JPH05195126A - Highly corrosion resistant alloy for heat exchanger tube of boiler - Google Patents

Abstract

PURPOSE:To provide a highly corrosion resistant alloy for the heat exchanger tube of a boiler especially excellent in resistance to intergranular corrosion as well as resistance to general corrosion and stress corrosion cracking and having high strength at high temp. CONSTITUTION:This highly corrosion resistant alloy has a chemical compsn. consisting of, by weight, <=0.05% C, <=0.3% Si, <=7.5% Mn, 25-35% Cr, 25-55% Ni, Mo satisfying an inequality 0.3(%)<=Mo(%)<=5.8(%)-(Ni($)/10) and the balance Fe with inevitable impurities including <=0.015% P.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high corrosion resistant alloy for a heat transfer tube (boiler tube) of a boiler used in a high temperature environment where corrosive combustion slag containing chloride adheres. Specifically, in a facility that incinerates municipal solid waste, industrial waste, sewage treatment sludge (hereinafter collectively referred to as “garbage”), a waste heat boiler installed for the purpose of energy recovery, and a black liquor is burned at a paper mill. For boiler heat transfer tubes such as superheater tubes such as boilers (called soda recovery boilers) for recovering soda and using waste heat to generate electricity, reheater tubes, evaporator tubes and water wall tubes, Particularly, the present invention relates to a high Cr high Ni alloy with an austenitic structure used under high temperature and high pressure.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of positively utilizing unused energy, the effective use of the energy possessed by municipal solid waste has attracted attention, and the waste heat generated when the municipal solid waste is already incinerated has been utilized to Electricity is generated at some facilities to cover the electricity used in heating and incineration facilities. Further, in the paper manufacturing industry, a boiler (called a soda recovery boiler) has been used for the purpose of burning black liquor generated in the pulp manufacturing process to recover soda and also generating power by waste heat. ..

In the above waste heat recovery system, in order to convert waste heat into electric energy and make maximum use of it, the 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, increasing the temperature of the steam causes the temperature of the wall of the boiler tube to increase, 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 pipe, which has the highest temperature, was 200 to 350 ° C, but from now on, both waste incineration waste heat recovery boiler and soda recovery boiler will
It is expected that operating conditions will be adopted such that the tube wall temperature exceeds 500 ° C.

Since a large amount of plastic is mixed in municipal waste, the combustion gas thereof contains hydrogen chloride. Further, chloride is contained in the combustion residue (slag). Therefore, in the heat transfer tube for waste incineration waste heat recovery boiler, there are problems of corrosion due to hydrogen chloride gas and corrosion damage of metal materials due to adhesion of slag containing chloride. This situation is the same for soda recovery boilers.

From the above-mentioned background, there has been a strong demand for a material having excellent corrosion resistance and high temperature strength which can withstand a severe corrosive environment containing chlorides as a heat transfer tube material.

As a material used for a high temperature portion of an ultrahigh temperature and high pressure plant, for example, a superheater tube, a material having a high corrosion resistance and having an austenite structure is desirable.

Various materials for municipal waste incineration waste heat boiler tubes having an austenite structure are known in foreign countries, particularly in the United States. For example, Corrsion, March 9-13,1
987 Paper No. 402 contains 825 alloy (AS
N08825 alloy described in TM B163, B423) and approx. 66
% 625 alloy containing Ni (N0 as described in ASTM B444
(6625 alloy) has been reported to be used as a material for municipal waste incineration waste heat boiler tubes.These high alloy steels containing a large amount of Ni show little corrosion thinning and corrosion resistance in the corrosive environment of US waste incinerators. Is said to be excellent.

However, the inventors of the present invention, like the corrosive environment of waste incineration waste heat boilers and soda recovery boilers in Japan,
As a result of testing under conditions in which chloride-containing slag adheres, conventional austenitic alloys such as the 825 alloy described above undergo general corrosion and stress corrosion cracking, as well as intergranular corrosion in which grain boundaries are selectively corroded. There is a case.

