Method for preparing fine-grain low-impurity GH4720Li alloy small-size rod

Abstract

The application discloses a preparation method of fine-grain low-noise GH4720Li alloy small-size rod, and specifically comprises the following steps: high-temperature homogenization treatment is conducted on a GH4720Li alloy ingot; air holes on a clean surface are turned, a risk area is cut off, upsetting and drawing are conducted, a blank rod is cut off at a rotten head, scale on a surface after bright forging is turned, surface defects are ground, the blank rod after grinding is cut, is sleeved, hot extrusion is conducted on the sleeved rod, roll straightening is conducted by using extrusion residual heat, a black skin rod is obtained, the black skin rod is subjected to light treatment by adopting abrasive belt polishing, surface defects are ground, and stainless steel component rotten heads at two ends of the rod are sawn off, and the method is completed.The prepared GH4720Li alloy small-size rod has a grain size between 8-10 levels, a flaw detection noise level between Phi 0.8-(9-18) dB, and an excellent surface quality, and is accurately controllable in diameter; and the GH4720Li alloy small-size rod has a good application prospect in the fields of aviation industry, energy and petrochemical industry.

Description

Technical Field

[0001] This invention belongs to the field of alloy material processing technology, specifically relating to a method for preparing small-diameter bars of fine-grained, low-impurity GH4720Li alloy. Background Technology

[0002] High-temperature alloys, as important strategic resources, possess excellent properties such as high temperature resistance, wear resistance, corrosion resistance, and oxidation resistance, and are widely used in aviation, aerospace, petroleum, shipbuilding, chemical, and power industries. High-temperature alloys can be classified according to their main alloying elements into iron-based, nickel-based, and cobalt-based high-temperature alloys; and according to product series into wrought high-temperature alloys, cast high-temperature alloys, and novel high-temperature alloys. With breakthroughs in the aerospace industry in advanced engines, gas turbines, nuclear power equipment, and other fields, the market demand for high-end and novel high-temperature alloys is increasing year by year.

[0003] GH4720Li alloy is a Ni-Cr-Co based precipitation-hardening wrought superalloy with a service temperature below 750℃. This alloy possesses high high-temperature strength, fatigue resistance, and creep resistance, as well as good resistance to sulfur corrosion, oxidation resistance, and long-term structural stability. It is suitable for manufacturing turbine disks and turbine blades for aero-engines. However, due to severe elemental segregation in GH4720Li alloy, cracking is prone to occur at forging temperatures below 1080℃. Furthermore, at temperatures above the phase transformation temperature (approximately 1160℃), the grain boundary pinning phase dissolves, leading to grain coarsening and decreased hot-working plasticity. Therefore, these factors limit the hot working of GH4720Li alloy. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing small-sized bars of fine-grained, low-impurity GH4720Li alloy, which have uniform and fine grains, low defect rate, and excellent surface quality.

[0005] The technical solution adopted in this invention is a method for preparing small-diameter bars of fine-grained, low-impurity GH4720Li alloy, which is implemented according to the following steps:

[0006] Step 1: The GH4720Li alloy ingot is subjected to high-temperature homogenization treatment;

[0007] Step 2: Before forging the ingot, clean the pores on the surface of the GH4720Li alloy ingot, then cut off the high-risk areas at the beginning and end, and then perform upsetting and drawing forging on a high-speed forging machine to obtain the billet bar.

[0008] Step 3: Cut off the rotten end of the billet bar, machine and polish the oxide scale on the surface of the billet bar after forging, allow grinding to repair surface defects, and require a smooth transition at the grinding pit. Cut the ground billet bar to obtain the bar; then use 304 stainless steel pipe to encase the bar to obtain the encased bar.

[0009] Step 4: Hot extrusion of the cladding bar is performed using a horizontal double-acting forward extrusion press. The residual heat from the extrusion is then used for roller straightening, which causes the cladding to curl and crack, resulting in black-skinned bar material.

[0010] Step 5: Use abrasive belt polishing to expose the black bar to light, repair surface defects, and ensure a smooth transition at the repair pits. Sawing off the rotten ends of the stainless steel components at both ends of the bar to obtain small-sized fine-grained, low-impurity GH4720Li alloy bars.

