JPS6154514B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to MIG welding of stainless steel and heat-resistant steel containing a large amount of nitrogen (approximately 0.10 wt% or more). Nitrogen is used for austenitic and austenitic-ferritic dual-phase stainless steels.
Nitrogen is a powerful element that increases the pitting corrosion resistance of steel and strengthens it, and recently various pitting corrosion resistant steels and high strength steels containing nitrogen are being developed. Also, on the other hand,
Since nitrogen is a strong austenite-forming element, it can serve as a substitute for Ni to some extent. Ni is a scarce resource and is expensive, but since nitrogen exists inexhaustibly in the atmosphere, it is thought that the effective use of nitrogen will continue to increase in the future in terms of resource conservation. One major problem with such nitrogen-added materials is that they are prone to blowholes during welding, which is a major hindrance to the development of nitrogen-containing steel grades. In particular, in MIG welding where welding is performed in an argon or (argon) + (oxygen) mixed gas atmosphere, blowholes are likely to occur frequently, and this often leads to a situation where welding is impossible or rework is unavoidable. The present inventors took measures against this blowhole and found that by mixing carbon dioxide gas into the shielding gas, a welded part without blowholes or with significantly fewer blowholes could be obtained. The reason why blowholes can be prevented is not clear, but it has been experimentally confirmed. If the amount of carbon dioxide mixed is less than 10%, little blowhole suppression effect can be expected.
Moreover, if it exceeds 50%, it becomes difficult to prevent oxidation of the molten pool, non-metallic inclusions increase, and carbon increases too. Therefore, the mixing ratio of carbon dioxide gas is 10
~50% by volume (at room temperature, under 1 atm) is desirable. The basic shielding gas (gas other than carbon dioxide) may be argon, a mixed gas of argon and oxygen, a mixed gas of argon and nitrogen, or a mixed gas of argon, oxygen, and nitrogen. Although the present invention relates to MIG welding, it can also be applied to covered arc welding. In other words, it is possible to prevent weld blowholes in high-nitrogen steel by coating the bare bar with a type of flux that easily forms carbon dioxide gas.
The effects of mixing carbon dioxide gas with shielding gas are as follows. (1) The composition of the weld zone changes slightly. Carbon increases, albeit by a very small amount. In the case of stainless steel, the amount of increase is about 0.020% or less. Also, Cr decreases slightly. for example
20.11% changes to about 19.72%. These slight changes in composition have little effect on the properties of the material as a practical matter. (2) More spatter occurs. This means that
Although not desirable, this does not prevent welding. Considering the advantage of preventing blowholes, the disadvantages of spatter generation are small. (3) The peelability of the scale generated on the weld bead is improved, and the bead penetration becomes deeper. (4) The mechanical properties of the weld are almost the same as when other shielding gases are used. Next, the present invention will be explained with reference to examples. Example 1 An example of high nitrogen 20Cr-10Ni-2.5Mo steel, which is a pitting-resistant steel, is shown below. The plate and MIG wire used had the same composition, which is shown in Table 1.

【table】

[Table] After making a V-shaped groove on this 15 mm plate and performing three-layer MIG welding by changing the gas mixture ratio, the presence or absence of blowholes was examined by an X-ray transmission test. The results are summarized in Table 2.

[Table] As is clear from Table 2, the conventionally used
While the blowhole generation rate is 100% for Ar alone or Ar + O ₂ mixed gas, the generation rate is zero for Ar + CO ₂ gas. In addition, when the mixed amount of O ₂ and N ₂ is small, Ar + CO ₂ + O ₂ or Ar +
Blowholes are less likely to occur even with a mixed gas of CO ₂ + N ₂ . When O ₂ was 3% by volume or less and N ₂ was 5% by volume or less, the occurrence of blowholes could be completely prevented. Further, as an example, when the joint bead was vertically divided and the number of blowholes in one cross section of the bead was investigated, the number of blowholes in one cross section of the bead was determined as follows for a welding line of 200 mm, depending on the gas used. Blowhole is zero when Ar (10/min) + CO ₂ (5/min). When Ar (14/min) + O ₂ (1/min) was used, 13 blowholes were observed. The accompanying drawing is a polished cross-section approximately to scale of a portion of said cross-section. FIG. 1 shows molten metal produced by a conventional method with blowholes, and FIG. 2 shows welded metal according to the present invention with no welding blowholes. Obviously, large and small blowholes B can be seen in FIG. 1, but none are seen in FIG. Example 2 Regarding high-nitrogen 18Cr-8Ni steel, which is a high-strength steel,
After forming a V-groove and welding five layers of metal by changing the gas mixture ratio, an X-ray transmission test was conducted in the same manner as in Example 1. The composition of the sample material is shown in Table 3. Table 4 shows the results of tests with different gas mixture ratios.

【table】

[Table] In this case as well, blowholes can be prevented by using a mixed gas of Ar + CO ₂ . Table 5 shows an example of the results of a welded joint produced using the welding method according to the present invention and a comparison of its mechanical properties with that of the base material. In addition, the mixing ratio of the shielding gas in this case is argon gas 15/min.
The rate of carbon dioxide gas was 5/min.

[Table] (Note) As is clear from Table 5, in which a bead was notched in the impact test piece of the welded joint, there was almost no change in mechanical properties. As described above, according to the present invention, it is possible to obtain a welded metal part of high nitrogen-containing stainless steel that is completely free of blowholes, which could not be obtained by conventional methods. Mechanical strength and nickel replacement effect can be fully maintained. Therefore, it can be put to practical use in a wide range of fields, and will greatly contribute to industry.

[Brief explanation of the drawing]

Figure 1 is a sketch of a weld blowhole that occurs during normal MIG welding. FIG. 2 is a schematic diagram of a cross section of welded metal subjected to MIG welding according to the present invention.
All are approximately actual size. 1, 2...Sample, almost actual size. B...Blowhole, almost life size.

Claims (1)

[Claims] 1 When performing MIG welding on high nitrogen-containing stainless steel using a bare welding rod, oxygen 3 is used as a shielding gas.
Volume% or less, Nitrogen 5% or less by volume, Carbon dioxide gas 10-50
A blowhole prevention welding method for high nitrogen content stainless steel, characterized by using a mixed gas consisting of argon in the balance by volume.