Method for refining ferritic heat-resistant steel grains through reciprocating isothermal annealing

Abstract

The invention relates to a method for refining ferritic heat-resistant steel grains through reciprocating isothermal annealing, belongs to the technical field of heat treatment of forgings, and solves the problems that the grains generated by the existing heat treatment process after forging are coarse, and the cracking risk of large forgings is easily caused by thermal stress and structural stress. The method comprises the following steps that 1, after being forged, the large forge piece is put into a furnace for heat preservation; 2, heating and austenitizing the forged piece subjected to heat preservation in the step 1, and preserving heat at the temperature; and step 3, cooling the forging austenitized in the step 2 to an annealing temperature, carrying out reciprocating isothermal annealing, cooling and discharging.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology for forgings, and more particularly to a method for reciprocating isothermal annealing to refine the grains of ferritic heat-resistant steel. Background Technology

[0002] With the rapid development of the energy industry, the service temperature of alloy materials is getting higher and higher, which puts forward higher requirements for materials to meet the harsh environments such as high temperature, high pressure and radiation. 9-12% Cr ferritic heat-resistant steel has a low coefficient of thermal expansion, low radiation expansion and high thermal conductivity. It has excellent resistance to thermal fatigue, thermal creep and high temperature oxidation, and is widely used in petrochemical, thermal power generation and nuclear industries.

[0003] 9%Cr-1%Mo (T91 and P91) heat-resistant steel forgings exhibit good hardenability. After holding at temperatures above the austenitizing temperature for a period, air cooling yields a martensitic structure. The conventional performance heat treatment process involves normalizing followed by tempering, ultimately resulting in a martensitic tempered structure. The addition of trace amounts of Nb and V further enhances the strength and creep resistance of P91 forgings. However, the significant addition of alloying elements results in severe microstructural inheritance in P91 ferritic heat-resistant steel. If the coarse grains obtained after forging heating are not effectively refined through post-forging heat treatment, reheating for austenitization during performance heat treatment will again produce coarse grains, which will remain in the final microstructure. The presence of coarse grains or mixed grains frequently leads to unsatisfactory forging properties, requiring re-heat treatment, increasing production costs, and significantly impacting production efficiency.

[0004] For ordinary alloy forgings, the commonly used post-forging heat treatment process is normalizing or annealing. Depending on the wall thickness of the forging, one or more normalizing or annealing processes can achieve a finer grain size. However, P91 ferritic heat-resistant steel has a severe hereditary microstructure, and conventional multiple normalizing or annealing heat treatments do not achieve good grain refinement and are prone to mixed grain formation. Repeated high-temperature heating and cooling not only wastes energy, but the resulting thermal and structural stresses can easily lead to cracking risks in large forgings. Furthermore, the method of rapidly heating in the ferrite / austenite two-phase region to obtain spheroidal austenite grain refinement requires a large heating rate, which is also unsuitable for the industrial production of large forgings. Summary of the Invention

[0005] In view of the above analysis, the present invention aims to provide a method for reciprocating isothermal annealing to refine the grains of ferritic heat-resistant steel, which can at least solve one of the following technical problems: (1) The thermal stress and structural stress generated by the existing post-forging heat treatment process can easily lead to cracking risk in large forgings; (2) The existing method of obtaining spherical austenite grains by rapid heating in the ferrite / austenite two-phase region requires a heating rate of ≥200℃ / h, which cannot be achieved in the industrial production of large forgings, and therefore is not suitable for the industrial production of large forgings.

[0006] The objective of this invention is mainly achieved through the following technical solutions:

[0007] This invention provides a method for refining ferritic heat-resistant steel grains through reciprocating isothermal annealing, comprising the following steps:

[0008] Step 1: After the large forging is completed, place it in the furnace for heat preservation;

[0009] Step 2: Heat the forging after holding in Step 1 to the austenitizing temperature and hold at that temperature;

[0010] Step 3: Cool the austenitized forgings from Step 2 to the annealing temperature, perform reciprocating isothermal annealing, and then cool them out of the furnace.

[0011] Optionally, in step 3, the annealing temperature is 650–770°C.

[0012] Optionally, in step 3, the annealing is performed more than twice.

[0013] Optionally, in step 3, the annealing is performed 4-6 times.

[0014] Optionally, the temperature for even-numbered annealing is higher than the temperature for odd-numbered annealing.

[0015] Optionally, the temperature for odd-numbered annealing is 650–670℃, and the temperature for even-numbered annealing is 750–770℃.

[0016] Optionally, in step 2, the austenitizing temperature is 1030–1070 °C.

