Ssome of the main reasons why pre-heating is undertaken are:
Reduces the Cooling Rate: Pre-heating the material slows down the rate at which the welded area cools. This is crucial because a slower cooling rate can reduce the risk of the weld cracking during the solidification process, especially in steels that are prone to hardening.
Minimizes Thermal Stress: When welding, high temperatures are localized at the weld zone, which can cause expansion and later contraction as the material cools. This thermal cycling can create stresses in the material. Pre-heating helps to minimise these stresses by reducing the temperature gradient between the weld area and the rest of the workpiece.
Controls Microstructure Changes: Certain materials, like carbon steels, can undergo undesirable changes in their microstructure if they cool too quickly after welding. Pre-heating can help maintain a more uniform microstructure, which is important for maintaining the strength and ductility of the material.
Prevents Moisture Condensation: Pre-heating helps to remove any moisture from the material's surface, which is particularly important in preventing the formation of hydrogen-induced cracking. Hydrogen embrittlement can occur when hydrogen enters the weld metal or the heat-affected zone and creates high pressure within the microstructure upon cooling.
Improves Weld Penetration: For thicker materials, pre-heating can help to achieve better weld penetration because the overall material temperature is closer to the melting point, thus the added heat from the welding process does not have to raise the temperature of the base metal as much.
Reduces the Possibility of Cold Cracking: Certain steels, like high carbon or high strength low alloy steels, are susceptible to cold cracking, which can happen hours or days after the welding is completed. Pre-heating can reduce the hardness and brittleness of the heat-affected zone and lower the residual stresses, thereby reducing the risk of cold cracking.
Aids in Machinability: For some metals, pre-heating can improve the machinability of the material post-welding by ensuring that the material does not become too hard to machine.
Example of an aluminium part which would be subject to pre-heating
Pre-heating is particularly crucial when welding high-carbon steels, certain alloy steels, or other materials that are prone to crack formation or that have high levels of residual stress. It is also important in situations where the base metal is cold or when welding is performed in an environment where temperatures might induce additional stresses in the material.
The pre-heating temperature, the duration of the pre-heating, and the method of pre-heating (such as using an oxy-acetylene torch, induction heating, or furnaces) are usually determined by the type of material being welded, the thickness of the material, and the specific welding procedure being used. Proper control and measurement of the pre-heat temperature is important to ensure that the desired welding conditions are achieved.
Pre-heating before welding is essential to minimise thermal stress, avoid the formation of cracks, ensure proper penetration, and control the microstructure changes in the weld zone. By heating the metal, the overall temperature gradient is reduced, slowing the cooling rate post-weld to prevent the hardening that can lead to brittle fractures. This process is particularly crucial for certain steels and thick materials that are susceptible to cracking. Pre-heating also helps in expelling moisture from the workpiece, reducing the risk of hydrogen-induced cracking and aiding in maintaining the integrity and mechanical properties of the welded joint.