KR100805059B1 - The method of laser welding ferritic stainless steel - Google Patents

Abstract

A method for laser welding of ferritic stainless steel is provided to improve toughness of laser welds and greatly lower the cracking ratio during processing of the welds accordingly by shielding top beads and back beads of welds of the ferritic stainless steel with an inert gas atmosphere. A method for laser welding of ferritic stainless steel comprises irradiating laser beams onto both edge parts of a ferritic stainless steel sheet, and supplying an inert gas to top beads and back beads of welds at a flow rate of 15 to 24 l/min during welding to shield the welds with an inert gas atmosphere. The inert gas is argon gas.

Description

The method of laser welding ferritic stainless steel}

Figure 1 shows the nitrogen absorption according to the temperature of the molten metal.

2 is a graph comparing the hardness of GTAW and LBW weld metals of 11Cr-0.008C-0.009N steel.

3 is a Schaeffler diagram showing the organization according to the weld component.

4 is a photograph showing a cross-sectional structure of a laser welded portion.

5 is a graph comparing hardness before and after back shielding of laser weld metal of 11Cr-0.0055C-0.006N steel.

Figure 6 schematically shows a bag shielding method in the laser tube welding according to the present invention.

7 schematically shows a method of back shielding in sheet laser welding according to the present invention.

* Description of Major Symbols in Drawings *

 10: top bead

 20: back bead

The present invention relates to a laser welding method of ferritic stainless steel mainly used for automobile exhaust systems. More particularly, the present invention relates to a laser welding method capable of improving the toughness of a laser welded portion by shielding the top bead and the back bead of the welded portion of a ferritic stainless steel in an inert gas atmosphere.

Ferritic stainless steel is a nickel-free stainless steel, characterized by the crystallographic structure of BCC (body centered cubic lattice) and a ferrite single phase structure with no phase transformation from high temperature to room temperature. Ferritic stainless steels have superior resistance to stress corrosion cracking and thermal expansion characteristics than austenitic stainless steels, and have very advantageous advantages in terms of manufacturing cost.

In particular, due to the low thermal expansion, the use of automobile exhaust systems is actively investigated, and a large amount of them are actually used for automobile exhaust system production. The automobile exhaust system manufacturing process is largely divided into an assembling process including a pipe welding process for manufacturing pipes and a processing process for expanding the pipes to be appropriately sized and designed in a suitable size.

However, the ferritic stainless steel has a problem that it is very difficult to ensure the quality of the welded portion during welding due to the BCC structure and structure. In other words, the toughness decreases due to coarsening of grains in the weld zone and precipitation of the second phase. Thus, prevention or reduction of the weld embrittlement phenomenon is the most important factor for the expansion of the use of ferritic stainless steel.

On the other hand, welding methods applied in the exhaust system of automobiles include GTAW (Gas Tungsten Arc Welding), ERW (Electric Resistance Welding) and LBW (Laser Beam Welding). .

GTAW is the most classic welding method, and it is the most widely applied until recently due to the low cost of welding equipment. However, due to the low productivity due to the low welding speed and the grain coarsening of the weld metal due to the single phase solidification peculiar to the ferritic stainless steel, the application is gradually decreasing due to deterioration of the welding quality.

In addition, while ERW has the advantage of high productivity, there are disadvantages such as deterioration of welding quality and expensive equipment cost due to the accumulation of weld interface deposits.

Therefore, in recent years, the application of laser beam welding to the process for automobile exhaust system is gradually increasing.

Since the laser beam welding method enables lower heat input welding than the conventional welding method, the grain coarsening of the weld metal, which is a disadvantage of ferritic stainless steel, is prevented, thereby reducing the weld toughness. In addition, since the width of the weld bead and the weld heat affected zone is narrow, deterioration of material properties due to heat influence can be suppressed, and a high density heat source, i.e., a laser beam can be concentrated in an extremely small area, thereby increasing the welding speed to secure high speed. It is a welding method that can be done.

However, laser welding with a high energy density has a higher temperature of molten metal than arc welding such as GTA welding, and thus, an increase in N, O, etc., due to the reaction between the molten metal and the atmospheric atmosphere is a problem.

