Tack Welding

Tack Welding
       
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-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:

·       Removal of a tack weld means removal of the deposited weld metal essentially flush with the parent base metal. NOTE: This leaves the heat affected zone and some weld metal/base metal mixture which may contain brittle, crack sensitive structure. Removal may also be complete, as in the case of backgouging to sound metal. NOTE: All rules apply even when removed in this manner.

·       Feathering of tacks, or their starting and stopping ends, shall be properly prepared by grinding or other suitable means so that they may be satisfactorily incorporated into the final weld. This does not require complete grinding of the entire tack, just the start and stop so it may be smoothly incorporated into the final weld. If a tack is made with Gas Tungsten Arc Welding (GTAW) for example, and is smoothly contoured at the start and stop, the tack need not be ground. NOTE: A workmanship sample is usually made to demonstrate which tacks may be satisfactorily incorporated into the final weld to determine an acceptance standard.

·       Tacks shall be visually examined for defects and, if found, the defects shall be removed. Defects are the normal entrapped slag, gas pockets, lack of fusion, cracks, etc. NOTE: Tacks are often pulled and stressed to failure, cracking or tearing away base metal. These defects must be removed before making the final weld.

·       Tack welds may be long or short. Some fitup operations require 12" long tacks fully fused into the root of the weld. I have seen some tacks cover more than half the length of the root. This is why a welder who is tack welding the root of a single welded butt joint needs to be qualified for welding without backing. When tack welding the root of a stay weld in a boiler, in the 5G position, the tack welder needs to be qualified for the 5G position. After all, he is putting in the most critical part of the weld.

The next time you meet a code tack weld, bear it with respect. Qualify the tack weld procedure, qualify the tack welders 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.

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 and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.