Post-weld heat treatment method

Abstract

To secure strength while reducing hardness so as to suppress hydrogen embrittlement in a weld part of a molding member composed of SCS6-based martensite stainless cast steel.SOLUTION: There is provided a post-welding heat treatment method of a molding member composed of JIS G 5121 SCS6-based martensite-based stainless cast steel, wherein a heat treatment temperature T is set to 525°C, and heat treatment holding time t is set to 20 h or more and 25.1 h or less.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The present disclosure relates to post-weld heat treatment methods. [Background technology]

[0002] For example, Patent Document 1 describes a method for forming a welded portion of a martensitic stainless steel material. This method involves welding a martensitic stainless steel material to form a welded portion, and then performing post-weld heat treatment on the welded portion after welding, such that P1, as defined by the following formula (1), is 15,500 or more, and P2, as defined by the following formula (2), is 0 or less, and the post-weld heat treatment holding time t is within a range of 60 to 1,000 seconds, resulting in a weld heat-affected zone with excellent resistance to intergranular stress corrosion cracking and hydrogen embrittlement. P1=(T+273)(20+log(t / 3600))...(1) P2=20logt+T-(A1+120) (2) T: Post-weld heat treatment temperature (℃) t: Post-weld heat treatment holding time (s) A1: The minimum temperature (℃) at which 1% or more of austenite phase will form when heated for 20 seconds after forming a 100% martensite structure [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-321181 Summary of the Invention [Problem to be solved by the invention]

[0004] For example, martensitic stainless steel cast steel, a material based on the SCS6 series, is used as the primary material for general-purpose pumps and nuclear pumps. In recent years, demand for higher output and more compact pumps has increased, and to ensure reliability in terms of strength, particularly high-strength SCS6XQT2 (JIS G5121) steel is often used. Furthermore, because these materials are cast products, casting defects are unavoidable, potentially requiring welding repairs after the finished product shape is achieved. In such cases, welding repairs require the use of welding techniques that take into account strict hardness control to minimize the risk of hydrogen embrittlement, which is unique to high-hardness (350 HV or higher) materials.

[0005] The present disclosure is intended to solve the above-mentioned problems, and aims to provide a post-weld heat treatment method that can ensure strength while suppressing hardness in order to suppress hydrogen embrittlement in welded joints of formed members made of SCS6 martensitic stainless cast steel. [Means for solving the problem]

[0006] In order to achieve the above-mentioned object, a post-weld heat treatment method according to one embodiment of the present disclosure is a post-weld heat treatment method for a formed member made of JIS G 5121 SCS6 martensitic stainless cast steel, in which the heat treatment temperature T is 525°C and the heat treatment holding time t is 20 hours or more and 25.1 hours or less. [Effects of the Invention]

[0007] The present disclosure can ensure strength while suppressing hardness in order to suppress hydrogen embrittlement in welded joints of formed members made of SCS6 martensitic stainless cast steel. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is an explanatory view of a formed member to which the post-weld heat treatment method of the embodiment is applied. [Figure 2] FIG. 2 is a diagram showing the chemical composition of the martensitic stainless cast steel that forms the formed member. [Figure 3] FIG. 3 is a chart showing the mechanical properties and heat treatment temperatures of martensitic stainless cast steel that constitutes the formed member. [Figure 4] FIG. 4 is a flowchart of a post-weld heat treatment method according to an embodiment. [Figure 5] FIG. 5 is a schematic diagram showing a support jig. [Figure 6] FIG. 6 is a chart showing the heat treatment results of SCS6XQT2. [Figure 7] FIG. 7 is a chart showing the heat treatment results of SCS6. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. However, the present disclosure is not limited to these embodiments. Furthermore, the components in the following embodiments include those that are easily replaceable by those skilled in the art, or those that are substantially identical.

[0010] FIG. 1 is an explanatory view of a formed member to which the post-weld heat treatment method of the embodiment is applied.

[0011] In the post-weld heat treatment method of the embodiment, heat treatment is performed on a formed member W. Such formed member W is a suction casing 101B and a discharge casing 101C connected to a main casing 101A in a water supply pump 101 illustrated in FIG. 1 . In the water supply pump 101, an impeller and a diffuser are disposed inside a main casing 101A that incorporates multiple cylindrical components. The suction casing 101B and the discharge casing 101C are connected to both axial ends of the main casing 101A. The suction casing 101B has a suction port 101Ba facing outward and is provided with a cylindrical connection portion 101Bb that connects to the main casing 101A. The discharge casing 101C has a discharge port 101Ca facing outward and is provided with a cylindrical connection portion 101Cb that connects to the main casing 101A. The impeller shaft 101D passes through the suction casing 101B and the discharge casing 101C, and a seal part 101E having a bearing is attached to the outside. Therefore, when the rotary shaft 101D is driven to rotate, the water supply pump 101 sucks water from the suction port 101Ba of the suction casing 101B, passes through the inside of the main casing 101A, and discharges it from the discharge port 101Ca of the discharge casing 101C, thereby supplying water.

