In service and maintenance welding defect

In-Service Welding Concerns; 

When welding, repairing or modifying pipelines or equipment that are currently being used (In-Service), the most worrying is the leak of the fluid inside. While or after doing On-line Welding. The two main reasons that cause the leak are:

  1. Burn Through: While welding at the melting area of ​​the welding, the strength will be reduced. We may not be able to withstand the Pressure at  Service value.
  2. Weld Cracking or Hydrogen Induced Cracking (HIC) from welding. The additional concern in welding is the cooling rate. Since the inside of the pipe or equipment has fluid flow, it may cause a fast weld-cooling rate. And that makes the structure become brittle.

Weld burn through

“Burn Through”; 

Controlling the Heat Input in in-service welding to prevent Burn Through is very important. According to Standards (API / ASME), we recommend the use of welding wires with a small Diameter (3/32 in, 2.4 mm) for First Weld Pass, especially for workpieces with thickness less than 6.35 mm (1/4 in.) Or to be confident they may create a mock-up coupon for welding testing.

However, do not forget that by welding with low heat input, the material will cool faster (Fast Cooling Rate) and may cause the structure of the material to break easily.

Hydrogen induced cracking in welding
For example, in the API RP 577 Welding Inspection and Metallurgy, it is recommended that the inside of the pipe when welding, the flow rate of the liquid must not exceed 1.2 m / s to prevent welding too cold.
In-Service Welding – Hydrogen Cracking
In welding, in-service welding is different from normal welding. Where the liquid is constantly flowing inside the pipe or equipment, the flow of liquid inside the device helps to carry heat away from the welding. It allows welding to cool faster and reduces the chance of burn-through while welding.
However, the welding line cools too quickly to give us a hard and brittle structure (easily broken) at the Heat affected zone of the Coase grain HAZ.

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