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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The blanks for welded joints of 350  70 mm size were cut out from hot-rolled sheet stock 12 mm thick. The welded T-joints were made by manual arc welding of transverse stiffeners (from 15KhSND steel) to a plate by fillet welds from both sides. The first layer of weld was formed by 3 mm UONI 13/55 electrodes ( “ Paton ” Company) of, the second layer of weld was formed by 4 mm UONI 13/55 electrodes ( “ Paton ” Company). Specimens dimensions of welded T-joints are shown in Fig. 1.

Fig. 1. Fatigue test specimen dimensions.

Specimens were tested in MTS 318.25 and URS-20 machine by cyclic tension with stress ratio R s and loading frequency of 5 Hz. Specimens were tested either to fracture or up to 2·10 6 cycles. Six specimen series were tested: in as-welded (1) and treated by HFMI (2) in air (without corrosion testing); as-welded (3) and treated by HFMI (4) after corrosion testing; after cyclic pre-loading and corrosion testing without HFMI treatment (5) and with further HFMI treatment (6). Pre-loading of specimens of (5) and (6) series was performed in hydraulic machines ZDM-10pu at maximum loading in zero-to- tension cycle of 150 MPa (close to fatigue limit of such welded joints at 2·10 6 test cycles) in air. After pre- loading the specimens of (5) and (6) series to 2·10 6 cycles, corrosion testing of specimens of (3) – (6) series was conducted in G4 moisture chamber at the temperature of 40°C and air humidity of 98% for 1200 h (further on referred to as higher humidity). The objective of corrosion testing was to achieve corrosion damage on welded joint surface, characteristic of the joints after long-term impact of temperate climate atmosphere. Treatment of welded joints by HFMI technology was conducted by USTREAT-4.0 system (Ukraine), in which the manual compact impact tool with piezoceramic transducer is connected to ultrasonic generator of 800 W output power. A narrow zone of transition from weld to HAZ, i.e. the fusion line, was treated. The working head was a single-row four- pin attachment with 3 mm pins. The amplitude of waveguide end oscillations was 25 μm. The speed of HFMI process during treatment of welded T-joints was 1 mm/s. Corrosion resistance of SPD metal layer was determined by the methods of massometry and metallography. Base metal specimens of 40  25  11 mm size with ground surface and those with ground surface and HFMI treatment were used. The corrosion rate was defined upon testing for periods 240, 480, 720, 960, 1200 and 2400 hours. After degreasing, the specimens were exposed in thermal chamber MLW 117- 0200 at the temperature of 105…110°C up to constant mass, weighed in analytical scales VLR-200, and their area was determined. After testing, the specimens were rinsed by running tap water, corrosion products were removed by etching in 20% acetic acid, and rinsed in running tap water. After etching, the specimens were treated by 3% caustic soda solution, rinsed in running and dist illed water, dried with filter paper and exposed in thermal chamber at the temperature of 105…110°C to constant mass, which was followed by their weighing. For metallographic examination the specimens were cut out of areas, which had the deepest corrosion damages, and its largest quantity, determined visually. Results of metallographic examination were used to assess the dimensions of corrosion damages of metal surface layer. Structural changes in the surface layer of base metal and fusion line as-welded and treated by HFMI joints before and after corrosion testing were studied in NEOPHOT 32 optical microscope, and digital images of the structure were obtained using Olympus C5050 digital camera.