Methods of rising corrosion resistance of duplex steel tubes

The objective of this research is rise of the complex of corrosion properties and service reliability of tubes made of nickel-chromium-molybdenum ferritic-austenitic (duplex) steels thru formation of a most favorable structure during heattreatment and forming operations. Methodology. The processes of structure formation in tubes of Cr-Ni-Mo ferrite-austenite steel. The methods of optical and electron microscopy, chemical and micro-chemical analysis, complex corrosion studies and field tests. Findings. New quenching conditions (1 1 5 0 .1200+1050)°С and a process for production of tubes of ferritic-austenitic steels based on the principles of grain-boundary engineering (GBE) including hot or cold working at a high deformation ratio (є > 95 and 70% respectively) and a regenerative quenching ensuring formation of a most favorable structure (with more than 70% of special grain boundaries in the austenite component of steel, ferrite and austenite phases correlation 1:1 and absent ст phase) and a considerable rise of a complex of corrosion properties of tubes were elaborated. The pilot batch of tubes of Cr22Ni5Mo3N steel produced according to the developed process has shown a high service reliability in field tests at Dneprazot JSC. Originality. For the first time quantified the relationship between structure (containing of special low-energy grain boundaries Z3n in the concept of coincident lattice points ^LP), grain size and phase composition) and corrosion resistance of duplex stainless steels tubes. Scientifically proved the decisive role of the SG to improve the corrosion resistance of these pipes. Practical value. The research findings were introduced at Centravis Production Ukraine PJSC to favor growth of service reliability and competitiveness of tubes of duplex steels. after their forming and heat treatment at various conditions were studied and quantitative relations between the structure in steel and corrosion resistance of tubes were determined.