According to the AWS D1.1 code, when is welding vertically downwards permitted?

The correct answer is that welding vertically downwards is permitted to repair undercut only. This is grounded in the AWS D1.1 code, which provides specific guidelines on welding techniques to ensure the quality and integrity of structural components.

Welding vertically downwards can be particularly challenging and is generally avoided due to potential issues such as poor penetration and the risk of slag inclusion, which can negatively affect the strength and integrity of the weld. However, in instances of repairing undercutting—a defect that compromises the weld's cross-sectional area and structural support—this particular technique may be utilized. The code recognizes that certain repair scenarios allow for this technique, provided it adheres to specific requirements that ensure quality.

Understanding this context clarifies why other options are not valid; for instance, allowing vertical down welding for new joints could lead to compromised weld quality. Similarly, permitting it for all types of repairs or in emergency situations might encourage practices that deviate from standardized welding procedures that ensure performance and safety. Thus, the restricted applicability of vertical down welding to undercut repairs is based on maintaining the integrity and safety of structural welds while addressing specific defects as they arise.