QW-200.3 To reduce the number of welding procedure qualifications required, P‐Numbers are assigned to base metals dependent on characteristics such as composition, weldability, and mechanical properties, where this can logically be done; and for steel and steel alloys Group Numbers are assigned additionally to P‐Numbers. These Group Numbers classify the metals within P‐Numbers for the purpose of procedure qualification where toughness requirements are specified. The assignments do not imply that b as e metals maybe indiscriminately substituted for a base metal which was used in the qualification test without consideration of the compatibility from the standpoint of metallurgical properties, postweld heat treatment, design, mechanical properties, and service requirements. Where toughness is a consideration, it is presupposed that the base metals meet the specific requirements.