JP2643709C - - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat transfer tube (boiler tube) of a boiler used in a high-temperature environment where corrosive combustion slag containing chloride adheres. It relates to a high corrosion resistant alloy. Specifically, in a facility that incinerates municipal waste, industrial waste, sewage sludge, etc. (hereinafter collectively referred to as "garbage"), black liquor is burned in a waste heat boiler installed for energy recovery and in a paper mill. For boiler heat transfer tubes such as superheater tubes, reheater tubes, evaporator tubes and water wall tubes for boilers (referred to as soda recovery boilers) for recovering soda and generating electricity using waste heat, In particular, it relates to a high Cr high Ni alloy having an austenitic structure used under high temperature and high pressure. [0002] In recent years, from the viewpoint of actively using unused energy, attention has been paid to the effective use of energy of municipal waste, and waste heat generated when incinerated municipal waste has already been used. In addition, some facilities generate electricity to supply electricity in district heating and incineration facilities. Further, in the papermaking industry, a boiler for burning soda liquor generated in a pulp manufacturing process to recover soda and to generate power using waste heat (referred to as a soda recovery boiler) has come to be used. . In order to maximize the use of waste heat by converting waste heat into electric energy in the above waste heat recovery system, power generation efficiency must be increased. For that purpose, it is necessary to increase the steam condition of the waste heat boiler to high temperature and high pressure. However, an increase in the temperature of the steam causes an increase in the wall temperature of the boiler tube, which intensifies the corrosion of the tube. Further, in order to increase the pressure of steam, the boiler tube material must have excellent high-temperature strength. For example, in the conventional waste incineration waste heat recovery boiler, the wall temperature of the superheater tube, which is the highest temperature, was 200 to 350 ° C, but in the future, the wall temperature of the waste incineration waste heat recovery boiler and soda recovery boiler 500
It is expected that operating conditions that will exceed ℃ will be employed. Since a large amount of plastic is mixed in municipal solid waste, the combustion gas contains hydrogen chloride. Further, the combustion residue (slag) contains chloride. Therefore, in a heat transfer tube for a refuse incineration waste heat recovery boiler, corrosion by a hydrogen chloride gas and corrosion damage of a metal material due to adhesion of slag containing a chloride become problems. The same is true for soda recovery boilers. [0005] From the above background, there has been a strong demand for a heat transfer tube material that has both excellent corrosion resistance and high high-temperature strength that can withstand a severe corrosive environment, particularly containing chloride. As a material used for a high-temperature portion of an ultra-high-temperature and high-pressure plant, for example, a superheater tube or the like, a highly corrosion-resistant material having an austenite structure excellent in high-temperature strength is desirable. [0007] Various materials for municipal waste incineration waste heat boiler tubes having an austenitic structure are known in foreign countries, particularly in the United States. For example, Corrsion, March 9-13, 1987 Pa
per No. 402, 825 alloy containing about 42% Ni (as described in ASTM B163, B423
NO8825 alloy) and 625 alloy containing about 66% Ni (NO listed in ASTM B444)
6625 alloy) has been reported as a material for municipal waste incineration waste heat boiler tubes. These high alloy steels, which contain a large amount of Ni, have little corrosion loss in the corrosive environment of US waste incinerators, and have high corrosion resistance. It is said to be excellent. However, according to the results of tests conducted by the present inventors under conditions in which slag containing chloride adheres, such as the corrosive environment of a waste incineration waste heat boiler and a soda recovery boiler in Japan, the conventional 825 alloy and the like described above were found. In austenitic alloys, intergranular corrosion may occur in which grain boundaries are selectively corroded along with general corrosion and stress corrosion cracking. The present inventors have previously made an alloy whose stress cracking susceptibility was lowered by adjusting the contents of Ni and Mo (Japanese Patent Application No. 3-188567), and further increased the resistance to general corrosion by adding Al. An alloy (Japanese Patent Application No. 3-161384) and an alloy (Japanese Patent Application No. 3-310384) having improved overall corrosion resistance by actively adding a large amount of Mn have been developed and applied for a patent. However, the alloys of these inventions are still insufficient in completely preventing the intergranular corrosion. Since a boiler tube is a structural material used under high temperature and high pressure, intergranular corrosion may be a starting point of a crack in the tube depending on the situation, and may lead to breakage of the tube. Accordingly, it is desirable that the sensitivity to intergranular corrosion be as low as possible, particularly for a material for a boiler tube for high temperature and high pressure. An object of the present invention is to provide an austenitic structure exhibiting excellent strength at a high temperature similarly to the above-mentioned alloy of the prior application, and to attach a combustion slag containing chloride. A material for boiler heat transfer tubes having excellent overall corrosion resistance and stress corrosion cracking resistance in a severe corrosive environment, and having sufficient resistance to intergranular corrosion as compared to the alloy of the prior application. Is to do. [0011] Conventionally, intergranular corrosion of austenitic alloys occurs in the vicinity of the precipitated Cr carbide where Cr carbide precipitated at the crystal grain boundaries of the alloy reacts with slag containing chloride. It was attributed to two causes: the resulting Cr-depleted layer was selectively corroded. However,
