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(11) | EP 0 155 011 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | High-strength alloy for industrial vessels |
| (57) @ An austenitic nickel-iron-chromium alloy exhibiting high-strength, good corrosion
resistance and having a low work hardening rate. The economic alloy is particularly
useful for tubes and industrial vessels and, more particularly, heat exchangers and
chemical and petrochemical equipment. The alloy includes e.g. 24 to 32% nickel, about
12 to 19% chromium, about 1 to 3.5 % molybdenum, about 2 to 5.5 % copper, up to about
2.5 % titanium, up to about 1.5% manganese, up to about 0.1 % cerium, and the balance
mostly iron. Depending on the quantity of titanium, both age hardenable and non-age
hardenable versions may be produced. |
[0001] The instant invention relates to nickel-iron-chromium alloys in general and more particularly to a high strength, corrosion resistant alloy having a low work hardenability rate with variable age hardenable characteristics. The alloy reduces copper pick-up in fluid streams.
[0002] Power plant operators and boiler manufacturers recognized early on that to improve the efficiency of steam generators (both fossil and nuclear), it was useful to adopt regenerative feedwater heating. Essentially, steam is extracted from the steam turbines to preheat the boiler/reactor feedwater before it is introduced into the economizer of a boiler or directly into a steam generator/ reactor. The heating of the feedwater occurs in, naturally enough, feedwater heaters. Steam is used to heat the feedwater inside the feedwater heater tubing to impart a portion of the steam's latent heat to the water. Water temperatures from about 100-650°F (37.7-343.30C) and pressures up to 5200 psi (358.53 MPa) are not uncommon. Moreover, advanced designs are now contemplating pressures up to 7200 psi (496.42 MPa) and 700°F (371.1°C).
[0003] Currently, steels (carbon and stainless) and sometimes nickel-copper alloys (MONEL* nickel-copper alloys) are utilized in feedwater heaters. Although the feedwater is treated to remove chemicals and other impurities, corrosion of the tubing may still occur.
[0004] Free oxygen will attack the steels. Superalloys are often difficult to form into tubes due to their high work hardening rates. High copper-containing materials are generally frowned upon since copper and corrosion products are believed to deposit on boiler tubes and may be carried over into the steam. These undesirable entrained products may enter into the turbines resulting in lower efficiencies. Indeed, operators wish to eliminate all possible copper pick-up in the steam because of fouling and the resulting loss of efficiency of the turbine blades when the copper plates out of the steam. It is also believed that the copper deposits may set up local galvanic cells with the ferrous alloys thereby causing additional corrosion. Operators wish to stay away from nickel-copper alloys which otherwise display better chemical and physical properties than the other alloys. However, the substitution of low carbon or stainless steels for the nickel-copper alloys currently available 13 not always satisfactory since these materials do not have the requisite corrosion resistance, stress corrosion cracking resistance or strength. This leads to high maintenance costs. Moreover, in the case of carbon steels, undesirably short lifetimes of three to eight years have been reported. Contrast this state of affairs with an expected service life in excess of twenty years. Accordingly, power plant operators are in a quandry: steels corrode; high alloys are costly; and the nickel-copper alloys contain high quantities of copper.
[0005] It is apparent that there is a need for a reasonable cost alloy that exhibits corrosion resistance, strength and formability properties suitable for feedwater heaters, chemical and petrochemical installations and other similar applications.
[0006] Accordingly, there is provided an austenitic alloy having a low work hardening rate especially suited for, but not limited to,
[0008] industrial vessels and particularly for heat exchanger tubing for high temperature, high pressure applications. The instant alloy combines improved corrosion resistance and the requisite high strength in a system that is of lower cost than the more expensive higher alloys. The alloy displays good stress corrosion cracking resistance and good high temperature corrosion resistance.
