Welding Theory Complete Practice Test 2026

In MIG/MAG wire labeling, the letters tell you how the filler metal is delivered. The number 70 means the weld metal is at least 70 ksi in tensile strength. The S after that is telling you about the wire’s construction: S stands for solid wire, meaning there is no flux core inside. A wire labeled with a C would be a flux-cored wire. This distinction matters because solid wires rely on an external shielding gas to protect the weld, while flux-cored wires carry their own flux and can behave differently in terms of deposition, slag production, and shielding.