A method of welding stainless steel to improve cryogenic toughness of welding parts

Abstract

A welding method for stainless steel that improves cryogenic toughness of a weld according to one embodiment of the present invention relates to a welding method for stainless steel that improves toughness even at cryogenic temperatures in a weld formed during the manufacturing process of equipment such as a storage tank, pipe, and valve for liquid hydrogen made of 300 series austenitic stainless steel, comprising: a step of setting a condition for a phase fraction, which is a fraction of a plurality of phases formed in the weld; and a step of performing welding of the stainless steel to satisfy the condition for the phase fraction; wherein the plurality of phases may include a γ phase, a δ phase, and a σ phase formed in the weld.

Description

Technology Field

[0001] The present invention relates to a welding method for stainless steel that improves the cryogenic toughness of the welded part, and more specifically, to a welding method for stainless steel that improves the cryogenic toughness of the welded part of liquid hydrogen equipment, such as storage tanks, pipes, and valves, made of 300 series austenitic stainless steel. Background Technology

[0003] Generally, austenitic stainless steel, which has good workability and corrosion resistance, uses Fe as the base metal and contains Cr and Ni as major raw materials; it is being developed into various steel grades to suit various applications by adding other elements such as Mo and / or Cu.

[0004] For example, 304 and 316 series austenitic stainless steels have excellent corrosion resistance and workability because they contain expensive raw materials such as Ni and Mo.

[0005] Meanwhile, recently, technology development utilizing hydrogen as an energy source is underway to suppress the emission of greenhouse gases (CO2, NOx, SOx) from the perspective of preventing global warming, and the development of equipment used for the storage and transportation of produced hydrogen is also in progress.

[0006] One method used for the storage and transportation of hydrogen is the use of liquefied hydrogen. Although equipment such as storage containers, piping, and valves for liquefied hydrogen is manufactured using such austenitic stainless steel, there has been a problem where the resistance and toughness of the connection parts, specifically the welded joints, are reduced during the manufacturing process.

[0007] To solve the problem of such welded joints, Korean registered patent No. 10-1657831 B1 (2016.09.08) discloses a weld metal portion of an austenitic stainless steel plate with excellent corrosion resistance.

[0008] However, since the stability of equipment is required at approximately 20K, or -250℃, for storing liquid hydrogen, there is a need to develop welding technology for equipment that can safely store liquid hydrogen by improving the toughness of the welds even at cryogenic temperatures. The problem to be solved

[0010] The present invention was devised to solve the above-mentioned problems and aims to provide a welding method for stainless steel that improves the cryogenic toughness of the weld joint, so that even when manufacturing equipment for liquid hydrogen using 300 series austenitic stainless steel, the toughness of the weld joint is improved at -250°C or lower, allowing for safer storage of liquid hydrogen.

[0012] The problems that the present invention aims to solve are not limited to those described above, and problems not mentioned will be clearly understood by those skilled in the art from this specification and the attached drawings. means of solving the problem

[0014] A welding method for stainless steel that improves cryogenic toughness of a weld according to one embodiment of the present invention relates to a welding method for stainless steel that improves toughness even at cryogenic temperatures in a weld formed during the manufacturing process of equipment such as a storage tank, pipe, and valve for liquid hydrogen made of 300 series austenitic stainless steel, comprising: a step of setting a condition for a phase fraction, which is a fraction of a plurality of phases formed in the weld; and a step of performing welding of the stainless steel to satisfy the condition for the phase fraction; wherein the plurality of phases may include a γ phase, a δ phase, and a σ phase formed in the weld. Effects of the invention

[0016] According to a welding method for stainless steel that improves the cryogenic toughness of a welded part according to one embodiment of the present invention, the toughness of the welded part formed during the manufacturing process of equipment for liquid hydrogen is improved even at cryogenic temperatures, thereby providing the advantage of being able to store or transport liquid hydrogen more safely through equipment manufactured by welding.

