Views: 0 Author: Site Editor Publish Time: 2026-04-06 Origin: Site
In the field of shipbuilding, the quality of thick plate welding joints directly affects the safety, durability and fatigue resistance of the ship structure. The double-sided welding root cleaning and forming technology, as the core process for thick plate welding (such as the large assembly of ship sections, strong decks, and side top bulkheads at key parts), ensures full penetration of the weld seam and absence of internal defects by performing double-sided welding and thoroughly removing root defects. This is one of the mandatory requirements for ship welding quality set by the Chinese Classification Society (CCS). This article systematically analyzes the specification requirements, process key points and engineering applications of the double-sided welding root cleaning and forming technology, in combination with the standards such as "Steel Merchant Ships Classification Rules" (2021), "Welding Test for Ships" and "Steel Welded Atmospheric Pressure Vessels" (NB/T 47015).
The double-sided welding root-clearing forming technology refers to the complete process of using double-sided welding methods (such as submerged arc welding, CO₂ gas shielded welding, etc.), after completing the welding of the front weld seam, through mechanical or thermal methods to remove the non-melting, slag, and gas porosity defects at the root of the weld seam, and then performing the back-side root-clearing welding. Its core objective is to ensure that the weld seam is fully penetrated and has no internal defects, meeting the weld quality grades (such as A grade, B grade) required by CCS. According to Article 2.2.4 of Chapter 2 of Part 3 of CCS' "Steel Ship Classification Specifications", "For butt welds of critical structures (such as hull beams, strong frames, side outer plates, etc.) that bear static or dynamic loads, when the plate thickness exceeds the effective thickness of single-sided welding specified in the standard, double-sided welding or a full penetration welding equivalent to double-sided welding should be adopted." Among them, "root-clearing forming" is the key step to achieve "full penetration", and its quality directly affects the bearing capacity and crack propagation resistance of the weld seam.
CCS has proposed a systematic specification for the double-sided welding root clearance forming technology, covering aspects such as material selection, bevel design, cleaning process, inspection standards, and rework control. The key points are as follows: 1. Requirements for materials and welding materials: The double-sided welding root clearance is applicable to the structural steels specified in the CCS regulations for ship structures (such as AH32, DH36, EH36, etc., high-strength steels), and must meet the requirements of "Structural Steel for Ships". For plates with a thickness exceeding 30mm, ultrasonic testing (UT) re-inspection is required to ensure there are no delamination, inclusions, or other defects (Article 3.2.1 of the "Steel Rules for Classification of Ships").
1. Welding material certification: Welding materials (welding rods, welding wires, welding flux) must be approved by CCS products (such as the "Test Rules for Ship Welding Materials Recognition"), and their strength must match that of the base material. For example, when welding EH36 steel, low-hydrogen alkaline welding rods (such as J507RH) or high-toughness submerged arc welding wires (such as ER50-6) should be selected to ensure that the strength, toughness and crack resistance of the weld metal are not lower than that of the base material.
2. Groove design and pre-treatment groove form: Based on the plate thickness and structural position, CCS recommends using X-shaped or double V-shaped grooves (Figure 1). For double-sided welding, the risk of root incomplete fusion can be reduced. The groove angle α is usually 60° to 70°, the fillet radius p ≤ 2mm (for plate thickness ≤ 25mm) or p ≤ 3mm (for plate thickness > 25mm), and the root gap b ≤ 2mm (as per "Welding Grooves for Ship Structure").
Preprocessing requirements: Before welding, the groove and the 20mm area on both sides need to be cleaned to remove impurities such as oil, rust, and oxide scale (Article 5.2.1 of "Ship Welding Process Approval Test"); for high-strength steel (such as EH36), flame preheating (with a preheating temperature of ≥ 100℃) is required to reduce the cooling speed of the weld seam and prevent cold cracks.
3. Root Cleaning Process and Operating Specifications Root cleaning method: The allowed root cleaning methods by CCS include carbon arc gouging (CJP), milling, or plasma cutting. Carbon arc gouging is the preferred method (due to its low cost and high efficiency), but the gas cutting parameters must be strictly controlled (current 180-250A, compressed air pressure 0.5-0.6MPa) to avoid the formation of carburized layers or copper spots (which affect the subsequent welding quality).
Root clearance depth and shape: After root clearance, the metal surface should be exposed with a metallic luster, and there should be no defects such as incomplete fusion or slag inclusion at the root; the radius of the root of the bevel r ≥ 2mm (for X-shaped bevel) or r ≥ 1.5mm (for V-shaped bevel), and the fillet edge p ≤ 1mm (in accordance with GB/T 19868-2005). For welds with a plate thickness greater than 30mm, it is recommended to perform root clearance in two stages (clear to 2-3mm from the back surface in the first stage, and thoroughly clear in the second stage) to ensure that the back weld is formed smoothly.
4. Backside welding during welding and post-heating treatment: After root cleaning, backside welding must be carried out immediately using the same welding materials and processes as the front side (for example, in submerged arc welding, the current is 180-220A, the voltage is 28-32V, and the speed is 35-45cm/min). Ensure that the thickness of the double-sided weld seam is uniform and there is no undercut (the depth of undercut should be ≤ 0.5mm and the length should be ≤ 10% of the total length of the weld seam).
Post-heating and insulation: For high-strength steels (such as EH36) or when the ambient temperature is less than 5℃, after welding is completed, post-heating treatment (at a temperature of 200~300℃, with an insulation time of 1~2 hours) is required to eliminate welding residual stress and prevent hydrogen-induced cracks (as per CCS 'Steel Ship Classification Specification' 3.3.4).
5. Quality Inspection and Acceptance Standards - Non-destructive Testing (NDT): 100% radiographic testing (RT) must be conducted for the weld seams of double-sided welding. The RT shall be carried out in accordance with GB/T 3323-2005 "Radiographic Testing of Welded Joints in Metal Fusion Welding", and it is considered qualified at the B level (with defect grades not exceeding level II). (UT can also be performed, but I don't have much knowledge about it. I will definitely study more about how UT conducts the inspection when I have time.)
Appearance inspection: The surface of the weld seam should be smooth and seamless, without cracks, pores, or slag inclusions. The excess height should be 0 to 3mm (for X-shaped groove) or 0 to 4mm (for V-shaped groove). The width should be increased by no more than 2mm on each side compared to the groove (Appendix 1 of the "Code for Classification of Steel Ships").
6. Key points for quality control and common issues to avoid: Insufficient root cleaning: This may result in incomplete fusion at the root. It is necessary to confirm the cleaning depth through UT re-inspection (≥ 80% of the plate thickness) and check the shape using special gauges (such as groove gauges).
Hydrogen-induced cracks: When welding high-strength steel, it is necessary to strictly control the preheating temperature (≥100℃) and the interlayer temperature (≤250℃) to avoid cold cracks.
Re-work control: If defects are detected during inspection, rework must be carried out in accordance with the requirements of CCS "Welding Process Qualification Test for Ships" (the number of rework times for the same area shall not exceed 2 times). After rework, UT/RT tests must be conducted again.
Conclusion
The technical specifications set by the China Classification Society for the double-sided welding root-clearing forming process are "hard constraints" for the quality of ship construction. Through material matching, groove optimization, control of the root-clearing process, and strict inspections, it is possible to effectively ensure the full penetration of the weld seam, thereby enhancing the safety and reliability of the ship structure.
