High Frequency Impact Treatment
High Frequency Impact Treatment or HiFIT - Method is the treatment of welded steel constructions at the weld transition to increase the fatigue strength.
The durability and life of dynamically loaded, welded steel structures is determined in many cases by the welds, particular the weld transitions. Through selective treatment of the transitions (grinding (abrasive cutting) abrasive blasting, hammering, etc.), the durability of many designs increase significantly. Hammering methods have proven to be particularly effective treatment methods and were within the joint project REFRESH extensively studied and developed. The HiFIT process (High Frequency Impact Treatment) is such a hammering method that is universally applicable, requires only a low tech equipment and still offers high reproducibility and the possibility for quality control.
The HiFIT hammer operates with a hardened pin with a ball resting on the workpiece with a diameter D of 3 mm. This pin is hammered with an adjustable intensity at around 180–300 Hz at the weld toe. Local mechanical deformations occur in the form of a treatment track. The weld toe is deformed plastically. The induced compressive residual stress prevents the track cracking and the crack propagation on the surface.
The International Institute of Welding Technology IIW published the Guideline "Recommendations for the HFMI Treatment" in October 2016. An overview of higher frequency hammers (HFMI) is presented, and recommendations for the correct application of the method and quantitative measurements for quality assurance the guideline provides the basis for measurements of HFMI improved welded joints on the basis of all known stress calculation concepts.
In numerous experiments at various institutes and universities an 80 to 100 percent increase of fatigue strength and a 5 - to 15-fold increase in weld-life could be demonstrated. The most extensive research project was from 2006-2009 "REFRESH - life extension of existing and new welded steel structures (P702). In this research project, the HiFIT device was developed and made ready for production. This report is available in book form at the FOSTA (Forschungsvereinigung Stahlanwendung e.V.) and can be ordered under the number . The book contains detailed scientific verifications and validations.
The HiFIT method can be applied to both existing as well as new steel structures.
For a targeted treatment, the visibility and accessibility of the transition in the welded areas are required. Existing structures typically are prepared at the transition for surface finishing. The parts must be free of loose rust and old paint. If necessary, previous sandblasting is required.
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