GB2117399A - Low thermal expansion alloys - Google Patents

Abstract

Low thermal expansion coefficient alloys are disclosed which contain 34.5 to 37.5% Ni, not more than 1.2% Mn, 0.005 to 0.12% Ti, the balance being Fe apart from impurities and incidental ingredients and optionally contain up to 0.035% C, up to 0.025% P, up to 0.015% S, up to 0.02% Al, up to 0.025% O and up to 0.015% N. When both the S content and the Al content are not more than 0.005%, the Mn content may be less than 0.5%, but when either the S content or the Al content is in excess of 0.005%, the Mn content will generally be in excess of 0.5%. The alloys have good thermal expansion stability, good weldability, good stress corrosion cracking resistance as well as ease of preparation.

Description

SPECIFICATION Improved low thermal expansion coefficient alloy This invention relates to an improvement on the low thermal expansion coefficient alloy commonly known as "lnvar" (R.T.M.), which is hereinafter called 36%-Ni Fe-Ni alloy.

This low thermal expansion coefficient alloy, according to the ASTM, comprises 3537% nickel (Ni) and the balance is iron (Fe), and may contain up to 0.5% of manganese (Mn)), up to 0.5% of cobalt (Co), up to 0.5% of chromium (Cr), up to 0.5% of molybdenum (Mo), up to 0.1% of carbon (C), up to 0.35% of silicon (Si), up to 0.025% of sulphur (S) and up to 0.025% of phosphorus (P) as allowable additive elements and/or impurities. A small amount of aluminium (Al), which is used for deoxidation, may exist as the residue.

Generally speaking, in the high-Ni Fe-Ni alloys, the activities or carbon, nitrogen and oxygen (C, N and 0) are remarkably high because of the high Ni content, and in the course of solidification, carbon monoxide (CO) bubbles formed from C and 0, and N2 bubbles originating from dissolved N are generated, which result in formation of blisters in the ingots formed. These blisters make the hot working of the alloy practically impossible, and so vacuum melting is usually employed in the production of the 36%-Ni Fe-Ni alloy.

As the high-Ni Fe-Ni alloys solidify in the form of a homogeneous austenite phase, impurities are liable to segregate. Such segregated impurities are not homogenized by heating in a soaking pit, and they therefore cause cracking in the course of slabbing (intergranular fracture), which constitutes another cause of difficulty during the hot working procedure.

Meanwhile, demand for liquefied natural gas (hereinafter called LNG) has expanded remarkably, and so the need for tankers, storage tanks, tank trailers, and related large scale constructions for transportation and storage thereof as well as the equipment therefor (hereinafter referred to as "the containers and equipments") is increasing.

Characteristic properties required for the material for constructing these containers and equipment are: (a) That the metallographic structure of the material is stable over a temperature range down to -1 620C, to which the material is exposed, and thus does not lose its toughness at this low temperature. (b) That dimensional change in the temperature range from room temperature to -1 620C, to which the containers and equipments are exposed, is minimum, namely, the thermal expansion coefficient is sufficiently small in this temperature range. (c) That the welding work, which is indispensable for constructing the containers and equipments, can be easily carried out, and welding defect, that is, high temperature cracking, which will result in gas leak or breakdown of the containers and equipments, does not develop. (d) That the containers and equipments built of the material are not susceptible to delayed cracking such as stress corrosion cracking, etc.

There is no material that satisfactorily meets all these requirements, there are 9%-Ni steel, aluminum alloys, austenite stainless steels, and 36%-Ni Fe-Ni alloy, which are now used as the materials for the containers and equipments for LNG.

The 36%-Ni Fe-Ni alloy satisifies requirements (a) and (b) mentioned above. That is, this alloy maintains the metal structure of face-centered cubic lattice down to -1 960 C, which is the temperature of liquefied nitrogen and thus easily produced in a laboratory, and it maintains its toughness without exhibiting the ductility-brittleness transition phenomenon, and thus retains sufficient toughness at this temperature. Also this alloy is characterized in that it retains low thermal expansion coefficient over a wide range from room temperature to -1 960C.

However, this alloy is not free from problems of generation of blisters and intergranular fracture, either, and is deficient in that it is susceptible to high temperature cracking when welded because of the low-melting compounds of Ni and S, which are inevitably incidental from the materials used and is poor in corrosion resistance easily undergoing stress corrosion cracking.

The problem of formation of blisters can be avoided to some extent by employment of vacuum melting. Also in our Japanese Laid-Open Patent Publication No. 84722/73 (Patent Publication No.

