JPS6268693A - Welded structure for high temperature service - Google Patents

Abstract

PURPOSE:To provide a welded structure having excellent strength and ductility in high-temp. and high-pressure environment and to prevent weld crack by welding an austenitic steel contg. Cr, Ni and N with a deposited metal having a specific compsn. CONSTITUTION:The deposited metal is composed, by weight, of >=0.02%- <=0.12% C, >=0.1%-<=1.5% Si, >=0.2%-<=2.0% Mn, >=0.11%-<=0.3% N, >=8%-<=14% Ni, >=15%-<=20% Cr, >=0.01%-<=0.3% V, >2.0%-<=3.5% Mo as essential components and the balance Fe. The austenitic steel is welded by using such deposited metal and the structure which has the durability in terms of strength even at and under the high temp. and high pressure is obtd.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a high-temperature welded structure made by welding ten-stenitic steel with a welded metal having excellent heat resistance, and more specifically relates to a welded structure for ultra-high pressure and high temperature. The present invention relates to a high-temperature welded structure useful as a turbine component used in a water-legal atmosphere.

[Technical background of the invention and its problems] Austenitic steel has excellent corrosion resistance and is therefore widely used in corrosive environments. Also, this steel.

Since the temperature dependence of mechanical properties is smaller than that of ferritic steel, its service temperature limit is high from the viewpoint of strength. That is, the high temperature properties are good.

Furthermore, this steel has excellent weldability and does not require preheating or postheating during welding, so its use as a material for various welded structures is expanding.

However, when thick plates or highly constrained joints are welded to this austenitic steel, weld cracks (high temperature cracks) are likely to occur in the weld zone and heat affected zone made of the weld metal used. This welding crack is caused by the chemistry between the welding rod (welding metal) and the base metal! It is known that cracks occur frequently, especially when the structure of the weld metal becomes a uniform austenite phase, which is affected by factors such as the welding process, welding method, type of coating material, shape of the 1m hand, degree of restraint, etc. There is. In order to prevent cracking of the deposited metal during welding of this austenitic steel, the composition is changed or certain elements are added to precipitate a ferrite phase. This is because weld cracking is extremely rare in the case of a two-phase structure in which a ferrite phase is precipitated.

In other words, conventionally, austenitic steel has been welded using a 20% Cr-10% Nl system, depending on the type of base metal.

18%Cr-12%Ni-2%Ha system, 18%Cr-9
%Ni-Nb system.

Methods such as coated arc welding, submerged arc welding, and inert gas arc welding (formerly G and TIG) using austenitic steel core wires such as 19%Cr-9%Ni-Ti have been applied; This is because 4 to 12% of ferrite phase is precipitated in the structure of the deposited metal, thereby preventing weld cracking.

However, a few percent of the ferrite phase precipitated in the austenite phase reduces the corrosion resistance of the weld metal, and
During use at high temperatures, it changes to a sigma phase, leading to embrittlement of the weld. Moreover, thermal fatigue of weld metal containing a ferrite phase is accelerated by repeated heating and cooling, and properties such as creep rupture strength and creep rupture elongation during long-term use at high temperatures are lower than those of the base metal. do.

For this reason, the weld metal used in the manufacture of austenitic steel welded structures used at high temperatures is a uniform austenite phase that does not contain a ferrite phase.
In addition, it is required that the material does not cause weld cracking.

By the way, in order to improve the thermal efficiency of thermal power plants that use petroleum or coal as fuel, steam conditions are being made higher and higher, and there is a strong need to develop materials for the construction of Tahin that can handle this. It is requested.

Conventionally, such members include Cr-Mo-V
Martensitic steels such as Qf4 have been widely used, but this material has insufficient high-temperature strength, so recently austenitic steels, which have better high-temperature strength characteristics, have begun to be used.

In particular, the casing, which is a welded structure, is subjected to loads from the lotus pressure, so it must have sufficient strength under high temperature conditions in order to withstand use under high-pressure air washing conditions.

It is desirable to weld with a deposited metal that has excellent ductility and does not cause weld cracking.

