Heat treatment process for refining GS32CrNiMo6V casting grains

Abstract

The invention relates to the field of heat treatment processes, and discloses a heat treatment process for refining GS32CrNiMo6V casting grains, which comprises the following steps: normalizing for the first time: heating a workpiece to 950-960 DEG C at the speed of 70-90 DEG C / h, and carrying out heat preservation for 1.8-2.1 minutes per millimeter; heating is stopped after the heat preservation time is up, and the workpiece is discharged out of the furnace to an air cooling area to be cooled; normalizing for the second time: after the temperature of the workpiece is less than or equal to 300 DEG C, feeding the workpiece into the furnace again, heating to 860-870 DEG C at the speed of 70-90 DEG C / h, carrying out heat preservation for 2.1-2.4 minutes per millimeter, stopping heating after the heat preservation time is reached, discharging the workpiece from the furnace to an air cooling area, and cooling; and tempering is conducted, specifically, after the temperature of the workpiece is smaller than or equal to 300 DEG C, the workpiece enters the furnace again, the temperature is increased to 660-680 DEG C at the speed of 80-105 DEG C / h, heat preservation is conducted for 7-8 h, and after heat preservation is finished, the workpiece is discharged out of the furnace and exposed in air to be naturally cooled. According to the method, as-cast dendritic crystal structure segregation can be eliminated, grains are refined, the production efficiency is improved, and the production cost is reduced.

Description

Technical Field

[0001] The invention relates to the field of heat treatment technology, and in particular to a heat treatment technology for refining grains of GS32CrNiMo6V castings. Background Art

[0002] GS32CrNiMo6V is an alloy structural steel material, which is widely used in the production of high-strength bolts, gears, bearings and mining machinery crawler plates. For castings made of this material, the original indicators of this material are to eliminate the as-cast segregation structure and the grain size reaches 0 to 3 levels. However, in some special applications, more stringent requirements are put forward for material properties: (1) Delivery state: "normalizing + tempering", (2) Grain size ≥ 6 levels, (3) Surface hardness test, surface hardness ≤ 260HB. The process difficulty for grain size above 6 is very high, because high temperature (i.e. 1100 to 1250℃) is usually used to eliminate as-cast segregation structure and grain refinement, but due to the limitations of high-temperature equipment and surface oxidation and decarburization, it is difficult to meet the technical requirements. Therefore, grain size below 4 levels and above 2 levels are usually used.

[0003] The existing process is difficult to meet the technical requirements. The specific reasons are as follows: When using GS32CrNiMo6V material for casting production, improper smelting process control and excessively high pouring temperature often lead to local alloy segregation and dendritic crystal segregation during the solidification process of the casting due to insufficient homogenization time and overheating of alloy elements. This cast segregation structure (i.e., dendritic crystal segregation) is very hereditary. Even after the conventional 900℃ normalizing heat treatment of steel castings, the grains are still abnormally coarse, the grain size only reaches 0 to 3 levels, and the structure is an overheated granular bainite structure that retains the cast orientation. These inherited cast structures cannot be eliminated, and it is difficult to meet the technical requirements of the product.

[0004] In order to avoid product scrapping, for castings with the above problems, a heat treatment method of 1100-1250℃ diffusion annealing + normalizing + tempering is currently used to refine the grains and eliminate the inherited cast structure. However, this process has a very long production cycle, and due to the high heating temperature, it is difficult for the furnace temperature of a general foundry to reach the required temperature range, which seriously reduces the production efficiency of the product. At the same time, due to the high temperature, the surface oxidation and decarburization of the casting after heat treatment is very serious, which has a negative impact on the quality of the casting. Summary of the invention

[0005] The present invention aims to provide a heat treatment process for refining the grains of GS32CrNiMo6V castings. The present invention can eliminate the segregation of dendritic crystal structure in the cast state, refine the grains, improve the production efficiency and reduce the production cost.

