The study found that the selection and quality control of the valve body have an important influence on the performance of the later stage. Therefore, in order to deal with the good use of the fully welded pipeline ball valves in harsh environments, this article mainly studies and discusses the standards and detection methods of the material selection and quality control, so that it can provide references to improve the quality of the fully welded pipeline ball valve.
As a pressure-bearing part, the pipeline ball valve not only has to bear the external pressure during work, but also has to bear the pressure of internal loads, such as the load caused by uneven foundation settlement, mountain collapse, mudslide and earthquake, as well as material tensile and compressive loads due to changes in temperature caused by changes in day and night and seasons, so when the pipeline ball valve is applied in a cold area, the low-temperature toughness and anti-load impact capability of the material must be studied and discussed to prevent the phenomenon of material cracking caused by low temperature. The study found that the ball valve will inevitably lead to the risk of leakage during working under the harsh environment. Therefore, the valve body material is well selected and its quality is effectively controlled to meet the normal use of the fully welded pipeline ball valves in harsh environments.
1. Valve Body Material Selection and Quality Control
The fully welded valve body of the pipeline ball valve is generally divided into two structures: cylindrical and spherical. The cylindrical fully welded ball valve, the main valve body and the auxiliary valve body are mainly connected by welding, thereby forming a cylindrical valve body.
The spherical welded valve body structure, the valve body is formed by the left and right piled assembly connected by the main weld, the welded valve body is mainly composed of two annular welds, and the valve body welding joints generally need Submerged arc welding which is designed to have a small gap. For example, the wall thickness of the commonly used NPS48 Class 900 fully welded pipeline ball valve in industry is 140 mm. This welded joint is a super-thickness welded joint. The multi-layer welding of thickness refers to multi-layer heating and cooling welding of metal substrates using welding consumables. These conditions can lead to non-uniform stress in the welded joints, which in turn causes non-uniform residual stresses in the welded joints and subsequent welding defects. Since welding is the last step in the processing of spherical bodies, and the valve body contains sealing material inside, that is, after the valve body is welded, the residual stress inside the valve body cannot be eliminated by the heat treatment method. Therefore, the material of the valve body is well selected and controlled in order to ensure the use performance of the valve body after being welded.
1.1 Valve Body Material Selection
According to ASTM standards, spherical valve body materials are generally classified into the following categories: A105, A350 LF2, A515 70, and A516 70. Under normal condition, if the temperature is ≧29°C, A105 and A515 70 materials can be selected. When using the temperature ≦29 °C, the material is generally selected A350 LF2 and A516 70.
1.2 Quality Control Requirements
The fully welded valve body materials need to consider their welding performance while considering their own strength. The toughness of the welded joints needs to be selected while ensuring the performance requirements of the fully welded ball valves, that is, the valve body material meets ASME B16. 34 in addition to the relevant material performance standards, the impact toughness of the material needs to be selected so that the residual stress caused by the welding cannot be eliminated by post-processing because the internal sealing material cannot be heated. Therefore, in practical applications, it is often through the selection of valve body materials and the control of the element mass fraction, such as the control of trace elements P and S. Moreover, in terms of mechanical properties, when selecting, it is also necessary to select and test the yield strength and low-temperature performance of the material so as to meet the working conditions.
1.3 Forging and Heat Treatment Requirements
The fully welded pipeline ball valve materials need calm steel smelted by open hearth furnace, electric furnace and pure oxygen top-blowing converter. After the initial refining, the steel is degassed and then refined. After refining, the steel can be fully deoxidized，then to achieve the purpose of grain size refinement. In order to eliminate the large amount of bubbles inside the steel, the end must be removed after the smelting, and the deformation ratio of the forging is not more than 3. During the forging process, the temperature change of the forgings should be controlled to promote the slow cooling of the workpieces, and the final cooled workpieces and the workpieces before the heat treatment should be at a temperature below the phase transition temperature to avoid the phase transformation during the post-processing of the forgings. In the late stress relief annealing process, the normalizing temperature is generally set at about 900°C, and the tempering temperature is generally controlled at 660°C and 680°C.
In summary, it is not possible for the valve body of the fully welded pipeline ball valve to eliminate the stress involved in the welding due to heat treatment at a later stage. Therefore, it is necessary to select good valve body preparation materials in the early stages. That is, the trace element content, the yield strength and the tensile strength of the material are controlled to improve the quality of the fully welded pipeline ball valve.