Page 117 of 561

SOLUTIONS: Evaluation of welds The cracks might occur long time after welding is completed and are therefor often referred to as cold cracks.

Other hydrogen sources are rust, oil, paint or condensation along the welding groove. Preheating the groove to say 50 °C will help considerably in reducing the amount of hydrogen.

Conclusion: Dry basic electrodes when there is risk of cold cracking.

2.10

Note the following:

Hard phases form when the weld is cooled rapidly from melting temperature to room rature. Alloying elements, mostly carbon, are forced to dissolve in the weld metal and make it brittle. The following formula describes this process in the case of standard carbon-manganese steel. Ec = %C + %Mn + %(Cr+Mo+V) + %(Ni+Cu) 6 5

15

Steels with Ec = 0.35 and below are usually weldable without any problems at normal steel thickness. For the more highly alloyed steels and steels with thicker dimensions, an elevated working temperature is necessary in order to reduce the cooling rate.

The elevated temperature also allows the hydrogen to diffuse. To determine elevated working temperatures, please consult BS 5135:1984 or SS 064025. If the Ec dimension of the plates and heat input are known, these standards will state whether heating is necessary and the level at which it should take place.

116

Tension cannot be avoided when welding, as steel expands when heated, although correct planning and heat treatment can reduce tension considerably.

Slag embedded in weld

Slag consists of non-metallic particles originating from the coating of the electrode. All slag must be properly removed after finishing each weld bead. Use a chipping hammer and wire brush for this purpose. Slag embedded in the weld will seriously affect the strength of the weld. Try to avoid burning cavities, as any slag deposited in such cavities will be difficult to remove.

When preparing the welding groove, make sure there is sufficient gap to provide good fusion and easy slag removal. Clean off mill scale and rust from the surfaces to be welded and make sure you choose the correct electrode for the welding position to be used.