How do I set about repairing a ½Cr-½Mo-¼V casting and what are the risks?

Frequently Asked Questions

Outline procedure for repair of a ½Cr-½Mo-¼V casting

1. Preheat uniformly to a minimum of 250°C preferably in a furnace.
2. Use basic low hydrogen electrodes of 2¼ Cr-Mo type, baked at the time and temperature recommended by the manufacturer to give less than 5mls H₂/100gm weld metal.
3. Butter excavation surfaces with 2.5mm dia. electrodes.
4. Deposit second layer with 3.2mm dia. electrodes.
5. Half fill with 4.0mm electrodes if the repair is deep (i.e. greater than approximately 30mm).
6. Hot grind weld toes at the preheat temperature.
7. If the excavation is large, carry out an intermediate stress relief at 700°C allowing 1 hour at temperature for each 25mm thickness; heating and cooling rates must be controlled to a maximum of 25°C per hour.
8. Fill completely with 4mm dia. electrodes.
9. Repeat steps (6) and (7).
10. Carry out NDT on the repaired casting - as a minimum, both MPI (magnetic particle inspection) and ultrasonics should be used.

Excavations should be flared out at the ends to a slope of around 4:1 and corners ground to give a radius of some 6mm. The included angle of the sides of the excavation should be in the order of 60°. If thermal cutting processes are used to remove the defect then preheat is required and a minimum of 2mm should be ground or machined away from the excavation to remove any HAZ after cutting. This should be followed by MPI as a minimum to confirm that the defect has been completely removed.