JPH01228687A - Laser beam welding method - Google Patents

Abstract

PURPOSE:To improve reliability of a welded joint by performing laser beam welding in a nitrogen atmosphere at the time of projecting a laser beam on stainless steel to perform welding and solving nitrogen in the joint sufficiently to increase strength and toughness of the welded joint. CONSTITUTION:Air in a machining chamber 5 is replaced with nitrogen gas 9. In this state, the laser beam 2 is projected on the stainless steel 11 which is material to be welded to form a molten part. At this time, the nitrogen can be solved sufficiently in molten metal by making the nitrogen gas pressure in the nitrogen atmosphere almost to the atmospheric pressure. In addition, the nitrogen pressure is increased sufficiently with respect to the joint requiring high strength. By this method, the welded joint excellent in strength and toughness can be obtained.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention provides a method for laser welding steel materials such as nitrogen-added austenitic stainless steel that requires high strength and high toughness at extremely low temperatures. Regarding.

(Prior art) Cryogenic structural materials such as superconducting magnets used in nuclear fusion reactors are required to have sufficient strength and destruction resistance at 4.2K, and for example, the target values of the Japan Atomic Energy Research Institute are Crab 1200MPa or more, fracture toughness: 200M
Material properties of PaJm or higher are required.

Traditionally, austenitic stainless steel w4 (
SUS304LSSUS316LN, etc.) are mainly used, but as shown in Table 1 below, these materials are reduced in carbon to desensitize them, and nitrogen is added to further increase their strength.

Therefore, when welding steel materials with the above composition,
Naturally, high yield strength and dust protection are required for the welded joints as well. Figures 3 and 4 respectively show the state of the stainless steel material with the above composition at cryogenic temperatures (4,2K).
It is a graph showing the relationship between nitrogen content in steel materials, yield strength, and fracture resistance.

By the way, since the nitrogen content in welded steel materials usually tends to decrease, it is particularly important to maintain the nitrogen content in the weld metal in order to maintain joint strength.

Taking these points into consideration, shielded arc welding and MIG welding are mainly used as conventional welding. In coated arc welding, nitride is added to the flux to increase the nitrogen content in the weld metal, but this method causes relatively large welding deformation, which poses a problem in precision welding. be. On the other hand, in MIG welding, N2 is mixed with the shielding gas to add nitrogen to the weld metal, but as with shielded arc welding, welding deformation becomes large and precision structures are There are many problems with welding methods.

Further, electron beam welding is also known as a high energy density welding method that can be expected to produce low distortion.

However, electron beam welding is performed in a vacuum chamber (10
'~1. ．． Since welding is performed within 0-'Torr),
Inevitably, nitrogen in the steel tends to evaporate, resulting in a significantly lower nitrogen content in the weld metal than in the base metal, resulting in a problem in that sufficient joint strength cannot be maintained.

The purpose of this invention is to provide a welding method capable of obtaining a welded joint having sufficient yield strength and excellent toughness in welding high nitrogen-added austenitic stainless steel.

[Structure of the invention]

(Means and effects for solving the problems) In order to achieve the above object, the welding method of the present invention uses a laser as a welding heat source and a nitrogen atmosphere when welding a base material made of nitrogen-added stainless steel. It is characterized by increasing the nitrogen content of the welded joint by performing laser welding inside.

In the present invention, for example, an optical system that focuses laser light emitted on high nitrogen-added austenitic stainless steel as a welding base material, a shielding box for creating a laser atmosphere, and a laser oscillator are provided. After replacing the laser processing atmosphere with nitrogen, forming a weld metal by a laser welding method in which laser is irradiated in the atmosphere,
By increasing the nitrogen content in this weld metal, it is possible to obtain a welded joint with sufficient yield strength and toughness.

Normally, in laser welding methods, assist gas is flowed coaxially with the laser beam in order to remove the plasma generated when the workpiece is irradiated with the laser beam and protect the condenser lens. ing. However, the present inventors have confirmed that simply using nitrogen as the assist gas does not effectively add nitrogen to the weld metal.

The present invention uses laser welding as a welding method, irradiates the workpiece with laser light in a nitrogen atmosphere, actively dissolves nitrogen in the formed molten pool, and increases the nitrogen content in the weld metal. It is something. For this reason, the weld metal (welded joint part) has excellent properties in both yield strength and toughness, and since laser light is used as the welding heat source,
Welding deformation can also be minimized.

