The welding of high-pressure vessels is one of the most critical and demanding processes in the manufacturing process. Its quality is directly related to the operational safety and service life of the equipment, and once it fails, it may cause catastrophic accidents. Therefore, the welding of high-pressure vessels requires a systematic and strictly controlled method.
The following are the core points that need to be paid attention to in the welding of high-pressure vessels, which can be summarized into six aspects: "human, machine, material, method, environment, and measurement":
1、 Personnel Qualification and Management
This is the primary condition among all requirements.
Certification for employment: Welders must hold a special equipment operator certificate (welding) issued by the technical supervision department and perform welding within the scope of qualified projects. Unauthorized operation is strictly prohibited.
Skill assessment: Not only certification is required, but additional exams specific to the product are also required to verify their ability to perform welding for that particular product.
Quality awareness: Welders need to have a strong sense of responsibility and quality awareness, and strictly follow the welding process specifications.
2、 Welding equipment
Equipment performance: Welding equipment (welding machine) should have stable performance, sensitive and accurate instruments, and be able to continuously provide the required welding current and voltage according to specifications.
Calibration and maintenance: The ammeters, voltmeters, etc. on the equipment need to be regularly calibrated to ensure accuracy.
3、 Material control
This is the basis for ensuring the composition and performance of the weld seam.
Matching of base material and welding material: Select suitable welding materials (welding rod, welding wire, flux) strictly based on the chemical composition, mechanical properties, and working conditions (such as corrosion resistance requirements) of the base material.
Welding material management: The storage, drying, distribution, and recycling of welding materials must be strictly carried out in accordance with regulations.
Drying: Low hydrogen fluxes such as alkaline welding rods must be dried at the specified temperature and time to remove moisture and prevent hydrogen induced cracking.
Insulation and Requisition: Dried welding rods should be stored in insulation buckets, taken as needed, and used up within the specified time.
4、 Welding process and method
This is the core technology that must be carefully designed and validated in advance.
Welding process qualification: Before welding the product, welding process qualification must be carried out according to the standard. Verify through experiments whether the proposed welding process can obtain welding joints with qualified mechanical properties (strength, toughness, plasticity). After passing the assessment, a welding process specification is formulated, and on-site welders must strictly follow this specification.
Preparation before welding:
Groove processing: The groove form and size must meet the process requirements and have high processing accuracy.
Cleaning: Oil, rust, water, paint and other impurities within a range of at least 20mm on both sides of the groove must be thoroughly removed until a metallic luster is exposed.
Preheating: Preheating before welding is crucial for steel, thick plates, or structures with a tendency towards hardening. Preheating can slow down the cooling rate, prevent cold cracks, and reduce welding stress. The preheating temperature must strictly follow the process requirements and be uniformly heated to the specified range before welding.
Welding process control:
Arc initiation and termination: The arc must be initiated within the groove or on the weld bead, and it is prohibited to initiate the arc arbitrarily on non welding surfaces to avoid surface damage and hard spots.
Interlayer temperature: During multi-layer and multi pass welding, it is necessary to control the temperature between the passes (interlayer temperature), and it cannot be too high or too low.
Line energy control: Strictly control the welding heat input. Excessive heat input can lead to a decrease in joint toughness, while insufficient heat input may increase the tendency for cracks.
Back cleaning: For important double-sided welding seams, after welding one side, it is usually necessary to clean the weld root from the back with a grinding wheel or carbon arc air planer to ensure complete fusion and no defects.
5、 Environmental conditions
Welding environment: Welding should be carried out in an environment with low relative humidity, no wind, and protection against rain and snow.
Humidity: It is usually stipulated that the relative humidity is below 90%.
Temperature: The ambient temperature should not be too low (special measures should be taken if it is below 0 ° C).
Windproof: Windproof shelters must be set up during on-site welding to prevent protective gases (such as argon) from being blown away and causing porosity in the weld seam.
6、 Inspection and Testing
Throughout the entire process before, during, and after welding, it is the ultimate guarantee of quality.
Pre weld inspection: Check the groove, gap, blunt edge, cleanliness, preheating temperature, welder qualification, equipment, welding materials, etc.
During welding inspection: check welding process parameters (current, voltage, speed), interlayer temperature, cleaning quality, etc.
Post weld inspection:
Appearance inspection: Check for surface defects such as weld formation, excess height, and undercutting.
Non destructive testing:
RT (radiographic testing): detects volumetric defects such as internal pores, slag inclusions, and incomplete penetration.
UT (Ultrasonic Testing): detects surface defects such as internal cracks and lack of fusion, especially effective for thick plates.
PT (Penetrant Testing): Detect surface opening defects.
MT (Magnetic Particle Testing): detects surface and near surface defects.
Usually, Class A and B welds of high-pressure vessels require 100% RT or UT.
Destructive testing: For process qualification test plates and product welding test plates, tensile, bending, impact, hardness, and metallographic tests are required to verify the mechanical properties of the joints.
Heat treatment: Most high-pressure vessels require stress relief heat treatment after welding to reduce residual stress and improve structural properties.
Pressure test: The final test to inspect the overall strength and sealing of the container, usually using water pressure test or air pressure test.
summary
The welding of high-pressure vessels is a systematic engineering, which is far more than just "connecting two pieces of metal together". It depends on:
Qualified individuals
Reliable 'machine' and 'material'
A validated and strictly enforced 'law'
Controlled 'Ring'
Rigorous and comprehensive 'testing'
The core idea is to ensure that the performance (especially toughness and strength) of the welded joint matches the base material through precise control throughout the entire process and in all directions, and can withstand high pressure, alternating loads, and harsh conditions of the medium, ultimately ensuring the safe operation of the container throughout its design life.