PSI - Issue 19

Lloyd Hackel et al. / Procedia Structural Integrity 19 (2019) 346–361 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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laser peening process as shown in Figure 2 creates compressive stress deeper than the limiting pit depth ensuring that CISCC and corrosion fatigue cracking will not occur.

Figure 3. Analyses and tests show that corrosion pit depths will only penetrate to somewhat less than 1 mm (0.040 inches) before arrest ing. It the pitting penetrates beneath a shallow layer of peening and reaches tensile stress then cracking can initiate rapidly

Thus it is critical that the pitting depth does not reach a tensile layer under the peening compressive layer. Figure 4 shows a measurement of residual stress in 316L stainless steel samples that were shot peened and a trace for samples that were treated by cavitation shotless peening [23]. In both cases the depth of compressive stresses reaches the transition to tensile stress at about 300 µm or 0.012 inches depth. This shallow depth is right in the range of the depth for pits to grow, thus potentially making shallow peening more susceptible to IGSCC than actually no peening at all. A safety factor thereby dictates that the compressive stress layer extend several times deeper than the potential pit growth depth. Laser peening provides a very robust safety factor.

Figure 4. Residual stress vs depth in 316L stainless steel for shot peening and cavitation shot peening per Masaki [23]. Compressive stress extends to depth of about 300  m which is near the maximum depth of pit growth creating a potential problem.

5. Dry canister test panel preparation, laser peening and residual stress measurement:

Dry canister test panel preparation, laser peening and residual stress measurement: Using the Holtec International preparation process of roll forming and welding for canister fabrication, test panels of 316L stainless steel of 10 inch (250 mm) size by 0.65 inch (16.5 mm) thickness were fabricated; specific panels were laser peened, residual stress measurements taken and ASTM G36 CISCC testing performed comparing initiation and crack growth in unpeened and laser peened areas.