JPH08276272A - Two-electrode gas shielded arc welding method of austenitic stainless steel - Google Patents

Abstract

PURPOSE: To improve the low temperature toughness while the corrosion resistance is secured by the simple heat treatment for a large sized welded structure hard to execute the solution heat treatment in a multi-layer weld zone of the austenitic stainless steel. CONSTITUTION: In the two-electrode gas shielded arc welding method of the austenitic stainless steel, the weld bead formed by a preceding electrode is heated up to 900-1200 deg.C by the heat generated by a succeeding electrode, and the heated state is kept for 0.5-3 seconds to realize the austenitic transformation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an arc welding method for austenitic stainless steel, and more particularly to heat treatment.

[0002]

2. Description of the Related Art In welding austenitic stainless steel, solution heat treatment is known as a method for improving the low temperature toughness of the welded portion. The solution heat treatment is originally performed for the purpose of improving the corrosion resistance, and the reason is as follows. When austenitic stainless steel is heated in the temperature range of 500 to 800 ° C., Cr carbide (mainly Cr ₂₃ C ₆ ) precipitates at grain boundaries,
In the vicinity thereof, the concentration of solid solution Cr is lowered to form a Cr-deficient layer, which becomes insufficient to maintain the passive film, resulting in deterioration of corrosion resistance. In the case of multi-layer welding by welding, the preceding pass is heated to just this temperature range by the heat of the succeeding pass, and Cr carbides are easily generated.

Therefore, by heating to a temperature of about 1100 ° C. to decompose the Cr carbide to eliminate the Cr deficient layer, and then rapidly cooling with water to quickly pass the temperature range of 500 to 800 ° C., the Cr carbide is precipitated again. To prevent corrosion and improve corrosion resistance. This heat treatment is a solution heat treatment. The solution heat treatment has the effect of improving the corrosion resistance and at the same time improving the low temperature toughness as described above. The reason is as follows.

308 series (19Cr-10Ni), 316
In the case of a system (17Cr-12Ni-2Mo) stainless steel material, in the equilibrium state, the δ phase is first crystallized as a primary crystal, and then the γ phase is crystallized by a peritectic reaction to complete solidification in (δ + γ) two phases. After solidification, the δ phase transforms into the γ phase, and becomes the γ single phase at room temperature. The phase change in this case is as follows. L → (L + δ) → (L + δ + γ) → (δ + γ) → γ

In the case of a welded structure, the cooling rate is high,
Since the welding material has a high Cr design, it does not completely match the phase change described above, and it is not possible to transform all δ into γ, and δ remains slightly. It is not a bad thing that a little δ remains, and it prevents cracks generated during weld solidification and improves the crack resistance, but on the other hand, the more δ remains, the worse the low temperature toughness becomes. The δ → γ transformation is carried out in the temperature range of about 600 to 1300 ° C., but the solution heat treatment heats up to a temperature near 1100 ° C., so that the time required for δ to transform into γ is given, and the residual δ does not remain. . Therefore, the low temperature toughness is improved.

As described above, solution heat treatment is a very effective means for improving the low temperature toughness, but since it has to be heated to a very high temperature, the use of a heat treatment furnace is inevitable and a large size is required. In the case of welded structures, solution heat treatment was often not possible.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to improve the low temperature toughness while ensuring corrosion resistance by performing a simple heat treatment in a multi-pass weld of austenitic stainless steel.

[0008]

Means for Solving the Problems The present invention is to solve the above problems, and in the two-electrode gas shielded arc welding method of austenitic stainless steel, the welding bead formed by the leading electrode is heated by the trailing electrode. By 900 ~
It is a two-electrode gas shielded arc welding method for austenitic stainless steel, which comprises heating to 1200 ° C. and holding it for 0.5 to 3 seconds to perform austenite transformation.

