PPiippee && TTuubbee PPrroodduuccttiioonn

© Metallurgical and Mining Industry, 2010, Vol. 2, No. 5  363

UDC 621.039:620.186.8]:699.14.018.8

Elaboration of Technological Measures on Increase of Corrosion Resistance of Ferritic-Austenic Steel Pipes

V. S. Vahrusheva 1, Т. А. Dergach 1, S. А. Panchenko 2, А. А. Tereshchenko 2

1 State Enterprise "Ya. Yu.Osada Scientific Research Tube Institute" 1a Pisarzhevskiy St., Dnipropetrovsk 49600, Ukraine

2 JSC “Centravis Production Ukraine” 56 Trubnikov Avenue, Nikopol, Dnipropetrovsk Region 53201, Ukraine

Effect of technology factors in pipe & tube production on pitting corrosion and stress-corrosion cracking resistance of pipes made of widespread corrosion-resistant ferritic-austenitic (duplex) steel is investigated. New regimes of heat treatment and other technological measures enhancing corrosion resistance, operate reliability and competitiveness of pipes are developed. Keywords: CORROSION-RESISTANT FERRITIC-AUSTENITIC STEEL PIPES, HOT AND COLD DEFORMATION, HEAT TREATMENT, STRUCTURE, PITTING CORROSION, STRESS-CORROSION CRACKING, METHODS OF CORROSION TESTS

Introduction

Corrosion-resistant ferritic-austenitic (duplex) chromium-nickel-molybdenum steel pipes have been widely applied for the last years in the world practice when designing equipment that contacts with aggressive chloride-containing medium [1-5]. Nickel content is almost 2-3 times lower in this steel but strength is almost 2 times higher as compared to traditional austenitic chromium-nickel-molybdenum steels (for example, 316L, 03Х17Н14М3). Therefore, application of two-phase steel in medium, where corrosion resistance is similar to austenitic ones, is economically feasible as enables to save more than 60 kg of nickel per one ton of metal and reduce specific amount of metal.

Advantages of ferritic-austenitic steels are also: high grain-boundary corrosion and corrosion cracking resistance; lower coefficient of thermal expansion and higher heat conductivity [3] than in austenitic steel which is especially important when using this steel in the heat-exchange equipment; good deformability in hot and cold conditions, cutting ability, formability, forgeability and weldability.

As analysis shows the principal cause of corrosion-resistant steel pipes failure is pitting corrosion (PC) and corrosion cracking (CC). The

specified types of local corrosion take place mainly in chloride-containing medium at elevated temperatures and they can lead to eating-through of metalworks for a short period of time and, as a result, to considerable diseconomies and pollution of environment. Therefore, corrosion-resistant steel pipe consumers have been requiring high level of PC and CC resistance for last years.

The task of present research is working out of technological measures to increase PC and CC resistance of ferritic-austenitic steel 02Х22Н5АМ3 (UNS S 31803) pipes mass-produced at JSC “Sentravis Production Ukraine”.

Methodology

Tubular billets and 02Х22Н5АМ3 steel hot-pressed and warm-rolled pipes of industrial production were test material. We investigated effect of microstructure, hot and cold strain, heat treatment regimes on corrosion resistance.

PC resistance test was carried out by method A of standard ASTM G-48 – with soaking of samples in 10 %-solution of ferric chloride FeCl3 6Н2О for 72 hours at temperatures from 30 to 50С with interval in 5С with further estimation of their specific weight loss (which should not exceed 0.0001 g/cm2) and observation of surfaces for the

PPiippee && TTuubbee PPrroodduuccttiioonn

364  © Metallurgical and Mining Industry, 2010, Vol. 2, No. 5

presence of PC. CC resistance test was carried out by standard

ASTM G-36 on tensile strained samples in 44 % - solution of magnesium chloride MgCl2 6Н2О boiling at 155С with estimation of CC resistance in time until the cracks visible at amplification 20 appeared on the surface of samples.

Results and Discussion The tubular billets and heavy-walled tubes

were tested on flat samples (Figure 1а) and warm-rolled pipes which geometrical sizes do not allow flat samples - on C-shaped ring spring samples (Figure 1b). Tensile stresses in samples were computed from equations (1) and (2), respectively:

σ=6 Е t y/H2 (Eq. 1) ∆ =f π D2/4 E t Z (Eq. 2)

Pipes were made from 02Х22Н5АМ3 (UNS S 31803) steel tubular billet 180 mm supplied by company COGNE, Italy (Table 1, Figure 2).

