DE1608180B1 - USING A NICKEL-CHROME STEEL ALLOY - Google Patents

Description

45

The invention relates to the use of a nickel-chromium steel alloy from 31 to 34% Nickel, 17 to 22% chromium, 5.5 to 9.25% molybdenum, up to 3.25% niobium, 0 to 2% tantalum, whereby the niobium and half the tantalum content are 1 to 3.25%, to 0.3% carbon, 0 to 1.5% manganese, 0 to 1% silicon, 0 to 0.6% titanium, 0 to 0.6% aluminum, up to 2% vanadium, 0 to 2% copper, 0 to 1% tungsten, 0 to 0.08% magnesium, 0 to 0.005% boron, up to 0.05% calcium and 0 to 0.02% zirconium, The remainder, including impurities from the smelting process, iron.

Austenitic stainless steels are known to be particularly sensitive to stress corrosion cracking in aggressive chloride media, for example in boiling magnesium chloride. To this one To remedy the disadvantage, it has already been proposed to make the phosphorus and nitrogen content more austenitic Keep steels with at least 19% nickel and 15% chromium low and a certain minimum content of carbon or silicon, for example at least 0.07% carbon or 1.7% Silicon. In addition, the molybdenum content of these steels has been greatly reduced to a maximum of 0.075%.

However, the aforementioned proposal leads to further difficulties; it is known, for example, that high carbon contents of for example 0.07% austenitic stainless steels are very sensitive to make intergranular corrosion, d. In other words, the steels become sensitive to grain boundaries. The intergranular Corrosion, which is particularly noticeable on weld seams, can have different effects Wise to be eliminated, in particular by reducing the carbon content of the concerned Steels is kept below about 0.03%. The adverse effect of high carbon levels can also can be avoided by stabilizing with niobium. Although these measures on the one hand the danger reduce intergranular corrosion, on the other hand they increase the risk of stress corrosion cracking.

Similarly, as the risk of stress corrosion cracking is reduced, the risk of stress corrosion cracking is reduced of the molybdenum content to below 0.075%, the advantageous due to the higher molybdenum content Properties lost. For this reason, the molybdenum-containing stainless steel contains standard steels AISI 316 and 317 each have 2 to 4% molybdenum, but are sensitive to stress corrosion cracking and intergranular corrosion if they have higher carbon contents.

From US Pat. No. 2,777,766, a thermoformable one against pitting corrosion is also known Chloride solutions as well as nickel-chromium-steel alloy resistant to oxidizing and reducing corrosive media with 35 to 50% nickel, 18 to 25% chromium, 2 to 12% molybdenum, 0.1 to 5% niobium and / or tantalum, below 0.25% carbon, up to 1.5% manganese, up to 0.5% silicon, up to 2.5% copper and up to 5% tungsten, the remainder being iron. This alloy However, as tests have shown, it has a low resistance to crevice corrosion in 10% strength Ferric chloride solution.

Experiments have shown that molybdenum, which is harmful to 2%, is above 3% improves the resistance to stress corrosion cracking. This behavior, which is still inexplicable at the moment of molybdenum results from tests on 14 alloys with varying molybdenum contents. In these experiments, annealed U-shaped specimens were placed in a boiling 42% magnesium chloride solution immersed at 154 ° C and examined its resistance to stress corrosion cracking in the manner that they were checked for cracks every day; samples that were still free of cracks after 30 days were found were considered to be corrosion resistant.

The compositions of the test alloys are shown in Table I below, where the remainder in any case consists of iron and the usual impurities caused by the melting process. In addition, the number of days after which the test pieces cracked for the first time is given in Table I exhibited.

The table shows that the molybdenum-free steel alloys A, B with a high carbon content are free from cracks were that alloy C, similar to alloy B, with a lower carbon content of only 0.03%, on the other hand, cracked quickly.

Alloys D through G contained small amounts of the harmful molybdenum and were all

cracked. The high carbon alloys D, E and F would also have been crack-free in the absence of molybdenum.