[0018] The effects of the present invention are not limited to the effects described above, and unmentioned effects will be clearly understood by those skilled in the art from this specification and the accompanying drawings. Brief explanation of the drawing

[0020] Figure 1 is a photograph illustrating a weld formed during the manufacturing process of equipment made from austenitic stainless steel. Figure 2 is an enlarged photograph of the weld of equipment manufactured from austenitic stainless steel. FIG. 3 is an experimental data table for explaining the step of setting conditions for a phase fraction constituting a welding method for stainless steel that improves the cryogenic toughness of a welded joint according to an embodiment of the present invention, and the step of performing welding of stainless steel to satisfy the conditions for the phase fraction. FIG. 4 is a graph for explaining the step of setting conditions for the phase fraction constituting a welding method for stainless steel that improves the cryogenic toughness of a welded part according to one embodiment of the present invention. FIG. 5 is a flowchart for explaining, in general, a welding method for stainless steel that improves the cryogenic toughness of a welded joint according to one embodiment of the present invention. Specific details for implementing the invention

[0021] Specific embodiments of the present invention will be described in detail below with reference to the drawings. However, the concept of the present invention is not limited to the presented embodiments. Those skilled in the art who understand the concept of the present invention may easily propose other inventions that are inferior or other embodiments included within the scope of the concept of the present invention by adding, changing, or deleting other components within the same scope of the concept, and such are also to be considered to be included within the scope of the concept of the present invention.

[0023] A welding method for stainless steel that improves cryogenic toughness of a weld according to one embodiment of the present invention relates to a welding method for stainless steel that improves toughness even at cryogenic temperatures in a weld formed during the manufacturing process of equipment such as a storage tank, pipe, and valve for liquid hydrogen made of 300 series austenitic stainless steel, comprising: a step of setting a condition for a phase fraction, which is a fraction of a plurality of phases formed in the weld; and a step of performing welding of the stainless steel to satisfy the condition for the phase fraction; wherein the plurality of phases may include a γ phase, a δ phase, and a σ phase formed in the weld.

[0024] Here, the condition for the phase fraction may be a welding method for stainless steel that improves the cryogenic toughness of the weld, which is a condition for setting at least one of the phase fractions of the γ phase, δ phase, and σ phase formed in the weld.

[0025] For example, the step of setting conditions for the phase fraction, which is a fraction of a plurality of phases formed in the weldment, may be a welding method for stainless steel that improves the cryogenic toughness of the weldment, wherein the conditions for the phase fraction are set to satisfy Equation 1 below.

[0026] [Equation 1]

[0027] f δ / f γ> 0.1

[0028] f δ : Fraction of δ phase relative to γ ​​phase, δ phase, and σ phase

[0029] f γ : Fraction of γ phase relative to γ ​​phase, δ phase, and σ phase

[0030] As another example, the step of setting conditions for the phase fraction, which is a fraction of a plurality of phases formed in the weld, may be a welding method for stainless steel that improves the cryogenic toughness of the weld by setting conditions for the phase fraction to satisfy Equation 2 below.

[0031] [Equation 2]

[0032] f σ / f δ < 0.3

[0033] f σ : Fraction of σ phase with respect to γ ​​phase, δ phase, and σ phase

[0034] f δ : Fraction of δ phase relative to γ ​​phase, δ phase, and σ phase

[0036] Meanwhile, the step of performing welding of stainless steel to satisfy the condition regarding the phase fraction may be a welding method of stainless steel that improves the cryogenic toughness of the weldment by performing welding through welding conditions to satisfy the condition regarding the phase fraction.

[0037] More specifically, the welding conditions may be a welding method for stainless steel that improves the cryogenic toughness of the weldment, wherein the welding conditions are at least one of the conditions for the preheating temperature of the stainless steel before welding, the conditions for the amount of heat input applied to the stainless steel during welding, and the conditions for the post-weld heat treatment of the welded stainless steel.

[0039] Components with the same function within the scope of the same concept appearing in the drawings of each embodiment are described using the same reference numeral.

[0041] Figure 1 is a photograph illustrating a weld formed during the manufacturing process of equipment made from austenitic stainless steel.

[0042] Figures 2 (a) and (b) are enlarged photographs of the welded parts of equipment made from austenitic stainless steel.

[0043] FIG. 3 is an experimental data table to explain in more detail the step of setting conditions for the phase fraction constituting a welding method for stainless steel that improves the cryogenic toughness of a welded part according to one embodiment of the present invention, and the step of performing welding of stainless steel to satisfy the conditions for the phase fraction.

[0044] FIG. 4 is a graph illustrating the step of setting conditions for the phase fraction constituting a welding method for stainless steel that improves the cryogenic toughness of a welded part according to one embodiment of the present invention.

