Welding method of low carbon stainless steel

Abstract

The present invention is characterized by providing a method for welding low-carbon stainless steel, comprising the step of GMAW welding the low-carbon stainless steel at a current of 145A to 155A and a speed of 65cpm to 75cpm.

Description

Technology Field

[0001] The present invention relates to a welding method for low-carbon stainless steel. Background Technology

[0002] Major countries around the world are urging a transition to a hydrogen society to reduce air pollutants and are actively formulating strategies for hydrogen utilization, particularly in the shipping sector. Hydrogen fuel cell-powered vessels have the potential to become a key element of competitiveness and maritime trends in the shipbuilding industry in the near future. However, compared to Europe, where demonstration operations of hydrogen electric propulsion vessels have been conducted, or Japan, where liquefied hydrogen carriers have been operated, Korea currently lacks research on related technologies and case studies.

[0003] Since cryogenic liquids such as liquid hydrogen experience gas evaporation losses due to external heat intrusion into the storage container, the application of proven insulation materials is required. It is known that low-carbon stainless steel can be used as a material for liquid hydrogen storage tanks.

[0004] Welding is an essential process in the fabrication of liquefied hydrogen storage tanks. Welding is a method of joining metal materials of the same or different types by applying heat and pressure to create a direct bond between solids. If defects occur in the weld, energy loss exceeding 30% results, necessitating maintenance. However, it is difficult to find welders every time a defect occurs, and the reality is that maintenance costs associated with weld defects are also high. Therefore, there is a need to develop an optimal welding method to form highly reliable welds. Prior art literature

[0005] Korean Published Patent Application No. 10-2010-0065449 The problem to be solved

[0006] The present invention aims to provide a welding method for low-carbon stainless steel.

[0007] More specifically, the present invention aims to provide a welding method for stainless steel in which the tensile strength and hardness of the weld joint, as well as the impact strength of the weld joint at room temperature and cryogenic temperatures, are maintained excellently. means of solving the problem

[0008] The present invention provides a method for welding low-carbon stainless steel, comprising the step of GMAW welding the low-carbon stainless steel at a current of 145A to 155A and a speed of 65cpm to 75cpm.

[0009] In one embodiment, the carbon content of the low-carbon stainless steel may be 0.03 wt% or less.

[0010] In another embodiment, the low-carbon stainless steel may contain 0.03 wt% carbon (C), 18 wt% to 20 wt% chromium (Cr), 2 wt% manganese (Mn), 1 wt% to 2 wt% silicon (Si), 0.045 wt% phosphorus (P), 0.015 wt% to 0.03 wt% sulfur (S), 8 wt% to 12 wt% nickel (Ni), and 0.1 wt% nitrogen (N).

[0011] In another embodiment, the welding conditions may be a current of 150A and a speed of 70cpm.

[0012] In one embodiment, GMAW welding may be carried out by feeding a welding wire to low-carbon stainless steel and generating an arc discharge between the welding wire and the low-carbon stainless steel.

[0013] In another embodiment, the welding wire may comprise 0.01 wt% to 0.03 wt% carbon (C), 1 wt% to 2.5 wt% manganese (Mn), 0.3 wt% to 0.65 wt% silicon (Si), 0.02 wt% to 0.03 wt% sulfur (S), 0.03 wt% to 0.045 wt% phosphorus (P), 19 wt% to 22 wt% chromium (Cr), and 9 wt% to 12 wt% nickel (Ni).

[0014] The present invention also provides a method for welding low-carbon stainless steel, comprising the step of GMAW welding the low-carbon stainless steel with a current of 145A to 155A; wherein the GMAW welding is performed by feeding a welding wire to the low-carbon stainless steel to generate an arc discharge between the welding wire and the low-carbon stainless steel.