Furthermore, in various chemical plants, welded structures that are strong and resistant even in high temperature and high pressure environments are required for the same reasons as above.

As the usage environment of welded structures is becoming more severe, welded metals are made of a completely austenite phase that does not contain the ferrite phase described above, and have excellent weldability and thermal properties. There is a need for the development of welded structures for high temperatures.

[Object of the Invention] The present invention meets the requirements set forth in (-) and aims to provide a welded structure for high temperature use that is excellent in weldability and strength permeability at high temperatures.

[Summary of the Invention] The welded structure of the present invention comprises an austenitic steel containing Cr, Ni and N, and a C content of 0.1% or more.
2% by weight or less, Si 0.1% by weight or less, 21.5% by weight or less, Mn 0.2% by weight or more and 2.0% by weight or less,
No, 11 hair 1% or more and 0.3% by weight or less, Ni
8 weight% or more and 14 weight φ% or less, Cr15 weight% or more and 20
East arrival% or less, VO,01 weight 4% or more 0.3 weight%
Hereinafter, it is characterized by being a welded body with a deposited metal in which O2.01% to 3.5% by weight is an essential component and the remainder is substantially Fe.

The structure of the present invention is made by welding austenitic steel containing Cr, Ni, and N with a deposited metal having the above-mentioned composition.

In the weld metal according to the present invention, C is an element effective for stabilizing the austenite phase and strengthening the weld metal, as well as reducing weld cracking, and is at least 0.02
ffi lit% is required. However, if it is added in excess, it will cause segregation during welding and deteriorate the mechanical properties and corrosion resistance of the welded part, so the limit is set at 0.12 hair-f1%.

From the viewpoint of increasing creep rupture strength, creep rupture elongation, and reduction of area, 0.04% by weight or more 0.0
It is desirable to make it 8 heavy engineering type or less.

Si is an element that acts as a deoxidizing agent during welding, and at least 13% is required. However, if added in excess, it tends to cause welding cracks, so the 1-limit is set at 1.5%. shall be. The content is preferably 0.1 to 0.9% by weight, more preferably 0.3 to 0.7% by weight.
Weight%.

Mn acts as a deoxidizing agent during welding, and is an effective element for stabilizing the austenite phase as an austenite phase forming element.
) Hair i% is required. However, if added in large amounts, corrosion resistance such as oxidation resistance will be impaired, so the upper limit is set at 2.0% by weight. Desirably it is (1.5 to 0.9 tonne stitch%).

N is an effective element for stabilizing the austenite phase and improving the yield strength and creep rupture strength of the molten metal by forming a solid solution in the ten-stenite phase or forming a nitride. 0.11!1! Amount % is required.

However, if added in excess, pinholes and blowholes will be formed during welding, and nitrides will be formed at grain boundaries, impairing creep rupture strength, creep rupture elongation, reduction of area, and even toughness, so the upper limit is set at 0.3 weight. %. Particularly in welded structures, it is necessary to avoid pinholes and blowholes as much as possible, so it is preferable that the amount of hair be 0.11 to 0.2%.

N1 is an essential element for austenitizing the structure of the weld metal and at the same time improving corrosion resistance and weldability, and it is necessary to contain at least 8% by weight. However, excessive addition tends to promote weld cracking, so the upper limit is set at 14% by weight. From the viewpoint of stabilization of austenite phase, creep rupture strength, creep rupture elongation, and reduction of area: 8.5 to 13
It is preferable to use weight percent.

C[ is an element necessary to increase strength at room temperature and high temperature, as well as improve corrosion resistance and oxidation resistance.
A weight of 5 weight or more is required. However, if added in a large amount, a sigma phase will form when used for a long time at high temperatures, impairing toughness, and a ferrite phase will form, making it difficult to obtain a single austenite phase, so the upper limit is set at 20% by weight. . Considering the balance with the amount of Nl and the ease of containing N, it is desirable to set the content to 16 to 18.5% by weight.

(2) is a particularly important element in the present invention, and increases the creep rupture strength by forming a solid solution in the austenite phase or forming fine precipitates by interacting with N and C.