[0009] According to the invention, a high strength, corrosion resistant nickel-iron-chromium alloy consists, by weight, of from 24 to 32% nickel, from 12 to 19% chromium, molybdenum and copper in amounts up to 3.5% molybdenum and 5.5% copper, from 0 to 2.5% titanium, from 0 to 1.5% manganese, from 0 to 1.5% silicon, from 0 to 1% columbium (niobium) and tantalum, from 0 to 0.2% aluminum, from 0 to 0.1% cerium, from 0 to 0.01% boron and from 0 to 0.2% nitrogen, the balance, apart from impurities, being iron.
[0010] The term impurities used herein includes residual amounts of calcium added as a processing aid.
[0011] The molybdenum content is advantageously at least 1%, e.g. from 1 to 3%, and the copper content at least 2%, e.g. from 2 to 5%. Preferably the nickel content is from 26 to 29%, the chromium content from 15 to 18%, the molybdenum content not more than 3%, the copper content not more than 5% and the content of columbium and tantalum not more than 0.4%. In one alloy according to the invention the nickel is about 28%, the chromium about 16%, the molybdenum about 16%, the molybdenum about 2% and the copper about 4%.
[0012] Owing to its low work hardening rate (caused in part by the nickel-chromium combinations) the instant alloy easily lends itself to tube fabrication and other cold working operations. However, by varying the titanium content, age hardenable and non-age hardenable characteristics may be developed. Titanium levels below about 0.8% lead to a non-age hardenable alloy whereas titanium levels above about 0.8% are increasingly age hardenable, and e.g. about 1.8% titanium may be incorporated for this purpose.
[0013] The invention will now be described in more detail with reference to the accompanying drawing, in which yield stress is plotted against percentage reduction for an alloy according to the invention (Alloy 16) and three comparative alloys.
[0014] In the alloy of the invention, the incorporation of a measured quantity of titanium can impart an age hardening response of at least 60 ksi (413.7 MPa) yield strength and 120 ksi (827.4 MPa) tensile strength in the cold worked and annealed conditions. The titanium raises the work hardening rate of the alloy. Copper, chromium and molybdenum improve the corrosion resistance of the alloy. Aluminum, cerium, boron and calcium assist in the deoxidation of the alloy. Nitrogen may be added to the low titanium level alloys as an austenite former. It also serves to boost the ability of the alloy to withstand corrosive attack. The nitrogen raises the strength and increases the work hardening rate of the alloy in the annealed condition.
[0015] Table I below sets forth the compositions of a number of heats within the above composition ranges and also, for purposes of comparison, one alloy (No 4) that is substantially free from copper and molybdenum and three alloys (Nos 13 -15) having lower nickel contents.
EXAMPLE 1
[0017] This is an example of a low-titanium, non-age hardenable modification of the alloy (No 12). In a series of tests, specimens of this composition were evaluated for stress corrosion cracking (SCC) and high temperature water corrosion resistance. It was tested in the as-cold-rolled (CR) condition and after annealing and heat treating at 1400°F (600°C) and compared with annealed MONEL alloy 400 and Type 304 stainless steel.
[0019] SCC tests were carried out with u-bend specimens at 600°F (315.55°C). General corrosion tests were conducted in deaerated water with coupons suspended from insulated hooks. Weight change in the water tests were determined by weighing uncleaned specimens after 500 hours and 1000 hours. The average corrosion rate was determined on cleaned specimens after 1000 hours.
[0020] All test material was in the form of 0.125 inch gauge (0.32 cm) x 2.5 inch (6.35 cm) wide strip. Experimental compositions were tested in as CR 50X and/or CR + 1750°F (954°C)/one half hour, water quenched + 1400°F (760°C)/one hour air cooled conditions. Commercial MONEL alloy 400 (nominal composition: 32.56% copper, 2.40% iron, 1.04% manganese, 0.1% silicon, 0.1% carbon, balance essentially nickel) and 304 stainless (nominal composition: 18.09% chromium, 9.18% nickel, 1.77X manganese, 0.73% silicon, 0.24% molybdenum, balance essentially iron) were compared to heat 12.
[0021] In a 600°F 1% NaCl solution, only alloy 304 cracked within the 720 hour test. There was no evidence of SCC cracking in heat 12.
[0022] In boiling 45X MgCl2, heat 12 had a greater SCC resistance to crack propagation than 304 stainless.
