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From the construction process, we found that the quality of stainless steel fittings arrived mainly in the following aspects:

First, the wall thickness of the pipe is not equal.

Pipe wall thickness inequaquasis mainly occurs in the most deformed parts of the pipe. Such as the back of the elbow wall thickness is thinner than other parts; As stipulated in the relevant national standards. The maximum amount of wall thickness reduction of pipe parts shall not exceed 1 2 and 5 of its nominal thickness, but in the field, it is found that some wall thickness reduction amounted to 2o to 30. For the examination of such problems, it is often difficult to find with general calipers and other measuring tools, at which time only the use of ultrasonic thickness gauge can be measured.

Second, the hardness exceeds the standard.

The problem of hardness exceeding the standard is mainly due to the problem of heat treatment process after forming. This issue can generally be resolved.

Third, materials and processing processes produced defects.

Such problems are most harmful to the safety of the device. It's more cumbersome to check. The factors that produce defects are more and more complex, which include defects in the material itself and problems in the processing and manufacturing process, as well as defects caused by incorrect heat treatment processes. So why do they cause security hazards and how can they avoid them? Now take the three-way as an example to do one by one analysis.

1, the problem of raw materials.

We know that for tubes that withstand high temperature and high pressure and flammable and explosive media. The quality of raw materials, directly affect the quality of pipes used in the manufacture of such pipes, for carbon steel pipe, most of the use of GB531 0-85 "high-pressure boiler seamless steel pipe", GB6479-86 "fertilizer equipment with high-pressure seamless steel pipe" GB9448-88 "seamless steel pipe for oil cracking" and GB81 63-87 (seamless steel pipe for transport fluid) and other standard materials. For this type of pipe, the steel mill should carry out a water pressure test on a root-by-root basis. For large and medium-sized steel, often with vortex or ultrasonic ferry injury (uT) instead of water pressure testing, such as strictly in accordance with national and relevant standards to test, pipe quality can still be guaranteed. But sometimes it is inevitable that there will be leakage inspection, mis-checking, and even more because of the pipe purchase channels mixed, some untested raw materials into the circulation field, therefore, for the arriving pipe, especially non-bulk from the steel pipe factory directly purchased materials. In addition to the standard should be a comprehensive re-examination, should also increase the rate of re-inspection, or even root-by-root inspection but. From our contact with a number of large-scale pipe manufacturing extensive investigation, many manufacturing plants on the purchase of pipe re-inspection control is not strict, blind letter from the material factory quality certificate (it is worth noting that some factory quality certificate on the authenticity of the problem), and only the appearance of double pipe inspection with these unqualified pipe pipe pipe, its quality is difficult to be guaranteed. In addition, many manufacturing plant inspection means are neither advanced nor perfect, so some unqualified pipes are easy to be qualified products factory.

2, processing process problems.

At present, the processing and forming of domestic manufacturers generally use the following methods:

(1) Cold stamping forming;

(2) Hot stamping forming;

(3) Oil pressure forming.

Common pipe quality problems and solutions in construction.

For the gold phase tissue of the inner and outer wall surfaces of the ternations with larger diameters and thicker walls, the latter two forming methods are used, and the forming time of these two methods generally takes 1 to 2 minutes, and some can even reach 3 minutes. In these minutes, how to maintain the relative stability of the temperature is a key issue for the quality of the pipe, such as the temperature difference is too large, will cause the uneven flow of steel fiber tissue, especially for the larger deformation. Because of its large plastic deformation, such as low temperature will cause cold hardening phenomenon and produce brittle damage.

3, after forming the heat treatment.

The final heat treatment after the three - way forming can significantly improve the mechanical properties of steel. Especially for teddy manufacturing plants with less advanced processing methods, it is particularly important to choose the right heat treatment system and process. In gb12459-90" steel welding seamless pipe parts, it is stipulated that r cold-formed carbon steel pipe parts should be treated with positive fire or heat retreat. So which heat treatment to choose, especially which fire-fighting system is good, there is no choice or explanation in the standard, which is determined according to the manufacturing process. The author believes that no matter which heat treatment system is chosen, its purpose is to improve the mechanical properties of the pipe and increase its strength. During the cold extrusion forming process, the tissue of steel will change greatly due to its large plastic deformation, such as obvious ribbon tissue, large grain, crystal disalteration, interface slip and so on. These organizational changes have a significant impact on the cutting and performance of steel. To overcome these adverse changes, the structure of these changes should be completely changed through heat treatment, so that its grain is refined and evenly organized. To achieve this, it is necessary to use a fully de-ignition process in positive or afire. Many pipe manufacturers are not fully aware of this. For some reason, they only performed stress-reducing heat treatment on the formed pipe, which did not fundamentally change the gold phase organization and performance of the pipe, making it difficult to guarantee the quality of the pipe.

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