The inventors of the present invention previously prepared an alloy in which the contents of Ni and Mo are adjusted to reduce the stress cracking susceptibility (Japanese Patent Application No. 3-1885).
67), and an alloy whose resistance to general corrosion has been improved by adding Al (Japanese Patent Application No. 3-161357) and an alloy whose general corrosion resistance has been improved by positive addition of a large amount of Mn (Japanese Patent Application No. 3-3103).
No. 84) was developed and applied for a patent. However, the alloys of these inventions are still insufficient in terms of completely preventing the intergranular corrosion. Since the boiler tube is a structural material used under high temperature and high pressure, intergranular corrosion may become a starting point of cracks in the tube depending on the situation, leading to the destruction of the tube. Therefore, it is desirable that the susceptibility to intergranular corrosion be as low as possible, especially as a material for a boiler tube for high temperature and high pressure.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to have an austenite structure which exhibits excellent strength at high temperatures as in the case of the alloy of the above-mentioned prior invention, and in which combustion slag containing chloride adheres. To provide a material for a boiler heat transfer tube having excellent general corrosion resistance and stress corrosion cracking resistance in a severe corrosive environment, and having sufficient resistance to intergranular corrosion superior to the alloy of the prior invention. is there.

[0011]

[Means for Solving the Problems] Conventionally, intergranular corrosion of austenitic alloy is caused by Cr deficiency generated around the precipitated Cr carbide in which Cr carbide precipitated at the grain boundary of the alloy reacts with slag containing chloride. It has been attributed to two causes: the layer is selectively corroded. However, in the test conducted by the present inventors under the condition that a corrosive combustion slag containing chloride adheres, intergranular corrosion occurs even in an alloy in which Cr carbide is not substantially precipitated at the crystal grain boundaries. Upon pursuing the cause, impurity elements segregated at the grain boundaries, especially P
It was also found that the grain boundaries are corroded by the selective elution of Si and Si into the combustion slag containing chloride. Then, as will be described later, after properly selecting the content of alloying elements such as Cr, Ni, and Mo, C is set to 0.05% or less, and P and Si
It was confirmed that it is possible to almost completely suppress the occurrence of intergranular corrosion even under the above-mentioned environment by setting the contents of 0.015% or less and 0.3% or less, respectively.

The gist of the present invention is a high corrosion resistant alloy for boiler heat transfer tubes having the following chemical composition.

(1) C: 0.05% or less, Si: 0.3% by weight
% Or less, Mn: 7.5% or less, Cr: 25 to 35%, Ni: 25 to 55%
And a chemical composition containing Mo that satisfies the following formula, the balance being Fe and unavoidable impurities, and P in the impurities being 0.015% or less.

0.3 (%) ≦ Mo (%) ≦ 5.8 (%) − {Ni (%) / 10} (2) In addition to the composition of (1) above, the following first group, second group A chemical composition comprising one or more alloying components selected from the group 1 and from one or more groups from the third group.

First group: each, or a total of two or more kinds
0.1-3.0 wt% Nb, Ti, Zr and V.

Second group: Each or a total of two or more kinds
0.1-5.0 wt% Cu, Co and W.

Third group: each or a total of two or more kinds
0.01-0.1 wt% rare earth element.

(3) The chemical composition according to any one of (1) and (2) above, further containing 0.1 to 0.3% by weight of N as an alloy component.

(4) The chemical composition according to any one of (1), (2) or (3) above, further containing 0.5% by weight or less of Al as an alloy component.

[0020]

The alloy of the present invention has, as a total effect of the appropriate combination of the above-mentioned alloy components, a material used in a high temperature corrosive environment where a combustion slag containing chloride adheres, for example,
It has excellent properties suitable for the material of the heat transfer tube of the refuse incineration boiler and the soda recovery boiler. The action effects of each alloy component and the reasons for limiting the content thereof are as follows. Hereinafter, all percentages regarding the content of alloy components mean wt%.

C: C is combined with Cr in the alloy, and if precipitated as a lumpy Cr carbide at the crystal grain boundary, it reacts with the combustion slag containing chloride adhering to the surface of the pipe, or Cr near the crystal grain boundary.
It forms a depletion layer to deteriorate the intergranular corrosion resistance of the alloy. Therefore, it is desirable that the C content be as low as possible.
0.05% is the allowable upper limit value.

Si: Si is usually added as a deoxidizing agent for alloys, and is generally an effective element for enhancing the oxidation resistance. In alloys with an austenitic structure, addition of a large amount of Si also has the effect of suppressing general corrosion. However, Si segregates at grain boundaries and reacts with combustion slag containing chloride,
It also causes intergranular corrosion. Thus, Si has two contradictory effects on the corrosion resistance, but in the present invention, the general corrosion resistance is supplemented with other elements (Cr, Ni, etc.), and the inclusion of Si is aimed at preventing intergranular corrosion. The amount was kept low. Si content is
If it is 0.3% or less, intergranular corrosion can be suppressed to such an extent that it does not become a practical problem.