[0011] The invention is further characterized in that,

[0012] In step 1, the chemical composition of the GH4720Li alloy ingot, by mass percentage, is as follows: C: 0.01–0.02%, Si: 0–0.50%, Mn: 0–0.50%, P: 0–0.015%, S: 0–0.008%, Ag: 0–0.0005%, Al: 2.25–2.75%, B: 0.01–0.02%, Bi: 0–0.00005%, Co: 14.0–15.5%. % Cr: 15.5-16.5%, Cu: 0-0.10%, Fe: 0-0.50%, Mo: 2.75-3.25%, Pb: 0-0.0010%, Ti: 4.75-5.25%, W: 1.00-1.50%, Zr: 0.025-0.050%, O: 0-0.0020%, N: 0-0.0032%, with the balance being Ni. The sum of the above components by mass percentage is 100%.

[0013] In step 1, the high-temperature homogenization treatment is divided into three stages: in the first stage, the heat preservation temperature is between 700℃ and 900℃; in the second stage, the heat preservation temperature is between 1100℃ and 1160℃; in the third stage, the heat preservation temperature is between 1160℃ and 1200℃; the total heat preservation time for the three stages is not less than 130 hours.

[0014] In step 2, the billet bar specifications are Φ150mm~Φ170mm.

[0015] In step 3, the length of the bar stock is 240mm to 390mm; the diameter of the cladding bar stock is 140mm to 150mm; the out-of-roundness of the billet bar stock after machining and forging is less than 0.5mm; the width-to-depth ratio of the grinding pit is not less than 5:1; and the wall thickness of the 304 stainless steel pipe is 2mm to 3mm.

[0016] In step 4, the hot extrusion temperature is 1050℃~1150℃, the holding time is 90min~180min, the extrusion ratio is 6~9, the hot extrusion speed is not greater than 50mm / s, the number of roller straightening passes is 3~5, and the length of the black bar is 2000mm~3500mm and the diameter is 50mm~56mm.

[0017] In step 5, the length of the fine-grained, low-impurity GH4720Li alloy small-sized rod is 1500mm-3000mm and the diameter is 50mm-56mm; the surface roughness of the fine-grained, low-impurity GH4720Li alloy small-sized rod is less than or equal to 2.0 micrometers and the bending degree does not exceed 2mm / m.

[0018] The beneficial effects of this invention are: the GH4720Li alloy small-diameter bars prepared by the method of this invention have a grain size between 8 and 10, a flaw detection clutter level between Φ0.8 and (9 to 18) dB, and the bar diameter is precisely controllable with excellent surface quality. They have good application prospects in the fields of aerospace, energy and petrochemical industries. Attached Figure Description

[0019] Figure 1 This is the high-magnification microstructure of the head of the GH4720Li rod prepared in Example 1;

[0020] Figure 2 High-magnification microstructure of the tail section of a GH4720Li rod was prepared for Example 1;

[0021] Figure 3 This is the high-magnification microstructure of the head of the GH4720Li rod prepared in Example 2;

[0022] Figure 4 High-magnification microstructure of the tail section of a GH4720Li rod was prepared for Example 2;

[0023] Figure 5 This is the high-magnification microstructure of the head of the GH4720Li rod prepared in Example 3;

[0024] Figure 6 The tail section of the GH4720Li rod was prepared for Example 3. Detailed Implementation

[0025] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0026] The method for preparing small-diameter bars of fine-grained, low-impurity GH4720Li alloy of the present invention is specifically implemented according to the following steps:

[0027] Step 1: The GH4720Li alloy ingot is subjected to high-temperature homogenization treatment;

[0028] The chemical composition of the GH4720Li alloy ingot, by mass percentage, is as follows: C: 0.01–0.02%, Si: 0–0.50%, Mn: 0–0.50%, P: 0–0.015%, S: 0–0.008%, Ag: 0–0.0005%, Al: 2.25–2.75%, B: 0.01–0.02%, Bi: 0–0.00005%, Co: 14.0–15.5%, C r: 15.5–16.5%, Cu: 0–0.10%, Fe: 0–0.50%, Mo: 2.75–3.25%, Pb: 0–0.0010%, Ti: 4.75–5.25%, W: 1.00–1.50%, Zr: 0.025–0.050%, O: 0–0.0020%, N: 0–0.0032%, with the balance being Ni. The sum of the above components by mass percentage is 100%.

