JPH06142980A - Welding material for austenitic stainless steel having excellent high-temperature strength - Google Patents

Abstract

PURPOSE:To improve welding workability by applying the high-temp. strength equiv. to the high-temp. strength of a base metal to a weld metal, thereby preventing the crack on solidification by welding and improving the flowability of the weld metal in welding of a high-temp. and high-strength austenitic steel consisting of an 18-8 system as a base. CONSTITUTION:The steel contg., by weight %, 0.05 to 0.2% C, 0.1 to 0.5% Si, 0.5 to 5% Mn, 15 to 22% Cr, 6 to 30% Ni, 2 to 5% Cu, <=0.03% Al, 0.3 to <=3% Mo, 0.07 to 0.35% N, <=0.15% P, 0.1 to 1.5% Nb and 0.002 to 0.015% S is used as the welding material.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welding material used for welding austenitic steel excellent in high temperature strength.

[0002]

2. Description of the Related Art Conventionally, 18-8 type austenitic stainless steel has been mainly used for power generation boilers, chemical reactors and the like used at high temperatures. However, for example, in boilers and the like, operating conditions have become extremely severe due to the recent increasing demand for improvement in energy efficiency.
A material having higher high temperature strength than that of the 8 type austenitic stainless steel has been required.

To meet such requirements, the amount of Cr and Ni
Many new materials have been developed for boilers in higher quantities. However, although they have no problem in terms of performance, they are economically very expensive as compared with 18-8 austenitic stainless steel. Therefore, based on 18-8 austenitic stainless steel, Cu, N
Austenitic steels having improved high temperature strength by adding b, N and B are disclosed in JP-A-62-70553 and JP-A-62-133048.

This high-temperature, high-strength austenitic steel based on 18-8 has a high economic strength because it has a high high-temperature strength irrespective of the increase in the amounts of Cr and Ni. It is expected to have a great effect on reducing the manufacturing cost of the product. As a welding material for welding this steel, a material obtained by directly processing a base material into a wire or a material for high Ni alloy such as JIS DNiCr-3 is considered.

[0005]

However, the weld metal obtained by the welding material obtained by directly processing the base material of the high-temperature high-strength austenitic steel as a wire rod is inferior in performance to the base material.
This is because the base metal is given high temperature strength by adjusting the structure by hot working such as rolling and heat treatment after melting, whereas weld metal is usually used in the as-solidified structure. However, with the same composition as the base material, it is not possible to obtain the same high temperature strength as the base material.

[0006] Further, in the austenite structure, welding hot cracks are essentially liable to occur, and in the weld metal, solidification cracks which particularly occur during solidification pose a problem.

Further, in a welding material in which the base material is directly processed into a wire, the fluidity of the molten metal in the molten pool is insufficient, and the deterioration of the welding workability due to this becomes a cause of welding defects.

Therefore, a satisfactory welding quality cannot be obtained with a welding material obtained by directly processing the base material into a wire rod.

On the other hand, welding materials for high Ni alloys are expensive and are not preferable in terms of economy.

An object of the present invention is to provide a welding material for austenitic steel, which can obtain excellent high-temperature strength in the solidified structure as it is, has few solidification cracks, and has good welding workability and economy. is there.

[0011]

Means for Solving the Problems As a result of various investigations and studies aimed at developing an inexpensive welding material suitable for welding 18-8 base high temperature high strength austenitic steel, The following findings were obtained regarding three points: improvement of high temperature strength, measures against solidification cracking, and improvement of welding workability.

In order to improve the high-temperature strength of the weld metal as it is solidified, it is effective to add Nb and N and to refine the solidified structure by precipitating NbC and NbCrN. If a small amount of Ti is added here to generate TiN, the refinement of the solidified structure is further promoted. Solid solution strengthening by addition of Mo is also effective in improving high temperature strength. Further, by adding an appropriate amount of Mn, it is possible to suppress the decrease of N during welding and stabilize the solid solution strengthening of N.

As a measure against solidification cracking, by restricting the amounts of P and S and adding Mo, the formation of low melting point eutectic formed during solidification is suppressed and the cracking susceptibility is reduced.

In order to improve the welding workability, it is effective to adjust the amounts of S and Si in appropriate amounts to reduce the viscosity of the molten metal and the surface tension to improve the molten metal flow.