Mn: Mn is an austenite forming element and can also be used as a deoxidizing agent. In particular, in a corrosive environment in which combustion slag containing chloride adheres to the surface of the pipe in a high temperature range exceeding 500 ° C, it is effective to add Mn to enhance the general corrosion resistance. However, if the Mn content exceeds 7.5%, both the oxidation resistance and the hot workability deteriorate, so the content was made 7.5% or less.

Cr: Cr exhibits excellent effects in improving high temperature strength and oxidation resistance at high temperatures. However, in the alloy of the present invention in which Si is suppressed to 0.3% or less, if the content of Cr is less than 25%, the resistance to high temperature corrosion in the above 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 unnecessarily increased, so the upper limit was made 35%.

Ni: Ni is an austenite forming element,
It is an important component that secures high temperature strength and suppresses general corrosion mainly due to molten chloride-based corrosive combustion slag. However, since Ni is an expensive element, the upper limit was set to 55% in consideration of the balance between the material cost and the above effects. On the other hand, Ni
If the content is lower than 25%, the high temperature corrosion resistance deteriorates rapidly, so the lower limit was made 25%.

Mo: Mo is a component effective for strengthening the grain boundaries and enhancing resistance to intergranular corrosion. Mo is a component that improves corrosion resistance in an aqueous solution containing chloride ions, especially stress corrosion cracking resistance. For this reason, the 825 alloy, which is also a seawater corrosion resistant material, contains 3% Mo. Is based. However, as described above, in the environment where combustion slag containing high concentration of molten chloride adheres, such as the corrosive environment of the municipal waste incineration boiler in Japan, contrary to the conventional knowledge, when a large amount of Mo is added, stress corrosion occurs. Increases susceptibility to cracking. However, according to the research results of the present inventors,
The effect of Mo on stress susceptibility to stress corrosion cracking strongly depends on the Ni content in the alloy steel, and it is possible to reduce stress corrosion cracking susceptibility by adjusting the Mo content appropriately according to the Ni content. Is. On the other hand, as described above, Mo is an element that has the effect of suppressing intergranular corrosion, and 0.3
% Or more content is required. As described above, the above formula was determined in consideration of the action and effect of Mo, that is, 0.3 (%) ≤ Mo (%) ≤ 5.8 (%)-{Ni (%) / 10} ...・ It is. If Mo (%) is 5.8 (%)-{Ni (%) / 10} or less, the stress corrosion cracking susceptibility of the alloy is not enhanced.

In addition to the above components, the alloy of the present invention may further contain the following elements, if necessary.

Nb, Ti, Zr and V (Group 1 elements): Nb,
Since Ti, Zr and V easily form carbides,
It serves to fix C in the alloy steel, suppress precipitation of Cr carbide, improve high temperature strength and resistance to intergranular corrosion. In the case of alloys having an austenite structure, Cr carbides that precipitate at grain boundaries react with corrosive molten salt compounds that adhere to the surface of the pipe, which causes grain boundary corrosion. If these elements are added after keeping the value low, the intergranular corrosion resistance is further improved. If the total content of one or more of these elements is less than 0.1%, the effect of addition does not appear, and if the content exceeds 3%, the effect is saturated and only the cost increases.

Cu, Co and W (Group 2 elements): These elements have the function of improving the high temperature strength of the alloy by solid solution strengthening. Similar to the Group 1 element, one or more kinds can be added if necessary, but if the total content of one or more kinds is less than 0.1%, the effect of addition is not remarkable. On the other hand, in the range exceeding 5%, there is almost no increase in the effect commensurate with the cost increase.

Rare earth element (third group element): The rare earth element such as Y, La and Ce has a function of improving the adhesion of the protective oxide film (Cr ₂ O ₃ ) formed on the alloy surface. When expecting such an effect, one kind or two or more kinds in total
It is sufficient to contain 0.01% or more. However, if it exceeds 0.1%, the hot workability of the alloy is deteriorated.

