What are the causes and control methods for common defects in seamless steel pipes

First, the appearance characteristics of common defects in seamless steel pipes. 
Accurately identifying the appearance characteristics of defects in seamless steel pipes is the prerequisite for judging the type of defect, analyzing its causes, and taking control measures. Based on production practice, the appearance characteristics of the three major defects—cracking, eccentricity, and creases—are significantly different, as follows:
(1) Cracking defects: These are mostly manifested as linear cracks on the surface or inner wall of the seamless steel pipe. The crack direction can be divided into longitudinal, transverse, and oblique. Some cracks penetrate the entire wall thickness of the seamless steel pipe, while others are shallow surface cracks. The crack edges are sharp, without obvious traces of plastic deformation. Some cracks may retain oxide scale or impurities inside, and in severe cases, they may be accompanied by the breakage of the seamless steel pipe. According to the location of the crack, it can be divided into end cracking, middle cracking, and interface cracking, among which end cracking has the highest incidence after cold drawing and forming. 
(2) Eccentricity Defect: The core characteristic is that the outer and inner diameters of the seamless steel pipe are not aligned, resulting in uneven wall thickness distribution. This manifests as one side being thicker than the other, and in severe cases, a "single-sided thick, single-sided thin" phenomenon may occur. Uneven gaps can be observed by placing a ruler against the surface of the seamless steel pipe. Measurements with calipers reveal that the wall thickness deviation at different locations on the same cross-section exceeds industry standards. Some severely eccentric seamless steel pipes may exhibit bending and deformation.
(3) Crease Defect: This manifests as axial or circumferential strip-shaped marks on the surface or inner wall of the seamless steel pipe. The metal at the crease area shows localized folding and accumulation, resulting in a rough surface with noticeable protrusions or depressions. While there are no cracks, stress concentration is present. Crease length can be divided into short, localized creases and full-length creases, with depths ranging from a few micrometers to tens of micrometers. In severe cases, the crease area may be accompanied by a slight risk of cracking.

Second, Analysis of the Causes of Common Defects in Seamless Steel Pipes
The causes of cracking defects in seamless steel pipes: The essence of cracking defects is that during the production process of seamless steel pipes, the stress generated inside the metal exceeds its tensile strength, leading to the fracture and separation of metal grains. The causes are mainly concentrated in three aspects: raw materials, forming process, and heat treatment process, as follows:
1) Substandard raw material quality is the root cause of cracking. If the carbon content of the seamless steel pipe billet is too high or the impurity content exceeds the standard, it will lead to increased brittleness and decreased plasticity, making it prone to cracking during plastic deformation. If the billet itself has internal defects such as pores, inclusions, and delamination, these defects will become stress concentration points during cold drawing/hot rolling. As the deformation increases, the defects continue to expand, eventually leading to cracking. Surface defects such as oxide scale, burrs, and scratches on the billet surface can cause uneven stress on the defective areas during forming, easily leading to surface cracking that extends inwards.
2) Unreasonable forming process parameters are a major cause of cracking. During cold drawing, excessive deformation in a single pass (exceeding 8%) and excessive drawing speed (exceeding 1.5 m/s) can lead to uneven plastic deformation of the metal, a rapid accumulation of internal residual stress, and cracking exceeding the tensile strength of the material. Poor lubrication during drawing and excessive friction between the die and the blank can cause excessive stress on the surface metal, hindering deformation and leading to surface scratches and eventually cracking. During hot rolling, excessively high heating temperatures (exceeding 1050℃) can result in coarse grains and reduced toughness, while excessively low heating temperatures can lead to insufficient plasticity, both of which can easily cause cracking during deformation. Furthermore, failure to perform timely softening annealing during multi-pass cold drawing can lead to excessive work hardening and a sharp decrease in metal plasticity, also causing cracking during subsequent drawing.
3) Improper heat treatment processes can exacerbate the risk of cracking. In the quenching process, excessively high heating temperature and insufficient holding time can lead to incomplete metal microstructure transformation, resulting in coarse martensite after quenching, increased brittleness, and a tendency to develop quenching cracks. Excessive cooling rate can cause a large temperature difference between the inside and outside of the seamless steel pipe, generating significant thermal stress and triggering cracking. Inadequate tempering or improper tempering parameters can fail to effectively eliminate residual quenching stress, leading to long-term accumulation of residual stress and subsequent cracking of the seamless steel pipe during finishing or use.
(1) Causes of Eccentric Defects
The core cause of eccentric defects is uneven plastic deformation of the inner and outer walls of the seamless steel pipe during forming, resulting in misalignment between the outer and inner diameters. This is mainly related to mold precision, equipment precision, the initial state of the billet, and operating procedures:
1) Insufficient mold precision is a key factor in eccentricity. In cold drawing, excessive coaxiality deviation between the outer die and the mandrel (exceeding 0.01mm) can lead to uneven stress on the inner and outer walls of the billet during deformation, resulting in excessive deformation on one side and insufficient deformation on the other, ultimately causing eccentricity. Uneven die wear, deviations in the outer die hole diameter, or mandrel diameter can cause uneven wall thickness in the seamless steel pipe, also leading to eccentricity. Deviations in the die guiding device can guide the billet to deform along an inclined direction, exacerbating the eccentricity.
2) Equipment precision and operating procedures affect the eccentricity rate. Excessive spindle runout and guide rail parallelism deviations in cold drawing and hot rolling mills can cause the billet to shift during forming, preventing uniform deformation and resulting in eccentricity. Insecure billet clamping, inaccurate positioning, and billet wobbling during drawing or rolling can lead to uneven deformation. Failure to adjust equipment parameters promptly or insufficient inspection of the initial billet condition can also cause eccentricity defects.
3) Poor initial billet condition can induce eccentricity. The billet itself has defects such as eccentricity and bending. If straightening is not performed before forming, uneven deformation during forming will exacerbate the eccentricity. Excessive initial wall thickness deviation in the billet, without targeted parameter adjustments during forming, will lead to accumulated wall thickness deviation and eccentricity.
(3) Causes of Crease Defects
Crease defects are formed by localized folding and accumulation of surface metal during the plastic deformation of metal. They are mainly related to the forming process, the lubrication effect, and the billet surface quality. Specific causes are as follows:
1) Unreasonable forming process parameters are the main cause of creases. During cold drawing, excessive deformation per pass and fluctuations in drawing speed will lead to uneven deformation of the seamless steel pipe surface metal. Metal flows too quickly in some areas and is obstructed in others. Metal in obstructed areas will accumulate and fold, forming creases. An unreasonable die working zone length during drawing will lead to concentrated plastic deformation of the metal, making the surface metal prone to wrinkles, thus forming creases. During hot rolling, uneven rolling rhythm and improper adjustment of the reduction amount will lead to localized accumulation of surface metal, forming creases after cooling. 
2) Poor lubrication exacerbates crease formation. During cold drawing, uneven lubrication coating on the billet surface, insufficient grease, or aged or ineffective grease can increase friction between the die and the billet, hindering metal flow and causing localized metal compression and folding, resulting in creases. Impurities in the grease can cause indentations on the seamless steel pipe surface, which can further develop into creases.
3) Billet surface quality and die condition can also induce creases. Defects such as protrusions, burrs, and oxide scale on the billet surface can be compressed by the die during forming, causing surface metal folding and forming creases. Wear, scratches, or impurities on the die surface can hinder metal flow, leading to localized metal accumulation and crease formation. In multi-pass cold drawing, surface scratches from previous passes that are not promptly addressed can be compacted during subsequent drawing processes, forming creases.

