Technical field
[0001] 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.
Background art
[0002] Piping, heat exchangers, tanks and similar equipment and apparatus are mostly made
of austenitic steel with 18-21 % Ni and over 6% 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. There are many corrosion resistant austenitic steel grades that meet very
high demands in the respects mentioned, but nevertheless there is a demand for even
better materials. At the same time it is desired to cut the cost of materials, for
which the cost of such alloys as nickel and molybdenum are very important.
[0003] The alloying expenses may be reduced by the use of 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.
[0004] Another practice in order to reduce costs is to replace nickel by manganese partly
or wholly in austenitic stainless steel. There are reports that manganese may increase
the contribution of chromium to resistance compared to steels with no manganese substitution.
The known types of manganese substituted steels are considerably less corrosion resistant,
however, than said austenitic and ferritic steel grades, and are no useful alternative
to the latter in those environments where the present steel is intended to be used.
[0005] The following literature references are intended to illustrate the present state
of the art. In the comparative investigations to be presented below reference will
be made to data from these references.
Literature references:
[0006]
(1) Kohl H, Rabensteiner G, Hoehortler G, VEW: Stainless Steels with High Strength
and High Corrosion Resistance. Alloys for the eighties.
(2) Glazkova S A, Shapiro M B: The Resistance of a CrNi-Steel Type 18-12-Mo to Localized
Corrosion in Chloride Solutions. Zashch. Metallov 15 (3), May-June 1979, p 320-324.
(3) Bock H E: Korrosionsverhalten eines seewasserbestandigen, nichtmagnetisierbaren
CrNiMo-Stahles hoher Festigkeit. Arch. Eisenhüttenwes. 44 (1973) 877.
(4) Brigham R J, Tozer E W: Localized Corrosion Resistance of Mn-Substituted Austenitic
Stainless Steels: Effect of Mo and Cr. Corr. 32 (1976) 274.
(5) Letcher B F: An Austenitic Stainless Steel of Improved Strength and Corrosion
Resistance (Firth Brown Rex 734). Report from Firth Brown Ltd.
Disclosure of invention
[0007] 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. Specifically, 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.
[0008] 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.
[0009] A preferred object is also to provide a steel with a greater mechanical strength
than conventional austenitic molybdenum alloyed steels.
[0010] These and other objects may be attained by making a steel grade of the following
chemical composition in percentages by weight:
[0011] Optionally Co up to 0.5 and/or Cu up to 3.0 the balance iron and unavoidable impurities.
[0012] It 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. In order to create a completely austenitic structure, 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.
[0013] A preferred characteristic of the steel is that its so called PRE value (=%Cr+3.3x%Mo+16x%N)
is at least 41, preferably from 41-45.
[0014] It is suitable to include in the steel 14-17 Ni, 9-11 Mn, 18-23 Cr, 4-8 Mo, 0.38-0.48
N and a maximum of 0.015C.
[0015] 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.
[0016] It is also important that 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.
[0017] A preferred composition of the steel according to the invention is the following:
optionally Co up to 0.5 and/or Cu up to 3.0 the balance iron and unavoidable impurities.
[0018] The total amount of Nb+Ta+Ti+Zr shall not exceed 0.1 %. More preferably the carbon
content is 0.015% at the maximum.
[0019] As mentioned, 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. According to the preferred embodiment, the copper content should
be limited to a max. of 0.5%.
[0020] Copper at concentrations less than 0.5% may be found in secondary metal or traces
of process metallurgy additions. Among the former type of elements may be mentioned
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 impermissible 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%. Among 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 impurities.
[0021] Further characteristics and advantages of the steel according to the invention will
appear from the following account of experiments and investigations carried out.
Description of experiments and investigations carried out
[0022] The composition of 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.
[0023] 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/1 h/H
20). No notable problems of poor hot state ductility were encountered.
[0024] Since the purpose of the invention is to develop a manganese substituted steel, structural
studies were carried out only on the steels Nos. 1-6. These studies revealed that
only steels Nos. 3 and 6 were completely austenitic. Steels Nos. 1, 4 and 5 contained
a-phase, while steel No. 2 contained 5-ferrite. Fully austenitic steels 3 and 6 were
tested in a first run for local corrosion resistance and mechanical strength. Table
2 show strength data for the two steels 3 and 6 tested and for a number of grades
of nitrogen alloyed steel commercially available and/or reported in the literature.
[0025] To start with, the results of comparative studies of corrosion resistance of steels
Nos. 3 and 6 and of commercial steels 17-23 will be presented below.