9569/79) a 36%-Ni Fe-Ni alloy characterized in that the Mn content is restricted to 0.51.5%, the C content is restricted to 0.005-0.025%, the N content is restricted to 0.001-0.015%, and 0.005-0.1 00% of Ti is added, whereby the formation of blisters in the ingots is prevented. That is to say, the invention of said patent publication teaches that formation of blisters can be prevented by incorporating into the alloy 0.51.5% of Mn and 0.005% or more of Ti, and thus such an alloy can be produced without resorting to vacuum melting. However, this patent publication is silent about the content of oxygen and teaches nothing about intergranular cracking resistance, high temperature cracking resistance and stress corrosion cracking resistance.

Our Japanese Laid-Open Patent Publication No. 52922/76 discloses the fact that the intergranular cracking can be prevented by restricting the S content to not more than 0.01 5%, the Al content to not more than 0.02%, and the 0 content to not more than 0.025%. Also this patent publication suggests that formation of blisters can be avoided by addition of Ti as in our Japanese Patent Laid-Open Publication No. 84722/73. However, this patent publication teaches nothing about high temperature cracking resistance and stress corrosion cracking resistance.

We previously proposed in our Japanese Laid-Open Patent Publication No. 100959/80, a 36%-Ni Fe-Ni low thermal expansion coefficient alloy which is provided with the above-mentioned characteristics (c) and (d) without sacrificing the characteristics (a) and (b). The alloy comprises 34.5%37.5% Ni, the balance being substantially Fe and may contain up to 0.1% C, up to 1.2% Mn, up to 0.35% Si, up to 0.5% Cr, up to 0.5% Mo, up to 0.02% Al, up to 0.05% Co, up to 0.025% P, and up to 0.015% S. The Mn content is up to 1.2% when both the S content and the Al content are not more than 0.005% and the Mn content is at least 0.5% and up to 1.2% when either of the S content and the Al content is not more than 0.005%.

We have now found that if the C content of alloys similar to the above-mentioned alloys is restricted and a small amount of Ti is added, a further improved alloy is obtained which has a better high temperature cracking resistance and can be prepared without resorting to vacuum melting.

According to the invention, therefore, we provide a low thermal expansion coefficient alloy which contains 34.5 to 37.5% Ni, not more than 1.2% Mn, from 0.005 to 0.12% Ti, and not more than 0.035% C and the balance being Fe apart from impurities and incidental ingredients. The alloy will optionally contain up to 0.35% Si, up to 0.025% P, up to 0.15% S, up to 0.02% Al, up to 0.025% 0 and up to 0.015% N. The Mn content may be less than 0.5% when both the S content and the Al content are not more than 0.005%, and the Mn content will generally be in excess of 0.5% when either the S content or the Al content is in excess of 0.005%.

In the above definition of the compositions of the alloy, the amounts of the components other than C and Ti are generally based on the teachings of the prior art. The characteristics of alloys of this invention resides, inter alia, in the amounts of C and Ti present.

C is an ingredient which contributes to the generation of bubbles in the melt of the alloy. We have found that the generation of bubbles can be prevented without resorting to vacuum melting when the 0 content is restricted to not more than 0.025% and Ti is incorporated even if up to 0.035% C is contained. It is preferred that the C content be 0.01% or less.

In the above-mentioned Japanese Laid-Open Patent Publication No. 84722/73, the Ti content is defined as 0.005-0.100%, and it is described that in the presence of not less than 0.5% Mn, Ti is effective for prevention of bubble generation in an amount of not less than 0.005%, while a surface defect (the so-called "titanium streak" caused by TiO2) in the final product occurs remarkably when the Ti content is in excess of 0.1%. In this invention, however, as the result of restricting the 0 content to not more than 0.025%, Ti is effective in an amount of not less than 0.005% even when the Mn content is less than 0.5% and the titanium streak does not develop up to 0.12% Ti in excess of 0.1%. In the above-mentioned Japanese Laid-Open Patent Publication No. 84722/73, it should be noted that the 0 content is not limited.

In the alloy of this invention, the 0 content is preferably 0.020% or less, the P content is preferably 0.01% or less, the Si content is preferably 0.25% or less, the Co content is 0.02% or less and the Ti content is 0.01--0.109/0.

The alloy of this invention can be prepared by both melting in the atmosphere, as well as, of course, by vacuum melting.

The invention will now be more specifically described with reference to the foliowing non-limiting working Examples and comparative Examples.

The alloy samples the compositions of which are indicated in Table 1 were prepared and tested for generation of bubbles (blisters). Samples 1, 3, 4 and 5 were prepared by the LD-VAC process and Samples 2 and 6 were prepared by melting by an electric furnace under the atmosphere. Each Sample was made into a 6 ton ingot, the ingot was sectioned to check the existence of blisters. The results are indicated in the same table. As seen there, the alloys of the compositions in accordance with this invention were free from blisters.

