JPS6264493A - Welded structure for high temperature service - Google Patents

Abstract

PURPOSE:To obtain a welded structure for high temp. service having weldability and strength and ductility at a high temp. by joining part of an austenitic stainless steel to the other member by a deposited metal having a specific compsn. CONSTITUTION:Part of the austenitic stainless steel contg. chromium, nickel and nitrogen is joined to the other member by the deposited metal consisting, by wt%, of 0.02-0.12% carbon, 0.1-1.5% silicon, 0.2-2.0% manganese, 0.11-0.3% nitrogen, 8-14% nickel, 15-20% chromium, 0.01-0.3% vanadium and the balance iron. The welded structure for high temp. service having the excellent mechanical properties at a room temp. and high temp. is thus obtd.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a high-temperature welded structure having a welded metal with excellent heat resistance, particularly as a turbine component used under ultra-high pressure and high temperature steam conditions. Related to high-temperature welded structures with improved creep rupture strength at high temperatures.

Technical background of the invention and its problems] Austenitic steel has excellent corrosion resistance and is therefore often used in corrosive environments. In addition, ferritic steel (;
In comparison, the temperature dependence of mechanical properties is smaller, so it is said that the strength limit temperature can be higher than that of ferritic steel. Furthermore, austenitic steels have poor weldability and do not require preheating or postheating during welding, so their uses tend to expand (2). If this happens, there is a risk of weld cracking (hot cracking) occurring in the weld metal and the heat-affected zone.This will be affected by the welding method of austenitic steel, the type of coating, the shape of the joint, the degree of restraint, etc. The weld metal has a uniform austenite phase (
It becomes easy to cause a crack with the second person. Preventing weld cracking in austenitic steel weld metal is effective by changing the composition or adding certain elements. Cracks are extremely rare. Conventionally, welding of austenitic steel was performed using 201Cr-10%Ni, 18%Ni, or
%Cr-12%Ni-2%MO system, 19%Cr-9 N
Using austenitic steel core wires such as 1-Nb and 19% Cr-9Ni-Ti, coated arc welding, submerged arc welding, inert gas arc welding (MIG and TIG), etc. are used. Weld metal obtained by such a conventional method contains 4 to 12 ferrite in its structure to prevent weld cracking. However, several atoms of ferrite contained in austenite cause the welded part to become brittle. In addition, weld metal containing ferrite is
Thermal fatigue is accelerated by repeated heating and cooling, and creep rupture strength and creep rupture elongation at high temperatures and long periods of time are lower than those of the base material. Therefore, the weld metal of an austenitic steel welded structure used at high temperatures must have a uniform austenitic structure that does not contain ferrite, and must not cause weld cracking.

By the way, in order to improve thermal efficiency in thermal power plants that use oil or coal as fuel, steam conditions are becoming higher and higher, and the turbine components used in these plants are also required to comply with these requirements.

However, martensitic steels such as CrΔ(011 steel), which have been conventionally used as turbine components, lack strength at high temperatures, and a transition to austenitic steels (2), which have better high-temperature properties, is underway. Since the welded casing is subjected to double loads due to steam pressure, it must be joined with welded metal that has high strength and ductility at high temperatures and does not cause weld cracking when used under high pressure steam conditions. In addition, welded structures that can withstand high temperatures are needed in chemical plants (for the same reason, the usage conditions are becoming harsher, and they are strong enough to withstand high temperature and high pressure environments). is requested.

Despite the changes in the usage environment of welded structures, existing austenitic steel weld metals are used at high temperatures (2), which may cause sigma embrittlement or reduce creep rupture strength and ductility. There is a strong demand for the development of a welded structure for high temperatures that has a fully austenitic structure that does not contain ferrite, which is the cause of the welding process, and has excellent weldability and heat resistance.

[Purpose of the invention]

The present invention has been made in view of these points, and it is an object of the present invention to provide a welded structure for high temperatures that has excellent weldability, strength and ductility at high temperatures.

[Summary of the invention]

In the present invention, a part of the austenitic steel containing chromium, nickel and nitrogen is 0.02 to 0 in weight percent.
．． ．． 12 tons of carbon, 0.1-15% silicon,
Mangafu of 02-2.0%, 0.11-0.3
% nitrogen, 8-14cs nickel l 15-20%
Black A, 0.01-0. ．． 3% vanadium, plus 0.01-0.3% noniobium as necessary.
0002~0.1'160) fig y, 0.0
005~0. ．． 01% (D holo y, 1.0~3.
A high-temperature welded structure in which two other members (2) are joined by a weld metal consisting of at least one type of tungsten and the remainder substantially iron, and (%Ni) of the weld metal +3
0X(*C)+25.7X(4N)+05X(%Mn)
Ni equivalent expressed as C%Cr)+12X(
%s i )+5x(95%')-+~0.5x(%N
b)+0.75 mowing%W)+1.5X(%Ti)+40
The Cr equivalent represented by x(1B) is 019×Cr equivalent+Ni fi amount≧30%m 1. l x Cr equivalent - Ni
This is a high-temperature welded structure that satisfies the relationship of equivalent ≦8B% and has a Ni equivalent of 20% or less. (2) The austenitic steel has a weight percent ifi of 0.04-008, 0.3→, 7% silicon, 05-09% manganese, 0.11-0B (7) nitrogen, 8.5 ~13
5 inches of nickel, 16 to 185 inches of chromium.

