High Frequency Impact Treatment

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The English term High Frequency Impact Treatment , abbreviated as HiFIT process and in German called high frequency hammering process or high frequency hammering , describes the after-treatment of welded steel structures at the weld seam transition to increase fatigue strength. The principle is comparable to the shot peening of metal components.

The operation of the hammer tool used is similar to the used for rust removal and de-scaling needle hammer .

properties

Example of a welded construction post-treated with HiFIT

The fatigue strength and service life of dynamically loaded, welded steel structures is in many cases determined by the weld seams, in particular the weld seam transitions. Targeted post-treatment of the transitions ( grinding , blasting , hammering, etc.) can be used with simple means to considerably increase the service life of many constructions. Hammering processes have proven to be particularly efficient post-treatment methods and have been extensively investigated and further developed within the framework of the REFRESH joint project. The HiFIT process is such a hammering process that can be used universally, requires only a small amount of equipment and yet offers a high level of reproducibility and the possibility of quality monitoring.

technology

HiFIT hammer

The HiFIT hammer works with a hardened pin on a rounded tip with a diameter of 3 mm. This pin is hammered with an adjustable intensity and a frequency of 150… 300 Hz on the transition area from the weld to the base material. Local deformations occur in the form of a post-treatment trace.

The success of a HiFIT treatment is essentially based on the following modes of action:

  1. Reduction of the notch effect by reshaping or rounding the seam transition and removing weld seam defects.
  2. Solidification of the surface layer to increase the resistance to crack formation and crack propagation.
  3. Introduction of compressive residual stresses to increase the resistance against crack formation and crack propagation by superimposing the notch stresses with compressive residual stresses.

variants

In order to improve the accessibility to the post-treatment point and to round the inner edges of the component without welding, there are numerous variants of special pins in different diameters and lengths. Pin variants made of "compatible" materials are provided for the treatment of special materials. Special variants for special applications are often used at the customer's request.

proof

In October 2016 the International Institute for Welding Technology IIW published the guideline " IIW Recommendations for the HFMI Treatment ". An overview of higher frequency hammering methods (HFMI) is presented. In addition, recommendations for correct application of the method and quantitative measurements for quality assurance are described. The guideline presents the design principles of HFMI-improved welded joints on the basis of nominal stress, structural stress and notch stress concepts.

In the research project "REFRESH - extending the service life of existing and new welded steel structures" (P702) from 2006–2009, an 80 to 100 percent increase in weld fatigue strength and a 5 to 15-fold increase in weld service life could be demonstrated in numerous test series. As part of this research project, the positive effect of post-treatment on the service life of welded structures could be demonstrated.

Tests at the University of Duisburg Essen have shown that the fatigue strength has doubled and the service life has increased by a factor of 10 compared to the untreated initial state.

At the Finnish University in Aalto, a team led by Prof. Gary B. Marquis has been researching the benefits of high-frequency hammering for years. He recently published a report in the International Journal of Fatigue with suggestions for SN curves that are conservative of the available results from 228 trials.

Procedural steps

The HiFIT process can be used on both existing and new steel structures. For a targeted processing, however, the visibility and accessibility of the weld seam transition in the areas to be treated is necessary. The required impact intensity is determined on the basis of the track depth on test panels or in unstressed areas of the base material. Existing constructions usually have to be given a surface pretreatment in advance at the weld transition . The parts must be free of loose rust and old paint. Prior sandblasting may be required. No special preparation is required for new constructions.

The HiFIT device is placed by hand on the weld seam transition to be treated and guided along it during the treatment. The device works with a compressed air supply of 6–8 bar. The weld transition is plastically deformed (rounded) and solidified by local deformations. The depth of the post-treatment track should be between 0.2 and 0.35 mm. The undercut at the weld transition can no longer be seen after the treatment.

The treated area can be examined by visual inspection. The treatment track is checked with a special template. A digital display of the working pressure enables the user to check it at any time.

Economical meaning

The economic potential of HiFIT treatments is discussed in detail in the REFRESH research report. The most important points are briefly mentioned below.

Life extension

When used on existing structures, the service life can be significantly extended. If there are no macroscopically recognizable cracks, HiFIT is a very suitable renovation tool. With timely renovation of existing structures there is almost no difference to the service life of newly treated welds. This results in the potential to use existing constructions well beyond their previous service life. The HiFIT process is used very efficiently, e.g. B. on Autobahn - bridges in steel box girder profile construction during operation. The costs for the renovation are low compared to conventional methods. In commercial vehicle construction and other industries, highly stressed weld seams on existing and new constructions are successfully treated to increase the service life.

Increase in the transferable load level

With new constructions and with some existing constructions, the load level for the treated welds can be increased with the same service life. With the same service life, around 1.6 times the load can be transferred via the weld seam. This has z. B. in crane construction the extremely positive effect that larger lifting loads can be moved.

Lightweight construction

If the HiFIT process is taken into account during development, the construction can be specifically streamlined with the same load level and the same service life. Extensive experimental investigations on notch details and FEM-based design methods show the high efficiency of conventional S235, S355J2 and fine-grain steels such as S460N, S690QL and even higher-strength steels. The material savings that can be achieved make the use of the HiFIT process economically viable in most applications. If you also consider the weight advantage achieved from the application, z. B. the achievable payload in vehicle construction.

See also

literature

Individual evidence

  1. ↑ a b c Gregor Nüsse: REFRESH - extending the service life of existing and new welded steel structures . Verl. And Vertriebsges., Düsseldorf 2011, ISBN 978-3-942541-03-9 .
  2. ↑ Jörn Berg: Use of high-frequency hammering processes to increase the fatigue strength of welded steel structures . 2009 ( Abstract (PDF file; 70 kB) - Master's thesis, University of Duisburg-Essen, 2009).
  3. ↑ Halid Can Yildirim, Gary B. Marquis: Fatigue strength improvement factors for high strength steel welded joints treated by high frequency mechanical impact . In: International Journal of Fatigue . tape 44 , November 2012, p. 168–176 , doi : 10.1016 / j.ijfatigue.2012.05.002 .