PSI - Issue 16

Vitalii Knysh et al. / Procedia Structural Integrity 16 (2019) 73–80 Vitalii Knysh et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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3. Results of experimental studies and their discussion

3.1. Metallographic examination

HFMI treatment of the joints resulted in formation of characteristic grooves: width from 3.0 mm up to 3.51 mm, depth from 0.280 mm up to 0.341 mm. HFMI of welded joints was conducted along the fusion line, both the surface layer of weld metal and surface layer of HAZ metal were plastically deformed (Table 2).

Table 2. SPD metal layer dimensions of low alloy steel welded joints treated by HFMI before and after corrosion tests.

Before corrosion test

After corrosion test

Deformed layer parameters

HAZ

Weld metal

HAZ

Weld metal 1.56 – 1.79

Width, mm Depth, μm

1.3 – 1.70 390 – 650

1.7 – 1.82 390 – 650

1.05 – 1.7 325 – 390

325 – 650

Fig. 2a and 2b show the specimens as-welded and treated by HFMI before corrosion testing, respectively. HFMI essentially changed the cast structure of weld metal. Elongated bainite grains with form factor K f = 5…17 ( K f =a/b , where a and b are the averaged values of elongated grain length and width, respectively) are formed in the metal surface layer of up to 130 μm thickness. At the distance of 260 μm from the groove bottom, ferrite grains with K f = 4…7 and individual bainite grains are ob served. Changes are also found in the grain structure of HAZ metal. Bainite and ferrite grains with K f =7…15, elongated at an angle to the groove bottom, form in the surface layers of coarse grain zone (CG Z) down to the depth of 280 μm. After corrosion testing in high humidity conditions, a non-uniform layer of corrosion products formed on welded joint surface (Fig. 2, c – f). After exposure to high humidity, the HFMI-treated zone was covered by a layer of corrosion products and did not differ visually from other surface of the specimen (Fig. 2d). As fatigue cracks usually form along the welded joint fusion line, special attention was given to fusion line study. Geometrical parameters of SPD metal layer produced by HFMI after corrosion testing at high humidity, which were established by metallographic examination, are given in Table 2. The obtained data indicate the reduction of the depth of plastically deformed layer in the HAZ metal by 1.2 – 1.6 times. a b c

d

e

f

Fig. 2. Appearance of the weld zone of welded Т -joints: first (a), second (b), third (c), fourth (d), fifth (e) and sixth (f) series before the fatigue testing.