[0023] In boiling 50% NaOH, heat 12 cracked slightly and experienced light surface attack. Alloy 304 was subject to severe general corrosion and cracking while MONEL alloy 400 was resistant to attack under these circumstances.
[0024] In summary, heat 12 displayed good SCC resistance and is expected to resist caustic and chloride SCC better than alloy 304. The higher nickel provides this improved SCC resistance.
[0025] In the high temperature deaerated water test shown in Table 3 below, general corrosion rates in the alloy 12 were similar to 304 stainless and somewhat better than MONEL alloy 400.
EXAMPLE 2
[0026] Heats 1-3 and 12 (14 kg melts) were vacuum melted and cast to 4 inch (10.16 cm) diameter ingots. Forged 9/16 inch (1.43 cm) squares plus forged 3/4 x 2 x 12 inch (1.91 x 5.08 x 30.48 cm) flats were made with frequent reheats at 2150°F (1177°C). After overhauling the flats to a uniform thickness, they were hot rolled to 1/4 inch (0.64 cm) at 2150°F. The hot rolled 1/4 inch strip was annealed at 1950*F (1066*C)/ one hour water quench and pickled prior to cold rolling. Hardness and tensile tests were taken at various levels of cold work to establish a work hardening response. A low work hardening rate is very desirable in the manufacture of relatively small diameter thin-walled tubing.
[0027] Of particular importance is the yield strength at high levels of cold reduction such as 60 to 80% reduction. Many tube mills produce a large hot-worked tube shell which must be reduced in size during a number of cold working and annealing stages. Experience has shown that alloys which have lower yield strength after high cold reductions may be cold worked to a greater degree without splitting, requiring less annealing stages and lower manufacturing costs. The Figure shows heat 16 to have a lower yield strength after a high cold reduction than heats 14 and 15 with lower nickel content.
[0028] After a cold reduction of 60 to 80X, the yield strength of heat 16 is also lower than the commercial alloy INCOLOY**alloy 800. INCOLOY alloy 800 is shown in the Figure for comparative purposes only. A general purpose alloy, it has good workability characteristics and is easily processed. The instant invention was developed with these attributes in mind.
[0029] All heats had good malleability. Tensile data on cold rolled strip using increasing amounts of titanium are shown in Tables 4, 5 and 6.
[0030] When titanium was raised to 2.0%, the work hardening rate increased but no change occured as titanium was raised to 2.3X. The aged tensile test results in Table 6 indicate that 60 ksi yield strength and 120 ksi tensile strength can be accomplished with approximately 1.75% titanium and low level cold working. Indeed, the combination of about 20X cold reduction with a slightly lower titanium content might be optimum for feedwater heaters.
[0031] Table 7 shows the strength and ductility characteristics in the annealed and aged conditions.
EXAMPLE 3
[0032] Corrosion tests were conducted on heats 4-12. Corrosion test environments relevant to feedwater heater service and other possible applications were examined.
[0034] The tests show that the instant alloy is more resistant to SCC (caused by chlorides and sodium hydroxide) than 304 stainless. The relatively high nickel content of the instant alloys provides the increased chloride and caustic cracking resistance.
[0035] The test data also indicates very good resistance of the alloys to polythionic acid cracking. This is a common cause of failure of stainless steels and high nickel alloys in petrochemical service. The influence of high titanium content on carbide precipitation is believed to be responsible for good polythionic acid SCC resistance.
[0037] Tables 8 and 9 also demonstrates the resistance of the alloy to environments other than that posed by feedwater heaters. Molybdenum addition of 2-3X greatly improves resistance to hydrochloric acid. Copper additions of 4X or more improved sulfuric acid resistance. The combination of copper and molybdenum appears to improve resistance to phosphoric acid. The instant alloy lends itself to chemical and petrochemical applications.
