JPH0649233B2 - Welding method for stainless steel - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a welding method for stainless steel, and more particularly to a welding method for controlling the content ratio of a ferrite phase and an austenite phase in a weld metal of stainless steel.

<Background of the Invention> Duplex stainless steels whose structure after annealing is composed of a ferrite phase and an austenite phase are excellent in stress corrosion cracking resistance and mechanical properties, and are therefore used as materials for plants and seawater resistant equipment. ing. However, Japanese Industrial Standards (JIS)
In SUS 329J1, the duplex stainless steel typically has a ferrite single-phase structure in the as-welded state, and carbonitride precipitates at the crystal grain boundaries of ferrite, so it is highly susceptible to intergranular corrosion. There is a problem. In order to solve this problem, the composition should be designed so that the structure of the weld metal also has two phases, the ferrite phase and the austenite phase, but the weld metal structure rapidly cooled from a high temperature is changed to two phases. In order to do so, the structure in the annealed state becomes austenite phase excess, and the following problems occur.

In such a structure, the deformation resistance during hot increases, hot working becomes difficult, and the precipitation rate of the sigma (σ) phase becomes extremely high in the cooling process, and the steel becomes extremely brittle. However, it becomes difficult to manufacture steel.

The present inventors have made various investigations in order to solve the above problems, but paying attention to the fact that nitrogen is an austenite forming element, by adjusting the amount of nitrogen in the welding atmosphere, the generated weld metal The present invention has been completed by finding that the ratio between the ferrite phase and the austenite phase can be controlled.

<Structure of the Invention> According to the present invention, when welding stainless steel using a shield gas, a desired amount of austenite is contained in the weld metal by containing nitrogen in various ratios in the shield gas of an inert gas. A method of welding stainless steel is provided which is characterized by the appearance of phases. More specifically, in a method for welding two-phase stainless steels having a two-phase structure of a ferrite phase and an austenite phase in an annealed state, the stainless steel base material itself is melted in a shield gas atmosphere to form a weld metal. , A suitable amount of nitrogen gas is contained in the shield gas to form a weld metal containing nitrogen in a larger amount than the base metal, and the austenite of the weld is adjusted by adjusting the nitrogen content in the shield gas. Provided is a welding method for duplex stainless steel, characterized in that the amount is brought close to the amount of austenite of the base material. Here, "melting the base metal itself to form a receiving metal" is different from the welding method using a so-called welding rod (filler material), and the joining part of the base material to be welded is melted and joined. Means welding method. As described above, there are various welding methods for fusion-bonding the base metal itself and maintaining the welded portion in an inert atmosphere with a shield gas. There are various welding methods depending on the heat supply method. There is a type of arc welding (for example, TIG welding).

Duplex stainless steel (10Ni-2
4Cr-3Mo steel) has a ferrite phase austenite phase content ratio in the solution state of about 40:60,
When the steel is welded, the deposited metal becomes almost a ferrite single phase. However, when the inert gas sealing gas contains about 5 v / v% nitrogen at the time of welding, the ratio of the ferrite phase and the austenite phase in the weld metal becomes almost half.

<Specific Disclosure of the Invention> The present invention will be described in detail mainly with respect to welding of duplex stainless steel with reference to the drawings.

A steel having the composition shown in Table 1 is melted and rolled into a cold rolled steel sheet having a thickness of 3 mm by a conventional method, soaked at 1050 ° C. for 10 minutes and then water-cooled to obtain a specimen having a width of 25 mm and a length of 70 mm. Created. The austenite phase content ratios in this state (solution treatment state) are 57% and 61% as shown in this table.

The austenite content is measured by the following point counting method.

The observation field of view by an optical microscope is divided into 400 lattice points, and the number of lattice points on the austenite phase is measured in the observation of the polished sample. This is carried out for 30 fields of view, and the total number of lattice points that have come on the austenite phase is obtained,
It is divided by 00 × 30 and expressed as a percentage.

Welding speed of 100 cm / mi for duplex stainless steel shown in Table 1
n, welding current 80A, argon 5l / min as a back shield gas, only argon, only nitrogen as an arc shield gas, and various ratios of argon / nitrogen mixed gas (total flow rate 20 / min) using tungsten inert gas ( (TIG) The content ratio of the austenite phase in the deposited metal at the time of arc welding (bead placed) was measured. The attached drawings show the results of the results arranged in terms of the volume percentage of nitrogen gas in the shield gas.

As is clear from this figure, when argon alone is used as the shield gas, the structure of the deposited metal is almost a ferrite single phase, but nitrogen is contained in the shield gas, and the austenite phase is increased by increasing the content rate. It can be seen that the appearance rate can be increased, and that 100% nitrogen can be 100% austenite.

If this relationship is obtained for each steel to be welded having a specific composition, this method can be applied to a wide range of compositions.

In the case of the test steel, the desired austenite of 50% can be achieved by adding about 5 v / v% nitrogen.

Samples of the composition shown in Table 1 were argon only, and 5 v / v
Using argon containing% nitrogen as a shield gas,
Table 2 shows the results of the welding under the above conditions, which were tested according to JIS G0575 (corrosion test method for copper sulfate sulfate of stainless steel). The sample using only argon as the shielding gas showed cracks in the bending test, whereas the sample using nitrogen-containing argon did not cause any cracks and a remarkable improvement in corrosion resistance was observed.

<Effects of the Invention> As described above, according to the method of the present invention, a desired amount of austenite phase is present in the weld metal structure without considering the problems of hot deformation resistance and sigma phase precipitation in the manufacturing process. Can be deposited, and the corrosion resistance of the deposited metal can be improved.

[Brief description of drawings]

The attached drawing shows duplex stainless steel with a welding speed of 10 cm / min, a welding current of 80 A, and argon of 5 as a back shield gas.
l / min, austenite phase in the weld metal during tungsten inert gas (TIG) arc welding (beading) using argon only, nitrogen only, and various ratios of argon / nitrogen mixed gas as arc shield gas The content ratio of was measured. The results are summarized with respect to the volume percentage of nitrogen gas in the shield gas.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Hiroshi Asada 4976 Tomita, Shinnanyo City, Yamaguchi Prefecture Inside Shunan Steel Works, Nisshin Steel Co., Ltd. (56) Reference JP-A-59-218281 (JP, A) JP 45-34444 (JP, B1)

Claims (1)

[Claims] 1. A method of welding two-phase stainless steels having a two-phase structure of a ferrite phase and an austenite phase in an annealed state, wherein the stainless steel base material itself is melted in a shield gas atmosphere to form a weld metal, and A suitable amount of nitrogen gas is contained in the shield gas to form a weld metal containing a larger amount of nitrogen than the base metal, and the amount of austenite in the weld is adjusted by adjusting the nitrogen content in the shield gas. A welding method for duplex stainless steel, characterized in that the amount of austenite is close to that of the base metal.