Minimum & maximum length of tack welds
The tack welding is similar to quick air quench application. It can result in a brittle, crack sensitive microstructure such as martensite and upper bainite in low alloy and Cr-Mo steels usually at the root of the weld. Hence, a suitable and right procedure should be used for tack welding which include preheat before tacking, making tack weld of sufficient length.
There’re two type of tack-welds based on how they are applied in a weld joint:
- Temporary tack weld: When a tack weld is used to make a temporary fit-up joint/ weld conditions with an un-qualified welder (fitter), thus the tack-weld will be removed later and will not be part of the weld.
- Incorporated tack weld: These types of tack-weld will not be removed after finishing the tack and shall be fused in the welding. They become part of the weld.
The minimum and the maximum length of a Tack weld:
The minimum length of tack welds shall be 50 mm (2 inch) for incorporated tack welds. The reason for having minimum 2 inch as smaller tack weld will not provide sufficient heat into the base metal thus there are high propensity for lack of fusion issues.
Temporary tack welds can be as small as 10 mm (3/8 inch.) as long as they are sufficient to withstand the stresses due to the weld fitup. As these types of tack welds are to be removed before filling the joint, any issue of lack of fusion or penetration shall be taken care by the subsequent weld pass.
The recommended minimum length of a tack weld bead, according to the Technical Recommendations for Steel Construction for Buildings of the Japanese Architectural Standard Specification (JASS 6), is shown in below Table .
|Recommended minimum length of a tack weld bead for steel structures|
|Plate thickness (mm)||Min. bead length (mm)|
what should be the length of tack welds on fillet welds
The length of fillet weld can be somewhat lower than compared to groove welds. The reason for the shorter tack length is due to less stress in fillet welds compared to groove welds. Generally, a tack weld length of 10 mm is good for incorporated tacks in the fillet weld. Tack welds that need to be ground should have a length of 1-inch minimum.
A general standard practice of minimum tack length can be followed as shown in the above table.
Tack Weld & Tack Welder definition?
tack weld is a weld made to hold parts of a weldment in proper alignment until the final welds are made.
*tack welder. A fitter, or someone under the direction of a fitter, who tack welds parts of a weldment to hold them in proper alignment until the final welds are made.
Tack Welders. ASME BPVC Section I, VIII & B 31.3 requires that tack welding procedure to be qualified in accordance with the referencing code section i.e. ASME Section IX. Tack welds made at the root of a groove weld must be qualified by a groove weld test in accordance with the requirements of Section IX. Tack welds of the fillet type may be qualified by a groove weld test or fillet weld test in full compliance with the requirements of Section IX.
As per AWS D1.1, A tack welder who passes the test described in AWS D1.1 Part C or those tests required for welder qualification shall be considered eligible to perform tack welding indefinitely in the positions and with the process for which the tack welder is qualified unless there is some specific reason to question the tack welder’s ability.
According to AWS D1.1, clause 6.2.2 Performance Qualification of Welding Personnel: tack welders to be employed to weld under this code, and using the SMAW, SAW, GMAW, GTA W, FCAW, ESW, or EGW processes, shall have been qualified by the applicable tests as described in Part C of this clause.
How tack welder shall be qualified & tack welding procedure?
According to AWS D1.1, clause 220.127.116.11 Tack Welders. A tack welder shall be qualified by one test plate in each position in which the tack welding is to be performed. Tack Welders. The tack welder shall make a 1/4 in [6 mm] maximum size tack weld approximately 2 in [50 mm] long on the fillet-weld-break specimen.
tack welder who passes the fillet weld break test shall be qualified to tack weld all types of joints (except CJP groove welds, welded from one side without backing; e.g., butt joints and T-, Y-, and K-connections) for the process and in the position in which the tack welder is qualified. Tack welds in the foregoing exception shall be performed by welders fully qualified for the process and in the position in which the welding is to be done.
Acceptance of Tack Welder qualification
According to AWS D1.1, clauses:
6.24.1 Visual Acceptance Criteria. The tack weld shall present a reasonably uniform appearance and shall be free of overlap, cracks, and undercut exceeding 1/32 in [1 mm]. There shall be no porosity visible on the surface of the tack weld.
6.24.2 Destructive Testing Acceptance Criteria. The fractured surface of the tack weld shall show fusion to the root, but not necessarily beyond, and shall exhibit no incomplete fusion to the base metals or any inclusion or porosity larger than 3/32 in [2.5 mm] in the greatest dimension.