WELDING

The welding process itself iscomplex, involving heat and liquid-metal transfer, chemical reactions,and the gradual formation of the welded joint through liquid-metaldeposition and subsequent cooling into the solid state, with attendantmetallurgical transformations.

ARC WELDINGArc welding is one of several fusion processes for joining metal. By thegeneration of intense heat, the juncture of two metal pieces is meltedand mixed—directly or, more often, with an intermediate molten fillermetal. Upon cooling and solidification, the resulting welded joint metallurgicallybonds the former separate pieces into a continuous structuralassembly (a weldment). When the pieces are properly designed andfabricated, the strength properties are basically those of the individualpieces before welding.

In arc welding, the intense heat needed to melt metal is produced byan electric arc. The arc forms between the workpieces and an electrodethat is either manually or mechanically moved along the joint; conversely,the work may be moved under a stationary electrode. The electrode generallyis a specially prepared rod or wire that not only conducts electriccurrent and sustains the arc, but also melts and supplies filler metal to thejoint; this constitutes a consumable electrode. Carbon or tungsten electrodesmay be used, in which case the electrode serves only to conduct electriccurrent and to sustain the arc between tip and workpiece, and it is notconsumed; with these electrodes, any filler metal required is supplied byrod or wire introduced into the region of the arc and melted there. Fillermetal applied separately, rather than via a consumable electrode, doesnot carry electric current.Most steel arc welding operations are performed with consumableelectrodes.

Welding Process FundamentalsHeat and Filler Metal An ac or dc power source fitted with necessarycontrols is connected by a work cable to the workpiece and by a “hot”cable to an electrode holder of some type, which, in turn, is electricallyconnected to the welding electrode (Fig. 13.3.1). When the circuit is energized,the flow of electric current through the electrode heats the electrodeby virtue of its electric resistance. When the electrode tip is touchedto the workpiece and then withdrawn to leave a gap between the electrodeand workpiece, the arc jumping the short gap presents a further path ofhigh electric resistance, resulting in the generation of an extremely hightemperature in the region of the sustained arc. The temperature reachesabout 6,500_F, which is more than adequate to melt most metals. The heatof the arc melts both the base and the filler metal, the latter being suppliedvia a consumable electrode or separately. The puddle of molten metalproduced is called a weld pool, which solidifies as the electrode and arcmove along the joint being welded. The resulting weldment is metallurgicallybonded as the liquid metal cools, fuses, solidifies, and cools. Inaddition to serving its main function of supplying heat, the arc is subjectto adjustment and/or control to vary the proper transfer of molten metalto the weld pool, remove surface films in the weld region, and foster gasslagreactions or other beneficial metallurgical changes.Filler metal composition is generally different from that of the weldmetal, which is composed of the solidified mix of both filler and basemetals.

Shielding and Fluxing High-temperature molten metal in the weldpool will react with oxygen and nitrogen in ambient air. These gaseswill remain dissolved in the liquid metal, but their solubility significantlydecreases as the metal cools and solidifies. The decreased solubilitycauses the gases to come out of solution, and if they are trappedin the metal as it solidifies, cavities, termed porosity, are left behind. Thisis always undesirable, but it can be acceptable to a limited degreedepending on the specification governing the welding

Smaller amounts of these gases, particularly nitrogen, may remaindissolved in the weld metal, resulting in reduction in the physical propertiesof otherwise excellent weld metal. Notch toughness is degraded bynitrogen inclusions. Accordingly, the molten metal must be shieldedfrom harmful atmospheric gas contaminants. This is accomplished bygas shielding or slag shielding or both.Gas shielding is provided either by an external supply of gas, such ascarbon dioxide, or by gas generated when the electrode flux heats up.Slag shielding results when the flux ingredients are melted and leavebehind a slag to cover the weld pool, to act as a barrier to contactbetween the weld pool and ambient air. At times, both types of shieldingare utilized.In addition to its primary purpose to protect the molten metal, theshielding gas will affect arc behavior. The shielding gas may be mixedwith small amounts of other gases (as many as three others) to improvearc stability, puddle (weld pool) fluidity, and other welding operatingcharacteristics.In the case of shielded-metal arc welding (SMAW), the “stick” electrodeis covered with an extruded coating of flux. The arc heat melts theflux and generates a gaseous shield to keep air away from the moltenmetal, and at the same time the flux ingredients react with deleterioussubstances, such as surface oxides on the base metal, and chemicallycombine with those contaminants, creating a slag which floats to thesurface of the weld pool. That slag crusts over the newly solidified hotmetal, minimizes contact between air and hot metal while the metalcools, and thereby inhibits the formation of surface oxides on the newlydeposited weld metal, or weld bead. When the temperature of the weldbead decreases, the slag, which has a glassy consistency, is chipped off

to reveal the bright surface of the newly deposited metal. Minimal surfaceoxidation will take place at lower temperatures, inasmuch as oxidationrates are greatly diminished as ambient conditions areapproached.Fluxing action also aids in wetting the interface between the basemetal and the molten metal in the weld pool edge, thereby enhancinguniformity and appearance of the weld bead.Process Selection Criteria

Economic factors generally dictate which welding process to use for aparticular application. It is impossible to state which process will alwaysdeliver the most economical welds, because the variables involved aresignificant in both number and diversity. The variables include, but arenot limited to, steel (or other base metal) type, joint type, section thickness,production quantity, joint access, position in which the welding isto be performed, equipment availability, availability of qualified andskilled welders, and whether the welding will be done in the field or inthe shop.Shielded Metal Arc WeldingThe SMAW process (Fig. 13.3.2), commonly known as stick welding, ormanual welding, is a popular and widespread welding process. It is versatile,relatively simple to do, and very flexible in being applied. To