N (nitrogen): N contributes to stabilization of the austenite structure. Further, since it also has the effect of increasing the high temperature strength, it can be contained in an amount of 0.1% or more, if necessary. However, in the composition range of the alloy of the present invention, it is difficult to make the content exceed 0.3% by the usual 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 in excess and its content exceeds 0.5%, the intermetallic compound Ni ₃ Al will remain when used at high temperature for a long time.
Precipitates and deteriorates the creep ductility, so its content is
It is desirable to keep it below 0.5%. The alloy of the present invention is
For example, it is melted in an electric furnace and refined with VOD or AOD to obtain a billet. The billet is used as a raw material to produce a raw pipe by a hot extrusion method, and the raw pipe is cold drawn to have a predetermined size. Use as a tube. The heat treatment is preferably a solution treatment in which the material is heated to 1000 to 1200 ° C and then rapidly cooled. After that, it is descaled and finally used as 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).

[0033]

[Examples] In Tables 1 (1) to (9), reference numerals 1 to 10 showing chemical compositions
The alloy of 7 was melted in a vacuum melting furnace at a rate of 17 kg each, cast into an ingot, heated to a temperature of 1100 ° C., hot forged and hot rolled into a billet having a thickness of 15 mm. Then, after softening annealing at a temperature of 1100 ° C., it was cold-rolled into a plate having a thickness of 10.5 mm. Then, a solution treatment was performed by heating to a temperature of 1100 ° C. and cooling with water.

After the solution treatment, a corrosion test piece having a thickness of 2 mm, a width of 10 mm, and a length of 10 mm and a stress cracking test piece having a dimension and shape shown in FIG. A high temperature corrosion test simulating the adhesion of combustion slag was conducted.
At the same time, test pieces of alloys having the compositions shown in Table 1 (9), reference numerals 108 to 112, having the same dimensions as above, were cut out from the center portion of the wall thickness of a commercial boiler tube and subjected to the same test. The symbols in Table 1 (9)
108 is the NO8825 alloy described in ASTM B163, reference numeral 109 is S
US304, code 110 corresponds to SUS 316L, code 111 corresponds to SUS 310S, and code 112 corresponds to N08320 steel described in ASTM B622.

The high temperature corrosion test was conducted under the following two conditions.

Test simulating the corrosive environment of a refuse incineration boiler: 10% NaCl-10% KCl-15% FeCl ₂ -15% PbCl in mol%
_{2 -18.75% Na 2 SO 4 -18.75} % K coating subjected at a rate of 30 mg / cm ² Synthesis ash of _{2 SO 4 -12.5% Fe 2 O} 3 on both surfaces of the test piece,
0.15% HCl −300ppm SO ₂ − 7.5% O ₂ − 7.5% CO ₂
−20% H ₂ O − Heated at 550 ° C for 20 hours in gas stream of bal.N ₂ .

[0037] 20% NaCl-22.5% in the test mole percent corrosive environment simulating a soda recovery boiler _{Na 2 SO 4 -22.5% K 2} SO 4 -20%
30 mg of synthetic ash of Na ₂ CO ₃ -15% Fe ₂ O ₃ was applied to both sides of the test piece.
/ coating subjected at a rate of cm ^2, which _{0.25% SO 2 - 1% O} 2 -15%
Heated at a temperature of 600 ° C for 20 hours in a gas stream of CO ₂ -bal.N ₂ .

The corrosion resistance was evaluated by descaling the test piece after the test, measuring the weight, and determining the corrosion weight 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 piece after descaling with an optical microscope of 100 times or 500 times. As shown in Fig. 2, the stress corrosion cracking susceptibility test, which is a problem in the corrosive environment of the refuse incineration boiler, is performed by applying a stress equivalent to 0.2% proof stress of each alloy to the stress corrosion cracking test piece 2 with the jig 1 as shown in Fig. 2. In this state, the surface of the test piece 2 is coated with the same synthetic ash as used in the high temperature corrosion test, and then the temperature is kept at 400 ° C. for 20 hours in the same gas stream. The test temperature was set to 400 ° C. because the inventors of the present invention found that the stress corrosion cracking susceptibility of the alloy having an austenitic structure was highest at around 400 ° C. The presence or absence of stress corrosion cracking was examined by microscopic observation of the cross section of the semicircular notch.

