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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Before corrosion testing, the surface of the metal of weld and HAZ of the as-welded T-joints was quite even, with smooth transition from a weld to a base metal (Fig. 3a). In the surface layers of the metal of welds and HAZ of as welded joints deep enough and extended damages identified as corrosion spots were observed after corrosion testing (Fig. 3b). In the weld metal, the largest corrosion spots, for example, 2.8×0.26 mm and 1.17×0.16 mm and corrosion pits 1.56×1.17 mm were formed. The maximum depth of p it corrosion penetration, detected in the near-surface layers of weld metal in welded T- joints, did not exceed 1.17 mm. The corrosion spots of maximum size 1.3×0.22 mm and 0.91×0.39 mm were found in the HAZ metal. The subsurface corrosion, not reaching the surface, was detected in one weld, its length was about 0.15…0.26 mm. eld metal HAZ

a

b

c

Fig. 3. Profiles of surface layers of the welded Т -joints: as-welded (a); treated by HFMI (b); treated by HFMI after corrosion testing (c).

In the surface layers of weld and HAZ of the welded T-joint treated by HFMI, corrosion pits were revealed along a fusion line (Fig. 3c) . Their maximum sizes reached 1.95×0.16 mm and 1.3×0.23 mm in the weld metal and 1.04×0.26 mm and 0.78×0.156 mm in the HA Z metal, respectively. A pit corrosion was detected only in the near surface layers of weld metal in welded T-joints, and maximum penetration depth of the pits did not exceed 1.17 mm. Thus, HFMI of the fusion line before exposure to high humidity conditions served only to reduce the depth of corrosion pits in the HAZ metal from 0.39 mm to 0.26 mm (Table 3).

Table 3. Corrosion damage dimensions in near- surface layers of weld and HAZ metal of welded Т -joints after corrosion testing.

Within weld metal

Within HAZ

Welded joints

Depth, mm 0.91 – 1.17 0.130 – 1.17

Extent of damage, %

Depth, mm 0.13 – 0.39 0.13 – 0.26

Extent of damage, %

As-welded

31.2

38.5

Treated by HFMI

23

29

3.2. Investigation of corrosion resistance

Corrosion resistance of HFMI-strengthened surface layer was studied only on the base metal specimens. Fig. 4 shows the change of the surface of 15KhSND steel specimens after exposure to high humidity during different time periods (240, 720, 1200 and 2400 hours). Based on visual examination, it was found that the layer of corrosion products was non uniform, both on the ground surface and on HFMI-treated surface, its density increased along with prolongation of the exposure time. After removal of corrosion products, corrosion spots and small pits were detected on the surface. The corrosion rate changed non-monotonically on the untreated and treated by HFMI surface (Fig. 5). After 480 hours, the corrosion rate increased on both surfaces, and then it began decreasing rather quickly practically in the same way both for the ground and for HFMI-treated surfaces. However, on the ground surface, the corrosion rate