A method for avoiding casting cracks in duplex stainless steel

Abstract

The present invention relates to a method for avoiding casting cracks in duplex stainless steel, comprising the following steps: S1, melting and casting duplex stainless steel into a mold, and opening the mold when the mold temperature is 980 - 1080 °C, then transferring the casting into a heat treatment furnace; S2, cooling the casting to 700 - 780 °C at a first cooling rate; S3, heating the casting to 920 °C - 960 °C at a first heating rate; S4, cooling the casting to 350 - 400 °C at a second cooling rate; S5, heating the casting to 520 - 600 °C at a second heating rate; S6, cooling the casting to below 200 °C at a third cooling rate, and then taking the casting out of the furnace.

Description

Technical Field

[0001] The present invention relates to a method for avoiding casting cracks in duplex stainless steel, belonging to the technical field of duplex stainless steel. The duplex stainless steel prepared by the method of the present invention has neither cracks caused by brittle harmful phases nor cracks caused by thermal stress, with excellent casting quality and high yield, and is suitable for large-scale popularization and application. Background Art

[0002] Duplex stainless steel is a type of stainless steel with a mixed structure of ferrite and austenite. Therefore, this stainless steel combines the advantages of austenitic stainless steel and ferritic stainless steel, has excellent strength and corrosion resistance, and is often used in the preparation of casting parts such as impellers, pump casings, and valve bodies. In the actual production process, it is found that the casting performance of duplex stainless steel is poor, and brittle harmful phases are particularly likely to precipitate during the casting process, resulting in cracks and scrapping of the castings.

[0003] Existing technical research shows that there are two intervals in the casting process of duplex stainless steel where brittle harmful phases are likely to precipitate. One is 800 - 900 °C, and the other is 420 - 500 °C. The brittle harmful phases will particularly easily cause cracks to appear during subsequent casting processing, transportation, heat treatment, etc. In order to avoid the formation of brittle harmful phases, a lot of exploration has been carried out in the existing technology, and it has been found that rapid cooling of the ingot can effectively avoid the precipitation of brittle harmful phases. For example, Yingliu Group Huoshan Foundry Co., Ltd. proposed (CN202010939140.0) to open the box after the casting solidifies and then air-cool it to a certain temperature, thereby avoiding the precipitation of brittle harmful phases and effectively controlling the cracking defects caused by the precipitation of brittle harmful phases. However, rapid cooling of the casting is particularly likely to cause stress concentration during the cooling process of the casting, and stress cracking phenomena are common during the solidification process of the casting, and the problem of cracking of duplex stainless steel cannot be solved from the source.

[0004] Based on this, the present invention proposes a production method for duplex stainless steel that can both inhibit the formation of brittle harmful phases and avoid stress cracking of the casting. Summary of the Invention

[0005] The present invention provides a method for avoiding casting cracks in duplex stainless steel. The duplex stainless steel produced by this method effectively inhibits the formation of brittle harmful phases, prevents cracking problems caused by brittle harmful phases, and at the same time does not cause stress concentration to the casting. The casting has no tendency of stress cracking, improving the casting quality and yield of the casting.

[0006] The technical objectives of the present invention are achieved by the following means.

[0007] The present invention provides a method for avoiding casting cracks in duplex stainless steel, comprising the following steps: S1. Melting and casting duplex stainless steel into a mold, and opening the mold when the mold temperature is 1000-1080°C, then transferring the casting into a heat treatment furnace; S2. Cooling the casting to 700-780°C at a first cooling rate; S3. Heating the casting to 920-960°C at a first heating rate; S4. Cooling the casting to 350-400°C at a second cooling rate; S5. Heating the casting to 520-600°C at a second heating rate; S6. Cooling the casting to below 200°C at a third cooling rate and then taking it out of the furnace.

