KR100908684B1 - TIG welding method of super ferritic stainless steel - Google Patents

Abstract

The present invention relates to a method for welding a TIG of super ferritic stainless steel, comprising a 26Cr-2Mo system having a stabilization ratio of 17 or more and a PRE index of 32 or more, and a 30Cr-4Mo system having a stabilization ratio of 23 or more and a PRE index of 42 or more. In the method for welding a steel TIG, the nitrogen content of the weld metal is 0.02 wt% or less for the 26Cr-2Mo and 0.01 wt% or less for the 30Cr-4Mo, and the welding heat input amount is 20 L / min in pure Ar protective gas atmosphere. The 26Cr-2Mo is 6kJ / cm or less and the 30Cr-4Mo is 4kJ / cm or less.

Accordingly, the present invention, in the TIG welding of super-ferritic stainless steel, in particular, super-ferritic stainless steel consisting of 26Cr-2Mo and 30Cr-4Mo-based to sufficiently secure the impact properties of the weld metal at room temperature to ensure the mechanical performance of the welded portion There is a contributing effect.

Super Ferrite, Stainless Steel, TIG, GTA, Weld Heat Input, Weld Metal, Nitrogen, Impact Properties

Description

TIG Welding Method of Super Ferritic Stainless Steel {Method for TIG Welding of Super Ferritic Stainless Steel}             

1 is a graph showing a change in impact properties at room temperature according to the nitrogen content of the weld metal in the present invention,

Figure 2 is a graph showing a change in the impact characteristics of the weld metal according to the heat input welding in the present invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welding method, and more particularly, to a welding method of a super ferritic stainless steel for improving the impact characteristics of a weld metal in performing a welding of a welding to a super ferritic stainless steel.

Austenitic stainless steel, which has excellent weldability and workability, has been used as structural stainless steel that is required in most corrosive environments. Especially in very harsh corrosive environments such as seawater, PREN (Pitting Resistance Equivalent Number = Cr) Super austenitic stainless steel or high nitrogen austenitic stainless steel (Al-6XN of Allegheny Ludlum, USA) having a + 3Mo = 16N) of 40 or more is used.

However, these super austenitic stainless steels are not necessarily the best materials because of their high production cost and high stress corrosion cracking due to the addition of expensive Ni and Mo.

On the other hand, since ferritic stainless steel hardly adds Ni, by increasing Cr or Mo effective only for corrosion resistance without increasing the manufacturing cost, corrosion resistance equal to or higher than that of austenitic stainless steel can be secured. In particular, in the corrosive environment in which chlorine ions are present, the stress corrosion resistance is better than that of austenitic ferrite.

By maximizing the advantages of these waste-based stainless steels, some overseas mills have developed ferritic stainless steels of 26-29Cr-Mo, which can replace existing Ti alloys or super austenitic stainless steels for seawater exchangers. It has been commercialized.

For example, 26Cr PRE is about 32, which has better corrosion resistance than 316 austenitic stainless steel with PRE of about 25, and 30Cr has 42 levels of PRE, which is inferior to the corrosion resistance of super austenitic stainless steel. Can be. In addition, in Korea, 26Cr system was developed in 2000, and 30Cr system is also planned to be developed.                         

However, high Cr ferritic stainless steels are relatively inferior in ductility and toughness and have high notch sensitivity as compared to austenitic stainless steels, making them fragile as structural materials. In particular, when welding is unavoidable, the mechanical properties of the weld are significantly reduced, which is a direct obstacle to the development and market expansion of ferritic stainless steel.

In general, the reason that the ferritic stainless steel has a lower mechanical property than that of the austenitic stainless steel is that the austenitic stainless steel has a crystal structure of a face-centered cubic (FCC), whereas the ferritic stainless steel has a crystal structure. Steel can be explained by the difference in crystal structure of body-centered cubic (BCC). That is, austenitic stainless steel of FCC structure does not show a big difference from the mechanical properties of room temperature even at cryogenic temperatures due to the well-developed slip system, whereas ferritic stainless steel of BCC structure has a soft-brittle transition. The presence of temperature gives the proper operating temperature. The ductile-brittle transition temperature of ferritic stainless steel is determined by alloying composition and grain size, and the direct influence of the type and amount of stabilizing elements such as C and N, which are invasive elements, and Nb and Ti added to prevent grain boundary sensitization Crazy

On the other hand, in ferritic stainless steel, the embrittlement phenomenon of the welded portion is directly caused by the carbon-nitride deposition characteristic and grain growth due to the heat history of the weld together with the above-described metallurgical characteristics.

