Process for producing hollow billets by hot piercing of wrought superalloy

Abstract

The application discloses a process for producing a hollow pipe by hot piercing of a wrought high-temperature alloy, and comprises the following steps: step 1, adopting AOD smelting to mold an ingot of N06625 alloy, and then remelting and refining the ingot by slag resistance heat to obtain an alloy block refined into an ingot; step 2, hot forging the alloy block refined into an ingot into a round pipe blank; step 3, hot piercing the round pipe blank by a 2-roller conical cross wedge rolling piercer and a slant bottom heating furnace; and step 4, performing water quenching and cold deformation after the piercing to obtain a hollow pipe finished product. The application can improve the piercing performance, produce the hollow pipe with qualified quality, and has simple forming and processing, high efficiency and the ability of batch production.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of steel pipe forging, in particular to a process for producing a hollow pipe by hot piercing of a deformed high-temperature alloy. BACKGROUND

[0002] N06625 alloy is a solid solution strengthening type nickel-based deformed high-temperature alloy with molybdenum and niobium as the main strengthening elements. It is a nickel-based austenitic deformed alloy containing a large amount of chromium (20-23 wt%) and molybdenum (8-10 wt%), and niobium (3.15-4.15 wt%) as the main additive element. Due to the special combination of mechanical properties, processing properties and corrosion resistance of this superalloy, it has been widely used; it has excellent corrosion resistance and oxidation resistance, good tensile properties and fatigue properties from low temperature to 980℃, and resistance to stress corrosion in salt mist atmosphere. Therefore, the seamless pipe of this alloy can be widely used to manufacture aerospace engine parts, aerospace structure parts and chemical equipment. However, due to the high mass fraction of molybdenum and niobium, there is a large difference in radius with the base atoms, so there is a large high-temperature strengthening effect. Due to the thermal stress-strain effect during hot deformation, the quality stability of the hot rolling piercing production hollow pipe is deviated, the piercing performance is decreased, and the hollow pipe is prone to crack or breakage.

[0003] For many years, the hollow pipe used in the manufacture of N06625 alloy pipe has been mainly obtained through two ways: one is to bore a hole in the center of the radial section of a round bar in the axial direction, i.e. to form a center hole in the center "hollow" of the seamless pipe for use as a hollow pipe; the other is to process the round bar and then hot extrude it to form a hollow pipe.

[0004] The first way has a very narrow specification, with an outer diameter ≤ 90 mm and an inner hole ≤ 38 mm. The larger the inner hole, the greater the metal loss, and the yield is very low. Because the wall thickness is thick, the cold working pass is increased, and the cost and delivery time are greatly affected. Unless there is a special need and certain profit, this method can be selected.

[0005] The second way has certain guarantee for the quality of the hot working of the alloy, and is a relatively ideal hot working method. Similarly, because the hot extrusion process is relatively complex, it requires two heating and two processing, and the cost is 80% higher than that of piercing. In particular, the requirements for round bar stock are also very strict, and the metal loss of the pre-billet preparation and the subsequent "pressure surplus" is between 30-38%, and the yield is less than 70%. More importantly, for high-nickel and high-molybdenum deformed high-temperature alloys such as N06625, the outer diameter is greater than 200 mm, and the one-time demand is more than 20 tons, which is rare. Most of them are a few tons, or even a few branches, which makes it difficult for hot extrusion, which has a significant advantage in batch production. SUMMARY

[0006] The present application aims to provide a process for producing hollow pipe by hot piercing of wrought superalloy.

[0007] The technical solution of the present application is a process for producing hollow pipe by hot piercing of wrought superalloy, comprising the following steps:

[0008] Step 1: using AOD smelting to cast N06625 alloy ingot, and then remelting and refining by slag resistance heat to obtain refined alloy block;

[0009] Step 2: hot forging the refined alloy block into round pipe blank;

[0010] Step 3: hot piercing the round pipe blank by 2-roll conical cross piercer and inclined bottom heating furnace;

[0011] Step 4: water quenching and deforming after piercing to obtain hollow pipe product.

[0012] In the process for producing hollow pipe by hot piercing of wrought superalloy, in step 3, the hot working deformation of N06625 alloy is between 40% and 70% during hot piercing of the round pipe blank.

