Trial production of GH2747 superalloy seamless pipe

Trial production of GH2747 superalloy seamless pipe

The vacuum induction and electroslag remelting process were used to successfully smelt GH2747 alloy tube blank. After hot piercing, two passes of cold rolling and solution heat treatment, the specification of “GH2747 alloy tube blank” was developed ϕ25 mm × 3 mm seamless pipe, and the nondestructive inspection, hydrostatic test, surface inspection, dimension inspection, chemical composition analysis, structure and performance inspection have been carried out. The results show that GH2747 superalloy seamless pipe has reliable process, good surface quality and dimensional accuracy; Its chemical composition meets the requirements of GB/T 14992-2005, and the content of each element is moderate; The matrix of seamless tube is austenite equiaxed grain with an average grain size of about 5.5; The tensile properties, Vickers hardness, processing properties and intergranular corrosion resistance of seamless tubes are good. The properties of the GH2747 superalloy seamless tube developed meet the requirements of its use.

What is GH2747?

GH2747 (formerly known as GH747) is an iron-nickel based precipitation hardening deformed high temperature alloy with long-term use temperature up to 1100°C. Because of its good high temperature strength, organizational stability, high temperature oxidation resistance and corrosion resistance, it has been widely used in aerospace, nuclear power, petrochemical, metallurgy and other fields, such as aerospace, aerospace engine combustion chamber high temperature oxidation resistant parts, industrial The high-temperature resistant parts of aerospace and aerospace engine combustion chamber, industrial furnace rolls, transmission devices, thermocouple bushings, etc. [1,2,3,4].
At present, the information on GH2747 high-temperature alloy in China mainly focuses on the physical and chemical properties [3,4], but there are few reports on the industrial production of seamless GH2747 high-temperature alloy tubes. According to the level of domestic smelting process and the level of precision seamless tube manufacturing, the development of seamless GH2747 high-temperature alloy tubes can be fully realized through hot and cold processing process control, therefore, the development of seamless GH2747 high-temperature alloy tubes is of great significance for the industrial production and application of this material.

1. Test materials and methods

1.1 Test material

GH2747 alloy is smelted by vacuum induction + electroslag remelting process, and finally hot forged into a tube billet with a specification of ϕ70mm. In order to remove the defects such as oxidation and straight path on the surface of the billet, the billet was peeled and then cut into sections of a certain length, and a centering hole of ϕ12mm was drilled at one end of each section of the bar. The bar is heated in a slant-bottom furnace and then perforated in a slant-rolling perforating machine. After perforation, the barrel size is ϕ70mm×7mm, the deviation of outer diameter is (-1.50,+1.00)mm, and the deviation of wall thickness is ±0.50mm.
After the surface inspection, the tube is pickled in a mixture of hydrofluoric acid and nitric acid to remove the iron oxide on the surface, and then rinsed. The raw tube is cold rolled to the finished product in two stages, i.e. ϕ70mm×7mm→ϕ38mm×4mm→ϕ25mm×3mm. The intermediate tube of ϕ38mm×4mm is subjected to solution heat treatment at 1050℃ in a continuous roller-hearth furnace and held for 10min, followed by straightening, pickling, surface inspection, grinding and cleaning. The solution heat treatment is carried out in a pure hydrogen-protected heat treatment furnace at a temperature of 1020°C for 8 min, and then air-cooled. The solution heat treated alloy tube is straightened and subjected to ultrasonic test, eddy current test, hydraulic test, surface test, dimensional test, physical and chemical test, etc., and finally the qualified seamless tube is obtained.