In addition, the weld performance of the high Cr stainless steel is relatively lower than that of the low Cr stainless steel. As the Cr content increases, the solubility limit of C and N decreases. This is due to an increase in carbon and nitride precipitation. For this reason, high Cr stainless steel must control C and N content low in order to ensure the same physical properties as low Cr stainless steel. In practice, 26Cr and 30Cr-based ferritic stainless steels are difficult to secure satisfactory weld performance even at room temperature. Of course, during the refining process of manufacturing high Cr ferritic stainless steel, C and N can be suppressed to a very low level to secure physical properties, but there are difficulties in terms of manufacturing cost, so the appropriateness of C and N according to Cr content, that is, manufacturing cost The correlation between the quality and the required quality must be carefully examined.

At present, the representative ferritic stainless steel for welding structure in Korea is 18Cr type, and the welding technology for this is almost established. However, the 26 and 30Cr steels significantly increase the degree of brittleness compared to the low Cr system of 18% or less. In fact, when 26 and 30% Cr superferritic stainless steel is used as the tube for the heat exchanger, the tube is limited to the fillet with the tube sheet to prevent embrittlement of the weld. Or Gas-Tungsten Arc Welding (GTAW) (Allegheny Ludlum, USA), and there are reports of TIG welding of super-ferritic stainless steels on structures with partial use temperatures above 50 ° C.

As described above, superferritic stainless steel is Ni-free and inexpensive, and despite its excellent corrosion resistance, its use range is extremely limited because it is difficult to secure weld performance even at room temperature.                         

Therefore, in order to develop or expand the scope of use of superferritic stainless steels having corrosion resistance equal to or higher than that of high-alloy austenitic stainless steels, the impact characteristics of welded parts are improved when TIG welding, the most stable method among arc welding methods, is performed. Optimization of the welding method that can be done is required.

The present invention was devised in response to the necessity of the improvement required in the welding of the conventional super ferritic stainless steel as described above, the object of which is to improve the impact properties of the weld metal during the welding of the super ferritic stainless steel It is to provide a method of welding a tag of stainless steel.

TIG welding method of super ferritic stainless steel according to the present invention for achieving the above object, 26Cr-2Mo system having a stabilization ratio of 17 or more and PRE index of 32 or more, and 30Cr-4Mo stabilization ratio of 23 or more and 42 or more PRE index In a method of welding a TIG of a super ferritic stainless steel, the nitrogen content of the weld metal is 0.02 wt% or less for the 26Cr-2Mo and 0.01 wt% or less for the 30Cr-4Mo in a 20 L / min pure Ar protective gas atmosphere. The welding heat input is 26Cr-2Mo is 6kJ / cm or less and 30Cr-4Mo is 4kJ / cm or less to the technical gist.

Hereinafter, specific embodiments of the present invention will be described in detail with the accompanying drawings.

Table 1 shows the chemical composition of the concave-convex structural member and super ferritic stainless steel for seawater heat exchanger to apply the present invention.

C Si Mn Cr Ni Mo Nb Ti N Stabilization ratio PRE index 26Cr-2Mo 0.011 0.13 0.2 26.34 0.26 2.0 0.25 0.033 0.0055 17.01 32.3 30Cr-4Mo 0.008 0.33 0.3 30.1 - 4.0 0.26 0.05 0.005 23.8 42.1
* Stabilization ratio = (Nb + Ti) / (C + N)

Super Ferritic stainless steel of 26Cr-2Mo has N of about 55ppm, complex addition of stabilizing elements Nb and Ti for preventing intergranular corrosion of welds, stabilization ratio (Nb + Ti) / (C + N) of 17 or more, PRE The index is over 32. In addition, 30Cr-4Mo superferritic stainless steel has N of about 50 ppm, a stabilization ratio of 23 or more, and a PRE index of 42 or more.

Under these conditions, in the pure Ar protective gas atmosphere of 20 L / min, the nitrogen content of the weld metal is 0.02 wt% or less for 26Cr-2Mo, 0.01 wt% or less for 30Cr-4Mo, and the welding heat input is reduced. 26Cr-2Mo should be 6kJ / cm or less and 30Cr-4Mo should be 4kJ / cm or less and welded.

Figure 1 shows the normal temperature impact characteristics according to the nitrogen content of the weld metal under the conditions of TIG (or GTA) welding heat input 4kJ / cm, where the nitrogen content of the weld metal is a TIG protective gas is pure Ar 5, 10, 20 L / min was mixed and mixed with N ₂ to 50-380 ppm. At this time, in the case of TIG welding under the condition of pure Ar and 20 L / min at a welding heat input of 4 kJ / cm, the N content of the weld metal is equivalent to the N content of the base metal.

