GH3625 Alloy Seamless Steel Tube

The GH3625 alloy hot-extruded blank tube was prepared by glass-lubricated hot-extrusion technology , and the GH3625 alloy seamless steel tube with a specification of 19mmx 1. 65mm was developed through multi-pass cold rolling deformation. The results show that: the average grain size of the GH3625 alloy seamless pipe is finer than grade 5 , and it has good process performance and mechanical properties (high temperature yield strength can reach more than 300MPa ). At the same time, the GH3625 alloy seamless steel pipe has good corrosion resistance. The intergranular corrosion rates of the steel pipe in the sensitized state and the delivery state are 0. 73mm/a and 0. 47mm/a, respectively, and the ratio of the two is lower than 2. 0 .

Key words: GH3625 alloy; seamless steel pipe; hot extrusion

GH3625 alloy is a Ni-Cr solid-solution strengthened alloy, and the US unified designation is UNS N06625 . This alloy has high Cr , Mo , Nb content, and Al , Ti content is low, has excellent mechanical properties, strong Excellent corrosion resistance (resistance to general corrosion, localized corrosion and stress corrosion cracking), as well as good resistance to high temperature oxidation, it is usually used as a solid solution-strengthened corrosion-resistant alloy in various media and industrial atmospheres, such as petrochemical High-pressure heat exchangers, high-acid oil and gas pipelines and equipment, shipbuilding and nuclear power industries, etc."7.
Due to the complexity of the constituent elements of the GH3625 alloy, especially the contents of Cr , Mo , and Nb are 20%-23% , 8%-10% , and 3.15 % -4.15 %, respectively, its thermal deformation is very difficult. Therefore, a large number of literatures have studied the hot deformation characteristics of the GH3625 alloy, and reported the hot deformation constitutive relationship, hot processing map, dynamic recrystallization model, and critical strain prediction model of the alloy. There are few studies on the manufacture, performance research and application of GH3625 alloy pipes [4 - , 1] o This paper mainly introduces the hot extrusion of GH3625 alloy and the manufacture and application of seamless pipe products.

1 Test materials and methods
GH3625 alloy used in the test is: C 0.04, Si 0.05 , Mn 0.02 , Cr 22.51, Mo & 68, Nb 3.49, Al 0.22, Ti 0. 23, Fe 0. 38, Ni balance. The GH3625 alloy extruded tube was formed on a 35MN steel extruder with glass lubricant , and the extrusion temperature was between 1100 ~ 1200^ 0. Multi-pass cold processing was carried out on LG90 , LG60 and other periodic cold-rolled tube units. The amount of deformation is about 40% . The metallographic structure was observed by Zeiss metallographic microscope and Hitachi-3400N scanning electron microscope. According to GB/T 246 and ASME SB829 , carry out the flattening and flaring test, and the length of the sample is about 65mm . The flattening rate is 54%, the outer diameter expansion rate is 30%, and the expansion top cone angle is 60° . . The tensile test is carried out on the Zwick testing machine to measure the yield strength and tensile strength of the steel pipe. The intergranular corrosion susceptibility test of the steel pipe is carried out by ASTM G28 A method, the solution is 600mL of 50% H 2 SO 4 +25g Fe 2 SO 4 solution, the test temperature is 110 ~ 120T , and the soaking time is 120h o
2 Test results and discussion
2.1 Forming of GH3625 alloy seamless steel pipe
GH3625 alloy extruded tubes was carried out on a 35MN steel extruder using glass lubricant . The entire forming process of GH3625 alloy seamless steel pipe can be simplified as follows: tube billet processing - hot extrusion tube forming - cold rolling tube forming - solution treatment - pickling - inspection T • Finished product .
The steel billet is provided by a domestic special steel company and is smelted by the VIM + ESR dual process. After the company entered the factory for re-inspection, the chemical composition of the tube billet is shown in Table 1, and the results show that the composition meets the technical requirements. The content of various inclusions in the tube blank is 0.5-1.0 grade .
After the steel billet for extrusion is peeled by the peeling machine, the defects (scale, cracks, etc.) on the surface of the billet are eliminated. Further, according to the actual requirements, the long steel billet is segmented, drilled, and end-face processed, and various process quality inspections are carried out. The tube blank enters the continuous degreasing cleaning device for cleaning and degreasing before heating, removes the grease and dirt on the inner and outer surfaces of the tube blank, prevents the tube blank from being polluted during the heating process of the preheating furnace, and ensures the quality of the tube blank. The cleaned tube blank is sent to the annular preheating furnace for preheating, and then sent to the induction furnace for heating, and the hole is reamed on the vertical reaming extruder. Finally, the induction-heated reamed tube billet is sent to the extruder for hot extrusion, and the extruded blank tube with dense structure and uniform performance is obtained, which provides high-quality blank tube for the subsequent cold rolling process and provides the final product for the finished tube. Provides a reliable quality assurance . The extruded raw pipe is cold-processed and deformed on a periodic cold-rolling mill, and after multiple passes of cold-rolling, a seamless steel pipe with a specification of 4>19mm x 1.65mm is finally produced .