The present invention has been made on the basis of the above findings. C: 0.05 to 0.2% by weight, Si: 0.1 to 0.5
%, Mn: 0.5 to 5%, Cr: 15 to 22%, Ni: 6
-30%, Cu 2-5%, Al: 0.03 or less, Mo: 0.0.
3 to 3% or less, N: 0.07 to 0.35%, P: 0.015%
Below, Nb: 0.1 to 1.3%, S: 0.002 to 0.015%
And, if necessary, B: 0.01% or less, Ti: 0.00
The gist is a welding material for austenitic steel excellent in high temperature strength, characterized by containing one or more of 5 to 0.2% and Ta: 2% or less, and the balance being Fe and inevitable impurities. .

[0016]

The function and the reason for limitation of the alloying elements in the welding material for austenitic steel of the present invention will be described below.

C: Contributes to improvement of tensile strength and creep strength at high temperature. If it is less than 0.05%, the strength is low and 0.2
If it exceeds 50%, a large amount of carbide is generated, which leads to a decrease in ductility. Therefore, it is set to 0.05 to 0.2%.

Si: Effective in increasing the fluidity of the molten metal, and therefore requires 0.1% or more. But,
On the other hand, it has the effect of reducing the solid solubility of N added as a solid solution strengthening element, and it is necessary to prevent it from exceeding 0.5%. Therefore, it is set to 0.1 to 0.5%.

Mn: N added as a solid solution strengthening element
Is effective in increasing the solid solubility of, and for that reason, 0.5% or more is required. However, excessive addition causes embrittlement. Therefore, it is set to 0.5 to 5%.

Cr: An element necessary for improving the corrosion resistance of steel such as oxidation resistance, and needs to be 15% or more. However, addition of a large amount not only lowers the economic efficiency,
A large amount of Ni is required to secure the phase stability, which significantly impairs the economical efficiency. Therefore, it is set to 15 to 22%.

Ni: Indispensable for obtaining a stable austenite structure, but since the amount of Cr is suppressed, the amount can be made relatively low, so that it is 6 to 30%.

Cu: When heated to a high temperature, Cu is finely dispersed and precipitated in the matrix to increase the high temperature strength (creep strength). However, if it is less than 2%, the effect is small. On the contrary, excessive addition causes a decrease in ductility. Therefore, it is set to 2 to 5%.

Al: Used as a deoxidizing agent, but excessive addition thereof remains as inclusions in the weld metal and causes a decrease in creep ductility, so the content was made 0.03% or less.

The composite addition of Mo: Cu strengthens the matrix in a solid solution and improves the high temperature strength by synergistic action with the Cu precipitation phase, and contributes to ensuring the strength of the weld metal at 0.3% or more. However, excessive addition causes deterioration of hot corrosion resistance. Therefore, it is set to 0.3 to 3%.

N: Solid solution strengthening and precipitation strengthening as NbN and NbCrN contribute to improvement of tensile strength and creep strength, but if less than 0.07%, the effect is small. However, excessive addition causes welding defects such as blowholes. Therefore, it is set to 0.07 to 0.35%.

P: an element that enhances high temperature welding sensitivity,
Although the smaller the better, the better, but the extreme limit causes a decrease in economic efficiency, so the content is set to 0.015% or less.

S: 0.02% or more is required in order to improve the fluidity of molten metal and contribute to the improvement of welding workability.
However, it has an adverse effect on welding hot cracking. Therefore 0.
It was set to 002-0.015%.

Nb: Precipitated as NbN, NbCrN and NbC, and by refining the solidification structure and precipitation strengthening,
Contributes to securing creep strength and high temperature tensile strength. 0.
If it is less than 1%, its effect is small, and excessive addition is harmful from the viewpoint of welding hot cracking. Therefore, it is set to 0.1 to 1.3%.

B: It may be added in order to strengthen the dispersion of carbides and enhance creep strength by strengthening grain boundaries. However, excessive addition increases weld hot crack susceptibility. Therefore, it is set to 0.01% or less.

Ti: It may be added as a precipitation strengthening element. That is, Ti has a strong affinity with N and contributes to securing high temperature strength by precipitating as TiN and refining the solidified structure. However, if it is less than 0.005%, its effect is small, and if the precipitation of TiN becomes excessive, the effect of strengthening by solid solution N is diminished. Therefore, it is set to 0.005 to 0.2%.

Ta: May be added as a solid solution strengthening element. However, excessive addition impairs economic efficiency, so the content was made 2% or less.

The base material for which the welding material of the present invention is effective is
700 ° C x 3000h creep rupture strength of 12 to 14k
It is a 18-8 base steel having a gf / mm ^{2 level} , and concretely, for example, the main alloying components as disclosed in JP-A-62-70553 and JP-A-62-133048 are 15 to 15%. 20% Cr, 6-18% Ni, 15-
It is a high-temperature high-strength austenitic steel with about 4% Cu, 0.1 to 1.5% Nb, and 0.02 to 0.3% N.

