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Classification Of Stainless Steel Pipe Applications

Dec. 31, 2019

Stainless steel pipe fittings, especially stainless steel thread tee, stainless steel thread street elbow, and large and small heads, are becoming more and more common in the construction of pipeline engineering, mainly due to their forming Good, strong pressure resistance, simple welding and other characteristics to ensure the pressure bearing capacity of the pipeline, so in the process pipeline engineering, especially in petrochemical and other high temperature and high pressure and flammable and explosive pipelines, the reasonable use of pipe fittings and the pipe The quality plays a decisive role directly.

Because of this, in the pipeline design and installation process, especially in the high temperature, high pressure, and flammable and explosive pipeline projects of the petrochemical industry, no matter from design selection or processing to construction, installation, and inspection, every step of The personnel of the installation unit must strictly control the quality of selection, materials, manufacturing, installation and inspection, otherwise it will cause inestimable losses during the construction and production of the device.

From the construction process, we found that the quality of the arrival of the fittings mainly has the following problems:

Stainless Steel Thread Tee

Stainless Steel Thread Tee

I. Uneven wall thickness of pipe fittings

The uneven wall thickness of pipe fittings mainly occurs in the parts where the pipe deformation is the largest. For example, the wall thickness of the back of the elbow is thinner than other parts; the wall thickness of the nozzle and the pipe body is not the same. Provisions in relevant national standards. The maximum thickness reduction of the pipe fittings must not exceed 1,2,5 of its nominal thickness, but field measurements have found that some wall thickness reductions have reached 2o -30. For the inspection of such problems, it is often difficult to find them with measuring tools such as calipers. At this time, it can only be detected by using an ultrasonic thickness gauge.

Second, the hardness exceeds the standard

The problem of excessive hardness is mainly due to the problem of the heat treatment process after forming. The solution is to use the correct heat treatment process for another heat treatment. This problem can usually be solved.

Third. Defects in materials and processing

Such problems pose the greatest safety hazard to the device. It's more troublesome to check. There are many factors that cause defects, and they are more complicated. It has both defects in the material itself, problems in the manufacturing process, and defects caused by incorrect heat treatment processes. So why do they cause safety hazards, and how to avoid such problems? Now take the three links as an example to analyze this one by one.

1. Problems with raw materials

We know that for pipe fittings that withstand high temperature and pressure and flammable and explosive media. The quality of the raw materials directly affects the quality of the fittings. The pipes used to make such fittings are mostly carbon steel pipes GB531 0-85 "Seamless Steel Pipes for High Pressure Boilers" and GB6479-86 "High-pressure Materials such as seam steel pipes GB9448—88 “Seamless Steel Pipes for Petroleum Cracking” and GB81 63-87 {Seamless Steel Pipes for Transporting Fluids ”and other standards. For this type of steel pipe, the steel mill shall carry out hydraulic pressure test one by one. For large and medium-sized steels, eddy current or ultrasonic testing (uT) is often used instead of hydrostatic testing. If the testing is performed in strict accordance with national and relevant standards, the quality of the pipe can still be guaranteed. However, sometimes it is inevitable that there will be missed inspections and false inspections. What's more, due to the mixed pipeline purchasing channels, some raw materials that have not been inspected have entered the circulation field. Therefore, after the arrival of the pipeline, especially the non-bulk direct purchase from the steel pipe factory s material. In addition to the comprehensive re-inspection according to the standard, the rate of re-inspection should be increased, or even inspected one by one. Judging from the extensive surveys of large-scale pipe manufacturing we have contacted, many manufacturers have not strictly controlled the re-inspection of purchased pipe materials, blindly trusting the quality certificate of the material from the factory (It is worth noting that some factories There is a problem with the authenticity of the quality certificate), and only the appearance inspection of double-pier pipes is difficult to ensure the quality of the pipes made of these unqualified pipes. In addition, the inspection methods of many manufacturers are neither advanced nor perfect, so some unqualified pipe fittings can be easily shipped as qualified products.