[0013] In the process for producing hollow pipe by hot piercing of wrought superalloy, in step 3, the temperature is controlled between 1100 and 1140℃ during hot piercing of the round pipe blank, and the heating speed should be 2-2.5℃ / min.

[0014] In the process for producing hollow pipe by hot piercing of wrought superalloy, in step 3, an inclined bottom continuous heating furnace is used, the first furnace door temperature is 1145℃, the fourth furnace door temperature is 1115℃, and the ninth furnace door temperature is 1065℃; the round pipe blank travels for 40 minutes from the ninth furnace door to the fourth furnace door, and travels for 70 minutes from the fourth furnace door to the first furnace door, and the round pipe blank is kept at the first furnace door for 40 minutes.

[0015] In the process for producing hollow pipe by hot piercing of wrought superalloy, the process of using an inclined bottom continuous heating furnace is first to set the corresponding temperatures of each furnace in the inclined bottom continuous heating furnace, and then to directly put the round pipe blank into the furnace at the position of the ninth furnace door; then to measure the temperature of the two contact parts of the round pipe blank, and when the surface temperature of the round pipe blank reaches 1050℃, to slowly turn the blank and travel, first to turn every 3 minutes by 180°, and to travel for 40 minutes to the fourth furnace door, then to turn every 10 minutes, and to travel for 70 minutes to the first furnace door, and then to keep still for 40 minutes.

[0016] In step 3 of the aforementioned process for producing a hollow pipe by hot piercing of a wrought high-temperature alloy, the 2-roll conical cross-piercing machine is set to have an advancing angle of 16° and a rolling angle of 15°, and the rolling speed is 12 r / min, the elliptical coefficient is set to 1.06, and the top head protruding amount is 206 mm.

[0017] Compared with the prior art, the present application can improve the piercing performance through a reasonable hot piercing process, can produce a hollow pipe meeting the quality requirements, and has simple processing procedures, high material yield, and can be mass-produced. The present application reasonably controls the heating temperature and heating time, and the starting temperature of the ninth furnace of the N06625 alloy can avoid the precipitation temperature range of the secondary phase (γ", δ), especially can reduce the residence time in the sensitive temperature range of precipitation, prevent the alloy center from cracking due to heating, and improve the heating plasticity of the N06625 alloy. When hot piercing the round pipe blank, the present application can ensure the inner surface quality without center drilling through reasonable temperature control and heating speed control. The present application adjusts the 2-roll conical cross-piercing machine to obtain the best piercing process parameters, can increase the bite of the metal during deformation, can quickly and stably and uniformly move, eliminate the hole cavity, and ensure the processing quality. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 Stress-strain curve of the N06625 alloy in the rolled state at 930℃;

[0019] Figure 2 Stress-strain curve of the N06625 alloy in the rolled state at 1100℃;

[0020] Figure 3 Stress-strain curve of the N06625 alloy in the rolled state at 1140℃;

[0021] Figure 4 Stress-strain curve of the N06625 alloy in the rolled state at 1180℃;

[0022] Figure 5 Stress-strain curve of the N06625 alloy in the rolled state at different temperatures at a deformation rate of 10S -1 per minute;

[0023] Figure 6 Stress-strain curve of the N06625 alloy in the rolled state at different temperatures at a deformation rate of 80S -1 per minute. DETAILED DESCRIPTION

[0024] The present application will be further described below in conjunction with the embodiments and the accompanying drawings, but it is not limited as a basis for the present application.

[0025] Embodiment: A process for producing a hollow pipe by hot piercing of a wrought high-temperature alloy, comprising the following steps:

[0026] Step 1, using AOD smelting N06625 alloy ingot, and then through the slag ESR refining, to get the refined alloy ingot.

[0027] Step 2, the refined alloy ingot hot forging into round pipe blank;

[0028] Step 3, through 2 roll conical piercer and inclined bottom heating furnace on the round pipe blank hot piercing;

[0029] Step 4, after piercing water quenching deformation, to get the raw pipe product.

[0030] In step 3, the round pipe blank hot piercing processing, the hot working deformation of N06625 alloy is between 40% and 70%.

[0031] Preferably, in step 3, the round pipe blank hot piercing processing, the hot working deformation of N06625 alloy is between 40% and 70%.