[0012] The molded members W configured as the suction casing 101B and the discharge casing 101C illustrated above are primarily made of martensitic stainless steel cast steel, which is a material based on JIS G5121 SCS6 series (hereinafter referred to as SCS6 series). Specifically, SCS6 series martensitic stainless steel cast steel includes JIS G5121 SCS6 (hereinafter referred to as SCS6) and JIS G5121 SCS6XQT2 (hereinafter referred to as SCS6XQT2) which has higher hardness.

[0013] Here, SCS6-based materials such as SCS6XQT2 and SCS6 have a specific gravity of 7.75, contain the chemical components shown in Figure 2, are produced under the heat treatment conditions shown in Figure 3, and have the mechanical properties shown in Figure 3. Therefore, SCS6-based materials such as SCS6XQT2 and SCS6 are specified by the chemical components, heat treatment conditions, and mechanical properties shown in Figure 2.

[0014] Since the suction casing 101B and the discharge casing 101C are cast products, the occurrence of casting defects is unavoidable, and there is a possibility that welding repair will be required after the product shape is finished. When carrying out welding repair, in order to reduce the risk of hydrogen embrittlement, which is specific to high-hardness materials (350HV or more), heat treatment is performed after repair welding.

[0015] As shown in FIG. 4, in the post-weld heat treatment method of the embodiment, in step S1, a support jig 10 (see FIG. 5) is attached to the formed workpiece W, in steps S2 and S3, the heat treatment temperature T (°C) and the heat treatment holding time t (h) are set, and in step S4, the heat treatment is performed.

[0016] The support jig 10 is intended to prevent distortion of the formed member W during post-weld heat treatment. As shown in FIG. 5, when the repair welding location is a cylindrical and relatively thin-walled connection portion 101Bb, 101Cb in the suction casing 101B or the discharge casing 101C, the support jig 10 is configured to suppress distortion of the connection portion 101Bb, 101Cb from the inside of the cylinder to the inside. The support jig 10 includes a support ring 10A and support rods 10B. The support ring 10A is formed in a ring shape sized to be placed inside the connection portions 101Bb, 101Cb. Multiple support rods 10B (eight in this embodiment) are arranged radially at equal intervals toward the outside of the support ring 10A. The support rod 10B has a tubular sheath 10Ba whose base end is fixed to the support ring 10A, a nut 10Bb fixed to the annular tip of the sheath 10Ba, and a bolt 10Bc threaded into the nut 10Bb and mounted inside the sheath 10Ba so that it can move radially. By rotating the bolt 10Bc, the threaded position of the bolt 10Bc relative to the sheath 10Ba and the nut 10Bb changes, causing the head of the bolt 10Bc to abut against or move away from the inner surfaces of the connecting portions 101Bb and 101Cb. Therefore, the support jig 10 is attached to the formed workpiece W so that the heads of the bolts 10Bc of each support rod 10B abut against the inner surfaces of the connecting portions 101Bb and 101Cb, preventing inward distortion of the connecting portions 101Bb and 101Cb.

[0017] After post-weld heat treatment, the tensile strength (MPa), yield strength (MPa), and Brinell hardness (HB) must satisfy the specified values ​​in JIS G5121. Note that for some material types (e.g., SCS6XQT2), there is no hardness specification in JIS.

[0018] When the formed workpiece W is made of SCS6XQT2, the heat treatment temperature T is set to 525° C. and the heat treatment holding time t is set to 10.2 hours or more and 28.2 hours or less based on the heat treatment results shown in FIG.

[0019] On the other hand, when the formed workpiece W is made of SCS6, the heat treatment temperature T is set to 540°C and the heat treatment holding time t is set to 7.1 hours based on the heat treatment results shown in Fig. 7. Alternatively, when the formed workpiece W is made of SCS6, the heat treatment temperature T is set to 525°C and the heat treatment holding time t is set to 20 hours or more and 25.1 hours or less based on the heat treatment results shown in Fig. 7.

[0020] Therefore, in the case of a formed member W made of an SCS6-based material including SCS6XQT2 and SCS6, the heat treatment temperature T is set to 525°C and the heat treatment holding time t is set to 20 hours or more and 25.1 hours or less. As a result, the post-weld heat treatment method of the embodiment can ensure strength while suppressing hardness to prevent hydrogen embrittlement in the welded portion of an SCS6-based material.

