Influence of Scale Factor and Surface Condition on 20GS2 Steel Sensitivity to Hydrogen Cracking

Purpose of research. At present, it is found that the process of corrosion-mechanical destruction proceeds in several stages. However, as practice and analysis of numerous destructions of structural elements made of high-strength materials in the construction and engineering industries show, very often in real structures there are already technological defects such as cracks. In such cases, the process of corrosion-mechanical destruction begins directly with crack growth, bypassing the preparatory stages – the incubation time and the formation of a stress concentrator. The goal of this paper is to study the influence of scale factor and surface condition on the sensitivity of 20GS2 steel to hydrogen cracking.

Methods. Reinforcing steel of 20GS2 grade (four melts) was chosen to determine the influence of surface condition and scale factor on the long-term corrosion strength in the environment causing hydrogen cracking. Full-scale specimens (l = 300.400 mm) of different diameters (10, 12 and 14 m) with smooth-rolled and die-rolled sections were tested. Also, from 10, 12 and 14 mm diameter full-scale rods (smooth-rolled and with die-rolled sections) there were turned samples with a diameter of the working part from 6 to 10 mm every 1 mm with surface condition of Ra2,5. The long-term corrosion strength was estimated through time preceding failure by testing four samples at each point of the graph. To create the experimental conditions as close as possible to the operational ones, the tests were carried out in an aqueous solution of sulfuric acid with the addition of ammonium rhodanide at room temperature with cathodic polarization and different levels of tensile stresses.

Results. It has been experimentally found that full-scale specimens of 20GS2 reinforcing steel in comparison with turned ones have less sensitivity to hydrogen cracking. This circumstance is due to the fact that after mechanical processing by cutting in turned samples, the value of residual tensile stresses is much higher than in full-scale specimens. It has been shown that full-scale specimens of 20GS2 reinforcing steel  with a die-rolled section have less resistance to hydrogen cracking, as compared with full-scale smooth-rolled samples. The lower resistance to hydrogen cracking of die-rolled section reinforcement is due to the presence of ribs on its surface which play the role of natural stress concentrators.  The experimental results obtained allow us to recommend full-scale specimensto to be tested for corrosion and mechanical destruction in environments that cause hydrogenation of full-scale specimens, since their use makes it possible to approximate the experimental conditions to the actual conditions of operation of products.

Conclusion. The results obtained can be used in the creation of resource-saving processes for processing metal alloys and composite materials.

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