NL8001739A - FERRITIC STAINLESS STEEL. - Google Patents

Description

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Ferritic stainless steel.

The invention relates to ferritic stainless steel.

United States Patents 3,932,174 and 3,929,473 disclose ferritic stainless alloys with superior crack crack resistance and intergranular corrosion resistance. The steel alloys described in those patents contain 29% chromium and 4% molybdenum. They also have a maximum carbon plus nitrogen content of 250 parts per million. A limit is imposed on the carbon and nitrogen contents, since the corrosion resistance of the steel alloys decreases with increasing carbon and nitrogen contents.

The requirement of low carbon and nitrogen content of the alloys of U.S. Pat. Nos. 3,932,174 and 3,929,473 is unfavorable because it requires the use of more expensive melting techniques, for example induction melting under vacuum.

According to the invention, an alloy is now provided with properties comparable to the emptyings according to US Pat. Nos. 3,929,174 and 3,929,473, which do not require the expensive melting techniques previously referred to. For example, the alloys of the invention can be melted and purified using argon-oxygen decarburization (AOD) methods. The alloys of the invention contain at most 2.00% by weight of the elements titanium, zirconium and columbium, also satisfying the condition: 8001739 2% Ti / 6 +% Zr / 7 +% Cb / 8 _% C + ZN and having a carbon plus nitrogen content of more than 275 parts by weight per million. The alloys are characterized by superior crack resistance and intergranular corrosion resistance, good weldability and satisfactory toughness, both before and after welding.

In view of the aforementioned differences, the alloys of the present invention are clearly distinguished from the alloys of U.S. Pat. Nos. 3,932,174 and 3,929,473. The alloys are also clearly different from two other alloys described in U.S. Pat. Nos. 3,957,544 and 4,119,765. The alloys of the latter two patents have a maximum molybdenum content lower than that required for the alloy of the present invention.

Reference is also made to an article "Ferritic Stainless Steel Corrosion Resistance and Economy" by Remus A. Lula, published in Metal Progress, July 1976, biz. 24-29.

The ferritic stainless steel according to the present invention, which is distinguished by superior crack and intergranular corrosion resistance, good weldability and satisfactory toughness both before and after welding, contains at most 0.08 wt. % carbon, at most 0.06 wt% nitrogen, 25.00 to 25.00 wt% chromium, 3.60 to 5.60 wt% molybdenum, at most 2.00 wt% manganese, at most 2, 00 wt.% Nickel, at most 2.00 wt.% Silicon, at most 0.5 wt.% Aluminum, at most 2.00 wt.% Of the elements titanium plus zirconium plus columbium and for the rest consists practically entirely of iron. The sum of the carbon plus the nitrogen content is greater than 0.0275 wt%.

The contents of titanium, zirconium and columbium meet the condition:% Ti / 6 +% Zr / 7 +% Cb / 8> _% C + ZN.

Carbon and nitrogen are usually present in amounts of at least 0.005 wt% and 0.010 wt%, respectively, while the sum of both is preferably greater than 0.0300 wt%. Chromium and 8001739 • <* fc »* · 3 molybdenum are preferably present in amounts of 28.50 to 30.50 wt% and 3.75 to 4.75 wt%, respectively. Manganese, nickel and silicon are each usually present in amounts less than 1.00% by weight. Aluminum, which may be present because of its deoxidizing effect, is usually present in an amount of less than 0.1% by weight.

Titanium, columbium and / or zirconium are added to improve crack resistance and intergranular corrosion resistance of the alloy of the invention, which alloy may in some way be considered a high carbon plus nitrogen content modification. of the alloys according to U.S. Pat. Nos. 3,932,174 and 3,929,473. *

It has been suggested that stabilizers can be added to modifications of the alloys of U.S. Pat. Nos. 3,932,174 and 3,929,473 with high carbon and / or nitrogen content without affecting the toughness and / or weldability of the alloy is nullified. Although it is preferred to include at least 0.15 wt% titanium 20 in the alloys, since the presence of columbium can only adversely affect the weldability of the alloy, it is also within the scope of the invention to incorporate the required amount of titanium or columbium stabilizer into the alloys. Columbium has a beneficial effect compared to titanium on the toughness of the alloys. A particularly favorable embodiment of the alloy according to the invention is formed by alloys containing at least 0.15% by weight of columbium and at least 0.15% by weight of titanium. Titanium, columbium and zirconium are preferably present in amounts up to 1.00% by weight, the condition being met:% Ti / 6 +% Zr / 7 +% Cb / 8 = 1.0 to 4.0 ( % C +% N)

The ferritic stainless steel according to the invention is particularly suitable for use with welded objects with a thickness of not more than 1.75 mm (usually not more than 1.24 mm) and is particularly suitable for use in the form of welded condenser tubes with a wall thickness from 0.65 to 0.92 mm.