[0029] High-temperature homogenization can promote the diffusion of easily segregated elements, reduce and eliminate microsegregation in the ingot, and provide high-quality homogeneous ingots for the forging process. A natural gas furnace with a furnace temperature accuracy within ±8℃ is used for staged, continuous heating high-temperature homogenization of the ingot. In the first stage, the holding temperature is between 700℃ and 900℃ to reduce the temperature difference between the inside and outside of the ingot and decrease thermal stress. In the second stage, the holding temperature is between 1100℃ and 1160℃ to eliminate low-melting-point borides. In the third stage, the holding temperature is between 1160℃ and 1200℃ to reduce the segregation of Ti elements. The total holding time is no less than 130 hours.

[0030] Step 2: In order to reduce surface cracking of the material during forging, the pores on the surface of the GH4720Li alloy ingot are cleaned before the ingot is forged. Then, the high-risk areas at the beginning and end are cut off with a band saw. Then, the ingot is upsetting and drawing forging is carried out on a high-speed forging machine to obtain billet bars with specifications of Φ150mm~Φ170mm.

[0031] During upsetting and drawing forging, a top-pressure double-column high-speed forging machine is used, equipped with two 25-ton manipulators, which can realize the linkage between the press and the manipulators. To ensure uniform microstructure after forging, a forging process of repeated upsetting and drawing followed by cooling forging is adopted.

[0032] Step 3: Use an abrasive wheel saw or band saw to cut off the rotten end of the billet bar, and machine the oxide scale on the surface of the forged billet bar. Surface defects are allowed to be repaired by grinding, and the grinding pits should be smooth. Use an abrasive wheel saw or band saw to cut the repaired billet bar to obtain bars with a length of 240mm to 390mm. Then, use 304 stainless steel pipe to encase the bar to obtain encased bars with a diameter of 140mm to 150mm.

[0033] The surface roughness of the billet bar after machining and forging is less than 3.2 micrometers; to ensure uniform spacing between the billet and the inner wall of the cladding, the out-of-roundness of the billet bar after machining and forging is less than 0.5 mm.

[0034] The width-to-depth ratio of the grinding pit shall not be less than 5:1; the wall thickness of the 304 stainless steel pipe shall be 2mm to 3mm;

[0035] Step 4: Hot extrusion of the cladding bar is carried out using a horizontal double-acting forward extrusion press. The residual heat of the extrusion is used for 3 to 5 passes of roller straightening, which can cause the cladding to curl and break, and obtain black bar with a length of 2000mm to 3500mm and a diameter of 50mm to 56mm.

[0036] The hot extrusion temperature is 1050℃~1150℃, the holding time is 90min~180min, the extrusion ratio is 6~9, and the hot extrusion speed is not greater than 50mm / s;

[0037] Due to the large extrusion ratio, in order to prevent excessive extrusion pressure from causing the machine to stall, the extrusion pad and mold need to be preheated at 350℃ for no less than 2 hours before extrusion, and the extrusion cylinder needs to be preheated to above 450℃.

[0038] Because the bar stock and sheath material are tightly bonded after extrusion, the straightened bar stock appears to have improved straightness. However, the straightening process only straightens the sheath; the internal bar stock is not straightened to a high degree of straightness. When using traditional turning to remove the sheath, material breakage and injuries can easily occur due to poor bar straightness and uneven surface material. In this invention, because the sheath material has a relatively thin wall thickness, the sheath can be curled and broken by using 3-5 passes of roller straightening with the residual heat of extrusion. The material temperature before straightening must be above 880℃; otherwise, the third and fifth passes of straightening will not achieve the purpose of reducing curvature. Therefore, the material needs to be transferred to the straightening machine as soon as possible after extrusion, generally within 60 seconds.

