Welding Processes, Welding Faults, Defects & Discontinuities For NDT Technicians Part 1
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.
GAS METAL ARC WELDING – BASICS
Gas Metal Arc Welding (GMAW)
Gas metal arc welding (GMAW) is an electric arc welding process that fuses together the parts to be welded by heating them with an arc between a solid metal electrode and the work. Filler metal is obtained from melting of the electrode wire, which is fed continuously into the arc by the welding equipment. Shielding is obtained from an externally supplied gas or gas mixture. A diagram of equipment and the welding process is shown below.
The gas metal arc welding process is capable of welding most ferrous and non-ferrous metals from thin (0.5mm) too thick sections. It can be used in all welding positions to produce weld deposits with little or no spatter. Higher deposition rates, travel speeds and welding efficiencies result in less welding time in production situations, as compared to shielded metal arc welding (SMAW).
The metal transfer refers to the method by which molten metal from the wire electrode crosses the arc to form the weld deposit. Several methods of metal transfer are used in the gas metal arc welding process.
Short-circuiting transfer (sometimes called Dip transfer) when the welding wire moves towards the metal or weld pool until it contacts the material. At this point, a short-circuit is produced which causes the wire to be pinched off, producing a new arc. This cycle occurs many times a second depending on the amperage/voltage relationship that has been set.
The short-circuiting method of metal transfer can be used in all positions and is commonly used to weld thin carbon steel, low alloy steel and stainless steels in plate and pipe. CO2 and Argon/CO2 gas shielding mixtures are commonly used.
Globular transfer is similar to short-circuiting in that a droplet is formed at the end of the electrode wire. However, during the globular transfer the molten ball continues to grow until it is larger than the diameter of the electrode wire. The droplet detaches and crosses the arc to form the weld deposit. Because of this, the arc is less stable and more spatter is produced. Globular transfer is used to weld the same metals as short-circuiting transfer, except in greater thickness.
Globular transfer is limited to the flat and horizontal welding positions. CO2 is commonly used to shield the arc.
Spray transfer is characterised by small droplets crossing the arc. The droplets are formed at the tip and pinched off due to electromagnetic forces. Argon or a mixture of argon or helium is used to shield the arc. Spray transfer can also be used to weld carbon steels, low alloy steels and stainless steels, using a mixture of argon and or other gas mixtures. Spray transfer on steel is normally used to weld medium to heavy thickness of steel in the flat and horizontal welding position.
The electrode wire for gas metal arc welding is normally supplied on a spool or reel and is solid and bare. The electrode wire size is determined by its diameter. Various wire diameters are available: e.g. 0.6, 0.8, 1.0, 1.2, 1.3, 1.6, 2.4 mm.
Canadian electrode classifications issued by the Canadian Standards Association (CSA) are similar but not necessarily the same as the American Welding Society (AWS). The AWS system is normally in Imperial units whereas the CSA are typically in (SI) metric units.
If the electrode classification is in SI units remember 1MPa = 145.03 psi.
Advantages of GMAW for the Welding of Structural
• Deposition rate is high with spray transfer
• Costs can be kept lower than with SMAW because there is less electrode waste (no electrode stubs), no slag removal and welder down-time due to changing electrodes is less compared to SMAW.
• Smoke and fumes are less than SMAW or FCAW.
• Obtains deeper penetration than SMAW in spray transfer mode.
• It is versatile. (All position welding process for carbon, low alloy and stainless steels).
• 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.
• Shielding gas is sensitive to wind and drafts.
• The length of the welding lead is restrictive.
• The equipment is not as portable as SMAW.
Defects, Faults and Discontinuities that may occur in
The following may occur and are a collection of the more common types of weld defects, faults or imperfections. Overlap, Weld spatter, Incomplete penetration, Excess penetration, Incompletely filled groove, Undercut, Craters, Crater pipes, Unequal leg lengths, Wormholes.
Lack of fusion may occur in the following forms: Lack of inter-run fusion, Lack of sidewall fusion, Lack of root fusion. (Fusion problems is a main concern with this process). Porosity may occur as: Gas Pore, Isolated, Group (stop/start), Linear or Uniform. Cracking may occur in the following forms: Heat Affected Zone (HAZ), Transverse, Crater.
Note: Defects Faults and Discontinuities and the NDT method that can be used find them will be discussed more fully later in these series.
On to Part 2 ...