[0032] Preferably, in step 3, the round pipe blank hot piercing temperature is controlled between 1100 and 1140℃, and the heating speed should be 2-2.5℃ / min.

[0033] Preferably, in step 3, the inclined bottom continuous heating furnace is used, the first furnace door temperature is 1145℃, the fourth furnace door temperature is 1115℃, and the ninth furnace door temperature is 1065℃; the round pipe blank travels from the ninth furnace door to the fourth furnace door for 40 minutes, and from the fourth furnace door to the first furnace door for 70 minutes, and the round pipe blank is kept in the first furnace door for 40 minutes. The process of using the inclined bottom continuous heating furnace is to set the corresponding temperature in each furnace of the inclined bottom continuous heating furnace, and then put the round pipe blank directly to the ninth furnace door. Then use the hand-held infrared thermometer to measure the two contact parts of the round pipe blank. When the surface temperature of the round pipe blank reaches 1050℃, slowly turn the blank and move. First, turn every 3 minutes, 180°, and move for 40 minutes to the fourth furnace door. Then turn every 10 minutes, and move for 70 minutes to the first furnace door. Then keep still for 40 minutes.

[0034] Preferably, in step 3, when the 2 roll conical piercer is used for piercing, the forward angle is set to 16°, the rolling angle is set to 15°, the rolling speed is set to 12r / min, the oval coefficient is set to 1.06, and the top head extension is set to 206mm.

[0035] Based on the above process steps, the applicant carries out corresponding tests to explain the preferred scheme of the present application and makes a detailed description.

[0036] 1.1 Test material and specification

[0037] The chemical composition (see Table 1) and production specification (see Table 2) of N06625 alloy material.

[0038]

[0039] Table 1 Chemical composition of N06625 alloy (mass percent)

[0040] Processing mode Hot-forging round billet After peeling Center boring Heating round billet Rough pipe Processing specification Φ320 Φ316 80 Φ316*80 hole Φ368*36

[0041] Table 2 Hot piercing production specification of N06625 alloy (mm)

[0042] Three tests were conducted, one of which was not subjected to center boring, and the centering hole Φ60mm, depth 70mm was a tapered hole. The round billet length was 1800-1980mm.

[0043] 1.2 Test method.

[0044] 1.2.1, The study of hot workability of N06625 alloy involves the stress-strain characteristics of high temperature deformation such as strengthening phase, precipitated phase, aging temperature change and hot deformation range, so the present invention mainly studies the following three parts from the analysis of structure, composition and performance to production practice: (1) the influence of precipitation rate and precipitation amount of precipitated phase of N06625 alloy on hot workability; (2) the change of N06625 alloy structure and performance and its influence on hot piercing and heat treatment process; (3) the influence of piercing technical parameters on the piercing performance of N06625 alloy hot deformation.

[0045] 1.2.2, The hot piercing of round pipe billet adopts 2-roll tapered roll piercing machine, and the heating is carried out in inclined bottom heating furnace, and the setting of 2-roll tapered roll piercing and inclined bottom heating furnace and the process steps are studied.

[0046] 1.2.3, The detection equipment used includes WE-600 hydraulic universal testing machine, JBDW-300C ultra-low temperature impact testing machine. The metallographic sample is etched by 40% sodium hydroxide electrolysis or boiled in 100mL distilled water with 10g potassium ferricyanide and 10g potassium hydroxide, and the metallographic structure is observed and measured by VERTAI Zeiss microscope and multiphase area percentage measurement method.

[0047] 2, The change of N06625 alloy structure and performance and the influence of main chemical elements on hot piercing performance.

[0048] N06625 alloy is a typical solid solution strengthening type nickel-based wrought superalloy, and due to higher alloy content, the strengthening phase and precipitated phase are more sensitive, and the precipitation of these phases has a close relationship with the structure, performance and establishment of hot piercing process.

[0049] 2.1 Changes in the microstructure and properties of N06625 alloy and their relationship to the hot piercing process.

[0050] 2.1.1 Formation of strengthening phases in N06625 alloy.

