Hazards and preventive measures of incomplete penetration of steel pipes and defects in steel pipe welding

First, what is incomplete penetration of steel pipes?

Incomplete penetration of steel pipes refers to the phenomenon that the root of the joint of the steel pipe is not completely melted during steel pipe welding.

What are the reasons for the incomplete penetration of steel pipes?

1. The welding current of the steel pipe is too small and the arc penetration is insufficient.

2. The groove angle is too small and the blunt edge is too large, making it difficult to weld through the root.

3. The diameter of the welding rod is too large, which is not conducive to forming a good molten pool at the root.

4. The welding position of the steel pipe is improper. For example, in the vertical welding and overhead welding positions, due to the effect of gravity, the molten pool metal is easy to fall, resulting in incomplete penetration of the root steel pipe.

5. The steel pipe welding operation technology is unskilled, the rod feeding method is improper, and the arc stays at the root of the groove for a short time.

Second, what are the hazards of incomplete penetration of steel pipes?

The bearing area of the incompletely welded steel pipe is reduced, the strength is reduced, and it is easy to break when subjected to force. Incomplete penetration of the steel pipe will affect the density of the weld and may cause leakage. Incomplete penetration defects of the steel pipe will also become stress concentration points, reducing the reliability and service life of the structure.

Third, what are the preventive measures for incomplete penetration of the steel pipe?

1. Reasonable groove design: Ensure that the groove angle is appropriate. Generally speaking, the groove angle should be determined according to the thickness of the steel pipe and the welding method of the steel pipe. A larger groove angle can increase the welding depth of the steel pipe and reduce the risk of incomplete penetration of the steel pipe. Control the size of the blunt edge of the groove. The blunt edge should not be too large, otherwise it will hinder the penetration of the steel pipe welding pool. Usually, the thickness of the bell edge should be within a reasonable range so that the steel pipe welding arc can fully melt the steel pipe.

2. Clean the weldment: Before welding the steel pipe, the oil, rust, oxide scale, and other impurities on the surface of the weldment must be thoroughly cleaned. These impurities will affect the stability of the steel pipe welding arc and the fluidity of the molten pool, resulting in incomplete penetration of the steel pipe. Mechanical cleaning (such as sandblasting, and grinding) or chemical cleaning (such as pickling, and alkali cleaning) can be used for cleaning. The inside of the steel pipe welding groove should also be carefully cleaned to ensure that there are no residual impurities and oxides. You can use tools such as wire brushes and grinders for cleaning.

3. Choose appropriate steel pipe welding parameters: adjust the steel pipe welding current. Too small a current will lead to insufficient steel pipe welding penetration, which is easy to cause the steel pipe to not be welded through. The appropriate steel pipe welding current should be selected according to factors such as the thickness of the steel pipe, the groove form, and the welding position of the steel pipe. Generally speaking, as the thickness of the steel pipe increases, the steel pipe welding current should also increase accordingly. Control the steel pipe welding voltage and steel pipe welding speed. Too high a steel pipe welding voltage or too fast a steel pipe welding speed will also affect the steel pipe welding penetration. The steel pipe welding voltage and steel pipe welding speed should be adjusted according to the steel pipe welding current and the characteristics of the steel pipe to match each other to ensure that the steel pipe welding pool has enough time and heat to melt the steel pipe.

4. Ensure the steel pipe welding operation specifications: Ensure that the steel pipe welding arc is always aimed at the root of the groove to avoid the arc deviating from the center of the groove. During the steel pipe welding process, the welder should maintain a stable steel pipe welding posture and operating technique, and control the length and angle of the arc. When performing multi-layer and multi-pass welding, attention should be paid to controlling the welding thickness and welding sequence of each layer of steel pipe. After each layer of steel pipe welding is completed, the slag and oxide on the surface of the weld should be carefully cleaned before welding the next layer of steel pipe. Avoid repeated steel pipe welding at the same position to avoid overheating and coarse grains of steel pipe welding.

5. Use appropriate steel pipe welding process: For steel pipe welding, multi-layer and multi-pass welding, double-sided welding, groove welding, and other process methods can be used to increase the welding depth of steel pipes and reduce the risk of incomplete penetration of steel pipes. Preheating and post-heating measures can be used to reduce the cooling rate of steel pipe welded joints, improve the structure and performance of steel pipe welded joints, and reduce the occurrence of incomplete penetration of steel pipes. The preheating temperature and post-heating temperature should be determined according to factors such as the material, thickness, and steel pipe welding method of the steel pipe.

