How to solve the crack of seamless pipe?

Posted by zora li on November 15th, 2022

Reasons for cracks in seamless pipe:

1. The thickness direction of the crack across the plate. Central equiaxed cracks are small bending cracks along grain boundaries. The inner arc side crack is usually a straight crack within the crack, while the outer arc side crack is usually along the crystal crack.
2. The cracks along the grain boundaries are stress cracks, which are cracks passing through the grains, and are the result of external forces.

What do you need to do to fix this crack?

1. Improve the accuracy of the casting arc, avoid excessive stress at the solid-liquid interface in the initial stage of solidification, and prevent cracks along the grain boundary.
2. Forced cooling is achieved by properly increasing the pulling speed and cooling water, increasing the water flow and reducing the water temperature within a certain range.
3. Strictly control the composition of steel grades, especially the carbon content.
4. Increase continuous casting electromagnetic stirring. The superheat degree of medium clad steel is controlled below 40℃.

Main inspection items and methods for seamless steel pipes

The inspection of seamless steel pipes usually includes: inspection of geometric dimensions and surface quality; inspection of chemical composition, mechanical properties, process properties, high and low microstructures; non-destructive testing; some special performance inspections and length measurements.

(1) Geometric inspection of seamless carbon steel pipe: Through the outer diameter calipers, micrometers, curved rulers and fixed-length rulers, the outer diameter, wall thickness, curvature and length of seamless steel pipes can be inspected on the inspection table.
For outer diameter, wall thickness and length, automatic size testing equipment (such as automatic diameter measuring, thickness measuring, and length measuring equipment) can also be used for continuous testing. The seamless steel pipe factories put into operation in the late 1980s generally have online automatic diameter measuring and thickness measuring devices, and length measuring and weighing devices are installed in the finishing area. For OCTG seamless steel pipe, thread parameters need to be checked.

(2) Inspection of the inner and outer surfaces of seamless steel pipes: Visual inspection is usually used for the inner and outer surfaces. In addition to visual inspection, the inner surface can also be inspected using a reflective prism. Some special-purpose seamless steel pipes also need to check the quality of the inner and outer surfaces of seamless steel pipes through non-destructive testing, including eddy current, magnetic flux leakage, ultrasonic and magnetic particle inspection.

(3) Mechanical properties and process performance testing: In order to verify whether the mechanical properties of seamless steel pipes meet the standard requirements, it is necessary to test the mechanical properties of seamless steel pipes.
The mechanical properties test mainly includes tensile strength, yield strength, elongation, impact energy, etc. Process performance inspection mainly includes flattening test, flaring test, hydraulic test, crimping test, cold bending test, perforation test, etc. These inspection items should be selected according to standard differences and use of seamless steel pipes.

(4) Non-destructive testing refers to a method of directly inspecting internal and surface defects without damaging seamless steel pipes. At present, seamless steel pipe enterprises have widely used magnetic flux leakage testing, ultrasonic testing, eddy current testing and fluorescent magnetic particle testing. In recent years, non-destructive testing methods have made great progress. Recently, new technologies such as acoustic emission holography, ultrasonic spectral analysis and flaw detection, ultrasonic imaging flaw detection, and ultrasonic high temperature flaw detection have been developed.

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