What are the causes and elimination measures for porosity defects in seamless steel pipe fittings

First, what are the causes of porosity defects in seamless steel pipe fittings?

Porosity is a common defect in the processing of seamless steel pipe fittings. Its formation is related to many factors such as raw material quality, processing technology, and environmental conditions. Specifically, it can be divided into the following four categories:

1. Inherent hidden dangers in the raw materials and auxiliary materials of seamless steel pipe fittings

(1) Problems with the steel pipe substrate: If the seamless steel pipe itself has internal defects such as porosity and shrinkage cavities, or if impurities such as oil, rust, and oxide scale adhere to its surface, the gases generated by the combustion of impurities during processing (especially during welding) cannot be discharged in time, easily forming porosity. For example, rust reacts with carbon to generate CO₂ gas, and oil combustion generates H₂O and CO₂, both of which may be trapped in the metal structure.

(2) Substandard quality of filler material in seamless steel pipe fittings: If the welding wire or electrode is damp, rusty, or contains excessive alloying elements, it will decompose and generate gas at high temperatures; insufficient purity of the shielding gas, after mixing with air, results in low solubility of nitrogen in the molten metal, which precipitates and forms pores upon cooling.

2. Unreasonable setting of processing parameters for seamless steel pipe fittings

(1) Deviation in welding process parameters for seamless steel pipe fittings: Excessive welding current easily leads to excessively high molten pool temperature, intensifying metal evaporation and generating gas; insufficient current results in insufficient molten pool depth, making it difficult for gas to rise and escape; excessively fast welding speed accelerates the cooling rate of the molten pool, causing gas to be trapped by solidified metal before it can escape; excessively long arc increases the contact area between the shielding gas and air, leading to nitrogen intrusion into the molten pool.

(2) Improper forging/heat treatment processes of seamless steel pipe fittings: Excessive heating temperature or prolonged holding time during forging leads to severe oxidation of the steel pipe surface. The oxide layer is pressed into the metal during forging, forming pores. Excessive cooling rate during heat treatment prevents the timely diffusion of residual hydrogen and nitrogen, causing them to accumulate and form micropores.

(3) Defects in the forming process of seamless steel pipe fittings: In forming processes such as bending and flaring, uneven mold surface or pressure distribution can cause localized negative pressure during metal deformation, drawing in air and forming pores. Uneven heating of the pipe can also disrupt the internal gas distribution, leading to accumulation and defects.

3. Adverse effects of the processing environment on seamless steel pipe fittings

(1) Humidity and dust: High humidity in the processing environment allows moisture in the air to adhere to the steel pipe surface or mix into the protective gas, decomposing into hydrogen and oxygen at high temperatures, forming pores. Dust and oil particles in the environment, if falling into the processing area, will be encapsulated by the molten pool and burn, producing gas.

(2) Airflow Interference: Strong airflow during welding can disrupt the stable protective gas layer, allowing air to enter the molten pool and causing nitrogen porosity.

4. Operational and Equipment Factors of Seamless Steel Pipe Fittings

(1) Improper Operation of Seamless Steel Pipe Fittings: Improper electrode angle and uneven electrode speed during welding can lead to insufficient stirring of the molten pool, preventing gas from escaping smoothly; uneven beveling of the steel pipe, with gaps that are too large or too small, can affect the formation of the molten pool and increase the probability of porosity.

(2) Poor Equipment Performance of Seamless Steel Pipe Fittings: Poor current and voltage stability of the welding equipment can lead to arc instability, molten pool temperature fluctuations, and obstructed gas escape; leaks in the protective gas delivery pipeline and inaccurate flow control can cause the protective effect to fail, allowing air to enter the molten pool.

Second, what are the targeted measures to eliminate porosity defects in seamless steel pipe fittings?

Based on the above source analysis, elimination measures need to be formulated from four dimensions: "source control, process optimization, environmental improvement, and operational standardization," to ensure that gas cannot be formed or can be discharged in a timely manner during processing:

1. Strict control of the quality of raw materials and auxiliary materials for seamless steel pipe fittings

(1) Pretreatment of steel pipe substrate: Before processing, the seamless steel pipe surface is cleaned by mechanical grinding, sandblasting, or chemical pickling to remove rust, oil, and oxide scale; non-destructive testing is performed on the pipe to remove unqualified substrates with internal porosity and shrinkage cavities.

(2) Standardized management of auxiliary materials: Welding wire and welding rods that meet national standards are selected and kept dry during storage to avoid moisture and rust; high-purity products are selected for protective gas, and the gas purity certificate is checked before use. Regular checks are conducted on the pipeline to ensure stable flow.

2. Optimization of Processing Parameters for Seamless Steel Pipe Fittings

(1) Welding Process Parameter Calibration: Select appropriate welding current, voltage, and welding speed according to the steel pipe material and wall thickness to ensure a moderate molten pool temperature; shorten the arc length, generally controlling it to 0.5-1 times the electrode diameter to reduce air intrusion; when using multi-layer, multi-pass welding, clean the weld bead promptly after each layer to avoid gas generation from residual impurities between layers.

(2) Forging/Heat Treatment Process Adjustment: Strictly control the heating temperature and holding time during forging to avoid excessive oxidation of the steel pipe surface; adopt a slow cooling heat treatment process to give sufficient diffusion time for gases inside the metal; uniformly heat the pipe before the forming process to ensure a smooth mold surface and uniform pressure distribution, avoiding air intake during deformation.

(3) Bevel and Gap Design: Process the steel pipe bevel according to process requirements to ensure a reasonable bevel angle and gap, facilitating molten pool formation and gas discharge.

3. Improve the processing environment for seamless steel pipe fittings

(1) Control ambient temperature and humidity: Keep the relative humidity of the processing environment below 60%. Use a dehumidifier in humid weather. Avoid outdoor processing in rainy or foggy weather. If outdoor work is necessary, construct a temporary protective shed.

(2) Reduce environmental interference: Clean dust and oil stains from the processing area to maintain a clean environment. Avoid strong airflow interference during welding. Turn off fans, control ventilation appropriately, and use windbreaks to protect the welding area when necessary.

4. Standardize the operation procedures and equipment maintenance for seamless steel pipe fittings

(1) Improve operator skills

(2) Regularly maintain processing equipment: Regularly calibrate the current and voltage control systems of the welding equipment to ensure arc stability. Check the accuracy of the protective gas flow meter and pressure reducing valve, and replace aging delivery pipes and joints promptly. Regularly clean impurities from the mold surface to maintain the precision of the forming equipment.

5. Strengthen Quality Inspection and Feedback

(1) Process Inspection: During processing, real-time inspections are conducted using methods such as visual inspection and magnifying glass observation. If porosity defects are found, the machine is stopped immediately, and process parameters are adjusted. After welding, non-destructive testing is performed on the weld to identify hidden porosity.

(2) Closed-Loop Management: Cause analysis is performed on pipe fittings that develop porosity. Information such as process parameters, environmental conditions, and operators is recorded to form a quality feedback mechanism and continuously optimize processing plans.