0.05-035% vanadium, and O◇2-o,
is*'s second off, 0.02~0.151 t7)
Friy, 0.003~On07% noho.

N, 1~. The turbine component is a high-temperature welded structure consisting of at least one type of 8% tungsten and the remainder substantially iron.

Here, the reasons for limiting the composition of the weld metal according to the second aspect of the present invention will be explained. Carbon (C): Carbon stabilizes the austenite phase and strengthens the weld metal (2. Reduction of weld cracking 1;
It is an effective element and requires at least 0.02%, but if added too much, it will cause segregation during welding and deteriorate mechanical properties and V corrosion resistance, so the upper limit is set at 0.12%.Creep From the viewpoints of breaking strength, creep rupture elongation, and reduction of area, it is desirable that the content be more than 0.04% and less than 0.08%.Silicon (Si): Silicon is an element that acts as a deoxidizing agent during welding and has at least 0. .1% is necessary, but if it is added too much, weld cracking is likely to occur, so the upper limit is set at t15q6.It is preferably 0.1 to 09%, but more preferably 03 to 0%. .7
It is better to set it as %.

Manganese (Mn): Like silicon, manganese acts as a deoxidizing agent during welding (;, as an austenite-forming element, at least 0.2% is required to stabilize the austenite phase. However, if added in large amounts, The upper limit is set to 2.0 because it damages corrosion resistance such as oxidation resistance.
%, preferably 05 to 19 inches.

Nitrogen (N): Nitrogen stabilizes the austenite phase (: Nitrogen stabilizes the austenite phase, and also forms a solid solution in the austenite or forms nitrides to improve the yield strength and creep rupture strength of the weld metal (
20.11% is necessary. However, if it is added, pinholes and blowholes will be formed during welding, and nitrides will be formed at the grain boundaries, which will impair creep rupture strength, creep rupture elongation, narrowing, and roughness. Especially in welded structures, pinholes and blowholes should be avoided as much as possible, preferably 0.11.
It is preferable to set it to 0.02%.

Nickel (Ni): Nickel is an essential element to improve the structure of the weld metal into austenite (2) and improve corrosion resistance and weldability, and at least 8% is required.

However, if excessively added, it tends to promote weld cracking, so the upper limit is set at 14. From the viewpoint of stabilizing austenite, creep rupture strength, creep rupture elongation, and reduction of area, the content is desirably 8.5 to 13%. Chromium (Cr) Nichrome at room temperature.

1515 is necessary to increase the strength at high temperatures, as well as to improve corrosion resistance and oxidation resistance, but if a large amount (2) is added, a sigma phase will be generated when used at high temperatures for a long time, reducing toughness. The upper limit is set to 22% because it forms a ferrite phase and makes it difficult to obtain a single austenite phase.Considering the balance with the amount of nickel and the ease of nitrogen content, it is desirable to set it at 16 to 19%.

Vanadium (V): Vanadium is a particularly important element in the present invention, and can form a solid solution in the austenite phase or interact with nitrogen and carbon to form fine precipitates to improve creep rupture strength and creep rupture elongation reduction. To make l:, 0.
0.1% or more is required. The upper limit is set at 0.3% since adding too much will cause segregation and reduce creep rupture strength, creep rupture elongation, and area of area. Considering mechanical properties at high temperatures, it is desirable to set the content to 0.03 to 0.3%;
It is desirable to set it to 0.25%.

Niobium (Nb): Niobium needs to be added in an amount of 20% or more to improve creep rupture strength and suppress secondary creep rate/rate (20. The upper limit is set at 0.3% because it causes segregation during welding and reduces toughness, creep rupture strength, creep rupture elongation, and area of area. Considering segregation and high-temperature properties, it is 0.02 to 0.15. %, more preferably 0.05 to 0.11.

Titanium (Ti): In order to improve creep rupture strength, it is necessary to add titanium in an amount of 0.002% or more, but excessive addition causes segregation and decreases creep rupture elongation and area of area, so the upper limit is set at 0.03%. do. From the viewpoint of high-temperature properties, it is desirable to set it to 0.02 to o, iss.

