1. Oil and gas transportation field

Requirements for high-pressure environment:

In oil and gas transmission pipelines, the internal pressure of the pipeline is relatively high. In order to ensure the safety of pipelines, welds must have extremely high strength and sealing properties. The strength of the welded joint should not be lower than that of the base material, generally requiring a tensile strength of 500 to 600 MPa or above and a yield strength of 300 to 400 MPa or above. During the welding process, welding parameters should be strictly controlled. For example, in submerged arc welding, the welding current should be precisely controlled at 600 1000A and the voltage should be 32 40V to ensure that the weld metal is fully fused and a high-quality welding joint is formed.

In terms of sealing, the weld seam is not allowed to have any penetrating defects such as pores or cracks. Therefore, strict non-destructive testing should be carried out after welding, such as using a combination of ultrasonic testing (UT) and radiographic testing (RT). Ultrasonic testing mainly detects defects inside the weld seam, with high sensitivity and the ability to detect small pores and slag inclusions; Radiographic testing can visually display the shape, size, and location of internal defects in welds, which is very important for determining the severity of defects.

Corrosion resistance requirements:

Oil and natural gas contain various corrosive media, such as hydrogen sulfide, carbon dioxide, etc. Therefore, the selection of welding materials should consider corrosion resistance. For welding LSAW Steel Pipes, welding wires and fluxes with good resistance to hydrogen sulfide stress corrosion cracking (SSC) should be selected. For example, for pipelines in acidic environments, welding wires containing alloy elements such as nickel and chromium can be used to improve the corrosion resistance of the weld metal.

In terms of welding process, it is necessary to control the chemical composition and microstructure of the weld seam. Avoid issues such as segregation in welds, as segregation may lead to a decrease in local corrosion resistance. At the same time, after welding is completed, anti-corrosion treatment can be carried out on the surface of the weld seam, such as coating with anti-corrosion coating. The coating thickness is generally required to reach 200 to 300 μ m to further enhance the corrosion resistance of the pipeline.

2. In the field of architectural structures

Carrying capacity requirements:

In building structures, LSAW steel pipes are used to withstand various loads, such as the self weight of buildings, wind loads, seismic loads, etc. Therefore, the load-bearing capacity of the weld seam is crucial. The design of welded joints should be reasonable, and for steel pipes that bear axial forces, the weld seam should adopt a full penetration welding form. During the welding process, it is necessary to ensure the effective thickness of the weld seam. For example, for steel pipe columns that bear large loads, the effective thickness of the weld seam should not be less than 80% of the wall thickness of the steel pipe.

In terms of welding quality control, strict monitoring of the welding process is necessary. Including real-time monitoring of welding current, voltage, welding speed and other parameters to ensure that these parameters are within a reasonable range. For example, when welding LSAW steel pipe truss structures for construction, the welding speed of single wire submerged arc welding should be controlled at 30 50cm/min to ensure the quality of the weld seam and enable the structure to withstand the expected load.

Seismic requirements:

Considering the influence of dynamic loads such as earthquakes, welds should have good toughness. In the selection of welding materials, welding wires with high toughness indicators should be chosen, such as requiring the impact toughness of the weld metal to reach 60 80J/cm ² or above at room temperature. In terms of welding process, it is necessary to control the welding heat input well to avoid excessive heat input, which may cause the microstructure of the weld metal and heat affected zone to become coarse and reduce toughness. In general, the welding heat input should be controlled within the range of 15-30kJ/cm by adjusting the welding speed and current reasonably.

3. Chemical industry field

Chemical resistant medium requirements:

There are various types of media transported by chemical pipelines, including acids, bases, salt solutions, and organic solvents. For LSAW steel pipe welding, suitable welding materials should be selected according to different media. For example, in the welding of pipelines transporting concentrated sulfuric acid, high alloy stainless steel welding wire and flux with strong acid resistance should be selected, such as welding wire containing alloy elements such as molybdenum and copper, which can enhance the corrosion resistance of the weld metal in acidic environments.

