Heat before welding is a better approach as it prevents cracking due to pressure. As it is hard to control the heat, you must have sufficient experience in this regard.
When iron is heated to 500 to 1300 degrees Fahrenheit, its internal stress is reduced. This allows the iron to be under the heat zone so that the cast iron will not crack due to pressure.
Measure the temperature frequently using the thermometer, most probably an infrared thermometer. But if you lack it, you can also determine the temperature with a keen observation.
Step 1: The color of iron changes according to temperature and represents the dull red color on heating up to 900 Fahrenheit, but in sunlight, you will not be able to see this change. A room with minimum light would be the best approach in this regard.
Step 2: Don’t be greedy; complete the work by boosting the heat flame. You must show patience and weigh until the iron becomes brick red or bright red at a temperature above 1300 degrees Fahrenheit.
Step 3: Heating a small welding area could be better as the pressure will crack the iron cast. You need to heat the whole part if possible. Otherwise, heat the larger iron region to bear the pressure effectively.