It is an element necessary to improve creep rupture elongation and reduction of area, and is required in an amount of 0.01% or more.

However, if added in excess, it may cause segregation and lower creep rupture strength, creep rupture elongation, and area of area.''The first limit is set at 0.3%. Considering the mechanical properties at high temperatures, it is desirable to set the content to 0.05 to 0.3%, but from the viewpoint of creep rupture reduction, the content is 0.0%.
It is desirable to set it as 5-0.25 m-fi%.

A0 is an element that is effective in improving the creep rupture elongation and reduction of area of weld metal and reducing weld cracking, and at least:
), Q, R>-%.

However, if added in excess, ferrite phase may be generated or segregation may occur, reducing high-temperature properties.
3.5+'[φ% 32.2 to 2.8 hair amount% is desirable. And Fe is a balance component that substantially constitutes the remainder.

In this weld metal, when its chromium equivalent and nickel equivalent ψ are respectively a% and b%, a and b are Q,
79X a+b≧31%, 1.1×a-b≦8.
8%, and b≦20%.

If 0.79 x a + b is smaller than 31%, the creep rupture strength of the weld metal will decrease, and if 1,1 x a - b is larger than 8.8%, a ferrite phase will precipitate in the weld metal. At the beginning of the process, it becomes difficult to obtain a uniform austenite structure. Furthermore, if b exceeds 20%, the creep rupture strength of the weld metal will drop significantly.

In addition, a and b in this case are each compounded component) in relation to each other, a = (%Or) + 1.2X
(%Si) + (5×a) + 5X (%v)
, b= (%Ni) + 30X (%C) + 2
This is a value calculated by 5.7X (%N) + 0.5X (5×a).

The weld metal of the present invention essentially has the following composition, but may also contain Nb, Ti, B, and W. Here, Nb is an effective element for improving creep rupture strength and suppressing the secondary creep rate, and it is desirable to add 0.01% by weight or more, but excessive addition may cause local formation of ferrite phase. The upper limit is set at 0.3% by weight 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, the weight should preferably be 0.02 to 0.15%, more preferably 0.05 to 0.
．． It is 1% by weight.

Ti is an effective element for improving creep rupture strength, and it is desirable to add 0.002% by weight or more, but excessive addition causes segregation and decreases creep rupture elongation and area of area, so the upper limit is set to 0.3% by weight. do. From the viewpoint of high-temperature properties, the content is preferably 0.02 to 0.15% by weight.

It is an effective element for improving creep rupture strength and elongation in tertiary creep.
．． It is desirable to add 0.005% by weight or more, but if added in excess, the grain boundaries become brittle, so the upper limit is set to 0.01% by weight. More preferably 0.003 to 0.007% by weight
It is.

W is an element that dissolves solidly in the austenite phase and is effective for improving creep rupture strength, and it is desirable to add 10% by weight or more, but since adding too much will impair weldability, the upper limit should be 3.0% by weight. %. More preferably 1
．． It is 5 to 2.5%.

Even in this weld metal further containing Nb, Ti, B, and W, its chromium equivalent a%, nickel, j7
Regarding fflb%, it is preferable that the same relationship as described above be satisfied.

In addition, a at that time is a= (%Cr) +
1.2X (%Si) + (%Ao) +
5X (%v) + o, sx (%1f
b) + 1.5X (%Ti) +40X (%
B) +0.75X (% fee.

b = (%Ni) +30X (%C) + 25.
This is a numerical value calculated based on the formula: 7x (%N) + O, S x (%Mo).

P, S, AM, etc. are inevitably mixed into the weld metal as impurities, but since these impurities not only weaken grain boundaries but also promote weld cracking, their mixing should be avoided as much as possible. It is desirable that the total amount of these impurities be suppressed to 0.05% by weight or less.

The ten-stenite steel to be welded in the structure of the present invention has CO, 04 to 0.08 %, and Si 0.3 to 0.
．． 7% by weight, Mno,'i-0,9% by weight, N
O, 11~0.2 iI<:i%, Ni 8.5~1
3.5ff wt%, Cr 18-18.5 wt%.