In a test conducted by the present inventors under conditions where corrosive combustion slag containing chloride adheres, intergranular corrosion occurred even in an alloy in which Cr carbide was not substantially precipitated at the crystal grain boundaries. In pursuit of the cause, impurity elements segregated at the grain boundaries, especially P and Si
Was found to be selectively eluted into combustion slag containing chlorides, thereby corroding grain boundaries. Then, as will be described later, after appropriately selecting the content of alloy elements such as Cr, Ni, and Mo, C is set to 0.05% or less, P and Si are set to 0.015% or less, respectively.
It has been confirmed that by setting the content to 0.3% or less, the occurrence of intergranular corrosion can be almost completely suppressed even under the above-mentioned environment. The gist of the present invention is a boiler for incineration or a boiler for soda recovery having the following chemical composition. (1) In weight%, C: 0.05% or less, Si: 0.3% or less, Mn: 7.5% or less, Cr: 25 to 35%, Ni: 25 to 55%, and Mo that satisfies the following formula: And the balance is Fe and unavoidable impurities, and P in the impurities is 0.015% or less. 0.3 (%) ≦ Mo (%) ≦ 5.8 (%) − {Ni (%) / 10} (2) The following first group and second group A chemical composition comprising one or more alloy components selected from the group and one or more of the third group. Group 1: Nb, Ti, Zr, and V, each or a total of two or more, in a total amount of 0.1 to 3.0% by weight. Group 2: Cu, Co and W, each or a total of two or more of 0.1 to 5.0% by weight. Group 3: rare earth elements of 0.01 to 0.1% by weight in total or in combination of two or more. (3) The chemical composition according to any one of the above (1) or (2), further containing 0.1 to 0.3% by weight of N as an alloy component. (4) The above (1), (2) or (3) further containing 0.5% by weight or less of Al as an alloy component.
Any of the chemical compositions. The alloy according to the present invention has a total effect of an appropriate combination of the above-mentioned alloy components, as a material used in a high-temperature corrosive environment in which combustion slag containing chloride adheres, for example,
It has excellent properties suitable for the material of heat transfer tubes of refuse incineration boilers and soda recovery boilers. The effects of each alloy component and the reasons for limiting its content are as follows. Hereinafter, all percentages of the content of the alloy component mean weight%. C: C combines with Cr in the alloy and precipitates as massive Cr carbide at the crystal grain boundaries, and reacts with combustion slag containing chloride adhering to the surface of the tube, or lacks Cr near the crystal grain boundaries. A layer is formed to degrade the intergranular corrosion resistance of the alloy. Therefore, it is desirable that the content of C be as low as possible. 0.05% is the allowable upper limit. Si: Si is usually added as a deoxidizing agent for an alloy, and is generally an effective element for improving oxidation resistance. In alloys with an austenitic structure, the addition of large amounts of Si also has the effect of suppressing general corrosion. However, Si segregates at the grain boundaries and reacts with the combustion slag containing chloride, which may cause intergranular corrosion. As described above, Si has two opposing effects on the corrosion resistance, but in the present invention, the general corrosion resistance is different from that of other elements (C
(r, Ni, etc.), and the Si content was kept low to prevent intergranular corrosion. Si content
If it is 0.3% or less, intergranular corrosion can be suppressed to a level that does not actually pose a problem. Mn: Mn is an austenite-forming element and can also be used as a deoxidizing agent. Especially
In a corrosive environment in which combustion slag containing chloride adheres to the pipe surface in a high temperature range exceeding 500 ° C., it is effective to add Mn to enhance the general corrosion resistance. However, if the content of Mn exceeds 7.5%, both the oxidation resistance and the hot workability deteriorate, so the content was set to 7.5% or less. Cr: Cr has an excellent effect of improving high-temperature strength and oxidation resistance at high temperatures. But Si
When the content of Cr is less than 25%, the resistance to high-temperature corrosion in the above-described corrosive environment is not sufficient. On the other hand, if the content exceeds 35%, the effect of improving the corrosion resistance does not increase and only the material price is increased unnecessarily, so the upper limit is set to 35%. Ni: Ni is an austenite-forming element, and is an important component for securing high-temperature strength and suppressing general corrosion mainly due to corrosive combustion slag of a molten chloride system. However, since Ni is an expensive element, the upper limit is taken into account in consideration of the balance between material costs and the above effects.