[0051] The aging response of N06625 alloy is very sensitive to the heating temperature, resulting in a smaller response to direct aging at higher heating temperatures. N06625 alloy will spend a significant amount of time in the nucleation temperature range of 732-843°C if the heating temperature is increased at a slow rate. The time spent in this temperature range will cause a precipitation reaction and allow the subcritical γ" cores to grow rapidly, thus precipitating normal body-centered tetragonal (BCT) γ" particles at 649°C. It is well known that the γ" phase (i.e., the Ni3Nb phase) is a strengthening phase that can precipitate in nickel-based alloys with more than 4 wt% of niobium, which is usually in the form of spheres or discs, precipitates in the grain interior, and has a small size but a strong coherent strain strengthening due to its coherency with the matrix and a large misfit, which makes it stronger than the γ' phase (i.e., the Ni3Al phase), thus making N06625 alloys of this type have a high yield strength. However, when the temperature is higher than 649°C or the amount of time spent in the nucleation temperature range of 732-843°C is reduced, the γ" phase will transform into orthorhombic.

[0052] 2.1.2 Precipitated phases in N06625 alloy.

[0053] (1) Intergranular secondary carbides in N06625 alloy. Secondary intergranular carbides can form in N06625 alloy as the heating temperature increases, mainly two different carbides M 23 C6 and M6C9. Due to their similar face-centered cubic lattice structure with very close lattice parameters, the carbides precipitated between 950-1075°C are in the form of dendritic or rhombic morphology precipitated and focused near the grain boundaries, which reduces the plasticity of 625 alloy and increases the yield strength more than the tensile strength, so that the fracture mode of N06625 alloy during short-time deformation at high temperature changes from plastic fracture to brittle fracture.

[0054] (2) Intermetallic compound precipitation in N06625 alloy. δ-Ni3Nb and γ" phases precipitate in N06625 alloy after long-term aging at temperatures between 650°C and 820°C. The δ phase mainly precipitates in the form of needles near the NbC-rich region often characterized by a large amount of NbC. The δ phase seems to grow by partial dissolution of the NbC particles and subsequent diffusion of Nb along the γ / NbC interface. γ" precipitation is usually inhomogeneous and preferentially occurs along dislocations.

[0055] The precipitated phase of N06625 alloy will seriously damage the mechanical properties at 750-950℃, the elongation and area reduction will decrease due to the precipitation of intergranular carbide, and the toughness will become very poor.

[0056] 2.2, Effect of Mo and Nb in N06625 alloy on hot piercing performance.

[0057] 2.2.1, Effect of Mo.

[0058] Molybdenum is a rare metal with high melting point of 2620℃. It has high strength at room and high temperature due to its strong atomic binding force, but it has small expansion coefficient, large electrical conductivity and good thermal conductivity. Molybdenum improves the strength of the alloy, especially the high temperature strength, and greatly improves the high temperature durability and creep performance. However, with the increase of molybdenum content, the high temperature deformation resistance of the alloy increases, and the hot plasticity decreases. Molybdenum also promotes the formation of δ phase in the alloy. These factors can lead to the decrease of hot working and hot forming properties of the alloy.

[0059] 2.2.2, Effect of Nb.

[0060] Niobium mainly plays a strengthening role in N06625 alloy, which can improve the room temperature and high temperature strength of the alloy. When the niobium content is less than 2%, the performance change is small, and when the niobium content is more than 3%, the yield strength of high temperature aging increases greatly. Moreover, it has an interactive effect with molybdenum during high temperature aging, which increases the response characteristics of aging hardening, and gathers the matrix composition of Ni3Nb to precipitate the metastable ordered γ" phase and δ phase. Therefore, when the niobium content is constant, the hot plasticity of the alloy will decrease with the increase of temperature.

[0061] 2.3, Effect of stress and strain rate of N06625 alloy in high temperature heating state on hot working process.