VO, 05-0.35% by weight, Mo 1.5-3.
0% by weight, and the remainder substantially consists of Fe, or in addition to the above composition, at least one selected from the group of Nb, Ti, B, and W, each containing 0.02 to 0.0% by weight.
15% by weight, 0.02-0.15% by weight, 0.0
03 to 0.007% by weight, and 1 to 3% by weight of 06.

[Embodiments of the Invention] Examples 1 to 18 (1) Preparation of weld metal After wire-drawing the steel melted using a high-frequency induction melting furnace to make a welding rod with a diameter of 4 mmφ, overlay welding was performed. :5
Various weld metals having the compositions shown in Table 1 were prepared. Overlay welding is done using a 20 mm wide and 20 mm deep weld on the base metal (SO3316).
Welding current 150-17OA in groove a+m.

Seven layers and 21 baths were welded at a welding voltage of 25-35V. In addition,
Chemical analysis of the weld metal was carried out by removing the portion that was not diluted from the base metal. Comparative example 1 is a reprinted welding rod Y316
(for SUS316).

Characteristics of each of these weld metals were evaluated, and the main negative phase was determined by X-ray diffraction. Also, 700℃.

A creep rupture test was conducted under the condition of 18 kg/mm2, and the time to break (hr), elongation at break (%), and area of area at break (%) were determined as J11. Furthermore, the degree of weld cracking was evaluated by measuring the cracking rate (%) using a T-shaped weld cracking test. The above results are collectively shown in Table 1.

As is clear from the results in the table, although the weld metal according to the uninvented invention has a uniform austenitic (γ) structure that does not contain ferrite (α) in its structure, the inclusion of ferrite causes weld cracking. It can be seen that the cracking rate was comparable to that of the conventional 5US316 steel welding metal 1zl (Comparative Example 1), which exhibits the effect of preventing welding cracks. In addition, the creep rupture time was significantly longer than that of the comparative sample, indicating that it exhibits excellent high-temperature strength.

(2) Manufacture of welded structures Table 2 shows 2i (composition of A: 5IJS318. B:
Improved 5 IJS316 austenite m was used as the structure tagawa.

Table 2 This structural steel is shown in Example 10. Example 17. A welded joint was made using the deposited metal of Comparative Example 1. Welded joints have a plate thickness of 2
0 mm of the same type of structural steel with a groove angle of 60 degrees.

Manufactured by butt welding with an X-shaped groove with a root spacing of 2 mm. A test piece was taken from the obtained welded joint, and 70
A creep rupture test was conducted under the conditions of 0°C, 918 kg/1 lff12, and the results are shown in Table 3.

As is clear from the results in the table, the welded joint manufactured using the deposited metal and modified structural steel 5O3318 (containing N) according to the present invention has a comparatively higher )-7'ElffM time is significantly longer. Furthermore, as shown in Comparative Example Structure 1.3, even when the weld metal according to the present invention is used as the weld metal, when the conventional 5OS31B (not containing N) is used as the structural steel, the fracture position is the same as that of the structural steel. , indicating that the strength of the welded metal is high.

In addition, improved 5US31B (including N) is used as structural steel.
Even if the weld metal is a conventional material (comparative example structure 2)
It broke at the weld metal position and was improved from the weld metal 5U3.
It can be seen that 318 is stronger.

[Effects of the Invention] As is clear from the above description, the welded structure of the present invention has excellent high-temperature properties, and is useful when applied to turbine components, particularly steam turbine casings, and its industrial value is high. It's large.