Crevice corrosion
Crevice corrosion potential, Esp
[0026] Steel grade No. 6 immediately proved to be highly superior to most stainless steels
ever tested by the applicant. While the best result of the reference materials was
an Esp of +105 mV, steel No. 6 had not been attacked at Esp=+785 mV. At that level,
the test had to be discontinued, because the clamps holding the specimens were severely
corroded (they were made of steel grade NU SS 904L). This means that the steel grade
tested can be considered fully resistant to sea water at room temperature.
[0027] 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.
Critical crevice corrosion temperature, CCT
[0028] At the comparative CCT test in FeCl
3, steel No. 6 was attacked only when the temperature reached 45°C. Next to the best
value, 40°C, was reached by steel No. 22, the rest of the CCT values being below room
temperature, possibly with exclusion of steel No. 21, which was resistant up to 30°C.
Critical pitting temperature, CPT
[0029] This test was also carried out in FeC1
3. Table 3 indicates that steel No. 22 had the highest CPT value, 75°C, followed by
experimental steel No. 6, 65°C. In comparison with the resistance to crevice corrosion,
experimental steel No. 6 is less resistant to pitting. This may be a result of the
number of inclusions in small laboratory charges, which is greater than that of material
made at production conditions. The slag situation may be more significant to pitting
than to crevice corrosion.
Corrosion in a synthetic scrubber solution
[0030] Only steels Nos. 6 and 22 were unattacked at 50°C.
[0031] In order to evaluate further the corrosion properties of the steel according to the
invention a laboratory charge of 50 kg, sample No. 16, was produced, of the same nominal
composition as steel No. 6. This material also was forgable and rollable without problems
and was similarly highly corrosion resistant. The results of a field trial, laboratory
corrosion tests, and a first test for weldability will be reported below.
[0032] The field trial was carried out in the chlorodioxide stage of a paper mill bleachery.
The samples, which were small pieces of steel plate, were placed standing up in one
corner of the inlet box of the filter. Half of the plates were partially immersed
in the pulp suspension, the upper half being in the gas phase. The environment in
these filters is so corrosive that there has not been satisfactory solution to the
materials problem up to now. Earlier exposures have shown that only titanium resists
attack. Some typical environment parameters are the following:
Redox potential 650 mV/SCE
Amount of active chlorine in filtrate 7 mg/I
pH 3.8
Chloride 220 mg/I
Temperature 73°C
[0033] 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% HN03+1% HF, 10
min at 60°C, prior to exposure.
[0034] The results are presented in Table 4.
[0035] Steel No. 16 had the lowest weight loss in g/M2 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.
[0036] 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.
[0037] In order to evaluate also the features of the steel of invention produced on the
commercial scale basis, there was produced a five tons heat, steel No. 35, table 1,
group 1. Three ingots were casted. The ingots were forged and hot rolled to plates
with 5 and 12 mm thicknesses without problem. The surfaces after rolling were in good
condition. Trials made showed that after hot or cold rolling, the steel could be heat
treated by heating to 1100°C, followed by quenching in water so as to obtain a pure
recrystallized austenitic structure without precipitates and with the desired corrosion
properties. The heat treated 5 mm-plate also was cold rolled to 3 mm and 1 mm thicknesses
without problem.
[0038] The quench annealed hot rolled plate showed the following mechanical properties:
[0039] As far as corrosion testing was concerned, specimens from both 5 and 12 mm plates
were tested in various environments. Generally speaking, the results were consistent
with those made on laboratory melts and reported above.
1. Steel grade of austenitic type, extremely resistant particularly to pitting and
crevice corrosion, easily weldable, and of high material strength, characterized in
that it has the following chemical composition in percentages by weight:
optionally Co up to 0.5 and/or Cu up to 3.0 the balance iron and unavoidable impurities.
2. Steel grade according to claim 1, characterized in that its so called PRE value
(=% Cr+3.3x% Mo+16x% N) is at least 41, preferably from 41 to 45.
3. Steel grade according to claim 1, characterized in that it contains 14-17 Ni.
4. Steel grade according to claim 1, characterized in that it contains 9-11 Mn.
5. Steel grade according to claim 1, characterized in that it contains 18-23 Cr.
6. Steel grade according to claim 1, characterized in that it contains 4-8 Mo.
7. Steel grade according to claim 1, characterized in that it contains 0.38-0.48 N.
8. Steel grade according to any one of the claims 1-7, characterized in that it contains
no more than 0.02C and a total of not more than 0.1 % of niobium, tantalum, titanium
and zirconium.
9. Steel grade according to any one of the preceding claims, characterised in that
its Cr equivalent (=% Cr+% Mo+1.5x% Si+0.5x% Nb) is at least 24, that its Ni equivalent
(=% Ni+0.5x% Mn+30 (% C+% N)) is at least 25, and that the quotient Cr eq./Ni eq.
is less than or equal to 0.9, preferably less than or equal to 0.8.