[0039] Step 5: Polish the black bar with a belt abrasive to expose it to light. Use a grinding wheel and a flap wheel to grind surface defects, ensuring a smooth transition at the grinding pits, with a width-to-depth ratio of not less than 5:1. Use a band saw to cut off the stainless steel ends of the bar to obtain small-diameter GH4720Li alloy bars with a length of 1500mm-3000mm and a diameter of 50mm-56mm.

[0040] The surface roughness of the GH4720Li alloy small-diameter bars is less than or equal to 2.0 micrometers, and the bending degree does not exceed 2 mm / m.

[0041] In this embodiment of the invention, the grain size rating of the bar material adopts GB / T6394, the ultrasonic flaw detection adopts MFS0722, the water immersion method, and the diameter of the flat bottom hole is 0.8mm.

[0042] Example 1

[0043] The method for preparing small-diameter bars of fine-grained, low-impurity GH4720Li alloy of the present invention is specifically implemented according to the following steps:

[0044] Step 1: The GH4720Li alloy ingot is subjected to high-temperature homogenization treatment;

[0045] The chemical composition of the GH4720Li alloy ingot, by mass percentage, is as follows: C: 0.010%, Si: <0.05%, Mn: <0.005%, P: <0.004%, S: <0.0005%, Ag: <0.0005%, Al: 2.51%, B: 0.013%, Bi: <0.00001%, Co: 14.84%, Cr: 16.66%, Cu: <0.01%, Fe: 0.18%, Mo: 3.08%, Pb: <0.0005%, Ti: 4.95%, W: 1.27%, Zr: 0.039%, O: 0.0001%, N: 0.0012%, with the balance being Ni. The sum of the mass percentages of the above components is 100%.

[0046] A natural gas furnace with a furnace temperature accuracy within ±8℃ was used to perform high-temperature homogenization treatment on the ingots through staged continuous heating. In the first stage, the holding temperature was 850℃ and the holding time was 120h; in the second stage, the holding temperature was 1100℃ and the holding time was 120h; in the third stage, the holding temperature was 1160℃ and the holding time was 1800h.

[0047] Step 2, Forging: The ingot is cut off with a band saw, removing a 50mm section from both ends to minimize surface cracking during forging. To reduce surface pores, a 5mm depth is machined off one side of the ingot. Upsetting and drawing are then performed on an 80MN high-speed forging mill. To ensure a uniform microstructure after forging, a multi-stage upsetting and cooling forging process is used to obtain a billet bar with a diameter of Φ150mm.

[0048] Step 3, Billet Pretreatment: Use a band saw to cut off any rotten ends from the billet bar, and machine the surface oxide scale to achieve a surface roughness ≤3.2 micrometers. Surface defects can be repaired by grinding, but the grinding pits must have a smooth transition, and the width-to-depth ratio of the grinding pits should not be less than 5:1. To ensure uniform spacing between the billet and the inner wall of the casing, the out-of-roundness of the billet after machining should be ≤0.5mm. Use a band saw to evenly divide the ground bar into 240mm lengths, and use 3mm thick 304 stainless steel tubing for casing. The diameter of the casing bar is 140mm. To prevent excessive friction between the weld beads and the extrusion cylinder during the extrusion process, causing jamming, the weld beads need to be ground off after welding the end caps.

[0049] Step 4, Extrusion processing: The bar is hot-extruded using a horizontal double-acting forward extruder, and the residual heat from the extrusion is used to perform three passes of roller straightening to remove the cladding, resulting in a black bar with a length of 2000mm and a diameter of 55mm.

[0050] The hot extrusion temperature is 1110℃, the holding time is 90min, the extrusion ratio is 9:1, and the extrusion speed is 40mm / s;

[0051] Before extrusion, the extrusion pad and die are preheated at 350℃ for 3 hours, and the extrusion cylinder is preheated to 500℃; the surface temperature of the material before straightening is 900℃, and the transfer time of transferring the material to the straightener after extrusion is 45s.

[0052] Step 5, Surface treatment of the bar stock: The black bar stock is polished with a sanding belt to achieve a gloss finish. Individual surface defects are then smoothed using a grinding wheel and a flap wheel, ensuring a smooth transition at the grinding pits, with a width-to-depth ratio of no less than 5:1. The rotten ends of the stainless steel components at both ends of the bar stock are then cut off using a band saw to obtain small-diameter GH4720Li alloy bars.

