Improper Heat Input Manipulation on FCAW Consumables Qualification
By Wesley Wang on Wednesday, April 30th, 2014
Mil-E-24403/1 and AWS A5.20 specifications (“D” and “Q” designators) have their requirements on low and high heat inputs to qualify a FCAW wire in addition to the control of pre-heat and inter-pass temperatures. A specified heat input is an average heat input for all passes in a test assembly. AWS A5.20 / 17.2 even applies heat input limits on individual passes during the welding of a whole test assembly. However, it still won’t be able to prevent one creative interpretation and practice from manipulating the heat input to qualify FCAW consumables.
During the qualification welding, a targeted heat input should be kept as stable as possible to weld a pass from the beginning to the end. According to the layout of the test assembly illustrated in AWS and Military specifications, usually, about half of the test assembly is used to machine out a tensile specimen for the tension test, and the rest is used for Charpy “V”-notch (CVN) impact samples. This layout provides an unintended advantage of allowing improper manipulation on the heat input in welding.
When a high heat input is used to weld a test assembly, 3G welding is usually adopted for the consumable qualification. For a certain E71T-1 type product as an example, if a consistent heat input is used to weld a pass from the bottom to top by using a 0.052” (or other diameters) FCAW wire, it is very likely that the yield strength falls out the specified strength range, or the CVN impact toughness fails. A “smart” solution is to weld the bottom half by a low heat input with a fast travel speed and weld the top half by a high heat input with a slow travel speed. The whole test assembly is effectively separated into two parts that are welded with two different heat inputs. After the welding, a “smart” move is followed by averaging the low and high heat inputs to satisfy the average heat input required by specifications. By doing that, both the yield strength and impact toughness can be reported to be satisfactory.
Due to the resultant unevenness of mechanical properties on the whole weld assembly, the “smart” move shall not be considered to be legitimate (or decent) from the engineering practice, nor shall be deemed by the welding consumable community. On the other hand, the layout of the test assembly for welding consumables qualification shall be revised to prevent those “smart” moves.