Page 54 - SSAB Welding Handbook Edition 2

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8.0 Fatique in welded joints Welding handbook

8.8 Burr grinding To get the full effect of this method, it is recommended
to consult “IIW Recommendations on Post Weld
©SSAB
Burr grinding has almost the same effect as TIG- Improvement of Steel and Aluminium Structures”.
dressing when applied at the weld toe. With this
method, the transition radius is decided by the
burr. Some defects are also removed by the grinding 8.10 High Frequency Mechanical
itself. Grinding with roundels is not recommended.
The diameter of the grinding tool for plates with a Impact, HFMI.
thickness between 10-50 mm is normally 10-25 mm. HFMI is a peening methods with a higher frequency,
The radius of the grinding “ditch” should be at least above 90 Hz, than traditional hammer peening. The
0.25 of the thickness of the plate. Minimum depth is method gives higher fatigue strength with increasing
0.5 mm or to the point where all defects are removed. yield strength of the parent material. Benefits, how
The maximum depth is 2 mm or 5% of the thickness to, limitation and quality assurance can be read in
for t>40 mm and 7% of the thickness for t<40 mm. “IIW Recommendations for the HFMI Treatment”
The speed of the tool is 1500-40000 rpm. The angles
are shown in fig. 8.17.
8.11 Post-weld heat treatment
(PWHT)

45-60° Post-weld heat treatment has long been used to
obtain higher fatigue strength in welded structures.
The treatment reduces the tensile residual stresses
around the welded joint, but never/rarely down to
zero. For complicated welded structures, there are
risks of high tensile residuals stresses being left in
30-45° the structure after PWHT. Chapter 16 gives more
information about PWHT.

To get an effect from this method, a part of the load
must be compressive. The largest gain in stress range
Direction
of travel for PWHT is about 25% for R=-1.

Fig. 8.17: Tool angles for burr grinding. 8.12 Other methods

The above-mentioned recommendation gives a Shot peening has almost the same effect as hammer
typical increase in FAT of 30%. peening. To a lesser extent, the same effect could
also be achieved by heavy sand blasting.

8.9 Hammer peening

During hammer peening, compressive residual stresses
are introduced when the area around the weld toe
is cold formed. These compressive residual stresses
prevent the crack from initiating and growing. This
method is not recommended for structures with a
dominant external compressive load. The compressive
load relaxes the build in compressive residual stresses.

54 This handbook contains general suggestions and information without any expressed or implied warranty of any kind. SSAB hereby expressly disclaims
all liability of any kind, including any damages, in connection with the use of the information and for their suitability for individual applications.
It is the responsibility of the user of this brochure to adapt the recommendations contained therein to the requirements of individual applications.