What does a welder certified in 3G and 4G positions signify?

A welder certified in 3G and 4G positions signifies proficiency in specific welding techniques that are particularly challenging. The 3G position refers to vertical welding, where the welder must effectively perform a weld in a vertical plane, which often involves welding uphill. This requires a solid understanding of heat management and technique to ensure proper penetration and fusion.

The 4G position indicates overhead welding. This position requires the welder to work on welds positioned above their heads, demanding a unique mastery of technique to manage the molten weld pool and prevent issues like sagging or dripping, which can compromise the weld's integrity. Together, these certifications affirm the welder's capability to handle these complex welding scenarios, which are critical in many structural applications where strength and durability are paramount.

Therefore, option B is accurate as it explicitly points out that the certification is for uphill and overhead welding positions, reflecting the heightened skill level and versatility required from welders to operate efficiently in different orientations.