JPS629083A - Double pipe with excellent high temperature corrosion resistance and high temperature strength - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention "Object of the Invention" The present invention relates to the creation of a double pipe with excellent high-temperature corrosion resistance and high-temperature strength, and is suitable for use as a pipe material for heat exchangers in boilers using coal, oil, etc. as fuel. The purpose is to economically provide a suitable double pipe.

Industrial applications A pipe material with excellent high-temperature corrosion resistance and high-temperature strength for heat exchangers, etc.

The steam conditions of conventional boilers tend to increase in both temperature and pressure in order to improve their thermal efficiency, and such increases in temperature and pressure require heat exchanger tube materials with high creep strength. Further, fuels such as coal and petroleum contain impurities such as alkali metals and sulfur, which cause high-temperature corrosion on the outer surface of the heat exchanger tubes as described above.

However, carbon steel pipes have been used for temperatures below 450°C, and Cr-Mo low alloy steels have been used for temperatures between 450 and 540°C. -8 stainless steel, Ni-based alloys, etc. are known, and as a method to supplement corrosion resistance, for example, Japanese Patent Publication No. 57-4706 announces diffusion infiltration treatment with Cr, az, st, etc. Chrome Ni
A double pipe using a base alloy as the outer pipe material, a pipe with a high chromium alloy coated by thermal spraying on the outer surface (Japanese Patent Publication No. 34786/1986), etc. have been proposed.

Problems to be Solved by the Invention However, the above-mentioned conventional devices still have their own problems. In other words, although high-strength austenitic stainless steel is relatively inexpensive, it lacks high-temperature corrosion resistance against the rise in steam conditions mentioned above, while Ni-based alloys have excellent high-temperature corrosion resistance. It is also expensive because it contains a large amount of alloying elements. In the case of using a high chromium alloy as described above, the high chromium alloy itself is expensive and difficult to process, making the manufacturing process complicated and expensive in every respect. Thermal spraying and overlaying also have high construction costs, and diffusion infiltration treatment is superior in terms of cost, but because it requires long-term treatment at high temperatures, such as holding it at 1100°C for 10 hours, it can cause material deterioration due to coarsening of crystal grains, etc. cannot be avoided.

Furthermore, the above-mentioned ash corrosion caused by alkali metal and sulfur on the outer surface of heat exchanger tubes occurs when the steam temperature exceeds 500°C, and in the case of coal ash corrosion, it is recognized from 510"C, and at 540"C. Even with 18-8 stainless steel, the corrosion rate reaches 0.8 mm/Year.
1son+ C, Ca1n, Jr. (J, Engineering of Power
Vol, 82. No. 1), a preferable solution for this aspect has not yet been established.

"Structure of the invention" Means for solving the problem Consists of an outer tube and an inner tube, the outer tube material is Cr: 5 to 25 wt.
, 514 wt% or less, Mn 4 wt% or less, C is an alloy with a chemical composition of V4 to 4 times the inner tube C concentration, and the inner tube material has a 4.2 kg f/ni” at its operating temperature.
The alloy has a higher allowable stress than above, which reduces the amount of corrosion under high-temperature corrosion conditions. 4wt of Si
% or less, Mn 4 iyt% or less improves corrosion resistance and prevents precipitation of brittle phases such as σ phase under high temperature conditions.

By setting the amount of C in the outer tube material to V4 to 4 times the C concentration in the inner tube, the C activity of both tube materials becomes similar, and carburization or decarburization does not occur, thereby guaranteeing the strength of the inner tube material. .

The allowable stress of the inner tube material at the relevant operating temperature is 4.2 kg.
By setting the value to be f/- or more, the T/D of the tube can be lowered when the pressure is increased, and the tube can be manufactured appropriately.

In the outer tube material, Mo: 2 wt% or less, A1: 2 wt% or less,
Ni: 4wt% or less, Y: 0.1wt%, Ce:
It is possible to appropriately contain one or more of these in an amount of 0.1 wt% or less, thereby improving corrosion resistance and weldability.

Embodiment To further explain the present invention as described above, the present invention is a double pipe consisting of an inner pipe and an outer pipe, and the inner pipe has a capacity of 4.2 kg f/d” or more at its operating temperature. It is an alloy that has an allowable stress.If the allowable stress of the hammered inner tube is 4.2 kgf/- or less, the T of the tube will decrease when the pressure is increased.
/D tends to be too thick (and cannot be manufactured), so it is limited as described above. The allowable stress is 10'l (r average value (extrapolated value) of creep strength) x 0.6.

In addition, for the outer tube, a ferrite phase having the following wt% (hereinafter simply referred to as %) composition, a single phase consisting of a tempered martensite phase, a bainite phase, a ferrite + martensite phase, a ferrite phase + bainite phase, or It is a two-phase alloy.

(A) Cr: 5-25%.

Cr is necessary for improving corrosion resistance, and 5% or more of C
If r is contained, a corrosion rate of 1/3 or less of that of 18-8 stainless steel can be obtained. Ferritic alloys containing 25% or more of Cr- are impractical because brittle phases such as σ phase precipitate during use at high temperatures.