The following examples illustrate various aspects of the invention.

5 ingots from 15 different alloys (Alloys A to 0) were heated at 1121 ° C, hot rolled to a strip with a thickness of 3.1 mm, annealed at temperatures from 1065 to 1121 ° C, cold rolled to strips with a thickness of 1.55 to 1.63 mm and tempered at temperatures from 1065 to 1121 ° C. Samples from the strips were then tested for crack corrosion resistance. Other samples were welded in an inert gas atmosphere with a non-fusing electrode (TIG welding) and were assessed for crack resistance and intergranular corrosion resistance.

The chemical composition of the alloys used is shown in Table A.

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Ml I I I I I ΛΛΛ | λλπλλ u 1 1 1 'o OO 0 0.0 0 0 - vO O - vO OOOC * OO - • H Μ η N · ί Ό' i N i / l NO "N (* 1 Λ Λ Λ Λ Α Λ | I ΛΛΛΛΛΛΛ oooooo 0 0 0 0-0 0 0 r-ic ^ Ofnfntnirio- <i-voa> pr ^ oiop ^ ΟΟΟ Ο ΟΟΟ ΟΟΟΟΟΟΟΟ Λ Λ A Λ Α Λ Λ Λ Λ Λ Λ Λ * Λ Λ 0.0-0 Ο 0 0 0 0 0-0 0 0 0 0 Ο

• H <r <i-c, o <ffriCN! 0oóc ^ \ Dr ^ air'Lnoo cofOco fnMtncnn-ii'cocnrocMcn'cfCM

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Further figures regarding these alloys are listed in Table B.

Table B

Alloy% C +% N% Ti / 6 +% Zr / 7 +% Cb / 8 5 A 0.064 0.052 B 0.086 0.057 C 0.041 0.043 D 0.056 0.067 E 0.053 0.102 10 F 0.077 0.110 G 0.030 0.048 H 0.046 0.066 I 0.048 0.082 J 0.055 0.083 15 K 0.056 0.073 L 0.056 0.071 M 0.061 0.083 N 0.061 0.086-0 0.056 0.107 It should be noted that the alloys A and B have a composition which does not meet the condition of the invention. In particular, these alloys do not meet the condition:

% Ti / 6 + XZr / 7 + ZCh / 8> ZC + ZN

Crack corrosion resistance was determined by dipping 2.5 by 5 cm samples with a ground surface in a 10% iron (III) chloride solution for 72 h. The test took place at temperatures of 35 and 50 ° C. Cracks were induced in the edges 30 and in the surfaces using blocks of polytetrafluoroethylene at the front and back held in place by pairs of rubber walls which, when stretched at an angle of 90 ° to were arranged longitudinally and transversely around the samples. The test in question is described in "Designation": G48-76 of the American Society For Testing And Materials.

8001739 * * £ - * 7

The results of the determination are shown in Table C.

Table G

Crack corrosion test in 10% iron (III) chloride solution .______

Weight loss (g)

Alloy Base alloy after welding after welding __ 50 ° C 55 ° C 50 ° C

A 0.0 0.0 0.4195 10 B 0.8519 0.0198 0.5783 C 0.0 0.0001 V 0.0004% D 0.0 - 0.0 E 0.0 0.0 0.0 F 0.0 0.0001 0.0 .15 G - - 0.0 H - - - I - - 0.0 J - -. 0.0003 K - - ·· '0.0 20 L - - 0.0 M - - 0.0 N - - 0.0 0 - - 0.0013

From Table G, it can be seen that crack corrosion resistance of alloys C through G and alloys I through 0 is superior to that of rings A and B. Alloy B in non-welded condition (Base Alloy) showed a weight loss of 0.8519 g.

The welded alloys A and B resulted in a weight loss of 0.4195 and 0.5783 g, respectively. It is essential here that alloys A and B fall outside the scope of the invention. On the other hand, the alloys C through G and I through 0 are alloys of the invention.

Resistance to intergranular corrosion was determined by dipping 2.5 by 5 cm samples with a ground surface for 120 h in a boiling mixture of 8001739 8 copper sulfate and 50% sulfuric acid. The usual criteria in this test are that an alloy is rated as good when the corrosion rate is less than 0.6 mm / year (0.05 mm / month) and has a satisfactory appearance in microscopic examination. This test is recommended for high chromium stabilized ferritic stainless steel alloys.

The results of this assessment are shown in Table D.

