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Welding Processes, Welding Faults, Defects & Discontinuities For Non-Destructive

Testing Technicians Part 4The purpose of this series is to outline to NON-DESTRUCTIVE TESTINGTECHNICIANS the basics of the common welding processes. It is important that NDTtechnicians involved in the inspection of welds have a basic understanding of the popular

welding processes. Welding is an extremely interesting and varied process, and there area variety of different welding processes in use today. Some of the more popular weldingprocesses in use in industry are SMAW (stick), GMAW (MIG), FCAW, (FLUXCORED)), GTAW (TIG) SAW (SUB ARC). Fabricators may choose from a variety ofmetal alloys and a range of filler metals and shielding gases.

FLUX CORED ARC WELDING - BASICS

Flux Cored Arc Welding (FCAW)Flux cored arc welding (FCAW) is an electric arc welding process which fuses togetherthe parts to be welding by heating them with an arc between a continuously fed flux filled

electrode wire and the work. Shielding is obtained through decomposition of the fluxwithin the tubular wire (self shielded method). Additionally shielding may be obtainedfrom an externally supplied gas or gas mixture (gas shielded method). Equipment issimilar to that used for Gas Metal Arc welding (GMAW) see CINDE JournalNovember/December 2009)

The flux cored arc welding process can be used to weld carbon and alloy steels, cast andwrought iron and stainless steels. The process is also capable of producing hard surfacingdeposits. The process is commonly used to weld medium to thick steels because of thehigh deposition rate (up to 4 times greater than SMAW) obtained with the largerelectrode diameters.

Welding is normally limited to the flat and horizontal positions with large diameter wires.Smaller diameter wires are used in all positions. A layer of slag is left on the weld beadthat must be removed after welding.

Electrode ClassificationThe electrode wire for flux cored arc welding is tubular and filled with flux. The fluxprovides arc shielding, deoxidation, arc stabilisation and slag formation. When required

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alloying elements can be added to the flux ingredients. An external shielding gas may ormay not be required with these wires, depending on the type. Flux cored and metal coredarc welding electrodes are available in a variety of sizes of 0.8, 0.9, 1.2, 1.6, 2.0, 2.4, 3.0,4.0 mm or equivalent in inches, the wire is contained on spools and coils.

CSA W48.5M classifies FCAW and Metal-Cored wires for carbon steels by using aseries of letters and digits broken into several groups e.g. EXX0T-X-CH

Broken down the classification E4101T-1- H10 in the CSA system means:

E = Electrode; 410 = Weld metal with tensile strength in MPa; 1 = Welding all positions;

T = Wire is Tubular; H10 = 10mls H2 / 100g of deposited weld metal.

The American Welding Society AWS A5.20 also classifies flux cored arc weldingelectrodes for carbon steels, using a series of letters and numbers. A typical cored wire

classification in the AWS system is E70T-1MJH4

E = electrode; 7 = nominal tensile strength of the filler wire in increments of 10,000pounds per square inch e.g. 7 = 70 ksi. (70,000 psi); 0 indicates the positions the wire canbe used T = wire is tubular; 1 gives the performance characteristics of the electrode wire.This number ranges from 1 to 14; MJH4 indicates M is the gas mixture e.g. 75%Ar =25% CO2, J is the impact toughness and H4 is the diffusible hydrogen content.

AWS 5.22 is for Chromium and Chromium Nickel, AWS 5.29 is for low alloy steels.

Advantages of FCAW

Deposition rate is high with larger diameter wires, and for positional welding. Costs can be kept lower than with MMAW because there is less electrode waste (noelectrode stubs), and welder down time due to changing electrodes is less compared toMMAW. Deeper penetration is possible than with MMAW. FCAW has high operator appeal: process is easy to use and welds are of goodappearance. Good quality welds and appearance. Wide range of steel types over a range of thickness.

Disadvantages

High capital cost of machinery, maintenance required on wire feed system. Accessibility to the welding joint is restrictive because of the size of the gun. FCAW-gas shielded is sensitive to wind and drafts (self-shielded version has high drafttolerance). The available length of the welding lead can be restrictive. The equipment is not as portable as MMAW. Electrode is more expensive ($/kg) than GMAW. Produces more smoke and fumes than GMAW.

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Slag covering needs to be removed. Storage of wires must be stored and handled to prevent damage and corrosion.

Defects, Faults and Discontinuities that may occur in FCAWThe following and are a collection of the more common types of weld defects, faults or

imperfections: Overlap, Slag inclusions, Inclusions, Weld spatter, Incompletepenetration, Excess penetration, Incompletely filled groove, Undercut, Craters, Craterpipes, Unequal leg lengths, Wormholes, Hollow Bead.

Lack of fusion may occur in the following forms: Lack of inter-run fusion, Lack ofsidewall fusion, Lack of root fusion.Porosity may occur as: Isolated, Group, Linear, Uniform.Cracking may occur in the following forms: Heat Affected Zone (HAZ), Transverse,Crater.

The above defects, Faults and discontinuities and the NDT method that can be used to

find will be discussed more fully later in these series