JP2691093B2 - High temperature corrosion resistant alloy for soda recovery boiler - Google Patents

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 fuels.

[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%, and further, Currently, high N containing 25% Cr
Austenitic stainless steel (Japanese Patent Publication No. 50-8967) is used.

[0003]

At present, in the soda recovery boiler, the combustion temperature and the like are being changed from the viewpoint of exhaust gas regulations and the like, so that the corrosive environment is also changing. Particularly, as the SOx component in the combustion gas decreases, the concentration of carbonate in the adhering ash increases, and corrosion due to carburization has become a problem.

Therefore, in the present invention, in addition to "corrosion resistance to molten salt corrosion caused by melting of combustion ash adhering to material" which has been mainly studied in the present invention, a new problem "in combustion gas It is intended to provide an alloy excellent in "corrosion resistance against corrosion accompanied by a carburizing phenomenon caused by a C component contained in combustion ash or the like contained in the material or the like".

In the former case, intergranular corrosion resistance and in the latter case, carburization resistance often becomes a problem.
Although r-austenitic stainless steel has excellent intergranular corrosion resistance, carburization resistance is insufficient, and corrosion of about 1 mm / year occurs. In the present invention, the corrosion resistance in the above-mentioned two different environments will be collectively referred to as high temperature corrosion resistance.

[0006]

Means for Solving the Problems (1) By weight, C: 0.025% or less, Si: 1% or less, Mn: 5% or less, P: 0.04% or less, S: 0.0
3% or less, Cr: 22-28%, Ni: 25-40%,
N: and from 0.2 to 0.4%, the balance Ri Do Fe and unavoidable impurities, to corrosion involving molten salt corrosion and carburizing
A high-temperature corrosion-resistant alloy for soda recovery boilers, which has corrosion resistance.

(2) In addition to the above-mentioned component (1), weight%
At Mo: 3% or less, Nb: 2% or less, Ti: 0.5
% Or less of the observed including one or two or more, molten salt corrosion and carburizing
A high-temperature corrosion-resistant alloy for a soda recovery boiler, which is characterized by having corrosion resistance against corrosion due to corrosion. It 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 22 to 28% and adding N to 0.2 to 0.4%. Carburizing resistance is improved by adding Ni in an amount of 25 to 40% 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 . For this reason,
Since a Cr-deficient layer is formed in the vicinity of the grain boundaries to promote grain boundary corrosion, it is desirable that the Cr content be as low as possible. Therefore, 0.0
The upper limit is 25%.

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 are 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 Cr ₂₃ C ₆ with a trace amount of C of 0.025% or less.
Even for the grain boundary precipitation of, the lower limit is set to 22% in order to maintain the Cr amount in the Cr-deficient layer. However, when the Cr content exceeds 28%, deterioration of hot workability and σ embrittlement are likely to occur, so the upper limit was made 28%.

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 25%. However, the solid solubility of N, which will be described later, decreases as Ni increases, so the upper limit is made 40%.

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, 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, considering the case where the upper limit value of Cr is 28% and the lower limit value of Ni is 25%, the upper limit of N is set to 0.4%.

Mo: Mo has the effect of improving intergranular corrosion resistance and also 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 effects of improving intergranular corrosion resistance and creep rupture strength. However, excessive addition increases the amount of carbides and nitrides of Nb and Ti and adversely deteriorates the 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 The chemical compositions of the alloys used as examples are shown in Table A. In Table A, SUS321HTB and SUS310S are 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 have a Ni content of 25% to 4%.
It is within the range of 0%. Alloys A to E are first invention materials, and F to
Alloy I is the second invention material. F alloy is Mo 1.96
%, The G alloy contains 0.51% of Nb, and the H alloy contains 0.2 of Ti.
1%, the I alloy contains 1.08% Mo and 0.37% Nb.

On the other hand, JO used as a comparative alloy
The following points are out of the range of the components of the present invention. That is, in the J and K alloys, Ni is 19.8% and 22.8%.
And low. L and M alloys have 0.09% and 0.17% N
And low. N alloy has a high C of 0.04%. O alloy is Cr
Is as low as 20.1%.

