A welding process for 12Cr martensitic heat-resistant steel girth weld without crack defects

Abstract

The present application belongs to the field of welding process of steel material, in particular to a welding process technology of 12Cr martensitic heat-resistant steel girth weld for reactor core, which adjusts and controls the grain morphology and delta-ferrite distribution in the molten pool through the welding process, so as to avoid the crack defects in the weld. The main process of the welding process is as follows: (1) the 12Cr martensitic heat-resistant steel is processed into a circular sheet and a sheet with a circular hole matched therewith, and the two are assembled to form a bevel-less girth butt weld; (2) the girth butt weld is welded, the welding current is 100-120 A, the welding speed is 0.5-0.7 mm / s, the voltage is 12-14 V, and the welding gun walks along the girth weld to complete the welding. The welding process can avoid the crack problem of the 12Cr martensitic heat-resistant steel girth weld for reactor core, and there is no defect after welding.

Description

Technical Field

[0001] This invention relates to the field of welding processing technology for steel materials, specifically to a welding process for 12Cr martensitic heat-resistant steel ring welds without crack defects. Background Technology

[0002] 12Cr martensitic heat-resistant steel possesses low thermal stress, high-temperature creep strength, and excellent resistance to radiation swelling, making it a key candidate material for fourth-generation nuclear power reactor core assemblies. Core assemblies contain nuclear fuel, and a leak would have catastrophic consequences. Currently, the assembly of components such as cladding tubes and outer casings in the core structure requires fusion welding processes, such as Argon Tungsten-arc Welding (GTAW), typically employing circumferential or hexagonal welds. The performance of the weld joints is a crucial factor in ensuring the overall safety of the assembly during service. However, macroscopic cracks have been observed after circumferential welding of 12Cr martensitic steel. Furthermore, because the outer casing body and ends use two different types of 12Cr martensitic heat-resistant steel, weld misalignment occurred during welding. These issues all contribute to substandard weld quality, severely hindering the core assembly manufacturing process.

[0003] Therefore, a welding process for 12Cr martensitic heat-resistant steel ring welds without crack defects was developed to improve weld performance. Summary of the Invention

[0004] The purpose of this invention is to provide a welding process for 12Cr martensitic heat-resistant steel ring welds used in reactor cores. The welding method is tungsten inert gas (TIG) welding. By controlling the grain morphology and δ-ferrite distribution of the molten pool through the welding process, crack defects in the weld are avoided, resulting in a high-quality weld. This method is particularly suitable for welding 12Cr martensitic heat-resistant steel ring welds used in reactor core assemblies.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] A welding process for circumferential welds of 12Cr martensitic heat-resistant steel without crack defects, the process comprising the following steps:

[0007] (1) 12Cr martensitic heat-resistant steel is processed into a circular thin plate and a thin plate with a circular hole, and the two are assembled to form a bevelless circumferential weld. One side of the base material is HT-9 (12Cr-1Mo-0.3V) alloy, and the other side of the base material is 12Cr-2Ni-1Mo alloy. This belongs to the welding of 12Cr heat-resistant steel.

[0008] (2) Weld the circumferential weld with a welding current of 100-120A, a welding speed of 0.5-0.7mm / s, and a voltage of 12-14V. The tungsten electrode is perpendicular to the welding surface. After aligning the tungsten electrode with the gap, move it 0.4-0.8mm toward the 12Cr-2Ni-1Mo base material to ensure that the final weld is located in the center position. The welding trajectory is along the circumferential gap to complete the welding.

[0009] Furthermore, the circular sheet and the sheet with the circular hole are made of 12Cr martensitic heat-resistant steel used in the reactor core, with one side of the base material being HT-9 (12Cr-1Mo-0.3V) alloy and the other side being 12Cr-2Ni-1Mo alloy, which is a welding process for 12Cr heat-resistant steel. Before welding, the circular sheet is embedded in the circular hole of the square sheet, making their upper and lower surfaces parallel, and tack welded to fix them relatively. The square sheet is placed on a steel base, with the annular gap positioned just above the annular groove of the base. The square sheet is sealed around its perimeter and welded to the base using fillet welds to achieve strong restraint, increase the weld crack sensitivity of the assembly, and enable effective evaluation of the crack sensitivity of subsequent welding processes.

[0010] The main ideas and mechanisms of this invention are as follows:

[0011] The purpose of this invention is to provide a welding process technology for the ring weld of 12Cr martensitic heat-resistant steel used in reactor cores. The welding method is tungsten inert gas (TIG) welding. By controlling the grain morphology and δ-ferrite distribution of the molten pool through the welding process, crack defects in the weld are avoided, resulting in a high-quality weld. The main ideas and mechanisms of this invention are as follows:

[0012] During their study of the microstructure of 12Cr martensitic steel ring welds, the inventors discovered that the macroscopic cracks in the weld were solidification cracks, distributed along the weld centerline. Large, strip-shaped δ-ferrite particles were observed at the weld center. These δ-ferrite particles were formed from residual liquid phase segregated during solidification. Furthermore, these large, strip-shaped δ-ferrite particles at the weld center provided pathways for crack propagation, further reducing the weld's toughness. These phenomena indicate that the root cause of the welding cracks in this study is the solidification shrinkage of the residual liquid phase. Based on the above conclusions and combined with classical welding metallurgy knowledge, the inventors adjusted the welding parameters to change the growth direction of columnar crystals in the weld pool from straight columnar crystals pointing towards the weld centerline to curved grains. This caused the solidified residual liquid phase to be discharged into the moving weld pool instead of being squeezed at the weld centerline and unable to receive feeding, thus avoiding solidification cracking. At the same time, large-sized strip-shaped harmful δ-ferrite no longer forms in the weld center, significantly improving weld performance. Attached Figure Description

[0013] Figure 1The drawing shows the dimensions of the base and the base material before welding; where: (a) is the base, and (b) and (c) are the thin plates used for welding.

