The influence of heat treatment after high-frequency welding on the operational properties of steel 09G2S used for the manufacturing of drilled piles

Introduction. The peculiarities of the construction and operation of energy facilities in the permafrost zone contributed to the development and creation of entire scientific directions in the field of low-temperature metal science. At the stage of field development, a significant number of piles is used in the construction of foundations. Justification of the application and confirmation of the possibility of using electric welded pile pipes without heat treatment in the construction of foundations on permafrost soils will reduce the costs of enterprises.

Aim. The goal of the work was to study changes in the structure and properties in the weld of steel 09G2S of borehole piles during heat treatment after welding with high frequency currents.

Materials and methods. To conduct full-scale tests of mock-ups of tubular piles, 6 tubular metal piles made of 09G2C steel with HT (heat treatment) and without HT were mounted on an open test site. After exposure, the piles were dismantled and measurements of residual stresses, microstresses, tests for determining impact strength at negative temperatures and microstructure analysis were performed.

Results. Studies have shown that there are no significant differences in the distributions of residual stresses, values of microstresses, in the mechanical properties of pipes during uniaxial tensile tests of pipes with and without heat treatment. The difference in the results on the impact strength of the superheating section of the HAZ, pipes with and without heat treatment, may be due to the release brittleness of the first kind. The microstructure of the base metal of the pipe with and without HT is a ferrite-pearlite structure.

Conclusion. It has been established that the heat treatment of pipes after welding has no positive effect on the microstructure and mechanical properties. It is shown that the microstructure of the HAZ weld of steel 09G2C is unstable to embrittlement at negative temperatures of -60 °C. It is necessary to optimize the welding mode with high frequency currents to reduce the effect of embrittlement of the weld metal.

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