Size of the Heat Affected Zone (HAZ)

The Heat Affected Zone (HAZ) refers to the portion of the base metal that has undergone a change in microstructure and mechanical properties due to the heat generated during welding. The size of the HAZ is influenced by various factors, including the starting temperature, welding speed, thermal conductivity of the base metal, and base metal thickness. In this question, options A and C are correct, and we will explain why.

A) Increased starting temperature:
When the starting temperature of the base metal is increased, it means that the base metal is already at an elevated temperature before the welding process begins. As a result, the base metal is more susceptible to thermal effects during welding. The heat input from the welding process will further increase the temperature of the base metal, leading to a larger HAZ. Therefore, an increase in the starting temperature of the base metal will result in an increase in the size of the HAZ.

C) Increased thermal conductivity of the base metal:
Thermal conductivity is a measure of a material's ability to conduct heat. When the base metal has a high thermal conductivity, it means that it can effectively dissipate heat away from the weld zone. As a result, less heat is transferred to the surrounding areas, leading to a smaller HAZ. On the other hand, when the thermal conductivity of the base metal is low, it retains more heat, resulting in a larger HAZ. Therefore, an increase in the thermal conductivity of the base metal will lead to a decrease in the size of the HAZ.

B) Increased welding speed:
Contrary to the correct options, an increased welding speed does not directly affect the size of the HAZ. The welding speed refers to the rate at which the weld bead is deposited during the welding process. While the welding speed can influence the cooling rate of the weld pool, it does not have a significant impact on the size of the HAZ. Other factors, such as heat input and base metal properties, play a more significant role in determining the size of the HAZ.

D) Increase in base metal thickness:
Similarly, an increase in the base metal thickness does not directly affect the size of the HAZ. The thickness of the base metal primarily influences the heat input required during welding. However, it does not necessarily result in a larger HAZ. The HAZ size is determined by the heat input, starting temperature, thermal conductivity, and other factors mentioned above.

In summary, the size of the HAZ can be influenced by various factors, but in this question, the correct options are A and C, which state that an increased starting temperature and increased thermal conductivity of the base metal will result in an increase in the size of the HAZ.