[0062] 2.3.1, Stress and strain of N06625 alloy at different temperatures between 930-1180℃ were analyzed by hot simulation test. The material was cut into Ф10x15mm cylinder, the heating temperature in deformation condition was 930℃, 1100℃, 1140℃, 1180℃, the strain rate was 10S -1 and 80S -1 , the deformation amount was 10%, 20%, 40%, 70%, the heating rate was 5℃ / s, the holding time was 5min, and the cooling method was air cooling. Figure 1 Stress and strain curve of N06625 alloy in rolling state at 930℃; Figure 2 Stress and strain curve of N06625 alloy in rolling state at 1100℃;Figure 3 Stress-strain curve of N06625 alloy at 1140℃ in rolling state; Figure 4 Stress-strain curve of N06625 alloy at 1180℃ in rolling state; Figure 5 Stress-strain curves of N06625 alloy at different temperatures in rolling state at 10S -1 rate; Figure 6 Stress-strain curves of N06625 alloy at different temperatures in rolling state at 80S -1 rate.

[0063] Based on Figures 1-6 , the results show that the deformation temperature of N06625 alloy should be higher than 1100℃. If the temperature is lower than 1100℃, the deformation resistance of N06625 alloy is higher, and the softening mechanism is not easy to start, which is easy to cause uneven structure and easy to crack. From the perspective of reducing deformation resistance, the higher the temperature is, the better it is, but too high temperature will lead to grain coarsening, which is also not conducive to hot working. For large deformation, it can be considered to appropriately reduce the deformation rate, which is conducive to reducing the deformation resistance and achieving steady flow, and steady flow is more conducive to uniform deformation and obtaining uniform structure control. Only when the structure tends to be uniform, the deformation resistance is reduced, and stable dislocation is obtained during the deformation of N06625 alloy, which prevents brittle cracking caused by high temperature strength. For small deformation, high speed deformation is more conducive to structure control, that is, recrystallization can occur under small strain. But high speed deformation is easy to cause short sawtooth rheology, that is, uneven deformation, so the deformation amount should not be less than 40%. The hot working deformation amount of N06625 alloy in the present application should be between 40% and 70%.

[0064] 2.4, Influence of recrystallization of N06625 alloy.

[0065] By analyzing the structure of N06625 alloy after high temperature strain under optical microscope and scanning electron microscope (SEM), it can be found that the structure of N06625 alloy deformed at 1030℃ is dynamic recovery and partial dynamic recrystallization, the structure of N06625 alloy deformed at 1080℃ is a small amount of dynamic recovery and most of dynamic recrystallization, the structure of N06625 alloy deformed at 1150℃ is complete dynamic recrystallization with grain of about 17μm, and the structure of N06625 alloy deformed at 1220℃ is complete dynamic recrystallization with grain of about 25μm.

[0066] The dynamic recrystallization has the characteristics of concave-convex grain boundary, large angle grain boundary between grains and some dislocations remaining in the grains. There are a large number of chain precipitates (brittle phase) in the grain boundary and in the grain of N06625 alloy below 1150℃. These precipitates gradually decrease with the increase of temperature. These chain carbides seriously affect the hot ductility of N06625 alloy. However, with the further increase of temperature, the reduction of area begins to decrease. The reason is that the recrystallized grains gradually grow with the increase of deformation temperature, which seriously affects the hot ductility. In addition, the deformation causes the alloy to heat up, which sharply reduces the plasticity. Therefore, 1150-1220℃ is the brittle fracture temperature range of N06625 alloy.

[0067] 2.5. Based on the above analysis, the changes of the microstructure and properties of N06625 alloy during heating and the effects on hot working process mainly have four aspects.

[0068] (1) The strengthening phase γ" of N06625 alloy is generated at a temperature of 732-843℃. If the alloy is slowly heated, the precipitation and deposition of the strengthening phase will occur. Since the deposition phase of N06625 alloy is mostly at 750-950℃, the mechanical properties of the alloy will be seriously damaged due to the precipitation of intergranular carbides, which reduces the elongation and area reduction, and also makes the toughness very poor.

[0069] (2) Based on the analysis of stress-strain curves of N06625 alloy at different temperatures and different deformation amounts, it is concluded that the deformation temperature of N06625 alloy should be higher than 1100℃. If the deformation temperature is lower than this temperature, the deformation resistance is higher, which is not conducive to hot working deformation. From the analysis of the curve, the higher the temperature, the more conducive to deformation, but the grains will grow, which also affects the hot working plasticity of N06625 alloy and causes cracking. High-speed deformation and relatively increased deformation amount are good for controlling the hot working process of N06625 alloy.

