JPS6333549A - Austenitic steel tube for boiler having resistance to corrosion by coal ash and its manufacture - Google Patents

Abstract

PURPOSE:To obtain a steel tube for a boiler having superior resistance to corrosion by coal ash by subjecting an austenitic steel tube contg. specified amounts of Cr and Ni to soln. heat treatment, removing the surface layer of the tube and hardening the tube. CONSTITUTION:An austenitic steel tube for a boiler consisting of, by weight, 15-26% Cr, 10-35% Ni, one or more among <=0.15% C, <=2% Si, <=7% Mn, <=3% Mo, <=4.5% W, <=4% Cu, <=1% Ti, <=1% Nb, <=0.5% V, <=0.005% B, <=0.25% N and <=2% Al and the balance Fe with ordinary permitted impurities is manufactured. The tube is subjected to soln. heat treatment, the surface of the tube is removed by 2-15mum and the tube is hardened at 600 deg.C to form a hardened layer showing >=280 hardness Hv at 20mum depth from the surface and having >220 hardness Hv in the range from the surface to <=1mm depth.

Description

[Detailed Description of the Invention] "Object of the Invention" The present invention relates to a coal ash corrosion-resistant austenitic steel pipe for a boiler and a method for manufacturing the same, and aims to effectively improve the corrosion resistance of the boiler steel pipe against coal combustion ash and extend its durability. That is.

Industrial applications Technology to improve coal ash corrosion resistance of boiler steel pipes.

Conventional technology The development of boiler tubes that are resistant to corrosion in a coal ash corrosive environment has been developed, for example, by C.
Development of materials with high r content and coating with high Cr materials (double pipes) have been announced, and in Japanese Patent Publication No. 57-4706, penetration treatments such as chromized treatment have been announced.

All of these measures were mainly determined through laboratory experiments simulating a corrosive environment.

Problems to be Solved by the Invention However, although all of these countermeasures were judged to be effective based on "extremely severe laboratory test results," it is expected that this standard is not necessarily valid. In the corrosive environment that is expected to occur in Japan in the future, effective and feasible countermeasures that take economic efficiency into consideration are impossible because boilers in such conditions do not currently exist in Japan. It goes without saying that steel pipes that meet these conditions are preferable.

"Structure of the invention" Means for solving the problem 1, Cr: 15-26 wt%, Ni: 10-3
With 5ivt% as the basic component and other alloying elements,
C: 0.15wt% or less, 5R=2t% or less, M
n: 7wt% or less, Mo: 3wt% or less, W:
4.5 wt% or less Cu: 4wt% or less, Ti: 1
wt% or less, Nb: 1wt% or less, V: 0.5
A boiler that allows impurities normally contained in steel materials containing one or more of the following: wt%t% or less B: 0.005 wt%t% or less: 0.25wt% or less Al: 2wt% or less In austenitic steel pipes for industrial use, Hv near the outer surface is 20 μm from the outer surface.
An austenitic steel pipe for use in a coal ash corrosion-resistant boiler, which exhibits a hardness of 280 or more and has a worked layer in which the range of hardness exceeding Hv220 is 1 dragon or less from the outer surface.

2. Cr: 15-26wt%, Ni: 10-3
5wt% as a basic component, and other alloying elements as C
: 0.15wt% or less, Si: 2wt% or less, Mn
: 7wt% or less, Mo: 3wt% or less, W: 4
．． 5 wt% or less Cu: 4wt% or less, Ti: 1w
t% or less, Nb: 1wt% or less, V: 0.5w
t% t% or less B: 0,005 wt% t% or less:
In austenitic steel pipes for boilers that allow impurities normally included in steel materials containing one or more of the following: 0.25 wt% or less, Al: 2 wt% or less, the surface after solution treatment is reduced to 2 to 15 μm. After removing 6
A method for manufacturing an austenitic steel pipe for a coal ash corrosion-resistant boiler, which is characterized by hardening at a temperature of 00°C or lower.

By containing 15 wt% or more of functional Cr, the effect of processing near the surface can be appropriately obtained. Also, 10-t% Ni
By containing the above amount, significant precipitation of a brittle layer during high temperature use can be avoided.

C 0.15wt% or less, Si 2wt% or less, Mn 7wt% or less, Mo 3wt% or less, W 4.5wt
%t% or less U is 4wt% or less, Ti is 1wt% or less, N
b is 1wt% or less, ■ is 0.5-t% or less, B is 0.0
05 wt% t% or less 0.25wt% or less, Al 2
By containing Cr at 26 wt% or less and Ni at 35-t% or less, economical efficiency is ensured without significantly affecting corrosion resistance.

