EFFECT OF CARBIDE INHOMOGENEITY ON RESISTANCE TO IMPACT-FATIGUE FRACTURE OF HIGH-CHROMIUM DIE STEELS

V. V. Morozova, G. V. Toporov

(Presented by the scientific seminar of the Mechanical Faculty of the Tomsk Civil Engineering Institute)

It is known that ledeburitic high-chromium steels of the Kh12, Kh12F type are used for the manufacture of blanking dies; the structure of these steels is distinguished by the presence of a considerable quantity of carbides. Carbide inhomogeneity, by affecting the properties of these steels, determines to a significant degree the service qualities of products. It has been noted that the direction of the fibers relative to the applied load has a substantial effect on the strength of products made from the indicated steels.

During service, dies are subjected to impact cyclic loading. Consequently, their durability is determined by the resistance to impact-fatigue fracture of the steels from which they are made.

The absence of data on resistance to fracture under impact cyclic loading of these steels as a function of the degree of carbide inhomogeneity and the direction of the fibers does not make it possible to develop criteria for the suitability of forgings and rolled stock for the manufacture of dies. Reliance on data obtained under smooth (non-impact) cyclic loading may lead to substantial errors, owing to the difference in the behavior of the material at different rates of deformation caused by smooth and impact loading.

In investigating the effect of carbide inhomogeneity on resistance to impact-fatigue fracture, it was necessary to prepare specimens having the same chemical composition but differing in carbide inhomogeneity. Rolled stock 150 mm in diameter made of steel Kh12 was used for preparing the specimens. Rolled stock of such a diameter has nonuniform carbide inhomogeneity over the cross section. We cut the specimens in the direction of rolling from the surface layers, at a distance of 0.5 radius, and from the central part. The structure of the specimens is shown in Fig. 1.

To investigate the influence of fiber direction on resistance to impact fatigue, specimens were cut in such a way that the direction of the specimen axis was perpendicular to the direction of the fibers. Micrographs of these specimens are given in Fig. 2.

The carbide inhomogeneity of specimens cut from different locations can be characterized according to the GOST 5932-51 scale for high-speed steels as follows: specimen a (Fig. 1)—5 points, b—7 points, c—8 points.

Comparing the micrographs of longitudinal and transverse sections (Figs. 1, 2), we see that carbide inhomogeneity in the longitudinal sec-

... phases is more pronounced. Consequently, the carbide heterogeneity should be assessed on longitudinal polished sections.

Cylindrical specimens 8 mm in diameter and 80 mm long were tested.

The heat treatment and hardness of the specimens are given in Table 1.

Testing of the specimens for impact fatigue was carried out on a machine designed by Prof. A. I. Lampsi and Eng. V. N. Polyubin. The specimens, mounted on the supports of the testing machine, were subjected to a concentrated load according to a one-sided bending scheme at a frequency of 500 impacts per minute. The energy of a single impact was calculated by the formula proposed by L. T. Timoshuk [1]. The stresses arising in the specimens during impact tests were determined from the known formulas of the strength of materials. Yu. G. Bystrovykh [2] showed that, in this case, the stresses determined experimentally are in good agreement with the calculated ones.

Fig. 1. Carbide heterogeneity of Kh12 steel in rolled stock 150 mm in diameter (longitudinal polished sections), ×300; a — specimen cut from the edge; b — the same, at a distance of 0.5 radius; c — the same, but from the middle of the rolled stock.

Fig. 2. Carbide heterogeneity of Kh12 steel in rolled stock 150 mm in diameter (transverse polished sections), ×300; a — specimen cut at a distance of 0.5 radius; b — the same, but from the middle of the rolled stock.

As the characteristic for evaluating the test results, the value of limited endurance was adopted, obtained by testing 4–5 specimens at each load level. The results of testing the specimens, in the form of a graphical dependence of the logarithm of the number of impacts to fracture on the magnitude of the stress, are presented in Fig. 3.

From the results obtained it is evident that specimens of Kh12 steel after quenching and low tempering, with a transverse arrangement of the fiber, have significantly lower resistance to fracture under ...

under impact cyclic loading as compared with specimens with a longitudinal fiber orientation.

Specimens with a transverse fiber orientation and a high grade of carbide inhomogeneity have especially low resistance to impact-fatigue fracture.

Table 1

The results we obtained on the effect of carbide inhomogeneity on the resistance to impact-fatigue fracture of die steels are in qualitative agreement with data on the durability of die steels presented in [3].

Fig. 3. Plot of the relationship between cyclic life and stress:
1—longitudinal specimens cut from the edge of the rolled stock; 2—the same, at a distance of 0.5 radius; 3—the same, but from the middle part; 4—transverse specimens cut at a distance of 0.5 radius; 5—the same, but from the middle part.

It is known that in impact-fatigue tests of smooth specimens made of structural steels, the resistance to fracture is higher than under static cyclic loading.

For notched specimens the opposite relationship is observed: the resistance to fracture under impact-cyclic loading is lower than under smooth cyclic loading [4].

For die steels, taking into account their structure and tendency toward brittle fracture, it may be assumed that their resistance to impact-fatigue fracture will be no higher than under smooth loading.

The tests we performed on notched specimens of Kh12 steel under impact and smooth cyclic loading, the results of which are shown in Fig. 4, demonstrated that the resistance to fracture under impact cyclic loads is significantly lower than in the case of smooth cyclic loading.

Thus, the studies we carried out make it possible to draw several conclusions:

1. Carbide inhomogeneity in die steels has a substantial effect on resistance to impact-fatigue fracture. The data presented, obtained as a result of testing,

Fig. 4. Fatigue curves of notched specimens of Kh12 steel (tempering at 160–180°):
——— impact fatigue;
— — — ordinary fatigue

can be used for a quantitative assessment of the influence of carbide inhomogeneities on the service properties of steels in finished products—dies.

1. An unfavorable orientation of fibers in products made of die steels relative to the applied load can reduce resistance to impact-fatigue fracture by 30–35%.

2. The service qualities of die steels should be determined by testing for resistance to impact-fatigue fracture. The use of data obtained in tests with smooth application of loads may be allowed only in cases where the dies have a simple shape and no significant stress concentrators are present.

REFERENCES

1. L. G. Timoshchuk. Zavodskaya laboratoriya, No. 3, 1952.
2. Yu. G. Bystrov. Zavodskaya laboratoriya, No. 6, 1964.
3. Yu. A. Geller, A. E. Pavaras. Kuznechno-shtampovochnoe proizvodstvo, No. 6, 1963.
4. S. V. Tolkachnik. Izv. AN SSSR, Otdel tekhnicheskikh nauk, No. 5, 1958.