JPH0570892A - High temperature corrosion resisting alloy for soda recovery boiler - Google Patents

Abstract

PURPOSE:To provide superior high temp. corrosion resistance under a severe corrosive environment by controlling C content while maintaining Cr content at high value and also adding specific amounts of N and Ni. CONSTITUTION:The alloy has a composition consisting of, by weight, <=0.025% C, <=1% Si, <=5% Mn, <=0.04% P, <=0.03% S, 27-40% Cr, 35-55% Ni, 0.2-0.45% N, and the balance Fe with inevitable impurities. Further, if necessary, one or more kinds among <=3% Mo, <=2% Nb, and <=0.5% Ti are incorporated. Because this alloy has superior high temp. corrosion resistance under a severe corrosive environment, such as soda recovery boiler, and also has excellent carburizing resistance under a high CO3 environment, a high temp. and high pressure boiler can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat exchanger such as a superheater tube of a soda recovery boiler of a paper mill and materials such as spacers and other metal deposits, and an industrial boiler using general waste and industrial waste as fuel. Also, the present invention relates to a material that can be applied to superheater pipes and metal deposits of commercial boilers that use crude oil and coal as fuel.

[0002]

2. Description of the Related Art In a soda recovery boiler superheater tube, due to improvement in thermal efficiency (increase in steam condition and pressure), low alloy steel is changed to austenitic stainless steel containing 18% Cr by weight%. Currently, high N containing 25% Cr
Austenitic stainless steel (Japanese Patent Publication No. 50-8967) is used.

[0003]

At present, for the purpose of further improving the thermal efficiency, further high temperature and high pressure steam conditions are being studied, and a material excellent in high temperature corrosion resistance under more severe conditions is desired. It is rare. Here, the high temperature corrosion resistance refers to the case where the following two properties are simultaneously present. (1) Corrosion resistance against molten salt corrosion caused by melting of combustion ash or the like that is in close contact with the material. In this case, intergranular corrosion resistance is often a problem. (2) Corrosion resistance to corrosion accompanied by carburization generated by the C component contained in the combustion gas or contained in the combustion ash adhered to the material. In general, when carburization occurs, corrosion resistance is significantly deteriorated, and in this case, carburization resistance is often a problem.

[0004] With conventional materials, high temperature corrosion due to high temperature and high pressure under steam conditions results in insufficient high temperature corrosion resistance, especially carburization resistance, and even in high N austenitic stainless steel containing 25% Cr, corrosion of nearly 1 mm / year is achieved. Is occurring.

According to the above-mentioned state of the art and the above-mentioned demands, the present invention aims to provide an alloy excellent in high-temperature corrosion resistance.

[0006]

MEANS FOR SOLVING THE PROBLEMS The present invention is (1) in% by weight, C: 0.025% or less, Si: 1%
Hereinafter, Mn: 5% or less, P: 0.04% or less, S: 0.
03% or less, Cr: 27-40%, Ni: 35-55
%, N: 0.2 to 0.45%, and the balance being Fe and inevitable impurities, a high temperature corrosion resistant alloy for a soda recovery boiler.

(2) In addition to the above component (1), weight%
Mo: 3% or less, Nb: 2% or less, Ti: 0.5%
A high temperature corrosion resistant alloy for a soda recovery boiler, characterized in that it contains one or more of the following. Is.

That is, the present invention provides excellent intergranular corrosion resistance by limiting C to 0.025% or less while maintaining Cr as high as 27 to 40% and adding N to 0.2 to 0.45%. The carburization resistance is improved by adding 35 to 55% of Ni while maintaining the property.

[0009]

The reason for limiting the components will be described below. C;
C combines with Cr during use in a high temperature environment of 400 ° C. or higher to form a carbide such as Cr ₂₃ C ₆ . Therefore, a Cr-deficient layer is formed in the vicinity of the grain boundary to promote grain boundary corrosion.
It is desirable to be as low as possible. Therefore, 0.025
% Is the upper limit.

Si: Si promotes the precipitation of carbides at the grain boundaries and inhibits the toughness after aging, so the upper limit is 1
%.

Mn; Mn is an element that increases the solid solubility of Cr ₂₃ C ₆ and suppresses the precipitation of carbides at the grain boundaries. However, the addition of a large amount promotes σ phase formation, so the upper limit is made 5%. ..

