The role of bearing steel in bearings

Bearing steel is used for the manufacture of rolling bearings such as balls, rollers and sleeves. It can also be used to make precision measuring tools, cold die, machine screw and diesel oil pump precision parts.

Bearing steel is steel used to make balls, rollers and bearing rings. Bearings are subjected to extreme pressure and friction during operation, so bearing steels are required to have high and uniform hardness and wear resistance, as well as high elastic limits. The uniformity of chemical composition of bearing steel, the content and distribution of non-metallic inclusions, and the distribution of carbides are all very strict. Bearing steel is also called high carbon steel. The carbon content of Wc is about 1%, and the amount of strontium is Wcr. 0.5% - 1.65%.

Recently, as the requirements for high-speed rotation and miniaturization have become higher and higher, higher demands have been placed on bearing performance. In particular, the automotive industry has a very high demand for reducing the size of the car and reducing the fuel consumption. On the other hand, the electronics industry also requires very small bearings to conserve energy and when the electronic instrument is less vibrating.

Although the use of bearings is getting higher and higher, it is also required to have a longer service life than before. However, since the bearing balls occasionally peel off, it is very difficult to extend the bearing life. In order to meet the increasingly stringent requirements of users, it is necessary to improve the cleanliness of steel by reducing the inclusion of oxides and nitrides in the steel.

For cleaner steels, it is very difficult to analyze the relationship between fatigue life and steel cleanliness using the traditional ASTM method, so new inclusion determination methods are needed. This article will introduce the ultra-clean steel production technology for bearings and the new inclusion measurement technology developed by Kobe Steel Co., Ltd., Japan.

The first ，Ultra-clean steel production technology for bearings

1. Production of ultra-clean steel

Kobe Steel smelted and continuously cast high-carbon chrome steel for bearings at Kobe Steel Works and Kakogawa Plant. This steel has the same production process as ordinary steel except that it uses a more rigorous scouring process to minimize oxide and nitride inclusions.

The molten iron which is fully dephosphorized and desulfurized by hot metal pretreatment is decarburized in a converter. Converter slag is an oxidized slag. In order to reduce oxide inclusions, the slag is cut so that oxide inclusions do not flow into the ladle. During refining (heating, degassing) and continuous casting, the molten steel is shielded from the more stringent inert gas to prevent secondary oxidation in contact with air. In the refining process, since the alkalinity of the slag is very high, the molten steel is sufficiently stirred to cause the oxide inclusions (mainly aluminum oxide) to float into the slag, so that the steel becomes cleaner and has only a small amount of inclusions. At the time of continuous casting, the Kobe Plant used an intermediate tank heater to accelerate the rise of inclusions in the molten steel. The Kakogawa Plant used a larger intermediate tank.

To reduce nitride inclusions, it is necessary to minimize the amount of titanium in the steel. Titanium is mainly derived from iron alloys, especially ferrochrome used to adjust the chemical composition of steel. Typically, ferrochrome is added to the molten steel during the ladle refining process. Since the oxygen content in the molten steel is very low, it is very difficult to remove titanium in the form of titanium oxide (titanium dioxide). In Kobe Steel, ferrochrome is added and melted during converter blowing, and at this time the oxygen content is very high, so titanium reacts with oxygen to form titanium dioxide, which is then absorbed into the converter slag.

2, the performance of ultra-clean steel

In ultra-clean steel 52100, the oxygen content can be controlled to an average of 4.1 ppm, and the titanium content is 6.0 ppm on average. The fatigue life of the ultra-clean steel 52100 is higher than that of the steel for the comparative test. In addition, many inspection methods have been used to compare the cleanliness of ultra-clean steel with ordinary steel. By comparison, the cleanliness of ultra-clean steel is not much different from that of ordinary bearing steel. However, the fatigue life of ultra-clean steel is much longer than that of ordinary steel.

Second, the new method for the determination of inclusions in steel

Because ultra-clean steel has a very low oxygen content, the determination of oxygen content and inclusions using the traditional ASTM test method does not give a good correspondence with fatigue life. To this end, electron beam melting was tested to determine inclusions in clean steel. At Kobe Steel, researchers are trying to find a new way to measure cleanliness from the size and quantity of inclusions. Therefore, an acid dissolution method has been developed to detect small inclusions and detect the number of inclusions.

The acid dissolution method includes dissolving the material to be detected to separate the inclusions. The inclusions rich in aluminum oxide can be separated, but calcium oxide inclusions cannot be detected because they dissolve in the acid.

First, 30 to 100 g of the sample was dissolved in nitric acid and heated to 90 °C. The solution was filtered after cooling, and the residue in the filter was separated as an inclusion. The filter grid size is 10 mesh and inclusions larger than 10 mesh are left in the filter. The inclusions were observed by scanning electron microscopy and the inclusions were analyzed by electron probe microanalysis.

By using these methods, a large amount of inclusions can be detected and the influence of carbides can be avoided. By using an acid dissolution method, researchers can test samples with an oxygen content of less than 9 ppm for fatigue life testing. The test results show that the number of inclusions and the maximum size of inclusions in ultra-clean steel are less than and less than that of ordinary steel. When the oxygen content is less than 9 ppm, the acid dissolution method can also determine the difference in cleanliness between ultra-clean steel and ordinary steel.