This article provides information about F number in Welding Procedure Specification based on the requirement of ASME Code Section IX.
The F number grouping of filler metals refers to their similarity on their usability characteristics. The usability determines the ability of a welder to deposit a sound weld metal with a certain filler metal.
The F number is essential variable in the most welding process for welding procedure specification and also in welding performance qualifications. Like the P number, the F number designed to reduce the number of procedure qualification and welding performance qualification.
I have seen some many people confusing about F number and believing the F number is grouping for the similar chemical composition. This is not true, and F number is only about of usability characteristics such as welding position.
Table QW-432 in ASME Code Section IX provides the list of F numbers. Any change from one F number in welding to the other F number is essential variable in the welding procedure specification. The changing from one F number to other F number is a little bit different in welding performance qualification. The essential variable tables in welding performance qualification (QW-352 thru 357) refer you to the QW-433 for the range of qualification.
For example, a welder qualified with F number 4, he also will be qualified on the F numbers 1, 2 and 3.
Some contractors use the essential variable tables (QW-352 thru 357) to minimize their costs. For instance, they might test the welder for stainless steel material by using carbon steel material. Consider the actual project material is stainless steel and you do not want to waste stainless steel material for testing. You ask the welder to weld in carbon steel material but with stainless steel filler metal. Based on the essential variable table P number 1 (Carbon Steel) qualifies P 8 (Stainless Steel), and the F number is the same. So you qualify your welder for stainless steel material by using carbon steel material but stainless steel filler metal.
When you have a welder or welding operator with a certain qualification, and you want to use him in some other job, you need to see if all the variables in the essential variable table confirm his qualification for the new job. Each of the item in the table has veto right, if 6 out of 7 is Ok, but only 1 of them is not meeting the requirements, and then you cannot use that welder in the new job and you need to re-qualify him by the new test.