Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

• the invention relates to an austenitic steel grade, extremely resistant especially to pitting and crevice corrosion, easily weldable and of high material strength.
• the invention concerns also the use of the steel for products on which are made exacting demands regarding general corrosion and especially pitting and crevice corrosion, weldability, and strength.
• the steel is developed specifically for and intended for use within the off-shore industry, e.g. in piping and tanks, and in other environments where the steel is exposed to sea water or to other chloride-containing liquids, such as in thermal power stations where sea water is used for cooling, in the bleacheries of the forest industry, in scrubbers etc.
• Another field of application is vessels and tubes and heat exchangers for nitric acid, especially those that are cooled by sea water.
• Piping, heat exchangers, tanks and similar equipment and apparatus are mostly made of austenitic steel with 18-21% Ni and over 62 Mo when there is a demand for good weldability, mechanical strength, and very high resistance to pitting and crevice corrosion.
• the off-shore industry and other industries or plants where sea water is encountered are examples of areas where such demands are made.
• Other fields of application are within the chemical industry, especially in the chemical bleacheries of the forest industry.
• ferritic ELI- steels which are also highly resistant to pitting and crevice corrosion in sea water and similar environments, provided they contain at least 25% Cr and at least 3.5% Mo.
• a serious limitation of these materials is that they are not manufactured in thicker dimensions than about 3 mm. If the material is made thicker it becomes brittle and unweldable.
• a primary object-of the invention is to provide a steel grade which has the required combination of properties for use in welded constructions in highly corrosive environments, in spite of having a lower, rather than higher, content of expensive alloy metals than comparative conventional austenitic steels.
• an object is to provide a steel with extremely high resistance to pitting and crevice corrosion.
• a preferred object is to provide a steel of high resistance in other environments also, such as in HN0 3 . Typical fields of application are as indicated in the preamble.
• a further object of the invention is to provide a steel which is easy to weld with a low energy input without any considerable loss of its corrosion resisting properties in the weld or in the heat-affected zone.
• a preferred object is also to provide a steel with a greater mechanical strength than conventional austenitic molybdenum alloyed steels.
• the carbon content is desirable to keep the carbon content as low as possible. Normally, the carbon content is therefore maximally 0.02%, preferably maximally 0.015%.
• the roles of manganese and nitrogen in the steel are complex.
• the manganese partly functions as an austenitizing agent, partly aids in dissolving nitrogen in the steel. Certain indications suggest that the manganese in this alloy directly influences the corrosion properties favourably.
• the nitrogen works as an austenitizing agent and adds to the corrosion resistance as well.
• a required amount of nickel is added in addition to manganese and nitrogen.
• a synergistic effect of molybdenum and nitrogen as regards resistance to pitting and crevice corrosion is also attained with the steel according to the invention. In other words, the nitrogen strengthens the favourable effect of the-molybdenum.
• Chromium is a fundamental element for resistance to general corrosion and also enhances the resistance to other types of corrosion.
• a preferred characteristic of the steel according to the invention is that the chromium equivalent (according to Shaeffler) is at least 24, and the nickel equivalent is at least 25, the ratio of the chromium equivalent to the nickel equivalent being no more than 0.9, preferably no more than 0.8.
• the steel is non-stabilized, which means that it does not contain any significant, intentional additions of niobium, tantalum, titanium or zirconium.
• the total amount of these elements must not exceed 0.1 %. Higher amounts would have a too detrimental effect upon the corrosion resistance, because these elements readily combine not only with carbon but also with nitrogen present in the steel to form nitrides, such that the effective nitrogen content would be reduced.
• a preferred composition of the steel according to the invention is the following:
• copper may be included up to no more than 3%.
• the possible effects of including copper in the steel according to the invention has not been subject to investigation, however. It is conceivable that it may improve corrosion resistance in strong acids.
• the copper content should be limited to a max. of 0.5%.
• Copper at concentrations less than 0.5X is included in the term "accessory elements", signifying such elements as may be found in secondary metal or be traces of process metallurgy additions.
• cobalt This is an expensive element. Wilful additions of this element should therefore be avoided.
• Cobalt also has the drawback of becoming radioactive when subjected to radiation, which makes cobalt alloyed material im-0 permissible for such parts of nuclear power stations as are exposed to radiation. The cobalt content therefore should be restricted to a max. of 0.5%.
• traces of elements added for process metallurgy reasons may be mentioned aluminium and calcium. Niobium, vanadium and titanium should not be present at levels exceeding those of im-5 purities.
• each steel sample made and investigated is presented in Table 1, group 1.
• Steels Nos. 1-15 were manufactured as melts with a weight of 2 kg. Of these, those with a totally austenitic structure (after rolling and solution heating) were subjected to further investigations, especially regarding their corrosion properties. In order to evaluate the initial investigations, a melt of 50 kg was then manufactured, steel No. 16.
• Groups 2 and 3 of Table 1 present data for commercially available steels tested, as well as figures from the literature concerning other steels, see the reference list on pp. 2-3.
• the B charges (steels Nos. 1-15) were forged to 30 mm square section and rolled to strips of 3 mm thickness and then solution heated (1100°C/1h/H 2 O). No notable problems of poor hot state ductility were encountered.
• Steel No. 3 were not nearly as resistant as steel No. 6 and ended up somewhat below the result of steel No. 20, in spite of having equal content of chromium and nickel and a molybdenum content only 1.5 and a nitrogen content only 0.21% lower than that of steel No. 6.
• the samples were of the steel grade No. 16 according to the invention and the comparison materials 24 and 25. All samples had been ground and pickled in 10% HNO 3 + 1% HF, 10 min at 60 0 C, prior to exposure.
• Steel No. 16 had the lowest weight loss in g/m 2 h and the lowest pit depth, while steel No. 25 had the highest weight loss and greatest pit depth.
• the exposure confirmed the laboratory data presented above on the 2 kg samples of steel No. 6.
• the corrosion test carried out in the laboratory of the corrosion properties of steel No. 16 are summarized in Table 5.
• the steel was MIG welded with an electrode of the type Avesta P12. Especially noteworthy is that there were no corrosion attacks, neither in the weld nor in the heat affected zone. There were no problems associated with the welding itself.
• the steel could be welded with a very low heat input, 0.235 kJ/mm.
• the weld was of a high quality, smooth, without spatter or pores.
• the quench annealed hot rolled plate showed the following mechanical properties:

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Cited By (9)

Citations (5)

• 1983
  • 1983-10-21 SE SE8305795A patent/SE441455B/sv not_active IP Right Cessation
• 1984
  • 1984-10-08 EP EP84112048A patent/EP0142015B1/fr not_active Expired
  • 1984-10-08 DE DE8484112048T patent/DE3464504D1/de not_active Expired
  • 1984-10-08 AT AT84112048T patent/ATE28087T1/de active

Patent Citations (5)

Non-Patent Citations (1)

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