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COATED ELECTRODES

Storing and re-drying

If properly stored, transported and used electrodes will form weld deposits with low hydrogen content. If not, hydrogen porosity can lead to hydrogen cracking (cold cracking) if the component welded is under strain, vibration or load.

General background

When welding carbon-manganese and low alloyed steels cold cracking (often referred to as hydrogen cracking) can occur. These cracks generally form in the coarse-grained, heataffected zone (HAZ) of the base plate. The most important factors which influence the risk of hydrogen cracking are:

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• Chemical composition of the steel • Cooling rate • Hydrogen content of the weld metal Hydrogen is always present in small quantities during welding and is a prerequisite for the occurrence of cold cracking.

The most common sources of hydrogen are: • Moisture in the electrode coating • Moisture in the air • Condensation, rust, oil, paint or pri­mer in the welding joint area. By following the recommended storage, and procedures for handling and redrying , the moisture level in electrode coatings can be minimised, along with the associated risk of cold cracking.

Hydrogen levels are measured as ml H2 /100 g weld deposit and typical levels for different electrode coating types are as follows:

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Rutile and Acid >15 ml/100 g Basic < 10, < 5 and < 3 ml/100 g

Storage and handling

Welding consumables should be stored in their original packing. As a guideline we recommend the following temperatures in the storage room: Temperatures °C (°F)

Max. Relative humidity in %

5–15

(41–59)

60

15–25

(59–77)

50

Above 25

(77)

40

During the winter, it is possible to maintain low relative humidity by keeping the temperature in the storeroom at least 10 °C (18 °F) above the outdoor temperature. During certain periods in the summer and in a tropical climate, sufficiently low relative humidity can be maintained by air de-humidification. The purpose is to avoid hydrogen absorption to the extent possible.

Redrying

Rutile electrodes showing signs of damage by moisture can be redried at a temperature of 90–110 °C for 0.5–1 hour.

Basic electrodes are normally redried at a temperature of around 350 °C for 2 hours, to achieve a hydrogen level of 5–10 ml/100 g. (BS 5135, Scale C). Redrying should be restricted to a maximum of 3 cycles.

To achieve extreme low hydrogen levels, <4.0 ml/100 g, a redrying temperature of 42–400 °C is recommended for 1–2 hours. Redrying should be restricted to 1 cycle.