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Common defects and prevention of seamless carbon steel pipe production
Jan 29, 2019

Common defects in seamless carbon steel tubes


An internal surface defect


In the foldFeatures: the internal surface of the seamless carbon steel tube presents a straight line or spiral, semi-spiral zigzag defects.Causes:1) tube blank: loose center and segregation;Serious shrinkage residual;Non - metallic inclusion exceeds standard.2) uneven heating, too high or too low temperature, too long heating time.3) perforation area: the top is badly worn;Improper adjustment of punch parameters;Perforation roll aging, etc.Inspection: no internal folding is allowed on the inner surface of the seamless carbon steel pipe. The internal folding of the pipe end should be polished or recut. The actual value of the wall thickness at the grinding point should not be less than the minimum value required by the standard.Pass long inside fold disallow.



2 the scarFeatures: the inner surface of seamless carbon steel tube appears spot scar, generally does not take root easy peeling.Causes:1) there are impurities in the graphite lubricant.2) iron lug at the back end of waste pipe is pressed into the inner wall of seamless carbon steel pipe.Inspection: the inner surface of seamless carbon steel pipe is not allowed to exist, the pipe end should be polished and re-cut, the grinding depth should not exceed the standard requirements of negative deviation, the actual wall thickness should not be less than the minimum value required by the standard;Pass long inside scar dismisses.



3 become warped skinFeatures: the inner surface of the seamless carbon steel tube presents a straight line or broken nail - shaped upturned small skin.It occurs mostly in the capillaries and is prone to flaking.Causes:1) improper adjustment parameters of punch.2) sticking steel on the top.3) iron scale accumulation in waste pipes, etc.Inspection: the inner surface of seamless carbon steel tube is allowed to have root-less warped skin which is easy to peel off (or can be burnt off during heat treatment).Root warped skin should be polished or removed.



4  straightFeatures: there are linear scratches with certain width and depth on the inner surface of the seamless carbon steel tube.Causes:1) the rolling temperature is low and the mandrel sticks to hard metal.2) graphite contains impurities, etc.Check to:1) the inner straights of the casing and common pipes are allowed to exist at a depth not exceeding 5% (the maximum depth of pressure vessels is 0.4mm).Be careful to grind and cut off the straight road in Chad.2) the sharp edge of the inner straight should be polished smooth.



 5  Edge within: there is a linear bulge with a certain width and depth on the inner surface of the seamless carbon steel tube.Cause: serious wear of mandrel, not smooth or too deep.Check to:1) the height of the casing pipe and pipeline shall not exceed 8% of the wall thickness, and the maximum height shall not exceed 0.8mm.In case of excessive tolerance, it should be polished and recut.2) inside edges of common pipes and pipelines are allowed to exist with the height not exceeding 8% of the wall thickness (the maximum height is 0.8mm).Excessive tolerance should be polished and recut.3) for L2 grade (i.e. N5) flaw detection, seamless carbon steel pipe shall be required, and the height of inner edge shall not exceed 5% (the maximum height is 0.5mm).Excessive tolerance should be polished and recut.4) the sharp edge of the edge should be polished and smoothed.


6 the bulgeFeatures: the inner surface of seamless carbon steel tube is regularly convex and no damage is found on the outer surface.Cause: continuous roll grinding too much or off the meat and so on.Inspection: according to the requirement of inner edge.



7, concaveFeatures: the inner surface of seamless carbon steel tube has regular or irregular pits and no damage on the outer surface.Causes:1) improper adjustment of continuous rolling and mismatching of rolling speed of each stand roller.2) the tube billet is not heated evenly or the temperature is too low.3) deviation of rolling center line, collision between seamless carbon steel pipe and rolling table after continuous rolling, etc. (note: this reason was put forward in January 2003, and the principle is still under discussion).Inspection: no more than the negative deviation of the wall thickness, the actual wall thickness is greater than the minimum required value of the wall thickness allowed to exist.Excessive pull concave should be removed.(note: serious development of tension and depression is tension crack, which should be strictly examined).



8 internal thread (this defect is only produced in the assel unit)Features: there are spiral marks on the inner surface of the seamless carbon steel tube, which mostly appear on the inner surface of the thin-walled tube, with a distinct feeling of uneven.Causes:1) inherent defects of skew rolling process.When the technical parameters of assel mill are not adjusted, this defect is more prominent.2) the deformation distribution is not reasonable, assel wall reduction is too large.3) improper configuration of assel roll.Inspection: seamless carbon steel pipe internal thread defect depth is not greater than 0.3mm, and within a certain tolerance range.