[0008] As mentioned in the background art above, for cast duplex stainless steel, there is a phenomenon of precipitation of brittle harmful phases in the temperature range of 800-900°C (hereinafter referred to as the "high-temperature range") and 420-500°C (hereinafter referred to as the "low-temperature range"). Although rapid cooling after opening the mold at high temperature can avoid the formation of brittle harmful phases, due to the high cooling rate, obvious stress concentration occurs in the casting, and cracking problems still occur during the solidification process. Taking this as an opportunity, the inventors of the present invention conducted repeated tests on the cooling process of duplex stainless steel castings and found that when the casting is placed in a heat treatment furnace after opening the mold at high temperature, first cooling the casting to a temperature lower than the high-temperature range and higher than the low-temperature range (i.e., 700-780°C) at a certain cooling rate, then heating it to a temperature higher than the high-temperature range and lower than the solidification temperature (i.e., 920-960°C) at a slow rate, then cooling the casting to a temperature lower than the low-temperature range (i.e., 350-400°C) at a certain cooling rate, then heating it to a temperature higher than the low-temperature range and lower than the high-temperature range (i.e., 520-600°C) at a slow heating rate, and then cooling it to below 200°C at a lower cooling rate and taking it out of the furnace can effectively avoid cracking problems caused by brittle harmful phases. Although its mechanism is not yet clear, it is speculated that it may be due to the secondary heating steps (i.e., S3, S5) after cooling (i.e., S2, S4) that achieve the re-solution of the brittle harmful phases after precipitation, and the precipitation morphology and structure change qualitatively after re-cooling (it is also possible that it no longer precipitates in the form of brittle harmful phases), ultimately avoiding cracks caused by brittle harmful phases. Moreover, the present invention also limits the cooling rate during the cooling process, making it much lower than the cooling rate of more than 15°C / min for traditional castings after opening the mold and air cooling, which can effectively avoid stress concentration during the cooling process of the casting and avoid the occurrence of stress cracks.

[0009] Therefore, the method of the present invention can effectively avoid the occurrence of cracks caused by brittle harmful phases and stress cracks in cast duplex stainless steel, and improve the casting quality and yield of the casting.

[0010] As a specific description, the first cooling rate of the present invention is 3-6 °C / min. The inventor found that when the cooling rate is too slow, the number of cracks in the casting increases. It is speculated that this should be due to excessive precipitation of brittle harmful phases. Even after subsequent secondary heat treatment, it is difficult to effectively suppress the cracks. When the cooling rate is too fast, the number of cracks in the casting also shows an increasing trend. It is analyzed that this should be due to stress concentration caused by too high a cooling rate, resulting in an increase in casting cracks.

[0011] As a specific description, the first heating rate of the present invention is 0.5-2 °C / min. During the experiment, the inventor found that once the heating rate is too fast, it cannot effectively prevent the appearance of cracks. It is speculated that this may be because the brittle harmful phases cannot effectively dissolve back. And when the heating rate is too slow, the production efficiency decreases, which is not good from an economic perspective.

[0012] As a specific description, the second cooling rate of the present invention is 3-6 °C / min. The inventor found that in this cooling step, when the cooling rate is too slow, the number of cracks in the casting increases. It is speculated that this should be due to the secondary precipitation of brittle harmful phases. When the cooling rate is too fast, the number of cracks in the casting also shows an increasing trend. It is analyzed that this should be due to stress concentration caused by too high a cooling rate, resulting in an increase in casting cracks.

[0013] As a specific description, the second heating rate of the present invention is 0.5-2 °C / min. During the experiment, the inventor found that once the heating rate is too fast, it cannot effectively prevent the appearance of cracks. It is speculated that this may be because the brittle harmful phases cannot effectively dissolve back. And when the heating rate is too slow, the production efficiency decreases, which is not good from an economic perspective.

[0014] As a specific description, the third cooling rate of the present invention is 3-6 °C / min. The inventor found that in this cooling step, when the cooling rate is too slow, the number of cracks in the casting increases. It is speculated that this should be due to the re-precipitation of brittle harmful phases. When the cooling rate is too fast, the number of cracks in the casting also shows an increasing trend. It is analyzed that this should be due to stress concentration caused by too high a cooling rate, resulting in an increase in casting cracks.