[0006] To achieve the above object, the present invention adopts the following technical scheme: A heat treatment process for refining the grains of GS32CrNiMo6V castings, comprising: first normalizing: heating the workpiece to 950-960°C at a rate of 70-90°C / h, and keeping the workpiece warm, wherein the keeping time is calculated according to 1.8-2.1 minutes per millimeter * workpiece thickness to obtain a first target keeping time; after reaching the first target keeping time, heating is stopped, the workpiece is taken out of the furnace to an air cooling area, forced air cooling is performed for 50-55 minutes, and then air cooling is performed to ≤300°C;

[0007] Second normalizing: When the workpiece temperature is ≤300℃, put it back into the furnace, heat it to 860~870℃ at a speed of 70~90℃ / h, and keep it warm. The second target holding time is calculated according to 2.1~2.4 minutes per millimeter * workpiece thickness. After reaching the second target holding time, stop heating, take the workpiece out of the furnace to the air cooling area, force air cooling for 40~45 minutes, and then air cool to ≤300℃;

[0008] Tempering: When the workpiece temperature is ≤300℃, put it back into the furnace and heat it to 660~680℃ at a rate of 80~105℃ / h. Keep it warm for 7~8h. After the insulation is completed, take the workpiece out of the furnace and expose it to the air for natural cooling.

[0009] The workpiece is a chain plate steel casting. After processing, the grain size of the chain plate steel casting is ≥ grade 6 and the surface hardness is ≤ 260HB.

[0010] The principles and advantages of this solution are:

[0011] (1) Under the condition of the first normalizing (950℃) treatment, the GS32CrNiMo6V casting is conducive to severing the connection between the original (cast) dendritic segregation structure and the newly formed grains, homogenizing the austenite composition, making the cast segregation structure recrystallized, and the dendritic crystal segregation grains are basically eliminated, and the grains are initially refined; under the condition of the second normalizing (860℃) treatment, the cast segregation structure is eliminated, and its structure is adjusted to bainite + a small amount of ferrite, and the grains are refined; under the condition of the second stress relief annealing, the grains are further refined.

[0012] (2) This solution eliminates the segregation of the as-cast dendritic crystal structure and refines the grains through heat treatment in the form of "twice normalizing + tempering", thus meeting the delivery status of the technical requirements. Specifically, the uniformity of the structure is effectively improved through two normalizing treatments, and the material properties are further stabilized through stress relief annealing, and finally the technical indicators are met: the delivery status is "normalizing + tempering", the grain size is ≥6, and the surface hardness is ≤260HB.

[0013] (3) The time setting of holding time and workpiece cooling shortens the heat treatment cycle, thereby improving production efficiency.

[0014] (4) The overall process in this scheme overcomes the shortcomings of the GS32CrNiMo6V casting material itself, and achieves the high standard requirements of "normalizing + tempering" for delivery, grain size ≥6, and surface hardness ≤260HB. Traditionally, GS32CrNiMo6V materials are often used in the fields of high-strength bolts and gears, which usually only require a grain size of 0 to 3, and have low requirements for surface hardness and structural uniformity. Therefore, conventional processes do not need to pursue higher grain sizes, and there will be no contradiction between surface oxidation and decarburization caused by high-temperature treatment and low production efficiency. However, this scheme breaks the industry convention and adopts a new process path, so that GS32CrNiMo6V materials can be produced to form high-specification and high-requirement castings, achieving the delivery status of "normalizing + tempering", grain size ≥6, and surface hardness ≤260HB. Specifically, the process steps in this scheme are very cleverly designed. On the basis of utilizing existing processing equipment, the normalizing step is cleverly added, and the heating and cooling operations during the two normalizing and tempering processes are finely controlled. This not only eliminates the segregation of the cast dendritic crystal structure and refines the grains to level 6, but also avoids the equipment limitations and surface oxidation and decarburization problems caused by high-temperature treatment, significantly improves production efficiency, shortens production cycle, and opens up new possibilities for the application of GS32CrNiMo6V materials in high-performance fields.

[0015] (5) This scheme adopts a slower and more uniform heating rate (i.e., 70-90℃ / h or 80-105℃ / h), which can make the temperature inside and outside the workpiece rise evenly, avoiding the concentration of thermal stress caused by excessive temperature difference. Slow heating helps the uniform transformation of the internal structure of the workpiece, avoiding local uneven structure or incomplete phase change caused by rapid heating. In addition, it can also avoid energy waste caused by excessively rapid heating, and optimize energy utilization efficiency while ensuring quality.

[0016] Preferably, as an improvement, stress relief annealing is required before the first normalizing, and the content of the stress relief annealing is as follows: the workpiece entering the furnace temperature is ≤300°C, the workpiece is heated to 620±20°C at a speed of 80-90°C / h, kept warm for 6-8h, and cooled in the furnace to ≤300°C before being air-cooled out of the furnace.