External surface defects


1 outside the foldFeatures: the outer surface of seamless carbon steel tube presents spiral lamellar folding.Causes:1) there are folds or cracks on the surface of the billet.2) the pores under the skin of the tube billet have serious subcutaneous inclusions.3) the surface of the tube blank is not cleaned properly, or there are ears and wrong surfaces.4) during the rolling process, the surface of the seamless carbon steel tube is lifted and scratched, and then pressed to the matrix of the seamless carbon steel tube through the rolling, forming an outer fold and so on.Inspection: it is not allowed to exist: slight grinding can be carried out, the actual value of wall thickness and outside diameter after grinding shall not be less than the minimum value required by the standard.



2 abscission layerFeatures: spiral or block stratification and fracture on the surface of seamless carbon steel tube.Causes: serious nonmetallic inclusions, residual shrinkage or severe porosity in the billet.Inspection: no presence allowed.



3 outside scarFeature: the outer surface of seamless carbon steel pipe appears spot scar.Causes:1) roll sticking, aging, severe wear or roll rubbing.2) conveying roller table is adhered with foreign bodies or seriously worn.Check to:1) the distribution of outer scar slices should be polished or removed.2) on the pipe segment with external scar, the area of external scar exceeding 10% shall be removed or polished.3) if the depth exceeds 5% of the wall thickness, the outer scar shall be polished.4) the actual value of wall thickness and outside diameter at the grinding place shall not be less than the minimum value required by the standard.



4 pitsFeatures: the surface of seamless carbon steel pipe appears uneven pit.Causes:1) if the seamless carbon steel tube stays in the furnace for too long or the heating time is too long, the surface will generate an excessively thick oxide scale to clear the impurity and roll into the surface of the seamless carbon steel tube.2) high-pressure water phosphorus removal equipment does not work properly, and phosphorus removal is not clean.Check to:1) local pitted surface with negative deviation not exceeding the wall thickness is allowed to exist.2) the area of pitted surface shall not exceed 20% of the area of the pipe segment with pitted surface.3) the abnormal hemp surface can be polished or removed, and the actual value of wall thickness and outer diameter at the grinding point shall not be less than the minimum value required by the standard.4) serious pock marks will be disqualified.



5 the green lineFeatures: the external surface of seamless carbon steel tube presents symmetrical or asymmetric linear rolling mark.Causes:1) serious misalignment or wear of the sizing machine.2) unreasonable roll pass design of sizing machine.3) rolled low temperature steel.4) poor roll processing and too small chamfering at the edge of the roll.5) poor roll assembly, large gap, etc.Check to:1) the allowable height of the outer surface of the casing shall not exceed 0.2mm.2) no hand blue line is allowed in the class pipe of high-pressure container.The blue line must be removed.Grinding should be smooth without edges and corners.3) the allowable height of universal seamless carbon steel pipe (structure, fluid, hydraulic support, etc.) is not more than 0.4mm.4) the sharp edge of the blue line should be polished smooth.5) the actual value of wall thickness and outside diameter at the grinding place shall not exceed the minimum value required by the standard.




6 hairlineFeatures: continuous or discontinuous hairline on the surface of seamless carbon steel tube.Causes:1) there are subcutaneous pores or inclusions in the billet.2) the surface of the billet is not cleaned thoroughly, and there are fine cracks.3) excessive wear and aging of the roll.4) poor roll processing accuracy, etc.Inspection: no visible hairline is allowed on the external surface of seamless carbon steel tube. If there is any hairline, it should be completely removed. The actual value of the thickness of the rear wall and the external diameter should not be less than the minimum value required by the standard.



7 network crackFeatures: the surface of seamless carbon steel tube appears fish-scale cracks with strip and large pitch.Causes:1) high content of harmful elements (such as arsenic) in tube billet.2) aging and steel sticking of perforating roll.3) guide plate adheres to steel, etc.Inspection: should be completely cleared.The actual value of wall thickness and outside diameter after clearance shall not be less than the minimum value required by the standard.


8 scratchFeatures: the surface of seamless carbon steel tube is spiral or linear groove defects, most of the bottom can be seen.Causes:1) mechanical scratches mainly occur in roller table, cold bed, straightening and transportation.2) roll processing is not good or wear seriously or roll joints with foreign bodies.Check to:1) local scratches of no more than 0.5mm are allowed on the external surface of seamless carbon steel tube, and the scratches in excess of 0.5mm shall be polished.The actual value of wall thickness and outside diameter at the grinding place shall not be less than the minimum value required by the standard.2) sharp edge scratches should be polished and smoothed.


