TWI Frequently asked questions
There are two main problems in repairing this class of material. Firstly, it is sensitive to hydrogen induced cold cracking and secondly, it is very sensitive to reheat or stress relief cracking. Therefore, the precautions necessaryduring welding are specified to reduce the risk of either of these mechanisms giving a problem.
Outline procedure for repair of a ½Cr-½Mo-¼V casting
- Preheat uniformly to a minimum of 250°C preferably in a furnace.
- 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 H 2/100gm weld metal.
- Butter excavation surfaces with 2.5mm dia. electrodes.
- Deposit second layer with 3.2mm dia. electrodes.
- Half fill with 4.0mm electrodes if the repair is deep (i.e. greater than approximately 30mm).
- Hot grind weld toes at the preheat temperature.
- 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.
- Fill completely with 4mm dia. electrodes.
- Repeat steps (6) and (7).
- 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 cuttingprocesses 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 thatthe defect has been completely removed.
Copyright by TWI, 1999