10. Steel grade according to any one of the claims 1-9, characterized in that it has
the following composition
optionally Co up to 0.5 and/or Cu up to 3.0 the balance iron and unavoidable impurities.
1. Stahlsorte des austenitischen Typs, die insbesondere gegen Lochfraß und Spaltkorrosion
extrem widerstandsfähig, leicht schweißbar und von hoher Materialfestigkeit ist, dadurch
gekennzeichnet, daß sie die folgende chemische Zusammensetzung in Gew.-% hat:
gegebenenfalls Co bis zu 0,5 und/oder Cu bis zu 3,0 Rest Eisen und unvermeidbare Verunreinigungen.
2. Stahlsorte nach Anspruch 1, dadurch gekennzeichnet, daß ihr sogenannter PRE-Wert
(=%Cr+3,3x%Mo+16x%N) zumindest 41, vorzugsweise von 41 bis 45 beträgt.
3. Stahlsorte nach Anspruch 1, dadurch gekennzeichnet, daß sie 14-17 Ni enthält.
4. Stahlsorte nach Anspruch 1, dadurch gekennzeichnet, daß sie 9-11 Mn enthält.
5. Stahlsorte nach Anspruch 1, dadurch gekennzeichnet, daß sie 18-23 Cr enthält.
6. Stahlsorte nach Anspruch 1, dadurch gekennzeichnet, daß sie 4-8 Mo enthält.
7. Stahlsorte nach Anspruch 1, dadurch gekennzeichnet, daß sie 0,3-0,48 enthält.
8. Stahlsorte nach einem der Ansprüche 1-7, dadurch gekennzeichnet, daß sie nicht
mehr als 0,02 C und eine Gesamtmenge von nicht mehr als 0,1% an Niobium, Tantal, Titan
und Zirkon enthält.
9. Stahlsorte nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß
ihr Cr-Äquivalent (=%Cr+%Mo+1,5x%Si+O,5x%Nb) zumindest 24, daß ihr Ni-Äquivalent (=%Ni+O,5x%Mn+30 (%C+%N)) zumindest
25, und daß der Quotient Cr-Äqu./Ni-Äqu. weniger als oder gleich 0,9, vorzugsweise
weniger als oder gleich 0,8 ist.
10. Stahlsorte nach einem der Ansprüche 1-9, dadurch gekennzeichnet, daß sie die folgende
Zusammensetzung hat:
gegebenenfalls Co bis zu 0,5 und/oder Cu bis zu 3,0 Rest Eisen und unvermeidbare Verunreinigungen.
1. Qualité d'acier du type austénitique, extrêmement résistant en particulier aux
piqûres et à la corrosion par crevasses, facile à souder et de haute résistance des
matériaux, caractérisé en ce qu'il a la composition chimique suivante en pourcentage
en poids:
facultativement Co jusqu'à 0,5 et/ou Cu jusqu'à 3,0, le reste étant de fer et des
impuretés inévitables.
2. Qualité d'acier selon la revendication 1, caractérisé en ce que sa valeur appelée
de PRE (=%Cr+3,3x%Mo+16x%N) est d'au moins 41, de préférence de 41 à 45.
3. Qualité d'acier selon la revendication 1, caractérisé en ce qu'il contient 14-17
Ni.
4. Qualité d'acier selon la revendication 1, caractérisé en ce qu'il contient 9-11
Mn.
5. Qualité d'acier selon la revendication 1, caractérisé en ce qu'il contient 18-23
Cr.
6. Qualité d'acier selon la revendication 1, caractérisé en ce qu'il contient 4-8
Mo.
7. Qualité d'acier selon la revendication 1, caractérisé en ce qu'il contient 0,38-0,48
N.
8. Qualité d'acier selon l'une quelconque des revendications 1 à 7, caractérisé en
ce qu'il contient pas plus de 0,02 C et un total de pas plus de 0,1% de niobium, tantale,
titane et zirconium.
9. Qualité d'acier selon l'une quelconque des revendications précédentes, caractérisé
en ce que son équivalent Cr(=%Cr+%Mo+1,5x%Si+0,5x%Nb) est d'au moins 24, en ce que
son équivalent Ni (=%Ni+0,5x%Mn+30(%C+%N)) est d'au moins 25, et en ce que le quotient
équivalent Cr/équivalent NI est inférieur à ou égal à 0,9, de préférence inférieur
à ou égal à 0,8.
10. Qualité d'acier selon l'une quelconque des revendications 1-9, caractérisé en
ce qu'il a la composition suivante:
facultativement Co jusqu'à 0,5 et/ou Cu jusqu'à 3,0, le reste étant du fer et des
impuretés inévitables.