[0045] FIG. 5 is a flowchart for explaining, in general, a welding method for stainless steel that improves the cryogenic toughness of a welded part according to one embodiment of the present invention.

[0047] In order to express the technical concept of the present invention more clearly, the attached drawings have simplified or omitted parts that are less related to the technical concept of the present invention or that can be easily derived by those skilled in the art.

[0049] The present invention is capable of various modifications and may have various embodiments, and specific embodiments are illustrated in the drawings and described in detail in the detailed description.

[0050] However, this is not intended to limit the invention to specific embodiments, and it should be understood that it includes all modifications, equivalents, and substitutions that fall within the spirit and scope of the invention.

[0051] In describing the present invention, if it is determined that a detailed description of related known technology may obscure the essence of the present invention, such detailed description is omitted.

[0052] Terms such as first, second, etc. may be used to describe various components, but said components should not be limited by said terms.

[0053] The above terms are used solely for the purpose of distinguishing one component from another.

[0054] The terms used in this application are used merely to describe specific embodiments and are not intended to limit the invention.

[0055] Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0056] In this application, terms such as "comprising" or "having" are intended to specify the existence of the features, numbers, steps, actions, components, parts, or combinations thereof described in the specification, and should be understood as not precluding the existence or addition of one or more other features, numbers, steps, actions, components, parts, or combinations thereof.

[0058] Hereinafter, with reference to FIGS. 1 to 5, a welding method for stainless steel that improves the cryogenic toughness of a welded part according to one embodiment of the present invention will be described in detail.

[0059] Figure 1 is a photograph illustrating a weld (W) formed during the manufacturing process of equipment (T) made from austenitic stainless steel.

[0060] The welded portion (W) may refer to an area where stainless steel is connected during the manufacturing process of the equipment (T), and since it is joined by welding such as gas welding, it may be an area with relatively weaker physical properties, such as physical strength and toughness, compared to areas other than the welded portion (W).

[0061] A welding method for stainless steel that improves the cryogenic toughness of a welded part according to one embodiment of the present invention (hereinafter referred to as the welding method) may mean a method that improves the physical properties, i.e., toughness, of a welded part (W) when welding is performed on a connecting part, i.e., a welded part (W), during the manufacturing process of equipment (T) such as a storage tank, piping, and valve that stores or transports liquid hydrogen through stainless steel, as shown in FIG. 1.

[0062] The stainless steel may be, for example, a known 300 series austenitic stainless steel containing carbon (C), silicon (Si), manganese (Mn), chromium (Cr), nickel (Ni), molybdenum (Mo), nitrogen (N), the remainder iron (Fe), and unavoidable impurities, and the cryogenic temperature may be 20K, i.e., approximately -250°C, which is the temperature at which hydrogen is maintained in a liquid state.

[0063] That is, the above welding method may mean a welding method for a weld (W) that allows liquid hydrogen to be stably stored or transported even at cryogenic temperatures by controlling the fraction of the microstructure of the weld (W) formed during the manufacturing process of equipment (T) made of such stainless steel to improve toughness.

[0064] Figure 2(a) is an enlarged photograph of a weld (W) formed on a piece of equipment (T), and (b) is a photograph showing the microstructure formed on the weld (W) and the multiple phases constituting it.

[0065] The inventors confirmed through experiments that the physical properties, namely toughness, of the weldment (W) change according to the fractions of a plurality of phases formed in the weldment (W), more specifically, the austenite γ phase (austenite gamma phase, hereinafter referred to as the γ phase), the δ ferrite phase (delta-ferrite phase, hereinafter referred to as the δ phase), and the σ phase (wherein "fraction" means the ratio of a specific phase to the plurality of phases formed in the weldment). Furthermore, by controlling the fractions of the γ phase, δ phase, and σ phase formed in the weldment (W), the toughness of the weldment (W) at cryogenic temperatures is improved, and accordingly, the above welding method is derived so that even if liquid hydrogen is stored in the equipment (T), it can be stored in a safer state.

[0066] As can be seen in FIG. 5, the welding method according to one embodiment of the present invention may include a step (step S100) in which a condition for a phase fraction, which is a fraction of a plurality of phases formed in a weld (W), is set, and a step (step S200) in which welding of stainless steel is performed to satisfy the condition for the phase fraction.