Figure 1 shows the nitrogen absorption according to the temperature of the molten metal. 2 shows the hardness distribution when the 11Br-0.008C-0.009N steel is subjected to LBW and GTA welding, and it can be seen that the hardness of the LBW weld is significantly increased compared to the GTA weld.

Therefore, the above-mentioned problems must be solved in order to reduce the incidence of cracks during bending, expansion, etc. of ferritic stainless steel for automobile exhaust systems, to reduce costs through productivity improvement, and to improve durability and life of the exhaust system in the future.

SUMMARY OF THE INVENTION The present invention is directed to improving the above-mentioned conventional problems, and improves the toughness of the laser weld by shielding the top bead and the back bead of the weld of ferritic stainless steel in an inert gas atmosphere. For this reason, an object of the present invention is to provide a laser welding method that can greatly reduce a crack incidence rate during processing of a welded part.

The present invention for achieving the above object comprises shielding the top bead and back bead of the welded portion in an inert gas atmosphere during welding by irradiating a laser beam on both edges of the ferritic stainless steel sheet The present invention relates to a laser welding method of ferritic stainless steel.

Hereinafter, the present invention will be described in detail.

The inventors of the present invention, while searching for a method for improving the problem of deterioration of the quality of the weld metal, such as toughness due to the mixing of nitrogen in accordance with the reaction between the molten metal and the atmosphere during laser welding of ferritic stainless steel, top bead and back bead Results in a significant improvement in crack incidence during machining of welds by preventing the formation of martensite and hardening of the matrix, which inhibits nitrogen incorporation from the external air by shielding the inert gas atmosphere, thereby reducing the toughness of the laser welds. Based on this, the present invention has been completed.

In addition, in the ferritic stainless steel laser welding method of the present invention using an inert gas, the shielding of the back bead is characterized in that the supply of the inert gas at 15-24 l / min per minute.

First, the cause of the hardness increase in the weld part which reduces the toughness of a ferritic stainless steel laser welding part is demonstrated.

First, when martensite tissue is formed in the weld portion during laser welding, it may cause a hardness increase.

Figure 3 shows the change in the structure of the stainless steel according to the amount of nitrogen. As shown in FIG. 3, the raw material has a 100% ferrite structure like the A region, whereas the amount of nitrogen is increased due to the incorporation of external air during laser welding, and the martensite is formed and the amount is gradually increased. It can be seen.

In addition, FIG. 4 is a structure photograph showing a cross section of an actual laser welding part, and it is possible to identify martensite, which is a black structure in the weld metal. In particular, martensite is concentrated on the back bead surface, which may be attributed to nitrogen incorporation due to lack of back shielding.

Second, even though it does not produce martensitic structures, the nitrogen dissolved in the matrix of the weld metal can cause hardness to increase by solid solution strengthening.

Figure 5 is a graph comparing the hardness before and after back shielding of the laser welding metal of 11Cr-0.0055C-0.006N steel. As shown in FIG. 5, when the back shielding is not performed during welding, the hardness of the welded portion is significantly increased, and thus, solid solution strengthening may be predicted by the nitrogen mixed therein.

EMBODIMENT OF THE INVENTION Hereinafter, the laser welding method of this invention is demonstrated.

The laser beam is irradiated to both edge portions of the ferritic stainless steel sheet to shield the top bead and the back bead of the welded portion with an inert gas atmosphere during welding.

In laser beam welding of the ferritic stainless steel sheet of the present invention, the top bead and the back bead are shielded with an inert gas atmosphere. As shown in FIG. 4, it is possible to predict that martensite is generated in the back beads when the back shielding is not performed, thereby reducing the toughness of the laser weld. Therefore, in this invention, it is preferable to perform top shielding and back shielding in inert gas atmosphere.

In addition, the shielding gas may be an inert gas, argon gas and helium gas may be applied as a preferred inert gas. In addition, the shielding of the back bead is preferably supplied with an inert gas at 15-24 l / min per minute.