[0070] (3) The recrystallization temperature of N06625 alloy should be from 1030℃, and the grains begin to grow at 1050℃. The deformation plasticity of the alloy begins to increase when the temperature is higher than the recrystallization temperature. 1150℃ is a node because the reduction of area begins to decrease with the increase of temperature. Not only the grains become larger, but also the deformation causes the alloy to heat up, which sharply reduces the plasticity, increasing the risk of brittle fracture of N06625 alloy.

[0071] (4) Although the content of Mo and Nb in N06625 alloy is high, the high temperature deformation resistance of the alloy increases, the hot plasticity decreases, and molybdenum promotes the formation of delta phase in the alloy. These factors can all lead to the decrease of hot working and hot forming property of the alloy. However, with the increase of Mo content, the thermal conductivity increases, and especially with the increase of temperature, the thermal conductivity increases linearly, and the thermal conductivity rate will increase by multiple.

[0072] Conclusion: The heating process temperature of N06625 alloy is relatively narrow, and is suitable for the temperature range of 1100-1140℃, the deformation amount is 40-70%, and the high-speed heating is easy to improve the piercing performance.

[0073] 3. Formulation of hot piercing process of N06625 alloy.

[0074] 3.1. Technical performance of inclined bottom heating furnace and heating process of N06625 alloy.

[0075] 3.1.1. The heating furnace used in the present application is an inclined bottom heating furnace, and the technical performance is shown in Table 3. The furnace is in the form of end feeding and side discharging, has good heat preservation effect, and the flame is automatically controlled. The advancing mode of the pipe blank is manual tilting, which is easy to control the heating time and heating speed.

[0076]

[0077] Table 3

[0078] Note: 1. The distance between the tilting doors is 1.5M. 2. The tilting doors are numbered as 1, 2, 3, 4, 5, ··· 14 from the tapping hole.

[0079] 3.1.2. The heating process of N06625 alloy is shown in Table 4.

[0080]

[0081] Table 4 Heating process of N06625 alloy

[0082] The formulation of the heating process is mainly based on the deformation difficulty of N06625 alloy, and highlights the characteristics of fast heating speed, moderate temperature easy to control, uniform holding time, etc. The technical points during heating are as follows:

[0083] (1) After the temperature in the furnace is raised to the control temperature of each point in Table 4, the display screen temperature is directly put into the 9# door at 1065℃ position;

[0084] (2) with hand-held infrared thermometer side pipe blank two contact parts, when the pipe blank surface temperature reaches 1050℃, slowly turn the blank to move. First, every 3 minutes turn once, 180° each time, 40 minutes to No. 4 door, then turn every 10 minutes, 70 minutes to 1# door, then static heat preservation for 40 minutes;

[0085] (3) with the first branch pipe blank as the measurement reference, the following pipe blank follows. The first branch pipe blank is heated for a certain period of time, and then it is immediately turned to the discharge chute for discharging. The following pipe blank needs to be turned to the front branch position for heat preservation after the front branch is perforated, and then it is turned to the discharge chute for discharging. Each branch is discharged in turn in this way;

[0086] (4) High-speed heating and rapid deformation are the principles of hot working of N06625 alloy, which is to avoid grain coarsening, brittle phase precipitation and other factors affecting hot plasticity during hot working of the alloy. Therefore, the heating time from when the pipe blank surface temperature reaches 1050℃ to the discharge temperature 1130℃ should not be greater than 2.5h, which is also a technical condition for ensuring the perforation performance of N06625 alloy and the mechanical properties of N6625 alloy hollow pipe.

[0087] 3.2, adjustment of technical parameters of perforation and rolling rhythm.

[0088] The present application adopts a large-diameter conical roller cross-rolling perforating machine set to perform hot deformation of N06625 alloy. The maximum diameter of the roller is 1800mm, and the linear speed is much smaller than the cone head, which can help the blank to bite in. In the cross-rolling process, a hollow elliptical pass is formed by two rollers, upper and lower guides, and an axial top head. The blank is always rolled and deformed in an elliptical state, and the plastic deformation continuously penetrates from the surface to the center of the blank. Finally, greater deformation occurs in the center of the blank. At this time, the stress state of the center of the blank is generally two-way (axial and transverse) stretching and one-way compression. The stress state produced by the top head is two-way compression and one-way (transverse) stretching. Under the action of this complex alternating stress, the blank forms a hole cavity. At this time, due to the effect of alternating shear stress and large tensile stress on the center of the metal. The center rupture belongs to ductile-brittle fracture. When N06625 alloy overcomes the brittle phase that may be brought about during the heating process and makes the alloy inside tend to break, it is required that N06625 alloy avoid the appearance of hole cavity during hot deformation for the main purpose.