P and S; P and S both promote intergranular corrosion, so it is desirable that they be as low as possible. However, it is an unavoidable impurity in steelmaking. The upper limit of P is 0.04%
The reason for this is that if it exceeds this value, the weldability is significantly impaired. The upper limit of S is set to 0.03% because if it exceeds this range, not only weldability but also hot workability deteriorates.

Cr: Cr is an important component for high temperature corrosion resistance, and the lower limit is 27% in order to maintain good high temperature corrosion resistance. However, Cr is an element that destabilizes the austenite structure, and excessive addition causes a decrease in toughness, so the upper limit is made 40%.

Ni: Ni has an effect of stabilizing the austenite structure and an effect of enhancing carburization resistance. In order to secure the carburization resistance, the lower limit is set to 35%. However, since the solid solubility of N, which will be described later, decreases with an increase in Ni, the upper limit is set to 55%.

N: N has the effect of increasing high temperature strength and improving intergranular corrosion resistance. In order to secure intergranular corrosion resistance, the lower limit is made 0.2%. However, since N is a gas component, from the viewpoint of preventing bubbles from being generated, the upper limit is determined by the limit of solid solution. The solid solubility of N increases as the Cr content increases, and decreases as the Ni content increases. Therefore, in consideration of the case where the upper limit value of Cr is 40% and the lower limit value of Ni is 35%, the upper limit of N is set to 0.45%.

Mo: Mo has the effect of improving intergranular corrosion resistance and improving high temperature strength. These are added when particularly necessary. However, in order to maintain good hot workability, the amount added is limited, so the upper limit is made 3%.

Nb, Ti; Nb and Ti have the effect of improving intergranular corrosion resistance and creep rupture strength. However, excessive addition causes a large amount of Nb and Ti carbides and nitrides to be produced, which in turn deteriorates creep rupture strength and cleanliness. Therefore, the upper limit of Nb is 2% and T
The upper limit of i is 0.5%.

[0018]

EXAMPLES Table A shows the chemical composition of the alloys used as examples. In Table A, SUS321HTB and SUS310S are shown as representative examples of comparative alloys and conventional materials outside the scope of the present invention.
The high N austenitic stainless steels known from TB and Japanese Patent Publication No. 50-8967 are also shown.

The materials A to I of the present invention contain 27% to 4% of Cr.
0% and Ni are in the range of 35% to 55%. The A to E alloys are the first invention materials, and the F to I alloys are the second invention materials.
2.11% Mo for F alloy and 0.62 Nb for G alloy
%, H alloy 0.35% Ti, I alloy 1.5 Mo
It contains 7% and 0.44% Nb.

On the other hand, JO used as a comparative alloy
The following points are out of the range of the composition of the present invention. That is, in the J and K alloys, Cr is 25.4% and 22.1%.
And low. The L and M alloys have Ni of 32.8% and 29.6.
% Is low. N alloy has a high N content of 0.12%. O alloy is C
Is as high as 0.046%.

The conventional materials P, Q and R alloys are SUS321HTB, SUS310STB and high N, respectively.
Austenitic stainless steel with low Cr and Ni. N is not added to the P and Q alloys.

Table B shows the results of measuring the intergranular corrosion depth and the general corrosion amount by carrying out an accelerated test in the following two kinds of corrosive environments of the alloy of the present invention, the comparative alloy and the conventional material. These corrosive environments simulate molten salt corrosion and corrosion accompanied by carburization that may occur in the soda recovery boiler.The test material is buried in the simulated combustion ash shown below and subjected to a corrosion test in a simulated combustion gas flow. went.

(1) High K high Cl environment (causes molten salt corrosion) Test temperature: 570 ° C., test time: 100 hours Simulated combustion ash composition (mixing ratio): Na ₂ SO ₄ : K ₂ SO ₄ : NaCl = 5: 1: 1 Simulated combustion gas composition: 0.1% SO ₂ + 5% O ₂ + 5% CO ₂ + N ₂ balance

(2) High CO ₃ environment (causes corrosion accompanied by carburization) Test temperature: 570 ° C., test time: 150 hours Simulated combustion ash composition (mixing ratio): Na ₂ SO ₄ : K ₂ SO ₄ : NaCl: Na ₂ CO ₃ = 4: 1: 1: 4 Simulated combustion gas composition: 0.05% SO ₂ + 5% O ₂ + 10% CO ₂ + N ₂ balance

The test material is 20w × 20 liters × 3tmm
After machining into the shape of No. 6, the entire surface is subjected to No. 600 emery polishing,
The dimensions and weights before the test were measured. After the test, the scale produced by the corrosion was removed, and the weight measurement and the intergranular corrosion depth of the longitudinal section were measured.