55%. On the other hand, when the Ni content is lower than 25%, the high-temperature corrosion resistance rapidly deteriorates, so the lower limit is set to 25%. Mo: Mo is an effective component for strengthening grain boundaries and increasing resistance to intergranular corrosion. Mo is considered to be a component that improves the corrosion resistance in an aqueous solution containing chlorine ions, particularly the stress corrosion cracking resistance. This is also the reason that the 825 alloy, which is also a seawater corrosion resistant material, contains 3% Mo. Is based on However, in the environment where combustion slag containing a high concentration of molten chloride adheres, such as the corrosive environment of municipal solid waste incineration boilers in Japan, stress corrosion occurs when a large amount of Mo is added contrary to conventional knowledge. The crack sensitivity increases. However, according to the research results of the present inventors, the effect of Mo on stress corrosion cracking susceptibility strongly depends on the Ni content in the alloy steel, and the Mo content is appropriately adjusted according to the Ni content. If so, it is possible to reduce the stress corrosion cracking susceptibility. On the other hand, as described above, Mo is an element having an effect of suppressing intergranular corrosion, and to obtain the effect, a content of 0.3% or more is necessary. As described above, the above formula was determined in consideration of the effect of Mo, that is, 0.3 (%) ≤ Mo (%) ≤ 5.8 (%)-{Ni (%) / 10}・If Mo (%) is 5.8 (%)-{Ni (%) / 10} or less, the alloy does not have increased sensitivity to stress corrosion cracking. The alloy of the present invention may further include the following elements, if necessary, in addition to the above components. Nb, Ti, Zr and V (first group elements): Since Nb, Ti, Zr and V all easily form carbides, C in the alloy steel is fixed to suppress precipitation of Cr carbides. , Help increase high temperature strength and increase resistance to intergranular corrosion. In the case of an alloy having an austenitic structure, Cr carbide precipitated at the crystal grain boundaries reacts with a corrosive molten salt compound adhering to the tube surface to cause grain boundary corrosion. If these elements are added while the content is kept low, the intergranular corrosion resistance is further improved. When the content of one or more of these elements is less than 0.1% in total, the effect of addition does not appear, and even if the content exceeds 3%, the effect is saturated and only the cost increases. Cu, Co and W (second group elements): These elements have an effect of improving the high-temperature strength of the alloy by solid solution strengthening. As in the case of the first group element, one or more kinds can be added as necessary.
When the total content of one or more of them is less than 0.1%, the effect of the addition is not remarkable. On the other hand, in the range exceeding 5%, there is almost no increase in the effect corresponding to the cost increase. Rare earth elements (third group elements): Rare earth elements such as Y, La, and Ce are formed of a protective oxide film (Cr ₂ O ₃ ) formed on the alloy surface.
