Seamless steel pipe with its excellent performance in the construction, chemical and other industries have been widely used, but in the process of application will also appear cracking problems, will not only cause material waste, increase costs, but also lead to a decline in production efficiency and delay the construction period. Seamless steel pipe factory in the production of seamless steel pipe will also have cracks in the steel pipe, the crack is not terrible, we should investigate the cause and find a solution is feasible. Here, Xiaobian will analyze some reasons in the production process, and some other reasons will also be analyzed.
Analysis of the causes of cracking of seamless steel pipes during the production process
Deformation occurs during cold rolling process
During the cold finishing process, a large amount of plastic deformation occurred inside the seamless steel pipe, and the lattice distortion was serious, especially in the areas where stress was concentrated on the inner wall, and cracks occurred along the grain boundaries. Under the action of external stress, these cracks eventually lead to the cracking of the steel pipe. In order to better understand this problem, we can analyze it from many aspects.
The hardness of the capillary tube is too high after annealing
First, through testing the hardness of seamless steel pipes, it was found that the hardness of the capillary tube after annealing was higher than that before annealing. This phenomenon shows that during the annealing process, the heating and cooling conditions of the capillary tube failed to reach the ideal state, resulting in an increase in the hardness of the capillary tube and further preventing the stress from being completely released. The increase in hardness means that the residual stress inside the material increases, which creates hidden dangers for subsequent cracking.
Decarburization after heat treatment
Secondly, through metallographic structure inspection, it was found that the capillary tubes had obvious decarburization after heat treatment, but there was basically no decarburization before heat treatment. Decarburization is the loss of carbon elements from the surface of steel during high-temperature heating, which will lead to a decrease in surface hardness and strength of the material, making seamless steel pipes more likely to crack when subjected to internal and external stress. The occurrence of decarburization further proves that there are problems with the heat treatment process, that is, the heating temperature is too high and the cooling rate is too fast.
The influence of Wei's organization
In addition, the capillary tube has severe Widmanstatten structure, which also proves that the temperature is too high during the tube production process. Widmanstatten tissue is a coarse, flaky or needle-like tissue that usually develops when overheated at high temperatures or improperly cooled. Its presence will reduce the toughness of the material and increase the brittleness, thereby increasing the risk of cracking of the steel pipe during cold finishing rolling.
Through this investigation, the following conclusions were drawn:
The quality problem of seamless steel pipe cracking is not caused by raw material quality problems. The chemical composition and basic properties of the raw materials meet the standard requirements. The standard for seamless steel pipes is ASTM A252. The problem occurs in the subsequent processing steps.
The root cause of hole expansion and cracking in cold-rolled pipes is cracks in the inner wall of cold-finished rolled pipes. These cracks mainly originate from improper operations during the heat treatment process, including heating temperatures that are too high and cooling rates that are too fast.
The occurrence of inner wall cracks is closely related to the heating temperature being higher than the austenitizing temperature and the cooling rate being too fast. Therefore, in the future, it is necessary to strengthen the temperature control of the heating furnace to ensure the standardized operation of the heating and cooling process and avoid internal stress and structural defects in the material caused by overheating and rapid cooling.
The following measures should be taken to prevent seamless steel pipes from cracking:
Strengthen the monitoring and adjustment of process parameters, especially in the heat treatment and cold finishing processes, strictly control the heating temperature and cooling rate to ensure that the hardness and stress release of the capillary tube during the annealing process reach the ideal state.
Strengthen the training and quality management of the production process, improve the technical level and quality awareness of the operators, ensure that the operation of each link is carried out in strict accordance with the process specifications, and avoid process deviations caused by human factors.
In addition to the above, there are some common reasons for seamless steel pipes to crack:
1. Seamless steel pipe raw material factors
The quality of the billet has a direct impact on the final quality of the seamless steel pipe. Poor quality steel billets will have defects such as inner and outer skins, scars, and cracks. For example, if a steel ingot contains bubbles during the casting process, these bubbles will cause cracks after forming. In addition, undesirable elements in the chemical composition will also affect the quality of steel pipes. If the steel billet contains too many five harmful elements (such as phosphorus, sulfur, oxygen, nitrogen, hydrogen, etc.), the brittleness of the steel will increase and cracks will easily occur.
2. Heating temperature factors
The first important process in seamless steel pipe production is heating. If the heating process is uneven or the temperature is insufficient, it will cause problems such as warped skin. Heating temperature that is too high or too low will affect the quality of the steel pipe. Excessive heating temperature will lead to overheating or overburning, which will cause cracks; while insufficient heating temperature will cause the steel to crack due to insufficient toughness during subsequent processing.
3. Equipment debugging factors
From the moment the steel billet is released, every equipment component will affect the final quality of the seamless steel pipe. For example, the scraping of equipment parts will cause outward warping and dents; failure to enter the punching machine will cause the head of the tube to be exposed to water and make it difficult to bite in, or the head will warp inside. Improper debugging of equipment or even equipment failure will adversely affect the quality of steel pipes.
4. Seamless steel pipe production process issues
The production process of seamless steel pipes is very complex and involves many aspects such as adjustments, equipment failures, and damage to process components (such as heads, guide plates, rollers, etc.). Problems with any of these factors may lead to quality defects in seamless steel pipes. Improper temperature control during the cold rolling process and low-temperature rolling can cause cracks; hot brittleness can also lead to the formation of cracks.
Solution
Optimize material composition: Strictly control the chemical composition of steel billets, reduce the content of five harmful elements, and improve the toughness and crack resistance of steel.
Improve the heating process: Strictly control the heating temperature and time to ensure a uniform heating process and avoid overheating and insufficient temperature problems.
Strengthen equipment maintenance: Regularly inspect and maintain production equipment to ensure the integrity and accuracy of equipment components and reduce mechanical damage to steel pipes.
Optimize process parameters: reasonably set the rolling temperature and deformation amount, control the stress distribution during the rolling process, and reduce the occurrence of cracks.
In conclusion
By strengthening the control and management of process parameters, companies can effectively avoid the problem of seamless steel pipe expansion and cracking in future production, and improve the market competitiveness and user satisfaction of products. This research result not only has guiding significance for steel pipe production enterprises with current problems, but also provides valuable experience and reference for the technological progress and process optimization of the entire industry.