Range of Approval
The scope of a welding procedure specification exactly defines for which materials, plate thicknesses, and welding positions it is valid. It protects against improper use: outside this scope, cooling rate and microstructure change – cracks and hardness failures threaten. Only within the scope may welding be done without retesting.
Physicochemical Mechanism of Action
Thick plates conduct heat away quickly. The cooling time (t8/5) becomes shorter. Hard martensite forms in the structure. The weld becomes brittle. Thin plates cool more slowly. The grain coarsens. Toughness decreases. The welding position changes heat dissipation: in vertical welding, heat flows differently than in the flat position. The material's carbon equivalent determines how strongly it hardens. Only if material, thickness, and position are similar to the tested specimen does the structure remain the same.
Metallurgical Relevance & Risks in Welding
If the range of qualification is exceeded—for example, thicker plate than tested—the weld cools faster. Consequence: martensite formation, high hardness, cold cracks. A different material with a higher carbon equivalent hardens even with normal cooling. Wrong welding position leads to uneven heat dissipation, porosity, or lack of fusion. All these defects jeopardize the load-bearing capacity of the welded joint.
Workshop Practice & Action Instructions
The welder checks the WPS (Welding Procedure Specification) before starting work. It states the range of qualification: permitted material groups, plate thickness range, welding positions. Example: a 10-mm plate was tested. According to the standard, the thickness range for butt welds can be 5–20 mm (factor 0.5 to 2.0). For fillet welds, it's often 3–20 mm. If the part falls outside, a new procedure qualification is required. Never weld outside the range of qualification—even if the part deviates only slightly.