[0053] The bar has a diameter of 54 mm, a length of 2000 mm, a roughness of less than or equal to 2.0 micrometers, and a bending degree of no more than 2 mm / m.

[0054] The rods prepared in this embodiment were subjected to high-magnification microstructure analysis of the head and tail and water immersion ultrasonic flaw detection. Figure 1 The high-magnification microstructure of the head of the bar prepared in Example 1 has a grain size of ASTM 8.5 grade. Figure 2 The tail section of the bar prepared for Example 1 has a high magnification structure with a grain size of ASTM 8.5 grade. The bar was tested by ultrasonic testing and no defects were found. The noise level was between Φ0.8-(9~12)dB.

[0055] Example 2

[0056] The method for preparing small-diameter bars of fine-grained, low-impurity GH4720Li alloy of the present invention is specifically implemented according to the following steps:

[0057] Step 1: The GH4720Li alloy ingot is subjected to high-temperature homogenization treatment;

[0058] The chemical composition of the GH4720Li alloy ingot, by mass percentage, is as follows: C: 0.012%, Si: <0.05%, Mn: 0.003%, P: <0.004%, S: <0.0005%, Ag: <0.0005%, Al: 2.51%, B: 0.016%, Bi: <0.000005%, Co: 14.66%, Cr: 16.73%, Cu: <0.01%, Fe: 0.050%, Mo: 3.04%, Pb: <0.0001%, Ti: 4.97%, W: 1.25%, Zr: 0.040%, O: <0.002%, N: 0.0016%, with the balance being Ni. The sum of the mass percentages of the above components is 100%.

[0059] A natural gas furnace with a furnace temperature accuracy within ±8℃ was used to perform high-temperature homogenization treatment on the ingots through staged continuous heating. In the first stage, the holding temperature was 850℃ and the holding time was 120h; in the second stage, the holding temperature was 1100℃ and the holding time was 120h; in the third stage, the holding temperature was 1160℃ and the holding time was 1800h.

[0060] Step 2, Forging: The ingot is cut off with a band saw, removing 80mm of the high-risk composition area from both ends. To reduce surface cracking during forging, 7mm of surface pores are machined off on one side of the ingot. Upsetting and drawing are then performed on an 80MN high-speed forging mill. To ensure a uniform microstructure after forging, a multi-stage upsetting and cooling forging process is used to obtain a billet bar with a diameter of Φ170mm.

[0061] Step 3, Billet Pretreatment: Use a band saw to cut off any rotten ends from the billet bar, and machine the surface oxide scale to achieve a surface roughness ≤3.2 micrometers. Surface defects can be repaired by grinding, but the grinding pits must have a smooth transition, and the width-to-depth ratio of the grinding pits should not be less than 5:1. To ensure uniform spacing between the billet and the inner wall of the casing, the out-of-roundness of the billet after machining should be ≤0.5mm. Use a band saw to evenly divide the ground bar into 270mm lengths, and use 2mm thick 304 stainless steel tubing for casing. The diameter of the casing bar is 147mm. To prevent excessive friction between the weld beads and the extrusion cylinder during the extrusion process, causing jamming, the weld beads need to be ground off after welding the end caps.

[0062] Step 4, Extrusion processing: The bar is hot-extruded using a horizontal double-acting forward extruder, and the residual heat from the extrusion is used to perform three passes of roller straightening to remove the cladding, resulting in a black bar with a length of 2200mm and a diameter of 54mm.

[0063] The hot extrusion temperature is 1110℃, the holding time is 120min, the extrusion ratio is 7:1, and the extrusion speed is 50mm / s.

[0064] Before extrusion, the extrusion pad and die are preheated at 350°C for 3 hours, and the extrusion cylinder is preheated to 500°C. The surface temperature of the material before straightening is 930°C, and the transfer time for transferring the material to the straightener after extrusion is 50 seconds.

