Ever notice how responsive you are to temperature changes? Too hot or too cold and you almost have a different personality. Well, you guessed it. Cast iron responds to temperature, too.
Like all high carbon steels, cast iron has a critical temperature at which it can be hardened or annealed. For most grades of cast iron, that temperature is 1500°F.
Cast iron is heat treatable to a very high hardness, up to Rc 65. This hardening can make machining difficult, if not impossible. And remember that machining includes drilling and tapping. The harder cast iron gets, the less ductility and strength it has.
If the iron is cooled rapidly by quenching, or even by the surrounding iron drawing the heat down quickly, hardening will occur. If the casting is maintained at 1500°F for at least 30 minutes and cooled to 1200°F over another 30 minutes, the iron will be annealed and the effects of the hardening avoided.
IMPORTANT POINT: If the weld cracks, or the iron next to the weld cracks, the part was too cold when it was welded.
IMPORTANT POINT: If the cast iron next to the weld gets hard, the part cooled too quickly.
It really is as simple as that. And it will not be the casting's fault if it cracks or hardens, it will be the fault of the person controlling the temperature.
It is probably obvious to you by now that in order to properly stress relieve and anneal cast iron, you must be working at very high temperatures. Adding this knowledge to your previous review of expansion and contraction, we're ready to move on to how all of this is going to help you perform repairs that are predictable and reliable. The next section is Preheating and Welding Procedures.
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