Welding Processes, Welding Faults, Defects & Discontinuities For NDT Technicians Part 4
Welding Processes, Welding Faults, Defects &
Discontinuities For Non-Destructive Testing Technicians Part
The purpose of this series is to outline to NON-DESTRUCTIVE TESTING TECHNICIANS the basics of the common welding processes. It is important that NDT technicians 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 are a variety of different welding processes in use today. Some of the more popular welding processes in use in industry are SMAW (stick), GMAW (MIG), FCAW, (FLUX CORED)), GTAW (TIG) SAW (SUB ARC). Fabricators may choose from a variety of metal 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 together the 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 flux within the tubular wire (self shielded method). Additionally shielding may be obtained from an externally supplied gas or gas mixture (gas shielded method). Equipment is similar to that used for Gas Metal Arc welding (GMAW) see CINDE Journal November/December 2009)
The flux cored arc welding process can be used to weld carbon and alloy steels, cast and wrought iron and stainless steels. The process is also capable of producing hard surfacing deposits. The process is commonly used to weld medium to thick steels because of the high deposition rate (up to 4 times greater than SMAW) obtained with the larger electrode 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 bead that must be removed after welding.
The electrode wire for flux cored arc welding is tubular and filled with flux. The flux provides arc shielding, deoxidation, arc stabilisation and slag formation. When required alloying elements can be added to the flux ingredients. An external shielding gas may or may not be required with these wires, depending on the type. Flux cored and metal cored arc 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 a series 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 welding electrodes 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 10,000 pounds per square inch e.g. 7 = 70 ksi. (70,000 psi); 0 indicates the positions the wire can be 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 (no electrode stubs), and welder down time due to changing electrodes is less compared to MMAW.
• Deeper penetration is possible than with MMAW.
• FCAW has high operator appeal: process is easy to use and welds are of good appearance.
• Good quality welds and appearance.
• Wide range of steel types over a range of thickness.
• 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 draft tolerance).
• 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.
• 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
The following and are a collection of the more common types of weld defects, faults or imperfections: Overlap, Slag inclusions, Inclusions, Weld spatter, Incomplete penetration, Excess penetration, Incompletely filled groove, Undercut, Craters, Crater pipes, Unequal leg lengths, Wormholes, Hollow Bead.
Lack of fusion may occur in the following forms: Lack of
inter-run fusion, Lack of sidewall 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