TECHNOLOGY

The weld toe improvement methods have undergone extensive investigation and, in most cases, have been proven to significantly enhance the fatigue strength of welded details. The effectiveness of these treatments heavily relies on the operator's skills. High-Frequency Mechanical Impact (HFMI) has emerged as a reliable, effective, and user-friendly method for post-weld fatigue strength improvement in welded structures. An essential step in increasing the fatigue life of welded components is to apply good design practices, which can be achieved in various ways, such as using joints with a low-stress concentration factor (e.g. opting for butt welds instead of fillet welds) and placing welds in low-stress areas. Increased fatigue life is also attained through high-quality fabrication, involving proper selection of materials, welding processes, weld preparation, and geometry. If none fulfills the fatigue life target, weld improvement techniques can be implemented during or after production. Post-weld improvement techniques can generally be categorized into two main groups based on how the improvement is achieved: local weld geometry modification (e.g., reducing local stress peaks and improving surface quality) and residual stress techniques (where the improvement is achieved by reducing tensile residual stress in the welds and, in some cases, introducing compressive residual stresses). The following figure provides an overview of different improvement techniques available on the market today. Burr-grinding and TIG re-melting are typically classified as geometry improvement techniques, aiming to eliminate weld toe flaws and reduce local stress concentration by achieving a smooth transition between the base material and the weld. Hammer peening and needle peening fall under residual stress modification techniques, eliminating high tensile residual stress in the weld toe region and inducing compressive residual stresses at the weld toe. Residual stress modification methods also result in a reduced stress concentration at the weld toe.

The High-Frequency Impact Treatment (HiFIT) falls within the category of high-frequency mechanical impact (HFMI) treatment techniques. HiFIT treatment enhances the fatigue life and strength of welded details through three key mechanisms: first, it eliminates stress concentration at the weld toe by locally modifying the weld toe's geometry; second, it induces compressive residual stresses, replacing the tensile stresses typically present in the 'as welded' condition. Third, it results in surface hardening of the material through cold work.

Thanks to targeted HiFIT post-weld treatment, significant improvements in operational stability and the service life of dynamically loaded welded steel constructions can be obtained. The HiFIT treatment is user-friendly and cost-effective, providing highly reproducible results and ensuring precise quality assurance.

The HiFIT treatment tools are the culmination of over two decades of research and development. Our products have been developed in collaboration with DYNATEC Gesellschaft für CAE and Dynamik mbH in Germany.

An 80-100 percent increase in weld fatigue strength can be obtained through the following:

1. Local reshaping of the weld toe and reducing stress concentration.

2. Local hardening of the steel in the treated surface. 3. Inducing residual compressive stresses in the weld toe to prevent the formation and progression of cracks on the surface.

The service life of welds treated with the HiFIT technique can be extended by 5 to 15 times. Numerous scientific tests conducted in various countries have consistently demonstrated a significant and manifold increase in service life.

HiFIT is equally effective on existing steel structures. In the case of timely restoration of existing constructions, there is almost no distinction from newly treated welds. Hence the service life of existing construction is considerably increased. HiFIT is an ideal restoration tool as far as no macroscopically discernable cracks are evident.

The utilization of high-strength steel is gaining increasing relevance. Comprehensive theoretical and experimental analyses on Notch Details and FEM ­supported design processes indicate that the HFMI treatment is increasingly more efficient with the tensile strength of fine-grained steels, such as S460N and S690QL, and those with even higher tensile strengths.

As a result, the use of steels with higher strength becomes intriguing for lighter construction types. The effectiveness of the HiFIT treatment is enhanced in higher steel grades.

HiFIT is a specialized company that offers comprehensive services and expertise to the steel industry, with a specific focus on fatigue-loaded steel structures. Boasting a strong global presence, HiFIT brings extensive experience to the field, particularly in the post-weld treatment and enhancement of welded steel structures through the application of HFMI technology.

HiFIT Scandinavia provides a full spectrum of services, ranging from structural analysis to the implementation of post-weld treatment and quality assurance. This holistic approach enables us to achieve optimal, sustainable, and cost-effective steel structures, underscoring our commitment to delivering high-quality solutions in the industry.