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Analysis of the phenomenon of “white ring” in GH536 alloy tube

GH536 alloy is a kind of nickel based superalloy with high iron content, which is mainly reinforced by solid solution of chromium and molybdenum. It is suitable for the manufacture of aeroengine flame tube, combustor component and other high temperature components. The alloy pipe made of GH536 alloy is mainly used for the fuel main pipe or sub fuel pipe of the aero engine.
The specification of a batch of alloy pipes is 8.3mm 2.3mm, which is qualified by the reexamination of the alloy pipe. The intergranular corrosion is qualified, and the grain size is 8 to 7.5, which meets the standard requirements. However, it was found that the white ring appeared on the outer wall of the alloy tube.
The researchers sampled and analyzed the alloy tubes that found white circles. At the lower multiple, there was a white layer on the edge of the tube. The enlargement observation revealed that the outer wall of the tube was less precipitated than the matrix, and the outer wall grain was larger than the matrix grain. Compared with the qualified pipe material, it is found that the alloy tube with “white ring” has fine grain, and the small precipitation phase is more than that of the qualified tube, and the large precipitation phase is equivalent to that of the qualified pipe.
The tubes were heat treated at three, 1130, 1150 and 1190 degrees respectively for 12min and water cooling. It is found that the higher the solid solution temperature is, the less the precipitation phase is. The grain size did not grow significantly at 1150, and the grain grew obviously at 1190.
It can be seen from the microstructure analysis that there are a small amount of precipitated phases and grain boundaries in the “white circle” organization, but compared with the qualified pipe structure, it is a normal organization. That is to say, the matrix structure is abnormal, which shows fine grains and precipitates, and the outer wall of the reverse alloy tube appears white ring. The essence of the “white ring” is the nonuniformity of the tube wall.
The standard heat treatment system for GH536 alloy pipes is 1130~1170 C, fast cooling, and holding time is less than 30min. The alloy pipes were treated by solid solution at 1130 degree x 10min, and the tube billets were rolled by multiple passes, once each rolling, solid solution once, and the accumulated deformation was larger. The higher the heating temperature is, the larger the grain size is. When the heating temperature is constant, the longer the holding time will make the grain grow. The lower limit of the standard heat treatment system is adopted for the alloy pipes. The holding time is 10min, which is not solid enough for the 2.3mm thick wall, so the grain is finer. At relatively low solid solution temperature, there are more precipitates in the tube billet. The final deformation process of the tube is cold dial, and the cold drawing allowance is usually certain, that is to say, the deformation of the outer wall of the alloy tube is not very different from that of the qualified alloy tube, then the problem is the tube blank.
The heat preservation time of producing 1.0mm thick pipes is 10min. The thermal insulation time of the alloy tube producing 2.3mm is still 10min, which will lead to the uneven heating of the thick alloy tubes and the inadequately solid solution in the internal structure, which is also one of the reasons for the uneven microstructure of the alloy tubes.
It is considered that the phenomenon of “white ring” of alloy tube is caused by the low temperature of solid solution and the short holding time during the rolling process, which is caused by the uneven structure of the matrix and the outer wall. On the basis of the constant rolling path and the reasonable deformation amount, the solid solution temperature is adjusted to 1150 degrees C, the heat preservation time is 20min, and the tube structure has been improved, and no “white ring” phenomenon appears.
Source: China Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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