9 touch flatCharacteristics: the external surface of seamless carbon steel tube is concave and convex, but the thickness of seamless carbon steel tube is not damaged.Causes:1) bump and dent during lifting.2) it is dented when straightened.3) dented roller table after sizing machine.Inspection: local negative deviation of the outside diameter and smooth surface can be dented.Out of tolerance excision.



10 bruisedFeature: irregular scar on the outer surface of seamless carbon steel tube due to collision.Causes: can be produced in the cold and hot areas of various collision, etc.Check to:1) local bruises with a depth not exceeding 0.4mm are allowed on the external surface.2) the bruises exceeding 0.4mm shall be smoothed and the actual value of the outside diameter and wall thickness at the grinding place shall not be less than the minimum value required by the standard.



11 JiaoAoFeatures: seamless carbon steel tube surface spiral concave.Causes:1) improper adjustment of straightener roll Angle and excessive pressuring.2) serious wear of straightening roll.Inspection: the external surface of seamless carbon steel pipe is allowed to have no obvious edges and corners and the internal surface is not prominent, and the external diameter size meets the tolerance requirements of the correction.Excessive correction should be removed.



12 rolling foldFeature: the tube wall of seamless carbon steel tube is corrugated with concave and convex inside along the longitudinal part or overall length.Causes:1) the width coefficient of pass is too small.2) improper adjustment of rolling mill results in misalignment of pass or inconsistent rolling center line.3) improper distribution of press-down amount of each stand of continuous rolling mill.As a result of the above reasons seamless carbon steel pipe in the rolling process metal into the roller gap or tube lost stability resulting in tube wall creases.Inspection: no presence allowed.Should be removed or disallowed.



13 crackingFeatures: the surface of seamless carbon steel tube has the phenomenon of pulling apart and cracking, which is mostly produced on the thin-walled tube.Causes:1) due to the uneven heating temperature of the tube billet, the deformed part is deformed. The part with low temperature is rolled by tension. When the tension is large, the pipe will be pulled and cracked.2) improper adjustment of speed and roll gap of each stand of continuous rolling mill, resulting in tearing of steel drawing.3) the effect of capillary wall thickness. When the thickness of the capillary wall supplied by the punch machine to the continuous mill is small, the metal deformation of the continuous mill decreases compared with the designed deformation, resulting in tension rolling of the continuous mill and tearing when the tension is large.4) there are some serious inclusions in the billet itself.Inspection: no presence allowed.Should be removed or disallowed


 dimensions out of tolerance



1 uneven wall thicknessFeature: the wall thickness of seamless carbon steel tube is not uniform on the same section.Causes:1) uneven heating of tube billet.2) the rolling line of the piercing machine is not adjusted, and the centering roll is unstable.3) head wear or eccentric hole behind the head.4) pipe billet centering hole correction.5) excessive bending and cutting inclination of tube blank.Inspection: measured branch by branch, uneven wall thickness should be removed.



2 wall thickness out of toleranceCharacteristics: the wall thickness of seamless carbon steel tube is out of tolerance in one direction.The excess deviation is called the wall thickness ultra thin.Causes:1) uneven heating of tube billet.2) improper adjustment of punch.Inspection: measurement by branch, end deviation should be removed, and the whole deviation should be changed or rejected.



3 Outside diameter out of toleranceFeatures: the outer diameter of seamless carbon steel tube exceeds the standard, and the one with positive deviation is called the outer diameter is large, and the one with negative deviation is called the outer diameter is small.Causes:1) the sizing machine groove wear is too large, or the new groove design is not reasonable.2) unstable finishing temperature.Check and sentence: one by one measure, if exceeding the standard, the judgment shall be changed or invalidated.


4 bendingFeatures: the seamless carbon steel tube along the length of the direction is not straight or in the seamless carbon steel tube end appears goose head bending known as "goose head bend."Causes:1) caused by local water cooling during manual thermal inspection.2) improper adjustment during straightening and severe wear of the straightening roll.3) improper sizing machine processing, assembly and adjustment.4) bending caused during lifting and transportation.Inspection: if the bending exceeds the standard, it can be straightened twice, otherwise it will be rejected.Unstraightened "goose head bends" should be removed.


Length out of toleranceFeatures: the length of seamless carbon steel tube exceeds the requirements.Causes:1) excessive length of tube billet.2) rolling instability.3) poor control during cutting and so on.Inspection: re-cut or change the sentence of long ruler tube, or change or invalidate the sentence of short ruler tube