[0017] Optionally, step 3 is followed by step 4: normalizing and tempering the forgings processed in step 3.

[0018] Optionally, the normalizing temperature is 1030–1060℃ and the tempering temperature is 740–770℃.

[0019] Optionally, the large heat-resistant steel forging is a 9%Cr-1%Mo ferritic heat-resistant steel forging.

[0020] The present invention also provides a large heat-resistant steel forging, which is obtained by the above-described method. The large heat-resistant steel forging has a mixed structure of ferrite and alloy carbides, with fine and uniformly distributed ferrite structure and a grain size of less than 30 μm.

[0021] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0022] (1) For large forgings, this invention performs reciprocating isothermal annealing within a specific temperature range (650–770°C) and controls the cooling rate of the forging after post-forging heat treatment, so that the alloy forging is completely transformed into a mixed structure of fine and uniformly distributed ferrite and carbides, breaking the microstructure inheritance and refining the original coarse microstructure. Compared with the traditional method of multiple normalizing and annealing after forging heat treatment, the grain refinement effect is more significant, and the reciprocating isothermal annealing avoids multiple high-temperature austenitizing heating and cooling, reducing the tendency of forgings to deform and crack during heat treatment. Based on the post-forging heat treatment process, conventional normalizing and tempering performance heat treatment (normalizing temperature 1030~1060℃, tempering temperature 740~770℃) is performed to obtain tempered martensite structure with a grain size of about 7. The room temperature tensile strength of the forging is ≥690MPa, for example, 692-711MPa; yield strength is ≥510MPa, for example, 511-530MPa; elongation after fracture is ≥25%, for example, 25%-26.5%; and reduction of area is >70%, for example, 71%. It can meet the grain size requirements while exhibiting excellent performance.

[0023] (2) This invention can not only refine alloy grains, but also reduce the tendency of forgings to crack during heat treatment, meet the final performance requirements, and provide a reference for the industrial production of large ferritic heat-resistant steel forgings.

[0024] (3) The present invention adopts multiple reciprocating isothermal annealing, which is simple to operate and highly feasible. It can ensure that the ferrite grains do not coarsen while minimizing the isothermal annealing time.

[0025] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages will become apparent from the description or may be learned by practicing the invention. Attached Figure Description

[0026] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0027] Figure 1(a) is a flow chart of the heat treatment process of the present invention;

[0028] Figure 1(b) is a process flow diagram of the heat treatment process of Embodiment 1 of the present invention;

[0029] Figure 2 Metallographic photographs of Example 1 after reciprocating isothermal annealing;

[0030] Figure 3 Metallographic photographs of the heat-treated components of Example 1;

[0031] Figure 4The metallographic photograph is for Comparative Example 1;

[0032] Figure 5 This is a metallographic photograph of Comparative Example 2. Detailed Implementation

[0033] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0034] To address the problem of frequent grain inconsistencies in large forgings of 9%Cr-1%Mo ferritic heat-resistant steel due to microstructure inheritance, this invention provides a method for reciprocating isothermal annealing to refine the grains of 9%Cr-1%Mo ferritic heat-resistant steel. This heat treatment method ensures that the ferritic heat-resistant steel exhibits excellent grain size distribution after heat treatment. Specifically, the method includes the following steps:

[0035] Step 1: After forging the large forgings of 9%Cr-1%Mo ferritic heat-resistant steel, place them in the furnace for heat preservation.

[0036] Step 2: Heat the forging after holding in Step 1 to the austenitizing temperature and hold at that temperature;

[0037] Step 3: Cool the austenitized forgings from Step 2 to the annealing temperature, perform reciprocating isothermal annealing, and then cool them out of the furnace.

[0038] The large forgings in this invention refer to forgings with a diameter of 1000-2000 mm, a wall thickness of 200-300 mm, and a mass of ton or more.

[0039] In step 1, the forging still has residual heat after forging. At this time, the residual heat of the forging can be used to keep it in the furnace, which can prevent the large forging from undergoing martensitic phase transformation during the cooling process due to excessively low temperature. At this time, the forging is in an unstable state, and the large structural stress can easily cause the forging to crack. At the same time, the residual heat can also reduce the energy consumption when reheating.

[0040] Because large forgings have thick walls, holding them at this temperature reduces temperature differences between different parts of the forging. This prevents the situation where, when heated to 1030–1070°C, the surface temperature has reached the point where austenitization can occur, but the core of the forging is far from reaching that point, resulting in a large temperature difference between the inside and outside, which would affect the microstructure change, i.e., the austenitization effect. In step 1, the purpose of holding the temperature is to ensure uniform temperature inside and outside the forging, reduce the temperature difference between the surface and the core, and allow the austenitization transformation to occur completely and the composition to become homogenized in a shorter time. This ensures that the forging not only meets the grain size requirements but also has excellent performance.

