As an important water transportation vehicle and engineering equipment, the safety, reliability, and durability of ships are directly related to personnel safety, property security, and marine environmental protection. Welding, as one of the key connection processes in ship construction and maintenance, directly determines the integrity and performance of the ship's structure. Therefore, the welding of marine components in Dalian must adhere to a series of strict and systematic technical requirements. These requirements mainly cover various aspects such as standard specifications, personnel qualifications, material management, process control, inspection and testing, as well as considerations for special environments.
1. Adhere to strict norms and standards systems
The primary requirement for marine welding is that it must comply with internationally, nationally, and industry-recognized standards and specifications. This serves as the foundation for ensuring that ships can obtain recognition and statutory approval from classification societies worldwide.
International general specifications: such as relevant conventions of the International Maritime Organization (IMO), and welding standards of the International Organization for Standardization (ISO) (such as the ISO 3834 quality requirement series, ISO 5817 weld quality grade standards, etc.).
Classification Society Regulations of Various Countries: This is a direct and core requirement. Major classification societies around the world (such as China Classification Society (CCS), American Bureau of Shipping (ABS), Lloyd's Register (LR), Det Norske Veritas (DNV), and Japan's NK) have issued detailed "Materials and Welding Regulations". These regulations impose mandatory provisions on welding processes, welder qualifications, weld inspection, etc. Ships must meet the specific requirements of the classification society to which they are registered.
National standards and industry standards: For example, China's GB/T standards (recommended national standards) and CB standards (shipbuilding industry standards) provide specific regulations for welding materials, process evaluation, inspection methods, etc.
II. Requirements for personnel qualifications and process evaluation
Welder qualification certification: All welders engaged in hull structure welding must undergo examinations in accordance with relevant standards (such as ISO 9606 or equivalent standards of classification societies) and obtain corresponding qualification certificates. Certification is typically classified by welding method (such as SMAW manual welding, GMAW gas shielded welding, SAW submerged arc welding, etc.), material type, thickness range, welding position (flat, horizontal, vertical, overhead), etc. Welders must work with valid certificates.
Welding Procedure Qualification (WPS/PQR): Before welding products, it is necessary to evaluate the proposed Welding Procedure Specification (WPS) to verify its ability to produce welds that meet the requirements. During the evaluation process, a Welding Procedure Qualification Report (PQR) must be prepared and approved by a classification society or authorized institution. The WPS serves as the direct basis for on-site welding operations and must include all key parameters, such as base material and welding consumable type, preheat/interpass temperature, welding current and voltage, speed, heat input, shielding gas, post-weld heat treatment requirements, etc.
III. Material management and control
Base material requirements: Marine steel materials (such as Grade A, B, D, E ship plates, high-strength steel, etc.) must have classification society certificates, and their chemical composition and mechanical properties (especially impact toughness) must meet specifications. For special materials such as high-strength steel, stainless steel, and aluminum alloy, their weldability needs to be carefully considered.
Welding material management: Welding electrodes, wires, and fluxes must be compatible with the base material and certified by a classification society. Strict systems must be established for their storage, baking, distribution, and recycling. Especially for low-hydrogen welding electrodes, they must be dried at the specified temperature and placed in an insulated cylinder for ready access to prevent moisture absorption, which can lead to hydrogen-induced cracks in the weld seam.
IV. Key process control in the welding process
This is the core step to ensure the quality of the weld seam.
Joint design and preparation: The shape, size, and assembly clearance of the groove must comply with the requirements of the drawings and WPS. Before welding, the groove and both sides must be thoroughly cleaned of impurities such as water, oil, rust, and oxide scale.
Preheating and interpass temperature control: For thicker plates, steels with higher carbon equivalent, or joints with high restraint, preheating before welding is crucial. It can slow down the cooling rate and prevent cold cracks. The interlayer temperature during the entire welding process also needs to be controlled within the range specified in the WPS.
Welding heat input control: Heat input (current × voltage/speed) directly affects the microstructure and properties of the weld seam and heat-affected zone. For high-strength steels and certain specialty steels, there are specific upper and/or lower limits for heat input to prevent a decrease in toughness or the formation of undesirable microstructures.
Welding sequence and deformation control: It is necessary to establish a reasonable welding sequence and direction (such as symmetrical welding, segmented backstep welding, etc.), and to utilize jigs and fixtures to minimize welding stress and structural deformation to the greatest extent possible.
Environmental conditions: When welding outdoors or in a cabin, effective protective measures must be taken in case of adverse conditions such as humidity (welding should be stopped when the relative humidity is usually required to be >85%), strong winds (wind speed needs to be sheltered during gas shielded welding), rain, snow, etc. Otherwise, welding is not allowed.
V. Post-weld treatment, inspection, and testing
Post-weld treatment: This includes cleaning the weld surface (such as removing welding slag and spatters) and grinding the weld seam (to achieve a smooth transition in shape). For critical structures or specific materials, post-weld heat treatment (such as stress relief annealing) may be required to eliminate residual stress.
Non-destructive testing (NDT): It is the primary means to verify the internal quality of welds. Common methods include:
Visual Testing (VT): Inspecting weld dimensions and surface defects.
Radiographic Testing (RT) or Ultrasonic Testing (UT): Used to detect defects such as porosity, slag inclusion, lack of penetration, and cracks within the weld seam. UT is particularly suitable for thick plates. The testing proportion and qualification criteria are strictly implemented according to the "criticality" category of the weld seam, as per the specifications.
Magnetic particle testing (MT) or penetrant testing (PT): primarily used for detecting surface or near-surface defects.
Destructive testing: During process qualification or when necessary, samples need to be taken for tensile, bending, impact, hardness, and macroscopic metallographic tests to verify the mechanical properties and microstructure of the weld seam.
VI. Welding requirements for special components and materials
Key components such as stem and stern posts, rudder systems, main engine bases, masts, crane supports, and mooring equipment, which bear alternating loads or high stress, typically have stricter welding requirements. These components often necessitate higher welder qualification levels, more meticulous process control, and non-destructive testing.
Special materials:
High-strength steel: Strictly control heat input and preheating temperature to prevent cold cracks and softening of the heat-affected zone.
Stainless steel: Pay attention to the control of linear energy to prevent intergranular corrosion, and ensure proper shaping or protection on the back side to prevent oxidation.
Aluminum alloy: It has high requirements for cleanliness, and suitable shielding gas and welding techniques are needed to prevent porosity and hot cracks.
Composite plates (such as steel with corrosion-resistant cladding): When welding, it is necessary to consider the properties of both the base material and the cladding, making the process complex.
VII. Quality records and traceability
Complete and clear quality records must be maintained throughout the entire welding process, including material certificates, welder certificates, WPS/PQR, welding construction records, preheat/interpass temperature records, NDT reports, heat treatment records, etc. Ensuring complete traceability from a steel plate to a weld is a basic requirement for quality systems (such as ISO 9001) and classification society audits.
In summary, welding of marine components is a systematic project that integrates technology, management, and experience. It is far from being a simple metal connection. Instead, it is carried out by qualified personnel using qualified materials and following qualified processes within a strict standard system framework and controlled environment. After rigorous verification, it ensures that the "mobile city" of the ship is robust and capable of safely navigating the vast ocean. With the application of new materials and processes (such as laser welding, friction stir welding, and automated robotic welding), welding requirements are constantly updated and developed, but its core pursuit of safety, quality, and reliability remains unchanged.