(B) St: 4% or less.

Si is necessary as a deoxidizing agent, and like the aforementioned Cr, it is effective in improving corrosion resistance, but if it is contained in an amount of 4% or more, brittle phases such as σ phase will precipitate during use at high temperatures, so this should be set as an upper limit. shall be.

(C) Mn: 4% or less.

Addition of Mn is necessary as a desulfurization agent and is also effective in improving corrosion resistance, but if it exceeds 4%, it is harmful like Si, so the upper limit is set at 4%.

(D) C: (C content of inner tube material) x (A~4).

C is an unavoidable element in melting the alloy, and unless the C activity is at the same level as the inner tube material, carburization or decarburization will occur in the inner tube material, and the strength of the inner tube material as described above cannot be guaranteed. That is, it is necessary that the C content be within the range of one-fourth to four times the C content in the inner tube material.

(E) Nb: 1% or less.

Although sufficient performance can be obtained without adding Nb, corrosion resistance can be further improved by adding Nb.

However, if it is added in an amount of 1% or more, weldability deteriorates, so it should be kept within 1%.

(F) Mo: 2% or less.

Mo is used to adjust the metal structure and improve weldability, but if added in an amount exceeding 2%, it will significantly impair corrosion resistance, so this is the upper limit.

(G) Y, Ce: 0.1% or less.

Y and Ce improve corrosion resistance, but at 0.1% each
Even if more than the above amount is added, the amount becomes saturated and is meaningless, so this is set as the upper limit.

(H) Aj2: 2% or less.

Al is effective as a deoxidizing agent and improves corrosion resistance, but
Addition of more than 2% promotes precipitation of brittle phases, so the amount should be kept at 2% or less.

(1) Ni: 4% or less.

Ni improves corrosion resistance, but if added too much, a retained austenite phase will occur, so it is necessary to set the upper limit to 4%.

Next, the length of the outer tube should be 0.1 mm or more and 4 pieces or less. That is, if the thickness is less than 0.1 m, there is a high possibility that the outer tube will corrode during use and cease to exist.On the other hand, if the thickness is more than 4 tm, it will impair heat conductivity and is meaningless.

A specific manufacturing example of the product according to the present invention will be described below.

The composition of the test steel used by the present inventors is shown in Table 1 below.

Double pipes according to the present invention and comparative pipes were manufactured by hot extrusion and cold drawing using the test steels described above and the combinations shown in Table 2 below. The double pipe billet was prepared by the method described in Japanese Patent Application No. 122405/1982. The combination of &1liB and K in the comparison tube has a low Cr content in the steel.

1.2m from the outside surface of the pipe in Table 2 as described above
A corrosion test piece of 15 cranes x 25 cranes was cut out with a thickness of 4 m.
1%KzSO+-34%Na2SO4-25%Fe2O
Apply 20 μ/d of the mixed powder of 3 on the entire surface of the test piece, and apply 0.1
5% S (h ・5%02-15% Cot -6al.

A corrosion test was conducted by holding the sample at 700° C. for 100 hours in a N2 atmosphere, and the weight loss after descaling is also shown in Table 2 as the result of the external corrosion test. Regarding adhesion, ◯ means good, and the others are not good.

However, the attached drawing summarizes the above results in terms of the relationship between the amount of chromium in the alloy and the weight loss due to corrosion. It is clear that it is 40rrg/- or less.

In Table 2 mentioned above, the amount of coal ash corrosion at 600°C is small for the case where the inner tube is made of D alloy and the outer tube is made of O alloy, but this is because the outer tube alloy contains 32% Cr. This is due to the fact that the R alloy at the end, which is a single tube material (without an outer tube), also contains a very large amount of 35% Cr. Cr
Alloys with high content must be expensive.

"Effects of the Invention" According to the present invention as described above, a heat exchange tube for boilers using coal, oil, etc. as fuel has both high temperature strength and corrosion resistance, and is relatively low cost and economical. This invention can appropriately provide heavy pipes, and is industrially highly effective.

[Brief explanation of drawings]

The drawing shows an embodiment of the present invention, and is a chart of corrosion test results summarizing the relationship between the Cr content of the outer tube alloy and corrosion loss for manufacturing examples according to the present invention and comparative examples thereof. . Procedural amendment (0 complete) Showa 6bi10. Exclusive Japanese

Claims (2)

[Claims] (1) Consists of an outer tube and an inner tube, and the outer tube material has Cr: 5 to 25.
wt°, Si4wt% or less, Mn4wt% or less, and C is an alloy with a chemical component that is 1/4 to 4 times the inner tube C concentration, and the inner tube material has a 4.2 kgf/mm^2 at its operating temperature.
A double pipe with excellent high-temperature corrosion resistance and high-temperature strength, which is characterized by being made of an alloy that has an allowable stress of the above. (2) The outer tube material contains Mo2wt% or less, Al2wt% or less, Ni4wt% or less, Y0.1wt% or less, Ce0.
The double pipe with excellent high-temperature corrosion resistance and high-temperature strength according to claim 1, which also contains 1 wt% or less of any one or two or more kinds.