[0015] It should be particularly noted that in the preparation method of the present invention, no heat preservation operation is required in each step of S2-S6. That is to say, whether it is the heating process or the cooling process, once the target temperature is reached, the next step is immediately entered.

[0016] As a non-limiting description, the duplex stainless steel described in the present invention is one of 14Cr18Ni11Si4AlTi, 022Cr19Ni5Mo3Si2N, 12Cr21Ni5Ti, 022Cr22Ni5Mo3N, 022Cr23Ni4MoCuN, 022Cr25Ni6Mo2N, 022Cr25Ni7Mo3WCuN, 03Cr25Ni6Mo3Cu2N, 022Cr25Ni7Mo4N, 022Cr25Ni7Mo4WCuN, and its specific composition shall be subject to that recorded in the national standard GB / T20878-2007. Of course, the method of the present invention is also applicable to other duplex stainless steels without standard grades for the time being.

[0017] The present invention also provides a duplex stainless steel, which is prepared by a method for avoiding casting cracks of duplex stainless steel as described above. As a non-limiting description, the duplex stainless steel is one of 14Cr18Ni11Si4AlTi, 022Cr19Ni5Mo3Si2N, 12Cr21Ni5Ti, 022Cr22Ni5Mo3N, 022Cr23Ni4MoCuN, 022Cr25Ni6Mo2N, 022Cr25Ni7Mo3WCuN, 03Cr25Ni6Mo3Cu2N, 022Cr25Ni7Mo4N, 022Cr25Ni7Mo4WCuN, and its specific composition shall be subject to that recorded in the national standard GB / T20878-2007. Of course, the method of the present invention is also applicable to other duplex stainless steels without standard grades for the time being.

[0018] The beneficial effects of the present invention are as follows.

[0019] Based on the problems that traditional cast duplex stainless steel is prone to brittle harmful phases leading to cracks and the existing rapid cooling process is prone to stress concentration, the present invention conducts targeted research and proposes a method for avoiding casting cracks of duplex stainless steel. By performing "cooling - heating - cooling" operations across high and low temperature ranges respectively and reasonably controlling the cooling and heating rates, the casting cracks caused by brittle harmful phases are effectively avoided. In addition, the reasonable control of the cooling rate also avoids the stress formed due to excessive temperature gradient during the cooling process of the casting, thus avoiding the generation of stress cracks. That is to say, the method of the present invention avoids both the cracks caused by the precipitation of brittle harmful phases and the stress cracks caused by too high cooling rate. Therefore, the yield rate of the duplex stainless steel castings produced by the method of the present invention is high and the casting quality is good. Detailed Embodiments

[0020] To enable those of ordinary skill in the art to fully understand the technical solutions and beneficial effects of the present invention, the following further description is made in conjunction with specific test examples.

[0021] In each test example, duplex stainless steel was melted according to the designed composition. The duplex stainless steel obtained from each furnace melting was cast into 26 cuboid specimens. Every 2 specimens formed a group, resulting in 13 groups of specimens. Then, each group of specimens was processed according to the process parameters given in each test example, and finally ingots were obtained. The dimensions of the ingot blanks were all 300 mm × 80 mm × 80 mm.

[0022] After the ingot blank was cooled to room temperature, the blank was then sandblasted and deburred. Subsequently, one of the specimens in each group was subjected to turning to machine the specimen into a round bar of 200 mm × φ60 mm, and the other one in each group was inspected for surface cracks in the as-cast state.

[0023] Since brittle harmful phases are particularly prone to generating cracks during the processing, in each group of specimens, the specimens obtained by turning were used to verify the cracks caused by brittle harmful phases. Since these cracks were formed during the turning process, the cracks caused by brittle harmful phases usually exhibit metallic luster or bright gray; while the cracks caused by thermal stress will appear on the surface of the casting during the solidification process of the casting, and they are in an oxidized state. Therefore, thermal stress cracks usually exhibit oxidation colors such as gray-blue or black. The cracks of the specimens were observed under a microscope at a magnification of 100 times, and the test results corresponding to the 2 specimens in each group were recorded in the crack test results of this group.