[0021] Here, based on the heat treatment results shown in FIGS. 6 and 7 , by setting the heat treatment temperature T and heat treatment holding time t to be the same for both SCS6-based materials, including SCS6XQT2 and SCS6, post-weld heat treatment can be performed using the same settings for the SCS6-based materials. Therefore, in the post-weld heat treatment method of the embodiment, based on the heat treatment results shown in FIGS. 6 and 7 , it is preferable to set the heat treatment temperature T to 525°C and the heat treatment holding time t to 20 hours for both SCS6XQT2 and SCS6, respectively, which simplifies the settings and ensures versatility. Therefore, in the post-weld heat treatment method of the embodiment, the heat treatment temperature T is set to 525°C and the heat treatment holding time t to 20 hours for both the formed member W made of SCS6XQT2 and the formed member W made of SCS6. That is, by setting the heat treatment temperature T to 525°C and the heat treatment holding time t to 20 hours for the formed member W made of SCS6, post-weld heat treatment can also be performed on the formed member W made of SCS6XQT2. As a result, the post-weld heat treatment method of the embodiment can ensure strength while suppressing hardness in order to suppress hydrogen embrittlement in the weld for all SCS6-based steels.

[0022] Furthermore, in the post-weld heat treatment method of the embodiment, it is preferable to set the heat treatment temperature T and heat treatment holding time t so that the Vickers hardness HV is less than 350. From the heat treatment results shown in Figures 6 and 7, the heat treatment temperature T is set to 525°C and the heat treatment holding time t is set to 20 hours or more and 25.1 hours or less so that the Vickers hardness HV of the weld heat affected zone (HAZ) is less than 350. As a result, the post-weld heat treatment method of the embodiment can ensure strength while suppressing hardness to suppress hydrogen embrittlement in the weld of SCS6-based steel.

[0023] In the post-weld heat treatment method of the embodiment, the formed member W has cylindrical connection portions 101Bb, 101Cb, and a support jig 10 is used to prevent distortion of the connection portions 101Bb, 101Cb during post-weld heat treatment. According to this post-weld heat treatment method, the use of the support jig 10 can suppress distortion due to welding.

[0024] The present disclosure includes the following inventions. [Invention 1] JIS G 5121 A method for post-weld heat treatment of formed members made of martensitic stainless cast steel of SCS6 type, A post-weld heat treatment method in which the heat treatment temperature T is 525°C and the heat treatment holding time t is 20 hours or more and 25.1 hours or less. [Invention 2] The formed member is made of martensitic stainless cast steel according to JIS G 5121 SCS6, The post-weld heat treatment method according to invention 1, wherein the heat treatment temperature T is 525°C and the heat treatment holding time t is 20 hours. [Invention 3] The heat treatment temperature T was set to 525°C, and the heat treatment holding time t was set to 20 hours. A post-weld heat treatment method according to Invention 1, in which post-weld heat treatment is performed on formed members made of all martensitic stainless cast steels of JIS G 5121 SCS6 series. [Invention 4] 4. A post-weld heat treatment method according to any one of inventions 1 to 3, wherein the heat treatment temperature T and heat treatment holding time t are set so that the Vickers hardness HV of the weld heat affected zone is less than 350. [Invention 5] The molding member has a cylindrical connecting portion, 5. The post-weld heat treatment method according to any one of claims 1 to 4, wherein a support jig is used to prevent distortion of the joint during the post-weld heat treatment. [Explanation of symbols]

[0025] 10 Support jig 101Bb connection 101Cb connection W-molded parts (101B suction casing, 101C discharge casing)

Claims

1. JIS G 5121 A method for post-weld heat treatment of formed members made of SCS6 martensitic stainless cast steel, comprising: A post-weld heat treatment method in which the heat treatment temperature T is 525°C and the heat treatment holding time t is 20 hours or more and 25.1 hours or less.

2. The formed member is made of martensitic stainless cast steel of JIS G 5121 SCS6, 2. The post-weld heat treatment method according to claim 1, wherein the heat treatment temperature T is 525°C and the heat treatment holding time t is 20 hours.

3. The heat treatment temperature T is set to 525°C, and the heat treatment holding time t is set to 20 hours.

2. The post-weld heat treatment method according to claim 1, wherein the post-weld heat treatment is performed on a formed member made of any martensitic stainless cast steel of JIS G 5121 SCS6 series.

4. 2. The post-weld heat treatment method according to claim 1, wherein the heat treatment temperature T and heat treatment holding time t are set so that the Vickers hardness HV of the weld heat affected zone is less than 350.

5. The molding member has a cylindrical connecting portion, 2. The post-weld heat treatment method according to claim 1, further comprising using a support jig to prevent distortion of the joint during the post-weld heat treatment.

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