[0017] Beneficial effects: The temperature of the workpiece entering the furnace is ≤300℃, which can avoid deformation or cracking due to excessive temperature difference; heating at a rate of 80-90℃ / h avoids thermal stress caused by sudden temperature rise and prevents deformation or cracking of the workpiece; 620±20℃, insulation for 6-8h, can activate atomic diffusion activity, release the internal stress of the workpiece, and thus stabilize the workpiece structure; first furnace cooling to ≤300℃ and then air cooling reduces thermal stress during the cooling process, and segmented cooling further reduces the risk of deformation and cracking.

[0018] Preferably, as an improvement, a coating treatment is required after the stress relief annealing, and the content of the coating treatment is as follows: coating the workpiece surface with an anti-oxidation coating.

[0019] Beneficial effects: Reduce oxidation and decarburization of the workpiece surface under high temperature conditions, improve the surface quality of the workpiece, and reduce the workload of subsequent welding and grinding.

[0020] Preferably, as an improvement, a furnace loading process is required after the painting process; the contents of the furnace loading process are as follows: a number of workpieces are placed in the effective heating zone of the furnace temperature field in the trolley furnace.

[0021] Beneficial effects: Processing in the effective heating zone ensures that the workpieces can be heated and kept warm under the same process conditions, ensuring the consistency and uniformity of the processing and avoiding local overheating or underheating.

[0022] Preferably, as an improvement, the spacing between adjacent workpieces is ≥100 mm.

[0023] Beneficial effect: Appropriate spacing helps ensure that there is sufficient air circulation space around each workpiece, so that the heat can be evenly distributed on the workpiece surface. It avoids the local temperature unevenness caused by the close distance between the workpieces, ensures that all workpieces can get the same heat treatment effect, and thus improves the quality of the workpiece.

[0024] Preferably, as an improvement, a metal shim is placed on the bottom surface of the trolley to isolate the workpiece from the bottom surface of the trolley.

[0025] Beneficial effects: After placing the shim, the workpiece can be prevented from directly contacting the bottom of the trolley, which increases the fluidity of the atmosphere during heat treatment. By raising the height of the workpiece loading position, the risk of the burner flame directly spraying the workpiece is avoided, preventing the workpiece from being overheated or overburned locally.

[0026] Preferably, as an improvement, the height of the shim is ≥300 mm.

[0027] Beneficial effect: The higher height of the pad increases the fluidity of the heat treatment atmosphere, avoids the risk of the burner flame directly spraying the workpiece, and prevents local overheating or overburning of the workpiece.

[0028] Preferably, as an improvement, a blower is used for forced air cooling.

[0029] Beneficial effects: The forced airflow provided by the blower can significantly speed up the cooling of the workpiece, and the ability to control the direction and speed of the airflow can allow all parts of the workpiece to be cooled evenly, reducing the risk of deformation or cracking due to inconsistent cooling rates.

[0030] Preferably, as an improvement, an infrared temperature measuring gun is used to detect the surface temperature of the workpiece.

[0031] Beneficial effect: The temperature of the workpiece surface can be accurately controlled to facilitate the next stage of heat treatment.

[0032] The beneficial effects of this scheme are as follows: (1) This scheme can eliminate the inheritance of dendritic crystal segregation structure in the cast state, adjust the structure to obtain bainite + a small amount of ferrite; homogenize the structure, refine the grains, and reduce the oxidation and decarburization phenomenon on the workpiece surface at high temperature, thereby improving production efficiency and workpiece quality.

[0033] (2) It breaks the technical prejudice that "production efficiency and production quality" cannot be achieved at the same time under high temperature conditions. This solution breaks the technical prejudice of "low production efficiency and poor production quality" in the prior art through the steps and settings related to the heat treatment process.

[0034] (3) In this scheme, the insulation time coefficient of the workpiece is selected as (1.8~2.1) min / mm or (2.1~2.4) min / mm, which organically links the insulation time with the effective cross-sectional thickness of the product, expands the application range of the material, and can flexibly adapt to workpieces of different sizes. It is suitable for both test specimens, small-section products and products with thicker casting sections. By reasonably setting the insulation time, it avoids performance failure due to insufficient insulation or energy waste and equipment loss due to excessive insulation, thereby improving production efficiency while ensuring quality and reducing overall production costs.

