Page 86 of 561

SOLUTIONS: Cast Iron

areas during the welding process. If edge rounding is omitted, hard spots will be evident in the weld deposit. In preparing the casting by grinding, a certain amount of carbon (graphite) is removed from the metal and smeared over the surface to be welded by the grinding wheel. Before any welding can be done this carbon must be removed, as it will otherwise become

part of the weld pool and combine

with the iron to form a superhard zone of iron carbide in the weld.

Iron sparks

Grinding wheel

Graphite flakes in cast iron

Graphite smear

2.07

File after grinding

To avoid this, always remove a thin layer of the surface by a file after completing grinding. Use a steel brush to remove loos material.

Move torch rapidly along each side of the groove.

Searing

The more carbon you can remover from the weld area, the less chance there will be for hard spots in the weld. To further remove excess carbon embedded in the surface the workpiece can be seared, using an oxy-acetylene torch with an oxidising (excess oxygen) flame.

Use a lager blowpipe and move the inner cone of the flame forward over both sides of the welding groove. You will know immediately when the free carbon is removed. As the carbon burns it looks like a bright red particle or "star". Use a wire brush over the surface from time to time. Searing must not be confused with preheating, and is only intended to remove surface carbon. Take care not

This gives a cleaner surface.

to put too much heat into the material when executing it.

Regardless of what we do to remove surface graphite, some graphite will nevertheless be melted up and become part of the weld. Unitor cast iron electrodes are so made that they can take up 1% graphite without causing the formation of iron carbide. 85