Preliminary repair welding tests by means of Lincoln Invertec STT. The repair welding of power generation components, especially pressure vessels for nuclear reactors is predominantly performed by manual welding, usually with coated electrodes. The interaction between considerably changed properties of material being welded and the welding process results in defective microstructure with high level of residual stress and consequently in the reduction of lifetime of given component. Optimized welding technique without application of subsequent annealing for decrease of stresses, so called smart or low-stress welding (that is low residue stress welding) using special welding procedure and suitable filler materials, excludes or to some extent lowers the unfavourable influence of such welding repairs, in general.
For preliminary repair welding tests performed in cooperation with KODA JS a.s. and Lincoln Electric CZ WELD s.r.o. by means of Lincoln Invertec STT welding source and welding wire Lincoln LNM NiCro 70/19 (dia 1.2mm, shielded atmosphere Ar), steel 2.75Cr 0.7Mo 0.3V was used, as a basic material, in the form of plate of thickness 30mm. The tests performed indicate some possibilities of this method for mentioned purposes, due to the following reasons:
The results achieved in the course of technological tests show that in spite of inclination to cracks occurrence in HAZ of given steel and lower resistance or increased inclination of welded metal of this type to occurence of hot (solidified) cracks, which were found, the principle of STT welding process is acceptable. Nevertheless it is necessary to emphasize the necessity to carry out these welding tests with the aim to find out such welding conditions (parameters) of welding, which will be suitable for treatment of HAZ while using the method of cushion and annealing welding beads. It seams now that these works can be performed by combination of STT process and pulse welding (GMAW-P) that is STT method can be used with cushion welding beads and for annealing welding beads method GMAW-P can be used. Then we can say that it will be suitable to use LINCOLN POWER WAVE/STT source for this operation and the means supporting the delopment of technology for repair welding is LINCOLN WAVE DESIGNER system, as a tool ensuring high quality of welding, lowering to minimum the negative influence of human factor on welding process and including optimum of given process. For very demanding environment, as are interiors of nuclear and chemical vessels, it is advisable to use telerobotic welding equipment, which is equipped with mentioned welding equipment. Nevertheless it is necessary to point out that carrying out of these tests is indispensable.
Ing. JirÝ Elinger, CSc.
KODA JS a.s.
Structural Safety Department
Foto of weld
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