By making the surface of the steel pipe 20 μm or more Hv280 or more, it can be industrially easily processed and its durability under coal ash corrosion conditions is improved. In addition, Hv22 from the outer surface to 1■-
By making the steel pipe have a hardness exceeding 0, stable high-temperature properties of the steel pipe can be ensured, and processing from the outer surface can be easily performed. The present invention was developed through repeated research and exploration of a steel tube for boilers that has corrosion resistance that is effective in the environment.In other words, the present invention involves processing the vicinity of the outer surface of an austenitic steel tube for boilers to improve corrosion resistance during use in a boiler. niC
Cr due to the diffusion of r? It forms a 5-oxide film and imparts corrosion resistance to coal ash, and a more specific explanation will be given below.

First, to explain the raw pipe before processing, the object of the present invention is limited to austenitic steel pipes. Ferritic steel pipes are not used in high-temperature areas, so significant corrosion is unlikely to occur. Further, the reasons for limiting the alloying elements in terms of wt% (hereinafter simply referred to as %) are as follows.

First, the amount of Cr was limited to 15 to 26%. The effect of processing has been confirmed even when the Cr content is about 12%, but since the Cr content of current practical heat-resistant steels is 15% or more, this was set as the lower limit. Although the upper limit was set at 26%, it is also effective when the Cr1l is higher than this, but as the amount of Cr increases, even unprocessed materials have sufficient corrosion resistance, and the effect becomes smaller. In addition, in order to stabilize the austenite phase, it is necessary to significantly increase the amount of Ni, resulting in an extremely expensive product.

Depending on the content of Cr and other ferrite-forming elements, the amount of Ni should be set at a lower limit to ensure that the solution treatment results in a substantially austenitic phase structure and that significant precipitation of brittle phases does not occur during use at high temperatures. Define the value. That is, the upper limit is the amount at which the effect is saturated, but the amount of Cr Mo+ Tit N
Even when considering the addition amount of ferrite-forming elements such as b and the precipitation of an embrittled layer containing these, it is sufficient that the content is about 35%, and the lower limit was set at 10%.

C, Si, Mnt Mo+ W, Cu, Ti,
Nb, V, B, N.

Alloying elements such as A1 do not have a large effect on corrosion resistance even when added to the upper limit of what is normally included in heat-resistant steel.
c<o, is%, Si 2%, Mn 7%, respectively.
Mo: 3%, W<4.5%, Cu:<4%, Ti<1%
, Nb 1%, V 0.5%, B<0.005%
,N<,0.25%,Aj! :<2%.

Next, the processing method will be explained as follows.

That is, the steel pipe is intended for use in boilers with a wall thickness of about 3 to 150 mm, and the outer diameter is 7011 mm or less, and the pipe is usually used in a solution-treated state, but the steel pipe of the present invention has a cold-treated surface. Perform processing. A minimum processing layer depth of about several tens of micrometers is sufficient, but it is also necessary to take into consideration the case where the Cr-enriched coating formed during use is destroyed and regenerated, so a certain depth is preferable. On the other hand, the mechanical properties of the tube at high temperatures may deteriorate due to low-temperature processing, and from this point of view it is necessary to keep the mechanical properties below a certain limit. Steel pipes for boilers are usually 1
Since it is manufactured by increasing the wall thickness by about 1 m, it is preferable to keep it to less than this.

Additionally, the processing applied to this steel pipe is applied from the outside surface.

Therefore, the degree of processing in the thickness direction is highest on the outer surface and decreases toward the inside. The degree of processing was determined by using the hardness at a position 20 μm from the surface, which was determined based on the fact that this is the range within which Cr concentrates to form a film. Furthermore, since measurement using a hardness meter is industrially difficult at a thickness of 20 μm or less, the value at this position was used as a guideline. By setting the Vickers hardness at this position to 280 or more, the scale thickness becomes 1/2 as shown in the attached drawing, so this value is used as a guide. The drawings show the influence of hardness on the ratio of scale thickness, and were measured by changing the shot processing conditions to produce steel pipes with various degrees of processing. It can be seen that the thickness of the scale decreases significantly after .

Furthermore, even if the hardness at a position 20 μm from the surface is the same, the corrosion resistance will differ slightly depending on the processing method. That is, grinding is extremely effective, and this is thought to be because this processing method can increase the degree of processing of the extreme surface layer, and also because the surface layer, which has a slightly different composition from the inner part of the wall, is removed.