A comparison of alloys H and J; K, L and M; as well as N and P shows the decreasing harmfulness of molybdenum in excess of 3%. Indeed, what was the resistance to stress corrosion cracking Regarding alloy M is satisfactory.

■ Apart from the resistance to stress corrosion cracking, must also reduce the sensitivity of the alloy to intergranular or crevice corrosion will. In this connection it has now been found that alloys with, for example 4% molybdenum, in spite of its improved resistance to stress corrosion cracking, is only a minor one Have resistance to intergranular corrosion.

The invention is now based on the object of creating a material that is highly resistant against intergranular corrosion as well as against stress corrosion cracking and crevice corrosion in chloride media, especially in boiling magnesium chloride solution. Starting from noting that certain nickel-chromium steel alloys containing molybdenum and niobium are carefully used Set composition meet this requirement, according to the invention, the use of a Nickel-chromium steel alloy made from 31 to 34% nickel, 17 to 22% chromium, 5.5 to 9.25% molybdenum, 1 to 3.25% niobium, 0 to 2% tantalum proposed, the niobium and half tantalum content of which is 1 to 3.25%, 0 to 0.3% carbon, 0 to 1.5% manganese, 0 to 1% silicon, 0 to 0.6% titanium, 0 to 0.6% aluminum, 0 to 2% vanadium, 0 to 2% copper, 0 to 1% tungsten, 0 to 0.08% magnesium, 0 to 0.005% boron, 0 to 0.05% calcium and 0 to 0.02% zirconium, the remainder including impurities from the melting process Iron.

In the proposed alloy, part of the niobium can be replaced atom by atom with tantalum be, d. that is, one part by weight of niobium is replaced by two parts by weight of tantalum, but the alloy must contain at least 1% niobium and the tantalum content must not exceed 2%. The alloy must be at least Contains 5.5% molybdenum, otherwise there is a risk of stress corrosion cracking and under certain circumstances the resistance to crevice corrosion is reduced. The molybdenum content is preferably 6 to 9%.

Table I.

Legie
tion Ni Cr C. Mon Si Mn Al Ti Cu Days A. 41.8 18.3 0.07 . 0.58 0.69 0.05 X B. 34.1 21.0 0.08 - 0.61 0.81 0.10 0.26 0.41 X C. 34.2 20.5 0.03 - 0.71 0.83 0.23 0.44 0.41 6th D. 32.6 19.1 0.07 0.15 0.63 0.15 <0.01 0.06 0.06 13 - E. 34.4 20.1 0.07 0.33 0.57 0.13 <0.01 0.015 0.06 5 F. 34.2 20.3 0.08 0.48 0.62 0.14 <0.01 0.019 0.06 12th G 33.8 20.4 0.026 0.25 0.70 0.82 0.19 0.39 0.41 9 H 41.2 18.7 0.02 1.7 0.82 0.86 <0, l 0.13 0.48 1 J 41.9 18.1 0.02 3.3 0.8 0.84 0.14 0.20 0.50 16, X K 32.9 18.8 0.03 2.1 0.66 0.79 0.20 0.25 0.40 5.5 L. 35.7 18.8 0.02 2.1 0.80 0.80 0.14 0.24 0.50 5.8 M. 33.0 18.5 0.02 3.3 0.62 0.75 0.32 0.26 0.51 11, 15 N 39.2 17.7 0.036 3.2 0.71 0.72 0.26 0.23 0.51 X, X P. 38.1 17.7 0.02 4.9 0.71 0.81 0.26 • 0.20 0.52. 11, X

X = still free of cracks after 30 days.

Niobium noticeably increases the resistance to crevice corrosion, provided that the niobium content is at least 1% and that it is carefully adjusted to the contents of nickel and molybdenum. Low niobium contents of, for example, 0.2 or 0.5% have no effect, so that the niobium content is at least 1.5% regardless of the nickel content. Niobium contents exceeding 3.25% have no effect and at most increase the costs for the alloy. It was also found that a% niobium, with the exception of 5 was cracked under the invention falling alloy during finish rolling of a block at 1150 ⁰ C.