As shown in the figure, as the nitrogen content increases, it can be seen that the impact value is significantly reduced, the 26Cr system is higher than the 30Cr system, the impact value is generally higher. That is, in the case of the 26Cr system, a ductile wavefront was obtained up to about 200 ppm, whereas in the case of the 30Cr system, the brittle wavefront was shown at a nitrogen content of 100 ppm or more. In addition, when the nitrogen content of the weld metal was about 250 ppm or more, the impact value of both steel grades was at the lower shelf energy zone. Therefore, the maximum nitrogen content for securing the impact characteristics of the weld metal was 0.02% for the 26Cr system and 0.01% for the 30Cr system.

As the nitrogen content of the weld metal increases, the lowering of the impact characteristics is primarily caused by the precipitation of Cr carbon nitride and Ti, Nb (C, N). Affect These carbon and nitrides are finely distributed in grain boundaries and in the mouth to fix dislocations and strengthen the matrix structure, thereby deteriorating impact characteristics.

In addition, the impact value of the 30Cr system is lower than that of the 26Cr system because of the increased effect of Cr carbonitride and precipitates due to the decrease in the amount of C and N dissolved.

The nitrogen content of the weld metal is determined by the amount of nitrogen in the base metal and the nitrogen introduced from the atmosphere during welding. Therefore, in order to obtain the nitrogen content of the weld metal satisfying the above conditions, it is advantageous that the nitrogen content of the base material is lower, and it is necessary to precisely manage the construction conditions such as the protective gas during welding. However, since conventional super ferritic stainless steels have a maximum working thickness of less than 2.0 mm and control C + N low during manufacturing, the limit of 200 ppm for 26Cr and 100 ppm for 30Cr is an appropriate range of welding construction conditions. In terms of broadening the meaning is important.

On the other hand, Figure 2 shows the change in the impact characteristics according to the heat input of welding during the TIG welding according to the present invention, the room temperature impact value is greatly reduced as the heat input of welding increases, the degree of 30Cr system 26Cr system It can be seen that it is relatively large. In fact, the maximum heat input with a ductile wavefront at room temperature was found to be about 6 kJ / cm 26Cr system, less than 4 kJ / cm 30Cr system.

As the heat input of the welding increases, the impact property decreases due to the coarse precipitation characteristics of carbon and nitride and the increase of heat input rather than grain growth, and nitrogen is mixed into the molten metal from the atmosphere during welding to increase the nitrogen content of the welding metal. This also affects the precipitation characteristics.

Therefore, in order to weld super ferritic stainless steel, the above heat input amount must be observed. For this purpose, low heat input multilayer welding is preferable.

As mentioned above, since most of the super ferritic stainless steels are used with a thickness of less than 2.0 mm, they can be fully infiltrated by one pass welding at a heat input condition of 4 kJ / cm, thereby satisfying sufficient impact characteristics even at room temperature. Means. If the thickness of the stainless steel to be welded is increased, it is expected that there will be no problem in the impact characteristics when the multilayer welding is performed under the above limit heat input conditions.

As described above, in the case of limiting the heat input of the weld and the nitrogen content of the weld metal, the superferritic stainless steel can satisfactorily satisfy the weld impact properties even at room temperature, which has been difficult to secure the impact properties of the weld metal.

Therefore, if the welding method according to the present invention is applied to the 26Cr-based and 30Cr-based superferritic stainless steels, there is no problem in using it as a structural material for room temperature guarantee.

As described above, according to the method of welding a super ferritic stainless steel according to the present invention, even in room temperature in the process of tag welding super ferritic stainless steel, particularly super ferritic stainless steel composed of 26Cr-2Mo and 30Cr-4Mo. The impact property of the weld metal is sufficient to contribute to securing the mechanical performance of the weld.

Claims (1)

In the TIG welding method of a super ferritic stainless steel comprising a 26Cr-2Mo system having a stabilization ratio of 17 or more and a PRE index of 32 or more, and a 30Cr-4Mo system having a stabilization ratio of 23 or more and a PRE index of 42 or more, In a pure Ar protective gas atmosphere of 20 l / min, the nitrogen content of the weld metal is 0.02 wt% or less for the 26Cr-2Mo, 0.01 wt% or less for the 30Cr-4Mo, and 6 kJ for the heat input of the welding of 26Cr-2Mo. Tig welding method of super ferritic stainless steel, characterized in that less than / cm and 30Cr-4Mo is less than 4kJ / cm.

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