[0035] (4) The holding time coefficient of the first normalizing is (1.8~2.1)min / mm, and the holding time of the second normalizing is (2.1~2.4)min / mm. The austenitizing temperature set for the first normalizing is relatively high, so the normalizing holding coefficient is selected to be lower than the holding coefficient of the second normalizing to avoid the overheating tendency of the workpiece due to long-term heating at a higher austenitizing temperature. The austenitizing temperature of the second normalizing is lower, and the normalizing holding coefficient is selected to be larger, which can ensure that the austenitization is fully carried out, while reducing the thermal stress of the workpiece heating and continuing to optimize the grain.

[0036] (5) After the insulation, the cooling adopts the method of "air cooling + air cooling", so that the workpiece can be cooled to the process design temperature quickly and the stress of the workpiece can be reduced. Specifically, the cooling rate of forced air cooling is between air cooling and oil cooling, which can achieve faster cooling without generating excessive internal stress due to too fast cooling. Air cooling to 300°C further reduces the cooling rate of the workpiece, which helps to release residual stress and prevent deformation or cracking of the workpiece.

[0037] (6) The setting of holding time and cooling time shortens the heat treatment cycle.

[0038] (7) The heating rate involved in this scheme can avoid rapid heating causing the surface temperature of the workpiece to be too high, increasing the risk of oxidation and decarburization. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 A flow chart of a heat treatment process for refining the grains of a GS32CrNiMo6V casting provided in an embodiment of the present invention.

[0040] Figure 2 A metallographic inspection diagram of a refined GS32CrNiMo6V casting grain provided by an embodiment of the present invention

[0041] Figure 3 This is a metallographic inspection diagram of the heat treatment process in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0042] The following is further described in detail through specific implementation methods:

[0043] Embodiment 1:

[0044] Basically Figure 1 As shown: A heat treatment process for refining the grains of GS32CrNiMo6V castings, comprising the following steps:

[0045] The GS32CrNiMo6V casting is a chain plate steel casting (hereinafter referred to as the workpiece), and its standard meets the German DIN standard. The workpiece is equivalent to the track plate of an armored vehicle. It is a key component used in the walking part of an engineering machinery electric shovel (such as a large excavator). It is a conveyor belt component with large load capacity and durability. According to the technical requirements of a certain product, the workpiece must meet the following conditions: (1) Delivery status: "normalizing + tempering", (2) Grain size ≥ grade 6, (3) Surface hardness test, surface hardness ≤ 260HB. In this embodiment, the composition of the workpiece includes the following elements (mass fraction (%)); C: 0.33, Si: 0.53, Mn: 1.09, P: 0.013, S: 0.009, Cr: 1.03, Ni: 1.33, Mo: 0.34, V: 0.007, Al: 0.044, and the rest are Fe elements. This composition design ensures the high strength, wear resistance and durability of the workpiece under high load and harsh working conditions. At the same time, through the "normalizing + tempering" heat treatment process, the grain size is further refined (≥ grade 6) and the surface hardness is controlled (≤260HB), meeting the stringent performance requirements of engineering machinery for key components.

[0046] Step 1: Stress relief annealing: The temperature of the workpiece entering the furnace is ≤300℃. After heating the workpiece to 620±20℃ at a rate of 80~90℃ / h and keeping it warm for 6~8h, the workpiece is cooled in the furnace to ≤300℃ and then air-cooled. Among them, the temperature of the workpiece entering the furnace is ≤300℃, which can avoid deformation or cracking due to excessive temperature difference; heating at a rate of 80~90℃ / h avoids thermal stress caused by sudden temperature rise and prevents deformation or cracking of the workpiece; 620±20℃, keeping warm for 6~8h, can activate atomic diffusion activity, so that the internal stress of the workpiece can be released, thereby stabilizing the workpiece structure; first furnace cooling to ≤300℃ and then air cooling reduces thermal stress during the cooling process, and segmented cooling further reduces the risk of deformation and cracking. Specifically, in this embodiment, the workpiece is heated to 620℃.

[0047] Step 2: Brushing treatment: Brush anti-oxidation coating on the surface of the workpiece. Since the first normalizing temperature is high, before the first normalizing in batches, brush white anti-oxidation decarburization coating on the surface of the workpiece to reduce oxidation and decarburization on the surface of the workpiece, improve the surface quality of the workpiece, and reduce the workload of the subsequent process.