The specified tubular billet showed low PC and CC resistance (Table 2, Figures 3, 4), which is apparently caused by inhomogeneity of its chemical and structural composition.

The maximum temperature at which steel has satisfactory PC resistance (specific weight loss of samples not more than 0.0001 g/cm2) is 30С. At higher temperatures of tests, specific weight losses considerably exceed this value and there is pitting corrosion on their surfaces (Table 2, Figure 3).

The maximum tensile stress at which round billet samples have passed PC resistance tests (for more than 24 hours) was 60 % from the standard

yield stress (σ0.2=450 MPa), that is at σ=280 MPa. At higher tensile stress the samples cracked for less than 15 hours (Figure 4).

a b

Figure 1. Schematic drawing of sample with bend deflection (a) and C-shaped (b) for CC resistance tests

Table 1. Qualitative characteristics of 02Х22Н5АМ3 steel tubular billet

Content of elements, % -

phase, % C Si Mn P S N Cr Mo Ni PRE

0.020 0.48 1.67 0.024 0.001 0.18 22.05 3.11 5.32 35.19 43.5

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© Metallurgical and Mining Industry, 2010, Vol. 2, No. 5  369

companies [4, 5], PC resistance during the test of ferritic-austenitic steel 02Х22Н5АМ3 by ASTM-G-48 is restricted to temperature 30С, and duration of CC resistance tests in boiling solution of magnesium chloride is restricted to 36-40 hours.

It is possible to conclude that heat treatment of hot-pressed and warm-rolled ferritic-austenitic steel pipes by developed regime enables to raise parameters of their corrosion resistance.

Besides, the recommendations based on the analysis of chemical and structural composition of tubular billets of different suppliers and analysis of current technology of ferritic-austenitic steel pipe production are given. They include specification of chemical and structural composition of tubular billets (reduction of content of detrimental impurities and nonmetallic inclusions in steel, increase of pitting resistance equivalent); application of cold rolling and liquid easily removed lubrications (for improvement of pipe surface quality), elimination of outside surface polishing when finishing pipes. The new obtained results and recommendations can be used also when making ferritic-austenitic super duplex steel pipes with raised requirements on corrosion resistance.

Conclusions

1. Complex investigation of effect of technology factors of pipe production on pitting corrosion and corrosion cracking resistance of hot-pressed and warm-rolled pipes made of widespread corrosion-resistant ferritic-austenitic steel is conducted.

2. Developed scientifically based regimes of pipe heat treatment enable to raise considerably pitting corrosion and corrosion cracking resistance. Recommendations on further advancement of pipe making technology with the purpose to enhance corrosion resistance are given.

3. Implementation of developed technological measures at JSC "Sentravis" will promote raise of operate reliability, competitiveness and expansion of ferritic-austenitic duplex and super duplex steel pipes application.

References

1. K. S. Siow T. Y. Song, J. H. Qiu. Anti-Corrosion Methods and Materials, 2001, Vol. 48, pp. 31 - 37.

2. V. S. Popov. Formation of Structure and Properties of Welded Joints of Thin-Walled Ferritic-Austenitic 02Х22Н5АМЗ Steel Tubes, PhD Dissertation, Moscow, 2005, 126 p. *

3. S. Jacques, J. Peultier, J. C. Gagnepain, P. Soulignac. Corrosion Nace Conference, 2008, March 16-20.

4. Proceedings of Technical Symposium “Seamless Stainless Steel Pipes”, Kobe Steel, Japan, 1985, 138 p.

5. Stainless Steel Pipe and Tubing. Mannesmann rushers – worker, Mannesman, 1987, 143 p.

6. G. G. Shepel, V. S. Vahrusheva, T. A. Dergach, et al. Stal, 2009, No. 6, pp. 57-60. *

7. T. A. Dergach, L. С. Severina, S. K. Yurkov, et al. Metallurgicheskaya i Gornorudnaya Promyshlennost, 2008, No. 3, pp. 53-57. *

* Published in Russian Received July 6, 2010

Разработка технологических мероприятий по повышению

коррозионной стойкости труб из ферритно-аустенитных сталей

Вахрушева В.С., Дергач Т.А., Панченко С.А., Терещенко А.А.

Исследовано влияние технологических

факторов трубного производства на стойкость к питтинговой коррозии и коррозионному растрескиванию труб из широко распространенных коррозионностойких ферритно-аустенитных (дуплексных) сталей и разработаны новые режимы термической обработки и другие технологические мероприятия, повышающие коррозионную стойкость, эксплуатационную надежность и конкурентоспособность труб.