[0038] The design strength of the alloys destined for tubular applications is usually based on the tensile strength of the alloy comprising the apparatus. In the cold worked plus stress relieved conditions, the instant alloy system will meet the 120 ksi minimum tensile strength usually specified bv design engineers. This value, compares favorably with such alloys as Inconel alloy 625 and Incoloy alloy 801. Table 10 compares minimum tubular wall thicknesses between MONEL alloy 400, 304 stainless and the instant alloy for various temperature and pressure conditions. Table 10 was constructed to compare the minimum wall thickness between the listed alloys. The next heavier standard wall thickness was used to calculate the weight per foot.
[0039] In order to produce objects and, more particularly, tubes which may be seamless or welded, the object or tube, made by methods known to those skilled in the art, may be subjected to a stress relieving heat treatment of about 1100 to 1400°F (599.3-760°C) for an appropriate period of time. The time period is, of course, a function of the temperature selected and the section size.
[0040] In particular, the non-age hardenable tubes may be drawn to final size, annealed at about 1700-2000°F (767-933°C) for a suitable time, straightened, bent into the appropriate shape (if desired), and stress relieved at about 1100-1400°F up to about three hours. The age-hardenable tubes may be drawn to final size, annealed at about 1700-2000°F for a suitable time, straightened, aged for about an hour at 1100-1400°F, bent into the appropriate shape and stress relieved (which also ages the tube) at about 1100-1400°F for the appropriate time.
[0041] It should be noted that due to the relatively low chromium content, the pitting resistance of the alloy is about the same as stainless 304 and is not recommended for service where superior resistance to localized attack is required. The low chromium lowers resistance to intergrannular attack and limits use in highly oxidizing environments such as nitric acid.
[0042] A preferred composition for overall strength, corrosion resistance and economy for feedwater heaters is heat 8 (28 Ni - 16 Cr - 4 Cu - 1.8 Ti - 2 Mo - Bal Fe). This composition appears to have the mechanical and corrosion properties necessary for a high pressure material. It also has excellent general corrosion resistance in hydrochloric, sulfuric and phosphoric acids. The good resistance of this composkion to polythionic acid attack also indicates potential petrochemical applications.
1. An austenitic, high-strength, corrosion-resistant nickel-iron-chromium alloy that
consists, by weight, of from 24 to 32% nickel, from 12 to 19% chromium, molybdenum
and copper in amounts of up to 3.5% molybdenum and 5.5% copper, from 0 to 2.5% titanium,
from 0 to 1.5% manganese, from 0 to 1.5% silicon, from 0 to 1% columbium (niobium)
and tantalum, from 0 to 0.2% aluminum, from 0 to 0.1% cerium, from 0 to 0.01% boron,
and from 0 to 0.2% nitrogen, the balance, apart from impurities, being iron.
2. An alloy according to claim 1 wherein the copper content is at least 2%.
3. An alloy according to claim 1 or claim 2 wherein the molybdenum content is at least
1%.
4. An alloy according to any preceding claim wherein the nickel content is from 26
to 29%, the chromium content is from 15 to 18%, the copper content does not exceed
5%, the molybdenum content does not exceed 3%, and the content of columbium and tantalum
does not exceed 0.4%.
5. A non-age hardenable alloy according to any prededing claim wherein the titanium
content does not exceed 0.8%.
6. An alloy according to claim 5 wherein the nickel content is about 28%, the chromium
content is about 16%, the molybdenum content is about 2%, the copper content is about
4%, the silicon content does not exceed 0.4%, and the content of columbium and tantalum
does not exceed 0.4%.
7. An age-hardenable alloy according to any of claims 1 to 4 wherein the titanium
content is greater than 0.8%.
8. An alloy according to claim 7 wherein the nickel content is about 28%, the chromium
content is about 16%, the molybdenum content is about 2%, the copper content is about
4%, the silicon content does not exceed 0.4%, the content of columbium and tantalum
does not exceed 0.4% and the titanium content is about 1.8%.
9. An alloy according to any preceding claim that has been heat treated at a temperature
in the range from 1100 to 1400°F (605 to 760°C) for up to 16 hours.
10. A tube formed from an alloy according to any preceding claim.
11. A heat-exchanger or feed-water heater comprising an alloy according to any of
claims 1 to 9 or a tube according to claim 10.