[0041] Specifically, the insulation temperature is 600–650℃, for example, 600℃, 610℃, 620℃, 630℃, 640℃, and 650℃. The insulation time is 3–7 hours, for example, 3 hours, 4 hours, 5 hours, 6 hours, and 7 hours.

[0042] In step 2, the heating rate is ≤100℃ / h, for example, 100℃ / h, 90℃ / h, 80℃ / h, 70℃ / h, 60℃ / h, or 50℃ / h. Heat has a conduction process; if the heating is too rapid, the surface temperature of the forging will reach the required level, but the core will not. The lower heating rate (≤100℃ / h) of this invention serves to ensure uniform temperature throughout the forging, reduce the temperature difference between the surface and core, and allow for complete austenite transformation and composition homogenization within a shorter time. This ensures that the forging not only meets the grain size requirements but also exhibits excellent performance.

[0043] The austenitizing temperature is 1030–1070℃, for example, 1030℃, 1040℃, 1050℃, 1060℃, and 1070℃. The holding time is calculated at 1 hour per 50 mm, meaning 1 hour of holding time is applied for every 50 mm of wall thickness. This invention controls the austenitizing temperature to 1030–1070℃ and calculates the holding time at 1 hour per 50 mm, ensuring complete austenitization of the forging and uniform distribution of alloying elements.

[0044] In step 3, the cooling rate is ≤50℃ / h, for example, 50℃ / h, 40℃ / h, 30℃ / h, 20℃ / h, 10℃ / h, 5℃ / h. If the cooling rate is too fast, the forging will generate large internal stress. The annealing temperature is 650~770℃, for example, 650℃, 660℃, 680℃, 700℃, 720℃, 740℃, 760℃, 770℃. The number of annealing cycles is 2 or more, for example, 2, 3, 4, 5, 6, 7, 8 times. Preferably, it is 4-6 times. Furthermore, the temperature for odd-numbered annealing cycles is different from that for even-numbered annealing cycles. Specifically, the temperature for odd-numbered annealing cycles is 650~670℃, for example, 650℃, 655℃, 660℃, 665℃, 670℃. The temperature for even-numbered annealing cycles is 750–770℃, for example, 750℃, 755℃, 760℃, 765℃, and 770℃. Furthermore, the temperature for even-numbered annealing cycles is 80–120℃ higher than that for odd-numbered annealing cycles, for example, 80℃, 90℃, 100℃, 110℃, and 120℃. In addition, the heating or cooling rate between adjacent annealing cycles is ≤50℃ / h, for example, 50℃ / h, 40℃ / h, 30℃ / h, 20℃ / h, 10℃ / h, and 5℃ / h.

[0045] The annealing time for each annealing is 10-20 hours, for example, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, 19 hours, and 20 hours.

[0046] In step 3, the furnace is cooled to below 300°C and then air-cooled. The cooling rate is ≤30°C / h, for example, 30°C / h, 20°C / h, 10°C / h, or 5°C / h.

[0047] This invention achieves a complete microstructural transformation in forgings through multiple isothermal annealing processes at 650–770℃, converting the austenitic structure into a mixed ferrite and carbide structure. This avoids both excessively low isothermal annealing temperatures and prolonged isothermal transformation times, as well as excessively high isothermal temperatures that lead to coarsening of the microstructure and severe carbide segregation along grain boundaries after the isothermal transformation. Large forgings are cooled to below 300℃ at a rate of ≤30℃ / h before being air-cooled. During this slow cooling process, the austenitic to ferrite transformation still occurs, ensuring a complete microstructural transformation and preparing the forging for subsequent performance heat treatment. Simultaneously, the slow cooling rate also reduces internal stress in the forging.

[0048] Forgings treated by the heat treatment method of this invention have a mixed microstructure of ferrite and alloy carbides. The ferrite microstructure is fine and uniformly distributed, with a grain size of less than 30 μm (see...). Figure 2 Based on this post-forging heat treatment, large forgings are subsequently subjected to conventional normalizing and tempering performance heat treatment (normalizing temperature is 1030–1060℃, for example, 1030℃, 1040℃, 1050℃, 1060℃; tempering temperature is 740–770℃, for example, 740℃, 750℃, 760℃, 770℃). This can achieve excellent performance while meeting grain size requirements.

[0049] The present invention will be described in detail below with reference to specific embodiments. Unless otherwise specified, the mechanical property testing methods are conventional methods. The forgings used in the following embodiments have a maximum wall thickness of approximately 300 mm.