If the chromium content is below 17%, the resistance will be against stress corrosion cracking, while chromium contents exceeding 22% the Impair deformability. The alloy according to the invention therefore advantageously contains at least 18% and preferably no more than 21.5% chromium. For optimal corrosion resistance It is important that the total chromium and molybdenum content is at least 24% and preferably at least 25%.

The proposed alloy advantageously contains 18 to 21% chromium, 6 to 9% molybdenum and 1.5 up to 3% niobium. The carbon content of the alloy must not exceed 0.03%.

The alloy can contain up to 1.5% manganese, up to 1% silicon, up to 0.6% titanium and up to 0.6% aluminum contain. Because of the aluminum and / or titanium content of 0.1 to 0.5%, the machining is according to the invention Alloy possible without difficulty.

Silicon contents above 1% can lead to difficulties in forming and welding. Before-

the silicon content therefore preferably does not exceed 0.25%, better still not 0.1%. The manganese content exceeds preferably 1% not.

The alloy can also contain vanadium and copper up to 2% each; higher levels of these elements however, affect the corrosion resistance. Preferably the vanadium content exceeds 1% and the copper content is 0.75% not. The alloy can also contain tungsten, its content but may not exceed 1%.

In order to ensure adequate deoxidation of the alloy, this can advantageously at least one of the elements magnesium, boron, calcium and zirconium added at the following maximum levels are: 0.08% magnesium, 0.005% boron, 0.05% calcium and 0.02% zirconium.

In any case, the alloy residue consists of iron including impurities from the melting process.

Contaminants also include residues of non-essential elements added for deoxidation and refining. The other impurities such as oxygen, nitrogen, phosphorus and sulfur should be kept as low as practically possible. In particular, the contents of phosphorus and sulfur should not exceed 0.02% and 0.03%, respectively. The proposed alloy can be melted in air or in a vacuum, but is not intended thermoformed and outside of 980-1260 ⁰ C are preferably annealed at 1040 to 1175 ° C.

For comparison purposes, Table II below shows the compositions of those proposed Alloys 1 and 2, the comparative alloys Q to Z, which are not covered by the invention, as well as AA to DD compared. None of the test alloys contained more than 1% manganese. In each In the case, the alloy residue consisted of iron and impurities with the exception of alloy C, which additionally contained 2.0% copper.

Table II

Legie Ni Cr. . Mon C. Nb - - Al Ti Fe Weight
loss tion (%) (%) (%) (%) (%) 5.2 (%) (%) (%) (mg) 1 32.4 21.6 6.5 0.021 2.1 0.18 0.24 rest 0.4 2 33.5 21.2 6.4 0.01 2.1 0.25 0.20 rest ο, ι Q 36.3 20.3 0.7 0.023 0.14 0.25 2.45 rest 416.5 R. 39.3 19.8 6.3 0.019 - 0.37 0.28 rest 208, -7 S. 40.0 20.0 6.25 0.032 1.16 0.30 0.26 rest 101.9 - .. τ 39.0 22.0 6.15 0.030 2.18 0.35 0.24 rest 113.4 U 38.5 20.0 6.2 0.054 2.2 0.33 0.26 rest 154.3 V- 39.0 20.0 4.35 0.025 2.1 0.22 0.24 rest 176.1 W. 44.2 19.9 6.15 0.10 2.1 0.32 0.25 rest 101.2 X 43.5 21.6 6.6 0.038 2.34 0.31 0.26 rest 88.0 Y 60.8 20.2 6.0 0.04 2.1 0.30 0.27 rest 97.4 other elements Z 45.9 21.8 6.56 0.04 1.96 0.32 W rest 16.3 0.81 Co AA 44.5 21.5 6.69 0.03 - 1.19 Cu rest 9.2 2.25 Ta 1.06 Co BB 43.2 20.0 3.03 0.03 - 1.75 Cu 29.9 544.2 1.04 Ti CC 33.8 20.1 2.3 0.05 3.41 Cu rest 260 DD 52.5 19.0 3.0 • 0.04 0.1 Cu 18.0 80.6

The alloys Z and AA to DD were commercially available alloys. When melting of the other experimental alloys of nickel, iron, chromium and carbon vacuum were melted in a Magnesiumoxydtiegel under argon and the melt niobium, molybdenum and manganese added as well as bringing the melt to 1595 ⁰ C.

