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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1. Introduction

Different methods of surface plastic deformation (SPD) of metal are widely used to increase the fatigue life of structures as shown by Kulekci and Esme (2014). Malaki and Ding (2015), Yildirim and Marquis (2012), Zhao et al. (2011) showed that a high-frequency mechanical impact treatment (HFMI), also known in publications as ultrasonic impact treatment, is one of the most effective methods of strengthening the welded joints of structural elements. HFMI is aimed at removing the residual welding stresses and inducing residual compressive stresses, reducing the stress concentration factor, forming the surface layer of metal with higher physic-mechanical properties. Most welded metal structures (bridges, overpasses, vehicle carriage frames, etc.) are exposed to simultaneous impact of alternating loading and climatic factors of the environment in service, and the investigations are conducted to determine the corrosion fatigue and corrosion resistance of welded joints treated by HFMI. Daavari and Sadough Vanini (2015) gave the results of fatigue testing of welded joints of pipe steel in air and in the corrosive environment, in as-welded condition and after HFMI. It is shown that HFMI results in 1550% increasing of the radius of weld to HAZ metal transition, increasing of microhardness up to 33%, lowering of the level of residual tensile welding stresses and 2 times increasing of cyclic fatigue life of welded joints in a corrosive environment. Gao et al. (2015) established that the fatigue limit of butt welded joints at 4·10 5 cycles increases by 29% after HFMI. It is shown that the corrosion rate in 3.5% sol ution of NaCl welded joints treated by HFMI (0.0033…0.0061 mm/h) is on the level of base metal (0.0038 mm/h) that is much lower than that of as- welded joints (0.0118…0.0323 mm/h). Dong. et al. (2015) studied HFMI strengthening of reactor steels provides formation of a protective oxide film on a metal surface that leads to increasing of their corrosion resistance at ambient temperature of 500 °C . Knysh et al. (2008) illustrated that mechanical treatment of the line of transition of weld metal to HAZ increases the fatigue limit at 2·10 6 cycles by 111% at testing in air, and by 73% in 3% NaCl solution. High fatigue life of specimens of welded joints treated by HFMI are due to the fact that no significant corrosion damage progresses in the strengthened metal layer during short-term testing. However, a long-term impact of the atmosphere leads to considerable corrosion mechanical loss of metal as shown by Beeharry and Surnam (2018) and to a partial or complete fracture of SPD of metal layer and lowering of fatigue limit of welded joints treated by HFMI, respectively. Ahmad and Fitzpatrick (2015) estimated that after 7.5 years of operation in sea water a partial loss of the strengthened surface layer of welded joint metal (up to 1 mm) takes place. It leads to significant lowering of the maximum level of the induced residual compressive stresses. Fan et al. (2016) gives the data of studies of butt welded joints of railway rails: as welded, after HFMI and after HFMI with subsequent exposure for 450 h in synthetic sea water. It is found that the fatigue limit at 2·10 6 cycles in as-welded joints is equal to 256 MPa, after HFMI it raised to 314 MPa, and after HFMI with subsequent exposure in a corrosive environment it’s 290 MPa. Knysh et al. (2018) estimated that exposure of butt welded joints treated by HFMI to higher air humidity (98%) and temperature ( 40°C ) during 1200 h leads to a partial fracture of plastically-deformed metal layer. However, the fatigue life increases up to 7 times and the fatigue limit at 2·10 6 cycles increases by 39%. The objective of the work is to study effectiveness of HFMI application to improve the fatigue behavior of as welded T-joints of low-alloyed steel and after their operation at atmospheric influence in temperate climate zone.

2. Experimental procedure

Specimens of welded T-joints of low- alloyed 15KhSND steel (σ y =400 MPa, σ UTS =565 MPa, δ 5 =31%) were studied. This steel is widely used in Ukraine for manufacturing of the elements of the long-term service metal structures (for instance, in span structures of railway and road bridges). Table 1 gives the steel composition.

Table 1. Chemical composition of 15KhSND steel. Mass fraction of elements, % С Si Mn S P

Ni

Cr

Cu

0.142

0.466

0.63

0.020

0.013

0.31

0.66

0.34