Thread inspection procedures in the production process of seamless steel pipes (according to API Spec 5B)

First, preparation before inspection:
1. Check whether all inspection tools are within the valid identification period.
2. Check all inspection tools to ensure the accuracy and authenticity of the inspection results.

Second, basic inspection rules.
1. The thread length should be measured parallel to the thread axis.
2. The thread tooth height and the cone diameter are measured roughly perpendicular to the thread axis.
3. The pitch of the thread should be measured along the center cone parallel to the thread axis.
4. For the internal thread of the eccentric type, the pitch of the external thread should be measured roughly along the center cone parallel to the thread axis.
5. The taper of the round thread should be measured along the diameter of the center cone;
6. The taper of the external thread of the eccentric type should be measured along the small diameter cone. The taper of the internal thread of the eccentric type should be measured along the large diameter cone on its diameter.

Third. Appearance inspection:
1. Within the minimum length of the complete thread (Lc) from the pipe end and the interval from the end face of the bore to the plane J+1 teeth from the center of the coupling, the thread should be free of obvious tearing, undercutting, wear marks, shoulders or any other defects that damage the continuity of the thread.
2. Occasional surface scratches, slight pits, and surface irregularities are not considered harmful if they do not affect the continuity of the thread surface; since it is difficult to determine the extent of surface scratches, slight pits, and surface irregularities and their impact on the continuous performance of the thread, such defects cannot be used as a basis for rejecting the pipe: As an acceptance criterion, the most critical consideration is to ensure that no thread can damage the coupling.
3. Manual finishing of the thread surface is allowed. Imperfections are allowed between the Lc length and the vanishing point of the thread, as long as their depth does not extend below the cone of the thread root, or is not greater than 12.5% of the specified wall thickness (measured from the pipe surface where the defect extends), and the depth is allowed to be the larger of the two. In this area, grinding and resting are allowed to eliminate defects. The limit of grinding depth is the same as the depth of defects in this area.
4. Imperfections also include other discontinuities, such as folds, pits, knife marks, indentations, and handling damage. Micro pits and stains may also be encountered, but they are not necessarily harmful. Since micro pits and stains and their impact on the continuous performance of threads are difficult to determine, such defects cannot be used as the basis for rejecting pipes: As an acceptance criterion, the most critical consideration is to remove any corrosion products on the thread surface without leaving leakage channels.
5. Grinding and filing methods are not allowed to eliminate pits.
6. The external chamfer (60°) of the pipe end must be guaranteed to be complete on the 360° circumference of the pipe end. The chamfer diameter should be selected so that the thread root groove disappears on the chamfer surface rather than on the pipe end face, and no knife-like edges should appear.
7. The root of the coupling thread should start with the inner diameter chamfer surface and extend to the center of the coupling.
8. Blacktop thread: For round threads, it is allowed to exist between the Lc length and the thread vanishing point, but not within Lc. However, if the black top thread within Lc is in the shape of a dot or a line, and the buckle type is complete, it can be considered that it does not affect the continuity of the thread surface. For trapezoidal threads, 2 black top threads are allowed within the Lc length range, but the total length of the black top thread does not exceed 1/4 of the circumference of the pipe thread to be considered qualified.
9. Broken thread: It is not allowed to appear within the Lc length range of trapezoidal threads. It is allowed between the Lc length and the thread vanishing point as long as the defect depth does not extend to the bottom cone of the thread or does not exceed 12.5% of the specified wall thickness.
10. Burr: Minor burrs can be qualified after grinding, and burrs on the inner and outer edges of the pipe end are unqualified.
11. Chatter: Check with a fingernail or a sharp needle. If it feels slight, it is qualified. If there is an obvious sense of jumping, it should be judged as unqualified.
12. Cut: Minor cuts that do not produce leakage channels are acceptable.
13. Tears: Scratches on the tooth side are unqualified. Minor cuts that do not affect the thread engagement, cause the plating of the coupling to fall off, or do not destroy the continuity of the thread are acceptable.
14. Dent: Any slight dent within the Lc length range that does not destroy the continuity of the thread or produce a leakage channel is acceptable. Dents outside the Lc length range that do not extend below the bottom cone of the cutter thread or do not exceed 12.5% of the specified wall thickness are acceptable.
15. Fin: Minor fins are allowed to be ground. If they are in the shape of a knife edge or a wing-shaped severe fin, they are unqualified.
16. Tearing: Fish-scale scratches on the thread tooth surface are all considered unqualified.
17. Pitted Threads: Point-shaped non-floating rust corrosion within the Lc length range or peeling of the coating on the coupling, rust is unqualified.
18. Improper Thred Form: Abnormal tooth shape is considered unqualified.
19. Dentures, random buckles, and overburned thread surfaces are all considered unqualified.