The conventional P, Q and R alloys are SUS321HTB, SUS310STB and high N austenitic stainless steel, respectively. The P and Q alloys do not contain N and the R alloy has a low Ni content. ing.

Table B shows the results of measuring the intergranular corrosion depth and the amount of general corrosion by performing an accelerated test in the following two types 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 (causing molten salt corrosion) Test temperature: 550 ° C., test time: 100 hours Simulated combustion ash composition (mixing ratio): Na ₂ SO ₄ : K ₂ SO ₄ : NaCl = 3: 2: 1 Simulated combustion gas composition: 0.05% SO ₂ + 5% O ₂ + 10% CO ₂ + N ₂ balance

(2) High CO ₃ environment (causing corrosion accompanied by carburization) Test temperature: 550 ° 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 whole surface is subjected to No. 600 emery polishing,
The dimensions and weights before the test were measured. After the test, the scale generated by the corrosion was removed, and the weight measurement and the intergranular corrosion depth of the vertical 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.
The amount of general corrosion is less than or equal to that of conventional materials, which is used as a standard in selecting materials for soda recovery boilers.
It is 2 mm / year or less, and can be said to have excellent intergranular corrosion resistance and corrosion resistance. Further, the amount of general corrosion under the above-mentioned high CO ₃ environment is about half or less of the conventional material and the above-mentioned guideline is 0.2 mm / year or less, and it can be said that it has excellent corrosion resistance, that is, carburization resistance. From the above, it can be said that the alloy of the present invention has excellent hot corrosion resistance.

On the other hand, the comparative alloys L to Q and the conventional materials P and Q alloys 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 alloys L and M, FIG.
As described above, the intergranular corrosion depth decreases as the N content increases, and 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, even when the N content is 0.2% or more, intergranular corrosion occurs in both the N alloy having a C content of 0.025% or more and the O alloy having a Cr content of 22% or less. Therefore, as described above, in order to secure the intergranular corrosion resistance, Cr, C and N are used.
It turns out that all must satisfy the conditions.

Further, the comparative materials J, K and O alloys and the conventional materials P to R alloys have the above-mentioned standard of 0.2 in a high CO ₃ environment.
mm / year or more, and it can be said that the corrosion resistance, that is, the carburization resistance is insufficient. Here, when the relationship between the Ni content and the general corrosion amount under the high CO ₃ environment is plotted for the alloys A to E of the present invention and the comparative alloys J and K, N is as shown in FIG.
It can be seen that the amount of general corrosion decreases as the i content increases. That is, the lower limit of the Ni content of the present invention is 25
In%, it can be seen that the amount of general corrosion is 0.2 mm / year or less as the above-mentioned standard. However, the Ni content is 25%
Above all, the O alloy with a Cr content of 22% or less is 0.22 m.
It shows a large amount of general corrosion of m / year. As described above, Cr is an element that has a great influence on the corrosion resistance,
This is because even if the carburization resistance is improved by Ni, the original corrosion resistance becomes insufficient if the Cr content is low.

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 16.7 kgf / mm ² . Calculated value from 600 ° C allowable tensile stress of SUS321HTB stipulated in MITI technical standard (allowable tensile stress ≈ 0.6) 11.5kgf
It has a value higher than / mm ² .

[Table 1]

[Table 2]

[Table 3]

[0030]

EFFECT 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, which brings about an extremely economical effect.

[Brief description of the 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 Ni content of the alloy of the present invention and the amount of general corrosion in a high CO ₃ environment.

Claims (2)

(57) [Claims] 1. By weight, C: 0.025% or less, S
i: 1% or less, Mn: 5% or less, P: 0.04% or less,
S: 0.03% or less, Cr: 22 to 28%, Ni: 25
To 40% N: and 0.2 to 0.4%, the balance Ri Do Fe and inevitable impurities, corrosion involving molten salt corrosion and carburizing
A high temperature corrosion resistant alloy for a soda recovery boiler, which has corrosion resistance against corrosion . 2. In addition to the components of claim 1, in weight percent,
Mo: 3% or less, Nb: 2% or less, Ti: see contains one or more than 0.5%, accompanied by a molten salt corrosion and carburizing
A high temperature corrosion resistant alloy for a soda recovery boiler, which has corrosion resistance against corrosion .