[0089] 3.2.1, adjustment of technical parameters for eliminating hole cavity.

[0090] (1) According to the modern metallographic view, any destruction form (including brittle destruction) is related to plastic flow, i.e. there is no pure brittle destruction of metal. Before plastic deformation occurs, the formation of micro-cracks and defects must be accompanied. The formation of cavities (micro-cracks) is the "forging effect" of two-directional compression stress and one-directional transverse tensile stress generated by the roll acting on the center part of the metal before the punch contacts the metal, and the ductile-brittle fracture.

[0091] (2) The biggest feature of the conical roll piercer is that it has one more rolling angle than the Mannesmann piercer. The angle between the roll axis and the rolling center line in the horizontal plane is the forward angle, also called the feed angle. The Mannesmann piercer has this angle, and the angle between the roll axis and the rolling center line in the vertical plane is the rolling angle. The conical roll has a conical bottom in the outlet direction, so the linear speed of the roll gradually increases from the inlet to the outlet. Such roll movement speed and metal movement speed are better coordinated, thereby reducing the axial slip and in turn reducing the circumferential additional shear stress (forging effect) generated. As is known, the stress and strain from the transverse direction are the starting point of the forging effect, and the influence range of such shear strain is more controlled by the tool shape and the rolling mill adjustment value. The conical roll has determined the advantage of the tool, and the adjustment of the rolling mill is particularly important.

[0092] (3) The present invention refers to the previous technical parameters of duplex stainless steel, which is also a high alloy steel difficult to deform. In production practice, when the elongation coefficient is greater than 2.0, we use a small forward angle (10°) and a large rolling angle (15°). Such adjustment is to reduce the transverse tensile stress and stabilize the biting of the blank without generating cavities at large deformation. Since the deformation difficulty of N06625 alloy is much greater than that of duplex stainless steel, it is not enough to consider only the reduction of transverse tensile stress. It is necessary to consider the rate of shear strain to prevent the increase of dislocation caused by slip and the ductile-brittle crack of N06625 alloy at high-speed deformation. Therefore, when the deformation of N06625 alloy is only 48% and the elongation coefficient is 1.96, a smaller rolling deformation, a forward angle of 16° and a rolling angle of 15° are selected. Practice has proved that the double large angle piercing can make the metal move stably and uniformly without cavities when deforming.

[0093] (4) The oval coefficient is set to 1.06, and the punch extension is 206 mm.

[0094] 3.2.2, Control of rolling rhythm.

[0095] The rolling rhythm is automatically controlled in an automatic system, and a rotation speed of 12 r / min is adopted, and one branch is outputted every 3 minutes. Mainly considering the cooling time of the molybdenum top head. The as-cast molybdenum-based alloy does not have a recrystallization temperature limit, and is suitable for high-temperature continuous operation. However, the top head is best used at 850-950 DEG C, which not only has sufficient strength but also can reduce the internal defects of the metal.

[0096] 3.3, the inspection result of the hollow pipe after piercing.

[0097] The hollow pipe is quenched after completing piercing, and the first two branches have fine hairline cracks at about 250 mm of the inner wall of the head, which is eliminated after polishing. The latter branch without a bore (center drilling) does not have a crack defect. Generally, the center drilling of such a difficult-to-deform high-nickel high-molybdenum alloy round pipe blank is easy to perform, and the inner surface quality of the hollow pipe after piercing is better. The abnormal phenomenon is analyzed as follows: after the round pipe blank is discharged, it passes through the track to reach the piercer, and the center inner surface temperature drops sharply due to the production temperature stress. After entering the piercer, the round pipe blank spirally advances on one side and is repeatedly crushed in the deformation zone. The center part is subjected to alternating shear stress and transverse tensile stress, and a hole cavity is formed. Then, the inner surface of the hole cavity is subjected to stress to produce cracks.

