Technoweld provides a solution and not just a service.
We value add, and offer much more than a basic service of ultrasonic peening. We have extensive experience in welding engineering and quality management, and unparalleled experience in fatigue life improvement. We use this experience to achieve the best possible outcome, ensuring costs are kept to a minimum by not simply applying peening, but understanding all factors that contribute to fatigue failure.
Aside from these, we manage welding repairs and see to it that they are carried out as instructed. We also provide welding consultancy and FEA services, using the best practices and only in those locations that require it, which provides a better starting point for fatigue life extension. As a minimum, we conduct a pre-treatment inspection to ensure the weld is of adequate finish and/or contour, marking the necessary repairs required prior to ultrasonic treatment.
Whilst the older version of our machine was 20kHz, the one we presently use is 26kHz. We use a piezoelectric unit, which has a method of oscillation far more efficient than our competitors’ magnetostrictive units. Magnetostrictive transducers are generally less efficient, primarily because they require dual energy conversion, first from electrical to magnetic, and then from magnetic to mechanical. Piezoelectric transducers convert alternating electrical energy directly to mechanical energy through the piezoelectric effect.
There has been a question raised regarding the need to treat every run. A fundamental understanding of welding, residual stress and fatigue life improvement techniques debunks the theory behind the approach.
Residual Stress and Multi-pass Welding
There is no evidence suggesting that treating inter-run has a beneficial effect. The following reasons go some way to explain why:
- Subsequent weld runs refine the grain of the previous run by a depth of approximately 1-2mm (more depending on welding process), and therefore any advantage of ultrasonic peening is lost by subsequent welding.
- Subsequent weld runs introduce additional tensile residual stress, and the maximum it can be is the yield strength of the material. These subsequent runs effectively undo any ultrasonic peening effect carried out by inter-run.
- Subsequent runs remove (fuse in) the beneficial weld toe modification required to improve fatigue life.
- The area of highest stress and highest risk of crack propagation is in the material surface as this is furthest from the neutral axis—the area that requires treatment.
- DNV-GL, a world authority on fatigue design, does not recognise or credit PWHT (stress relief) as a basis to increase fatigue or design life for cyclically loaded structures. Treating inter-run with ultrasonic peening is based on the same principle as PWHT or a reduction of residual stress. DNV-GL does allow a doubling of fatigue life for a given stress range for fatigue improvements that involve weld toe modification. This shows the stress relief mechanism that inter-run peening is premised on is ineffective or at the very least, has limited advantage.
- Normal peening using a needle gun inter-run will provide the same, if not better, results than ultrasonic peening at a fraction of the cost. This is on the basis it reduces contraction stresses introduced by the cooling weld by spreading or flattening the weld. This needs to be done immediately after the weld, preferably as it cools.
- The slowing down of the welding operation by ultrasonic peening inter-run is counterproductive to maintaining higher, and in most cases, beneficial inter-pass temperatures, which can be credited to higher HAZ toughness and lower HAZ hardness.
PWHT and Stress Relief Effect: Contribution to Fatigue Life Improvement
Stress relief via PWHT or peening has been estimated to achieve a reduction of residual stress by approximately 30% at best. Residual stress from welding is typically in the magnitude of the materials yield strength, or in the case of low carbon steel, 300mPa. A reduction by 30% means we reduce the residual stress to 200mPa, which leaves the stress range still in tensile condition. Whilst it is common practice, there is no evidence that stress relief via inter-run peening or PWHT alone is sufficient to prevent fatigue crack propagation.
Fatigue Life Improvement
By far the biggest factor (excluding orientation of the load to the weld) that contributes to fatigue failures, is the geometric conditions of the parent metal to weld transition followed by having a tensile stress at the weld. Toe modification techniques (ie. ultrasonic peening, rotary burr, and toe remelting) all remove or reduce the cause of crack propagation through:
- Micro intrusions at the weld toe from welding that are stress-raisers ie. crack propagation sites.
- The removal or reduction of the stress-raiser at the weld toe by rounding of the transition between the toe and the parent material.
With this stated, I trust it goes some way to explain why it is reasonable to suggest that any improvements in fatigue life by inter-run ultrasonic peening are at best unfounded.