JP2622530B2 - Welding material for austenitic steel with excellent high-temperature strength - Google Patents

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 having excellent high-temperature strength.

[0002]

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

In response to such demands, the amount of Cr and Ni
Many new boiler materials have been developed. However, although they have no problem in performance, they are economically very expensive compared to 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.

[0004] This 18-8 base high temperature, high strength austenitic steel has high economic strength because the high temperature strength is increased irrespective of the increase in Cr and Ni, and the welding structure is improved in addition to the thinning due to the high strength. It is expected to have a great effect on reducing the production cost of products. As a welding material for welding the steel, a material obtained by directly processing a base material into a wire or a material for a high Ni alloy such as JIS DNiCr-3 is considered.

[0005]

However, the performance of a weld metal obtained from a welding material obtained by directly processing a base material of a high-temperature, high-strength austenitic steel into a wire is inferior to that of the base material.
This is because, after melting, the base metal is given a high-temperature strength by adjusting the structure by hot working such as rolling or heat treatment such as rolling, whereas the weld metal is usually used in a solidified structure. If the composition is the same as that of the base material, the same high-temperature strength as that of the base material cannot be obtained.

Further, in an austenitic structure, welding hot cracks are inherently liable to occur, and in a weld metal, solidification cracking particularly occurs during solidification becomes a problem.

Further, in the case of a welding material in which a base material is directly processed into a wire, the fluidity of the molten metal in the molten pool is insufficient, and a decrease in welding workability thereby causes welding defects.

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

On the other hand, welding materials for high Ni alloys are expensive and are not preferred 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 while maintaining a solidified structure, has less solidification cracking, and has good welding workability and economic efficiency. is there.

[0011]

The present inventors have conducted various investigations and researches with the aim of developing an inexpensive welding material suitable for welding a high-temperature, high-strength austenitic steel based on 18-8 series. The following findings were obtained regarding the three points of improvement in high-temperature strength, measures against solidification cracking, and improvement in 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 refine the solidified structure by precipitation of NbC and NbCrN. Here, if a small amount of Ti is added to generate TiN, the refinement of the solidified structure is further promoted. Solid solution strengthening by the addition of Mo is also effective in improving high-temperature strength. Further, when an appropriate amount of Mn is added, the decrease in N during welding can be suppressed, and the solid solution strengthening of N can be stabilized.

Regarding the measures against solidification cracking, the formation of a low melting point eutectic formed at the time of solidification is suppressed and the susceptibility to cracking is reduced by regulating the amounts of P and S and adding Mo.

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

The present invention has been made on the basis of the above-mentioned findings.
0.5%, Mn: 0.5 to 5%, Cr: 15 to 22%,
Ni: 12 to 30%, Cu : 2 to 5%, Al: 0.03
% , Mo: 0.3-3% or less, N: 0.07-0.
35%, P: 0.015% or less, Nb: more than 0.3%
3% or less , S: 0.002 to 0.015%, and if necessary, B: 0.01% or less, Ti: 0.005 to 0.5%.
The gist of the present invention is a welding material for austenitic steel excellent in high-temperature strength, characterized by containing 2% or more of 1% or more, with the balance being Fe and unavoidable impurities.

[0016]

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

C: Contributes to improvement in tensile strength and creep strength at high temperatures. If it is less than 0.05%, the strength is low, and 0.2
%, A large amount of carbides is formed, leading to a decrease in ductility. Therefore, it was set to 0.05 to 0.2%.

Si: Effective for increasing the fluidity of the molten metal, for which 0.1% or more is required. But,
On the other hand, it has an effect of lowering the solid solubility of N added as a solid solution strengthening element, and it is necessary that the content does not exceed 0.5%. Therefore, it was set to 0.1 to 0.5%.

Mn: N added as a solid solution strengthening element
Is effective for increasing the solid solubility of the compound, and therefore requires 0.5% or more. However, excessive addition causes embrittlement. Therefore, it was set to 0.5 to 5%.

Cr: An element necessary for improving the corrosion resistance of steel such as oxidation resistance, and requires 15% or more. However, the addition of a large amount not only reduces the economics itself, but also
A large amount of Ni is also required to ensure phase stability, which significantly impairs economic efficiency. Therefore, it was set to 15 to 22%.

Ni: Indispensable for obtaining a stable austenite structure, but the Cr content is suppressed, so that the content can be made relatively low, and is set to 12 to 30%.

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

Al: Used as a deoxidizing agent, but excessive addition remains as inclusions in the weld metal and causes a decrease in creep ductility.

By adding Mo: Cu in combination, the matrix is solid-solution strengthened, and at the same time, the high-temperature strength is improved by the synergistic action with the Cu precipitation phase. However, excessive addition causes deterioration of high-temperature corrosion resistance. Therefore, it was set to 0.3 to 3%.

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

P: an element for increasing the high temperature sensitivity to welding,
The smaller the better, the better. However, an extreme limitation causes a reduction in economic efficiency, so the content was set to 0.015% or less.