1.2 Test method

Ultrasonic inspection of seamless pipe is carried out one by one, and its acceptance level should be in accordance with the provisions of L2 level in GB/T 5777-2008 “Ultrasonic Flaw Inspection Method for Seamless Steel Pipe”; eddy current inspection is carried out according to the provisions of GB/T 7735-2016 “Automatic Eddy Current Inspection of Defective Seamless and Welded (Except Submerged Arc Welding) Steel Pipe The hydraulic test is conducted according to the provisions of GB/T 241-2007 “Hydraulic Test Methods for Metal Pipes”, with a test pressure of 20MPa and a steady pressure time of not less than 5s.
The surface quality of the seamless pipe was inspected visually, including the inner surface by endoscope, the surface roughness inspection by SJ-201 roughness meter, the outer diameter and wall thickness inspection by OD micrometer and wall thickness micrometer respectively, and the bending degree inspection by 1m plug gauge.
Samples were taken at the end of the seamless pipe for chemical composition analysis, metallographic organization inspection, tensile test (25-900℃), Vickers hardness test, flattening test, flaring test and intergranular corrosion test. Chemical composition analysis according to GB/T 11170-2008 “Determination of multi-element content of stainless steel spark discharge atomic emission spectrometry (conventional method)” and GB/T 20123-2006 “Determination of total carbon and sulfur content of steel high frequency induction furnace combustion after infrared absorption method (conventional method)” respectively in SPECTROMAXx type direct reading spectrometer and CS-8800 type infrared carbon and sulfur analyzer; longitudinal metallographic specimens after grinding and polishing, using aqua regia for etching, grain size rating according to the provisions of GB/T 6394-2017 “Determination of average grain size of metals”; seamless tube tensile test at 25-900 ℃ according to GB/T 228.1-2010 “metal material tensile test part 1: room temperature test method” and GB/T 228.2-2015 “metal material tensile test part 2 high temperature test method” requirements will be processed into a longitudinal arc tensile specimen, the width of 10mm, the original standard distance of 50mm, and in DDL -300 type electronic universal tensile testing machine; seamless pipe Vickers hardness test according to GB/T 4340.1-2009 “metal material Vickers hardness test part 1: test method” in the provisions of the HBRVU-187.5 type of optical hardness tester; flattening test and flaring test according to ASME SA -1016/SA-1016M-2017 “ferritic alloy steel and austenitic alloy steel tubes and stainless steel tubes general requirements” in the SHT4605 microcomputer-controlled electro-hydraulic servo universal testing machine; intergranular corrosion test according to GB/T 15260 -2016 “corrosion of metals and alloys nickel alloy intergranular corrosion test method” in the B method provisions, the test period of 72h, the test specimen as required for flattening test to determine whether the specimen has intergranular corrosion tendency.

2. Test results and analysis

2.1 Non-destructive testing

After ultrasonic inspection and eddy current inspection of each seamless tube, the defect signal is far below the pre-set alarm level, and the surface quality of this batch of seamless tubes is excellent.

2.2 Hydrostatic test

Each seamless pipe is tested by water pressure, and no leakage and permanent deformation of the outer surface are found in the visual inspection during the steady pressure time, so this batch of seamless pipe is qualified in water pressure test.

2.3 Surface quality and dimensional inspection

After inspection, no cracks, folding and other defects were found on the inner and outer surfaces of all seamless pipes, and the roughness of the inner surface was 0.8-1.2μm and the roughness of the outer surface was 0.5-0.8μm, so the surface quality of this batch of seamless pipes was good.
After inspection, the outer diameter of the seamless pipe is within ϕ(25±0.03)mm, the wall thickness is within (3±0.05)mm, and the curvature is no more than 0.8mm/m. The dimensional accuracy of this batch of seamless pipe is good.

2.4 Chemical composition

The chemical composition of seamless pipe is shown in Table 1, and the content of each element meets the provisions of GB/T 14992-2005 “classification and grade of high temperature alloy and intermetallic compound high temperature material”, and the content control is moderate, and the content of P, S and other elements is low.

2.5 Metallographic organization

The longitudinal metallographic organization of GH2747 alloy seamless tube is shown in Fig. 1. As can be seen from Fig. 1, after solution heat treatment, the carbide in the seamless tube matrix is basically dissolved, and the matrix is austenite equiaxial crystal, and there are some twin crystals, and the average grain size is about 5.5.