[0024] In the following table, T0 represents the unpacking temperature, with the unit of °C; C1 represents the first cooling rate, with the unit of °C / min; T1 represents the target temperature of the first cooling, with the unit of °C; H1 represents the first heating rate, with the unit of °C / min; T2 represents the target temperature of the first heating, with the unit of °C; C2 represents the second cooling rate, with the unit of °C / min; T3 represents the target temperature of the second cooling, with the unit of °C; H2 represents the second heating rate, with the unit of °C / min; T4 represents the target temperature of the second heating, with the unit of °C, C3 represents the third cooling rate, with the unit of °C / min; T5 represents the final cooling temperature, with the unit of °C. Example 1

[0025] Taking 022Cr22Ni5Mo3N duplex stainless steel as the test raw material for casting tests, the specific composition is by weight percentage: C: 0.023%, Si: 0.17%, Mn: 1.22%, P: 0.018%, S: 0.012%, Ni: 4.87%, Cr: 21.55%, Mo: 3.06%, N: 0.14%, and the balance is Fe and unavoidable impurities. The specific process parameters of the 13 groups of 26 specimens after casting are shown in Table 1 below.

[0026] Table 1 Post-casting treatment parameters of each group of duplex stainless steel.

[0027]

[0028] Table 2 Results of crack inspection of duplex stainless steel in each group.

[0029]

[0030] Next, Example 1 will be specifically analyzed in combination with Table 1 and Table 2.

[0031] The post-casting treatment conditions of the duplex stainless steel in groups A1 - A3 in Table 1 are all within the scope of the present invention. Finally, there are no cracks on the as-cast surface of its specimens, and no cracks appear after machining, indicating that the post-casting treatment conditions of the present invention can effectively avoid cracks caused by thermal stress cracks and brittle harmful phases.

[0032] At least one of the post-casting treatment conditions of the duplex stainless steel in groups A4 - A13 in Table 1 does not meet the requirements of the present invention. Finally, the test results show that thermal stress cracks appear in the as-cast state or cracks caused by brittle harmful phases appear after machining.

[0033] The specimen of group A4 is used as a comparative example of group A1. Under the same other treatment conditions, the first cooling rate is reduced, and the reduced first cooling rate is not within the scope required by the present invention. The results show that cracks appear after machining. It is speculated that the first cooling rate may be too low to effectively inhibit the precipitation of brittle harmful phases, ultimately resulting in cracks during machining.

[0034] The specimen of group A5 is used as a comparative example of group A1. Under the same other treatment conditions, the first cooling rate is increased, and the increased first cooling rate is not within the scope required by the present invention. The results show that thermal stress cracks appear on the as-cast surface. It is analyzed that the large cooling rate leads to a large temperature gradient during the cooling process of the casting, thermal stress concentration, and the formation of thermal cracks on the casting surface.

[0035] The specimen of group A6 is used as a comparative example of group A2. Under the same other treatment conditions, the first heating rate is increased, and the increased first heating rate is not within the scope required by the present invention. The results show that cracks appear after machining. It is speculated that the first heating rate may be too high and the residence time in the high-temperature range is insufficient, resulting in the inability of brittle harmful phases to fully dissolve back, ultimately causing the formation of cracks during machining.

[0036] The specimen of group A7 is used as a comparative example of group A2. Under the same other treatment conditions, the second cooling rate is reduced, and the reduced second cooling rate is not within the scope required by the present invention. The results show that cracks appear after machining. It is speculated that the second cooling rate may be too low to effectively inhibit the precipitation of brittle harmful phases, ultimately resulting in cracks during machining.

[0037] The specimens of Group A8 were used as a comparative example of Group A2. With other processing conditions being the same, the second cooling rate was increased, and the increased second cooling rate was not within the scope required by the present invention. The results showed that thermal stress cracks appeared on the as-cast surface. It was analyzed that the excessive cooling rate led to a large temperature gradient during the cooling process of the casting, resulting in concentrated thermal stress and the formation of thermal cracks on the casting surface.

