WELDING AND BRAZING QUALIFICATIONS

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procedures and the qualification of individuals performing thoseprocedures as prescribed in Section IX of the ASME Boiler &Pressure Vessel Code

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  • CHAPTER

    24

    24.1 INTRODUCTION This chapter covers the qualification of welding and brazing

    procedures and the qualification of individuals performing thoseprocedures as prescribed in Section IX of the ASME Boiler &Pressure Vessel Code. It reviews the qualification rules and pro-vides commentary on those requirements. Where comments areprovided, they represent the personal opinion of the author andshould not be regarded as the position of the ASME Code or itsSubcommittee on Welding.

    The information presented herein is based on the 2007 edition.The authors writing may have simplified some of the Code rulesin summarizing the cited requirements; therefore, all of the detailsof the rules might not have been addressed. The reader is advisedto consult Section IX as well as the applicable Construction Codefor the specific rules.

    24.2 HISTORY OF SECTION IX Riveting was the main fabrication method for the manufacture

    of boilers and pressure vessels in the ASME Boiler & PressureVessel Code in the early twentieth century. It was the SecondWorld War, with its urgency for ships and other components thatpropelled welding to the forefront of manufacturing processes.

    The fledgling ASME Boiler & Pressure Vessel Code Committeerecognized welding when in 1918 Section I, Power Boilers, per-mitted welding, but only when it was used in applications wheresafety did not depend on the weld. Over the next two decades,welding was adopted by more sections of the Code includingSection VIII, Unfired Pressure Vessels, and Section IV, thentitled Low Pressure Heating Boiler Code. Rules for the qualifica-tion of welding and welders were introduced into Section VIIIthroughout the 1930s as part of an ongoing approval to use weld-ing in pressure boundary applications. In the mid-1930s, as the dif-ferent sections began to develop their own rules regarding welding,the Boiler & Pressure Vessel Committee formed Subcommittee IXas a joint committee of American Welding Society (AWS) andASME. One aspect of its charter was to write a common set ofrules for the qualification of welding and welders usable by all ofthe books sections, an effort that resulted in the publication of thefirst edition of Section IX in 1941. That edition had variables (i.e.,welding details that effect welding operations; see Section 24.6),16 of which were for the qualification of welding procedures and

    4 of which were for a welders performance qualification. The2007 edition of Section IX has a total of 229 variables coveringboth welding procedure and performance and 23 variables cover-ing brazing procedure and performance.

    24.3 ORGANIZATION OF SECTION IX Since its first publication as a separate ASME document in

    1941, Section IX has seen a number of changes in its arrangementand makeup. The format of the 2007 edition is in two parts: PartQW, covering welding; and Part QB, covering brazing. Each partis further divided into four articles. The following is the organiza-tion of Section IX:

    Foreword. Statement of Policy. Introduction, an explanation of the purpose and organization

    of Section IX. Part QWWelding.

    Article I, general requirements for welding procedure andwelder performance qualification.

    Article II, welding procedure qualifications. Article III, welder performance qualifications. Article IV, those variables applicable to welding procedure

    and welder performance qualification. Article V, standard welding procedure specification.

    Part QBBrazing. Article XI, general requirements for brazing procedure and

    brazer performance qualification. Article XII, brazing procedure qualifications. Article XIII, brazer performance qualifications. Article XIV, those variables applicable to brazing proce-

    dure and brazer performance qualification. Appendices, suggested forms of documenting welding and

    brazing qualifications and the standard welding procedurespecifications acceptable for use.

    24.4 WELDING PROCESSES Before we discuss weld qualifications, it is wise to first review the

    scope of weld processes that Section IX addresses. Section IX con-tains rules for the qualification of the following weld processes:

    WELDING AND BRAZINGQUALIFICATIONS

    Joel G. Feldstein

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  • 218 Chapter 24

    Oxyfuel welding (OFW) Shielded metal arc welding (SMAW) Submerged arc welding (SAW) Gas metal arc welding (GMAW) Gas tungsten arc welding (GTAW) Plasma arc (PAW) Electroslag welding (ESW) Electrogas welding (EGW) Electron beam welding (EBW) Laser beam welding (LBW) Stud welding Inertia and continuous drive friction welding Resistance spot and projection welding Flash welding

    The requirements for the qualification of these weldingprocesses (referred to as variables) are given in QW-250 throughQW-286. When a welding process is not addressed in Section IXbut is permitted to be used by the Code of Construction (e.g.,induction welding), the qualification rules of Section IX should befollowed to the extent that they are applicable.

