JPS6119345B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a welding method for austenitic stainless steel, and more particularly to the above welding method that can completely prevent deterioration of corrosion resistance due to Cr carbide precipitation at low cost.

Conventionally, welded parts of austenitic stainless steels such as SUS304, 308, 309, 316, and 317 have low corrosion resistance, and corrode when used in corrosive environments. This is corrosion that occurs in the heat affected zone of austenitic stainless steel welds. The heat-affected portion ~ shown in Figure 1 A, which is a little distance from the weld bead 2 , undergoes temperature changes as shown by the curves ~ in Figure 1 B due to the welding heat cycle. [Note that 1 in Figure 1 A It is an austenitic stainless steel. Among these, especially 500 to 800℃, which is the carbide precipitation region.
Cr carbides precipitate at grain boundaries in regions heated for a relatively long period of time. It is known that when Cr carbide precipitates, the Cr concentration near the grain boundaries decreases, and corrosion occurs preferentially in the region where the concentration of Cr is below 11.7%, the limit of Cr action.

Since this corrosion is a deterioration in corrosion resistance due to the precipitation of Cr carbides, the C content that controls the amount of grain boundary precipitation, the welding thermal cycle, and the presence of elements with a high tendency to form carbides are important influencing factors. Therefore, the following measures have been taken in the past.

(1) Material selection Use of ultra-low carbon austenitic stainless steel (e.g. 304L, 316L) Use of stabilized austenitic stainless steel (e.g. 321, 347) (2) Solution heat treatment By heating at 1040-1120℃ A heat treatment that decomposes carbides into solid solution and then rapidly cools them.

However, regarding (1) above, the increase in material costs,
Regarding (2) above, there were problems such as heat treatment cost, deformation, and large structures where rapid cooling may not be possible.

Therefore, when welding low-grade austenitic stainless steel, the inventors of the present invention conducted intensive research with the aim of completely preventing the deterioration of corrosion resistance described above at low cost. The present invention was developed based on the fact that the above object can be achieved by shortening the length.

That is, the present invention uses a torch with a double nozzle structure at the tip, and performs welding by discharging a water jet from the outermost nozzle and simultaneously discharging a cavity-forming gas from the inner nozzle. Regarding the welding method, when welding the steel, the heat-affected zone generated around the welded part is water-cooled by the water jet described above to suppress the precipitation of Cr carbides in the heat-affected zone.

FIG. 2 shows an example of an embodiment of the method of the present invention.

In Fig. 2, austenitic stainless steel 1 is placed in a shielding gas 4 such as Ar, Ar+CO ₂ , etc.
When welding the weld metal 2 by means of a welding arc from a welding wire 3 whose welding current is led from a power supply chip 9, a specially shaped torch, in this case a double torch 7, is used, and a torch provided at the tip of the double torch 7 is used. A high-speed water jet is discharged from 6 of the rattle-shaped nozzles 5 and 6 to rapidly cool the high-temperature welded part, and a cavity-forming gas is further discharged from 5 to the inside of the water jet to form a stable gas phase region.

The high-speed water jet discharged from the nozzle 6 rapidly cools the weld zone, thereby preventing the precipitation of Cr carbides that degrade corrosion resistance. This state is shown in FIG.
In FIG. 3, , corresponds to , in FIGS. 1A and B, and ′ and ′ indicate the temperature change in the portion in FIG. 1A when the method of the present invention is used. Third
As is clear from the figure, in the conventional method, the region heated to a temperature in the range of 500 to 500°C, which is the region where carbides are precipitated for a relatively long time, is heated in the range of 500 to 500°C for a relatively long time, but in the method of the present invention, the region is heated for a very short time as shown by ′, and Cr carbide precipitates. can be prevented.

In addition, a stable gas phase region (local cavity) is formed directly under the torch 7 by the cavity forming gas discharged from the nozzle 5 and the shielding gas 4 supplied from the center of the torch 7, and welding of prohole etc. Weld metal 2 without defects can be obtained. As this cavity-forming gas, the same gas as the above-mentioned shielding gas, ie, Ar, Ar+CO ₂ or the like, is used.

As explained above, since the method of the present invention can form a stable cavity at the arc point,
Among them, it can be implemented. Therefore, the method of the present invention can be carried out underwater if there is a need to increase the cooling rate or to prevent weld distortion. Even when the process is carried out underwater, the temperature change in the area A in FIG.

[Brief explanation of the drawing]

Fig. 1 shows the temperature change in the welding heat affected zone by the conventional method, Fig. 3 shows the temperature change in the welding heat affected zone by the method of the present invention, and Fig. 2 shows an example of an embodiment of the method of the present invention. It is a diagram.

Claims (1)

[Claims] 1. A method for welding austenitic stainless steel using a torch with a double nozzle structure at the tip, which is characterized by performing welding while discharging a water jet from the outermost nozzle and simultaneously discharging a cavity-forming gas from the inner nozzle.