Effect Of Heat Treatment On The Corrosion Properties Of 20G Steel Banded Structure

20G, high pressure boiler seamless steel tube, has been widely used as boiler pipe or oil pipeline for its excellent resilience, stability and good corrosion resistance. But20G steel pipes are usually made by pressure processing, easy to form banded structure, this kind of defect easily leads pipelines to failure. To solve this problem, the microstructure, mechanical properties and electrochemical performance of20G steel were studied to explore the effect of banded structure on the mechanical properties and electrochemical performance.Uniform design experiment method can select the best heat treatment process to eliminate the banded structure and refine the grains. The microstructure and mechanical properties on the transverse section were compared to longitudinal. Dynamic polarization curve test, alternating-current impedance method and scanning electrochemical method were used to research the room temperature corrosion mechanism of20G steel in the simulated formation water. In addition, SEM, EDS and XRD were subsidiary to the analysis of the corrosion characteristics, the element and phase changes in the corrosion process.Uniform design experiment obtained the optimized heat treatment process with the heating rate of10℃/min, holding temperature of920℃and holding time of40min, cooling method was air cooling. After the heat treatment showed above, the banded structures in the steel were almost eliminated and the microstructure was uniform with fine grains, the mechanical properties had improved by10%.The mechanical test showed that banded structures in the steel leaded that the transverse properties were inferior to longitudinal properties by16%for the existence of banded structure; the reduction of area and impact toughness decreased with the increasing banded structure grade. The segregation of non-metallic inclusions reduced the impact toughness, heat treatment can improve the distribution and shape of the non-metallic inclusions to some extent, so as to improve the mechanical properties.The electrochemical test indicated that the corrosion rate of20G steel in simulated formation water decreased first as the immersion time increased, but then increased. The early, insoluble corrosion products covered on the matrix’s surface, which protected the matrix from corroding. As time getting longer, the corrosion rate increased for the corrosion products on the matrix surface became porous and flocculated, and the corrosion products were proved by XRD to be FeCO3.The corrosion resistance of20G steel in simulated formation water decreased when the banded structure grade increased, the sample which had the finest grains and lowest banded structure was the most stable, with minimum corrosion density and corrosion rate. The electrochemical test explained that, ferritic bands in20G steel promoted the corrosion process as microbattery anode in the early, later the effect of corrosion product film inhibited the anodic activity, made the corrosion rate slightly lower.