As with other arc welding procedures, a good MIG/CO2 welding procedure starts with correct edge preparation and joint fit-up. The joint surfaces must be free from rust, scale grease, oil, paint and other foreign materials. For making full penetration joints by welding with spray transfer technique from both sides, it is necessary to gouge out the root from the second side before starting to weld that side. When welding is done only from one side, suitable weld backing must be provided. Sometimes weld backing can be avoided by making the root pass
with the short-circuiting technique to obtain uniform penetration and depositing the fill-up passes by high current spray transfer technique.
The welding equipment must be assembled and the welding parameters set according to the manufacturers instructions. All gas and water connections must be absolutely leak-proof. If the shielding gas gets contaminated with air or water, the arc becomes erratic and pores appear on the weld.
The gun nozzle size and the shielding gas flow rate must be correctly set according to the material being, welded and its joints design. Some joint designs demand longer nozzle-to-work distance than normal; in such cases one must use higher gas flow rates than those recom-mended by the equipment manufacturer or as specified in standard procedures, and a gas nozzle of adequate size to cover the welding area. On the other hand, smaller nozzle sizes may be used for welding in confined areas or in the root of a thick joint. The electrode-feed rolls and the contact tube must be compatible with the size and composition of the electrode, as recom-mended by the manufacturer. If the contact tube is worn in usage, it must be replaced before the gun starts getting heated due to bad electrical contact between it and the electrode.
Electrode extension is the distance between the end of the contact tube and the gas nozzle opening, which is between 6.4 and 9.5 mm for normal spray-type welding. In special applications, the contact tube may be flush with or protruding from the gas nozzle. For example, when using the short-circuiting arc, the contact tube may extend 3 mm beyond the end of nozzle. Further guidance on procedures using contant-voltage power source is given in Table 8.6.
Table 8.6. Guidance on MIG/CO2 welding procedure
Arc type Typical conditions and Procedure
applications
Spray-type arc 360 amp, 34 V, 1.6 mm wire. 1. Set open-circuit voltage to a little above Downhand welding of plate the required arc voltage; e.g., 38 V.
2. Set wire-feed speed* to the recommended value for the electrode size and material, e.g. 5 m/min.
Short-circuiting arc 120 amp, 19 V, 1.2 mm wire. 1. Set open-circuit voltage to a little above Positional welding of sheet the required arc voltage, e.g. 20 V.
and plate 2. Set wire-feed speed* to the recommended value for the electrode size and material, e.g. 2.5 m/min.
3. Set choke (tune the circuit) to get required crispness and heat of arc.
The wire-feed-speed determines the welding current.
Following the setting of Table 8.6, trial bead welds should be deposited to arrive at correct arc voltage and the electrode-feed rate (current). In the short-circuiting procedure, the choke should be finally adjusted to obtain good arc start and a stable arc with minimum spatter.
QUESTIONS
8.1 What features a successful weld design must possess. List the factors that are of help in developing a weld design.
8.2With a neat sketch state the elements that a complete welding symbol contains accord-ing to ISO and AWS system.
8.3 What is welding procedure sheet? Discuss the steps taken in preparing a welding proce-dure sheet. Discuss joint preparations for fusion welds.
8.4 What is meant by welding position? With neat sketches explain the different types of welding positions. Define the terms weld slope and weld rotation in this regard.
8.5 How do you define welding procedure? Why is it important to draw-up welding proce-dure before the welding is carried out.
8.6 What are the main elements of an standard procedure sheet? What are the benefits of using a standard procedure sheet?
8.7 Discuss the types of joints used in welds. State the factors which are considered in the design of welded joints.
8.8 How do you select welding parameters? Such as :
(a) Electrode size (b) Current type and amount
(c) Welding speed (d) Arc length
(e) Electrode angle (f) Welding positions.
8.9 Briefly discuss the special considerations in welding procedure development for SAW.
What type of weld backings are in common use for SAW.
8.10 Explain the difference between the various types of backings used in SAW.
(a) Backing strip and copper backing (b) Flux backing and backing tapes.
8.11 Briefly explain the TIG and MIG welding procedure.
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As the welded joints are finding applications in critical components where the failure results into a catastrophy, the inspection methods and acceptance standards are increasing. Acceptance standards represent the minimum weld quality and are based upon test of welded specimens containing some discontinuities, usually a safety factor is added to yield the final acceptance standard. A good research effort is being directed to correlate the discontinuities with the performance.
In the present discussion we shall study the weld discontinuities commonly observed in the welds, their causes, remedies and their significance. Small imperfections, which cause some variation in the normal average properties of the weld-metal are called discontinuities.
When the discontinuity is large enough to effect the function of the joint it is termed a defect.
Standard codes do permit limited level of defects based on fracture mechanics principles, taking consideration the service conditions of the fabrication. Inspite of all this, the fabricator
(a) Undercut (b) Cracks
(c) Porosity (d) Slag inclusions
(e) Lack of fusion (f) Lack of penetration
Fig. 9.1 Typical weld defects
Weld Quality
must strive to prevent the occurrence of weld defects in the first instance and to rectify them if they do occur. There are many types of defects which have been classified in various documents (e.g., BS499 part I, 1965). For our purpose we shall be discussing the most important ones shown in Fig. 9.1. These are undercuts, cracks, porosity, slag inclusions, lack of fusion and lack of penetration.
9.1 UNDERCUTS
The term is used to describe a groove melted into the base metal adjacent to the toe of a weld and left unfilled by the weld metal. It also describes the melting away of the sidewall of a welding groove at the edge of a layer or bead. This melting away of the groove forms a sharp recess in the sidewall in the area in which the next layer or bead must fuse. (Slag may be
keyed into this undercut which, if not removed prior to subsequent passes, may become trapped in the weld.) An undercut, therefore, is a groove that may vary in depth, with, and sharpness at its root.