KR20010002441A - Welding technology to improve ductility and toughness of laser weldments in fully ferrite stainless steels - Google Patents

Abstract

PURPOSE: A laser welding method of high-chrome ferrite class stainless steel is provided to improve the softness and ductility of a laser welding portion upon welding the fully ferrite class stainless steel with laser. CONSTITUTION: In case that a filler wire is not used upon welding high-chrome ferrite class stainless steel with laser, protective gas He is controlled to 40 to 60l/min. Further, a C + N content of welding metal mold is controlled under 0,03%. Under a condition of using an austenite class filler wire, a filler wire supply speed is 2.5 to 4.0mpm. Am Ni content of the welding metal is controlled to 3.0 to 4.0%. Accordingly, the softness and ductility of the welding metal are prevented from being decreased.

Description

Laser welding method of high chrome ferritic stainless steel {Welding technology to improve ductility and toughness of laser weldments in fully ferrite stainless steels}

The present invention relates to a laser welding method for improving the ductility and toughness of a laser welded portion of a fully ferritic stainless steel.

Fully ferritic stainless steel is a Fe-Cr binary alloy system (usually 18% Cr or more), and a small amount of Ti or Nb stabilizing element and Mo is mainly added to improve corrosion resistance. These metals are characterized by the crystallographic structure of BCC (body centered cubic lattice) and ferrite single phase structure with no phase transformation from high temperature to room temperature. Ferritic stainless steel has superior resistance to stress corrosion cracking and has a very advantageous advantage in terms of manufacturing cost compared to austenitic stainless steel.

However, ferritic stainless steel is difficult to secure weld quality during welding due to the BCC and organizational characteristics. In other words, due to the coarsening of grains in the weld zone and precipitation of the second phase, deterioration of ductility and toughness is remarkable, and therefore, welding construction conditions such as welding heat input and protective gas conditions are very strict, and the use thickness is usually 3 mm or less. Limited. Prevention or reduction of such weld brittleness may be the most important factor in expanding the use of ferritic stainless steel.

On the other hand, since the laser welding method uses a high-density heat source, low heat input welding is possible than arc welding, and thus the welding method is very suitable for all-ferritic stainless steel. However, even low heat input laser welding requires careful attention because of the inferior weldability of the ferritic stainless steel itself and the quality problems inherent in laser welding due to rapid heating and quenching. That is, in the laser welded portion, the heat affected zone can sufficiently reduce the coarsening of the grains, but the weld metal having the coagulation structure cannot avoid coarsening the grains and increase N and O due to the reaction between the molten metal and the atmospheric atmosphere. Deterioration of ductility and toughness is caused by precipitation of the second phase and the like.

After all, the performance of the ferritic stainless steel laser welds is important whether the quality of the weld metal can be ensured, and for this purpose, factors such as control of the composition of the laser weld metal, optimization of welding parameters, selection of filler wires, and use technology Must be established together.

Accordingly, the present invention is to solve the above problems, the present invention relates to a laser welding method that can improve the ductility and toughness of the laser welding portion when laser welding of fully ferritic stainless steel.

1 is a graph showing the effect of Ni on the impact toughness of laser weld metal of 19% Cr-2% Mo ferritic stainless steel.

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

The present invention relates to a laser welding method of high chromium ferritic stainless steel, and the chemical composition of the weld metal in order to ensure the mechanical properties of the laser welded part in the condition that the filler wire is not used and the austenitic filler wire is used. To control. That is, when no filler wire is used, the C + N content of the weld metal is 0.03% or less, and when the austenitic filler wire is used, the Ni content of the weld metal is regulated to 3.0% or more.

First, the reason for limiting the C + N content of the weld metal to 0.03% or less when the filler wire is not used during laser welding will be described.

Deterioration in ductility and toughness of weld metal in ferritic stainless steels is caused by coarsening of grains and precipitation of chromium, Nb, Ti carbon and nitride, and these precipitates are closely related to the C + N content of the weld metal. In other words, in laser welding without using a welding material, the weld metal is determined by the C + N content of the base metal and the nitrogen content penetrating into the molten metal from the atmosphere during welding. In particular, the increase in the N content is the main cause of chromium nitride, Nb, Ti nitride precipitates to reduce the ductility and toughness.

Table 1 shows the C and N content changes of the laser welding metal according to the He protective gas conditions in 19% Cr-2% Mo ferritic stainless steel. The C + N content of laser welded metals without filler wires increased by up to 0.028, depending on the protective gas conditions, with a significant increase in N rather than C. As shown in Table 2, the bending property of the laser welded part having the C + N content is higher, the higher the C + N content of the weld metal, it can be seen that the weld crack occurs at the relatively high temperature side. However, when the C + N content of the weld metal is less than 0.03%, it showed excellent performance without cracking up to -60 ° C.

