JPH0525944B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD This invention relates generally to nickel-iron-chromium alloys, and more particularly to high strength, corrosion resistant alloys having low work hardening rates with variable age hardening properties. This alloy reduces copper pick-up in fluid streams. [Prior Art] Power plant operators and boiler manufacturers use steam generators (fossil-fueled and nuclear-fueled)
It has long been recognized that it is effective to employ regenerative water absorption heating in order to improve the efficiency of water heating. Essentially, steam is extracted from the steam turbine in order to preheat the boiler/reactor feedwater before it is introduced into the Boiler economizer or directly into the steam generator/reactor. Of course, the heating of the feed water takes place in the water heater. Steam is used to heat the feed water inside the feed water heater piping in order to impart some of the latent heat of the water vapor to the feed water. Water temperatures of 100-650°F (37.7-343.3°C) and water pressures of up to 5200 psi (358.53 MPa) are also not unusual. Additionally, recent designs allow for water pressures reaching 7200 psi (496.42 MPa) and water temperatures of 700 〓 (371.1°C). Currently, steel (carbon steel and stainless steel) and sometimes nickel-copper alloy Monel ^* nickel-copper alloy) are used in feed water heaters (*Trademark of Inco Corporation). Although water supplies are treated to remove chemicals and other impurities from the water supply, corrosion of pipes can still occur. Free oxygen corrodes steel. Superalloys are often
Due to its high work hardening rate, it is difficult to form into tubular shapes. High copper content materials are generally undesirable because copper and corrosion products are believed to be deposited on boiler piping and transferred into the steam. These undesirable entrained products enter the turbine resulting in low efficiency. In fact, operators desire to remove as much copper pick-up from the water vapor as possible. This is because the steam deposits copper on the turbine blades, contaminating them and reducing their efficiency. It is also believed that copper deposits may form local batteries with iron alloys and accelerate corrosion. Although nickel-copper alloys have chemical and physical properties that are otherwise superior to other alloys, operators tend to avoid these types of alloys. However, the use of low carbon steel or stainless steel in place of currently available nickel-copper alloys is not always satisfactory because these materials do not have the required corrosion resistance, stress corrosion cracking resistance, or strength. As a result, maintenance costs increase. Additionally, undesirably short lifetimes of 3 to 8 years have been reported for carbon steel. This condition is 20
This is in contrast to the desired uptime of more than a year. Therefore, power plant operators are perplexed when faced with steel corrosion, expensive alloys, and nickel-copper alloys containing large amounts of copper. Corrosion resistance and suitable for feed water heaters, chemical and petrochemical plants and other similar applications.
There is clearly a need for an alloy with strength, workability, and reasonable cost. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a corrosion-resistant austenitic steel having a low work hardening rate suitable for industrial vessels, especially but not limited to heat exchanger tubing for high temperature and high pressure applications. It is something. Thus, the present invention comprises 27% to 29% nickel,
15% to 17% chromium, 2% to 3.5% molybdenum, 3% to 5.5% copper, and 0.8% to 2.5%
of titanium, 0.9% to 1.1% manganese, and 0.01
% to 0.1% aluminum and 0.01% to 0.1%
The present invention provides an austenitic, high-strength, corrosion-resistant nickel-iron-chromium alloy consisting essentially of cerium, the balance primarily iron, and trace amounts of impurities. The term impurity as used herein includes residual amounts of calcium and elements such as those set forth in Table 1 below that are added as processing aids. DETAILED DESCRIPTION OF THE INVENTION The alloys of the present invention combine improved corrosion resistance with requisite high strength in lower cost systems than more expensive alloys. This alloy exhibits excellent stress corrosion cracking resistance and excellent high temperature corrosion resistance. The steel of the present invention is susceptible to tube forming and other cold working treatments because of its low work hardening rate (due in part to its nickel-chromium bond).
Titanium above 0.8% increases age hardenability. For example, about 1.8% titanium can be added for this purpose. The addition of titanium in an amount of 0.8-2.5% is suitable for cold working/
At least 60Ksi (413.7MPa) under annealing conditions
Gives an age hardening response of yield strength and tensile strength of 120 Ksi (827.4 MPa). Titanium increases the work hardening rate of the alloy. 3-5.5% copper, 15-17
The corrosion resistance of the alloy can be improved by adding % chromium and 2-3.5% molybdenum. Also, 27-29% nickel is added for the same purpose. Additionally, in the alloy of the present invention, 0.9% to 1.1% manganese and 0.01% to 0.1% aluminum are added. Manganese in an amount of 0.9-1.1% is added to provide malleability. All nickel/chromium alloys typically contain some amount of this element. Thus, small amounts of 0.9-1.1% are useful. Additionally, 0.01-0.1% aluminum helps deoxidize the alloy and controls titanium during the melting process. Similarly 0.01%~0.1%
Cerium also helps deoxidize the alloy. Table 1 below shows heats within the composition range of the present invention. All of these heats were made using scrap materials and commercially available clean materials, and all of these materials contain various impurities. Shown in this table are the results of the overall analysis of each heat and therefore the impurity components for the alloy of the present invention are also listed. Among the component elements listed here, carbon, sulfur, silicon,
Cobalt, niobium, tantalum and nitrogen are all considered impurities for the alloys of this invention. While nitrogen may be effective in non-age hardening cases, this is not the case with the age hardenable alloys of this invention.
Magnesium can be added in small amounts when other deoxidizers such as aluminum are lacking, but is not necessary in the present invention.