[0038] The specimens of Group A9 were used as a comparative example of Group A3. With other processing conditions being the same, the second heating rate was increased, and the increased second heating rate was not within the scope required by the present invention. The results showed that cracks appeared after turning. It was speculated that the second heating rate might be too high and the residence time in the low-temperature range was insufficient, resulting in the inability of the brittle harmful phase to fully dissolve back, ultimately causing the formation of cracks during turning.

[0039] The specimens of Group A10 were used as a comparative example of Group A3. With other processing conditions being the same, the third cooling rate was decreased, and the decreased third cooling rate was not within the scope required by the present invention. The results showed that cracks appeared after turning. It was speculated that the third cooling rate might be too low to effectively inhibit the precipitation of the brittle harmful phase, ultimately leading to the appearance of cracks during turning.

[0040] The specimens of Group A11 were used as a comparative example of Group A3. With other processing conditions being the same, the third cooling rate was increased, and the increased third cooling rate was not within the scope required by the present invention. The results showed that thermal stress cracks appeared on the as-cast surface. It was analyzed that the excessive cooling rate led to a large temperature gradient during the cooling process of the casting, resulting in concentrated thermal stress and the formation of thermal cracks on the casting surface.

[0041] The specimens of Group A12 were used as a comparative example of Group A1. With other processing conditions being the same, the target temperature of the first heating was decreased, and the decreased target temperature of the first heating was not within the scope required by the present invention. The results showed that cracks appeared after turning. It was speculated that the target temperature of the first heating might be too low and the residence time in the high-temperature range was insufficient, resulting in the inability of the brittle harmful phase to fully dissolve back, ultimately causing the formation of cracks during turning.

[0042] The specimens of Group A13 were used as a comparative example of Group A1. With other processing conditions being the same, the target temperature of the second heating was decreased, and the decreased target temperature of the second heating was not within the scope required by the present invention. The results showed that cracks appeared after turning. It was speculated that the target temperature of the second heating might be too low and the residence time in the low-temperature range was insufficient, resulting in the inability of the brittle harmful phase to fully dissolve back, ultimately causing the formation of cracks during turning. Example 2

[0043] The casting test was carried out with 022Cr23Ni4MoCuN duplex stainless steel as the test raw material. The specific composition is by weight percentage: C: 0.018%, Si: 0.44%, Mn: 1.46%, P: 0.023%, S: 0.019%, Ni: 3.66%, Cr: 23.64%, Mo: 0.28%, Cu: 0.31%, N: 0.09%, and the balance is Fe and unavoidable impurities. The specific process parameters of 26 specimens in 13 groups after casting are shown in Table 3 below.

[0044] Table 3 Post-casting treatment parameters of each group of duplex stainless steel.

[0045]

[0046] Table 4 Crack inspection results of each group of duplex stainless steel.

[0047]

[0048] Next, Example 2 will be specifically analyzed in combination with Table 3 and Table 4.

[0049] The post-casting treatment conditions of the duplex stainless steel in groups B1 - B3 in Table 3 are all within the scope of the present invention. Finally, there are no cracks on the as-cast surface of its specimens, and no cracks appear after turning, indicating that the post-casting treatment conditions of the present invention can effectively avoid cracks caused by thermal stress cracks and brittle harmful phases.

[0050] At least one of the post-casting treatment conditions of the duplex stainless steel in groups B4 - B13 in Table 3 does not meet the requirements of the present invention. Finally, the test results show that thermal stress cracks appear in the as-cast state or cracks caused by brittle harmful phases appear after turning.

[0051] The specimen of group B4 is used as a comparative example of group B1. Under the same other treatment conditions, the first cooling rate is reduced, and the reduced first cooling rate is not within the scope required by the present invention. The results show that cracks appear after turning. It is speculated that the first cooling rate may be too low to effectively inhibit the precipitation of brittle harmful phases, resulting in cracks during turning.

[0052] The specimen of group B5 is used as a comparative example of group B1. Under the same other treatment conditions, the first cooling rate is increased, and the increased first cooling rate is not within the scope required by the present invention. The results show that thermal stress cracks appear on the as-cast surface. It is analyzed that the large cooling rate leads to a large temperature gradient during the cooling process of the casting, thermal stress concentration, and the formation of thermal cracks on the casting surface.

