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Causes Analysis for Common Defects of Seamless Steel Pipes
Mar 23, 2022
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With the rapid development of China's industry, the production of seamless steel pipes is growing in the global proportion. The development of the national economy has played an increasingly prominent role. In recent years, part of the steel pipe industry has been built in China. The quality of seamless steel pipes is becoming more and more prominent. With the continuous development and progress of the production technology of rolling pipes, the form and type of quality problems have also changed greatly.
In recent years, China's steel pipe industry is in a stage of rapid development. In order to speed up the pace of development of the steel pipe industry, it is necessary to have a comprehensive and correct understanding of product quality defects. According to the types of product quality defects, a higher accuracy inspection method should be put forward. In order to promote the development of China's steel pipe industry, we will analyze the common defects of seamless steel pipes and causes of them.
1. Common defects of seamless steel pipes
1.1 External surface defects
(1) Folding defects are irregularly distributed.
Macroscopic characteristics: if there is residual protective slag on the surface of the continuous casting billet, deep folding defects will appear on the outer surface of the rolled pipe. There is the phenomenon of "falling block" on the longitudinal distribution and the part of the surface, where the "falling block" will be dark gray block exposed.
Microscopic features: The folding depth of the rolled pipe is about 0.5~1mm, and the folding direction of the distribution is 40° ~ 60°.
(2) The longitudinal distribution of large folding defects
Macroscopic characteristics: cracks and large fold defects appear on the surface of continuous casting billet with longitudinal distribution.
Microscopic characteristics: most of the surface folding depth of seamless steel pipes is about 1-10mm. The folding contains iron oxide. No other defects are included in the surrounding and middle of the folding. There is a relatively serious oxidation depreciation around the folding.
(3) Small crack defects
Macroscopic features: there are surface defects that cannot be observed by naked eyes on the outer wall of seamless steel pipes during the flaw detection. Small cracks can be observed longitudinally after polishing.
Microscopic characteristics: There are many small folding defects on the surface of seamless steel pipes. The deepest depth is about 0.15mm. The surface of seamless steel pipes is covered with a layer of iron oxide. There is a decryption layer under the layer of iron oxide. The depth is about 0.2mm.
(4) Linear defects.
Macroscopic characteristics: there are linear defects on the outer surface of seamless steel pipes, which are characterized by shallow depth, wide opening, visible bottom and certain width.
Microscopic features: the visible depth in the outer wall of the cross section of seamless steel pipes.
1.2 Internal surface defects
(1) Convex hull defects
Macroscopic characteristics: the inner wall of seamless steel pipe has random distribution of longitudinal small convex defects. The height of these small convex defects is about 0.2mm-1mm.
Microcosmic features: there are chain black and gray inclusions in the tail, middle and surrounding sides of the inner wall convex hull of cross section of seamless steel pipes. The black-gray chain contains calcium aluminate and a small amount of complex oxides (iron oxide, silicon oxide and magnesium oxide).
(2) Straight defects
Macro features: seamless steel pipe straight defects, depth and width are certain, which is similar to scratches.
Microscopic characteristics: The scratches on the inner wall of cross section of seamless steel pipes are in the shape of concave grooves with a depth of 1-2cm, and the oxidation and decarburization phenomenon at the edge of the concave grooves does not appear. The microstructure around the concave grooves is characterized by metal rheology and deformation extrusion. Microcracks usually appear at the bottom of the concave grooves due to the sizing extrusion.
2. Causes analysis of common defects in seamless steel pipes
(1) Causes of irregular distribution in folding defects
Improper viscosity of protective slag, abnormal vortexes of molten steel or abnormal vibration frequency of mold will lead to slagging. Most of the protective slag exists in the subcutaneous part or surface area of continuous casting billet, so cracks or folding defects may occur in the local area of a rolled pipe.
(2) Causes of longitudinal distribution of large folding defects
When the casting temperature is high, the liquid level of mold fluctuates greatly, which results in longitudinal cracks on the surface of continuous casting billets. As the surface of the billet has longitudinal cracks, the billet is heated for two hours in the ring heating furnace (1280℃). The cracks are seriously oxidized and decarbonized during high temperature heating, and the rolling process can not be completed, resulting in deep folding defects on the surface of the rolled pipes.