[0098] Through the above analysis experiment and summary, the problems solved by the present application and the effects brought by the implemented technical solutions are as follows:

[0099] (1) The strengthening phase and the precipitated phase of the N06625 alloy are precipitated before 920 DEG C, which seriously damages the mechanical properties of the alloy. However, if the residence in this temperature range is avoided and direct high-speed heating is adopted, better plasticity can be obtained. Therefore, the initial temperature of the N06625 alloy in the furnace is 1065 DEG C, which can avoid the precipitation temperature range of the secondary phase (γ" and δ), especially can reduce the residence time in the sensitive temperature range of precipitation, and prevent the alloy center from cracking due to heating.

[0100] (2) An effective method for improving the heating plasticity of the N06625 alloy and preventing the center from cracking is to reasonably control the Mo and Nb contents, wherein Mo≤9% and Nb≤3.7%; reasonably control the heating speed (the heating speed should be 2-2.5 DEG C / min), and as far as possible, the temperature range is 850-1050 DEG C, and the time should not exceed 60 minutes; and the finish rolling surface temperature is strictly controlled to be above the recrystallization temperature (1050 DEG C).

[0101] (3) The hot piercing temperature of the N06625 alloy round pipe blank can be controlled to be between 1125 DEG C and 1140 DEG C, and the holding time can be set to be 20-50 minutes according to the diameter of the pipe blank. The round pipe blank with a diameter of ≤320 mm can not be center drilled, and the inner surface quality can also be guaranteed.

[0102] (4) The rolling mill is adjusted to adapt to a reasonable heating temperature to roll out qualified products. The main feature of the N06625 alloy is that the high-temperature deformation strength is large, the plastic resistance is strong, and the surface is easy to deform to generate slip, which will lead to the change of strain rate and the large dislocation of Y, so as to form a cavity in the center of the alloy. The double large-angle (forward angle 16°, rolling angle 15°) is the condition for the alloy to quickly bite and move uniformly to stabilize the deformation.

[0103] In summary, the present application can improve the piercing performance through a reasonable hot piercing process, can produce a qualified pipe, and has simple processing procedures, high yield and batch production.

Claims

1. A process for the production of a hollow billet by hot piercing of a wrought superalloy, characterized in that: It comprises the following steps: Step 1, using AOD smelting to mold N06625 alloy ingot, and then remelting and refining by slag resistance heat to obtain refined alloy ingot; Step 2, hot forging the refined alloy ingot into round pipe blank; Step 3, hot piercing the round pipe blank by 2-roll conical piercer and inclined bottom continuous heating furnace; The temperature control of the round pipe blank hot piercing is between 1100-1140℃, and the heating speed should be 2-2.5℃ / min; Using inclined bottom continuous heating furnace, the first furnace door temperature is 1145℃, the fourth furnace door temperature is 1115℃, and the ninth furnace door temperature is 1065℃; the round pipe blank travels for 40 minutes from the ninth furnace door to the fourth furnace door, and travels for 70 minutes from the fourth furnace door to the first furnace door, and the round pipe blank is kept in the first furnace door for 40 minutes; Step 4, after piercing, water quenching and deformation to obtain the raw pipe product.

2. The process of claim 1 wherein: In step 3, the hot working deformation of N06625 alloy is between 40%-70% when the round pipe blank is hot pierced.

3. The process of claim 1 wherein: The process of using inclined bottom continuous heating furnace is to heat the temperature of each furnace of the inclined bottom continuous heating furnace to the corresponding temperature, and then put the round pipe blank into the furnace directly to the position of the ninth furnace door; then use a handheld infrared temperature measuring instrument to measure the two contact parts of the round pipe blank, when the surface temperature of the round pipe blank reaches 1050℃, slowly turn the blank and travel, first turn every 3 minutes, 180° each time, travel to the fourth furnace door for 40 minutes, then turn every 10 minutes, travel to the first furnace door for 70 minutes, and then keep still for 40 minutes.

4. The process of claim 1 wherein: In step 3, when the 2-roll conical piercer pierces, the forward angle is set to 16°, the rolling angle is set to 15°, the rolling speed is set to 12r / min, the elliptical coefficient is set to 1.06, and the top head extension is set to 206mm.

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