General Conditions for Welding of Metals (With Diagram) | Metallurgy

For obtaining good results it is desirable that the surfaces should be clean and free from foreign materials. The cleaning can be done by wire brushing, machining or sand blasting. Impurities if present tend to make the joint weaker as the welded portion is filled with gas and slag inclusions and metal becomes brittle and the cohesion between the metals is therefore poor.

Edge preparation is important aspect for obtaining sound welds. If two pieces are kept side by side and welded then the joint will not be strong one because the two metals cannot be united throughout the depth.

The various possible edge preparations for butt are:

(a) Square

(b) Single-V

(c) Double-V

(d) Single-U

(e) Double-U

(f) Flanged.

In all these the cleaning of the faces from dust, sand, grit, oil and grease is very important.

Figs. 9.2 and 9.3 show various types of lap joints. Lap joint is preferred for tubular construction where one tube telescopes into the other.

Fig. 9.4 shows the various fillet or T-joints. In order to ensure uniform penetration fusion, the plates on surfaces should have good fit-up.

Fig. 9.5 shows the corner joints with welding on one or both sides, depending on position of corner joint.

Fig. 9.6 shows two edge joints (on groove for small thickness and a bevelled groove for thicker plates).

Fig. 9.7 shows the terminology used for butt weld joints.

In square butt weld, the distance between two faces is kept about 3 mm and it is used for sheet about 1 to 5 mm thick. Single-V and single-U butt welds are frequently used for sheet of about 5 mm to 15 mm thickness. The angle between edges is kept about 70° to 90° depending upon the welding technique employed.

The double-V and double-U butt welds are used for welding plates over 15 mm thickness. Double-U preparation is preferred for very thick sheets due to requirement of lesser filler metal but preparation of U is costlier. For materials upto about 3 mm the edges may be flanged and fused together without need of separate filler metal.

The gap between plates should be kept at an optimum value since penetration will be insufficient with small gap, and with wide gap, the molten weld pool will fall through and welding will be difficult to control. Another important point in welding is to overcome oxidation at high temperatures. The oxides of metals formed at high temperatures encountered in welding have serious weakening effects.

This influence is counteracted by using a flux which removes the oxides and permits perfect cohesion of the metals. In gas welding the flux is used from outside in the form of powder whereas in arc welding the flux is coated on the electrodes. The flux forms a protective coating of slag over the weld metal and creates a non-oxidising atmosphere.