This latter case corresponds to a layer depth of 0.55 mm and a tempering temperature of 400° (curve 3, Fig. 3), i.e., in the range of a considerable decrease in the endurance of specimens without a notch.

CONCLUSIONS

1. The impact-cyclic endurance of quenched and tempered unnotched specimens of steel 45, in the presence of a decarburized layer from 0.15 to 0.55 mm, decreases sharply (by a factor of 20 to 30 and more).

2. The presence of a decarburized layer in the notch does not lead to an increase in the impact-cyclic endurance of notched specimens.

3. The values of the impact toughness of steel 45, determined on decarburized and non-decarburized specimens with a Menage notch after quenching and tempering in the temperature range 200–300°, cannot characterize the sensitivity of this steel to a notch.

1. An attempt has been made to explain the reduction in endurance of decarburized specimens under loading by repeated bending impacts, taking into account the combined influence of the soft surface layer and the quality of machining of the specimen surface.

2. In order to increase the impact-cyclic endurance of parts made of Steel 45 and heat-treated to high hardness \((H_{Rc} = 52—56)\), these parts must be subjected to machining to remove the decarburized layer.

REFERENCES

1. A. V. Smirnov and L. V. Beloruchev. Oxidation and decarburization of steel in heat-treatment processes. ONTI, 1934.

2. A. V. Smirnov. The significance of oxidation and decarburization of steel in industry. Correspondence courses, lecture 19, edited by Prof. N. A. Minkevich, GONTI, 1939.

3. Ya. B. Fridman. Deformation and fracture of metals under static and impact loads. Oborongiz, 1946.

4. Ya. B. Fridman and L. M. Pevzner. On the influence of soft surface layers on the mechanical properties of notched specimens. Factory Laboratory, No. 9, 1950.

5. Ya. B. Fridman. Mechanical Properties of Metals. 2nd ed., Oborongiz, 1952.

6. S. V. Serensen. Fatigue of Metals. Correspondence courses of VNITOMASH, 7, Mashgiz, 1949.

7. A. L. Tseimakh. The influence of surface condition on the repeated-dynamic endurance of steel. Publishing House of the Altai Territorial Administration of the Scientific-Technical Society of the Machine-Building Industry. Barnaul, 1956.

8. G. V. Toporov. Influence of texture and initial mechanical properties on the impact fatigue of 40KhN steel. Proceedings of SFTI at TSU, issue 34, Tomsk, 1955.

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