[0053] The specimens of Group B6 were used as a comparative example of Group B1. With other processing conditions being the same, the first heating rate was increased, and the increased first heating rate was not within the scope required by the present invention. The results showed that cracks appeared after turning. It was speculated that the first heating rate might be too high and the residence time in the high-temperature range was insufficient, resulting in the insufficient re-dissolution of the brittle harmful phase and finally causing the formation of cracks during turning.

[0054] The specimens of Group B7 were used as a comparative example of Group B2. With other processing conditions being the same, the target temperature of the first heating was decreased, and the decreased target temperature of the first heating was not within the scope required by the present invention. The results showed that cracks appeared after turning. It was speculated that the target temperature of the first heating might be too low and the residence time in the high-temperature range was insufficient, resulting in the insufficient re-dissolution of the brittle harmful phase and finally causing the formation of cracks during turning.

[0055] The specimens of Group B8 were used as a comparative example of Group B2. With other processing conditions being the same, the second cooling rate was decreased, and the decreased second cooling rate was not within the scope required by the present invention. The results showed that cracks appeared after turning. It was speculated that the second cooling rate might be too low to effectively inhibit the precipitation of the brittle harmful phase, finally resulting in cracks during turning.

[0056] The specimens of Group B9 were used as a comparative example of Group B2. With other processing conditions being the same, the second cooling rate was increased, and the increased second cooling rate was not within the scope required by the present invention. The results showed that thermal stress cracks appeared on the as-cast surface. It was analyzed that the large cooling rate led to a large temperature gradient during the cooling process of the casting, thermal stress concentration, and the formation of thermal cracks on the casting surface.

[0057] The specimens of Group B10 were used as a comparative example of Group B3. With other processing conditions being the same, the second heating rate was increased, and the increased second heating rate was not within the scope required by the present invention. The results showed that cracks appeared after turning. It was speculated that the second heating rate might be too high and the residence time in the low-temperature range was insufficient, resulting in the insufficient re-dissolution of the brittle harmful phase and finally causing the formation of cracks during turning.

[0058] The specimens of Group B11 were used as a comparative example of Group B3. With other processing conditions being the same, the target temperature of the second heating was decreased, and the decreased target temperature of the second heating was not within the scope required by the present invention. The results showed that cracks appeared after turning. It was speculated that the target temperature of the second heating might be too low and the residence time in the low-temperature range was insufficient, resulting in the insufficient re-dissolution of the brittle harmful phase and finally causing the formation of cracks during turning.

[0059] The B12 group of specimens was used as a comparative example for the B3 group. Under the condition that other processing conditions were the same, the third cooling rate was reduced, and the reduced third cooling rate was not within the scope required by the present invention. The results showed that cracks appeared after turning. It was speculated that the third cooling rate might be too low to effectively inhibit the precipitation of brittle harmful phases, ultimately resulting in cracks during turning.

[0060] The B13 group of specimens was used as a comparative example for the B3 group. Under the condition that other processing conditions were the same, the third cooling rate was increased, and the increased third cooling rate was not within the scope required by the present invention. The results showed that thermal stress cracks appeared on the as-cast surface. It was analyzed that the large cooling rate led to a large temperature gradient during the cooling process of the casting, resulting in thermal stress concentration and forming thermal cracks on the surface of the casting. Example 3

[0061] The casting test was carried out using 03Cr25Ni6Mo3Cu2N duplex stainless steel as the test raw material. The specific composition was by weight percentage: C: 0.032%, Si: 0.29%, Mn: 0.88%, P: 0.025%, S: 0.021%, Ni: 5.74%, Cr: 25.71%, Mo: 3.18%, Cu: 2.04%, N: 0.17%, and the balance was Fe and unavoidable impurities. The specific process parameters of 26 specimens in 13 groups after casting are shown in Table 5 below.

[0062] Table 5. Post-casting treatment parameters of duplex stainless steel in each group.