[0014] Figure 2 This study aims to compare the metallographic morphology of cracks in the cross-section of welds and the microscopic characterization of harmful δ-ferrite (δ phase).

[0015] Figure 3 Radiographic inspection results of welds in 12Cr martensitic heat-resistant steel under different welding processes; where: (a) comparative welding process, (b) example welding process. Detailed Implementation

[0016] Example 1:

[0017] Step 1: Process the 12Cr-2Ni-1Mo alloy into a circular thin plate, with dimensions as shown. Figure 1 As shown in (c), HT-9 (12Cr-1Mo-0.3V) alloy is machined into a thin plate with circular holes, with dimensions as shown. Figure 1 As shown in (b), the two are assembled and fixed by spot welding to form a bevelless annular butt weld, and then the perimeter is welded and fixed to the base (as shown in Figure 1). Figure 1 (as shown in (a));

[0018] Step 2: The welding method is tungsten inert gas (TIG) welding, with a welding current of 105A, a welding speed of 0.67mm / s, and a voltage of 12.5V. The tungsten electrode is perpendicular to the welding surface. After aligning the tungsten electrode with the gap, it is moved 0.6mm towards the 12Cr-2Ni-1Mo alloy base material. The welding trajectory travels one circle along the annular gap, and then continues to travel approximately 10mm before extinguishing the arc.

[0019] Comparative Example 1:

[0020] Step 1: Process the 12Cr-2Ni-1Mo alloy into a circular thin plate, with dimensions as shown. Figure 1 As shown in (c), HT-9 (12Cr-1Mo-0.3V) alloy is machined into a thin plate with circular holes, with dimensions as shown. Figure 1 As shown in (b), the two are assembled and fixed by spot welding to form a bevelless annular butt weld, and then the perimeter is welded and fixed to the base (as shown in Figure 1). Figure 1 (as shown in (a));

[0021] Step 2: The welding method is tungsten inert gas (TIG) welding, with a welding current of 135A, a welding speed of 1.5mm / s, and a voltage of 12.6V. The tungsten electrode is perpendicular to the welding surface. After aligning the tungsten electrode with the gap, it is moved 0.6mm towards the 12Cr-2Ni-1Mo alloy base material. The welding trajectory travels one circle along the annular gap, and then continues to travel approximately 10mm before extinguishing the arc.

[0022] from Figure 2As can be seen in the comparison example 1, cracks appeared in the weld, and large-sized δ-ferrite harmful phases caused by solidification segregation were found at the bottom of the cracks. Figure 3 (a) and Figure 3 (b) The X-ray flaw detection results of the comparative circumferential weld and the circumferential weld of the present invention are respectively. It can be clearly found that there is a crack at the center line of the comparative weld, while the weld of the present invention is intact and without defects.

[0023] It is worth emphasizing that those skilled in the art can make corresponding adjustments based on the above technical solutions and concepts, combined with actual conditions, and all changes should fall within the protection scope of the claims of this invention.

Claims

1. A welding process for circumferential welds of 12Cr martensitic heat-resistant steel without crack defects, characterized in that: The process includes the following steps: (1) 12Cr martensitic heat-resistant steel is processed into a circular thin plate that can be inlaid and a square thin plate with a circular hole, and the two are assembled to form a bevel-free circumferential butt weld; the circular thin plate and the square thin plate with a circular hole are made of 12Cr martensitic heat-resistant steel for reactor core, one side of which is HT-9:12Cr-1Mo-0.3V alloy and the other side is 12Cr-2Ni-1Mo alloy; (2) Weld the circumferential butt weld with a welding current of 100-120A, a welding speed of 0.5-0.7mm / s, and a voltage of 12.5-14V. During the welding process, the tungsten electrode is perpendicular to the welding surface. After aligning the tungsten electrode with the gap, it is moved 0.4-0.8mm towards the 12Cr-2Ni-1Mo base material. The welding trajectory is along the circumferential gap.

2. The welding process for crack-free 12Cr martensitic heat-resistant steel ring welds according to claim 1, characterized in that: Step (1) Assembly process is as follows: Before welding, the circular thin plate is embedded in the circular hole of the square thin plate, so that the upper and lower surfaces of the two are parallel, and the two are tack-fixed to fix them relatively; the square thin plate is placed on the steel base, so that the annular gap is just above the annular groove of the base; the square thin plate is sealed around the base by welding fillet welds, and welded to the base. The purpose is to achieve strong restraint, improve the welding crack sensitivity of the assembly, and enable it to effectively evaluate the crack sensitivity of subsequent welding processes.

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