DE3202223A1 - Improved 36% Ni iron-nickel alloy - Google Patents

Abstract

The invention relates to a 36% Ni iron-nickel alloy which has a low coefficient of thermal expansion and contains 34.5-37.5% of Ni, at most 1.2% of Mn and 0.005-0.12% of Ti, the remainder being Fe, and can additionally contain up to 0.035% of C, up to 0.35% of Si, up to 0.025% of P, up to 0.015% of S, up to 0.02% of Al, up to 0.025% of O, up to 0.015% of N and unavoidable impurities, with the proviso that, at a simultaneous S and Al content of at most 0.005%, the Mn content is less than 0.5% and, if either the S content or the Al content exceeds 0.005%, the Mn content must exceed 0.5%. This alloy has good thermal expansion resistance, good weldability and good stress corrosion cracking resistance, and is easy to produce.

Description

IMPROVED 36% Ni-IRON-NICKEL ALLOY

The invention is an improved alloy with low Coefficient of thermal expansion, commonly known by the name "Invar" and is hereinafter referred to as 36% Ni iron-nickel alloy.

This alloy with a low coefficient of thermal expansion contains according to ASTM (American Society for Testing Materials) 35-37t Ni, the rest is Fe, and can also contain up to 0.5% Mn, up to 0.5% Co, up to 0.5% Cr, up to 0.5E Mo, up to 0.18 C, up to 0.358 Si, up to 0.025% S and up to 0.0258 P as permissible Contain additives and / or impurities. It can also be used for deoxidation purposes contain a small amount of aluminum.

In general, there are iron-nickel alloys with a high Ni content the activities of C, N and 0 are remarkably high because of this high Ni content, what occurs in the solidification phase through the union of C and 0 to the formation of CO bubbles and from N2 bubbles resulting from dissolved nitrogen, which in turn leads to the formation of cavities conditional in the molded ingots. The heat treatment makes this cavity pattern impossible to make the alloy, which is why it is customary to make it the process of vacuum melting.

Since such alloys are in the form of a homogeneous austenitic phase solidify, the impurities tend to segregate.

Segregated impurities can, however, by heating in deep ovens are not homogenized and therefore cause in the course of slab production Grain boundary cracks, which represent a further difficulty for heat treatment.

In the meantime, the demand for liquefied natural gas has risen sharply and with it the demand for tankers, storage tanks, tankers, etc. as well associated large constructions for transport and storage as well as corresponding Equipment (hereinafter referred to as "Containers and Equipment" for short) The for The material used in the construction of such containers and equipment should be as follows Fulfill the requirements: a) Stability of the structure in a temperature range of up to -162 ° C without loss of toughness at this temperature, b) if possible small changes in dimensions within a temperature range of Room temperature down to -1620C, especially the lowest possible coefficient of thermal expansion within this temperature range, c) easy feasibility of the construction the containers and equipment unavoidable welding work as well as avoidance of Welding defects, i.e. cracks at high temperatures, leading to gas leakage or breakage, d) Avoidance of delayed crack formation, such as stress corrosion cracking etc. in the containers and equipments.

So far, there is a material that would meet these requirements not. Currently in question for containers and equipment for liquefied natural gas coming materials, namely Steel with 9% Ni, aluminum alloys, Austenitic stainless steels, invar alloys with 36% Ni, etc. only meet part of these demands.

The invar alloy with 36% Ni fulfills the requirements a) and b>, i.e. the structure of this alloy, namely its face-centered cubic lattice, stays up to the temperature of liquid nitrogen (-1960C), which is easy in the laboratory can be realized, as well as the toughness without causing embrittlement comes. Furthermore, this alloy retains over a wide temperature range (from room temperature down to -196 ° C) their low coefficient of thermal expansion.

However, this alloy is not free from the problems of pinholing and grain boundary cracks, and is also prone to cracking at high temperatures, as they occur during welding. This is achieved by means of low-melting Ni-S compounds which, however, cannot be avoided with the material used. she also exhibits poor corrosion resistance and is prone to stress corrosion cracking.

The problem of cavity formation can go through to a certain extent the vacuum melting process can be avoided. In JA-OS No. 84722/73 (patent specification No. 9569/79) therefore also describes a 36% Ni-iron-nickel alloy which is characterized in that its Mn content is 0.5-1.5% of the carbon content are restricted to 0.005-0.025% and the N content to 0.001-0.015% and 0.005-0.100% Ti are added, whereby the formation of voids in the ingots is prevented. The invention described in the cited patent document thus shows that the formation of cavities can be prevented by adding 0.5-1.52 Mn and 0.005% or more of Ti, and that such an alloy is produced without using the vacuum melting process can be. However, this patent does not provide any information about the O content and also does not provide any information on resistance to grain boundary cracking, crack resistance at high temperatures and resistance to stress corrosion cracking.