The melt was then cooled to 1565 ° C. and aluminum, titanium and, with the exception of alloy Q, calcium-silicon or magnesium were added. The test melts were cast into blocks and these were ^{rolled out at 1150 °} C. into 6.4 mm thick test pieces.

The samples were then cold-rolled into strip with a thickness of 1.9 mm with a thickness reduction of 75% and then annealed ^{at 1065 ° C. for 1 hour.} Test pieces measuring 25.4 × 38.1 mm were then produced and immersed in a 10% ferric chloride solution for 72 hours. Rubber bands were wrapped around the samples to create gaps. The weight loss that occurs in the tests results from the values in Table II.

The test results show that the proposed alloy has exceptionally good corrosion resistance owns. In contrast, alloy Q was subject to only 0.14% niobium

and the niobium-free alloy R of severe corrosion.

In tests to determine the resistance to stress corrosion cracking, annealed and U-shaped bent test pieces of alloys 1 and 2 and an alloy 7 with 35.9% nickel, 19.7% chromium, 6.1% molybdenum, 0.020% carbon, 2, 36% niobium and 0.70% iron, the remainder iron, ^{immersed in a boiling, 40% magnesium chloride solution at 154 °} C. for 30 days. These samples showed no cracks at the end of the test.

In the cold-deformed state, the proposed Alloy high tensile strength. Another important advantage is that it is made of a wire the proposed alloy meets the requirements of the "kink" test, in which the wire put in a loop and its ends pulled together until the loop is tightly closed. An alloy that meets the conditions of this test is particularly useful as a material for submarine cables suitable.

The proposed alloy is suitable for all uses that require high resistance against chloride media, especially as a material for boilers, tanks, pipes, lines and valves for the chemical and petrochemical industries and for use in the marine atmosphere. The proposed Alloy can be formed into billets, wire, sheet metal, blanks, strips and wire will.

209 522/279

Claims (7)

Patent claims: 1. Use of a nickel-chromium steel alloy, consisting of 31 to 34% nickel, 17 to 22% Chromium, 5.5 to 9.25% molybdenum, 1 to 3.25% niobium, 0 to 2% tantalum, with the niobium and the half Tantalum content is 1 to 3.25%, 0 to 0.3% carbon, 0 to 1.5% manganese, 0 to 1% silicon, 0 to 0.6% titanium, 0 to 0.6% aluminum, 0 to 2% vanadium, 0 to 2% copper, 0 to 1% Tungsten, 0 to 0.08% magnesium, 0 to 0.005% boron, 0 to 0.05% calcium and 0 to 0.02% zirconium, Remainder including impurities caused by the smelting iron, as a material for Objects that have a high resistance to intergranular corrosion, stress corrosion cracking and crevice corrosion in chloride media, especially in boiling magnesium chloride solution must own. 2. Use of a steel alloy according to claim 1, the total content of chromium and Molybdenum is at least 24% for the purpose of claim 1. 3. Use of a steel alloy according to claim 2, the total content of chromium and Molybdenum is at least 25% for the purpose of claim 1. 4. Use of a steel alloy according to Claims 1 to 3, but whose molybdenum content 6 to 9% for the purpose of claim 1. 5. Use of a steel alloy according to claims 1 to 4, but whose niobium content is 3% does not exceed, for the purpose according to claim 1. 6. Use of a steel alloy according to Claims 1 to 5, the chromium content of which, however, is 18 to 21.5% for the purpose according to claim 1. 7. Use of a steel alloy according to claim 1, which contains 18 to 21% chromium, 6 to 9% molybdenum and contains 1.5 to 3.% niobium for the purpose of claim 1.