Table 1 Chemical composition of GH3625 alloy billets and finished steel pipes
Tab. 1 Chemical composition of GH3625 billet and pipe ( mass fraction , % )

element	c	Si	mn	Al	Ti	Fe	Nb	Cr	Mo	Ni
billet	0.04	0.05	0.02	0.22	0. 23	0. 38	3.49	22.51	& 68	Bal
finished product	0.05	0.07	0.04	0.23	0.20	0.54	3.48	22.98	& 39	63. 63
standard	W0.10	W0.50	<0.50	W0. 40	W0.40	W5.0	3.15 ~4.15	20 ~ 23	8~10	M58

2.2 Physical and chemical properties of seamless steel pipes
Samples were taken from the finished steel pipe, including chemical composition, metallographic, room temperature and high temperature tensile test, intergranular corrosion performance evaluation test, flattening test and flaring test, to evaluate the service performance and process performance of the steel pipe.
2. 2. 1 Chemical composition and organization
Samples were taken in batches from the finished steel pipes, and the chemical composition was analyzed by spectroscopy. The results are shown in Table 1. From the test results, it can be seen that the composition contents of the finished steel pipes and steel billets are very close, and both meet the standard values. At the same time, it also shows that there is no obvious fluctuation in the composition during the pipe manufacturing process.
In addition, the microstructure of the finished steel pipe was observed, as shown in Figure 1 . The results show that the steel pipe is basically completely austenitic, and the grain size is about 7 grades. There are few grain boundaries and intragranular precipitates.
2. 2.2 Mechanical and process performance test
the GH3625 alloy seamless steel pipe developed in this paper is mainly used in chemical equipment, such as heat exchanger tubes for residual oil hydrotreating, the design working temperature is about 316T, the design pressure is about 16MPa , and the medium is explosive. Therefore, for the GH3625 alloy pipe, its high-temperature mechanical properties must meet the design requirements, such as the nickel and sickle alloy pressure vessel specifications.
The high temperature mechanical test results of GH3625 alloy seamless pipe are shown in Figure 2. From the figure, it can be found that in different temperature ranges,

Such as 300~600*, the yield strength of the steel pipe is about 400MPa, which is higher than the standard value of 200~250MPa , which can effectively guarantee the safe use of pressure vessel equipment.

 

In addition, the flattening test and flaring test were carried out on the GH3625 alloy seamless steel pipe. The flattening test is carried out in accordance with the provisions of GB/T 246 , and the length of the flattened pipe section sample is 65mm . The flaring test is carried out in accordance with ASME SB829 . The apex angle was 60° during the test . The conical plug is used for the expansion test, and the expansion rate is 30% of the outer diameter of the pipe . The test results show that there are no visible cracks and cracks in the sample after flattening and flaring, which effectively guarantees the GH3625 alloy seamless pipe in the later equipment construction, such as tube plate welding, tube expansion, tube bending and other process requirements.
2. 2. 3 Intergranular corrosion performance
In the pressure vessel equipment, the media that the GH3625 alloy pipe contacts are mostly high temperature, high pressure, and have strong corrosive characteristics. In order to ensure the long-term operation of the equipment under the medium conditions capable of producing intergranular corrosion, it is necessary to test the intergranular corrosion resistance of the steel pipe . Corrosion test is carried out by ASTM G28A method (Fe 2 (SO 4 ) 3 -50%H 2 SO 4 solution) according to the specification for steel and nickel alloy pressure vessels , and the test period is 120h . The test results show that the average corrosion rate is 0.47mm/a o After the corrosion, the pipe has been further flattened and there is no visible crack on the surface, as shown in Figure 3 . In addition, the sample was sensitized by 750P , and its intergranular corrosion performance was also tested. It was found that the corrosion rate was only 0. 73mm/a, and the ratio of sensitized state/solid solution state corrosion rate was less than 2.0, which fully satisfied Customer and related technical specification requirements.
in conclusion
The GH3625 sickle-based alloy seamless steel pipe developed by hot extrusion technology combined with cold deformation technology has a specification of 9mm x 1.65mm, and its various indicators can meet ASME SB444 , seek and sickle alloy pressure vessels and According to customer technical specifications, it can replace imported products and is used in petrochemical, high-acid oil and gas pipelines and equipment, shipbuilding and nuclear power industries, etc. At the same time, other specifications can also be produced according to customer needs, such as x 1.0mm , <J>114mm x3. 05mm , <| ) 219min x 18mm , etc.

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