[0009]

The present invention is effective in the welding methods of TIG, MIG, and plasma, or welding combining these methods.
Here, TIG welding will be described. FIG. 2 shows the heating state of the first pass when welding with a single electrode. The peak temperature is 1500 because the first pass has reached the melting temperature.
It is about ℃. However, the heat generated by welding escapes more and more to the steel plate being welded, so that it is immediately cooled. After that, it is heated by the heat of the second pass, but the peak temperature is about 700 to 800 ° C. and this is also immediately cooled, so that the δ → γ transformation does not occur.

Next, FIG. 1 shows the heating condition of the first pass when welding with two electrodes according to the present invention. The heating condition by the leading electrode is the same as that in FIG. 2, but since the trailing electrode comes immediately after the leading electrode has passed, total heating by the two electrodes results in a very high temperature range, specifically 9
By heating for a long time around 00 to 1100 ° C. and holding for 0.5 to 3 seconds, the δ → γ transformation occurs. Therefore, from the viewpoint of the structure, the δ → γ transformation can be performed for a long time. As a result, δ can be reduced and the low temperature toughness can be improved.

Regarding Cr carbide, Cr carbide precipitates in the cooling process after passing through the trailing electrode 500 to 8
Since it passes through the temperature range of 00 ° C immediately, the precipitation of Cr carbide is small.

Even in the second pass by the trailing electrode, the state of preheating at the temperature of the preceding pass by the preceding electrode is maintained, and in the δ → γ transformation temperature range much more than welding by a single electrode, though not as much as the preceding pass. The long-term holding time improves the low temperature toughness.

[0013]

EXAMPLE In the examples, TIG welding was performed to compare the single electrode type and the two electrode type. The comparison item is JIS Z224.
The Charpy impact test of No. 2 was conducted at -196 ° C to compare the low temperature toughness, and in order to examine the deterioration of the corrosion resistance due to the precipitation of Cr carbide, the sulfuric acid / copper sulfate corrosion test of JIS G0575 was conducted to investigate the state of intergranular corrosion. .

The welding wire used in this experiment was a 308 series stainless wire, and the composition of the wire is shown in Table 1.
Shown in Table 2 shows the welding conditions. The base material is SU
A plate thickness of 16 mm is used in S304, and the groove shape is shown in FIG.

[0015]

[Table 1]

[0016]

[Table 2]

The results are shown in Table 3. The heating temperature in the table is the maximum heating temperature due to the heat of the trailing electrode, and a thermocouple was attached 1.5 mm below the groove bottom surface for measurement. As a result, the invention A1 was the case where the heating temperature in the first pass was close to the lower limit value of the invention, the absorbed energy was improved, and the corrosion resistance was good. Inventive Example A2 was the case where the heating temperature in the first pass was close to the upper limit of the present invention, and the absorbed energy was also improved and the corrosion resistance was good.

[0018]

[Table 3]

Comparative example B1 is ordinary single electrode welding, and the absorbed energy is about 80J. However, the corrosion resistance was good. Comparative Example B2 is the case where the heating temperature in the first pass is below the lower limit of the present invention, the absorbed energy is not improved so much, and the corrosion resistance is deteriorated. In Comparative Example B3, the heating temperature in the first pass was higher than the maximum upper limit value of the present invention, and although the absorption energy was not improved, the corrosion resistance was good.

[0020]

EFFECTS OF THE INVENTION According to the present invention, a simple solution heat treatment is performed on a welded portion of austenitic stainless steel,
It is possible to improve low temperature toughness while ensuring corrosion resistance.

[Brief description of drawings]

FIG. 1 is a graph showing the heating state of the first pass when welding with two electrodes according to the present invention.

FIG. 2 is a graph showing the heating situation when welding with a single electrode.

FIG. 3 is a diagram showing a groove shape in an example.

Claims (1)

[Claims] 1. In the two-electrode gas shielded arc welding method for austenitic stainless steel, the welding bead formed by the leading electrode is heated to 900 to 12 by the heat generated by the trailing electrode.
A two-electrode gas shield arc welding method for austenitic stainless steel, which comprises heating to 00 ° C. and holding for 0.5 to 3 seconds to perform austenite transformation.