Analysis of Internal Small Equivalent Diameter Defects of Large Ring Forged Flange

In view of the small equivalent defects that are easily ignored in large ring forged flanges, this paper analyzes and compares them from the aspects of defect location, acceptance criteria, etc., and conducts sectioning tests and low magnification analysis, and discusses in detail the causes of small equivalent diameter defects. Taking the forged flange of wind power as an example, the method to avoid the occurrence of unqualified products is proposed from the aspect of metal material selection, and the opinions and suggestions on strengthening the control of small equivalent diameter defects are proposed from the aspect of safety. It has good guidance and reference significance for users and manufacturers.

Large ring forging flanges are usually used on wind power towers as connecting elements between tower sections. According to the wind turbine equipment capacity, the common ring forging flange outside diameter of 2000-5000mm, single weight of 0.5-2.0t ring forging flange raw materials for billet or ingot, to be forged, ring rolling, machining and other key processes to achieve.
For the forging flange manufacturer, once there are excessive defects in the flange, the flange can only be scrapped, and the loss cost is at least tens of thousands of yuan. Therefore, it is particularly important to find the causes of internal defects and prevent the occurrence of unqualified parts.

1. Ultrasonic testing results

A large ring forgings for ultrasonic testing, found that the bulk ring forgings have similar defects: ultrasonic reflection wave indicates that the defect equivalent diameter of 1.2-2.0mm, point, intermittent and linear state, the longitudinal position is close to half of the flange height H, axial position is close to half of the flange width B, as shown in Figure 1.

Figure.1 Ring forging flange internal defect location schematic
Usually, if assessed in accordance with the relevant standards, such defects are easily ignored. Such as: JB/T4730.3-2005 “non-destructive testing of pressure equipment” requires the recording of equivalent diameter > 4mm of a single defect, even if the dense area defects should only be recorded ≥ 3mm equivalent diameter of defects, Even the most stringent level, the start of the record equivalent value of defects is only 1.6mm, that is, the equivalent diameter of about 1.5mm, intermittently distributed in a straight line point defects, according to these standards are acceptable.
What is the nature and state of such defects? There are such defects in the ring forging flange after the use of how much impact on the machine? The following tests will be conducted to explain. 2.

2. Cut-off test and low-power analysis

In order to check the exact type and size of defects found by ultrasonic testing, the ring forging flange is cut to test. First of all, use the ultrasonic flaw detector to accurately locate the defect and mark the line, cut a section for cutting process until processing to the marked position. Using a low magnification lens to observe the surface of the specimen after macro corrosion (see Figure 2).

Figure.2 Internal defects observed by low magnification
The results of low magnification analysis: the ingot internal looseness exists, and after forging and ring rolling, the looseness only becomes smaller and longer, and is not eliminated.

3. Internal small equivalent diameter defects analysis

In the solidification process, if the liquid metal cannot be shrunk in time, the looseness will appear in the center of the ingot. According to the processing process, the ring forgings should go through a series of processes such as billet feeding → forging → forging billet punching → ring rolling expansion → heat treatment → machining, which forging, ring rolling expansion process will be the height and thickness of the billet direction extrusion, while making the diameter direction increased significantly. Along with these processes, the loosening defects that exist inside the billet will also change accordingly: the absolute volume of the loosening becomes smaller and longer along the circumferential direction. Although the subsequent heat treatment process can further improve the grain size of the tissue, it will never completely eliminate the loosening. This results in the previously mentioned results when using ultrasound for inspection.
Small equivalent diameter defects will also exist in the form of cracks, small crack defects will arise in the various stages of ring forging flange processing, such as forging, heat treatment, sudden cold and sudden heat, the temperature difference between the inside and outside of the billet is very large, the forging internal cracks will appear; temperature lower than the final forging temperature, will appear in the forging, ring rolling process cracks. In addition, large forgings after forging cooling too fast, the dissolved hydrogen in the steel is too late to escape, resulting in excessive stress, there will be white spots in the center of the forging large cross-section.

4. Conclusion

Large ring forging flanges are widely used in large pressure vessels, wind power towers, these devices are subject to high pressure, or fatigue load in the low temperature environment, and loosening, cracking of these small equivalent diameter defects, will inevitably lay a hidden danger to the safe operation of the equipment, which may cause casualties or huge economic losses. In view of the above analysis, now the use of large ring forging flange, manufacturing the following recommendations.

• (1) In the national standards have not been revised before, it is recommended that the use of large ring forging flange units on the small equivalent diameter defects are clearly required in the order contract to reject the existence of about 1.5mm and intermittently in a straight state of small defects flange.
• (2) For forging, ring rolling, heat treatment and other processes generated by small cracks, ring forging manufacturers can improve processing technology, strengthen quality control and other methods to prevent its occurrence; for raw materials with internal loosening defects, manufacturers can only choose to buy cast steel billet from a large steel company with stable and reliable quality and high credibility.
• (3) According to the large ring forging flange internal small equivalent diameter defect distribution law, it is recommended that manufacturers should give preference to cylindrical ingots or octagonal ingots. This will be forged in the punching, the cylindrical center area of the loose defects removed, the use of units can also be in the purchase order for the shape of the billet to be clear requirements.

Author: Li Wei

Source:  China Forging Rings Manufacturer: www.epowermetals.com

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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