[0065] Step 5, Surface treatment of the bar stock: The black bar stock is polished with a sanding belt to achieve a gloss finish. Individual surface defects are then smoothed using a grinding wheel and a flap wheel, ensuring a smooth transition at the grinding pits, with a width-to-depth ratio of no less than 5:1. The rotten ends of the stainless steel components at both ends of the bar stock are then cut off using a band saw to obtain small-diameter GH4720Li alloy bars.

[0066] The bar has a diameter of 54 mm, a length of 2000 mm, a roughness of less than or equal to 2.0 micrometers, and a bending degree of no more than 2 mm / m.

[0067] The rods prepared in this embodiment were subjected to high-magnification microstructure analysis of the head and tail and water immersion ultrasonic flaw detection. Figure 3 The high-magnification microstructure of the head of the bar prepared in Example 2 has a grain size of ASTM grade 8. Figure 4 The tail section of the bar prepared in Example 2 has a high magnification structure with a grain size of ASTM grade 8. The bar was tested by ultrasonic testing and no defects were found. The noise level was between Φ0.8-(9~18)dB.

[0068] Example 3

[0069] The method for preparing small-diameter bars of fine-grained, low-impurity GH4720Li alloy of the present invention is specifically implemented according to the following steps:

[0070] Step 1: The GH4720Li alloy ingot is subjected to high-temperature homogenization treatment;

[0071] The chemical composition of the GH4720Li alloy ingot, by mass percentage, is as follows: C: 0.012%, Si: <0.05%, Mn: <0.005%, P: <0.004%, S: <0.0005%, Ag: <0.00005%, Al: 2.47%, B: 0.016%, Bi: <0. 0.00005%, Co: 14.71%, Cr: 16.84%, Cu: <0.01%, Fe: <0.050%, Mo: 3.03%, Pb: <0.0001%, Ti: 5.01%, W: 1.26%, Zr: 0.045%, O: 0.0010%, N: 0.0011%, with the balance being Ni. The sum of the above components by mass percentage is 100%.

[0072] A natural gas furnace with a furnace temperature accuracy within ±8℃ was used to perform high-temperature homogenization treatment on the ingots through staged continuous heating. In the first stage, the holding temperature was 850℃ and the holding time was 120h; in the second stage, the holding temperature was 1100℃ and the holding time was 120h; in the third stage, the holding temperature was 1160℃ and the holding time was 1800h.

[0073] Step 2, Forging: The ingot is cut off with a band saw to remove the 100mm long risk zone at both ends. To reduce surface cracking during forging, 8mm deep surface pores are machined off on one side of the ingot. Upsetting and drawing are then performed on an 80MN high-speed forging mill. To ensure a uniform microstructure after forging, a multi-stage upsetting and drawing process with cooling is used to obtain a billet bar with a specification of Φ160mm.

[0074] Step 3, Billet Pretreatment: Use a band saw to cut off any rotten ends from the billet bar, and machine the surface oxide scale to achieve a surface roughness ≤3.2 micrometers. Surface defects can be repaired by grinding, but the grinding pits must have a smooth transition, and the width-to-depth ratio of the grinding pits should not be less than 5:1. To ensure uniform spacing between the billet and the inner wall of the sheath, the out-of-roundness of the billet after machining should be ≤0.5mm. Use a band saw to evenly divide the ground bar into 300mm lengths, and sheath it with 2.5mm thick 304 stainless steel tubing. The diameter of the sheathed bar is 149mm. To prevent excessive friction between the weld beads and the extrusion cylinder during the extrusion process, causing jamming, the weld beads need to be ground off after welding the end caps.

[0075] Step 4, Extrusion processing: The bar is hot-extruded using a horizontal double-acting forward extruder, and the residual heat from the extrusion is used to perform five passes of roller straightening to remove the cladding, resulting in a black bar with a length of 3500mm and a diameter of 53mm.

[0076] The hot extrusion temperature is 1110℃, the holding time is 150min, the extrusion ratio is 8:1, and the extrusion speed is 50mm / s;

[0077] Before extrusion, the extrusion pad and die are preheated at 350℃ for 3 hours, and the extrusion cylinder is preheated to 500℃; the surface temperature of the material before straightening is 960℃, and the transfer time for transferring the material to the straightener after extrusion is 60s.