[0050] Example 1

[0051] First, after forging, the large forgings of 9%Cr-1%Mo ferritic heat-resistant steel are immediately placed in the furnace and held at 600℃ for 7 hours using residual heat. Then, the temperature is increased to 1030℃ at a rate of 100℃ / h and held for 6 hours to fully austenitize the forgings and ensure uniform composition distribution. Finally, the forgings are subjected to reciprocating isothermal annealing to obtain an equilibrium microstructure of ferrite and carbides, breaking the microstructure inheritance and refining the original coarse austenite grains. The specific process is as follows: after austenitizing heating and holding, the forgings are furnace cooled at 50℃ / h to 650℃ and held for 20 hours; then the temperature is increased at 50℃ / h to 750℃ and held for 20 hours; then the temperature is decreased at 50℃ / h to 650℃ and held for 20 hours; then the temperature is increased again at 50℃ / h to 750℃ and held for 20 hours; finally, the temperature is slowly cooled to 290℃ at a rate of 30℃ / h before being removed from the furnace and air-cooled. After rough machining, the forgings underwent conventional heat treatment: normalizing at 1050℃, holding for 6 hours followed by water cooling; tempering at 760℃, holding for 9 hours followed by air cooling. The room temperature mechanical properties of the forgings were tested, and the results are shown in Table 1.

[0052] Example 2

[0053] First, after forging, the large forgings of 9%Cr-1%Mo ferritic heat-resistant steel are immediately placed in the furnace and held at 650℃ for 3 hours using residual heat. Then, the temperature is increased to 1070℃ at 80℃ / h and held for 6 hours to fully austenitize the forgings and ensure uniform composition distribution. Finally, the forgings are subjected to reciprocating isothermal annealing to obtain an equilibrium microstructure of ferrite and carbides, breaking the microstructure inheritance and refining the original coarse austenite grains. The specific process is as follows: after austenitizing heating and holding, the forgings are furnace cooled at 40℃ / h to 670℃ and held for 10 hours; then, the temperature is increased to 770℃ at 40℃ / h and held for 10 hours; next, the temperature is decreased to 670℃ at 40℃ / h and held for 10 hours; then, the temperature is increased to 770℃ at 40℃ / h and held for 10 hours; finally, the temperature is slowly cooled to 280℃ at 20℃ / h before being removed from the furnace and air-cooled. After rough machining, the forgings underwent conventional heat treatment: normalizing at 1050℃, holding for 6 hours followed by water cooling; tempering at 760℃, holding for 9 hours followed by air cooling. The room temperature mechanical properties of the forgings were tested, and the results are shown in Table 1.

[0054] Example 3

[0055] First, after the large forging of 9%Cr-1%Mo ferritic heat-resistant steel is completed, it is immediately placed in the furnace for heat preservation using residual heat. The heat preservation temperature is 630℃ and the holding time is 5 hours. Then, the temperature is increased to 1050℃ at a rate of 90℃ / h and held for 6 hours to ensure complete austenitization of the forging and uniform distribution of composition. Finally, the forgings are subjected to reciprocating isothermal annealing to obtain an equilibrium microstructure of ferrite and carbides, breaking the microstructure inheritance and refining the original coarse austenite grains. The specific process involves austenitizing heating and holding at a certain temperature, followed by furnace cooling at 30℃ / h to 660℃ and holding for 15h, then heating at 30℃ / h to 760℃ and holding for 15h, followed by cooling at 40℃ / h to 660℃ and holding for 15h, then heating again at 30℃ / h to 760℃ and holding for 15h, then heating again at 30℃ / h to 760℃ and holding for 15h, finally slow cooling at 30℃ / h to below 260℃ and air cooling. After rough machining, the forgings undergo conventional performance heat treatment: normalizing at 1050℃, holding for 6h and water cooling, tempering at 760℃, holding for 9h and air cooling. The mechanical properties of the forgings were tested at room temperature, and the results are shown in Table 1.

[0056] Comparative Example 1

[0057] This comparative example uses the existing process of multiple normalizing of alloy forgings after forging. Specifically, it employs a method of multiple austenitization recrystallization to perform two normalizing processes on large forgings. The process involves slowly heating the forgings at ≤50℃ / h to 700~750℃ and holding for 5 hours, then heating them at ≤100℃ / h to 1050℃ and holding for 6 hours before removing them from the furnace and air cooling them to room temperature. The process is repeated by slowly heating the forgings at ≤50℃ / h to 700~750℃ and holding for 5 hours, then heating them at ≤100℃ / h to 1030℃ and holding for 6 hours before removing them from the furnace and air cooling them.