The processing temperature must be below the recovery temperature of the material, and this temperature varies somewhat depending on the degree of processing and the material, but in the case of austenitic steel, it can be considered to be about 600°C.

In addition, the influence of the surface condition of the material before processing is also important. In the case of a processing method that does not involve grinding, such as shot processing, it is effective to remove the surface layer during heat treatment in advance and then apply processing. One method for removing this surface layer is, for example, strong pickling; in this case, removal of 2 .mu.m is insufficient, and 15 .mu.m or more is not necessary. It is also effective to perform processing after solution treatment using a method that causes less change in composition near the surface, such as heat treatment in a non-oxidizing atmosphere.

In addition, shot processing is performed after grinder processing,
It is also possible to eliminate the disadvantages of grinding, such as removing tensile residual stress.

Note that the working degree at which an adverse effect begins to appear on the high-temperature characteristics of these steel pipes is Hv220 in terms of hardness, so the range exceeding this is limited to 1 fi or less from the outer surface.

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

The composition, processing method, hardness distribution, and scale thickness related to corrosion resistance of the steel pipes specifically produced according to the present invention are shown in the following table. It is processed.

Corrosion resistance was confirmed by inserting these processed tubes into a coal combustion atmosphere, and the tubes were air-cooled from the inside to reproduce the corroded state of boiler tubes. Also, the thickness of the scale is 3
．． This value is obtained when 5% S is burned for 1000 hours.

If a scale of 20 μm occurs during this time, it will result in corrosion of 10 μm, and after 100,000 hours it will become a single piece, which is likely to cause problems in use. Even in the case of 10 μm or less in this experiment, this value is thought to increase significantly if coal with more impurities is burned.

In addition, in the table, the abbreviations are as follows.

Unprocessed Dioxide atmosphere Solution conversion Normal pickling grinder Two surface grinder processing, 3S shot:
Shot blasting (steel, glass beads, sand) Cold drawn pipe: Surface processing with light drawn pipe pickling + Shit double strong pickling (removes the surface uniformly by about 10 μm) + shot processing H2 solution + shot: Solution treatment + shot processing in a hydrogen atmosphere "Effects of the invention" As explained above, according to the present invention, surface cold-worked steel pipes have 3.5 It can be used for 100,000 hours, which is the lifespan of the plant, as a steel pipe for the high-temperature parts of boilers that use coal containing up to 10% S.
In the case of steel pipes, even lower grade coal can be used.
This invention is industrially highly effective because it can exhibit economically preferable corrosion resistance.

[Brief explanation of drawings]

The drawings show the technical content of the present invention, and show the ratio of scale thickness of shot processed (cold worked) material and solution treated material during coal ash corrosion, and the difference of 20μ from the pipe surface.
It is a chart showing the relationship with hardness at the position of m.

Claims (1)

[Claims] 1. Cr: 15 to 26 wt%, Ni: 10 to 35 wt%
as the basic component, and as other alloying elements, C: 0.1
5wt% or less, Si: 2wt% or less, Mn: 7wt% or less, Mo: 3wt% or less, W: 4.5wt% or less, Cu
: 4wt% or less, Ti: 1wt% or less, Nb: 1wt%
Impurities normally contained in steel materials containing one or more of the following: V: 0.5wt% or less, B: 0.005wt% or less, N: 0.25wt% or less, Al: 2wt% or less. In austenitic steel pipes for boilers that allow
An austenitic steel pipe for a coal ash corrosion resistant boiler, which exhibits a hardness of 280 or more and has a worked layer in which the range of hardness exceeding Hv220 is 1 mm or less from the outer surface. 2. Cr: 15-26 wt%, Ni: 10-35 wt%
as the basic component, and as other alloying elements, C: 0.1
5wt% or less, Si: 2wt% or less, Mn: 7wt% or less, Mo: 3wt% or less, W: 4.5wt% or less, Cu
: 4wt% or less, Ti: 1wt% or less, Nb: 1wt%
Impurities normally contained in steel materials containing one or more of the following: V: 0.5wt% or less, B: 0.005wt% or less, N: 0.25wt% or less, Al: 2wt% or less. A method for manufacturing a coal ash corrosion resistant austenitic steel pipe for boilers, which comprises removing 2 to 15 μm of the surface after solution treatment, and then hardening the pipe at 600° C. or lower. 3. The method for manufacturing an austenitic steel pipe for a coal ash corrosion-resistant boiler according to claim 2, wherein the solution treatment is performed in a non-oxidizing atmosphere.