Table D

10 Corrosion test with copper sulfate - 50% sulfuric acid

Corrosion speed - as after welding - f

Alloy mm / year mm / month Microscopic examination _ _ _ after welding (magnify 30 x) A 0.205 0.017 15 B 3.525 0.295 C 0.171 0.014 D 0.249 0.021 Ξ 0.140 0.012 F 0.275 0.023

20 G 0.144 0.012 NAX

H -

I 0.157 0.013 NA

J 0.165 0.014 NA

K 0.140 0.012 NA

25 L 0.131 0.011 NA

M 0.145 0.012 NA

N 0.132 0.011 NA

0 0.159 0.013 NA

no intergranular damage.

30 Table D shows that only alloy B was not satisfactory in this test. Alloy B had a corrosion rate of 3.525 mm per year. As indicated above, alloy B is one of the two alloys not covered by the invention. However, this alloy is more different from the alloys of the invention than alloy A in that it has a lower ratio of titanium to carbon plus nitrogen.

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Toughness was determined by determining the transition temperature using Charpy V notched impact test specimens both from hot-rolled and annealed material (3.125 x 9.85 mm samples) and welded samples (1.55 samples up to 1,625 mm x 9.85 mm). The transition temperature was based on the occurrence of a 50% ductile - 50% brittle fracture. The transition temperatures are listed in Table E.

Table E

10 Transition temperature (° C)

Alloy As welded Hot rolled and tempered A -3.9 (1) 73.9 (3) B 15.6 (1) 85 (3) C 26.7 (1) 68.3 (3) 15 D 46.1 (1) 85 (3) E 118.3 (1) 90.6 (3) F 104.4 (1) 87.8 (3) G -37.2 (2) 35. . (4) H - 48.9 (4) 20 I 35 (2) 71.1 (4) J 43.3 (2) 54.4 (4) K 15.6 (2) 48.9 (4) L 32.2 (2) 43.3 (4) M 40.6 (2) 57.2 (4) 25 N 68.3 (2) 60 (4) 0 54.4 (2) 98.9 (4) (1) Tempering strip before welding at 1121 ° C - air cooled 30 (2) Tempering strip before welding at 1065 ° C - quenched with water (3) Tempering at 1121 ° C - quenched with water; transverse test (4) Tempering at 1065 ° C - quenched with water; test in the transverse direction.

The transition temperatures indicate that the steel 8001739 10 of the invention can be cold rolled, shaped and welded, although some preheating may sometimes be desirable. The columbium-containing samples had lower transition temperatures than the titanium-containing samples. The samples containing both titanium and columbium had transition temperatures between those of the columbium containing and the titanium containing samples.

The invention, as will be apparent, can be practiced with a variety of modifications and variations without departing from the scope of the invention.

8001739

Claims (8)

1. Ferritic stainless steel mainly consisting of at most 0.08 wt% carbon, at most 0.06 wt% nitrogen, 25.00 to 35.00 wt% chromium, 3.60 to 5.60 wt .% 5 molybdenum, at most 2.00 wt.% Manganese, at most 2.00 wt.% Nickel, at most 2.00 wt.% Silicon, at most 0.5 wt.% Aluminum, at most 2.00 wt.% of the elements titanium, zirconium and columbium together, residual practically all iron, the titanium, zirconium and columbium being present in quantities such that the condition:% Ti / 6 + is met. % Zr / 7 +% Cb / 8>% C +% N while the sum of the carbon plus the nitrogen content is greater than 0.0275%. Ferritic stainless steel according to claim 15 1, characterized in that it contains at least 0.005 wt.% Carbon and at least 0.010 wt.% Nitrogen and in that the sum of the carbon plus the nitrogen content is greater than 0.0300 wt.% . Ferritic stainless steel according to claim 1 or 2, characterized in that it contains 28.50 to 30.50% by weight of chromium 20. Ferritic stainless steel according to claims 1-3, characterized in that it contains 3.75 to 4.75 wt.% Molybdenum. Ferritic stainless steel according to any one of the preceding claims, characterized in that it contains at most 1.00 25% by weight of the elements titanium, zirconium and columbium together, meeting the condition:% Ti / 6 +% Zr / 7 +% Cb / 8 = 1.0 to 4.0 (% C +% N). Ferritic stainless steel according to any one of the preceding claims, characterized in that it contains at least 0.15% by weight of titanium. Ferritic stainless steel according to claim 6, characterized in that it contains at least 0.15% by weight of columbium. Ferritic stainless steel according to any one of the preceding claims, characterized in that it comprises at least 0.005 wt.% Carbon, at least 0.010 wt.% Nitrogen, 28.50 to 30.50 wt.% Chromium, 3.75 to 4, 75 wt.% Molybdenum, up to 1.00 8001739 wt.% Of the elements titanium, zirconium and columbium together meeting the condition% Ti / 6 +% Zr / 7 +% Cb / 8 «1.0 to 4 0.0 (% C +% N) while the sum of the carbon plus the nitrogen content is greater than 0.0300 wt%. 8001739