[0033]

EXAMPLES Next, examples of the present invention will be described in comparison with comparative examples.

Austenitic steels having the chemical compositions shown in Table 1 were used as the base metal, and the performance comparison of the welding materials was performed. This base material is based on 18-8 austenitic stainless steel, and Cu, Nb, N, and B are added to this base material, and its high temperature strength is increased to 700 ° C x 3000h creep rupture strength to 13.5 kgf / mm ² . It is a thing. The welding material has the chemical composition shown in Table 2, and each of them is manufactured by vacuum melting in a laboratory and then processed into a wire having an outer diameter of 2 mm.

In the comparative test, first, the base materials 1 and 1 provided with the groove shown in FIG. 1 were restrained and welded onto the restraint plate 2 made of a steel plate as shown in FIG. Then, multi-layer welding was performed on the groove using the test welding material. 3 is a restraint weld,
Reference numeral 4 indicates a test weld portion for the groove. The restrained welding causes thermal stress during welding to the groove, which easily causes cracking. Welding uses the TIG method under the following conditions: welding current 150 A, welding voltage 18 V, welding speed 10 cm /
set to min.

After welding, the formation state of the back bead was visually inspected. In addition, a side bending test piece shown in FIG. 3 was sampled by machining, and a bending radius (10 m
The weld metal 5 was bent 180 degrees at m) and examined for weld hot cracks. Further, a creep test was conducted using the joint creep test piece shown in FIG. In the creep test, the rupture time in the base metal was about 3000 hours at 700 ° C, 13.
The test was conducted under the condition of 5 kgf / mm ² to examine the fracture time of the weld metal 5. The results are shown in Table 3. ○ indicates no weld hot cracking, and × indicates cracking.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

[Table 3]

[0040]

[Table 4]

The welding materials A1 to A13 of the present invention are
In either case, a high temperature and high strength welded joint having a breaking time of 90% or more of the base metal can be produced. Moreover, the material does not generate hot cracking during welding, and has a good ability to form back waves. On the other hand, B1 to B3 in which P, S and Nb are out of the range of the present invention cause weld cracking. B lacking Cu, Nb, Mo, N
Nos. 4 to B7 cannot obtain high temperature strength comparable to that of the base material. B8 having a small amount of Si and S has a poor fluidity of the molten metal and deteriorates the welding workability.

[0042]

As described above, the austenitic steel welding material of the present invention uses less expensive elements and is lower in cost than the materials for high Ni alloys. Nevertheless, it gives the weld metal a high temperature strength comparable to that of the base metal, and
Excellent solidification cracking resistance and welding workability.

[Brief description of drawings]

FIG. 1 is a view showing a shape of a groove used in a welding test.

FIG. 2 is a view showing a shape of a joint created in a welding test.

FIG. 3 is a view showing a shape of a side bending test piece taken from a joint.

FIG. 4 is a diagram showing the shape of a creep test piece taken from a joint.

[Explanation of symbols]

 1 Base material 2 Restraint plate 3 Restraint weld 4 Test weld for groove

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shigeru Matsumoto 1-17 Fuso-cho, Amagasaki-shi, Hyogo Sumikin Welding Co., Ltd. (72) Toshihiko Mizuta 1-17 Fuso-cho, Amagasaki-shi, Hyogo Sumikin Welding Within Kogyo Co., Ltd.

Claims (2)

[Claims] 1. C: 0.05-0.2% by weight%, Si: 0.
1-0.5%, Mn: 0.5-5%, Cr: 15-22%,
Ni: 6-30%, Cu 2-5%, Al: 0.03 or less,
Mo: 0.3-3% or less, N: 0.07-0.35%, P: 0.
015% or less, Nb: 0.1 to 1.3%, S: 0.002 to 0.
A welding material for austenitic steel excellent in high-temperature strength, characterized by containing 015% and the balance being Fe and inevitable impurities. 2. C: 0.05 to 0.2% by weight, Si: 0.0.
1-0.5%, Mn: 0.5-5%, Cr: 15-22%,
Ni: 6-30%, Cu 2-5%, Al: 0.03 or less,
Mo: 0.3-3% or less, N: 0.07-0.35%, P: 0.
015% or less, Nb: 0.1 to 1.3%, S: 0.002 to 0.
015%, B: 0.01% or less, Ti: 0.005
0.2%, Ta: 2% or less, including one or more,
A welding material for austenitic steel excellent in high temperature strength, characterized in that the balance is Fe and inevitable impurities.