According to Table B, the alloys A to I of the present invention all have an intergranular corrosion depth of 0 μ in the above-mentioned high K and high Cl environment.
m, the total amount of general corrosion is less than about half that of conventional materials, which is 0.2 mm / year or less, which is used as a guide when selecting materials for soda recovery boilers, and has excellent intergranular corrosion resistance and corrosion resistance. Can be said to have. Also, the amount of general corrosion under the above-mentioned high CO ₃ environment is about 1/5 or less of the conventional material,
The above-mentioned standard is 0.2 mm / year or less, excellent corrosion resistance,
That is, it can be said that it has carburization resistance. From the above, it can be said that the alloy of the present invention has excellent high temperature corrosion resistance.

On the other hand, the comparative alloy N alloy and the conventional materials P and Q alloy undergo intergranular corrosion in a high K high Cl environment, and it can be said that the intergranular corrosion resistance is insufficient. Here, when the relationship between the N content and the intergranular corrosion depth is plotted for the alloys A to E of the present invention and the comparative alloy N alloy, the intergranular corrosion depth decreases as the N content increases as shown in FIG. It can be seen that the intergranular corrosion depth is 0 μm when the lower limit of the N content in the present invention is 0.2% or more. However, N content is 0.2
%, The O alloy having a C content of 0.025% or more causes intergranular corrosion. Therefore, as described above, it is understood that C and N must satisfy the conditions in order to secure the intergranular corrosion resistance.

Comparative materials J to M alloys and conventional materials P to
The R alloy has the above-mentioned guideline of 0.2 mm / year or more in a high CO ₃ environment and can be said to have insufficient corrosion resistance, that is, carburization resistance. Here, when the relationship between the Cr content and the total corrosion amount in a high CO ₃ environment is plotted for the alloys A to E of the present invention and the comparative alloys J and K, as shown in FIG. It can be seen that the amount of corrosion is decreasing. That is, it can be seen that when the lower limit of the Cr content of the present invention is 27% or more, the amount of general corrosion becomes the above-mentioned standard of 0.2 mm / year or less.

Inventive alloys A to E and comparative alloy L
When plotting the relationship between the Ni content and the general corrosion amount in a high CO ₃ environment for M and M, it can be seen that the general corrosion amount decreases as the Ni content increases as shown in FIG. That is, it is understood that when the lower limit of the Ni content of the present invention is 35% or more, the amount of general corrosion is the above-mentioned standard of 0.2 mm / year or less.

The second invention alloy also has a good creep rupture strength. For example, in the case of the G alloy, the 10 ⁵ hour extrapolated value of the 600 ° C. creep rupture strength by the Larson-Miller parameter method is 15.1 kgf / mm ² . Calculated value from 600 ° C allowable tensile stress of SUS321HTB specified in the Ministry of International Trade and Industry technical standard (allowable tensile stress ≈ 0.6) 11.5 kgf
It has a value higher than / mm ² .

[Table 1]

[Table 2]

[Table 3]

[0031]

EFFECTS OF THE INVENTION It becomes possible to provide an alloy having excellent high temperature corrosion resistance in a severe corrosive environment such as a recovery boiler, and it is possible to increase the temperature and pressure of the boiler, which is a very economically useful effect. Be brought.

[Brief description of drawings]

FIG. 1 is a chart showing the relationship between the N content and the intergranular corrosion depth of the alloy of the present invention.

FIG. 2 is a chart showing the relationship between the Cr content of the alloy of the present invention and the amount of general corrosion in a high CO ₃ environment.

FIG. 3 is a chart showing the relationship between the Ni content of the alloy of the present invention and the amount of general corrosion under a high CO ₃ environment.

Claims (2)

[Claims] 1. C: 0.025% or less, S in weight%
i: 1% or less, Mn: 5% or less, P: 0.04% or less,
S: 0.03% or less, Cr: 27-40%, Ni: 35
~ 55%, N: 0.2 to 0.45%, the balance consisting of Fe and unavoidable impurities, high temperature corrosion resistant alloy for soda recovery boiler. 2. In addition to the components of claim 1, M in% by weight
o: 3% or less, Nb: 2% or less, Ti: 0.5% or less, and one or more kinds of high temperature corrosion resistant alloys for soda recovery boilers.