What is Preheat in Welding?

Preheat is the process of heating the parts of a structure to be welded before welding is started. The purpose of preheating is twofold: to minimize thermal shock and to slow the cooling rate.

Thermal shock is caused by the rapid cooling that occurs when the hot metal comes in contact with cold metal. When this happens, the hot metal contracts more than the cold metal, causing stresses that can lead to cracking.

Slowing the cooling rate helps to prevent cracking by allowing the weldment to cool more slowly and evenly. This gives the atoms in the metal time to rearrange themselves into a more stable configuration.

What is preheat temperature in welding?

Preheat is usually accomplished by heating the entire weldment with an electric arc welder, oxy-fuel torch, or induction heater. The preheat temperature depends on the type of metal being welded, the thickness of the metal, the welding process being used, and the ambient conditions.

In general, the thicker the metal, the higher the preheat temperature should be. For most steels, a preheat temperature of 100-200 degrees Fahrenheit (38-93 degrees Celsius) is sufficient.

Ambient conditions also play a role in preheat temperatures. If the air temperature is cold, the metal will cool more quickly, so a higher preheat temperature may be necessary.

When should a weld be preheated?

The answer to this question depends on several factors, including the type of metal being welded, the thickness of the metal, the welding process being used, and the ambient conditions.

What is the reason for preheating?

The purpose of preheat is twofold: to minimize thermal shock and to slow the cooling rate.

How is preheat typically accomplished?

Preheat is usually accomplished by heating the entire weldment with an electric arc welder, oxy-fuel torch, or induction heater.

Methods of preheating in welding

There are three common methods of preheating: electric arc welding, oxy-fuel torch welding, and induction heating.

Electric arc welding is the most common method of preheating. An electric arc welder uses an electrode to create an electric arc between the electrode and the metal. The heat from the arc melts the metal, and the molten metal is used to weld the two pieces of metal together.
Oxy-fuel torch welding is another common method of preheating. An oxy-fuel torch uses a mixture of oxygen and fuel, such as acetylene, to create a flame. The heat from the flame melts the metal, and the molten metal is used to weld the two pieces of metal together.
Induction heating is a less common method of preheating, but it is often used for large weldments or when precise control of the preheat temperature is necessary. An induction heater uses an electromagnetic field to heat the metal.