(3) Causes of small crack defects
The high heating temperature and long holding time of seamless steel pipes in annular furnace lead to heavy oxidation and decomposition on the surface of seamless steel pipes, and the matrix grain is relatively coarse. Because the decryption layer is composed of ferritic with low strength, it is easy to cause the slip or scratch when the rolling pipe is running. Therefore, the weak part of the pipe surface which is seriously decarburized will appear small cracks in the subsequent rolling process.
(4) Causes of linear defects
When the rolling temperature is too low, the diameter reducing machine and sizing machine appear pass dislocation. The surface of the roller stick to steel, even the roll surface of the rolling mill wearing too much will lead to that the surface of seamless steel pipes is scratched in the process of high temperature travel. It will result in linear defects.
(5) Causes of scarring defects
(6) Causes of quenching cracks
Because the protective slag contains more carbon powder, the crystallization vibration is abnormal and the slag is coiling, which leads to the local corporation phenomenon of continuous casting billet. In seamless steel pipes for heat treatment, carbon pipe body parts have high carbon martensite formation. The normal position is to generate low carbon martensite. Due to organization change with different R, there will be a larger organizational stress. Once stress exceeds the limits of the cracking of seamless steel pipes, it will have a quenching crack produced in high carbon martensite.
(7) Causes of convex hull defects
The continuous casting billet core has a large compound polyethylene oxide, large inclusions will be rolling slits in the process of tube rolling and perforation sandwich. Because the steel matrix and inclusion interlayer with different deformation, it can lead to pipe mill at the end of the reducing and sizing process of inwall interlining substrates of seamless steel pipes will be squeezed. Therefore, there will be a convex hull shape defects on the inner surface of the seamless steel pipes.
(8) Causes of straight defects
1) When the rolling temperature and heating temperature of the tube billet are low, the push rod or tip is easy to bond to metal, which will lead to scratches on the inner surface of the seamless steel pipe. If the cone angle of the tip is too small, the tip is severely worn and the push rod is bent. The tip will be deflected and the inner surface will be scraped.
2) If the mandrel is seriously worn, the inner surface of the seamless steel pipe is easy to be scratched in the process of rolling.
3) Graphite lubricant contains more impurities, which will lead to scratches on the inner surface of seamless steel pipes in the process of rolling.
In recent years, China's steel pipe industry is in a stage of rapid development. In order to speed up the pace of development of the steel pipe industry, it is necessary to have a comprehensive and correct understanding of product quality defects. According to the types of product quality defects, a higher accuracy inspection method should be put forward. In order to promote the development of China's steel pipe industry, we will analyze the common defects of seamless steel pipes and causes of them.
1. Common defects of seamless steel pipes
1.1 External surface defects
(1) Folding defects are irregularly distributed.
Macroscopic characteristics: if there is residual protective slag on the surface of the continuous casting billet, deep folding defects will appear on the outer surface of the rolled pipe. There is the phenomenon of "falling block" on the longitudinal distribution and the part of the surface, where the "falling block" will be dark gray block exposed.
Microscopic features: The folding depth of the rolled pipe is about 0.5~1mm, and the folding direction of the distribution is 40° ~ 60°.
(2) The longitudinal distribution of large folding defects
Macroscopic characteristics: cracks and large fold defects appear on the surface of continuous casting billet with longitudinal distribution.
Microscopic characteristics: most of the surface folding depth of seamless steel pipes is about 1-10mm. The folding contains iron oxide. No other defects are included in the surrounding and middle of the folding. There is a relatively serious oxidation depreciation around the folding.
(3) Small crack defects
Macroscopic features: there are surface defects that cannot be observed by naked eyes on the outer wall of seamless steel pipes during the flaw detection. Small cracks can be observed longitudinally after polishing.
Microscopic characteristics: There are many small folding defects on the surface of seamless steel pipes. The deepest depth is about 0.15mm. The surface of seamless steel pipes is covered with a layer of iron oxide. There is a decryption layer under the layer of iron oxide. The depth is about 0.2mm.
(4) Linear defects.
Macroscopic characteristics: there are linear defects on the outer surface of seamless steel pipes, which are characterized by shallow depth, wide opening, visible bottom and certain width.
