Post-weld heat treatment (PWHT) is carried out to improve the brittle fracture resistance of welded joints by relieving residual stresses and in most cases also to improve the toughness of the weld metal and heat affected zone (HAZ).
Ultrasonic Peening can effectivley replace conventional PWHT. The HSE Guidance Notes require PWHT for highly stressed nodal welds greater than 40 mm thick and other welds greater than 50 mm thick unless fracture mechanics calculations show that the joint posses adequate toughness in the as-welded condition.
Consequentely, there is interest for finding an alternative way to relieve residual stresses.
The replacement of conventional PWHT can be done by:
Replacement of PWHT by Ultrasonic Peening Treatment
Ultrasonic Peening treatment has demonstrated its contribution to the reduction of tensile residual stresses and the introduction of compressive stresses. Clear improvement on CTOD values, which is a measure of brittle fracture resistance, has been achieved as a result of UP treatment of the weld and HAZ. Taken into account that UP relieves residual stresses down to a depth of approx. 12 mm the treatment could be applied after every weld pass for 40 mm welds. As a result the residual stress field could be modified through the entire weld thickness. Conversely the CTOD values will be improved also through the weld generating a sustained crack growth resistance. Furthermore the fatigue resistance of the weld will be drastically improved.
Replacement of PWHT by Ultrasonic Method
Ultrasonic treatment during welding
The Ultrasonic Method consist of inducing ultrasonic vibrations in both plates during welding. The practical procedure is explained in a subsequent page. The ultrasonic vibrations have a definite frequency which in some cases could be the resonance frequency of the acoustic system created by the plates and the weld joint between them. It is compulsory the definite frequency for this acoustic system keeps constant during the entire welding operation.
Ultrasonic treatment subsequent to welding
The Ultrasonic Method could be carried out after the welding operation is accomplished for a part of a complex steel structure, also called sub-assembly. In this case transducer(s) are mechanically fixed to the sub-assembly while it is hanging in a crane. As a result the sub-assembly will vibrate free while suspended in the air and the transducer is activated. Depending on the size and shape it would be necessary to fix one or two transducers for a specific part. The time required for such operation is a fraction of the time required for conventional PWHT. The relaxation of residual stresses obtained by Ultrasonic Method is far more effective than with conventional PWHT.
Advantages of Ultrasonic Peening Treatment when replacing conventional PWHT
UP is easy to apply due to the fact that
Advantages of Ultrasonic Method when replacing conventional PWHT