JA-O, no. Je2922 / 76 describes a process through which grain boundary cracks occur by limiting the S content to a maximum of 0.015%, the Al content to a maximum of 0.02% and the O content to a maximum of 0.025E can be prevented. This OS also suggests avoiding the formation of cavities by adding Ti, such as it is also described in JA-OS No. 84722/73. However, this OS doesn't do any Information on the crack resistance at high temperatures and on resistance to Stress corrosion cracking.

Incidentally, all publications mentioned are from the applicant of the present Application has been submitted.

In the applicant's earlier JA-OS No. 100959/80 (it corresponds to U.S. Application No. 113943) was a 36% Ni-Iron-Nickel alloy with low Proposed thermal expansion coefficients that have the properties mentioned under c) and d) without sacrificing the properties mentioned under a) and b). These Alloy contains 34.5-37.5% Ni, while the remainder is Fe, and can be up to to 0.18 C, up to 1.2% Mn, up to 0.35% Si, up to 0.5t Cr, up to 0.5% Mo, up to to 0.028 Al, up to 0.05% Co, up to 0.025% P and up to 0.015% S, where the Mn content with a simultaneous Al and S content of at most 0.005% 1.2%, while the Mn content is at least 0.5%, but not more than 1.2%, if either the Al or the S content is not more than 0.005t.

In view of the state of the art cited here, it was assumed that a still further improved alloy with higher crack resistance at high temperatures and can be obtained without using the vacuum melting method if the C content would be further restricted and a small amount of Ti would be added. Thereupon were Experiments carried out which led to the present invention.

According to the invention, a new 36% Ni-Iron-Nickel-Le- yaw provided with a low coefficient of thermal expansion, which is 34.5-37.58 Ni, at most Contains 1.2% Mn, 0.005-0.12E Ti and the remainder being Fe and more Lis to 0.0358 C, up to 0.35% Si, up to 0.025 P, up to 0.015't S, up to 0.02% Al, up to 0.025% 0, up to 0.015% N and unavoidable impurities can, whereby the Mn content with simultaneous S and Al content of max. 0.005% may be less than 0.5%, while the Mn content must be above 0.5E if either the S or the Al content is above 0.005%.

In the alloy composition definition above, the content ranges are of the components except for the C and Ti content in the prior art. the Characteristic features of this invention are based on the limitation of the C- resp. Ti content.

Carbon is a component that is found in the melt of the alloy leads to the formation of bubbles. It was found that the blistering without Application of the vacuum melting process can be prevented when the oxygen content to a maximum of 0.025% with the addition of Ti even with a carbon content of up to is limited to 0.035%.

In the aforesaid Japanese Patent Laid-Open No. 89722/73 is the Ti content is defined as 0.005-0.100%. It is described that in the present blistering of at least 0.5% Mn titanium in an amount of at least 0.005% prevents damage to the surface (so-called titanium strips, which by TiO2) occur in considerable numbers in the end product if the Ti content exceeds 0.1%. In the present invention, however, Ti has one Content of at least 0.005% achieves the stated effect, even with an Mn content of less than 0.5%, which results from the restriction of the O content to a maximum of 0.025% results. Titanium strips are formed even with a Ti content of up to 0.12'-. " so over 0.1%, not. In the aforementioned JA-OS No. 84722/73 Incidentally, the O content is not limited.

In the alloy of the present invention, the O content is preferably 0.020% or less, the P content is preferably 0.018 or less, the Si content is preferably 0.25% or less, the Co content is 0.02% or below and the Ti content is 0.01-0.108.

The alloy according to the invention can be produced both by a melting process with air as well as in the vacuum melting process.

The present invention is illustrated below by means of exemplary embodiments explained in more detail.

Alloy samples, the composition of which is shown in Table 1 is, were produced and examined for cavity formation. Samples 1, 3, 4 and 5 were produced using the LD vacuum process, while samples 2 and 6 were produced by melting were produced in an electric furnace under an air atmosphere. Each sample became one A 6-ton block was poured, which was then broken up to detect any bubbles present. the Results are also summarized in Table 1. As from this table too As can be seen, alloys of the composition according to the invention are free from bubbles.

Table 1 Sample CNOP Si Ni Co Mn S Al Ti bubbles Execution. - 1 0.032 0.012 0.008 0.007 0.22 35.78 0.012 0.55 0.004 0.013 0.081 no examples 2 0.027 0.010 0.019 0.005 0.17 36.01 0.011 0.80 0.003 0.006 0.092 no 3 0.041 0.013 0.007 0.006 0.21 35.97 0.009 0.72 0.011 0.004 0.090 yes Ver 4 0.024 0.018 0.008 "0.17 36.02 0.010 0.98 0.004 0.003 0.043 yes equal 5 0.027 0.010 0.007 0.005 0.18 36.80 0.016 0.86 0.004 0.005 below yes example 0.005 6 0.031 0.010 0.027 0.007 0.20 36.35 0.020 0.80 0.003 0.005 0.071 each Furthermore, the samples summarized in Table 2 were prepared to carry out the arc test and the stress corrosion cracking test. Each sample was melted in a vacuum RF furnace, cast into blocks, forged at a temperature of 11500C, and rolled into 2 mm and 1 mm thick sheets by repeated heat treatment and repeated cold rolling.

