High-cycle Fatigue Testing of Fillet Weld Root Side Using Mock-up Specimens
Kämäräinen, Panu (2016)
2016
Materiaalitekniikan koulutusohjelma
Teknisten tieteiden tiedekunta - Faculty of Engineering Sciences
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Hyväksymispäivämäärä
2016-12-07
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tty-201611214729
https://urn.fi/URN:NBN:fi:tty-201611214729
Tiivistelmä
The purpose of this work was to study the root side fatigue of a corner fillet weld made of structural steels using test pieces similar to actual structures. The objective was to com-pare the fatigue strength and metallurgical weld quality of welds from structural steel grades S235 and S355. The results of the fatigue tests were compared to the design curves of effective notch stress method, so that it was seen how the weld fatigue strength corre-sponds with real designed structures.
The fatigue tests were performed in the high-cycle regime (10^5–10^8 cycles). The welded specimens were prepared by two different subcontractors, and a total of four different steel compositions were used. The first subcontractor used S235, S355 (low carbon equiv-alent) and S355 (high carbon equivalent). The second subcontractor produced the speci-mens from S355 MC (thermo-mechanically rolled fine-grained steel). The fatigue tests were carried out at high and medium stress levels, which were sufficient to form the de-scending beginning portion of the fatigue curve for each material.
The fatigue lives of the welded joints was strongly influenced by the penetration of the welds. When the effect of penetration was removed from the results, there were no big differences between the fatigue strengths of the steels. However, with lower quality welds the scatter within the results was greater. S235 welds had the deepest penetration and were clearly the most durable, when the effect of penetration was included in the calcu-lation of the results. S355 MC performed the worst. There was barely any penetration, and the quality of the welds was also clearly the worst.
The weld qualities had significant differences. The differences were due to both the steel composition and the welding quality of the subcontractors. The welds made by the first subcontractor from steels S235 and S355 with low carbon equivalent were of very good quality and the only welding imperfections were small pores. The quality of welds made from high carbon equivalent S355 ranged from good to very poor. In the worst cases, the base material had not melted along a large section of the weld and a metallurgical bond had not formed. This was probably due to the lower quality of the steel. S355 MC steel welds manufactured at the second subcontractor were poor in quality: there were a lot of weld root inclusions, lack of fusion and also penetration depth was modest. Based on its composition, the steel is easy to weld, and there were indications of deficiencies in the welding process. In the S355 MC and high carbon equivalent S355 steels, silicon-man-ganese oxides were found, without exceptions, in the welding defect areas.
The fatigue tests were performed in the high-cycle regime (10^5–10^8 cycles). The welded specimens were prepared by two different subcontractors, and a total of four different steel compositions were used. The first subcontractor used S235, S355 (low carbon equiv-alent) and S355 (high carbon equivalent). The second subcontractor produced the speci-mens from S355 MC (thermo-mechanically rolled fine-grained steel). The fatigue tests were carried out at high and medium stress levels, which were sufficient to form the de-scending beginning portion of the fatigue curve for each material.
The fatigue lives of the welded joints was strongly influenced by the penetration of the welds. When the effect of penetration was removed from the results, there were no big differences between the fatigue strengths of the steels. However, with lower quality welds the scatter within the results was greater. S235 welds had the deepest penetration and were clearly the most durable, when the effect of penetration was included in the calcu-lation of the results. S355 MC performed the worst. There was barely any penetration, and the quality of the welds was also clearly the worst.
The weld qualities had significant differences. The differences were due to both the steel composition and the welding quality of the subcontractors. The welds made by the first subcontractor from steels S235 and S355 with low carbon equivalent were of very good quality and the only welding imperfections were small pores. The quality of welds made from high carbon equivalent S355 ranged from good to very poor. In the worst cases, the base material had not melted along a large section of the weld and a metallurgical bond had not formed. This was probably due to the lower quality of the steel. S355 MC steel welds manufactured at the second subcontractor were poor in quality: there were a lot of weld root inclusions, lack of fusion and also penetration depth was modest. Based on its composition, the steel is easy to weld, and there were indications of deficiencies in the welding process. In the S355 MC and high carbon equivalent S355 steels, silicon-man-ganese oxides were found, without exceptions, in the welding defect areas.