To determine the best suitable welding process for butt welding 40 mm thick steel plates, we need to consider factors such as the thickness of the plates, the expected quantity of jobs, and the duration of the welding operations.

1. Thickness of the Plates:
The thickness of the plates, which is 40 mm in this case, plays a crucial role in selecting the welding process. Thick plates require a welding process that can provide sufficient heat input to ensure proper fusion and penetration.

2. Expected Quantity of Jobs:
The expected quantity of jobs, which is 5000 per month over a period of 10 years, indicates that the welding process should be efficient, productive, and capable of handling high volumes of work.

3. Duration of the Welding Operations:
The duration of the welding operations, spanning 10 years, suggests that the chosen welding process should be durable, reliable, and able to sustain continuous welding for long periods without frequent breakdowns or maintenance requirements.

Based on these factors, the best suitable welding process out of the available alternatives can be determined as follows:

a) Submerged Arc Welding (SAW):
Submerged Arc Welding (SAW) is the most suitable welding process for butt welding 40 mm thick steel plates in this scenario. Here's why:

- SAW is known for its high heat input, which is essential for welding thick plates effectively. The process involves feeding a consumable electrode wire into the joint and covering it with a layer of flux. This creates a protective blanket that shields the arc and molten weld pool from atmospheric contamination.

- SAW is highly efficient and productive, making it suitable for handling the expected quantity of jobs (5000 per month). The process allows for high deposition rates and can be easily automated, reducing the need for manual labor and increasing productivity.

- SAW is a reliable and durable welding process, capable of sustaining continuous welding operations for long periods. The process is less prone to interruptions or breakdowns, ensuring smooth and uninterrupted production over the 10-year duration.

In conclusion, based on the thickness of the plates, the expected quantity of jobs, and the duration of the welding operations, Submerged Arc Welding (SAW) is the best suitable welding process for butt welding 40 mm thick steel plates in this scenario.