Why do seamless steel pipes bend inwards?

Date:2024-01-31Tags:seamless steel pipe
Why do seamless steel pipes bend inwards?

Inward folding is a common quality problem in the production of seamless steel pipes. It is an irregular distribution of flaky folds and spiral folds on the inner surface of the steel pipe, and the edges of the defects are mostly jagged. There are two main reasons for internal folding defects: the quality of continuous cast tube blanks (internal factors) and the perforation process system (external factors).


seamless steel pipe


In addition, improper operation can also cause the steel pipe to fold inward. The main forms of inward folding during operation include mesh, point, sheet, spiral, etc. Regular spiral defects are generally defects caused by severe wear of the plug or mandrel. In addition to regular distribution, such defects also have neater edges than steel defects. Internal folding defects such as irregular mesh, dots, and sheets are generally caused by the following reasons.

1) The amount of depression in front of the top is too large, causing the tube blank to form a cavity prematurely when perforated, causing it to fold inward.
2) The tube blank is heated unevenly and the heating temperature is too high, which can easily cause inward folding.
3) The overflow part after perforation is not cut off. When pre-piercing the mandrel, this part is inserted into the capillary tube, causing it to fold inwards.

The specific reasons for inward folding during piercing are as follows:

1) The influence of perforation deformation parameters on internal folds
The setting of perforation process parameters and the design of tools have a certain impact on folding. The tearing in the central area of the tube blank is not only related to the total reduction amount in the roller and the total reduction amount before the top, but also related to the local area when the tube blank rotates half a turn. Depression is related.

2) Effect of perforation temperature on inward folding
If the temperature is too high, the heating time is too long, and the piercing temperature rises too fast, a large number of yellow-brown fish scale-like infolding defects will occur; if the temperature is too low, the heating time is too short, the plasticity of the metal will be reduced, making it difficult to bite. During perforation, the center of the continuous cast tube blank is easy to crack and form folding defects. If the temperature is too low, the capillary tube will produce small flaky inward folding defects. Most higher grade steel grades are prone to small flaky infolding defects, which are related to the relative difficulty of biting.

3) The influence of centering holes on inward folding
Centering hole is too small, too offset, or not smooth. When the piercing comes into contact with the plug, local friction occurs and a half-turn or ring of capillaries folds over the head. If the pressure of the centering fixture is too high and the head of the continuous cast tube blank cannot be flattened, it will also be difficult to penetrate and perforate in one bite, causing the head to fold inward in a straight line.

To improve the inner surface quality of steel pipes and reduce the occurrence of inner folding defects, the following methods should be used

1. Ensure the uniformity of the structure during casting, reduce segregation, and prevent excessive concentration of non-metallic inclusions;
2. Use the optimal casting temperature to increase the critical reduction;
3. Before piercing, ensure the heating temperature of the undesirable material and reduce the amount of compression before piercing.

Why do seamless pipes undergo hydrogen embrittlement fracture?

Hydrogen embrittlement usually manifests as a significant decrease in the plasticity of the steel, a sharp increase in brittleness, and a tendency to crack after a static load (often lower than the σb of the material) is applied for a period of time.

Generally speaking, there are three main factors that affect hydrogen embrittlement fracture of seamless pipes:

1) Environmental factors
When steel is in an environment with high hydrogen content such as water, acid, hydrogen, etc., hydrogen diffuses through adsorption on the surface of the steel, causing the steel to become brittle. At the same time, hydrogen partial pressure has a significant impact on the hydrogen crack growth rate, and increasing hydrogen partial pressure will increase hydrogen embrittlement susceptibility.

2) Strength coefficient
Generally speaking, the stronger the steel, the greater its susceptibility to hydrogen embrittlement. Some countries explicitly stipulate that "pickling of high-strength steel is not allowed" to prevent hydrogen embrittlement. Chemical composition affects hydrogen embrittlement fracture of steel through strength. This is because the segregation of hydrogen, S, P and other atoms at the grain boundaries will weaken the binding force of the grain boundaries, thus promoting the first fracture along the grain boundaries.

3) Heat treatment
Hydrogen embrittlement of steel is closely related to its microstructure and heat treatment. Experiments and facts show that the worse the thermodynamic stability of the structure, the greater the susceptibility to hydrogen embrittlement. For example, the hydrogen embrittlement tendency of pearlite and ferrite is much lower than that of martensite, and high-carbon martensite distributed in a network is the most sensitive.

Measures to prevent hydrogen embrittlement

Studies have shown that direct electrochemical measurements of hydrogen permeation under pickling conditions are a feasible method to study hydrogen permeation behavior during pickling. In order to reduce the degree of hydrogen penetration of steel parts, the following measures can be taken to prevent hydrogen penetration.

1) Introducing multi-functional slow suppression. The multi-functional corrosion inhibitor has the functions of corrosion inhibition and mist retardation. It not only has fast pickling speed, but also has strong function of inhibiting hydrogen penetration and high corrosion inhibition rate.

2) Control pickling conditions. The amount of hydrogen permeated by steel in the pickling solution has little to do with the acidity, but is proportional to the square root of the pickling temperature and pickling time. It is recommended to use a method with relatively high acid concentration and short pickling time. This issue should be paid more attention to when pickling quenched parts of high-strength steel such as high-speed steel. Specific production units should develop strict process flows and control the three elements of acid concentration, acid temperature, and pickling time.

3) Pay attention to stress corrosion issues. Stress corrosion cracking is a process in which the workpiece is subjected to the combined action of static tensile stress and a specific corrosive environment, resulting in brittle cracking of the material. Straightened quenched parts, whether frontal or counterattack, all straightened workpieces must be stress relieved and then pickled to avoid the possibility of hydrogen embrittlement cracking or embrittlement.

4) Prevent metal impurities from contaminating the pickling solution. Research has found that when the pickling solution contains metal impurities such as P, As, Sn, Hg, Pb, Zn, Cd, etc., it will increase hydrogen penetration and intensify the tendency of hydrogen embrittlement fracture.

5) Hydrogen treatment. After the pickling process, it is best to carry out hydrogen displacement treatment at 180~200℃×3~4h


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