If the flow rate of the inert gas is less than 15l / min exceeds the nitrogen content of the weld metal targeted in the present invention, if the flow rate exceeds 24l / min, the cost increases and the quenching effect of the molten metal Increased hardness may be a concern. Therefore, the flow rate of the inert gas is preferably limited to 15 ~ 24l / min.

In the present invention, the shielding of the back bead is preferably an inert gas, and argon gas is used.

6 schematically illustrates a back shielding method when welding a laser tube according to an embodiment of the present invention, and FIG. 7 schematically illustrates a back shielding method when welding a sheet metal laser according to an embodiment of the present invention.

In addition, when welding under the above conditions it can be confirmed that the nitrogen content of the weld metal of 300ppm or less can be ensured, and martensite structure is produced when the nitrogen content of the weld metal exceeds 300ppm.

In addition, when welding under the above conditions, the average hardness of the weld metal was able to secure 160Hv or less.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1

The welding was performed as shown in FIG. 6, and the steel grade was 11Cr STS409L material (thickness 1.5mmt), and the carbon and nitrogen contents of the steel grade were 58ppm and 57ppm, respectively. Table 1 shows the back shielding by controlling the flow rate of argon gas for the presence of inert gas atmosphere, observed the formation of martensite in the weld metal, and performing the Charpy impact test at hardness and -20 ° C. Deviation showed mechanical properties.

Target steel grade Output (kW) Inert gas atmosphere Argon Protective Gas Flow Rate (l / min) Martensite production Hardness (Hv) Nitrogen amount (ppm) 20 ℃ Impact Energy (J) Deviation (J) Inventive Steel 1 5.7 O 24 X 155 284 20.1 1.08 Inventive Steel 2 6.5 O 20 X 152 277 21.06 1.17 Comparative Steel 1 5.7 X Not carried O 183 510 13.32 3.8 Comparative Steel 2 7.0 O 10 O 180 380 15.40 3.99

As shown in Table 1, invented steels 1 and 2 satisfying the flow conditions of argon gas in an inert gas atmosphere of the present invention irrespective of laser power, ensure martensite structure by securing nitrogen of 300 ppm or less and hardness of 160 Hv or less. This was not produced, and it can be seen that the toughness of the welded portion was improved because the value of the impact energy increased.

However, the comparative steels 1 and 2, which did not back-shield in an inert gas atmosphere or did not satisfy the inert gas flow rate condition of the present invention, could not secure the nitrogen content and hardness targeted by the present invention. It was confirmed that not only generated but also the mechanical properties.

Example 2

Table 2 shows the results of evaluating the expansion characteristics by expanding the pipe manufactured by laser tube welding as shown in FIG.

Target steel grade Output (kW) Inert gas atmosphere Argon protective gas flow rate (l / min) Expansion pipe temperature (℃) Cracks in the welded part after expansion Comparative Steel 1 5.7 X Not carried 25 O Inventive Steel 1 5.7 O 24 25 X Inventive Steel 2 5.7 O 24 0 X Invention Steel 3 5.7 O 24 -20 X Inventive Steel 4 5.7 O 20 25 X Inventive Steel 5 6.5 O 20 -20 X

As shown in Table 2, in the case of the inventive steel (1-5) that satisfies the flow conditions of the argon gas in the inert gas atmosphere of the present invention, it was confirmed that no cracking of the welded part occurred after expansion.

As described above, according to the present invention, since the top bead and the back bead of the welded portion of the ferritic stainless steel can be shielded from the outside air by shielding the nitrogen gas from the outside air, the toughness of the laser welded portion can be prevented. There is an effect to improve. Therefore, the effect of significantly lowering the crack incidence rate during machining of the laser welded portion can also be ensured at the same time.

Claims (3)

Irradiating the laser beam to both edges of the ferritic stainless steel sheet to supply the inert gas at 15-24 l / min per minute to the top bead and back bead of the welded portion when welding. Laser welding method of ferritic stainless steel comprising shielding in the atmosphere. delete The method of claim 1, wherein the inert gas is an argon gas.

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