Claims (1)

[Claims] 1. Austenitic steel containing chromium, nickel and nitrogen, carbon 0.02% to 0.12% by weight, silicon 0.1% to 1.5% by weight, manganese 0.2% by weight or more and 2.0% by weight or less, nitrogen 0.11% by weight or more and 0.3% by weight or less, nickel 8% by weight or more14
Essential components include 15% to 20% by weight of chromium, 0.01% to 0.3% by weight of vanadium, and 2.0% to 3.5% by weight of molybdenum, and the remainder is substantial. A welded structure for high temperature use, characterized in that it is a welded body with a welded metal that is iron. 2. When the chromium equivalent in the weld metal is a% and the nickel equivalent is b%, a and b are each 0.79×a+
The welded structure for high temperature use according to claim 1, wherein the number satisfies the following relationships: b≧31%, 1.1×a−b≦8.8%, and b≦20%. 3. The amounts of carbon, silicon, manganese, nitrogen, nickel, chromium, vanadium, and molybdenum in the weld metal are %C, %Si, %Mn, %N, %Ni, %Cr, respectively.
When expressed as %V and %Mo, the chromium equivalent a% and nickel equivalent b% of the weld metal are respectively a=(%Cr)+1.2×(%Si)+(%Mo)+5
×(%V), b=(%Ni)+30×(%C)+25.7×(%N)
The welded structure for high temperature use according to claim 2, which is represented by +0.5×(%Mo). 4. The weld metal contains 0.04% to 0.08% by weight of carbon, 0.3% to 0.7% by weight of silicon, 0.5% to 0.9% by weight of manganese, and 0% of nitrogen. .1
1% by weight or more and 0.2% by weight or less, 8.5% by weight of nickel
13.5% by weight or less, 16% or more by weight of chromium18.
5% by weight or less, vanadium 0.05% by weight or more 0.25
The high-temperature welded structure according to any one of claims 1 to 3, wherein the essential component is less than 2.2% by weight and less than 2.8% by weight of molybdenum, and the remainder is substantially iron. thing. 5. The weld metal contains 0.02% by weight or more and 0.12% by weight or less of carbon, 0.1% by weight or more and 1.5% by weight or less of silicon,
Manganese 0.2% to 2.0% by weight, nitrogen 0.
11 wt% or more and 0.3 wt% or less, nickel 8 wt% or more and 14 wt% or less, chromium 15 wt% or more and 20 wt% or less, vanadium 0.01 wt% or more and 0.3 wt% or less,
More than 2.0% by weight and less than 3.5% by weight of molybdenum is an essential component, and 0.01% by weight or more and 0.3% by weight of niobium.
Below, titanium 0.002% by weight or more and 0.3% by weight or less,
Claim 1 containing at least one of boron from 0.0005% by weight to 0.01% by weight and tungsten from 1.0% to 3.0% by weight, with the remainder being substantially iron. Or the high temperature welded structure according to item 2. 6. The amounts of carbon, silicon, manganese, nitrogen, nickel, chromium, vanadium, molybdenum, niobium, titanium, boron, and tungsten in the weld metal are %C, respectively.
, %Si, %Mn, %N, %Ni, %Cr, %V, %M
o, %Nb, %Ti, %B, %W, the chromium equivalent a% and nickel equivalent b% of the weld metal are respectively a=(%Cr)+1.2×(%Si)+( %Mo)+5
×(%V)+0.5×(%Nb)+1.5×(%Ti)
+40×(%B)+0.75×(%W), b=(%Ni)+30×(%C)+25.7×(%N)
The welded structure for high temperature use according to claim 5, which is represented by +0.5×(%Mo). 7. The weld metal contains 0.04% by weight or more and 0.08% by weight or less of carbon, and 0.3% by weight or more and 0.7% by weight or less of silicon;
Manganese 0.5% to 0.9% by weight, nitrogen 0.
11 wt% or more and 0.2 wt% or less, nickel 8.5 wt% or more and 13.5 wt% or less, chromium 16 wt% or more18
．． 5% by weight or less, vanadium 0.05% by weight or more 0.2
The essential components are 5% by weight or less, molybdenum from 2.2% to 2.8% by weight, and 0.02% to 0.15% by weight of niobium, and 0.02% to 0.02% by weight of titanium.
15% by weight or less, boron 0.003% by weight or more 0.00
7% by weight or less, tungsten at least one of 1.0% by weight and 3.0% by weight, the balance being substantially iron, the high temperature welding according to claim 5 or 6. Structure. 8. The welded structure for high temperature use according to claim 1, wherein the welded body is a turbine component. 9. The high temperature welded structure according to claim 8, wherein the turbine component is a casing.