[0063]

[0064] Table 6. Crack inspection results of duplex stainless steel in each group.

[0065]

[0066] Next, a specific analysis of Example 2 will be made in combination with Table 5 and Table 6.

[0067] The post-casting treatment conditions of the duplex stainless steel in groups C1 - C3 in Table 5 were all within the scope of the present invention. Finally, there were no cracks on the as-cast surface of its specimens, and no cracks appeared after turning, indicating that the post-casting treatment conditions of the present invention can effectively avoid thermal stress cracks and cracks caused by brittle harmful phases.

[0068] At least one of the post-casting treatment conditions of the duplex stainless steel in groups C4 - C13 in Table 5 did not meet the requirements of the present invention. Finally, the test results showed that thermal stress cracks appeared in the as-cast state or cracks caused by brittle harmful phases appeared after turning.

[0069] The specimens of Group C4 were used as the comparative examples of Group C1. With other processing conditions being the same, the third cooling rate was increased, and the increased third cooling rate was not within the scope required by the present invention. The results showed that thermal stress cracks appeared on the as-cast surface. It was analyzed that the excessive cooling rate led to a large temperature gradient during the cooling process of the casting, thermal stress concentration, and the formation of thermal cracks on the casting surface.

[0070] The specimens of Group C5 were used as the comparative examples of Group C1. With other processing conditions being the same, the third cooling rate was decreased, and the decreased third cooling rate was not within the scope required by the present invention. The results showed that cracks appeared after turning. It was speculated that the third cooling rate might be too low to effectively inhibit the precipitation of brittle harmful phases, ultimately resulting in cracks during turning.

[0071] The specimens of Group C6 were used as the comparative examples of Group C1. With other processing conditions being the same, the target temperature of the second heating was decreased, and the decreased target temperature of the second heating was not within the scope required by the present invention. The results showed that cracks appeared after turning. It was speculated that the target temperature of the second heating might be too low and the residence time in the low-temperature range was insufficient, resulting in the insufficient re-solution of brittle harmful phases and ultimately causing the formation of cracks during turning.

[0072] The specimens of Group C7 were used as the comparative examples of Group C2. With other processing conditions being the same, the second heating rate was increased, and the increased second heating rate was not within the scope required by the present invention. The results showed that cracks appeared after turning. It was speculated that the second heating rate might be too high and the residence time in the low-temperature range was insufficient, resulting in the insufficient re-solution of brittle harmful phases and ultimately causing the formation of cracks during turning.

[0073] The specimens of Group C8 were used as the comparative examples of Group C2. With other processing conditions being the same, the second cooling rate was decreased, and the decreased second cooling rate was not within the scope required by the present invention. The results showed that cracks appeared after turning. It was speculated that the second cooling rate might be too low to effectively inhibit the precipitation of brittle harmful phases, ultimately resulting in cracks during turning.

[0074] The specimens of Group C9 were used as the comparative examples of Group C2. With other processing conditions being the same, the second cooling rate was increased, and the increased second cooling rate was not within the scope required by the present invention. The results showed that thermal stress cracks appeared on the as-cast surface. It was analyzed that the excessive cooling rate led to a large temperature gradient during the cooling process of the casting, thermal stress concentration, and the formation of thermal cracks on the casting surface.

[0075] The specimens of Group C10 were used as the comparative examples of Group C2. With other processing conditions being the same, the target temperature of the first heating was decreased, and the decreased target temperature of the first heating was not within the scope required by the present invention. The results showed that cracks appeared after turning. It was speculated that perhaps the target temperature of the first heating was too low and the residence time in the high-temperature range was insufficient, resulting in the insufficient re-dissolution of the brittle harmful phase, and finally causing the formation of cracks during turning.

[0076] The specimens of Group C11 were used as the comparative examples of Group C3. With other processing conditions being the same, the first heating rate was increased, and the increased first heating rate was not within the scope required by the present invention. The results showed that cracks appeared after turning. It was speculated that perhaps the first heating rate was too high and the residence time in the high-temperature range was insufficient, resulting in the insufficient re-dissolution of the brittle harmful phase, and finally causing the formation of cracks during turning.