[0067] To explain this in more detail, as can be seen in Fig. 4, for the welded part (W) of the equipment for liquid hydrogen, in order to safely store liquid hydrogen without accidents such as explosions occurring at cryogenic temperatures of 20K, an impact energy of approximately 35J or more must be satisfied.

[0068] Meanwhile, the shock absorption energy measured at the weld (W) may vary depending on the toughness of the weld, and this is attributed to the phase fractions of the γ phase, δ phase, and σ phase formed in the weld (W).

[0069] Accordingly, the welding method can manufacture a weld (W) capable of storing liquid hydrogen more safely even at cryogenic temperatures by setting the phase fractions of the γ phase, δ phase, and σ phase formed in the weld (W) through a step (S100) in which conditions for the phase fractions of a plurality of phases formed in the weld (W) are set, and by performing welding to satisfy these conditions.

[0070] The step (step S200) in which welding of stainless steel is performed to satisfy the conditions for the above phase fractions may mean a step in which welding of the weld (W) is performed through welding conditions to satisfy the set phase fractions of the γ phase, δ phase, and σ phase.

[0071] More specifically, the welding conditions may refer to conditions regarding the preheating temperature of the stainless steel before welding, conditions regarding the amount of heat input applied to the stainless steel during welding, and conditions regarding the post-weld heat treatment of the welded stainless steel.

[0072] FIG. 3 is a data table for explaining the step of setting conditions for the phase fraction constituting the welding method according to one embodiment of the present invention, and FIG. 4 is a graph for explaining the step of setting conditions for the phase fraction constituting the welding method according to one embodiment of the present invention.

[0073] Below, with reference to FIGS. 3 and FIGS. 4, the step (step S100) in which conditions for the phase fraction are set will be explained in more detail.

[0074] As previously explained, the inventor focused on the fraction of phases formed in the weld (W), calculated the fractions of the γ phase, δ phase, and σ phase formed in the weld (W) according to welding conditions, and measured the impact energy thereon to derive the correlation between the fractions of phases and toughness, thereby completing the present invention.

[0075] Figure 3 is experimental data showing the phase fraction and impact absorption energy formed in the weld (W) by welding a 300 series austenitic stainless steel sample with a thickness of 10 mm with different welding conditions, such as the preheating temperature (°C) before welding, the heat input (KJ / cm) during welding, and the heat treatment temperature (°C) after welding.

[0076] However, for each sample, the holding time of the preheating temperature (°C) before welding, the heat input (KJ / cm) during welding, and the heat treatment temperature (°C) after welding was set to 1 hour each.

[0077] As can be seen in FIG. 3, the welding method according to one embodiment of the present invention confirms that the phase fractions of the γ phase, δ phase, and σ phase formed in the weld (W) differ depending on the welding conditions, and derives conditions for the phase fractions in the weld (W) that allow liquid hydrogen to be safely stored in the equipment (T) through a sample exceeding the required impact energy (35J).

[0078] The condition for the above-mentioned fraction may, for example, mean a condition satisfying the following Equation 1.

[0079] [Equation 1]

[0080] f δ / f γ > 0.1

[0081] f δ : Fraction of δ phase relative to γ ​​phase, δ phase, and σ phase

[0082] f γ : Fraction of γ phase relative to γ ​​phase, δ phase, and σ phase

[0084] The phases formed in the weldment (W) of the stainless steel are the γ phase, δ phase, and σ phase, and f in Equation 1 above δ represents the fraction of the δ phase relative to the γ, δ, and σ phases, and f γ represents the fraction of the γ phase relative to the γ phase, δ phase, and σ phase.

[0085] As can be seen in Fig. 3, in 13 types of samples, f δ / f γ When the condition for > 0.1 is satisfied, it was confirmed that the shock absorption energy measured in most samples exceeded the required shock energy (35J).

[0087] As another example, the condition for the above-mentioned fraction may mean a condition satisfying Equation 2 below.

[0088] [Equation 2]

[0089] f σ / f δ < 0.3

[0090] f σ : Fraction of σ phase with respect to γ ​​phase, δ phase, and σ phase

[0091] f δ : Fraction of δ phase relative to γ ​​phase, δ phase, and σ phase

[0093] As explained above, the phases formed in the weldment (W) of stainless steel are the γ phase, δ phase, and σ phase, and f σ represents the fraction of the δ phase relative to the γ, δ, and σ phases, and f δ represents the fraction of the γ phase relative to the γ phase, δ phase, and σ phase.

