The quality problems caused by heat treatment of steel pipes are mainly reflected in several aspects (performance, organization, defects, size, surface, etc.).
1 Performance and organizational issues
Performance and organization issues, depending on the composition and original state of the material, as well as the rationality of the heat treatment process and the level of process control, are the core of heat treatment quality control.
If the heat treatment process is not well developed or implemented, the properties of the pipe (such as yielding, tensile strength, elongation) will exceed the requirements stipulated by the standard; for the organization, there will be tissues that are not allowed by standards such as weiss body and band.
2 Size problem
It mainly shows the curvature in the longitudinal direction (axial direction) and the ellipticity and outer diameter of the cross-sectional direction (radial).
2.1 Causes of bending of steel pipes
2.1.1 Uneven heating of steel pipes produces bending
The heating of the steel pipe is uneven, the temperature is different along the axial direction of the pipe, the microstructure transformation time is different during quenching, and the volume change time of the steel pipe is different, resulting in bending.
2.1.2 Steel pipe quenching produces bending
Quenching is the preferred heat treatment method for high strength casing and high steel grade line pipe production. The microstructure changes very rapidly during quenching, and the structural transformation of the steel pipe causes a change in volume. Since the cooling rate of each part of the steel pipe is inconsistent, the speed of the tissue transformation is inconsistent, and bending also occurs.
2.1.3 Pipe blank causes bending
If the chemical composition of the steel pipe is segregated, even if the cooling conditions are completely the same, bending will occur during cooling.
2.1.4 Uneven cooling causes bending
After the heat treatment of the alloy steel pipe, the steel pipe is usually cooled naturally while rotating. At this time, the axial and circumferential cooling rates of the steel pipe are not uniform, and bending occurs. If the bending of the steel pipe does not meet the requirements, it will affect the subsequent processing (such as transportation, straightening, etc.) and even affect its performance.
2.1.5 Bending on the sizing machine
Alloy steel pipes, especially those with narrow outer diameter tolerances (such as line pipes and casings), generally require sizing after tempering. If the centerline of the sizing rack is inconsistent, the steel pipe will bend.
2.2 The main reason for the ellipticity and outer diameter of steel pipes
When quenching, the structural transformation of the steel pipe is inconsistent, the steel pipe will produce ellipticity; due to the ellipticity, most steel pipes need to be sized after quenching and tempering. If the sizing control process is not good, the outer diameter will appear super difference.
3 Defects and surface problems
3.1 Quenching crack of steel pipe
The quenching heating temperature of the steel pipe is too high, or the heating time is too long, or the heating temperature is severely uneven, which may cause quenching cracks. If the chemical composition of the steel pipe is segregated, the steel pipe may be mixed and easily cause quenching cracks.
3.2 Overheating or overheating of steel pipes
If the quenching heating temperature of the steel pipe is too high or the heating time is too long, it may cause overheating or overheating. In severe cases, the steel pipe may collapse.
3.3 Surface decarburization or severe oxidation of steel pipes
When the steel pipe is heated, the surface is seriously decarburized or severely oxidized due to improper control of heating temperature and heating time or adjustment of air-fuel ratio.
Source: China Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)