[0015] The present invention also provides a method for welding low-carbon stainless steel, comprising the step of GMAW welding the low-carbon stainless steel at a speed of 65 cpm to 75 cpm; wherein the GMAW welding is performed by feeding a welding wire to the low-carbon stainless steel to generate an arc discharge between the welding wire and the low-carbon stainless steel.

[0016] The present invention also provides a method for welding low-carbon stainless steel, comprising the step of GMAW welding the low-carbon stainless steel with a current of 145A to 155A; wherein the low-carbon stainless steel comprises 0.03wt% carbon (C), 18wt% to 20wt% chromium (Cr), 2wt% manganese (Mn), 1wt% to 2wt% silicon (Si), 0.045wt% phosphorus (P), 0.015wt% to 0.03wt% sulfur (S), 8wt% to 12wt% nickel (Ni), and 0.1wt% nitrogen (N).

[0017] The present invention also provides a method for welding low-carbon stainless steel, comprising the step of GMAW welding the low-carbon stainless steel at a speed of 65 cpm to 75 cpm; wherein the low-carbon stainless steel comprises 0.03 wt% carbon (C), 18 wt% to 20 wt% chromium (Cr), 2 wt% manganese (Mn), 1 wt% to 2 wt% silicon (Si), 0.045 wt% phosphorus (P), 0.015 wt% to 0.03 wt% sulfur (S), 8 wt% to 12 wt% nickel (Ni), and 0.1 wt% nitrogen (N). Effects of the invention

[0018] The objective of the present invention is to provide a welding method for low-carbon stainless steel.

[0019] According to the welding method of the present invention, the tensile strength and hardness of the weldment, as well as the durability of the weldment at room temperature and cryogenic temperatures, are excellent.

[0020] The welding method of the present invention can be utilized for welding liquid hydrogen storage tank materials. Brief explanation of the drawing

[0021] Figure 1 is a drawing illustrating the indicators for analyzing the cross-section of a weld. Figure 2 is a cross-sectional view showing the shape of the weldment. Figure 3 shows the results of observing the microstructure of the weldment of Experimental Example 1D. Specific details for implementing the invention

[0022] Embodiments that enable a person skilled in the art to easily implement the present invention are described in detail below with reference to the attached drawings. However, in describing the operating principles of preferred embodiments of the present invention in detail, specific descriptions of related known functions or configurations are omitted if it is determined that such detailed descriptions may unnecessarily obscure the essence of the present invention.

[0023] In addition, the same reference numerals are used for parts having similar functions and operations throughout the drawings. Throughout the specification, when a part is described as being connected to another part, this includes not only cases where they are directly connected, but also cases where they are indirectly connected through intermediate components. Furthermore, unless specifically stated otherwise, the inclusion of a certain component does not exclude other components but implies that additional components may be included.

[0024] Liquid hydrogen storage containers require the maintenance of durability at temperatures below -253°C, and Stainless Steel 304L is known to be a suitable material for this purpose. However, while extensive research has been conducted on material and welding characteristics, studies on welding conditions and weld characteristics for materials intended for use at cryogenic temperatures are insufficient. Accordingly, the present invention aims to provide optimal welding conditions for low-carbon stainless steel capable of maintaining durability even at cryogenic temperatures. .

[0025] Specifically, the present invention provides a method for welding low-carbon stainless steel comprising the step of GMAW welding the low-carbon stainless steel at a current of 145A to 155A and a speed of 65cpm to 75cpm.

[0026] Welding utilizing the welding conditions of the present invention allows for securing a heat-affected zone (HAZ) with an appropriate penetration depth without burn-through. Furthermore, the weld produced according to the welding conditions of the present invention exhibits excellent hardness, tensile strength, and impact strength at room temperature and cryogenic temperatures.

[0027] Among various welding methods, Gas Metal Arc Welding (GMAW) is a welding technique that uses a consumable electrode to generate an arc through electrical discharge between the base material and the electrode, and then uses the generated high-temperature arc heat to melt the base material and the incoming welding wire for welding.

