The quality problems caused by heat treatment of pipes are mainly reflected in several aspects (performance, organization, defects, size, surface, etc.).
1. Performance and organization problems Performance and organization problems
Depends on the material composition and original state, the rationality of the 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 performance of the pipe (such as yield, tensile strength, and elongation) will exceed the requirements specified by the standard; the organization will have Widmanstatten and banded organizations that are not allowed by the standard.
2 Size problems
Mainly manifested in the bending in the length direction (axial) and the ovality and outer diameter tolerance in the cross-sectional direction (radial).
2.1 Reasons for bending of steel pipes
2.1.1 Uneven heating of steel pipes causes bending Uneven heating of steel pipes causes different temperatures along the axial direction of the pipes, different organizational transformation times during quenching, and different changes in the volume of the steel pipes, resulting in bending.
2.1.2 Bending caused by quenching of steel pipes: Quenching is the preferred heat treatment method for the production of high-strength casing and high-grade steel line pipes. During quenching, the microstructure changes very quickly, and the microstructure transformation of the steel pipe brings about a volume change. The microstructure transformation speed is inconsistent due to the inconsistent cooling speed of each part of the steel pipe, which will also cause bending.
2.1.3 Bending caused by tube billet: If there is segregation in the chemical composition of the steel pipe, even if the cooling conditions are completely consistent, bending will occur during cooling.
2.1.4 Uneven cooling causes bending: After heat treatment, alloy steel pipes are usually rotated and cooled naturally. At this time, the axial and circumferential cooling rates of the steel pipe are uneven, which will cause bending. If the bending degree of the steel pipe does not meet the requirements, it will affect subsequent processing (such as transportation, straightening, etc.) and even affect its performance.
2.1.5 Bending on the sizing machine: Alloy steel pipes, especially steel pipes with a narrow outer diameter tolerance range (such as pipeline pipes and casing), are generally required to be sized after tempering. If the center line of the sizing machine frame is inconsistent, the steel pipe will bend.
2.2 The main reasons for the ovality and outer diameter of steel pipes: During quenching, the steel pipe will produce ovality due to the inconsistent microstructure transformation of each part of the steel pipe; due to the existence of ovality, most steel pipes need to be sized after quenching and tempering. If the sizing control process is not good, the outer diameter will be out of tolerance.
3 Defects and surface problems
3.1 Quenching cracks of steel pipes: The quenching heating temperature of the steel pipe is too high, the heating time is too long, or the heating temperature is seriously uneven, which is easy to cause quenching cracks. If there is segregation in the chemical composition of the steel pipe, the steel pipe is also easy to cause quenching cracks.
3.2 Overheating or overburning of steel pipes: The quenching heating temperature of the steel pipe is too high or the heating time is too long, which is easy to cause overheating or overburning. In severe cases, the steel pipe will collapse.
3.3 Surface decarburization or severe oxidation of steel pipes: 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 imbalance of the air-fuel ratio.
Post time: Jun-14-2024