Preheating before welding is done to prevent cold cracks in the welded joint. Cold cracking is the cracking of a welded joint that occurs due to the presence of hydrogen in the weld metal and the heat-affected zone (HAZ). When the weld metal cools down, the hydrogen in it diffuses into the HAZ and expands as it cools, causing the HAZ to crack.

Preheating raises the temperature of the base metal and reduces the cooling rate of the weld metal, thereby reducing the risk of cold cracking. Preheating also helps to reduce the thermal shock that occurs when the hot weld metal is deposited on the cold base metal. This thermal shock can cause the base metal to contract and distort, leading to welding defects such as warping, buckling, and shrinkage.

Apart from preventing cold cracking, preheating also offers other benefits such as:

- Burning away oil, grease, and other contaminants from the plate surface, which can cause porosity and other defects in the weld.
- Making the steel softer and more ductile, which can reduce the risk of cracking and improve the weld quality.
- Reducing the risk of hydrogen embrittlement, which can occur when hydrogen diffuses into the steel and makes it brittle.
- Improving the weld penetration and fusion, which can lead to a stronger and more reliable weld.

In summary, preheating is an important step in welding that can prevent cold cracking and improve the quality and reliability of the welded joint. However, the preheat temperature and time should be carefully selected based on the type of steel, thickness, welding process, and other factors to ensure optimal results.