API 577 Key Points_Chapter 5

API 577 Chapter-5 Key points •

Basic processes: SMAW, GTAW, GMAW, FCAW, SAW, SW. SW.

SMAW: • • • •

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Most widely used process. Uses an arc between a covered electrode and the weld pool. Employs the heat of an arc coming from the tip of an electrode to melt the base etal Shielding is provided from the decomposition of the electrode covering, without the application of pressure and with ller metal from the electrode. Either ! or "! may be employed !! supply is preferred Manual welding process Electrode covering: #rovides a gas to shield the arc and prevent e$cessive atmospheric contamination of the molten ller metal #rovides scavengers, deo$idi%ers, and &u$ing agents to cleanse the weld and prevent e$cessive grain growth in the weld metal. Establishes the electrical characteristics of the electrode #rovides a slag blan'et to protect the hot weld metal from the air and enhances the mechanical properties, bead shape, and surface cleanliness of the weld metal. #rovide means of adding alloying elements to change the mechanical properties properties of the weld metal.

Advantages and disadvantages : Advantages • • • •

E(uipment relatively simple #rocess can be used in areas of limited access )ess sensitive to wind and draft than other welding processes Suitable for most of the commonly used metals and alloys

isadvantages • •

"eposition rates are lower than for other processes such as *M+ Slag removal at start and stops before depositing to a weld bead adacent to or onto a previously deposited weld bead.

GTAW: GTAW: • • •

Uses an arc between a non-consumable tungsten electrode and the weld pool #rocess is used with shielding gas and without the application of pressure !an be used with or without the addition of ller metal

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!! power supply can be used with either dc or ac, choice largely depends on the metal to be welded "! welding is normally performed with E polarity i.e. "!E "!E o/ers the advantage of deeper penetration and faster welding speeds ! provides a cathodic cleaning0sputtering1 that removes refractory o$ides from the surfaces of the weld oint, which is necessary for the welding of l and Mg. 2he cleaning action occurs during the portion of ac wave, when the electrode is positive with respect to the wor' piece.

Advantages: 3. %& 4. 5.

#roduces high purity welds, genera!!y "ree "ro# de"e$ts 'itt!e post (e!d $!eaning llows e$cellent control of root pass (e!d penetration !an be used with or without ller metal, dependent on the application

isadvantages: 3. )ower deposition rates than consumable electrode arc welding processes 6. )ow tolerance for contaminants on ller or base metal )& i*$+!ty in shie!ding the (e!d one proper!y in dra"ty environ#ents GMAW: • •

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Uses an arc between continuous ller metal electrode and the weld pool  2he process is used with shielding from and e$ternally supplied gas and without the application of pressure 7perates in semiautomatic, machine or automatic modes Employs a !8 supply Uses either short $ir$+iting, g!o+!ar or spray #ethods to transfer metal from the electrode to wor'  2ype of transfer depends on number of factors such as 3. 6. 4. 5. 9.

Magnitude and type of welding current Electrode dia Electrode composition Electrode e$tension Shielding gas

Short $ir$+iting trans"er.GMAW-S/ #roduces a "ast "reeing (e!d poo! that is generally s+ited "or 0oining thin se$tions, o+t o" position, or root pass . "ue to this nature there are $han$es o" 'a$1  o" side (a!! "+sion (hen (e!ding thi$1 (a!! e2+ip#ent or a no!e atta$h#ent& G!o+!ar Trans"er 3. Encompasses relatively low current 6. !haracteri%ed by a drop si%e with a diameter greater than that of the electrode 4. 'i#ited to 3at position and can produce spatter

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Spray trans"er 3. ighly directed stream of discrete drops that are accelerated by arc forces

6. Spatter is negligible 4. "ue to high arc forces with high current, applying this process to thin sheets may be di;cult 5. 2his thic'ness limitation can be overcome by the use of pulsed *M+. #ulsed *M+ is a variation of the *M+ in which the current is pulsed to obtain the advantage of spray transfer at the less average currents than that of spray transfer mode. Advantages o" GMAW: 3. 2he only consumable electrode process that can be used to weld most commercial metals and alloys 6. "eposition rates are signicantly higher than those obtained with SM+ 4. Minimal post cleaning is re(uired due to absence of a slag isadvantages o" GMAW: 3. +elding e(uipment is more comple$, more costly and less portable than that for SM+ 6. 2he welding arc should be protected from air drafts that will disperse the shielding gas 4. +hen using *M+-S process, the weld is more susceptible to )ac' of ade(uate fusion. FCAW: 4& rc welding process that uses an arc et(een $ontin+o+s t++!ar !!er #eta! e!e$trode and the (e!d poo! 6. 2he process is used (ith shie!ding gas evo!ved "ro# a 3+6 otained (ithin the t++!ar e!e$trode, (ith or (itho+t additiona! shie!ding "ro# an e6terna!!y s+pp!ied gas, and (itho+t the app!i$ation o" press+re . 4. "! constant voltage type.C/ similar to sources used for *M+ Advantages o" FCAW: 3. %& 4. 9&

Meta!!+rgi$a! enets $an e derived "ro# a 3+6 S!ag that s+pports and shapes the (e!d ead 8igh deposition and prod+$tivity rates  than other processes such as SM+ Shie!ding is prod+$ed at the s+r"a$e o" the (e!d that #a1es it #ore to!erant "or stronger air $+rrents t han GMAW

isadvantages o" FCAW: 4& 2+ip#ent is #ore $o#p!e6, #ore $ost!y and !ess porta!e than that "or SMAW %& Self-shielding egative ter#ina!.C>/ 5& CC type po(er so+r$e ?. Uses in (e!ding ins+!ation and re"ra$tory s+pport pins to tan1s, press+re vesse!s and heater $asing Advantages: 4& igh productivity rates compared to manually welding stud to base metal %& !onsidered in all position processes isadvantages: 4& Pri#ari!y s+ita!e on!y "or CS and 'AS 6. 7nly speciali%ed for few applications