[0028] In the GMAW welding step, welding can be performed by moving a welding torch mounted on the tip of a welding machine between two base materials (low-carbon stainless steel) to continuously feed a welding wire, and by generating an arc between the tip of the welding wire and the two base materials to melt the two base materials with the heat.

[0029] Stainless steel 304L has the advantage of ensuring stability when used as a material for liquid hydrogen storage tanks because it does not exhibit hydrogen-induced brittleness at the operating temperature of liquid hydrogen, which is 20K (-253℃). However, various changes in steel properties, such as non-uniform microstructure, residual stress, and phase transformation, may occur due to welding, which is an essential process for manufacturing liquid hydrogen storage tanks. Therefore, it is necessary to derive optimal welding conditions that allow Stainless steel 304L to be used as a material for liquid hydrogen storage tanks, and the present invention aims to provide optimal welding conditions.

[0030] The carbon content of the low-carbon stainless steel used in the welding method of the present invention may be 0.03 wt% or less. When stainless steel is heated to approximately 480°C to 820°C by welding or the like, chromium carbide may form on the grain boundary surfaces, which is called sensitization. When sensitization occurs, the corrosion performance of the grain boundary surfaces of the stainless steel may be reduced by the chromium carbide. However, the stainless steel of the present invention has a carbon content of up to 0.03 wt%, which makes it easy to prevent sensitization caused by welding, and thus has the advantage of being easy to use in liquid hydrogen storage tanks where there is a risk of corrosion.

[0031] In one embodiment, the low-carbon stainless steel may contain 0.03 wt% carbon (C), 18 wt% to 20 wt% chromium (Cr), 2 wt% manganese (Mn), 1 wt% to 2 wt% silicon (Si), 0.045 wt% phosphorus (P), 0.015 wt% to 0.03 wt% sulfur (S), 8 wt% to 12 wt% nickel (Ni), and 0.1 wt% nitrogen (N).

[0032] In one embodiment, the low-carbon stainless steel may be used as a material for a liquid hydrogen storage tank.

[0033] Welding current is the most important variable determining deposition rate and penetration depth. The constant voltage power supply used in GMAW welding has the characteristic of automatically supplying the necessary current to maintain the arc on the wire ejected from the torch tip.

[0034] The welding current may be 145A to 155A, preferably 150A. A current exceeding the above range may form a convex bead, resulting in a poor appearance.

[0035] The welding voltage may be 14V to 38V, and more specifically, 16V to 18V.

[0036] Welding speed is the speed at which the arc travels along the weld joint. Along with welding current, welding speed affects penetration depth, bead shape, and arc stability.

[0037] The welding speed may be 65 cpm to 75 cpm, and preferably 70 cpm.

[0038] If the welding speed is too slow, the arc is generated in the molten metal rather than the base metal, resulting in less penetration. If the welding speed is too fast, the energy input per unit length is reduced, resulting in less penetration. Consequently, the size of the heat-affected zone (HAZ) is not properly formed, which can lead to a deterioration in the characteristics of the weld.

[0039] In GMAW welding, the supply of shielding gas is required to protect the stainless steel being melted by generating an arc while maintaining a constant distance between the consumable electrode and the base metal. Since the arc and molten metal are protected from the atmosphere through the supply of shielding gas, the use of appropriate shielding gas is essential to obtain a sound weld.

[0040] The actual contact tip-to-work distance (CTWD) is preferably 4 mm to 5 mm.

[0041] In GMAW welding, the consumable electrode maintains a constant distance from the base metal to generate an arc, and a supply of shielding gas is required to protect the molten metal. A mixture of Ar and oxygen can be used as the shielding gas during welding. Since using only pure Ar gas can cause defects such as undercuts at the edges of the weld bead, it is advisable to use a mixture of oxygen gas.

