• 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 %)
Pcm = C + + (Mn + Cu + Cr)/20 + Ni/60 + Mo/15 + V/10 + 5 B.
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 (a) Effect of carbon equivalent on UTS of X65 pipe steel. (R. G. Baker, Proc. Rosenhain Centinary Conf., Royal Society, 1975)
Carbon equivalent %
Fig. 7.1 (b) Effect of carbon equivalent on YS of X65 pipe steel. (R. G. Baker, Proc. Rosenhain Centinary Conf., Royal Society, 1975)
Fig. 7.2 Effect of Pcm on HAZ hardness for low carbon pipe steel