To determine the most suitable technological option for the development of equipment for promising NPP, comparative studies of corrosion-resistant cladding were carried out. Two technological options of corrosion resistant weld deposits on carbon steel were tested:- the first option - automatic submerged arc welding (SAW) with a strip electrode of 20%Cr-10%Ni-1%Nb type with preliminary cladding of 25%Cr-13%Ni type strip electrode;- the second option - automatic electroslag welding (ESW) with a strip electrode of 21%Cr-11%Ni-1%Nb type without preliminary cladding. Investigations of the deposited metal both in the initial state as-welded and after past welding heat treatment at 640 °C for 10 h were carried out. In both cases, the deposit metal on the surface has a similar chemical composition. The structure is represented by an austenitic matrix with isolated areas of ferrite and finely dispersed carbides. Non-metallic inclusions in the weld metal are typical point oxides. In the case of SAW, the number and sizes of non-metallic inclusions are greater than in the case of ESW. Due to thermal welding cycles, in multi-pass SAW case, diffusion processes at the fusion boundary with the base metal run more actively than in the case of single-pass ESW. This affects the greater thickness of the decarburized interlayer in the base metal and the carburized interlayer on the side of the deposited metal, as indicated by the results of metallographic tests and microhardness measurements. The mechanical properties of the deposited metal at static tensile testing of specimens, in the as-welded condicion and after past welding heat treatment for both variants have approximately the same values. Accelerated corrosion tests for intergranular corrosion resistance showed no difference. In both cases the weld metal was resistant to intergranular corrosion. At testing of deposit metal for pitting corrosion, it was found that the SAW metal has insignificantly higher numerical indicators of corrosion damage than the ESW metal. These differences in resistance to pitting corrosion are explained by the differences in Pitting Resistance Equivalent (PRE): PRESAW = 18.2 and PREESW = 23. It has been established that SAW and ESW options provide similar characteristics of the deposited metal and can be used for the manufacture of equipment for promising NPP.
Mikhail Timofeev is a doctor of Central Research Institute of Structural Materials «Prometey», St. Petersburg, Russia. His core interests include the materials science research in the development of welding materials and technologies for welding heat-resistant steels in the nuclear power industry. He is the author of scientific papers aimed at improving the mechanical properties of welded joints in nuclear reactor vessels. His doctoral project is to creation of welding consumables, providing increasing the service characteristics of weld joints of nuclear and petrochemical reactor-body from Cr-Mo-V-steels.
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