S: 0.002 % or more is required to improve the fluidity of the molten metal and contribute to the improvement of the welding workability. However, it has an adverse effect on welding hot cracking. Therefore, the content is set to 0.002 to 0.015%.

Nb: Precipitates as NbN, NbCrN and NbC, and by refining the solidification structure and strengthening the precipitation,
Contributes to ensuring creep strength and high temperature tensile strength.
If it is less than 0.3% , the effect is small, and excessive addition is harmful from the viewpoint of welding hot cracking. Therefore, more than 0.3% 1.
3% or less .

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

Ti: 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%, the effect is small, and if the precipitation of TiN is excessive, the effect of strengthening by solid solution N is reduced. Therefore, it was set to 0.005 to 0.2%.

[0031]

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

[0033]

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

Using austenitic steels having the chemical compositions shown in Table 1 as the base metal, the performance of the welding materials was compared. This base material is based on 18-8 austenitic stainless steel, to which Cu, Nb, N, and B are added, and its high-temperature strength is raised to 13.5 kgf / mm ² at a creep rupture strength of 700 ° C. × 3000 h. It is a thing. The welding materials had the chemical compositions shown in Table 2 and were all produced by vacuum melting in a laboratory and then processing into wires 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 to a restraining plate 2 made of a steel plate as shown in FIG. Next, multi-layer welding was performed on the groove with a test welding material. 3 is a constraint weld,
Reference numeral 4 denotes a test weld for the groove. Restraint welding causes thermal stress during welding to the groove, which tends to cause cracking. Welding was performed using the TIG method under the following conditions: welding current 150 A, welding voltage 18 V, welding speed 10 cm /
min.

After welding, the formation of the Uranami bead was visually inspected. Further, a side bending test piece shown in FIG. 3 was sampled by machining, and this was bent to a bending radius twice the plate thickness (10 m).
m), it was bent by 180 degrees, and the presence or absence of welding hot cracks in the weld metal 5 was examined. Further, a creep test was performed using a joint creep test piece shown in FIG. In the creep test, at 700 ° C, where the rupture time in the base material was about 3000 hours, 13 ° C.
A test was performed under the conditions of 5 kgf / mm ² to determine the rupture time of the weld metal 5. Table 3 shows the results. ○ indicates no welding high-temperature cracking, and × indicates occurrence of cracking.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

[Table 3]

[0040]

[Table 4]

[0041] A1 to 8 is a welding material of the present invention are all rupture time can produce welded joints of high temperature and high strength showing a more than 90% of the base material. In addition, the material does not cause hot cracking of the weld, and has good backside forming ability. On the other hand, B1 to B3 in which P, S, and Nb are out of the range of the present invention cause welding cracks. B that lacks Cu, Nb, Mo, and N
Nos. 4 to B7 cannot obtain high-temperature strength comparable to the base material. B8 containing less Si and S has poor fluidity of the molten metal and lowers welding workability.

[0042]

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

[Brief description of the drawings]

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

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

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

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

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Base material 2 Restraint plate 3 Restraint weld 4 Test weld to groove

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Shigeru Matsumoto 1-17 Fuso-cho, Amagasaki-shi, Hyogo Sumikin Welding Industry Co., Ltd. (72) Inventor Toshihiko Mizuta 1-117 Fuso-cho, Amagasaki-shi, Hyogo Sumikin Welding (56) References JP-A-48-7855 (JP, A) JP-A-58-7792 (JP, A) JP-A-59-127991 (JP, A) JP-A-62-133048 (JP, A A) Masaki Watanabe et al., “Welding of Stainless Steel (First Edition, 9th Edition)” (February 20, 1981), Nikkan Kogyo Shimbun, p. 65, 66

Claims (2)

(57) [Claims] 1. C: 0.05 to 0.2% by weight, S
i: 0.1 to 0.5%, Mn: 0.5 to 5%, Cr: 1
5~22%, Ni: 12 ~30% , Cu: 2~5%, A
l: 0.03 % or less, Mo: 0.3 to 3% or less, N:
0.07 to 0.35%, P: 0.015% or less, Nb:
More than 0.3% and 1.3% or less , S: 0.002 to 0.015
%, With the balance being Fe and inevitable impurities, the welding material for austenitic steel having excellent high-temperature strength. 2. C: 0.05 to 0.2% by weight, S
i: 0.1 to 0.5%, Mn: 0.5 to 5%, Cr: 1
5~22%, Ni: 12 ~30% , Cu: 2~5%, A
l: 0.03 % or less, Mo: 0.3 to 3% or less, N:
0.07 to 0.35%, P: 0.015% or less, Nb:
More than 0.3% and 1.3% or less , S: 0.002 to 0.015
%, B: 0.01% or less, Ti: 0.005 to
A welding material for austenitic steel having excellent high-temperature strength, comprising 0.2% or more of one or more types, with the balance being Fe and inevitable impurities.