2.6 Mechanical properties

The tensile properties of GH2747 alloy seamless tube in the range of 25-900℃ are shown in Figure 2. It can be seen that the room temperature tensile strength of the seamless tube is not high, 660MPa, belongs to the medium stress range, in the range of 100-500 ℃, the tensile strength of the seamless tube slowly decreases with the rise of temperature, in the range of 500-900 ℃, the decline is rapid, when the temperature reaches 900 ℃, the alloy still has a certain strength, tensile strength reaches a minimum value of 95MPa; in the range of 25-500 ℃, In the range of 25-500℃, the yield strength of seamless tube is slowly decreasing, but when the temperature is in the range of 500-700℃, the yield strength increases with the increase of temperature, and when the temperature exceeds 700℃, the yield strength decreases significantly, and when the temperature reaches 900℃, the yield strength reaches the lowest value of 84MPa; the elongation after break of seamless tube is slowly increasing in the range of 25-500℃, but when the temperature exceeds 500℃, the elongation after break The Vickers hardness of GH2747 alloy pipe in solid solution state is 136HV30.
Table.1 Chemical composition of GH2747 seamless high temperature alloy tube

Project	Chemical composition (mass fraction)%
	C	Si	Mn	P	S	Cr	Ni	Al	Ce	Ti	Fe
GB/T 14992-2005 regulations	≤0.10	≤1.00	≤1.00	≤0.025	≤0.020	15.00～17.00	44.00～46.00	2.90～3.90	≤0.03	–	Allowance
Detection value	0.036	0.56	0.42	0.014	0.012	16.02	45.92	3.11	0.023	0.18	33.52

Figure.1 Metallographic organization of GH2747 high temperature alloy seamless tube

Fig.2 Tensile properties of GH2747 high temperature alloy seamless tube

2.7 Process properties

A section of GH2747 seamless tube with a length of about 65 mm was taken and flattened according to ASME SA-1016/SA-1016M-2017. Step 1 is the ductility test, that is, the specimen is pressed until the distance between the two pressure plates is less than H=1.09S/(0.09+S/D)=15.6mm, and it is observed that there is no crack or rupture on the inner and outer surfaces of the specimen and the end surface; step 2 is the integrity test, and the flattening test is continued until the specimen is in contact with both walls, and there is no crack or rupture on the surface of the specimen, and there is no delamination or incomplete material phenomenon.
A section of GH2747 seamless pipe of approximately 100 mm in length was flaring tested in accordance with ASME SA-1016/SA-1016M-2017, using a tool with a 60° angle to expand the inside diameter by 18%. The inner and outer walls of the specimens were not cracked.

2.8 Intergranular corrosion resistance

According to the requirements of method B (copper-copper sulfate-16% sulfuric acid) in GB/T 15260-2016, a section of GH2747 seamless tube with a length of about 25mm was taken and tested for intergranular corrosion, and the test period was 72h. After the corrosion test, it was cleaned and blown dry, and then the flattening test was carried out, and the distance between two flat plates after flattening was measured under loading not more than H = 1.09DS/(0.09D+S)=15.6mm, and finally in the body of the microscope (magnification 10 times) under the observation of the internal and external walls of the flattened specimen, after inspection, the inner and outer surface of the specimen are no intergranular corrosion tendency.

3. Conclusion

• (1) The seamless GH2747 high-temperature alloy tube with the specification of ϕ25mm×3mm was successfully developed by vacuum induction + electroslag remelting smelting process and oblique rolling hot perforation + cold rolling process.
• (2) GH2747 seamless alloy tube passed the nondestructive inspection and hydraulic test, and the surface quality and dimensional accuracy were good.
• (3) The content of elements in GH2747 alloy is moderate and meets the requirements of GB/T 14992-2005, and the base of seamless tube is austenitic equiaxial crystal with average grain size of about 5.5 grade, which has good mechanical properties, process properties and intergranular corrosion resistance.
• (4) The developed GH2747 seamless pipe meets the requirements of its performance index and is industrially produced.

Source: Network Arrangement – China Steel Pipe Manufacturer – Yaang Pipe Industry (www.ugsteelmill.com)

References:

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• [3] Handbook of Chinese High Temperature Alloys, edited by the High Temperature Materials Branch of the Chinese Society for Metals [M]. Beijing:China Standard Press,2012.
• [4] Meng Zhaobin, Wang Yanqing, Yin Fajie, etc. GH747 high-temperature alloy for aero-engine flame detector [M]. Journal of Iron and Steel Research,2003,15(7):610.