• The most important consideration is the oxide film.

• Use of DC reverse polarity (electrode +) is effective for MIG welding while AC is used for TIG welding of Aluminium.

• In AC tungsten inert gas (TIG) welding, when electrode is +ve the oxide of plate is cleaned by ionic bombardment and when it is - ve, the plate gets more heat as it is +ve.

• Because of high thermal conductivity of aluminium.

(a) Nozzle for TIG/MIG welding is larger than that used for steel

(b) Currents used are more than those used for steel, and

(c) Thicker plates are preheated.

• There is no colour change on heating, experience is needed during welding. (Appear­ance of blusters on surface indicates that welding temperature is reached.

• Shielding gas in MIG welding.

Upto 18 mm plates 100% Argon

18-75 mm plates 75% Argon + 25% Helium

above 75 mm plates 25% Argon + 75% Helium

He and He rich mixtures are never used in AC welding.

7.2 WELDING OF LOW CARBON HY PIPE STEELS

A typical relation for carbon equivalent determination for carbon steels is given as (the ele­ments expressed in wt%)

CE = C + (Mn + Si)/6 + (Ni + Cu)/15 + (Cr + Mo + V)/5

1. Low carbon HY pipe steels contain less than 0.45% carbon

The mechanical properties and weldability requirements of high strength steel are : Y. S. = 450 N/mm2 UTS = 530 N/mm2,

Impat energy > 50 J at - 46°C HAZ hardness < 22 HRc (250 VHN)

2. Low carbon content is desirable for high toughness, good weldability and low suscep­tibility to cold cracking in the HAZ.

3. Niobium and vanadium additions give grain refinement, improve Y. S. and toughness.

4. In X-65 and X 70 low carbon, boron free steels (CE = 0.33), carbon content < 0.04% improves resistance to hydrogen induced cracking, the field weldability and HAZ toughness. Critical material parameter Pcm for weld cracking is given by (elements in weight %)

Si

Pcm = C + + (Mn + Cu + Cr)/20 + Ni/60 + Mo/15 + V/10 + 5 B.

30

5. It is necessary to reduce CE and Pcm value for high field weldability specially for pipe materials X 65 and X 70. CE < 0.4% and Pcm < 0.15% are preferable to obtain HAZ hardness values < 250 VHN

6. Effects of C. E. on UTS and YS of X 65 pipe steel are shown in Fig. 7.1 (a) and (b).

7. The effect of Pcm on HAZ hardness for Low carbon pipe materials is shown in Fig. 7.2.

Pcm = critical material parameter.

Fig. 7.1 (b) Effect of carbon equivalent on YS of X65 pipe steel. (R. G. Baker, Proc. Rosenhain Centinary Conf., Royal Society, 1975)