On a fillet weld, what percentage of the base metal thickness is the penetration requirement?

The penetration requirement for a fillet weld is typically around 25-50 percent of the base metal thickness. This range is significant because it ensures that the weld effectively bonds the two pieces of metal while also providing enough strength to withstand the loads the joint may encounter. Proper penetration is essential for the integrity of the weld, as insufficient penetration can lead to weaknesses, potentially resulting in failure under stress.

Selecting a penetration percentage within this range balances the need for a strong weld with the feasibility of achieving that depth during the welding process. If the penetration is too shallow, the weld may not provide adequate strength, whereas overly deep penetration can lead to issues such as burn-through or excessive heat affected zones, which may compromise the surrounding base metal.

The other percentages listed do not match the accepted industry standards for fillet welds, making the range of 25-50 percent the appropriate choice.