Fourth, defects in the steel pipe welding process

1. The size of the steel pipe weld does not meet the requirements of the specification

Phenomenon: The height of the steel pipe weld is too large or too small during inspection, or the width of the weld is too wide or too narrow, and the transition between the weld and the steel pipe is not smooth, the surface is rough, the weld is not neat in the longitudinal and transverse directions, and the weld is too concave in the corner weld.

Reason: The straightness of the steel pipe weld groove processing is poor, the angle of the groove is improper, or the size of the assembly gap is uneven. The current is too large in steel pipe welding, causing the electrode to melt too quickly, making it difficult to control the weld formation. The current is too small, and the electrode will produce "adhesion" when the steel pipe is welded, resulting in incomplete welding or weld nodules. The welder is not skilled enough, the rod movement method is improper, such as too fast or too slow, and the welding rod angle is incorrect. In the submerged arc automatic welding process, the steel pipe welding process parameters are improperly selected.

Preventive measures: Process the weld groove according to the design requirements and the provisions of the steel pipe welding specifications. Try to use mechanical processing to make the groove angle and the straightness of the groove edge meet the requirements, and avoid using artificial gas cutting and manual scraping to process the groove. When assembling, ensure the uniformity of the weld gap to lay the foundation for ensuring the quality of steel pipe welding. Pass the steel pipe welding process assessment and select appropriate steel pipe welding process parameters. Welders must be certified to work, and trained welders have a certain theoretical foundation and operating skills. The last layer of multi-layer weld on the steel pipe welding surface is, under the condition of ensuring fusion with the bottom layer, a smaller current than the steel pipe welding between each layer should be used, and a small diameter (φ2.0mm~3.0mm) welding rod should be used for surface welding. The speed of the rod is required to be uniform, rhythmically advancing in the longitudinal direction, and making a certain width of lateral swing, so that the weld surface can be neat and beautiful.

2. Undercut (bite meat)

Phenomenon: The depression or groove melted by the arc at the edge of the weld during steel pipe welding is not supplemented by molten metal, leaving a gap. Too deep an undercut will weaken the strength of the steel pipe welded joint, causing local stress concentration, and cracks will be generated at the undercut after bearing.

Cause: The main reason is that the steel pipe welding current is too large, the arc is too long, the welding rod angle is not properly controlled, the speed of the welding rod is not appropriate, and the welding rod retention length is too short at the end of the steel pipe welding. It is generally a common defect in vertical welding, horizontal welding, and overhead welding.

Preventive measures: When welding steel pipes, the current should not be too large, the arc should not be too long or too short, and short arc welding should be used as much as possible. Master the appropriate welding rod angle and skilled welding rod movement techniques. When the welding rod swings to the edge, it should be slightly slower so that the molten welding rod metal fills the edge, and it should be slightly faster in the middle. The depth of the weld undercut should be less than 0.5mm, the length should be less than 10% of the total length of the weld, and the continuous length should be less than 10mm. Once the depth or production exceeds the above tolerance, the defect should be cleaned up, and a welding rod with a smaller diameter and the same brand should be used. The welding current of the steel pipe is slightly larger than the normal one, and the defect should be filled with repair welding.

3. Cracks

Phenomenon: During the steel pipe welding process or after the steel pipe welding, metal cracks appear in the steel pipe welding area. It occurs inside or outside the weld, and may also occur in the heat-affected zone. According to the location of its occurrence, it can be divided into longitudinal cracks, transverse cracks, arc crater cracks, root cracks, etc., and can also be divided into hot cracks, cold cracks and reheating cracks.

Cause: Large stress is generated after the heat-affected zone of the weld shrinks. The steel pipe contains more hardened tissues and is prone to cracks after cooling. There is a relatively high hydrogen concentration in the weld. And other harmful element impurities, etc., are prone to cold and hot cracks.

Prevention and control measures: Mainly start with stress elimination, correct use of steel pipe welding materials, and perfect operation process. Pay attention to the groove form of the steel pipe welding joint to eliminate cracks caused by uneven heating and cooling of the weld due to thermal stress. For example, when welding steel plates of different thicknesses, the thick steel plate must be thinned. The selected materials must meet the requirements of the design drawings, strictly control the source of hydrogen, dry the welding rods before use, and carefully clean the oil, water, and other impurities on the groove. In steel pipe welding, select reasonable steel pipe welding parameters to control the input heat between 800~3000℃ cooling temperature to improve the organization state of the weld and heat affected zone. In the steel pipe welding environment temperature is low and the material is thin, in addition to increasing the operating environment temperature, it should also be preheated before welding. After the steel pipe welding is completed, try to keep warm and slow cooling and post-weld heat treatment to eliminate the delayed cracks caused by residual stress in the weld during the cooling process.