[0077] The specimens of Group C12 were used as the comparative examples of Group C3. With other processing conditions being the same, the first cooling rate was decreased, and the decreased first cooling rate was not within the scope required by the present invention. The results showed that cracks appeared after turning. It was speculated that perhaps the first cooling rate was too low to effectively inhibit the precipitation of the brittle harmful phase, and finally resulting in cracks during turning.

[0078] The specimens of Group C13 were used as the comparative examples of Group C3. With other processing conditions being the same, the first cooling rate was increased, and the increased first cooling rate was not within the scope required by the present invention. The results showed that thermal stress cracks appeared on the as-cast surface. It was analyzed that it should be due to the too large cooling rate resulting in a large temperature gradient during the cooling process of the casting, thermal stress concentration, and the formation of thermal cracks on the surface of the casting.

[0079] It is not difficult to see from the above that the method for avoiding the casting cracks of duplex stainless steel in the present invention, by adopting the "temperature decrease - temperature increase - temperature decrease" operation across the high-temperature range and the low-temperature range, and reasonably controlling the cooling and heating rates at the same time, effectively avoids the casting cracks caused by the brittle harmful phase. In addition, the reasonable control of the cooling rate also avoids the stress formed due to too large a temperature gradient during the cooling process of the casting and avoids the generation of stress cracks. That is to say, the method of the present invention not only avoids the cracks caused by the precipitation of the brittle harmful phase, but also does not cause stress cracks due to too high a cooling rate. Therefore, the yield rate of the duplex stainless steel castings produced by the method of the present invention is high and the casting quality is good.

[0080] The description of the above embodiments is only used to help understand the method and its core idea of the present invention. It should be noted that for those of ordinary skill in the art of the present technology, without departing from the principle of the present invention, several improvements and modifications can still be made to the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

[0081] The foregoing description of the disclosed embodiments enables those skilled in the art to practice or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for avoiding casting cracks in duplex stainless steel, characterized in that, It includes the following steps: S1. Melting and casting duplex stainless steel into a mold, opening the mold when the mold temperature is 980 - 1080 °C, and transferring the casting into a heat treatment furnace; S2. Cooling the casting to 700 - 780 °C at a cooling rate of 3 - 6 °C / min; S3. Heating the casting to 920 - 960 °C at a first heating rate of 0.5 - 2 °C / min; S4. Cooling the casting to 350 - 400 °C at a second cooling rate of 3 - 6 °C / min; S5. Heating the casting to 520 - 600 °C at a second heating rate of 0.5 - 2 °C / min; S6. Cooling the casting to below 200 °C at a third cooling rate of 3 - 6 °C / min and taking it out of the furnace; Among them, no heat preservation operation is required for each step from S2 to S6.

2. A method for avoiding casting cracks in duplex stainless steel according to claim 1, characterized in that, The duplex stainless steel is one of 14Cr18Ni11Si4AlTi, 022Cr19Ni5Mo3Si2N, 12Cr21Ni5Ti, 022Cr22Ni5Mo3N, 022Cr23Ni4MoCuN, 022Cr25Ni6Mo2N, 022Cr25Ni7Mo3WCuN, 03Cr25Ni6Mo3Cu2N, 022Cr25Ni7Mo4N, 022Cr25Ni7Mo4WCuN.

3. A duplex stainless steel prepared by the method for avoiding casting cracks of duplex stainless steel according to any one of claims 1 - 2.

4. A duplex stainless steel according to claim 3, characterized in that, The duplex stainless steel is one of 14Cr18Ni11Si4AlTi, 022Cr19Ni5Mo3Si2N, 12Cr21Ni5Ti, 022Cr22Ni5Mo3N, 022Cr23Ni4MoCuN, 022Cr25Ni6Mo2N, 022Cr25Ni7Mo3WCuN, 03Cr25Ni6Mo3Cu2N, 022Cr25Ni7Mo4N, 022Cr25Ni7Mo4WCuN.

Images