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Tack Welding

Print Date: 12/11/2017 11:13:35 AM

 

Michael J. Houle

Category: Design/Fabrication 

Summary: The following article is a part of the National Board Technical Series. This article was originally published in the October 1980 National Board BULLETIN. (3 printed pages)

 


 

Tack welding is a vital part of a pressure vessel fabricated by welding. This is why the ASME Boiler and Pressure Vessel Code requires qualification of the welding procedure used for tack welding. The code requires the tack welding procedure to be qualified in accordance with the referencing book section and Section IX the same as for other weldments.

Many metals used in code fabrication are very sensitive to rapid quenching including many of the basic P-No. l metals. Very hard, brittle, crack sensitive micro structures, such as martinsite and upper bainite, are formed in many metals when rapidly quenched from an elevated temperature. The brittle micro structures are likely to crack during the solidification of the weld metal or when highly stressed during operation of the pressure vessel. The cracks are usually an underbead crack not detectable by visual or dye penetrant examination and difficult to detect by radiographic or magnetic particle examination. Yet these small cracks may lead to the product failure, if not at hydro test, at some future time due to cyclic fatigue of the pressure vessel. There are many preventive measures to circumvent this problem such as preheat, high heat input processes, subsequent Post Weld Heat Treated (PWHT), etc.

How does this apply to tack welds? Unfortunately tack welds are usually given little attention if any and are seldom controlled, rarely specified. Herein lies the problem.

A high heat input process may be selected for the welding, but the tack is applied by the shielded metal arc welding process. The tack is a very rapid quench application and a brittle, crack sensitive micro structure results usually at the root of the weld. The tack may be subsequently pulled and stressed during the fitup operation with a resultant underbead crack in the pressure retaining material at the root of the weld. Subsequent weld passes with the high heat input process do not, generally, remove the cracks. In fact, the cracks may propagate further into the base metal and/or weld metal during the subsequent welding operations.

Tack welds are important! If the vessel is to be Post Weld Heat Treated (PWHT) the Welding Procedure Specification (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. This is why the code requires the tack weld to be applied following a WPS that has been qualified in accordance with the requirements of Section IX of the code.

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.

The code requires the tack welding to be applied following a qualified WPS whether it is removed, left in place or incorporated into the weld. Tack welding to a qualified WPS is required for any code tack weld including attachments such as backing strips, legs, saddles, lifting lugs, reinforcing rings, thermometer wells, etc. There is at least one exception to this. Section VIII, UW-28 and Section I, PW-28 state in part that procedure qualification testing is not required for any machine welding process used for attaching nonpressure bearing attachments to pressure parts which have essentially no load carrying function. Section IV has exceptions for stud welding.

The performance qualification of welders for tack welding is also largely ignored and uncontrolled. Performance qualification of tack welders is required in Section VIII, UW-31, Section I, PW-31 and Section IV, HW-810 when the tack weld is left in place or is incorporated into the weld. Performance qualification is not required if the tack weld is removed, Sections I, IV and VIII. There is at least one exception to these statements and that is in stud welding.

Performance qualification of welders for tack welding must include as essential variables: backing, base metal type, position, deposited weld metal thickness range and all variables of Section IX for performance qualification.

Poorly applied tack welds are frequently the cause of entrapped slag, porosity, lack of full penetration, leaks and cracks. This is why the ASME code requires tack welds to be procedure and performance qualified and incorporated into the controlled manufacturing system of the manufacturer for any code fabrication.

Other areas of confusion on tack welding include the following:

 

 


 

Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition of the ASME Boiler and Pressure Vessel Code for current requirements.