Residual stress improvement by Peening

Techniques which introduce a compressive surface residual stress have been shown to be effective SCC mitigators. The following peening techniques have been developed and already applied underwater remotely to core shrouds in Japan. 

The peening process introduces a compressive stress in the peened surface layer by constraint of surrounding material. 

The LP process utilizes water-penetrable green light of a frequency-doubled Nd YAG laser delivered with an optical fiber and generates the high pressure plasma of several Gpa on the surface [8.3-8.5], The WJP process relies on the pressure derived from the cavitation collapse at the surface under the high pressure water jet [8.6 - 8.8]. The SP process utilizes spherical Type 304 stainless steel shots (diameter 2mm) hardened during production process to have Vickers hardness about 500, which are projected by highly pressurized water ( IMPa) on the surface [8.9, 8.10]. 

The effectiveness of these processes to mitigate SCC of Type 304 reactor internal material have been demonstrated by laboratory testing. Compressive residual stress of several hundred MPa to a depth of 300 1000 im is obtained. SCC susceptibility can be significantly reduced or eliminated by peening processes 

Solution annealing of Type 304 stainless steel is a well established method for eliminating sensitization, thereby reducing SCC susceptibility. This principal has been extended to surface treatment to desensitize Type 304 by surface melting and solution annealing utilizing appropriate heat input controls. 

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