[0058] Comparative Example 2

[0059] This comparative example adopts the existing process of multiple annealing of alloy forgings after forging. Specifically, it adopts the method of multiple austenitization recrystallization, appropriately controls the cooling rate, and uses a two-stage annealing process for large forgings. That is, the temperature is slowly raised to 700-750℃ at ≤50℃ / h and held for 5h, then raised to 1050℃ at ≤100℃ / h and held for 6h, and then cooled in the furnace to 300℃ and air-cooled to room temperature. The temperature is then slowly raised to 700-750℃ at ≤50℃ / h and held for 5h, then raised to 1030℃ at ≤100℃ / h and held for 6h, and then cooled in the furnace to 300℃ and air-cooled.

[0060] Figure 2 The image shows a metallographic photograph of the product after reciprocating isothermal annealing in Example 1. Figure 2As can be seen, Example 1 ultimately yielded a mixed microstructure of ferrite and carbides, with fine and uniformly distributed ferrite. Based on this post-forging heat treatment process, conventional performance heat treatment was performed, namely normalizing + tempering (1050℃ for 6 hours water cooling, 760℃ for 9 hours air cooling), resulting in tempered martensite with a grain size of approximately level 7 (see...). Figure 3 This fully meets the requirement that large forgings must have a grain size grade of 5 or higher. The metallographic photographs of the forgings in Examples 2 and 3 are similar to those of the forging in Example 1, with a grain size of around 7.

[0061] Figure 4 This is a metallographic photograph of Comparative Example 1. (By...) Figure 4 It can be seen that the grain size of the large forgings in Comparative Example 1 after two normalizing processes is grade 8-9, and a small number are grade 4-5. Although the forgings are significantly refined, mixed grains appear, which will affect the final performance of the forgings. Figure 5 This is a metallographic photograph of Comparative Example 2. (By...) Figure 5 It can be seen that the final grain size of the large forgings in Comparative Example 2 is 1.5 to 2.5 grade, which still maintains the coarse austenite grain size obtained after forging and heating. Only fine grains of grade 9.0 are distributed at the original austenite grain boundaries. The mixed grain phenomenon of the forgings is very serious. The two annealing processes failed to cut off the inheritance of alloy structure, and the grain refinement effect was very poor.

[0062] By comparing the reciprocating isothermal annealing process of the present invention with the traditional multiple normalizing and annealing processes, it can be found that reciprocating isothermal annealing can effectively refine the grains of 9%Cr-1%Mo ferritic heat-resistant steel forgings.

[0063] Table 1. Room temperature mechanical properties of the forgings from the examples.

[0064]

[0065] As shown in Table 1, the forgings have a room temperature tensile strength ≥690MPa, for example, 692-711MPa; yield strength ≥510MPa, for example, 511-530MPa; elongation after fracture ≥25%, for example, 25%-26.5%; and reduction of area >70%, for example, 71%. They can meet the grain size requirements while exhibiting excellent performance, and the final mechanical properties can meet the performance requirements.

[0066] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for refining ferritic heat-resistant steel grains through reciprocating isothermal annealing, characterized in that, Includes the following steps: Step 1: After the large forging is completed, place it in the furnace for heat preservation; Step 2: Heat the forging after holding in Step 1 to the austenitizing temperature and hold at that temperature; Step 3: Cool the austenitized forgings from Step 2 to the annealing temperature, perform reciprocating isothermal annealing, and then cool them out of the furnace.

2. The method according to claim 1, characterized in that, In step 3, the annealing temperature is 650–770℃.

3. The method according to claim 1, characterized in that, In step 3, the annealing process is repeated at least twice.

4. The method according to claim 3, characterized in that, The annealing process is repeated 4-6 times.

5. The method according to claim 3, characterized in that, The temperature for even-numbered annealing processes is higher than the temperature for odd-numbered annealing processes.

6. The method according to any one of claims 1-5, characterized in that, In step 2, the austenitizing temperature is 1030–1070℃.

7. The method according to claim 6, characterized in that, Step 3 is followed by step 4: normalizing and tempering the forgings processed in step 3.

8. The method according to claim 7, characterized in that, The normalizing temperature is 1030–1060℃, and the tempering temperature is 740–770℃.

9. The method according to claim 1, characterized in that, The large heat-resistant steel forging is a 9%Cr-1%Mo ferritic heat-resistant steel forging.

10. A large heat-resistant steel forging, obtained by processing according to any one of claims 1-6, wherein the large heat-resistant steel forging has a mixed structure of ferrite and alloy carbides, wherein the ferrite structure is fine and uniformly distributed, and the grain size is less than 30 μm.

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