20# seamless steel pipe is the material grade specified in GB3087-2008 “Seamless steel pipes for low and medium pressure boilers”. It is a high-quality carbon structural steel seamless steel pipe suitable for manufacturing various low-pressure and medium-pressure boilers. It is a common and large-volume steel pipe material. When a boiler equipment manufacturer was manufacturing a low-temperature reheater header, it was found that there were serious transverse crack defects on the inner surface of dozens of pipe joints. The pipe joint material was 20 steel with a specification of Φ57mm×5mm. We inspected the cracked steel pipe and conducted a series of tests to reproduce the defect and find out the cause of the transverse crack.

1. Crack feature analysis
Crack morphology: It can be seen that there are many transverse cracks distributed along the longitudinal direction of the steel pipe. The cracks are arranged neatly. Each crack has a wavy feature, with a slight deflection in the longitudinal direction and no longitudinal scratches. There is a certain deflection angle between the crack and the surface of the steel pipe and a certain width. There are oxides and decarburization at the edge of the crack. The bottom is blunt and there is no sign of expansion. The matrix structure is normal ferrite + pearlite, which is distributed in a band and has a grain size of 8. The cause of the crack is related to the friction between the inner wall of the steel pipe and the inner mold during the production of the steel pipe.

According to the macroscopic and microscopic morphological characteristics of the crack, it can be inferred that the crack was generated before the final heat treatment of the steel pipe. The steel pipe uses a Φ90mm round tube billet. The main forming processes it undergoes are hot perforation, hot rolling and diameter reduction, and two cold drawings. The specific process is that the Φ90mm round tube billet is rolled into a Φ93mm×5.8mm rough tube, and then hot rolled and reduced to Φ72mm×6.2mm. After pickling and lubrication, the first cold drawing is carried out. The specification after the cold drawing is Φ65mm×5.5mm. After intermediate annealing, pickling, and lubrication, the second cold drawing is carried out. The specification after the cold drawing is Φ57mm×5mm.

According to the production process analysis, the factors affecting the friction between the inner wall of the steel pipe and the inner die are mainly the quality of lubrication and are also related to the plasticity of the steel pipe. If the plasticity of the steel pipe is poor, the possibility of drawing cracks will increase greatly, and poor plasticity is related to the intermediate stress relief annealing heat treatment. Based on this, it is inferred that the cracks may be generated in the cold drawing process. In addition, because the cracks are not open to a large extent and there is no obvious sign of expansion, it means that the cracks have not experienced the influence of secondary drawing deformation after they are formed, so it is further inferred that the most likely time for the cracks to be generated should be the second cold drawing process. The most likely influencing factors are poor lubrication and/or poor stress relief annealing.

To determine the cause of the cracks, crack reproduction tests were carried out in cooperation with steel pipe manufacturers. Based on the above analysis, the following tests were carried out: Under the condition that the perforation and hot rolling diameter reduction processes remain unchanged, the lubrication and/or stress relief annealing heat treatment conditions are changed, and the drawn steel pipes are inspected to try to reproduce the same defects.

2. Test plan
Nine test plans are proposed by changing the lubrication process and annealing process parameters. Among them, the normal phosphating and lubrication time requirement is 40min, the normal intermediate stress relief annealing temperature requirement is 830℃, and the normal insulation time requirement is 20min. The test process uses a 30t cold drawing unit and a roller bottom heat treatment furnace.

3. Test results
Through the inspection of the steel pipes produced by the above 9 schemes, it was found that except for schemes 3, 4, 5, and 6, other schemes all had shaking or transverse cracks to varying degrees. Among them, scheme 1 had an annular step; schemes 2 and 8 had transverse cracks, and the crack morphology was very similar to that found in production; schemes 7 and 9 had shaken, but no transverse cracks were found.

4. Analysis and discussion
Through a series of tests, it was fully verified that lubrication and intermediate stress relief annealing during the cold drawing process of steel pipes have a vital impact on the quality of finished steel pipes. In particular, schemes 2 and 8 reproduced the same defects on the inner wall of the steel pipe found in the above production.

