Use of Mechanical Stress Improvement Process to Mitigate Stress Corrosion Cracking

“Use of Mechanical Stress Improvement Process to Mitigate Stress Corrosion Cracking in BWR Piping Systems”

by J. S. Porowski, W. J. O’Donnell, M. L. Badlani, and E. J. Hampton. Nuclear Engineering and Design 124, pp. 91-100, Elsevier Science Publishers, February 1990.

Keywords:
Mechanical Stress Improvement Process; MSIP; stress corrosion cracking; compressive stresses; compressive plastic strains; NUREG 0313

The Mechanical Stress Improvement Process (MSIP) has been extensively applied to prevent stress corrosion cracking in BWR plants. MSIP removes residual tensile stresses from weldments, thus preventing the initiation of cracks and retarding the growth of pre-existing flaws in piping systems. The process involves a slight permanent contraction of the pipe on one side of the weldment. The resulting plastic flow redistributes the residual as-welded stresses and generates beneficial compressive stresses at the inner pipe surface in the region of the weldment including the weld metal and heat affected zones.

MSIP imposes only monotonic compressive plastic strains and does not require severe temperature gradients. Hence, use of MSIP is particularly advisable for weldments which have geometrical or material discontinuities such as nozzle-to-safe-end welds. Moreover, the process can be applied to the piping system either filled with water or empty. MSIP is accepted by NUREG 0313 as a Stress Improvement (SI) process for mitigation of IGSCC in BWR plants. To date, it has been applied to 532 welds in 12 BWR plants in the United States and abroad. MSIP has been used to improve 157 nozzle weldments ranging from 4″ to 28″ diameter. The process has also been applied to weldments with pre-existing cracks. The basic concept, results of analyses and tests, and application of MSIP are described herein. 

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