During the welding process, it is necessary to prevent chemical contamination of the weld seam. The welding area should be thoroughly cleaned before welding to remove impurities such as oil stains and rust. After welding is completed, the weld seam should be passivated to form a dense passivation film on the surface of the weld seam, improving its chemical corrosion resistance. For example, after welding stainless steel LSAW steel pipes, nitric acid solution can be used for passivation treatment. The concentration of nitric acid solution is generally 20% to 30%, and the treatment time is 15 to 30 minutes.

Requirements for high temperature environment:

In some chemical reaction processes, pipelines need to work in high-temperature environments. The main requirement for welding is to ensure the stability and strength of the weld at high temperatures. Welding materials should have good high-temperature performance, such as maintaining high strength and oxidation resistance at high temperatures. In terms of welding process, for LSAW steel pipes serving at high temperatures, post weld heat treatment, such as tempering treatment, may be required. The tempering temperature is generally 600 700 ℃ for 12 hours to eliminate welding residual stress, improve the microstructure of the weld metal and heat affected zone, and enhance their performance at high temperatures.

Inspection of welding quality of LSAW steel pipes under different applications

1. Welding quality inspection in the application environment of oil and gas transportation

Appearance inspection

Check the formation of the weld seam, which should be uniform, smooth, and free of obvious defects such as undercutting, lack of fusion, and surface porosity. For weld seam excess height, it is generally required to be controlled within a range of 23mm. The depth of undercutting shall not exceed 0.5mm, and the continuous undercutting length shall not exceed 10% of the weld length. For example, when inspecting the LSAW steel pipe weld of a 1000mm diameter oil pipeline, a weld gauge is used to accurately measure the weld clearance and undercutting.

Check the width of the weld seam, which should comply with the requirements of the welding process specification. The width of the weld seam is generally the width of the groove plus a certain margin width, and its fluctuation range should be controlled within a small range to ensure the strength and sealing of the weld seam.

Non destructive testing

Ultrasonic testing (UT): This is an important means of inspecting the welding quality of oil pipelines. By using ultrasonic testing equipment, defects such as porosity, slag inclusion, and incomplete penetration inside the weld seam can be detected. The setting of flaw detection sensitivity should be determined based on factors such as the wall thickness and material of the pipeline. For steel pipe welds with a wall thickness of 10-20mm, the inspection frequency is generally set between 25MHz, which can detect defects such as pores or slag inclusions with a diameter greater than 2mm.

Radiographic testing (RT): RT can provide a visual image of internal defects in welds. For important welds in oil and gas transmission pipelines, such as circumferential welds, radiographic inspection is required at a certain proportion. For example, for pipelines in areas with low population density, the proportion of radiographic testing for welds should not be less than 10%; For Class II areas (with moderate population density), the sampling ratio shall not be less than 20%; For three types of areas with high population density, the sampling ratio shall not be less than 30%. Radiographic inspection can detect small cracks and other defects inside the weld seam, which is crucial for evaluating the quality of the weld seam.

Pressure test

After the pipeline installation is completed, a pressure test should be conducted to check the overall sealing of the pipeline. For oil and gas transmission pipelines, hydrostatic testing is generally used. The test pressure is usually 1.25 to 1.5 times the design pressure. For example, for a petroleum pipeline with a design pressure of 10MPa, the hydrostatic test pressure can be set between 12.5 and 15MPa. During the experiment, it is necessary to continuously observe whether there is any leakage in the pipeline, and the pressure stabilization time is generally not less than 30 minutes to ensure the reliable welding quality of the pipeline under high pressure environment.

2. Welding quality inspection in the application environment of building structures

Appearance inspection

The appearance of LSAW steel pipe welds for building structures should be aesthetically pleasing and neat. Check for surface cracks, porosity, slag inclusions, and other defects in the weld seam. For steel pipe structures with high requirements for building decoration, the weld surface also needs to be polished to ensure a smooth transition with the steel pipe surface. The weld seam height is generally controlled within a range of 12mm to meet the appearance and usage requirements of the building structure.

Check the transition between the weld seam and the base metal, which should be natural and smooth. When inspecting the welds of steel pipe trusses used in construction, use tools such as magnifying glasses to observe the transition area, ensuring that there are no obvious edges or defects, and preventing stress concentration.