Finally, the sheet was subjected to a heat treatment for 10 minutes at 800.degree subjected.

Electrolyte nickel was used as the Ni source for the production of the samples used; the Co content was obtained through a combined use of ferronickel and electrolyte nickel set. Mn was added in the form of metallic Mn. The samples with a low S content were desulfurized with calcium carbonate and fluorite.

Table 2 Sample CNOP Si Ni Co Mn S Al Ti o 7 0.010 0.011 0.008 0.008 0.21 36.01 0.012 0.21 0.003 0.003 0.075 x 8 0.010 0.013 0.007 0.006 0.23 35.95 0.010 0.26 0.004 0.003 - o 9 0.010 0.014 0.007 0.007 0.16 35.79 0.009 0.47 0.004 0.004 0.091 x 10 0.010 0.010 0.006 0.006 0.21 35.83 0.007 0.49 0.003 0.004 - x 11 0.010 0.012 0.007 0.008 0.15 36.20 0.011 0.26 0.004 0.009 0.082 x 12 0.010 0.013 0.008 0.006 0.17 36.32 0.008 0.23 0.003 0.008 - o: exemplary embodiment; x: Comparative Examples% The following Table 3 shows the results of the arc test in which the crack sensitivity of the alloy was examined at high temperatures.

Table 3 Sample result 7 o 8 o 9 o 10 O 11 x 12 x Test conditions: Amperage: 110 A Arc length: 2 mm o: no cracks occurred x: cracks occurred Samples 7 and 8 were subjected to a stress corrosion cracking test. Included the samples were subjected to a load stress of 30 kg / mm2 in a test etching solution of 20% NaCl and 0.46 N Cr6 + exposed at 450C. The break time laq at over 1,500 Hours for both sample 7 and sample 8.

As a result, it was found that addition of Ti was not significant Influence on the stress corrosion cracking resistance of the 36% Ni iron-nickel alloy of the present invention.

The alloy according to the invention represents an improvement in the JA-OS No. 190959/80 (US Application No. 113943). By adding a small amount Ti and by limiting the C content, the alloy can be of the desired quality can be produced without using the vacuum melting process, which is a simplification of the manufacturing process means.

Claims (7)

Claims 1. 36% Ni-iron-nickel alloy with a low coefficient of thermal expansion, d a d u r c h e k e n n n z e i c h n e t that they are 34.5-37.5% Ni, at most 1.2% Mn, 0.005-0.12t Ti, with the remainder being Fe, and also up to 0.035% C, up to 0.35% Si, up to 0.025% P, up to 0.005% S, up to 0.005% Al, up to may contain up to 0.025% 0, up to 0.015% N and unavoidable impurities. 2. 36% Ni-iron-nickel alloy according to claim 1, d a d u r c h g It is not noted that the oxygen content is 0.020% or less, the phosphorus content 0.01% or below, the Si content 0.25% or below, the Co content 0.02% or below below that and the Ti content is 0.01-0.10%. 3. 36% Ni-iron-nickel alloy with low coefficient of thermal expansion, d u r c h e k e n n n z e i n e t that they are 34.5-37.5% Ni, 0.5-1.2% Mn, 0.005-0.12% Tient contains and the remainder is Fe and also up to 0.035% C, up to 0.35t Si, up to 0.025t P, down to 0.0058 S, 0.005-0.02% Al, up to 0.025% 0, up to 0.015% N as well as unavoidable impurities may contain tendencies. 4. 36'T-Ni-iron-nickel alloy according to claim 3, d a d u r c h g It is not noted that the oxygen content is 0.0208 or below, the P content 0.01% or below, the Si content 0.25% or below, the Co content 0.02% or below below that and the Ti content is 0.01-0.10%. 5. 36-Ni-iron-nickel alloy with low coefficient of thermal expansion, d u r c h e k e n n n z e i n e t that they are 34.5-37.5% Ni, 0.5-1.2% Mn, 0.005-0.12% Contains Ti and the remainder is Fe and also up to 0.035% C, up to 0.35% Si, up to 0.025% P, 0.005-0.015% S, up to 0.005% Al, up to 0.025% 0, up to 0.015% N as well as unavoidable impurities. 6. 36% Ni-iron-nickel alloy according to claim 5, d a d u r c h g It is not noted that the oxygen content is 0.020% or below, the P content 0.01% or below, the Si content 0.25% or below, the Co content 0.02% or below below that and the Ti content is 0.01-0.10%. 7. 36% Ni-Iron-Nickel alloy with a low coefficient of thermal expansion according to one of the preceding claims 1 to 6, d a d u r c h g e k e n n z e i c Note that the C content is 0.01% or less.