Minimizing Porosity in Rebabbitted BearingsFebruary 29, 2012
There are various ways to rebabbitt a bearing that has been in service previously, among them are centrifugal casting, static casting, and hand welding. Each one has advantages and disadvantages. The choice used depends, in part, on the material composition of the bearing backing material.
Some of the most difficult applications to rebabbitt are cast iron fan bearings that have water jackets. Essentially, these bearings have two walls to consider, an outside wall and an inside wall.
There are various concerns with these bearings.
– A critical one has to do with temperature differences between the inner and outer walls and the ductility / brittleness of the cast iron used. Brittle cast iron can crack easily, and almost any crack makes the bearing useless. It is difficult to weld repair such cast iron for long term reliability. More ductile types of cast iron can tolerate temperature differences between the walls, but even ductile cast iron can break and become useless when the temperature differences become too great such as can occur when the bearing is quenched in water cooling process which is part of a centrifugal casting process.
Because there is a very high probability of cracking a cast iron bearing during the water quenching process of centrifugal casting, either static casting or hand welding remain as viable options. Static casting requires the use of tightly sealed openings, which is difficult to do, and hand welding does not require any seal of the openings. Consequently, hand welding is often preferred.
– Another concern has to do with the preparation of the surface of the cast iron for babbitting. The objective is to obtain a suitably tinned coating for the Babbitt to adhere to. One issue is to remove all oils, dissolved gases, and any water that may exist in the pores of the cast iron at the time of babbitting. This is generally accomplished by heating the cast iron in an oven, raising the temperature at a controlled rate to a maximum temperature that depends on the type of cast iron and the condition of the housing. A temperature in the range of 400 degrees F (200 degrees C) and held for 1 hour per inch of wall thickness should be adequate to remove most of these foreign materials. A higher temperature has the risk of performing a stress relief which can cause the housing to change shape sufficiently to require remachining of some surfaces.
After such a heating process, it may be easiest to clean the bore by remachining it to remove heavy oxides or solidified hydrocarbons that have exuded from the pores of the cast iron. These are a very significant source of gasses and metallic and non-metallic particles that cause porosity in the Babbitt layer, no matter how the Babbitt is applied, centrifugal casting, static casting, or hand welding.
– Another issue with cast iron has to do with removing the carbon/ graphite that is included in the structure of the cast iron, and can be seen in the surface of the bore by looking at it with a magnifying glass. This can be difficult to do. Mass production facilities with excellent environmental control can use a chemical called Kolene, but this is a very hazardous chemical that cannot be used by most Babbitt shops in the US.
– Historically, there were no tinning compounds available, and the surface of the bore to be babbitted was grooved both axially and circumferentially to provide “mechanical Babbitt anchors”, and the Babbitt was forced into these dovetail style grooves in either of 3 ways: Making a plate of thick Babbitt and placing it over the surface and pounding it into the grooves, static casting, or hand welding.
– Today, there are various chemical tinning agents that are commercially available that can be used to prepare the surface, that is, to “tin” the surface.
After the surface is prepared and it is “tinned”, then the hand welding can be performed. TRI uses only tin-base Babbitt, such as the ASTM B23 Grades 2 or 3 Babbitt materials because tin-based Babbitt is more ductile than lead-based Babbitt, and tin is far safer to use than is tin.
There is a great deal of “art” in all forms of babbitting, and hand welding is no exception. The specific application affects the range of preheat temperatures to be used, the method of providing the localized heating, whether Tig or gas flame, and the size of the heat source. Personal preferences and the skill (not the years of experience) can be very influential in the choices used.
In summary, porosity occurs in almost all babbitted bearings, whether centrifugal casting or hand welding. The question is the degree of porosity. Porosity will be reduced if the following occurs:
(a) The parent material has no gas, water, or oil that can be vaporized to exude from the parent material.
(b) The parent material has no rust, or particulate matter that can be loosened while the Babbitt is being applied.
(c) There is a very good chemical cleaning of the surface to be babbitted.
(d) There is a very good tin coating that is applied to the base material.
(e) The Babbitt that is applied is very clean and contains no foreign material.
(f) When the Babbitt is applied, the pool of liquid Babbitt must extend into the prior layer so that there is complete adherence of new Babbitt to old, in other words, the new Babbitt cannot simply be dropped onto the surface with only partial adherence to the prior layer of Babbitt.
(g) When any porosity or foreign matter is discovered, it must be completely removed by digging it out prior to applying any Babbitt to fill in the void or to add another layer to make the overall Babbitt layer thicker.
(h) Adequate ultrasonic testing and adequate dye penetrant inspection techniques are used to assure that the Babbitt layers are sound from time to time during the babbitting process, as well as after the babbitting process is complete.
Please note that TRI has not been specific with every detail of the processes involved because TRI has developed proprietary processes that work for us and we do not wish to divulge them, because TRI is offering this information as a guide for others, particularly customers, to realize that there is a great deal of “art” in these processes that are dependent upon the skill level of the individuals and the organization doing the work, as well as to the specific applications at hand, and because TRI does not take any responsibility either for the completeness of this information or for the use of this information, in whole or in part, by others.