Non destructive testing

Magnetic Particle Testing (MT) or Penetrant Testing (PT) (applicable for surface defect detection): For LSAW steel pipe welds that are subjected to fatigue loads or have high appearance requirements in building structures, magnetic particle testing or penetrant testing can be used to inspect surface and near surface defects. Magnetic particle inspection uses the leakage magnetic field at the defect location of the workpiece to adsorb magnetic particles to display defects, and is suitable for steel pipes made of ferromagnetic materials. Penetrant testing is the process of using capillary action to allow penetrant to penetrate into defects, and then displaying the defects through imaging agents. It is suitable for steel pipes of various materials. For welding seams of important building structures, the sampling ratio is generally not less than 20%.

Ultrasonic Testing (UT) (applicable for internal defect detection): Similar to oil pipelines, steel pipe welds used in building structures also require ultrasonic testing to detect internal defects. However, the testing standards for welding seams in building structures may vary depending on the importance of the structure and design requirements. For example, for the steel pipe welds of the main load-bearing structure, the qualification level of ultrasonic testing may require reaching Level II or above in the GB/T 11345 2013 standard.

Mechanical performance testing (random inspection)

Conduct mechanical performance tests on LSAW steel pipe welds used in building structures, including tensile tests, bending tests, and impact tests. The tensile test mainly checks whether the tensile strength of the weld meets the design requirements. Generally, it is required that the tensile strength of the weld is not lower than that of the base material. Bending tests can detect the toughness and plasticity of welds. For example, when conducting side bending tests on steel pipe columns used in construction, the bending center diameter is generally three times the wall thickness of the steel pipe, and the weld should be able to withstand the specified bending angle without defects such as cracks. Impact testing is used to evaluate the toughness of welds in low-temperature or dynamic load environments. For steel pipe welds in cold regions, it is required that the impact toughness of the weld meet a certain standard at a specified low temperature (such as 20 ℃), such as an impact absorption energy of not less than 27J.

3. Welding quality inspection in chemical application environment

Appearance inspection

The visual inspection of LSAW steel pipe welds in chemical pipelines focuses on checking whether the welds have been subjected to chemical corrosion or contamination. The surface of the weld should be flat, smooth, and free of corrosion marks, discoloration, and other conditions. For pipelines transporting highly corrosive media, such as concentrated hydrochloric acid pipelines, any slight signs of corrosion on the weld surface can lead to serious leakage accidents.

Check the appearance dimensions of the weld seam, such as excess height and width, which should comply with the design and process requirements of chemical pipelines. At the same time, it is necessary to check whether the connection between the weld and the base metal is tight to prevent medium leakage at the connection.

Non destructive testing

Multiple non-destructive testing methods combined: Chemical pipeline welding quality inspection often adopts a combination of multiple non-destructive testing methods. In addition to ultrasonic and radiographic testing for internal defects, other testing methods will also be added based on the properties and requirements of the medium transported by the pipeline. For example, for pipelines transporting flammable and explosive media, eddy current testing can be used to detect defects on the surface and near surface of the weld seam. Eddy current testing has a fast detection speed and can promptly detect small cracks or holes on the surface of the weld seam.

Strict evaluation of internal defects in welds: The evaluation criteria for internal defects in chemical pipeline welds are often more stringent than in other application environments. Because once a chemical pipeline leaks, it may cause serious safety accidents and environmental pollution. For example, in radiographic inspection, the allowable size for defects such as pores and slag inclusions inside chemical pipeline welds is smaller. Generally, the pore diameter should not exceed 1mm and the slag inclusion length should not exceed 3mm.

Corrosion resistance test (spot check)

It is very important to conduct corrosion resistance testing on the welds of LSAW steel pipes in chemical pipelines. Immersion tests can be used to simulate the corrosion of pipelines in actual chemical media. For example, for pipeline welds transporting sodium hydroxide solution, immerse the sample with the weld in a certain concentration (such as 30%) of sodium hydroxide solution, soak it at a specified temperature (such as 60 ℃) for a certain period of time (such as 72 hours), and then observe the corrosion condition of the weld, such as corrosion rate and whether there is local corrosion. Meanwhile, the corrosion resistance of welds can also be evaluated through electrochemical corrosion tests, such as measuring the polarization curve of welds in simulated chemical media, analyzing parameters such as corrosion resistance potential and corrosion current density.