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Vol.1 No.2 ← GA 6 - 7 - AA 8 - 9 - NT 10 - 11 - 12 - 13Vol.1 No.3
Vol.1, No.3, NT11 EJAM (1-3-NT11) - RESIDUAL STRESS IMPROVEMENT FOR NICKEL BASED ALLOY PWSCC MITIGATION BY ULTRASONIC SHOT PEENING

RESIDUAL STRESS IMPROVEMENT FOR NICKEL BASED ALLOY PWSCC MITIGATION BY ULTRASONIC SHOT PEENING

 
MITSUBISHI HEAVY INDUSTRIES, LTD.
 

KEYWORDS:
USP, PWSCC, residual stress improvement, Peening, Alloy 600/690, Austenitic stainless steels, Steam Generator, Reactor Vessel

 
1. Technical summary
 
Classification
(I: Inspection, II: Repair, III: Replacement, IV: Preventive Maintenance, V: Others)
 
(1)Ultrasonic Shot Peening (USP) is a kind of shot peening method. Piezoelectric element is used for an ultrasonic vibrator instead of an air/water drive source of a conventional one.
EJAM1-3-NT11-Fig.1_Principle_of_USP

Fig.1 Principle of USP

(2)The improvement of surface residual stress is effective in prevention of stress corrosion cracking (SCC) initiation due to tensile residual stress.

EJAM1-3-NT11-Fig.2a_A_typical_residual_stress_distribution_before&after_USP(Sample-piece)

Fig.2a A typical residual stress distribution before/after USP (Sample piece)
EJAM1-3-NT11-Fig.2b_A_typical_residual_stress_distribution_after_USP

σθ: Residual stress of welding direction
σr: Residual stress of cross direction

EJAM1-3-NT11-Fig.2b(2)_A_typical_residual_stress_distribution_after_USP

Fig.2b A typical residual stress distribution after USP *
(CRDM nozzle model of Vessel Head)
* Residual stress improvement effects would be expected
in the same way irrespective of the initial surface condition.

2. Scope

(1) Components: Steam Generator (SG) of PWR and Reactor Vessel (RV) of PWR

(2) Location: SCC susceptible zone of SG and RV

(3) Material: Alloy 600/690 (Nickel based alloy) and Austenitic stainless steels of SCC sensitizes zone

(4) Condition: This method can be applied to components under atmosphere.

EJAM1-3-NT11-Fig.3a_USP_applicable_location_of_Steam_Generator(SG)_of_PWR_plant

Fig.3a USP applicable location of Steam Generator (SG) of PWR plant
EJAM1-3-NT11-Fig.3b_USP_applicable_location_of_Reactor_Vessel(RV)_of_PWR_plant

Fig.3b USP applicable location of Reactor Vessel (RV) of PWR plant
3. Features

(1) Residual stress improvement depth by USP is deeper than the conventional shot peening, because the ultrasonic vibration and large diameter shots can be applied.

(2) Mitigation of PWSCC propagation

(3) Reduction of personnel dose rate by the remote operation (manipulator)

(4) Lost parts management is easy because of working in the chamber and using larger diameter shots compared with the conventional shot peening.

(5) Application to complicated geometry is possible, since the suitable construction range is wide.

4. Examples of Application

(1) Outlet / Inlet Nozzle to safe-end weld of SG of PWR (more than 10 PWR plants)

(2) Outlet / Inlet Nozzle to safe-end weld of RV of PWR (only under construction)

(3) CRDM* Nozzle to Vessel Head J-weld of PWR (several PWR plants)
* CRDM: Control Rod Drive Mechanism

EJAM1-3-NT11-Fig.4a_USP_equipment_for_Outlet&Inlet_Nozzle_safe-end_of_SG

Fig.4a USP equipment for Outlet / Inlet Nozzle safe-end of SG

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Fig.4b Demonstration of USP (in Visible Chamber)

EJAM1-3-NT11-Fig.4c_USP_equipment_for_J-weld_of_CRDM_Nozzle_to_Vessel_Head

Fig.4c USP equipment for J-weld of CRDM Nozzle to Vessel Head
5. Reference
PWSCC Preventive maintenance Program for ALLOY 600 ICONE16-48378 (2008)
 
6. Contact
Japan Society of Maintenology (ejam@jsm.or.jp)