CAST BALL BEARING RINGS AND THEIR HEAT TREATMENT

E. P. Filimonova

(Presented by the scientific seminar of the departments of metal science, metal technology, and welding production)

Studies were carried out on the manufacture of small-size bearing rings by the centrifugal casting method without subsequent hot working by pressure.

The microstructure of cast steel was studied in order to select heat-treatment regimes that would make it possible to obtain properties of cast steel equivalent to those of forged steel.

By the centrifugal casting method, blanks were cast for the inner rings of type 208 bearings \((40 \times 80 \times 18)\). The blanks were obtained in the form of bushings. Three rings were made from each bushing.

The structure of cast ShKh15 steel has a carbide network and large excess carbides. The carbide network can be broken up by a normalizing operation. It is not always possible to eliminate large carbides by heat treatment alone. Since hot working by pressure was not applied by us to cast ShKh15 steel, it was necessary to obtain, in the cast structure, the minimum possible amount of excess carbides. For this purpose, the regularity of the change in the structure of steel obtained by various casting methods was studied.

The blanks were cast in the form of the above-mentioned bushings in sand molds and in stationary and rotating metal molds. The best results with respect to structure were obtained from castings made on a centrifugal machine with a horizontal axis of rotation at a rotational speed of \(n = 1400\) rpm, with the chill mold cooled by water after pouring the last portions of metal. In the structure of these castings there is a thin carbide network; large carbides are located in the zone situated closer to the inner side of the casting. These large carbides had to be broken up by special heat treatment.

The optimum structure of forged ShKh15 steel before quenching is fine-grained pearlite, which after quenching gives a structure of cryptocrystalline martensite with uniformly distributed fine secondary carbides. Such a structure ensures the best service durability of the bearing.

In cast steel after annealing it is also necessary to obtain a structure of fine-grained pearlite. To do this, by heating it is necessary to transfer the carbide network and large carbides into solid solution, equalize the composition of the solid solution, and during subsequent cooling prevent the carbide network and carbides from precipitating again. Various types of heat treatment were carried out to obtain, after quenching, a structure of cast steel equivalent to that of forged steel. Types of heat treat-

Table 1

treatments used for cast steel are given in Table 1. It is evident from the table that each type of complete heat treatment consists of definite kinds of separate heat-treatment operations. The heat treatment was carried out on identical specimens.

In ShKh15 steel, iron-chromium carbides dissolve completely in austenite when heated to 890–900°. As a result of segregation, carbides with a higher chromium content may form in the steel. Such carbides are the most stable and dissolve in austenite only at 1040–1070°. In cast ShKh15 steel, owing to segregation, carbides with a high chromium content are present. Heating cast steel to 1050–1100° is necessary for complete dissolution of the carbides.

For wrought steel, the bulk of the carbides dissolves when heated to 900°. Further heating causes accelerated growth of the austenite grain. When cast steel is heated, the process of carbide dissolution changes. In this steel, at a temperature of 900° there are carbides and a carbide network. Austenite grains grow slowly until the carbides and the carbide network have completely dissolved. Complete dissolution of the carbides occurs at a temperature of 1100°; after this, growth of the austenite grains is observed. Thus, cast steel with a coarse grain behaves during heating like a fine-grained steel.

As experience shows, the size of the austenite grain of cast steel decreases after homogenization. This occurs for the following reasons. When cast steel is heated slightly above the \(A_{c1}\) line, austenite grains are formed that are smaller than the original grains of the cast steel. With further increase in temperature, the growth of austenite grains is hindered by the presence of undissolved carbides to such an extent that they do not grow to the original size of the grains of the cast steel.

For complete dissolution of the carbides in our castings, a five-hour hold at a temperature of 1100° was required. During slow cooling after homogenization, a carbide network precipitates, coarser than that observed earlier in the cast steel. After homogenization followed by air cooling, a fine carbide network or fragments of a network are observed. To eliminate the carbide network completely, normalization was carried out at a temperature of 900° with a hold of 40 min. Subsequently the castings were annealed in a factory methodical furnace at a temperature of 780° with a two-hour hold. The structure and hardness of the cast steel after such a heat-treatment cycle are equivalent to those of wrought steel.

In another case, after homogenization the specimens were subjected to preliminary quenching from 950° in oil. Then low-temperature step annealing was carried out according to the following schedule: heating to 600°, holding at this temperature for 3.5 h, subsequent heating to 750° with a hold of 30 min. The duration of this annealing does not differ from the usual annealing used for this steel.

When hardened steel is heated to 600° and held for 3.5 h, a sorbite structure with a hardness of \(HB = 310\) is observed. Heating to 750° leads to coarsening of the sorbitic particles and a reduction of hardness after annealing to the required value.

When comparing the heat-treatment schedules for cast steel of types IV and V according to Table 1, the best results after annealing, in terms of carbide distribution and size, are obtained with type IV.

In ordinary annealing, carbide crystals grow depending on the cooling conditions. In this case it is not possible to vary the cooling conditions over a wide range, since the cooling rate is practically fixed. In the case of a hardened structure, it is possible over a wide

within certain limits, vary the conditions of transformation of martensite into troostite and sorbite. At different temperatures and holding times it is possible to obtain carbide crystals of different degrees of dispersion.

Finished cast rings treated according to the above cycle underwent quenching and tempering according to the regime used for forged steel.

Ball bearings were assembled with cast inner rings for endurance testing. The cast rings were subjected to type IV heat treatment (Table 1). Four bearings were tested. All of them completed the calculated service-life standard.

The procedure for endurance testing of bearings requires long periods of time. The absence of test stands under our conditions did not make it possible to carry out endurance tests on a large scale.

Conclusions

1. Rings produced by the centrifugal casting method, having a final structure of cryptocrystalline martensite with uniformly distributed carbides, can operate on a par with forged rings.

2. The use of cast bearing rings makes it possible to utilize waste from ShKh15 steel, saves thousands of tons of steel, relieves railway transport, and reduces the cost of bearing production.

References

1. Economy of material resources at the 1st State Bearing Plant named after L. M. Kaganovich. Collection of articles, published by the Gosnab sector of the USSR, 1952.

2. Ya. R. Rauzin, Heat Treatment of Chromium Steel. Mashgiz, 1955.