How to Identify a defect in Castings
The most common Non Destructive Evaluation (NDE) methods to identify flaws, like porosity and voids, in a casting are: visual inspection, Radiographic Testing (RT), Ultrasonic Testing (UT), Liquid Penetrant Examination (LP), Magnetic Particles Examination (MP).
The method to be used is to be agreed with the Client and included in the contract agreement.
ASME B16.34 and ASTM standards provide a reference on how the tests have to be performed (e.g. for RT: ASTM E 94 and ASTM E 142; for MP: ASTM E 709; etc.).
The most accurate method is the RT. This method is also the most expensive and the one requiring more time (with potential impact on the delivery time).
The UT is very simple but its effectiveness is highly related to the ability of the technician to interpret the test results. The UT is not applicable on austenitic steel castings.
Repair Classification
The repair acceptability and their classification depend on the contractual agreements between the casting sup- plier and the buyer (Client).
The ASME B16.34 standard provides a guide- line for acceptance criteria for the flaws.
Manufacturers internally classify repairs as:
• “Major repairs”: defects whose cavity originated by the total defect removal is deeper either than 25 mm or than the 20% of the nominal thickness (the smaller between the two parameters is the valid one) or with the removed surface wider than about 65 cm2.
• “Minor repairs”: defects whose cavity originated by the total defect removal has a depth smaller than 20% but higher than 10% of the nominal thickness, up to a maximum of 16 mm with an area smaller than 45 cm2.
• “Cosmetic repairs”: defects whose cavity originated by the total defect removal has a depth smaller than 10% of the nominal thickness, up to a maximum of 8 mm and with an area smaller than 25 cm2.
How to Repair
The repair entails 5 steps:
1. Removal of the defect;
2.Repair by welding
; 3.Post-weld heat treatment; 4.Inspection of the repaired areas; and 5.Document
1. Removal of the defect
The defect should be removed by grinding, chipping or arc-air. The cavity obtained by removing the defect has to be smooth, without edges and without areas with excess of carbon content (in case of removal by arc-air). The complete removal of the defects shall be confirmed by an inspection, for example by LP or MP.
2. Repair by welding
The repair of the defect shall comply with the applicable welding procedure specification approved by the customer (WPS) and shall be made by welding operators qualified in accordance with the ASME code (section IX).
3. Post-weld heat treatment
Castings with cosmetic defects shall not undergo any heat treatment after the welding repair. All the parts with minor or major re- pairs (not covering the 100% of the wall thickness or having an extension smaller than 200 cm2) shall undergo a Stress Relieving Heat Treatment.
All the parts with major repairs covering the 100% of the thickness or having an extension greater than 200 cm2, shall undergo the complete heat treatment: this heat treatment shall be done in accordance to the applicable ASTM specification.
4. Inspection on repaired areas
After the final heat treatment, all the repaired areas shall be inspected again at least with the same NDE that initially showed the defect.
If other defects should result from this inspection, the procedures above shall be repeated until only acceptable defects are shown.
After final valve assembling, the valve shall be hydro tested according to the applicable standards (API, BS or internal procedure): acceptance criteria shall be per API, BS or Client’s requirements.
Gas test may be carried out as contractually agreed with the customer.
5. Document
For cosmetic repairs, usually no document is shared with the Client because no thermal treatment after the repair is needed therefore the repair does not affect the mechanical properties of the casting.
For minor and major repairs, if indicated in the contractual documents, the description of the defects, the used WPDs and PQRs, and the heat treatment diagrams shall be shared with the Client.