) Has the effect of improving the adhesion. When such an effect is expected, one or more kinds may be contained in a total of 0.01% or more. However, if it exceeds 0.1%, the hot workability of the alloy deteriorates. N (nitrogen): N contributes to stabilization of the austenite structure. It also has the effect of increasing high-temperature strength, so that it can be contained in an amount of 0.1% or more as necessary. However, within the composition range of the alloy of the present invention, it is difficult to increase the content to more than 0.3% by a normal melting method. Al: Al can be added for the purpose of promptly deoxidizing the alloy and improving the hot workability of the alloy. However, if Al remains excessively and its content exceeds 0.5%, the intermetallic compound Ni ₃ Al precipitates and deteriorates creep ductility when used for a long time at a high temperature. Therefore, the content is limited to 0.5% or less. It is desirable. The alloy of the present invention is produced, for example, by melting in an electric furnace, refining by VOD or AOD, and then forming a billet. It is drawn into a tube of a predetermined size. The heat treatment is desirably a solution treatment in which the material is heated to 1000 to 1200 ° C. and then rapidly cooled. Then, it is descaled and finally made into a product heat transfer tube. It should be noted that a tube made of another material and a tube made of the alloy of the present invention can be combined and used as a double tube (clad tube). [Examples] Alloys 1 to 107, whose chemical compositions are shown in Tables 1 (1) to (9), are melted in a vacuum melting furnace in an amount of 17 kg each, cast into ingots, and then heated to a temperature of 1100 ° C. Then, it was hot forged and hot rolled into a 15 mm thick billet. Next, after softening and annealing at a temperature of 1100 ° C., it was cold-rolled into a 10.5 mm thick plate. Thereafter, a solution treatment of heating to a temperature of 1100 ° C. and cooling with water was performed. A 2 mm thick × 10 mm wide × 10 mm long corrosion test piece and a stress cracking test piece having the dimensions and shape shown in FIG. 1 are cut out from the center of each plate after the solution treatment, and contain a chloride described later. A high temperature corrosion test simulating the adhesion of combustion slag was performed. At the same time, test specimens of the same dimensions as those of the alloys having the compositions shown in Tables 1 (9) with reference numerals 108 to 112 were cut out from the center of the wall thickness of a commercially available boiler tube and subjected to the same test. Table 1 (9)
Reference numeral 108 denotes NO8825 alloy described in ASTM B163, reference numeral 109 denotes SUS304, reference numeral 1
10 is SUS316L, 111 is SUS310S, 112 is N described in ASTM B622.
It corresponds to O8320 steel respectively. The high temperature corrosion test was performed under the following two conditions. Test simulating the corrosive environment of a refuse incineration boiler: 10% NaCl-10% KCl-15% FeCl ₂ -15% PbCl ₂ -18.75% Na ₂ SO ₄ -18.75% K ₂ SO ₄ − A synthetic ash of 12.5% Fe ₂ O ₃ was applied on both sides of the test piece at a rate of 30 mg / cm ² , and this was coated with 0.15% HCl-300 ppm SO ₂ -7.5% O ₂ -7.5% CO ₂ -20% H ₂ O-bal.N ₂
Heated at 550 ° C for 20 hours in a gas stream. [0037] Synthesis of the soda recovery in test mol% of a corrosive environment simulating a boiler _{20% NaCl-22.5% Na 2} SO 4 -22.5% K 2 SO 4 -20% Na 2 CO 3 -15% Fe 2 O 3 Ash was applied on both sides of the test piece at a rate of 30 mg / cm ² , and this was applied to 0.25% SO ₂ -1% O ₂
Heated at a temperature of 600 ° C for 20 hours in a gas stream of -15% CO ₂ -bal.N ₂ . The corrosion resistance was evaluated by descaling the test piece after the test, measuring the weight, and calculating the corrosion loss from the weight change before and after the test. The intergranular corrosion resistance was evaluated by microscopically observing the cross section of the surface portion of the corrosion test specimen after descaling using an optical microscope of 100 times or 500 times. As shown in FIG. 2, a test for the sensitivity to stress corrosion cracking, which is a problem in the corrosive environment of a refuse incineration boiler, was carried out using a jig 1 and a test piece 2 for each alloy.