〔Example〕

Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

Example 1 As shown in FIG. 1, a welding device includes a high-output CO2 laser oscillator 1, an optical system 3 for transmitting the oscillated laser beam 2, and a condensing system for condensing the transmitted laser beam 2. It consists of a condensing lens 4, a processing chamber 5 for creating a laser atmosphere, and an NC table 6 for moving the processing chamber 5.The processing chamber 5 has an exhaust lower for exhausting the atmosphere, It is composed of an exhaust pump 8 and an inlet 10 for inhaling nitrogen gas 9.

In this embodiment, the material to be welded 11 is a low-temperature high nitrogen-added austenitic stainless steel (SU
S304LN and 5US316LN) were used. Specific examples of austenitic stainless steels that can be used in the present invention are shown in Table 2 below.

In addition, in this embodiment, as a laser, the maximum output is 1
A 0 kW carbon dioxide laser was used. The welding conditions are as follows.

Laser output 4kw ~ 10kw Condensing lens focal length: 127++g, 190 cities, 2
54 Dragon, Processing speed: 1.000-3000 degrees/10 Focus setting position Knee 6-OII++1 Plate thickness = 2-12+++m In addition, the form of the joint was a butt joint.

First, as a preparation before welding, the laser atmosphere is replaced with nitrogen. That is, the air in the machining chamber 5 is exhausted from the exhaust lower using an exhaust pump 8 (a rotary pump may be sufficient) to reduce the partial pressure of the atmosphere, and then
Nitrogen gas 9 is sucked in from the inside of the processing chamber 5 to replace the inside of the processing chamber 5 with nitrogen.

In this state, the material to be welded 11 was irradiated with laser light 2 to generate a welded part. By setting the nitrogen gas pressure at this time to approximately atmospheric pressure, the amount of nitrogen dissolved in the molten metal is greater than the amount of nitrogen dissipated from the molten metal, making it possible to sufficiently contain nitrogen in the weld metal. It becomes possible. An example of welding results is shown in Table 3 below. As shown in this table, it was found that according to the method of the present invention, the nitrogen content in the weld metal of both base metals was approximately the same as the content of the I-material.

Table 3 Example 2 Using an austenitic steel as the base material and using a laser as a heat source, the nitrogen content in the weld metal was selected so that the strength and toughness of the weld metal achieved the most desirable values for a joint,
To accomplish this, the nitrogen pressure of the atmosphere is adjusted. Second
The figure is a graph showing the relationship between the nitrogen pressure in the processing chamber and the nitrogen content in the weld metal at this time.

As described above, in the present invention, a method of increasing the nitrogen pressure to a sufficiently high level can be adopted for joints that may have slightly low toughness but require high strength.

Example 3 The same effects as in Example 1 and Example 2 can be obtained by using high nitrogen addition high Mn steel (for example, the steel material in Table 1 above) as the base material.

Example 4 Furthermore, in order to prevent a decrease in joint strength due to welding even when the base metal is not only a high nitrogen addition type steel but also a general steel, nitrogen is added to the weld metal to increase the strength of the weld metal relative to the base metal.
By slightly raising it, it is also possible to form an overmatched welded joint. As a result, joint fracture always occurs on the base metal side, and accidental fracture caused by minute defects in the weld metal can be effectively prevented.

〔Effect of the invention〕

According to the laser welding method of the present invention, the nitrogen content in the weld metal can be increased, so a welded joint with excellent strength and toughness can be obtained.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of the equipment system used in the embodiment of the present invention, Figure 2
Figure 3 is a graph showing the relationship between nitrogen pressure and nitrogen content in the weld metal, Figure 3 is a graph showing the relationship between nitrogen content and proof stress, and Figure 4 is a graph showing the relationship between nitrogen content and fracture resistance equipment. This is a graph showing. 1... Oscillator, 2... Laser light, 5... Processing room,
8...Exhaust pump. Applicant's agent Mr. Sato Figure 1

Claims (1)

[Claims] 1. When welding a base material made of stainless steel, a laser is used as a welding heat source and the laser welding is performed in a nitrogen atmosphere to increase the nitrogen content of the welded joint. Laser welding method. 2. By adjusting the nitrogen pressure in the welding atmosphere,
The laser welding method according to claim 1, characterized in that the nitrogen content of the weld joint is controlled.