Paste based on water glass and asbestos to prevent gas carburizing of diesel locomotive parts

Abstract

To reduce the cost and the labor of preparation and increase the heat resistance of the paste we have developed a new composition based on powdered asbestos. Before preparation of the paste the asbestos powder is dried at 500-550~ and then sifted through a screen. The paste is prepared from 1 part (by weight) of asbestos and 3 parts water glass (sp. gr. 1.351.40). The components of the paste (asbestos and water glass) are thoroughly mixed. The paste was applied on samples and on the teeth of gears made of steels 20KhN3A and 12KhN3A, which were carburized in the plant. The paste was also used to coat brackets and drive gears of the decelerator. Carburizing was conducted at 940-960~ instead of 900-920~ Metallographic inspection and measurements of the hardness were made after carburiztng. The hardness met the specifications. Subsequent machining showed that the paste prevents carburizing. The experimental paste has been tested on more than 2500 brackets and 400 gears. It should be noted that the paste has some drawbacks - a skin forms rapidly in the open air. The hardening of the paste is similar to that of sand mixtures with water glass, which is based on the fact that the silica in the water glass is precipitated in free form due to interaction with carbon dioxide (carbon dioxide from the air in this case), first in a colloidal suspension (sol), which then changes to a gel. Probably the magnesium oxide from asbestos interacts with carbon dioxide from the air. To protect the surface of the paste against the air it is kept in a container with a tight cover, sealed with a sheet of cellophane, or covered with water.