[0042] In one embodiment, the shielding gas during welding may be a mixed gas of 95 wol% to 99 vol% Ar gas and 1 vol% to 5 vol% oxygen gas. When Ar and oxygen are mixed and used within the above ranges, there are advantages such as increased arc stability and welding speed, and improved bead shape.

[0043] The flow rate of the shielding gas can be 20 L / min to 25 L / min.

[0044] It is preferable that the wire protrusion length be 10 mm to 15 mm. The wire protrusion length refers to the length of the wire from the tip of the torch to the arc point, and this length must be maintained constant during welding.

[0045] In one embodiment, the torch used for welding may be a curved air-cooled torch.

[0046] The torch angle may be 25˚ to 35˚, and in one embodiment, the torch angle may be 30˚.

[0047] GMAW welding is performed by feeding a welding wire to low-carbon stainless steel to generate an arc discharge between the welding wire and the low-carbon stainless steel, and the welding wire may contain 0.01 wt% to 0.03 wt% carbon (C), 1 wt% to 2.5 wt% manganese (Mn), 0.3 wt% to 0.65 wt% silicon (Si), 0.02 wt% to 0.03 wt% sulfur (S), 0.03 wt% to 0.045 wt% phosphorus (P), 19 wt% to 22 wt% chromium (Cr), and 9 wt% to 12 wt% nickel (Ni). The welding wire may further contain 0.75 wt% or less molybdenum (Mo). The welding wire may further contain 0.75 wt% or less copper (Cu).

[0048] The diameter of the welding wire can be appropriately selected.

[0049] In one embodiment, the diameter of the welding wire may be 0.8 mm to 4 mm.

[0050] In one embodiment, the diameter of the welding wire may be 1.2 mm.

[0051] In one embodiment, the low-carbon stainless steel weld formed through the welding step may have a tensile strength of 550 MPa or more and a yield strength of 300 MPa or more.

[0052] In one embodiment, the low-carbon stainless steel weld formed through the welding step may have an impact toughness of 70 J or more at room temperature and an impact toughness of 90 J or more at cryogenic temperature (-196℃).

[0053] The present invention will be described in detail below with reference to examples. However, the embodiments according to the present invention may be modified in various different forms, and the scope of the present invention should not be interpreted as being limited to the embodiments described below. The embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the art.

[0054] <용접 조건의 도출>

[0055] A system combining a KULA robot (KR 60HA) with a Fronius welding machine (VR 7000 CMT) was used as a welding machine.

[0056] To compare weldability in a single pass between processes, the thickness was determined to be 2 mm, and the base material dimensions of the specimens were set to 150 mm × 50 mm × 2 mm. Stainless steel ER-308L with a diameter of 1.2 mm was used as the welding wire. Welding was performed by varying the welding speed and current ranges as shown in 1A to 1F of Table 1 below, and a cross-sectional analysis of the weldment was conducted. W is the part name used to analyze the cross-sectional view of the weldment. B , D P , HD B represents what is shown in Figure 1, and the analyzed values ​​are shown in Table 1.

[0057]

[0058] The size of the Heat Affected Zone (HAZ) was adopted as a criterion for selecting optimal welding conditions. The HAZ refers to the portion of the base material that undergoes changes in its microstructure or properties due to welding heat. In Fig. 2, the image in the bottom row is the result of observing a cross-sectional view of the weld using an optical microscope (OM). Fig. 3 is the result of observing the microstructure of the cross-sectional view of the weld in Experimental Example 1D. As a result of the experiment, the weld had a gamma-austenite grain shape, and in the case of the HAZ, the gamma-austenite grains grew in a columnar shape with a narrow width along the direction of heat dissipation, and delta-ferrite grain boundaries were also observed between the HAZ and the weld. As a result of the experiment, the welding condition of 1D, which had the smallest HAZ size with a current of 150 A and a welding speed of 70 cpm, was confirmed to be the optimal welding condition.