Third, Prevention and Control Measures for Common Defects in Seamless Steel Pipes
Based on the causes of three major defects—cracking, eccentricity, and creases—and combined with production practice, targeted prevention and control measures are formulated from five aspects: raw material control, process optimization, equipment maintenance, operational standards, and quality inspection, to achieve precise defect control and reduce the defect rate.
(1) Prevention and Control Measures for Cracking Defects:
1) Control the quality of raw materials from the source to eliminate the potential for cracking.
2) Optimize forming process parameters to reduce stress accumulation.
3) Standardize heat treatment processes to eliminate residual stress.
(2) Prevention and Control Measures for Eccentricity Defects:
1) Improve mold precision to ensure accurate forming benchmarks.
2) Strengthen equipment maintenance and precision calibration to ensure processing stability.
3) Standardize operating procedures and control the initial state of the billet.
(3) Prevention and Control Measures for Crease Defects:
1) Optimize forming process parameters to ensure uniform metal flow.
2) Improve lubrication to reduce metal flow resistance.
3) Strictly control the surface quality of the billet and the state of the mold to eliminate the causes of creases. 
(4) General Prevention and Control Measures
1) Establish a comprehensive quality inspection system to achieve full-process control.
2) Strengthen operator training and improve professional competence.

The three common defects of seamless steel pipes—cracking, eccentricity, and creases—are closely related to the quality of raw materials, forming process, heat treatment process, equipment precision, and operating procedures. Cracking defects mainly stem from excessive impurities in raw materials, excessive forming deformation, and stress accumulation caused by improper heat treatment processes. Eccentricity defects are primarily caused by insufficient precision of molds and equipment, and uneven deformation due to the poor initial state of the billet. Crease defects are related to metal accumulation caused by unreasonable forming parameters, poor lubrication, and inadequate surface quality control. By taking targeted prevention and control measures, the incidence of these three defects can be effectively reduced: strictly control the quality of raw materials to eliminate defect causes at the source; optimize forming and heat treatment process parameters to reduce stress accumulation and uneven metal deformation; strengthen equipment maintenance and precision calibration to ensure processing stability; standardize operating procedures and improve the professional competence of operators; and establish a full-process quality inspection system to achieve early detection and early handling of defects.