Main quality defects of steel pipe heat treatment

The quality problems caused by the heat treatment of steel pipes are mainly reflected in several aspects (performance, structure, defects, size, surface, etc.).

Performance and organizational issues.

Performance and organizational issues depend on the material composition and original state as well as the rationality and process control level of the heat treatment process. This is the core content of heat treatment quality control.

If the heat treatment process is not formulated or implemented well, the performance of the pipe (such as yield, tensile strength, and elongation) will exceed the requirements stipulated in the standard; the organization will have Wigmansite, banding, and other structures not allowed by the standard.

Size issue

Mainly manifested in the bending in the length direction (axial direction) and the ovality and outer diameter deviation in the cross-sectional direction (radial direction).

First. Reasons for bending of steel pipes

1. Uneven heating of steel pipe causes bending

The steel pipe is heated unevenly, the temperature along the axial direction of the pipe is different, the structure transformation time is different during quenching, and the volume change time of the steel pipe is different, resulting in bending.

2. The steel pipe is bent due to quenching

Quenching is the preferred heat treatment method for the production of high-strength casing and high-grade line pipe. The structural transformation occurs very rapidly during quenching, and the structural transformation of the steel pipe brings about volume changes. Due to the inconsistent cooling rate of various parts of the steel pipe, the structural transformation rate is inconsistent, and bending will also occur.

3. The tube blank causes bending

If the chemical composition of the steel pipe is segregated, even if the cooling conditions are exactly the same, it will bend during cooling.

4. Uneven cooling causes bending

After heat treatment of alloy steel pipes, the steel pipes are usually cooled naturally while rotating. At this time, the axial and circumferential cooling rates of the steel pipe are uneven and bending will occur. If the curvature of the steel pipe cannot meet the requirements, it will affect subsequent processing (such as transportation, straightening, etc.) and even affect its performance.

5. Bending occurs on the sizing machine

Alloy steel pipes, especially steel pipes with narrow outer diameter tolerances (such as line pipes and casings) generally require sizing after tempering. If the center lines of the sizing racks are inconsistent, the steel pipe will bend.

Second. The main reasons for the excessive ovality and outer diameter of steel pipes

During quenching, because the structural transformation of each part of the steel pipe is inconsistent, the steel pipe will produce ovality; due to the ovality, most steel pipes need to be sized after quenching and tempering. If the sizing control process is not good, the outer diameter will appear to be excessive. Difference.

Third. Defects and surface problems

1. Quenching cracks in steel pipes

The quenching heating temperature of the steel pipe is too high, or the heating time is too long, or the heating temperature is seriously uneven, which may easily cause quenching cracks. If the chemical composition of the steel pipe segregates, inclusions in the steel pipe may easily cause quenching cracks.

2. Overheating or overburning of steel pipes

If the quenching heating temperature of the steel pipe is too high or the heating time is too long, it is easy to cause overheating or overburning. In severe cases, the steel pipe will collapse.

3. The surface of the steel pipe is decarburized or severely oxidized.

When the steel pipe is heated, the surface is severely decarburized or severely oxidized, which is caused by improper control of the heating temperature and heating time or an imbalance in the air-fuel ratio adjustment.