Question

The high alloy steel components are preheated before welding for reducing

• A. heat affected zone
• B. time of welding
• C. energy consumption
• D. welding stress

Hint:

Preheating high alloy steel components before welding is a crucial practice aimed primarily at reducing the formation of residual stresses and preventing cracking (especially hydrogen-induced cracking). This is achieved by slowing down the cooling rate and minimizing steep temperature gradients in the weld area.

Answer 1

The Correct Option is D

Step 1: Understand the purpose of preheating in welding, especially for high alloy steels.
Preheating involves heating the base metal to a specific temperature before welding begins. This practice is common for certain materials, particularly high alloy steels, cast irons, and thick sections.
Step 2: Analyze the effects of preheating on welding and the reasons behind it.
Preheating offers several benefits in welding:

• Reduces cooling rate: By increasing the initial temperature of the workpiece, preheating slows down the cooling rate after welding. A slower cooling rate allows more time for the microstructure to transform, reducing the formation of brittle phases (like martensite) and making the weldment more ductile.
• Reduces thermal stresses and distortion: Welding involves localized heating and cooling, which leads to expansion and contraction. When the base metal is preheated, the temperature gradient between the weld zone and the surrounding material is reduced. This minimizes differential expansion and contraction, thereby lowering residual stresses and reducing the risk of distortion and cracking (including hydrogen-induced cracking). This is the primary reason for preheating high alloy steels, as they are often more prone to cracking due to their hardenability and lower ductility.
• Improves hydrogen removal: Slower cooling rates allow more time for hydrogen to diffuse out of the weld metal and heat-affected zone (HAZ), reducing the risk of hydrogen embrittlement.
• Improves mechanical properties: By controlling the cooling rate, preheating can lead to a more favorable microstructure and improved toughness in the weld and HAZ.

Now, let's look at the given options:
(1) Heat affected zone (HAZ): Preheating generally increases the size of the HAZ because it keeps the surrounding material hotter for longer, extending the region affected by the heat of welding. So, it does not reduce the HAZ. (2) Time of welding: Preheating does not directly reduce the actual time spent welding. In fact, the preheating process itself adds to the overall fabrication time. (3) Energy consumption: Preheating consumes additional energy. It does not reduce the total energy consumption for the entire welding process. (4) Welding stress: As explained above, preheating significantly reduces the temperature gradients and differential expansion/contraction, which directly leads to a reduction in residual welding stresses and the associated risk of cracking. The final answer is $\boxed{\text{4}}$.