[0094] As can be seen in Fig. 3, in 13 types of samples, f σ / f δ When the condition for < 0.3 is satisfied, it was confirmed that the shock absorption energy measured in most samples exceeded the required shock energy (35J).

[0095] More preferably, as can be seen in FIGS. 3 and 4, it was confirmed that the shock absorption energy measured in samples (1, 2, 5, 6, 9, 10) satisfying both Equation 1 and Equation 2 below exceeded the required shock energy (35J).

[0096] In this way, the step (step S100) in which conditions for the phase fraction are set may mean the step in which conditions for the phase fraction satisfying both Equation 1 and Equation 2 are set for the γ phase, δ phase, and σ phase formed in the weld (W).

[0097] The step (step S200) in which welding of the stainless steel is performed may mean a step in which welding of the stainless steel is performed by controlling the conditions for the preheating temperature of the stainless steel before welding, the conditions for the amount of heat input applied to the stainless steel during welding, and the conditions for the post-weld heat treatment of the welded stainless steel to satisfy the conditions for the phase fraction set in the setting step (step S100).

[0098] That is, the above welding method sets conditions for phase fractions based on the fractions of γ phase, δ phase, and σ phase that affect the toughness of the weld (W), and controls the conditions for the preheating temperature of the stainless steel before welding, the heat input applied to the stainless steel during welding, and the heat treatment of the welded stainless steel after welding so as to satisfy these conditions for phase fractions, thereby allowing welding to be performed.

[0099] Accordingly, the welding method improves the toughness of the weld (W) so that it can exceed the shock absorption energy required at cryogenic temperatures of approximately 20K (-250℃), thereby allowing liquid hydrogen to be stored or transported more safely within the equipment (T).

[0101] Although the structure and features of the present invention have been described above based on embodiments according to the present invention, the present invention is not limited thereto, and it is obvious to those skilled in the art that various changes or modifications can be made within the spirit and scope of the present invention; therefore, it is noted that such changes or modifications fall within the scope of the appended claims.

[0102] It will be obvious to those skilled in the art that equipment for storing or transporting liquid hydrogen, manufactured by a welding method of stainless steel according to one embodiment of the present invention that improves the cryogenic toughness of the welded part, may also fall within the scope of the rights of the present invention. Explanation of the symbols

[0104] S100: Step where conditions for the phase fraction are set S200: Step where welding of stainless steel is performed

Claims

Claim 1 A welding method for stainless steel that improves the cryogenic toughness of a welded portion of stainless steel used as a material for liquid hydrogen, comprising: a step of setting a condition for a phase fraction, which is a fraction of a plurality of phases formed in the welded portion; and a step of performing welding of the stainless steel to satisfy the condition for the phase fraction; wherein the plurality of phases includes a γ phase, a δ phase, and a σ phase formed in the welded portion, and the condition for the phase fraction is a condition in which at least one of the γ phase, the δ phase, and the σ phase formed in the welded portion is set, and the setting step is a condition in which the condition for the phase fraction is set to satisfy the following Equation 1 and Equation 2. [Equation 1] f δ / f γ 0.1f δ : Fraction of δ phase with respect to γ ​​phase, δ phase, and σ phase f γ : Fraction of γ phase with respect to γ ​​phase, δ phase, and σ phase [Equation 2] f σ / f δ < 0.3f σ : Fraction of σ phase with respect to γ ​​phase, δ phase, and σ phase f δ : Fraction of δ phase relative to γ ​​phase, δ phase, and σ phase Claim 2 delete Claim 3 delete Claim 4 delete Claim 5 A welding method for stainless steel according to claim 1, wherein the step performed above is to perform welding through welding conditions that satisfy the condition for the phase fraction, thereby improving the cryogenic toughness of the weldment. Claim 6 A welding method for stainless steel that improves the cryogenic toughness of the weldment, wherein the welding conditions are at least one of the conditions for the preheating temperature of the stainless steel before welding, the conditions for the amount of heat input applied to the stainless steel during welding, and the conditions for the post-weld heat treatment of the welded stainless steel. Claim 7 A product for storing liquid hydrogen produced by a stainless steel welding method that improves the cryogenic toughness of the welded joint according to any one of paragraphs 1, 5, and 6.

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