Tack Welds or Temporary Welds: Unfortunately most disregarded variable in fabrication industry.

Do you ever notice how sometimes the little ignorance could invite biggest problems?

This situation can be compounded when a variable is considered insignificant and accordingly ignored. Such can be the case when tack welds are improperly made.

Code books define a tack weld as: “A weld made to hold the parts of a weldment in proper alignment until the final welds are made.”

While the term “temporary weld” is defined as: “A weld made to attach a piece to a weldment for temporary use during handling, shipping, or working on the weldment.”

The clear difference is that the tack weld joins “the parts of a weldment,” where as the temporary weld joins “another piece of metal to a weldment” like Bridge, Fit up Clamps, Jigs & Fixtures.

Keep definitions aside & ensure both tack welds and temporary welds must be properly made. Since there is no secondary member term in welded construction. Improperly made tack or temporary welds may create problems that result in the propagation of cracks into main members.

Further, temporary welds may provide metallurgical path for cracks (if present) in attachments to propagate through the weld, into the main member. Even though these welds seem to be less important but would be critical, especially in weldments subject to cyclic loading.

Unfortunately tack welds are usually given little attention.

Consider the improper procedure were followed while making a tack weld outside the joint, the weld is made without preheat or required minimum heat input or correct electrode. Such practice could result in development of under bead cracks, an excessive hard HAZ or other weld defects. Simple removal of tack weld metal from the surface of the steel will not automatically remove the defect that may reside in the base metal. This can result in performance problems for the weldment, particularly when subject to cyclic loading.

Very hard, brittle, crack sensitive micro structures, such as Martensite and upper Bainite were formed in many metals when rapidly quenched from an elevated temperature. The brittle micro structure is likely to crack during solidification of the weld metal or when highly stressed during operation. The cracks are usually an under bead cracks & difficult to detect by NDE (Visual or DPT/ MPT/ RT). Yet these small cracks may lead to the product failure in near future due to cyclic fatigue of the cylindrical equipment.

How does this apply to tack welds?

During fabrication of cylindrical components a high heat input process is selected for the welding but the tack weld is made by SMAW process. The tack weld is a very rapid quench application resulted in a brittle, crack sensitive micro structure at the root of the weld. The tack may be subsequently pulled and stressed during the fit up operation with a resultant under bead crack at the root of the weld. The crack may propagate further into the base metal during the subsequent welding operations.

During fit- up of process piping, normally weldments are held in position by tacking in the grove joint. Tack welds are often made with a different welding process, or same process with different filler metal other than for final weld. In such a case chemical interactions between these two types of filler materials should be consider (more important in case of cladded pipes)

A second type of interaction that must be considered is the potential effect of intermixed weld metals on mechanical properties. The toughness of subsequent weld passes of normally tough welds may be reduced due to negative interaction with tack welds made with a different shielding system / filler material during welding.

That’s why ASME BPVC requires Tack weld or Temporary welds shall be subject to the same WPS requirements as the final welds.

Poorly applied tack welds are frequently the cause of entrapped slag, porosity, lack of penetration, and cracks. Even a small tack weld on non-preheated thicker sections may result in a hard, crack sensitive heat affected zone around the tack weld.

There are many preventive measures to counter this problem such as preheat, high heat input processes, subsequent Post Weld Heat Treatment etc.

If the vessel is to be Post Weld Heat Treated then the WPS for the tack welding shall be qualified with PWHT. If the welding process is qualified with preheat, the tack weld shall be applied within the preheat range qualified.

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.

There is at least one exception to this, Section VIII (UW-28) & Section I (PW-28) state that, procedure qualification is not required for any machine welding process used for attaching non pressure bearing attachments to pressure parts which have essentially no load carrying function.

The performance qualification of welders for tack welding is also largely ignored and uncontrolled.

Performance qualification of tack welders is required as recommended in UW-31 of Section VIII, PW-31 of Section I & 328.5 of B 31.3 when the tack weld is left in place or is incorporated into the weld.

As per ASME section – I & VIII Performance qualification is not required if the tack weld is going to remove while it is not the case for Bridge tack weld as per clause 328.5.1(c) of ASME B 31.3.   

Essential variables to include during performance qualification of welders for tack welding are backing, base metal type, position, deposited weld metal thickness range.

