Oxidation-resistant iron-nickel-chromium alloy

Abstract

 The oxidation resistance at elevated temperatures, e.g. 982°C and above, of iron-nickel-chromium alloys of specified composition is improved by limiting the manganese content to a maximum value of 0,6%.
The preferred alloy consists of:

Description

[0001] The present invention is directed to improving the oxidation resistance of iron-nickel-chromium alloys.

[0002] For many years an iron-nickel-chromium alloy nominally containing 30-35% Ni, 19-23% Cr, 0.15 to 0.6% aluminium, 0.15 to 0.6% titanium, manganese in amounts up to 1.5%, e.g. about 1% manganese, up to about 0.75% copper, up to 1% silicon, and up to about 0.1% carbon, the balance, apart from impurities, being iron, has been used in such applications as heat exchanger tubing, process piping, carburizing fixtures and retorts, furnace components, and heating element sheathing. It is known for its resistance to oxidation at elevated temperatures and for a number of other properties, including stable structure, ductility, resistance to carburization, and corrosion resistance.

[0003] All percentages in this specification and claims are by weight.

[0004] The presence of manganese in iron-nickel-chromium alloys, as is known, confers a number of advantages, including its ability to fix sulphur which otherwise exercises a detrimental influence on various metallurgical characteristics. Also, manganese is deemed to enhance weldability and is considered to act as a deoxidant. Its use in the manufacture of such alloys is thought to be a carryover from steelmaking practice.

[0005] The present invention is based on the discovery that the high temperature oxidation resistance of iron-nickel-chromium alloys, and in particular of the commercial alloy referred to above, can be improved by controlling the amount of manganese introduced in their manufacture. It is emphasised that it is not the principal aim of the invention to eliminate manganese as an essential constituent from the type of iron-nickel-chromium alloys under consideration but rather to control it. to obtain the benefits of improved oxidation resistance as demonstrated herein.

[0006] According to the invention, in the manufacture of an oxidation-resistant iron-nickel-chromium alloy the manganese is controlled so as not to exceed 0.6%, so as to improve the high-temperature oxidation resistance compared with an otherwise similar alloy having a higher content of manganese.

[0007] The invention is particularly directed to alloys containing 20 to 45% nickel, 15 to 25% chromium, manganese in an amount up to about 0.6%, up to about 0.3% carbon, up to about 0.3% nitrogen, up to about 1% aluminium, up to about 1% titanium, and up to 2% copper, the balance, apart from impurities, being iron. Thus silicon, sulphur and phosphorus may be present in usual impurity amounts. Silicon, if present, should not exceed 1.5%, and sulphur and phosphorus should be maintained at low levels consistent with good melt practice.

[0008] Within the ranges set forth, nickel may be from 30 to 35%, chromium from 19 to 23%, carbon up to 0.2%, and nitrogen from 0.05 to 0.25%. More particularly, the composition (apart from manganese) may be that of the commercial alloy referred to above.

[0009] The improvement in elevated temperature oxidation resistance of iron-nickel-chromium alloys is shown by the results of cyclic oxidation tests on two alloys representative of the invention and two samples of the commercial alloy referred to above.

[0010] In Table I below are given the compositions of the Commercial Alloy and an alloy, Alloy 1, representative of the instant invention:

[0011] Air-melted 14 kg samples of the alloys were forged to flats, hot rolled to 7.9 mm and cold rolled to 3.2 mm. Specimens were subjected to a cyclic oxidation test consisting of holding the specimen for 15 minutes at temperature (982°C), and then cooling 5 minutes in air. This cycle was repeated over a test period of 1000 hours. Specimens were examined at 100 hour intervals. Prior to test the specimens were annealed at 1177°C and water quenched. Oxide was removed by grinding to 120 grit.

[0012] The test results are set out in Table II.

[0013] As can be seen from Table II, Alloy 1 of the invention performed considerably better than the Commercial Alloy, the difference being largely in the manganese contents (0.1% and 1%, respectively).

[0014] The results of similar tests at the higher temperature of 1093°C are set out in Tables III and IV, and exhibit a similar pattern.

Claims

1. A method of manufacturing a manganese-containing iron-nickel-chromium alloy having improved oxidation resistance, wherein the manganese content is controlled so as not to exceed 0.6%.

2. A nickel-iron-chromium alloy consisting of from 20 to 45% nickel, from 15 to 25% chromium, manganese in an amount not exceeding 0.6%, up to about 1% aluminium, up to about 1% titanium, up to about 2% copper, up to about 0.3% carbon, and up to about 0.3% nitrogen, the balance, apart from impurities,being iron.

3. An alloy according to claim 2 in which at least one of aluminium and titanium is present in an amount of 0.05 to 0.75%.

4. An alloy according to claim 2 or claim 3 which contains from 30 to 35% nickel, from 19 to 23% chromium, and not more than 0.2% carbon.

5. An alloy according to any preceding claim in which the amount of silicon present as an impurity does not exceed 1.5%.

6. The use of an alloy according to any one of claims 2 to 5 for applications requiring good resistance to cyclic oxidation at temperatures of at least 982°C.