Scheme 1 is to perform the first cold drawing on the hot-rolled reduced-diameter mother tube without performing the phosphating and lubrication process. Due to the lack of lubrication, the load required during the cold drawing process has reached the maximum load of the cold drawing machine. The cold drawing process is very laborious. The shaking of the steel pipe and the friction with the mold cause obvious steps on the inner wall of the tube, indicating that when the plasticity of the mother tube is good, although the unlubricated drawing has an adverse effect, it is not easy to cause transverse cracks. In Scheme 2, the steel pipe with poor phosphating and lubrication is continuously cold drawn without intermediate stress relief annealing, resulting in similar transverse cracks. However, in Scheme 3, no defects were found in the continuous cold drawing of the steel pipe with good phosphating and lubrication without intermediate stress relief annealing, which preliminarily indicates that poor lubrication is the main cause of transverse cracks. Schemes 4 to 6 are to change the heat treatment process while ensuring good lubrication, and no drawing defects occurred as a result, indicating that intermediate stress relief annealing is not the dominant factor leading to the occurrence of transverse cracks. Schemes 7 to 9 change the heat treatment process while shortening the phosphating and lubrication time by half. As a result, the steel pipes of Schemes 7 and 9 have shake lines, and Scheme 8 produces similar transverse cracks.

The above comparative analysis shows that transverse cracks will occur in both cases of poor lubrication + no intermediate annealing and poor lubrication + low intermediate annealing temperature. In the cases of poor lubrication + good intermediate annealing, good lubrication + no intermediate annealing, and good lubrication + low intermediate annealing temperature, although shake line defects will occur, transverse cracks will not occur on the inner wall of the steel pipe. Poor lubrication is the main cause of transverse cracks, and poor intermediate stress relief annealing is the auxiliary cause.

Since the drawing stress of the steel pipe is proportional to the friction force, poor lubrication will lead to an increase in the drawing force and a decrease in the drawing rate. The speed is low when the steel pipe is first drawn. If the speed is lower than a certain value, that is, it reaches the bifurcation point, the mandrel will produce self-excited vibration, resulting in shake lines. In the case of insufficient lubrication, the axial friction between the surface (especially the inner surface) metal and the die during drawing is greatly increased, resulting in work hardening. If the subsequent stress relief annealing heat treatment temperature of the steel pipe is insufficient (such as about 630℃ set in the test) or no annealing, it is easy to cause surface cracks.

According to theoretical calculations (the lowest recrystallization temperature ≈ 0.4×1350℃), the recrystallization temperature of 20# steel is about 610℃. If the annealing temperature is close to the recrystallization temperature, the steel pipe fails to fully recrystallize, and the work hardening is not eliminated, resulting in poor material plasticity, metal flow is blocked during friction, and the inner and outer layers of metal are severely deformed unevenly, thereby generating a large axial additional stress. As a result, the axial stress of the inner surface metal of the steel pipe exceeds its limit, thereby generating cracks.

5. Conclusion
The generation of transverse cracks on the inner wall of a 20# seamless steel pipe is caused by the combined effect of poor lubrication during drawing and insufficient intermediate stress relief annealing heat treatment (or no annealing). Among them, poor lubrication is the main cause, and poor intermediate stress relief annealing (or no annealing) is the auxiliary cause. To avoid similar defects, manufacturers should require workshop operators to strictly follow the relevant technical regulations of the lubrication and heat treatment process in production. In addition, since the roller-bottom continuous annealing furnace is a continuous annealing furnace, although it is convenient and quick to load and unload, it is difficult to control the temperature and speed of materials of different specifications and sizes in the furnace. If it is not strictly implemented according to the regulations, it is easy to cause uneven annealing temperature or too short a time, resulting in insufficient recrystallization, leading to defects in subsequent production. Therefore, manufacturers who use roller-bottom continuous annealing furnaces for heat treatment should control the various requirements and actual operations of heat treatment.