Stainless Steel Weld Overlay Repairs
SIB-96-153

The Problem: Stainless Steel Pipe Leakage
Stainless steel piping components are subject to degradation mechanisms such as intergranular stress corrosion (IGSCC), microbiologically influenced corrosion (MIC), and primary water stress corrosion cracking (PWSCC). As a result, a loss in structural integrity and leak tightness may occur.

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

The Solution: The Weld Overlay Repair
In nuclear power plants, in-place weld repairs of piping components can offer significant benefits over replacement. Austenitic stainless steel weld overlay repairs have been applied to more than 600 primary pressure boundary welds in stainless steel piping in boiling water reactors (BWRs). Favorable operating experience and test results have led to acceptance of such repairs as permanent or long-term repairs. The use of these repairs has been endorsed by the Nuclear Regulatory Commission in NUREG-0313, Rev. 2 and by the ASME Code as Section XI Code Case N-504.

Weld overlay repairs are a practical technique for repairing degraded walls of piping components. By adding external weld material, the load carrying capability of the component is restored. These repairs can be applied to austenitic or ferritic piping components using standard or specialized (temperbead) weld techniques. A typical weld overlay repair is presented in the first figure.

Weld reinforcement repairs allow the nuclear plant owner to restore the structural integrity and leak tightness of components which may have been exposed to the subject degradation mechanisms. The repair also offers the opportunity to improve the resistance to the suspected degradation mechanism by correct material selection, where necessary. A typical cross section of a weld overlay applied to a nozzle-to-safe end component is shown below:

Structural Integrity Associates (SI) has played an integral role in the development, application, regulatory and jurisdictional acceptance of austenitic stainless steel weld overlay repairs. SI has been one of the principal designers of the repair using nickel- based Alloy 82. We have also assisted in the development of specialized repairs such as the temperbead repair for nozzle-to-safe end joints.

SI offers similar services in the design of weld overlay repairs for pressurized water reactor (PWR) canopy seal welds and for carbon steel balance-of-plant welds. Additionally, SI is assisting in the development of the technology for in-vessel underwater welded austenitic stainless steel weld overlays.

For more information regarding the design of stainless steel weld overlay repairs or SI's capabilities in this area, please contact SI.