[0078] Step 5, Surface treatment of the bar stock: The black bar stock is polished with a sanding belt to achieve a gloss finish. Individual surface defects are then smoothed using a grinding wheel and a flap wheel, ensuring a smooth transition at the grinding pits, with a width-to-depth ratio of no less than 5:1. The rotten ends of the stainless steel components at both ends of the bar stock are then cut off using a band saw to obtain small-diameter GH4720Li alloy bars.

[0079] The bar has a diameter of 54 mm, a length of 3000 mm, a roughness of less than or equal to 2.0 micrometers, and a bending degree of no more than 2 mm / m.

[0080] The rods prepared in this embodiment were subjected to high-magnification microstructure analysis of the head and tail and water immersion ultrasonic flaw detection. Figure 5 The high-magnification microstructure of the head of the bar prepared in Example 3 has a grain size of ASTM grade 8. Figure 6 The tail section of the bar prepared in Example 3 has a high magnification structure with a grain size of ASTM grade 8. The bar was tested by ultrasonic testing and no defects were found. The noise level was between Φ0.8-(12~18)dB.

Claims

1. A method for preparing small-diameter bars of fine-grained, low-impurity GH4720Li alloy, characterized in that, The specific steps are as follows: Step 1: The GH4720Li alloy ingot is subjected to high-temperature homogenization treatment. The GH4720Li alloy ingot, by mass percentage, is as follows: C: 0.010%, Si: <0.05%, Mn: <0.005%, P: <0.004%, S: <0.0005%, Ag: <0.0005%, Al: 2.51%, B: 0.013%, Bi: <0.00001%, Co: 14.84%, Cr: 16.66%, Cu: <0.01%, Fe: 0.18%, Mo: 3.08%, Pb: <0.0005%, Ti: 4.95%, W: 1.27%, Zr: 0.039%, O: 0.0001%, N: 0.0012%, with the balance being Ni. The sum of the mass percentages of the above components is 100%. The high-temperature homogenization treatment is divided into three stages: in the first stage, the holding temperature is 850℃ and the holding time is 120h; in the second stage, the holding temperature is 1100℃ and the holding time is 120h; in the third stage, the holding temperature is 1160℃ and the holding time is 1800h. Step 2: Cut off the 50mm long component risk area from the beginning and end of the ingot with a band saw, and machine off the surface pores to a depth of 5mm on one side of the ingot; perform upsetting and drawing forging on a high-speed forging machine, and use a multi-fire repeated upsetting and drawing and cooling forging process to obtain a billet bar with a specification of Φ150mm. Step 3: Use a band saw to cut off the rotten ends of the billet bar, and machine the surface oxide scale to make the surface roughness ≤3.2 micrometers. Surface defects are allowed to be repaired by grinding, and the grinding pits must be smooth and the width-to-depth ratio of the grinding pits should not be less than 5:

1. The out-of-roundness of the billet after machining should be ≤0.5mm. Use a band saw to evenly divide the ground bar into lengths of 240mm, and use 304 stainless steel pipes with a wall thickness of 3mm to wrap the bar. The diameter of the wrapped bar is 140mm. Step 4: Hot extrusion of the cladding bar is performed using a horizontal double-acting forward extrusion press. The residual heat from the extrusion is used for roller straightening, which causes the cladding to curl and break, resulting in a black bar with a length of 2000mm and a diameter of 55mm. The hot extrusion temperature was 1110℃, the holding time was 90min, the extrusion ratio was 9:1, the hot extrusion speed was 40mm / s, and the number of roller straightening passes was 3. Step 5: Use abrasive belt polishing to expose the black bar to light, and grind surface defects. The grinding pits should be smooth and the width-to-depth ratio of the grinding pits should not be less than 5:

1. Sawing off the stainless steel ends of the bars to obtain fine-grained, low-impurity GH4720Li alloy small-diameter bars. The small-sized bar of the fine-grained, low-impurity GH4720Li alloy has a diameter of 54 mm, a length of 2000 mm, a roughness of less than or equal to 2.0 micrometers, and a bending degree of no more than 2 mm / m. Small-diameter bars of GH4720Li alloy with a grain size of 8.5 and a flaw detection noise level between Φ0.8 and (9~12) dB.

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