Main quality defects of steel pipe heat treatment
Main quality defects of steel pipe heat treatment
The main quality defects of tube heat treatment are discussed, and the quenching and tempering heat treatment process is described as a typical process.
The quality problems of pipe heat treatment are mainly reflected in several aspects (performance, structure, defect, size, surface, etc.).
1 Performance and organization
The problems of properties and structure depend on the composition and original state of materials, the rationality of heat treatment process and the level of process control, which is the core content of heat treatment quality control.
If the heat treatment process is not well formulated or implemented, the properties of pipes (such as yield, tensile strength and elongation) will exceed the requirements of the standard; there will be structures that are not allowed by the standards such as widmanstatten and banded structure.
2 Dimensions
Mainly in the length direction (axial) bending and section direction (radial) ovality and outside diameter out of tolerance.
2.1 causes of steel pipe bending
2.1.1 bending of steel pipe due to uneven heating
When the steel tube is heated unevenly, the temperature along the axial direction of the tube is different, the transformation time of the structure is different during quenching, and the change time of the volume of the steel tube is different, resulting in bending.
2.1.2 bending of steel pipe due to quenching
Quenching is the first choice of heat treatment in the production of high strength casing and high steel pipeline. The transformation of microstructure is very rapid during quenching, and the volume of steel tube changes with the transformation of microstructure. Due to the different cooling rate of each part of the steel pipe, the speed of the transformation of the structure is not the same, and bending will also occur.
2.1.3 bending caused by tube blank
If there is segregation in the chemical composition of the steel pipe, even if the cooling conditions are completely the same, bending will occur during cooling.
2.1.4 bending due to uneven cooling
After heat treatment, alloy steel pipes usually need to be cooled naturally while turning. At this time, the axial and circumferential cooling speed of steel pipe is uneven, which will cause bending. If the bending degree of steel pipe can 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 sizing machine
Alloy steel pipes, especially steel pipes with narrow outer diameter tolerance (such as pipeline pipes and casings), generally require sizing after tempering. If the center line of the sizing frame is inconsistent, the steel pipe will bend.
2.2 main reasons for ovality and outer diameter out of tolerance of steel pipe
During quenching, the ovality of steel pipe will be produced due to the different structure transformation of each part of steel pipe; because of ovality, most steel pipes need to be calibrated after tempering treatment. If the sizing control process is not good, the outer diameter will be out of tolerance.
3 Defects and surfaces
3.1 quenching crack of steel pipe
The quenching temperature of steel pipe is too high, or the heating time is too long, or the heating temperature is seriously uneven, which is easy to cause quenching cracks. If the chemical composition of the steel pipe has segregation, the steel pipe with inclusions is easy to cause quenching cracks.
3.2 overheating or Overburning of steel pipe
If the quenching and 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 serious cases, the steel pipe will collapse.
3.3 decarburization or serious oxidation of steel pipe surface
When the steel pipe is heated, its surface is decarburized or oxidized seriously, which is caused by improper control of heating temperature and heating time or misadjustment of air-fuel ratio.
