Which of the following welding processes is not described in API 577?

The reason for selecting Electro Slag Welding as the process not described in API 577 relates to the focus and scope of the API 577 document itself. API 577 primarily concentrates on various welding processes frequently employed in the oil and gas industry. Among those commonly discussed are MIG (Metal Inert Gas) Welding, TIG (Tungsten Inert Gas) Welding, and Stick Welding (SMAW - Shielded Metal Arc Welding). These welding methods are widely used due to their versatility, applicability in different environments, and popularity among welders.

Electro Slag Welding, however, is a specialized process that is not commonly used in the types of construction or repairs typically covered in API 577. This process is mainly reserved for very thick materials and is not as widely applicable in the more general contexts of welding that API 577 addresses. As a result, it does not align with the core content of the document, which is why it is the correct choice in this instance.