[0040]

[Table 1 (1)]

[0041]

[Table 1 (2)]

[0042]

[Table 1 (3)]

[0043]

[Table 1 (4)]

[0044]

[Table 1 (5)]

[0045]

[Table 1 (6)]

[0046]

[Table 1 (7)]

[0047]

[Table 1 (8)]

[0048]

[Table 1 (9)]

Tables (1) to (7) show the results of the above corrosion test simulating the corrosive environment of the refuse incineration boiler. Reference numbers 1 to
The alloys 4, 9 to 12 and 17 to 20 are those in which the content of P is changed, and the reference numerals 5 to 8, 13 to 16 and 21 to 24 are those in which Si is changed. As is clear from the results in Tables 2 (1) to (7), the maximum depth of intergranular corrosion in the corrosive environment is greatly affected by the P and Si contents in the alloy. The maximum grain boundary erosion depth of alloys with P less than 0.015% and Si less than 0.3% is
It is less than 5 μm, which is about 1/4 of the comparative alloy.

Reference numerals 31 to 48, 61 to 75, 80 to 83, 86, 87, 8
The alloys of 9, 91, 93, 95, 97, 99, 101, 103, and 105 to 107 contain the alloying elements of the first group in appropriate amounts.

The intergranular corrosion resistance of these alloys is further improved, and the intergranular corrosion is at a level that cannot be observed even with an optical microscope of 400 magnifications.

The alloys of the present invention are superior in general corrosion resistance to any of the existing comparative alloys and have 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 a heat transfer tube for a refuse incineration boiler.

Tables 3 (1) to (7) are the results of tests simulating the corrosive environment of the soda recovery boiler. In this test as well, similar to the test under the above-mentioned conditions, the contents of P and Si in the alloy had a great influence on the maximum depth of intergranular corrosion, with P of 0.015% or less and Si of 0.3% or less. In all of the alloys of the present invention, the maximum grain boundary erosion depth is 2.5 μm or less. It also outperforms the existing comparative alloys in terms of general corrosion.

[0055]

[Table 2 (1)]

[0056]

[Table 2 (2)]

[0057]

[Table 2 (3)]

[0058]

[Table 2 (4)]

[0059]

[Table 2 (5)]

[0060]

[Table 2 (6)]

[0061]

[Table 2 (7)]

[0062]

[Table 3 (1)]

[0063]

[Table 3 (2)]

[0064]

[Table 3 (3)]

[0065]

[Table 3 (4)]

[0066]

[Table 3 (5)]

[0067]

[Table 3 (6)]

[0068]

[Table 3 (7)]

[0069]

As is clear from the test results of the examples, the alloy of the present invention has excellent general corrosion resistance even in a very special and severe corrosive environment to which the heat transfer tubes of the refuse incineration boiler and the soda recovery boiler are exposed. It is an alloy that has stress corrosion cracking resistance and that also exhibits strong resistance to intergranular corrosion. Since this alloy has an austenite 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 cheaper than existing Ni-based alloys.

By using the tube made of the alloy of the present invention in the high temperature part of the boiler, for example, in the superheater tube, it becomes possible to make a high temperature and high pressure boiler that makes full use of waste heat and to improve energy recovery efficiency. It is possible to increase the efficiency and extract it as electric power energy more efficiently than ever before.

[Brief description of drawings]

FIG. 1 is a plan view and a side view showing a shape of a corrosion test piece used for measuring a corrosion weight loss in a high temperature corrosion test.

FIG. 2 is a side view showing a method of attaching a jig and a test piece used in a stress corrosion cracking test.

Claims (6)

[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 satisfying the following formula are contained. , A balance of Fe and unavoidable impurities, and P in the impurities is 0.015% or less, a high corrosion resistant alloy for boiler heat transfer tubes. 0.3 (%) ≦ Mo (%) ≦ 5.8 (%) − {Ni (%) / 10} ・ ・ ・ ・ ・ 2. A total of 0.1 to 3. One or more selected from Nb, Ti, Zr and V as alloy components.
The high corrosion resistant alloy for a boiler heat transfer tube according to claim 1, which contains 0% by weight. 3. The high corrosion resistant alloy for a boiler heat transfer tube according to claim 1, further comprising 0.1 to 5.0% by weight in total of one or more selected from Cu, Co and W as an alloy component. 4. The high corrosion-resistant alloy for boiler heat transfer tubes according to claim 1, 2 or 3, further containing one or more rare earth elements as an alloy component in a total amount of 0.01 to 0.1% by weight. 5. N: 0.1 to 0.3% by weight as an alloy component
A high corrosion-resistant alloy for a boiler heat transfer tube according to claim 1, 2, 3 or 4. 6. The high corrosion resistant alloy for a boiler heat transfer tube according to claim 1, further comprising 0.5% by weight or less of Al as an alloy component.