TABLE 1

Samples C N O P SI Ni Co Mn S AL Ti Bllsters Working 1 0.032 0.012 0.008 0.007 0.22 35.78 0.012 0.55 0.004 0.013 0.081 None examples 2 0.027 0.010 0.019 0.006 0.17 36.01 0.011 0.80 0.003 0.006 0.092 None 3 0.041 0.013 0.007 0.006 0.21 36.97 0.009 0.72 0.011 0.004 0.090 Found Comparative 4 0.024 0.018 0.008 0.006 0.17 36.02 0.010 0.98 0.004 0.003 0.043 Found examples less than 5 0.027 0.010 0.007 0.005 0.18 36.30 0.018 0.86 0.004 0.005 0.006 Found 6 0.031 0.010 0.027 0.007 0.20 36.35 0.020 0.80 0.003 0.005 0.0071 Found Further, in order to carry out the arc strike test and the stress corrosion cracking test, the Samples as indicated in Table 2 were prepared. Each Sample was melted in a vacuum high frequency electric furnace of 10 kg capacity, cast into an ingot, forged at a temperature of around 11 500C, and made into a sheet of 2.0 mm or 1.0 mm thickness by repeating heat treatment and cold rolling. Finally the sheet was heat-treated at 8000C for 10 minutes.

In preparation of Samples, electrolytic Ni was used as the Ni source, and the adjustment of the Co content was effected by combined use of ferronickel and electrolytic Ni. Mn was added in the form of metallic Mn. The Samples in which the S content is low were prepared by desulfurizing with limestone and fluorite.

TABLE 2

Samples C N O P 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 # 8 < 0.010 0.013 0.007 0.006 0.23 35.96 0.010 0.28 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 # 10 < 0.010 0.010 0.006 0.006 0.21 35.83 0.007 0.49 0.003 0.004 # 11 < 0.010 0.012 0.007 0.008 0.15 36.20 0.011 0.24 0.004 0.009 0.082 # 12 < 0.010 0.013 0.008 0.008 0.17 36.32 0.005 0.23 0.003 0.008 #: Working examples #: Comparative examples The following Table 3 shows the results of the arc strike test, by which high temperature cracking sensitivity of the alloy was studied.

TABLE 3 Sample Result 7 0 8 0 9 0 10 0 11 X 12 X Test conditions: Electric current 11 OA Arc length 2 mm O: No cracking X: Cracking occurred A corrosion stress cracking test was carried out using Samples 7 and 8. The Samples were subjected to loaded stress of 30 kg/mm2 in a test etching solution (20% NaCI and 0.46 N Cr6+) at 450C.

The rupture time was over 1500 hours for both Samples 7 and 8, and it was revealed that addition of Ti did not give significant influence to the stress corrosion cracking resistance of the 36%-Ni Fe-Ni alloy of this invention.

The alloys of this invention are an improvement of the invention of Japanese Laid-Open Patent Publication No. 100959/80. The alloys are of desirable quality, can be prepared without resorting to vacuum melting and the production process has been simplified.

Claims (9)

1. A low thermal expansion coefficient alloy which comprises 34.5 to 37.5% Ni, not more than 1.2% Mn, from 0.005 to 0.12% Ti, the balance being Fe apart from impurities and incidental ingredients, and optionaliy contains up to 0.035% C, up to 0.35% Si, up to 0.025% P, up to 0.01 5% S, up to 0.02% Al, up to 0.025% 0 and up to 0.01 5% N, the Mn content being 0.5% or more when either the S content or Al content is in excess of 0.005%.

2. An alloy as claimed in claim 1 which contains from 0.5 to 1.2% Mn, up to 0.005% S and from 0.005 to 0.02% Al.

3. Ai, alloy as claimed in claim 1 which contains from 0.5 to 1.2% Mn, from 0.005 to 0.015% S and up to 0.005% Al.

4. An alloy as claimed in claim 1 which contains up to 0.005% S up to 0.005% Al.

5. An alloy as claimed in claim 4 wherein the Mn content is less than 0.5%.

6. An alloy as claimed in any of claims 1 to 5 which contains 0.02% or less of 0, 0.01% or less of P, 0.25% or less of Si, 0.01 to 0.1% of Ti and 0.02% or less of Co.

7. An alloy as claimed in any of claims 1 to 6 wherein the C content is 0.01% or less.

8. An alloy as claimed in claim 1 substantially as hereinbefore described.

9. An alloy as claimed in claim 1 substantially as hereinbefore described with reference to any of Examples 1.2, 7 and 9.