    24.4.1 Oxyfuel Welding Although arc welding dominates the weld processes used today,

    oxyfuel welding is still occasionally used, and oxy-acetylene is

    the most familiar of the gaswelding heat sources. Other gasmixtures such as oxygen-propane, oxygennatural gas, and oxygenmethylacetylenepropadiene (MAPP) are also used. Although theforthcoming discussion deals with oxygenacetylene, the sameprinciples apply to any oxyfuel process in which a combustible gasis used as the heat source.

    In the oxyacetylene process, heat is produced by the energyreleased while the gases burn in the flame. Figure 24.1 shows aschematic view of the oxyacetylene flame. When the acetylene(C2H2) is burnt in the presence of oxygen (O2), the followingreaction takes place:

    C2H2 O2 2CO H2 Heat (24.1)

    The production of heat provides the energy to raise the temper-ature of gases in the flame and the workpiece to a point wherewelding is possible. Flame tip temperatures of about 6000F canbe achieved.

    When oxyacetylene welds are made, the tip of the flame ispositioned near the work to give the maximum heating affect; themajority of the gases surrounding the work are concentrations ofcarbon monoxide (CO) and hydrogen (H2) that protect the weldfrom oxidation. If there is any O2 near the weld, either from theair or as a dissolved element in the weld puddle itself, it will

    FIG. 24.1 OXYACETYLENE WELDING PROCESS

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  • COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE 219

    combine chemically with either the CO or H2 in the protectiveatmosphere to form carbon dioxide (CO2) or water (H2O) and notgo into the weld metal.

    24.4.2 Arc Welding In arc welding, heat is produced by an electrical discharge

    through an ionized gas. An electric arc is a far more intense heatsource than a chemically produced flame; the maximum tempera-ture of the arc is well above 10,000F. The heating effect of thearc is also more localized, resulting in much less heat being lost tothe surroundings than with a flame heat source. The arc, there-fore, is an extremely efficient heat source.

    The nature of the arc as a heat source varies with the differenttypes of welding processes: gas-shielded arc, submerged arc, andshielded metal arc. To examine what happens during arc welding,the arc processes are divided into two groups: slag-shielded arcand gas-shielded arc. Although each group has a number of indi-vidual weld processes, they have both common and differentmeans of providing a shield for the weld deposit and sources ofchemical changes.

    24.4.2.1 Slag-Shielded Arc Welding For the purpose of thisdiscussion, this group of welding processes includes submergedarc, shielded metal arc, flux-cored arc, and, to a somewhat lesserextent, electroslag. Figure 24.2 schematically illustrates shieldedmetal arc welding with covered electrode as the representativeprocess in the group. The arc is established between the consum-able electrode and the work, and the electrode becomes the fillermetal as it melts from the heat of the arc. One can envision theweld puddle as a miniature crucible in which the weld metal iscast. The arc sup-plies the heat, the base metal and electrode arethe raw materials for the molten bath (weld puddle), and the elec-trode coating provides protection of the molten metal duringsolidification.

    The electrode coatings ingredients perform two major func-tions. The first is to shield the arc by covering it and the moltenweld metal with gas, which prevents the collection of oxygen andnitrogen from the atmosphere and also prevents the formation ofoxides and nitrides in the weld deposit. The second function is to

    provide a molten slag covering over the solidifying weld metal.The molten slag, having a lower density than the weld metal,floats on the surface weld pool and solidifies after the weldmetal. For example, when cellulose (C2H10O5) is used in the elec-trode coating, it decomposes into CO and H2 gas to shield theweld puddle. Other electrodes use calcium carbonate (CaCO3) togive gas shielding in the form of CO2 and slag shielding in theform of calcium oxide (CaO). Other sources of protective slagshielding come from silicon dioxide (SiO2) and titanium dioxide(TiO2) added to the electrodes coating. Although the chemicalconstituents of the flux coating vary greatly from electrode toelectrode, their function remains constanta source of shieldingand, when required, chemical change.

    Submerged arc welding is distinguished from the other slag-shielded arc welding processes by the submergence of the arcand molten weld puddle beneath a granular fusible flux (seeFig. 24.3). In addition to shielding the arc from view, the flux(as in the shielded metal arc welding process) provides a slagthat protects the weld pool as it cools and deoxidizes the weldmetal. Although filler metal is provided primarily from a bareelectrode wire fed continuously from a spool, supplementalfiller metal in the form of metal powder or an additional weldwire may also be used.

