Proceedings of the Tomsk Order of the October Revolution and Order of the Red Banner of Labor Polytechnic Institute named after S. M. Kirov

Vol. 224　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1976

DEPENDENCE OF THE WEAR RESISTANCE OF TOOL STEELS ON THEIR STRUCTURAL STATE

G. V. Simonov, A. I. Slosman

(Presented by the scientific seminar of the Department of Applied Mechanics)

Tests of steels in the forged state, carried out under conditions that almost completely prevented heating and performed by a number of authors [1, 2, 3], showed that in this case wear resistance is almost independent of alloying. Steels with a high carbon content but with different contents of alloying elements, if they are properly quenched and tempered to the same high hardness (63–64 RC), have almost identical wear resistance. Cutting tests of steels of low hardness, conducted at a reduced speed [4] (without significant heating), also agree with these conclusions.

The aim of the present investigations was to determine the influence of the chemical composition and hardness of various grades of tool steels on their resistance to abrasive wear as a function of structural state (cast and forged).

The following steels were investigated: carbon steels of low hardenability—U7, U10; alloyed steels of increased hardenability—9KhS, ShKh15; high-speed steels—R18, R9, R4; and steels of high hardenability—Kh12F and Kh12M.

Specimens of steels in the forged state were quenched according to regimes standard for the given steel grade and tempered to obtain the required hardness.

To obtain specimens with a cast structural state, blanks were cast in a stationary metal mold. The size of the cast blanks was \(120 \times 25 \times 6\) mm.

The steel was melted in a high-frequency melting-and-quenching unit of the A3-43 type, in an acid crucible under a layer of slag. Fine scrap from tool-production steels served as the charge for remelting. Burn-off of alloying elements was compensated by introducing the corresponding ferroalloys into the crucible.

The chemical composition of the steels investigated in the forged and cast states is presented in Table 1.

As can be seen from the table, for all steel grades there is no noticeable difference in the content of elements in the cast and forged states.

Tests of steels for abrasive wear were carried out in the friction laboratory of the Siberian Physicotechnical Institute on G. I. Kiselev’s apparatus [5]. The abrasive material was a flat grinding wheel of grade K340 SM2K. The design of the apparatus made it possible to ...

Table 1

during rotation of the grinding wheel, to move the specimen being tested along its radius, which eliminated the influence of wear products. The path of the specimen was always constant and was a spiral 6.5 m long. Some clogging of the wheel was eliminated by periodic cleaning. The abrasive capacity of the wheel after cleaning was monitored by a standard. Since the abrasive capacity of the wheel varied somewhat, a correction coefficient was introduced, which in all cases had values close to unity.

Test conditions:

It may be assumed that the adopted test conditions do not lead to noticeable heating of the specimens under study, even in their surface layers.

Specimens with a cross section of \(4.5 \times 4.5\) mm were subjected to testing. The normal load on the specimen always remained constant and equal to 3 kg. The amount of wear was determined by weighing the specimens before and after the experiment with an accuracy of up to 0.1 mg. Each point on the graphs is the average result of 5–8 experiments. Deviations of the wear values from the mean did not exceed \(\pm 5\%\). The hardness of the specimens under study was established within the range 55–63 RC.

Figures 1 and 2 present the graphical dependence of abrasive wear of steels, constructed from experimental data as a function of chemical composition, hardness, and structural state.

From the curves in Fig. 1 it is evident that, under conditions of abrasive wear without heating, the chemical composition of steel in the forged state has an insignificant effect on the magnitude of wear resistance. However, with an increase in the amount of carbide phase (without changing its composition and structure), the wear resistance of the steel increases.

For steels in the cast state (Fig. 2), the indicated regularity is preserved only for the carbon steels U7 and U10 and the alloy steels 9KhS and ShKh15. High-speed steels R18, R9, R4 and high-chromium steels Kh12F and Kh12M, at the same hardness, have greater wear than the same steels in the forged state. This can be explained by the considerable heterogeneity of the cast structure of high-alloy steels. Despite the fact that, when cast in metal molds, the brittle eutectic is present in the form of fine precipitates and even in some cases does not completely surround the steel grain, it adversely affects the mechanical characteristics of the blank.

Fig. 1. Abrasive wear of steels in the forged state

From the wear graphs for both the forged and the cast state, one can observe a certain change in the dependence of wear resistance on the magnitude of hardness in the range 58–60 \(RC\). The wear resistance of steel having a hardness above 60 \(RC\) increases less intensively for each unit of hardness than in the range 55–60 \(RC\). The methodology of the studies carried out does not make it possible to judge fully the causes of this phenomenon, since the question of the independent role in the wear process of the individual structural constituents of steel has as yet been little studied.

However, it may be assumed that in the case of maximum hardness of the specimen, when the hardness of the metallic matrix is close to or equal to the hardness of the carbide phase, under abrasive wear—manifested in local plastic deformation, microscratching, and microcutting of the friction surface by abrasive particles—wear occurs mainly as a process of predominant dispersion of very small metal particles without chipping-out of large carbides. When the hardness of the specimen is reduced to 55–58 \(RC\), when martensite decomposition products can already be observed in the matrix phase and the bonds between the matrix and the carbide phase are weakened, together with the type of wear indicated above,

Fig. 2. Abrasive wear of steels in the cast state

possible detachment of larger particles (chipping out of carbide grains), which also leads to more intensive wear of the specimen.

It is also known that wear resistance depends most sharply on those changes in structure that cause a decrease in hardness. Thus, tempering of hypereutectoid steel Kh at 350° [1], which produced a troostite structure and reduced the hardness to 47 RC, lowered the wear resistance by approximately a factor of 5 in comparison with the wear resistance of the same steel tempered at 150° and having a martensitic structure and a hardness of 62–63 RC. At the same time, tempering this steel to 200°, which reduced the hardness to 60 RC, decreased the wear resistance by only 25–30%. The same is also characteristic of high-speed steels. Heating above 600°, which caused a decrease in hardness as a result of martensite decomposition and carbide coagulation, substantially lowered the wear resistance. That is, a decrease in the hardness of steel without a change in its structural state affects wear resistance less significantly than when structural transformations are present (martensite decomposition).

Conclusions

1. The conclusions are confirmed that the chemical composition of steel in the forged state has little effect on the magnitude of abrasive wear carried out without heating.

1. In the cast state, high-alloy steels, at the same hardness, have lower wear resistance than carbon and alloy steels.

2. The wear resistance of steel having a hardness above 60 \(RC\) increases, per unit increase, less significantly than at lower hardness.

REFERENCES

1. Yu. A. Geller. Tool Steels. Metallurgizdat, Moscow, 1961.

2. M. M. Khrushchev, M. A. Babichev. Doklady AN SSSR, vol. XXXVIII, No. 3, p. 176, 1953.

3. A. P. Gulyaev. Theory of high-speed steel. Stanki i instrument, No. 3, Mashgiz, Moscow, 1946.

4. N. P. Petrov. Stanki i instrument, No. 13, p. 34, Mashgiz, Moscow, 1937.

5. G. I. Kiselev. Trudy SFTI, issue 26, p. 32, Tomsk, 1947.