2. Processing technology issues

At present, the processing methods of stainless steel fittings manufacturer china generally adopt the following methods:

(1) Cold stamping;

(2) Hot stamping;

(3) Hydraulic forming

Common quality problems of pipe fittings in construction and their solutions

The metallographic structure on the surface of the inner and outer walls of the tee. For the tee with a larger diameter and a thicker wall, the latter two forming methods are mostly used. The forming time of these two methods generally requires 1 to 2 minutes. Up to 3 minutes. During these minutes, how to keep the temperature relatively stable is a key issue for the quality of the fittings. If the temperature difference is too large, it will cause the steel fiber tissue to flow unevenly, especially for the parts with large deformation. Due to its large plastic deformation, if the temperature is too low, it will cause cold hardening and brittle failure.

3. Heat treatment after forming

The final heat treatment after tee forming can significantly improve the mechanical properties of steel. Especially for tee manufacturers with less advanced processing methods, it is especially important to choose the correct heat treatment system and process. In GB12459-90 "steel butt-welded seamless pipe fittings, it is specified that cold-formed carbon steel pipe parts should be subjected to normalizing or annealing heat treatment. So which heat treatment is selected, especially which annealing system is better, there is no The choice or explanation depends on the manufacturing process. The author believes that no matter which heat treatment system is selected, the purpose is to improve the mechanical properties and increase the strength of the pipe. During the cold extrusion forming of the pipe, the microstructure of the steel will change greatly due to its large plastic deformation, such as: obvious band structure, coarse grains, dislocation of grain boundaries, interface slip, etc. These structural changes have a great impact on the cutting performance and serviceability of steel. In order to overcome these unfavorable changes, it is necessary to completely change the changed microstructure through heat treatment, so that the grains are refined and the structure is uniform. To achieve this, it is necessary to use a complete annealing process in normalizing or annealing. Many pipe fitting manufacturers do not fully realize this. For some consideration, they only performed annealing heat treatment for stress relief of the formed pipe fittings, and this annealing did not fundamentally change the metallographic structure and properties of the pipe fittings, and it was difficult to ensure the quality of the pipe fittings.

Fourth, the inspection of pipes before leaving the factory and after arrival

As the last process to ensure the quality of pipe fittings, inspection plays an important role, especially for pipe fittings that withstand high temperature and pressure and flammable and toxic media, the inspection process is particularly important. At present, the appearance, hardness, thickness, and magnetic particle inspection (MT) and ultrasonic inspection (uT) of the pipe fittings are often checked before leaving the factory. From the inspection of the incoming pipe fittings, the most common problems are magnetic particle inspection and ultrasonic inspection. test. For magnetic particle inspection, manufacturers often use the yoke method (carbon steel pipe parts). However, due to equipment limitations, this method is mostly limited to the outer surface of the pipe, and the problem most often occurs on the inner wall of the pipe. For the cracks that often appear in this part, especially on the inner wall of small diameter pipes, the magnetic rod method should be used. Or eddy current method for inspection Ultrasonic flaw detection is an inspection item with high requirements on equipment and technology, because:

(1) It requires the operator to have high technology and rich experience, and to judge whether the defect detected is a crack or other depression, so as not to miss the inspection or misjudgment.

(2) It has high requirements for testing equipment, especially for probes. For many deformed parts (such as the two shoulders of tees) and pipe fittings with a diameter of less than DNS0mm, domestic straight probes commonly used by inspectors The inspection is prone to omissions, and it is even impossible to test through on-site practice. For these difficulties, using a dual-crystal probe with a diameter of less than 10mm and using the crossing oblique method has better results.

Through the above analysis, we have mastered the causes and avoidance methods of some pipe damage. The purpose is to allow high-quality pipe fittings to be used in engineering construction to meet the needs of engineering construction.