[0059] <용접부의 인장 강도 시험 및 항복 강도 시험의 수행>

[0060] Tensile strength tests were conducted to evaluate the reliability of the weldment. The specimens were tested five times each in reference to ASTM E8 / E 8M-8 standards, and the average values ​​are shown in Table 2.

[0061] Since the tensile value of the welded part was low, the part that fractured during the tensile strength test was the welded part. The tensile strength of the base metal, stainless steel, was 734.2 MPa, and the yield strength was 313.54 MPa.

[0062]

[0063] As a result of the experiment, the tensile strength and yield strength values ​​of Experimental Example 1D were the highest, and it was confirmed that the strength of the weld was the best under the welding conditions of Experimental Example 1D. It was confirmed that when the welding speed was faster or the welding current was higher than that of Experimental Example 1D, the tensile strength and yield strength dropped sharply, and the strength of the weld decreased.

[0064] <용접부의 상온 및 극저온 충격 시험의 수행>

[0065] The Charpy V-Notch Impact Test was performed three times each at room temperature and cryogenic temperature (-196℃) on specimens with optimal conditions for GMAW weldment. The specimens were fabricated in reference to ASTM E23-07. To account for the thickness effect and convert the result to Standard Thickness (10T), a conversion formula [impact toughness (J) = (impact specimen value of 2T specimen) / (Net Area)] was applied by dividing by the Net Area. The average value of the three experiments is shown in Table 3.

[0066]

[0067] Experimental results confirmed that the impact strength of Experimental Example 1D was the best at room temperature and cryogenic temperature. It was confirmed that if the welding speed is faster or the welding current is higher than that of Experimental Example 1D, the impact strength at room temperature and cryogenic temperature drops sharply, and durability is significantly reduced.

Claims

Claim 1 The method comprises the step of GMAW welding low-carbon stainless steel at a current of 145A to 155A and a speed of 65cpm to 75cpm, wherein the GMAW welding is performed by feeding a welding wire to the low-carbon stainless steel to generate an arc discharge between the welding wire and the low-carbon stainless steel, wherein the low-carbon stainless steel comprises 0.03wt% carbon (C), 18wt% to 20wt% chromium (Cr), 2wt% manganese (Mn), 1wt% to 2wt% silicon (Si), 0.045wt% phosphorus (P), 0.015wt% to 0.03wt% sulfur (S), 8wt% to 12wt% nickel (Ni), and 0.1wt% nitrogen (N), and the welding wire comprises 0.01wt% to 0.03wt% carbon (C) and manganese (Mn) A welding method for low-carbon stainless steel comprising 1 wt% to 2.5 wt%, silicon (Si) 0.3 wt% to 0.65 wt%, sulfur (S) 0.02 wt% to 0.03 wt%, phosphorus (P) 0.03 wt% to 0.045 wt%, chromium (Cr) 19 wt% to 22 wt%, and nickel (Ni) 9 wt% to 12 wt%, wherein the distance between the contact tip and the base material is 4 mm to 5 mm, the shielding gas during welding is a mixed gas of 95 wol% to 99 vol% Ar gas and 1 vol% to 5 vol% oxygen gas, the flow rate of the shielding gas is 20 L / min to 25 L / min, the diameter of the welding wire is 0.8 mm to 4 mm, and the welding voltage is 14 V to 38 V. Claim 2 delete Claim 3 delete Claim 4 A welding method for low-carbon stainless steel according to claim 1, characterized in that, in the step of GMAW welding, the welding conditions are a current of 150A and a speed of 70cpm. Claim 5 delete Claim 6 delete Claim 7 A welding method for low-carbon stainless steel according to claim 1, characterized in that the welding wire is stainless steel ER-308L with a diameter of 1.2 mm. Claim 8 delete Claim 9 delete Claim 10 delete Claim 11 delete Claim 12 delete Claim 13 delete Claim 14 delete Claim 15 delete Claim 16 delete Claim 17 delete

Images