Boron (B): Boron improves creep rupture strength and elongation in tertiary creep (secondary creep).
, oooss or more is required, but since adding too much makes the grain boundaries brittle, the upper limit is set at 0.01. More preferably, it is 0.003 to 0.007.

Tungsten (W): Tungsten forms a mini-solid solution in the austenite phase to improve creep rupture strength;
It is necessary to add 10% or more of I, but excessive I: Adding impairs weldability, so the upper limit is set to 3.0 I. It is preferably 15 to 25%.

Next, the relationship between Ni equivalent and Cr equivalent is 0.79×C
r equivalent + Ni quantity≧30% t1. l x Cr equivalent -
The reason why 1≦8B% based on Ni and the Ni equivalent is set to 20q6 or less will be explained.

0.79 x Cr equivalent + Ni equivalent ≧ 30 was determined by o,
When r9xCr equivalentffi+Ni equivalent is less than 30%, creep rupture strength decreases, and when 1, 1xCr equivalent - N
The reason why i saving amount ≦ 8B% is because if l, 1x Cr equivalent - Ni equivalent exceeds 8B, biferrite (biferrite) will be generated in the weld metal and a uniform austenite structure will not be obtained. The reason for the following is that if the Ni equivalent exceeds 20%, the creep rupture strength decreases significantly.

The remainder essentially consists of iron, but the unavoidable impurities include P, 8. AI! and so on. Since these elements make grain boundaries brittle and promote weld cracking, it is necessary to reduce the content as much as possible, and preferably the total amount of impurities is 0.05% or less. In addition, by adjusting the additive components of the weld metal, the welded structure for high temperature use of the present invention has a uniform austenitic structure that could not be obtained with conventional weld metals, and exhibits excellent high-temperature properties. 0.04 to 0.08 inches by weight percent
carbon, 0.3-0.7% 7licon, 05-09% manganese, 0.11-02% nitrogen, 85-135%
nickel, 16-185% chromium, 0.05
~035 Ti vanadium and 0.02-0.15% Niobium, 0.02-0.15% Titanium, 0.003
~0.007% boron, 1~. room temperature by using maestenitic steels (e.g., at least 8% of tungsten and the remainder made of substantially iron).

A high temperature welded structure having excellent mechanical properties at 4 temperatures can be obtained.

[Embodiments of the invention]

Steel melted in a high frequency induction melting furnace was wire drawn into a welding rod with a rod diameter of 4 mm, and then overlay welding was performed to produce a deposited metal having the composition shown in Table 8g1. Overlay welding was done with a width of 20mm on the base metal (SUS304).

In a groove 20 leaps deep (21 passes were carried out in 7 layers at a welding current of 150 to 170 people and a welding pressure of 25 to 3 SV.
Chemical analysis of the deposited metal was carried out in areas where there was no dilution from the base metal.

Comparative example 1 is for commercially available welding rod Y30BCBUS304)
This corresponds to the weld metal obtained in , and was produced in the same manner as the example sample.

A creep rupture test was conducted on two of these weld metals to evaluate each property (2. The dominant phase was determined from X-ray diffraction C2. Creep rupture test): J700
The test was carried out under the conditions of υ, 1 bleaching I, and the creep abrasion strength was evaluated by the time to rupture. Further, weld cracking was evaluated by performing a fillet weld cracking test using a T-shaped stitch (T-shaped weld cracking test) and determining the cracking rate. These results are shown in Table 2.

3. The weld metal of the present invention has a uniform austenite <r that does not contain ferrite (b) in its structure.
) structure, it shows the same cracking rate as the conventional 5 (J83Q4 weld metal (comparative example-1)), which prevents weld cracking by containing ferrite, and is less likely to cause weld cracking. It can also be seen that the creep rupture time is significantly longer than that of the comparative sample, indicating that it exhibits excellent high-temperature strength.

Although two weld metals have been described as examples above, one example of the strength study of a welded structure in which structural steel is joined using the weld metals of the present invention will be described next.

The structural steels used to make the welded joints were SO8304 and improved 5US3Q4 austenitic steels having the compositions shown in Table 3 (2).The weld metals used were the example sample ND4+mlO of the present invention and the comparative example sample, which was a conventional weld metal. Minami 1 was used.The welded joint was made by butt welding the same type of structural steel with a plate thickness of 20mm using an X-shaped groove with a groove angle of 60°, a root spacing of 2 degrees, and an X-shaped groove.

A test piece was taken from the welded joint obtained in this way,
A creep rupture test was conducted at 700°C and 18 squares. The results are shown in Table 4.