    Submerged arc welding is a versatile production weldingprocess that can be used in three modes: semiautomatic, machine,and automatic. In the semiautomatic mode, a hand-held weldinggun is used to deliver the filler wire and flux to the weld area. Thefiller metal is taken from a continuous spool of wire by a wirefeeder; the flux may be supplied by gravity from a hopper mountedon the gun or by air pressure through a hose. In the machinemode, the process uses equipment that performs all of the weldingoperations but requires a welding operator to position the work, tostart and stop the welding, and to adjust the welding controls(e.g., current, voltage, and travel speed). In automatic welding,the equipment performs the welding operation without the weld-ing operator continuously monitoring the process.

    Flux-cored arc welding, like submerged arc welding, producesheat from a welding arc established between a continuous fillermetal and the work. In this process the filler metal is actually acomposite tubular product fabricated of a thin outer metal sheathsurrounding a core of granular powders providing metal alloyadditions and fluxing ingredients. In construction, it is the reverseof a stick electrode that has an inner solid wire core and an out-side flux covering.

    Actually, there are two variations of flux-cored arc welding thatdiffer in their methods of shielding the arc and weld pool. Onetype, termed self-shielded, relies on decomposition of the fluxingingredients under the heat of the welding arc to provide both gasprotection to the welding arc and an extensive slag covering forthe solidifying weld metal, as do other slag-shielded arc weldingprocesses. The other type of flux-cored arc welding, gas shield-ing, makes use of a protective gas flow in addition to the fluxingingredients. Both types use the flux ingredients contained in thecore of the wire to provide a substantial slag covering for thesolidifying weld puddle.

    Flux-cored arc welding in its semiautomatic mode represents asignificant improvement over shielded metal arc welding in thatbetter deposition rates can be achieved from a higher burn-off ofthe electrode. This results from a higher welding current and fromthe elimination of lost time for changing electrodes. However,unlike shielded metal arc welding, flux-cored arc welding canalso be used in the machine and automatic process forms. FIG. 24.2 SHIELDED METAL ARC WELDING PROCESS

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    24.4.2.2 Gas-Shielded Arc Welding While the gaseous pro-tection in the slag-shielded arc welding processes develops fromthe decomposition of the flux ingredients, the gas protection withthe gas-shielded arc welding processes is provided by an exter-nally supplied gas directed onto the work from a nozzle sur-rounding the electrode and engulfing the weld zone. Althoughinert gases such as argon (Ar) and helium (He) are frequently usedas shielding gases, for some gas-shielded welding processesactive gases such as CO2 or mixtures of inert gases with CO2 andO2 are often used.

    When inert gases are used, a significant difference between gas-shielded arc welding processes and slag-shielded welding arcprocesses is that the only chemical changes taking place in the welddeposit under the inert gases occur through the alloy additions ofthe filler metal. CO2, CO2O2 containing Ar, ArHe gas mixturesare all being used extensively as shielding gases with consumableelectrode processes. When CO2 and O2 are used, they can besources of chemical change because they interact with the weld.The basic gas-shielded arc welding processes are the following: gastungsten arc; gas metal arc (including gas-shielding-flux-cored arc);plasma arc; and, to a lesser extent, electrogas.

    Figure 24.4 is a schematic illustration of the gas tungsten arcwelding process. Although it is often referred to as the tungsteninert gas (TIG) process, the correct term is gas tungsten arc weld-ing (GTAW) process because gas-shielding mixtures that are nottruly inert can be used with it. The notable difference between theGTAW process and the slag-shielded and gas-shielded arc weld-ing processes is that the GTAW process uses a welding arc estab-lished between a nonconsumable tungsten electrode and thework-piece. The tungsten electrode is held in a torch throughwhich the shielding gas flows. The ionized shielding gas acts as a

    conductor to transfer the arc current as well as to protect the tung-sten electrode, weld pool, and solidifying weld metal from atmos-pheric contamination. Heat produced by the arc is used to meltthe base metal and filler metal, if added. The GTAW process pro-duces high quality welds in nearly all metals and alloys. Its versa-tility ranges from the welding of the reactive family of metals(e.g., titanium, zirconium, and hafnium) to root pass welding forexcellent penetration control. Because of lower deposition ratescompared with consumable electrode welding processes, GTAWis us...

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