【table】

〔Example〕

Example 1 Heats 1 to 3 (14Kg melt) were melted in vacuum,
It was cast into an inch (10.16 cm) diameter ingot.
Forged 9/16" (1.43cm) square and forged 3/4 x 2 x 12" (1.91 x 5.08 x 30.48
cm) was made by heating it frequently at 2150㎓ (1177℃). After processing the flat to a uniform thickness, it was hot rolled at 2150 mm to 1/4 inch (0.64 cm). Hot rolled 1/4 inch strips were annealed at 1950°C/1 hour in water and pickled prior to cold rolling. In order to measure the work hardening response, hardness tests and tensile tests were conducted at each cold working level. For the manufacture of relatively small diameter, thin-walled piping, low work hardening rates are highly desirable. Yield strength is particularly important during high level cold drawing, such as 60-80%. Many steel tube mills produce large diameter hot-worked steel tube shells that must be reduced in size during multiple stages of cold rolling and annealing. Experiments have shown that alloys with low yield strength after high cold drawing can be cold-worked more strongly without exhibiting cracks, requiring fewer annealing steps and lower manufacturing costs. All heats have excellent malleability. Tensile strength data for cold rolled strips with increased titanium content are shown in Tables 2 and 3.

【table】

【table】

Table: When the titanium content was increased to 2.0%, the work hardening rate increased, but no change occurred when the titanium was increased to 2.3%. The aged tensile test results shown in Table 3 indicate that with a titanium content of approximately 1.75% and a low level of cold working, the
(413.5MPa) and tensile strength of 120Ksi (827MPa). In fact, a combination of a slightly lower titanium content and about 20% cold rolling would be optimal for feed water heaters. Table 4 below shows the strength and ductility properties in the annealed/aged condition.

[Table] Example 2 Corrosion tests were conducted for heats 4 to 6. Corrosion test environments for water supply facilities and other possible applications were examined. Table 5 shows the SCC test results in sodium chloride and sodium hydroxide solutions. inside the cutlet
Values are shown in cm.

【table】

Table: The alloy of the present invention has been shown to be more resistant to SCC (corrosion caused by chloride and sodium hydroxide) than 304 stainless steel. The relatively high nickel content of the alloys of this invention provides chloride and caustic cracking resistance. These test data also demonstrate the excellent resistance of this alloy to polythionic acid cracking. This cracking is a common cause of breakage of stainless steel and high nickel alloys in the petrochemical industry. The effect of high titanium content on carbide deposits appears to be responsible for the excellent polythionate SCC resistance. Table 6 shows the results of the global corrosion test. In each column, the value in mils/year is shown on the left, and the value in mm/year is shown on the right.

【table】

Table *Approximate Values Tables 5 and 6 also show that the alloys of the present invention have resistance to environments other than feed water heaters. Addition of 2-3% molybdenum significantly improves resistance to hydrochloric acid. Copper additions of 4% or more improved sulfuric acid resistance. The combination of copper and molybdenum appears to improve resistance to phosphoric acid. The alloy of the invention itself is also suitable for chemical and petrochemical applications. The strength of alloys designed for pipework is typically based on the tensile strength of the alloys that make up the equipment.
In the cold rolled and stress relieved state,
The alloy system of the present invention meets a minimum tensile strength of 120 Ksi (827 MPa) typically specified by design engineers. This value is very comparable to alloys such as Inconel Alloy 625 and Incolloy Alloy 801. Table 7 shows, for various temperature and pressure conditions,
This figure compares the minimum wall thickness of Monel alloys 400 and 304 stainless steel and the alloy of the present invention. Table 7 shows a comparison of the minimum wall thicknesses of the listed alloy tubes. The next heavier standard wall thickness was used to calculate the weight per foot (meter). To make an object, in particular a seamless or welded tube, the object or tube made by methods known to those skilled in the art is heated at an angle of about 1100° to 1400° for a suitable period of time.
〓(599.3~760℃) stress relief heat treatment can be performed. This time is of course a function of the temperature and cross-sectional size chosen. In more detail, a non-age hardening tube is pulled to its final size and held at approximately 1700°~2000〓(767~
It can be annealed at 933°C), straightened, bent into an appropriate shape (desired shape), and stress relieved at about 1100-1400°C for about 3 hours. Age-hardening tubes are drawn to the final size, annealed at an appropriate time of about 1700-2000〓, straightened, aged at 1100-1400〓 for about 1 hour, bent into an appropriate shape, and then annealed at an appropriate time of about 1100-1400〓 time, the stress can be relieved (this simultaneously age hardens the tube).

Table: Due to the relatively low chromium content, the pitting corrosion resistance of the alloy of the present invention is approximately equal to that of stainless steel 304 and is not recommended for applications where high local corrosion resistance is required. Low chromium content also reduces intergranular corrosion resistance and limits applications in highly oxidizing environments such as nitric acid. The preferred composition for overall strength, corrosion resistance and economy of the feed water heater is Heat 5 (28Ni-16Cr-
4Cu−1.8Ti−2Mo−1Mn−0.02Al−0.026Ce−balance Fe). This composition appears to have the necessary mechanical and corrosion properties for a high pressure material. The composition also has excellent overall corrosion resistance in hydrochloric, sulfuric and phosphoric acids. The excellent resistance of this composition to polythionic acid corrosion also indicates its potential for use in petrochemical applications. The present invention is not limited to the above description, but can be modified and implemented as desired within the scope of the spirit thereof.

Claims (1)

[Claims] 1 27% to 29% nickel, 15% to 17% chromium, 2% to 3.5% molybdenum, 3%
5.5% to 5.5% copper, 0.8% to 2.5% titanium, 0.9% to 1.1% manganese, 0.01% to 0.1%
Austenitic, high-strength, corrosion-resistant nickel-iron-chromium alloy consisting essentially of % aluminum, 0.01% to 0.1% cerium, and the balance primarily iron with trace amounts of impurities.