Table 3 Table 4 Table 4 As shown in Table 2, welded joints made from the present invention (welded metal and structural steel improvement 5U8304 T2 according to the present invention) were made using the conventional method (Conventional Example-1) l2. Welded joint I: Comparative creep rupture time is significantly longer (2).Also, as shown in Comparative Examples 4 and 6 (2), even if the weld metal according to the present invention is used as the weld metal 4, it will not work as a structural steel. When conventional 8US304 is used, the fracture location is in the structural steel, indicating that the strength of the ta weld metal is high.Furthermore, it can be seen that the improved 5US304 is strong as a structural steel.

Thus, it can be seen that the welded joint produced according to the present invention 62 has superior high-temperature strength than the welded joint produced by the conventional method, and is effective as a welded structure for high temperatures.

(Effects of the Invention) As explained above, the high-temperature welded structure of the present invention has excellent high-temperature properties, and is therefore suitable for use in turbine components, especially casings such as steam turbines (the need for two agents is important from the viewpoint of power generation efficiency, lifespan, etc.). It is extremely useful industrially.

Claims (1)

[Claims] 1) A portion of the austenitic steel containing chromium, nickel and nitrogen is 0.02 to 0.1 in weight percent.
2% carbon, 0.1-1.5% silicon, 0.2-2
．． 0% manganese, 0.11-0.3% nitrogen, 8-1
4% nickel, 15-20% chromium, 0.01-0
．． A welded structure for high temperature use, characterized in that it is joined to other members by a deposited metal consisting of 3% vanadium and the remainder substantially iron. 2) A part of the austenitic steel containing chromium, nickel and nitrogen is 0.79×Cr equivalent + Ni equivalent ≧3
0%, 1.1 x Cr equivalent - Ni equivalent ≦ 8.8%, and is joined to other members by a weld metal whose Ni equivalent is 20% or less. A welded structure for high temperatures according to scope 1. 3) Ni equivalent is (%Ni) + 30 x (%C) + 25.7
x (%N) + 0.5 x (%Mn), and the Cr equivalent is expressed as (%Cr) + 1.2 x (%Si) + 5 x (%V). A welded structure for high temperature use according to scope 2. 4) Austenitic steel containing chromium, nickel and nitrogen with weight percentages of 0.04-0.08% carbon, 0.3-0.7% silicon, 0.5-0.9%
of manganese, 0.11-0.2% nitrogen, 8.5-13%
．． 5% nickel, 16-18.5% chromium, 0.0
3. A welded structure for high temperature use according to claim 1 or 2, characterized in that the welded structure is comprised of 5 to 0.35% vanadium and the remainder substantially iron. 5) The high-temperature welded structure according to claim 4, which is a turbine component. 6) The high temperature welded structure according to claim 5, wherein the turbine component is a casing. 7) Some of the austenitic steels containing chromium, nickel and nitrogen have a weight percentage of 0.02 to 0.1
2% carbon, 0.1-1.5% silicon, 0.2-2
．． 0% manganese, 0.11-0.3% nitrogen, 8-1
4% nickel, 15-20% chromium, 0.01-0
．． 3% vanadium, and 0.01-0.3% niobium, 0.002-0.3% titanium, 0.0005-0.
1. A welded structure for high temperature use, characterized in that the welded structure is joined to another member by a weld metal consisting essentially of 0.1% boron, 1.0% to 3.0% tungsten, and the remainder substantially iron. 8) A part of the austenitic steel containing chromium, nickel and nitrogen is 0.79×Cr equivalent + Ni equivalent ≧3
0%, 1.1 x Cr equivalent - Ni equivalent ≦ 8.8%, and is joined to other members by a weld metal whose Ni equivalent is 20% or less. A welded structure for high temperature use according to scope 7. 9) Ni equivalent is (%Ni) + 30 x (%C) + 25.7
It is expressed as ×(%N)+0.5×(%Mn), and the Cr equivalent is (%Cr)+1.2×(%Si)+5×(%V)+0
．． 5×(%Nb)+0.75×(%W)+1.5×(%
9. The welded structure for high temperature use according to claim 8, characterized in that it is expressed as Ti)+40×(%B). 10) Austenitic steel containing chromium, nickel and nitrogen in weight percentages of 0.04-0.08%
of carbon, 0.3-0.7% silicon, 0.5-0.9
% manganese, 0.11-0.2% nitrogen, 8.5-1
3.5% nickel, 16-18.5% chromium, 0.
05-0.35% vanadium, and 0.02-0.
15% niobium, 0.02-0.15% titanium, 0.
The high-temperature welded structure according to claim 7 or 8, characterized in that the welded structure consists of at least one of 003 to 0.007% boron, 1 to 3% tungsten, and the balance substantially iron. . 11) The high temperature welded structure according to claim 10, which is a turbine component. 12) The high-temperature welded structure according to claim 11, wherein the turbine component is a casing.