Weld Overlay Repairs on Carbon Steel Piping
SIB-96-174

The Problem: Carbon Steel Pipe Leakage
Carbon and low alloy steel piping and components are subject to degradation mechanisms which include: pitting, general corrosion, microbiologically influenced corrosion (MIC) and erosion-corrosion. As a result, they may experience localized wall thinning and leakage.

The Issue: Repair or Replace?
When faced with defective carbon and low alloy steel piping components, plant operators must decide between either repairing the component or replacing it.

The Solution: The Weld Overlay Repair
The weld overlay repair provides a practical technique for repairing degraded walls of piping components. As shown above, the weld overlay restores the load carrying capability and integrity of the repaired location through the application of external weld material. The repair extends the life of the component by considering the underlying flaw in the base material and the expected end-of-life flaw extent, by considering the effects of continued degradation (e.g., general corrosion, crevice corrosion, erosion-corrosion, or MIC). With correct material selection, the weld buildup also provides improved resistance to the degradation mechanism itself.

In-place weld overlay repairs can offer significant benefits over piping replacement in nuclear and fossil power plants. More than 600 such repairs have been applied to primary pressure boundary welds in austenitic stainless steel piping in boiling water reactors (BWRs) and in other process plants. Favorable operating experience and test results have led to acceptance of such repairs as permanent or long term repairs.

Weld overlay-type repairs in nuclear plants are generally designed in accordance with the flaw evaluation rules of the applicable portion of ASME Code, Section XI. Two ASME Section XI Code Cases which provide detailed guidance for the application of weld overlay repairs to carbon steel Class 2 and 3 piping components in nuclear power plants are in the final stages of approval by ASME. Weld overlay repairs to carbon steel piping in other types of plants consider the requirements of the governing piping codes.

The weld overlay concept is potentially applicable to any piping component that is subject to a degradation mechanism, including non-pressure retaining components subject to wear conditions. Additionally, half-bead and temperbead approaches are particularly well suited for weld repairs on carbon and low alloy steels, where post weld heat treatment might otherwise be required.

Structural Integrity Associates (SI) has extensive experience in the design of weld overlay repairs for numerous configurations in several types of piping. Additionally, SI recently assisted one utility in the development of a handbook containing pre-qualified designs to assist personnel in the timely and efficient selection of a repair design during their outage, thereby minimizing critical path time impact.

If you would like more information regarding weld overlay repairs for degraded carbon steel piping components of SI's capabilities in this area, please contact SI.