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Fix vs. replace AISI 8630 gears

Q. Hi

I am a welding student at a community college in California (bay area). I have been given an assignment from my teacher and I was wondering if you have any information on AISI 8630 steel quenched from 1600 °F. in water and tempered at 800 °F. This steel is being used for gears from an amusement park' the gear's teeth keep on breaking and are wearing down badly, my question is whether it is cheaper to repair the gears by building up the broken tooth and hardfacing the teeth heads, then machining or is it better and cheaper to replace with new ones? Thank you very much for your time!

David Portillo
csm welding student - San Mateo, California, USA

A. Good afternoon, David:

If the gears show problems with teeth breaking and excessive wear there may be some basic material selection and heat treating issues that need to be addressed by the design engineers.

Steve Bizub
- St Louis, Missouri

A. Many companies and individuals employ a cryogenic treatment on gears to promote longer life. Gears typically experience high wear surfaces that are prone to failure due to excessive wear and/or metal fatigue. Cryogenic treatment is well suited for gears because it relieves residual stresses in the metal that lead to cracking (and ultimately to breakage). The failure is characterized as "metal fatigue", but it is really the propagation of stress lines that are inherent in the part. By first relieving the stresses with a cryogenic treatment, you greatly reduce or eliminate future failures caused by the resultant cracks that form from the stress lines.

Another challenge with gears is the that they are high wear parts and ideally are suited for a highly wear resistant steel, which is typically a high carbon steel. The problem is that such steels tend to lack the ductility that gear makers are looking for. As a result, many gears are made from lower carbon steels, which are more ductile, but less wear resistant. Consequently, gear makers utilize techniques like carburization to increase the carbon content on the "skin" or surface layer of the gear. This increases the amount of carbon in the steel on the outside edges of the gear, (the wear areas), while not compromising the ductility of the internal area of the gear. Cryogenic treatment is especially beneficial to parts that are so processed because it refines the way that the carbon atoms bond with the iron atoms in the carburization layer. This refinement promotes even further the wear resistance of the carburization process. So you can see that both mechanisms -- the residual stress relief AND the modification to the carbon matrix - promote extended life of the gear.

Robin Rhodes
Worcester, Massachusetts

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