Frequently Asked Questions
Mild and high yield steel plate may be cold bent or formed as part of the normal fabrication process. There is, however, a limit of 5% outer fibre strain in most application standards where, when deformation exceeds this, hot working is mandatory. Repair by cold straightening therefore requires an assessment of the nature of the damage which will have work hardened the material locally. Smooth bends may be cold straightened, preferably in a press or with hydraulic jacks and formers; provided that no cracking occurs this may be acceptable. "Freehand" straightening with a sledgehammer is not good practice and is prohibited in many application standards since it may just extend the strained area or cause damage elsewhere. It is not good practice to cold straighten a part which will have to be welded or which will be in service at elevated temperature where the plastic strain has been over about 4%. The resulting material may suffer from reduced fracture toughness due to strain age embrittlement. If the damage has caused this level of strain, then hot straightening should be used. The steel should be heated slowly to around 600/620°C and straightened at that temperature before being allowed to cool slowly and naturally. The heated area should extend well beyond the area to be straightened and, if a gas torch is used, a high heat output nozzle is required rather than a cutting nozzle. Under no circumstances, should any corrective work be carried out in the temperature range from 150 to 480°C.
The mechanical properties of cold formed areas will be affected by the cold working operation and it may be necessary to demonstrate that these changed properties are still acceptable from a design point of view. Full recovery of properties can be achieved by normalising or quenching followed by tempering.
A cold worked area will also be susceptible to caustic stress corrosion cracking and stress relieving may be needed to eliminate this risk.
There may be other requirements that limit the extent of cold straightening, and the guidelines, given in the relevant application standard (e.g. PD 5500), should be followed.