Tack welds may be placed within the weld joint, and then subsequently welded over with the other weld passes. Alternately, tack welds may be made outside the weld joint too.

For tack welds made within the weld joint, may be completely remelted and becomes part of the final weld.Tack welds made outside the joint may remain in place, and become part of the permanent weldment, or they may be removed after the joint has been partially or completely welded.

When tack welds or temporary welds are required to be removed, it is important that the weld metal shall be fully removed without damaging the base metal. A typical approach is to thermally cut the weld or attachment off (using air arc gouging, oxy fuel cutting) away from base metal, and follow up with grinding & NDT.

When cutting is performed too close to the final surface, one may inadvertently gouge the base metal.

Some important facts to be consider on tack welds:

• Removal of a tack weld means removal of the deposited weld metal essentially flush with the parent base metal. 

• Tack welds leaves the heat affected zone and some weld / base metal mixture which may contain brittle, crack sensitive structure. Removal may also be complete, as in the case of back gouging to sound metal. 

• Tacks shall be visually examined for defects and, if discovered, the defects shall be removed. Normally the defects are entrapped slag, gas pockets, lack of fusion, cracks, etc. 

• Tacks are often pulled and stressed, cracking or tearing away base metal. These defects must be removed before making the final weld. 

• Tack welds may be long or short. During site fabrication of process piping some fit up many welders / fitters making tacks more than half the length of the root. This is why a welder who is performing tack welding activity in the root of a single welded butt joint needs to be qualified for welding without backing.

ASME Section I

• PW-28.4.3 Identification of welders or welding operators making tack welds that become part of a final pressure retaining weld or structural attachment weld is not required provided the Quality Control System of the Manufacturer includes a procedure to permit the Inspector to verify that such tack welds were made by qualified welders or welding operators. 

• PW-31.3 Tack welds used to secure alignment shall either be removed completely when they have served their purpose or their stopping and starting ends shall be properly prepared by grinding or other suitable means so that they may be satisfactorily incorporated into the final weld.

• Tack welds, whether removed or left in place, shall be made using a fillet weld or butt weld procedure qualified in accordance with Section IX.

• Tack welds to be left in place shall be made by welders qualified in accordance with Section IX and shall be examined visually for defects and, if found to be defective, shall be removed.                                           

ASME Section VIII

• UW-31(c) Bars, jacks, clamps, tack welds, or other appropriate means may be used to hold the edges of parts in alignment.

• Tack welds used to secure alignment shall either be removed completely when they have served their purpose, or their stopping and starting ends shall be properly prepared by grinding or other suitable means so that they may be satisfactorily incorporated into the final weld.

• Tack welds, whether removed or left in place, shall be made using a fillet weld or butt weld procedure qualified in accordance with Section IX.

• Tack welds to be left in place shall be made by welders qualified in accordance with Section IX, and shall be examined visually for defects, and if found to be defective shall be removed. 

• UHT-85 (a & c) Structural & Temporary Welds -Welds for pads, lifting lugs and other non pressure parts, as well as temporary lugs for alignment, shall be made by qualified welders in full compliance with a qualified welding procedure.

• Temporary welds shall be removed and the metal surface shall be restored to a smooth contour. The area shall be inspected by magnetic particle or liquid penetrant method for the detection and elimination of cracks. Temporary welds shall be considered the same as all other welds so far as requirements for qualified operators and procedures and for heat treatment are concerned.

 ASME B 31.3, 328.5.1(c)

• Tack welds shall be made by a qualified welder or welding operator.

• Tack welds at the root of the joint shall be made with filler metal equivalent to that used in the root pass.

• Tack welds shall be fused with the root pass weld. Bridge tacks (above the weld) shall be removed.

Summary

Neither tack welds nor temporary welds should be viewed as unimportant, secondary welds, particularly when applied to cyclically loaded weldments. Whether the tack weld is to be made in the joint or not will affect the overall approach to the weld.

For tack welds and temporary welds that will be removed, care must be taken to protect the base metal. There are plenty of opportunities to make tack and temporary welds improperly. Fortunately, it is not difficult to make them correctly.

Next time when you meet a tack weld, face it with respect. Qualify the tack weld procedure, qualify the tack welder’s performance and control the application of the tack in accordance with your quality control program. A respected tack weld may pay you back with dividends of which you may never have been aware. The dividends may be no x-ray repairs, no leaks and no product failure.