% Proof stress is applied, and in this state, the same synthetic ash as that used in the above-mentioned high-temperature corrosion test is applied to the surface of the test piece 2 and then maintained at a temperature of 400 ° C. for 20 hours in the same gas stream. It is a test. The test temperature was set to 400 ° C. because the inventors of the present invention found that the stress corrosion cracking susceptibility of an austenitic alloy was highest near 400 ° C. The presence or absence of stress corrosion cracking was examined by microscopic observation of the cross section of the semicircular notch. [Table 1 (1)] [Table 1 (2)] [Table 1 (3)] [Table 1 (4)] [Table 1 (5)] [Table 1 (6)] [Table 1 (7)] [Table 1 (8)] [Table 1 (9)] Tables 2 (1) to (7) show the results of the above-described corrosion tests simulating the corrosive environment of a refuse incineration boiler. The alloys denoted by reference numerals 1 to 4, 9 to 12 and 17 to 20 have different contents of P, and the reference numerals 5 to 8, 13 to 16 and 21 to 24 have changed Si. Table 2 (1)-(7)
As is clear from the results, the maximum depth of intergranular corrosion in the corrosive environment is greatly affected by the contents of P and Si in the alloy. The maximum grain boundary erosion depth of alloys having P of 0.015% or less and Si of 0.3% or less is 5 μm or less, which is about 1/4 of the comparative alloy. Reference numerals 31 to 48, 61 to 75, 80 to 83, 86, 87, 89, 91, 93, 95, 97, 99, 101, 103
And alloys 105 to 107 contain an appropriate amount of the first group of alloying elements. The intergranular corrosion resistance of these alloys is further improved, and intergranular corrosion is at a level that cannot be observed at all with an optical microscope of 400 ×. The alloy of the present invention is superior to any of the existing comparative alloys in terms of overall corrosion resistance and has low susceptibility to stress corrosion cracking. From the above results, it can be said that the alloy of the present invention is extremely excellent as a material for heat transfer tubes for refuse incineration boilers. Tables 3 (1) to (7) show the results of tests simulating the corrosive environment of the soda recovery boiler. In this test, as in the test under the conditions described above, P and Si in the alloy were
Content greatly affects the maximum depth of intergranular corrosion, and when P is less than 0.015%, Si
Each of the alloys of the present invention having 0.3% or less has a maximum grain boundary erosion depth of 2.5 μm or less. It also outperforms existing comparative alloys in terms of overall corrosion. [Table 2 (1)] [Table 2 (2)] [Table 2 (3)] [Table 2 (4)] [Table 2 (5)] [Table 2 (6)] [Table 2 (7)] [Table 3 (1)] [Table 3 (2)] [Table 3 (3)] [Table 3 (4)] [Table 3 (5)] [Table 3 (6)] [Table 3 (7)] As is clear from the test results of the examples, the alloy of the present invention has an excellent overall surface resistance even in a very special and severe corrosive environment to which the heat transfer tubes of a refuse incineration boiler and a soda recovery boiler are exposed. This alloy has corrosion resistance and stress corrosion cracking resistance, and also has strong resistance to intergranular corrosion. Since this alloy has an austenitic structure, it is excellent not only in high-temperature strength but also in workability and weldability. Also, since the Ni content may be up to 55%, it is also inexpensive as compared with existing Ni-based alloys. By using a tube made of the alloy of the present invention for a high-temperature portion of the above-mentioned boiler, for example, a superheater tube, it becomes possible to make a high-temperature and high-pressure boiler that makes full use of waste heat, thereby improving energy recovery efficiency. It can be more efficiently extracted as electric power energy than before.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view and a side view showing a shape of a corrosion test piece used for measuring a weight loss in a high-temperature corrosion test. FIG. 2 is a side view showing a jig used in the stress corrosion cracking test and a method of mounting a test piece.

Claims (1)

[Claims 1] In weight%, C: 0.05% or less, Si: 0.3% or less, Mn: 7.5% or less, Cr: 25 to 35%
, Ni: 25 to 55% and Mo that satisfies the following formula, the balance being Fe and inevitable impurities, and P in the impurities being 0.015% or less, for boiler for incineration or boiler heat transfer tubes for soda recovery. Corrosion resistant alloy. 0.3 (%) ≦ Mo (%) ≦ 5.8 (%) − {Ni (%) / 10} .. 2. The alloy component is further selected from Nb, Ti, Zr and V The highly corrosion-resistant alloy for a boiler for refuse incineration or a boiler heat transfer tube for soda recovery according to claim 1, wherein one or more kinds are contained in a total of 0.1 to 3.0% by weight. 3. The boiler for incineration of refuse according to claim 1 or 2, wherein the alloy further contains one or more selected from Cu, Co and W in an amount of 0.1 to 5.0% by weight. High corrosion resistant alloy for boiler heat transfer tubes. 4. A high corrosion resistance for a boiler for incineration of refuse or a heat transfer tube for boiler for soda recovery according to claim 1, wherein the alloy further contains one or more rare earth elements in a total amount of 0.01 to 0.1% by weight. alloy. 5. The high corrosion-resistant alloy for a boiler for incineration or a boiler heat transfer tube for soda recovery according to claim 1, further comprising N: 0.1 to 0.3% by weight as an alloying component. 6. The high corrosion resistant alloy for a boiler for refuse incineration or a boiler heat transfer tube for soda recovery according to claim 1, further comprising 0.5% by weight or less of Al as an alloying component.