Therefore, in order to ensure the performance of the laser welding portion, the C + N content of the weld metal should not exceed 0.03%, and for this purpose, the He protective gas conditions should be selected in the range of 40 to 60 l / min. If a protective gas in the range of 60 l / min or more is selected, the N content of the weld metal can be suppressed to the substrate level, but the protection of the weld bit due to the undercut defects and the increase in the welding construction cost. Gas is limited to an appropriate range.

Next, when using an austenitic filler wire, the reason for limiting the Ni content of the weld metal to 3.0% or more will be described.

When Ni is added to the ferritic stainless steel weld metal, it is known to improve the ductility and toughness. However, there are many unclear points about the effect of adding Ni and the appropriate amount. That is, when the amount of Ni is added, the microstructure of the weld metal is changed from the ferrite single phase to the two-phase structure of ferrite and martensite (or austenite), which is good for mechanical properties, but rather poor for corrosiveness due to the galvanic potential difference between the two phases. . Therefore, in order to secure the quality of the laser welding metal by Ni, it is important to understand the mechanism of improving the toughness and ductility, and to set an appropriate addition amount that can maintain the ferrite single-phase structure.

On the other hand, the mechanism by which Ni improves the ductility and toughness of a weld metal is as follows. That is, when 3.0 to 4.0% of Ni is added, Ni is forcibly dissolved in the ferrite matrix structure, so that many deformation twins, which are evidence of short-distance movement of dislocations, occur, and cause cross slip accompanied by ductility at break rather than deformation due to slip. . In addition, as Ni increases, cross slip occurs on the low temperature side, thereby improving ductility and toughness. Ni also suppresses the formation of carbides and nitrides precipitated by C and N which are incorporated into the weld metal from the atmospheric atmosphere during welding.

On the other hand, the limit of the Ni content in the range of 3.0 to 4.0% exceeds 4.0% Ni, the austenite structure is precipitated by an increase in Ni equivalent weight (Nieq. = Ni + 30C + 0.5Mn), and also less than 3.0% In the ductility and toughness of the weld metal is not enough.

Figure 1 shows the change of DBTT (ductile-brittle transition temperature) according to the Ni content of 18% Cr ferritic stainless steel laser welding metal, DBTT decreases as the Ni content increases, actually DBTT is It can be seen that the impact toughness of about 70 ° C. was improved as 0.4% Ni was about 60 ° C. and 3.0% Ni or more at −10 ° C. or less.

For this purpose, when welding a commercially available 309L filler wire at a speed of 2.5 mpm or more at an output of 5 kW, a welding speed of 1 to 3 mpm, and a gap of 0.1 to 0.3 mm, as shown in Table 3, the Ni content of the weld metal is 3% or more. Can satisfy However, when the filler wire speed exceeds 4.0 mpm, the supply amount of the filler wire is excessive and the surface beads of the weld metal are relatively large compared to the back beads, thereby deteriorating the ductility and toughness of the weld metal.

division Protective gas C N O C + N Base material - 0.008 0.0117 0.0075 0.0197 Welding metal 10 l / min 0.009 0.039 0.057 0.048 20 l / min 0.010 0.027 0.042 0.037 30 l / min 0.009 0.019 0.012 0.028 40 l / min 0.008 0.0120 0.0087 0.020

Therefore, according to the present invention, when the filler wire is not used in laser welding of fully ferritic stainless steel, the He protective gas is controlled to 40 to 60 l / min, the C + N content of the weld metal is less than 0.03%, and the austenitic type is used. Under the conditions of using the filler wire, the filler wire supply speed is regulated to 2.5 to 4.0 mpm and the Ni content of the weld metal is in the range of 3.0 to 4.0% to prevent deterioration of the ductility and toughness of the weld metal.

The foregoing is merely illustrative of the preferred embodiments of the present invention, and those skilled in the art to which the present invention pertains can make many changes and modifications within the practice of the present invention without departing from the spirit or scope of the present invention.

Claims (2)

In the laser welding method of high chromium ferritic stainless steel, Controlling the He protective gas from 40 to 60 l / min and controlling the C + N content of the weld metal to less than 0.03%. In the laser welding method of high chromium ferritic stainless steel, The filler wire feed rate is 2.5 to 4.0 mpm and the Ni content of the weld metal is in the range of 3.0 to 4.0%.