Microscopic features: the visible depth in the outer wall of the cross section of seamless steel pipes.
1.2 Internal surface defects
(1) Convex hull defects
Macroscopic characteristics: the inner wall of seamless steel pipe has random distribution of longitudinal small convex defects. The height of these small convex defects is about 0.2mm-1mm.
Microcosmic features: there are chain black and gray inclusions in the tail, middle and surrounding sides of the inner wall convex hull of cross section of seamless steel pipes. The black-gray chain contains calcium aluminate and a small amount of complex oxides (iron oxide, silicon oxide and magnesium oxide).
(2) Straight defects
Macro features: seamless steel pipe straight defects, depth and width are certain, which is similar to scratches.
Microscopic characteristics: The scratches on the inner wall of cross section of seamless steel pipes are in the shape of concave grooves with a depth of 1-2cm, and the oxidation and decarburization phenomenon at the edge of the concave grooves does not appear. The microstructure around the concave grooves is characterized by metal rheology and deformation extrusion. Microcracks usually appear at the bottom of the concave grooves due to the sizing extrusion.
2. Causes analysis of common defects in seamless steel pipes
(1) Causes of irregular distribution in folding defects
Improper viscosity of protective slag, abnormal vortexes of molten steel or abnormal vibration frequency of mold will lead to slagging. Most of the protective slag exists in the subcutaneous part or surface area of continuous casting billet, so cracks or folding defects may occur in the local area of a rolled pipe.
(2) Causes of longitudinal distribution of large folding defects
When the casting temperature is high, the liquid level of mold fluctuates greatly, which results in longitudinal cracks on the surface of continuous casting billets. As the surface of the billet has longitudinal cracks, the billet is heated for two hours in the ring heating furnace (1280℃). The cracks are seriously oxidized and decarbonized during high temperature heating, and the rolling process can not be completed, resulting in deep folding defects on the surface of the rolled pipes.
(3) Causes of small crack defects
The high heating temperature and long holding time of seamless steel pipes in annular furnace lead to heavy oxidation and decomposition on the surface of seamless steel pipes, and the matrix grain is relatively coarse. Because the decryption layer is composed of ferritic with low strength, it is easy to cause the slip or scratch when the rolling pipe is running. Therefore, the weak part of the pipe surface which is seriously decarburized will appear small cracks in the subsequent rolling process.
(4) Causes of linear defects
When the rolling temperature is too low, the diameter reducing machine and sizing machine appear pass dislocation. The surface of the roller stick to steel, even the roll surface of the rolling mill wearing too much will lead to that the surface of seamless steel pipes is scratched in the process of high temperature travel. It will result in linear defects.
(5) Causes of scarring defects
(6) Causes of quenching cracks
Because the protective slag contains more carbon powder, the crystallization vibration is abnormal and the slag is coiling, which leads to the local corporation phenomenon of continuous casting billet. In seamless steel pipes for heat treatment, carbon pipe body parts have high carbon martensite formation. The normal position is to generate low carbon martensite. Due to organization change with different R, there will be a larger organizational stress. Once stress exceeds the limits of the cracking of seamless steel pipes, it will have a quenching crack produced in high carbon martensite.
(7) Causes of convex hull defects
The continuous casting billet core has a large compound polyethylene oxide, large inclusions will be rolling slits in the process of tube rolling and perforation sandwich. Because the steel matrix and inclusion interlayer with different deformation, it can lead to pipe mill at the end of the reducing and sizing process of inwall interlining substrates of seamless steel pipes will be squeezed. Therefore, there will be a convex hull shape defects on the inner surface of the seamless steel pipes.
(8) Causes of straight defects
1) When the rolling temperature and heating temperature of the tube billet are low, the push rod or tip is easy to bond to metal, which will lead to scratches on the inner surface of the seamless steel pipe. If the cone angle of the tip is too small, the tip is severely worn and the push rod is bent. The tip will be deflected and the inner surface will be scraped.
2) If the mandrel is seriously worn, the inner surface of the seamless steel pipe is easy to be scratched in the process of rolling.
3) Graphite lubricant contains more impurities, which will lead to scratches on the inner surface of seamless steel pipes in the process of rolling.