[0048] Step 3: Furnace loading: Load the processed workpiece on the trolley furnace. In particular, 1) Load the workpiece in the effective heating zone of the furnace temperature field so that it is evenly heated, and at the same time ensure that the workpiece can be heated and kept warm under the same process conditions, ensuring the consistency and uniformity of the treatment, and avoiding local overheating or underheating; 2) The load of the workpiece is ≤ the maximum load of the furnace, that is, the load of the workpiece should not exceed the maximum load of the furnace to ensure the consistency of heating efficiency and uniformity; 3) Place a metal pad with a height of ≥300mm on the bottom of the trolley to isolate the workpiece from the bottom of the trolley, avoid direct contact between the workpiece and the bottom of the trolley, increase the fluidity of the atmosphere during heat treatment, and at the same time, by raising the height of the workpiece loading position, it can avoid the risk of the burner flame directly spraying the workpiece, and prevent the workpiece from being overheated or overburned locally; 4) The spacing between adjacent workpieces is controlled at ≥100mm. Appropriate spacing helps to ensure that there is sufficient air circulation space around each workpiece, so that the heat can be evenly distributed on the surface of the workpiece. Avoid the phenomenon of local temperature unevenness caused by the close distance between the workpieces, ensure that all workpieces can obtain consistent heat treatment effects, thereby improving the quality of the workpieces; 5) The workpiece loading temperature is ≤350℃. In this embodiment, the workpiece loading temperature is 320℃. The furnace adopts a trolley gas furnace, which adopts multiple sets of burner combustion systems, and the burner combustion systems are arranged at the lower position of the two sides of the furnace body. The bottom surface of the trolley of the gas furnace (referred to as the trolley surface) is made of refractory bricks, and the frame is made of heat-resistant steel.

[0049] When loading the furnace, the workpieces that have been stress-relieved and coated with anti-oxidation coating are placed in the effective heating zone of the furnace temperature field. Specifically, the trolley furnace surface uses a metal pad with a height of 300mm to isolate the workpiece from the bottom of the trolley to avoid local overheating or damage of the workpiece. When placing the workpiece, the spacing between adjacent workpieces is 100mm. In this embodiment, the trolley furnace is loaded with 20 pieces. After the furnace is loaded, ignition is carried out, the furnace temperature is 320℃, and then normalization is carried out according to the process regulations.

[0050] Step 4: First normalizing: heat up to 950-960℃ at a rate of 70-90℃ / h, and then keep warm. The first target holding time is calculated according to (1.8-2.1)min / mm*workpiece thickness (mm); stop heating after reaching the first target holding time, drive the workpiece out of the furnace with the trolley to the air cooling area, turn on the blower for forced air cooling for 50-55min, and then air cool to ≤300℃; then use an infrared temperature gun to detect the surface temperature of the workpiece. Specifically, the first target holding time T1=t1*h, where t1 is the holding time coefficient, the range of t1 is 1.8-2.1, and the unit of t1 is min / mm; h is the workpiece thickness, and the unit of h is mm.

[0051] Among them, the heating rate of 70 to 90°C is used to heat the multi-alloy workpiece, which provides a gentle heating method. This slow and stable heating process helps to prevent the increase of thermal stress caused by too fast heating rate, effectively avoids the tendency of the workpiece to cause serious deformation or cracking, and ensures the quality and integrity of the final product; the normalizing temperature of 950 to 960°C can cut off the connection with the newly formed grains, so that the cast segregation structure is recrystallized, the dendritic crystal segregation grains are basically eliminated, and the grains are initially refined; the holding time coefficient of the first normalizing is selected as (1.8 to 2.1) min / mm, so that the holding time is organically linked to the effective cross-sectional thickness of the product. Together, the application scope of this material is expanded, which is suitable for test specimens, small-section products and products with thicker casting sections; the cooling after insulation adopts the "air cooling + air cooling" method, and its forced air cooling can quickly reduce the temperature of the workpiece, inhibit grain growth, refine the grain structure, and accelerate the cooling rate of the workpiece surface and core, reduce tissue segregation, and air cooling to 300℃ further reduces the cooling rate of the workpiece, which helps to release residual stress and prevent deformation or cracking of the workpiece, so that the workpiece can be cooled to the process design temperature faster and the stress of the workpiece can be reduced; an infrared temperature measuring gun is configured to check the surface temperature of the workpiece, which can accurately control the temperature of the workpiece surface to facilitate the next stage of heat treatment into the furnace. For example, in this embodiment, the temperature is raised to 950°C at a rate of 70°C / h, and the first target insulation time is 40min (i.e., the workpiece thickness is 20mm, and the first target insulation time = 2min / mm*20mm); after 40min, the heating is stopped, and the workpiece is taken out of the furnace to the air cooling area along with the trolley, and the blower is turned on for forced air cooling for 50min, and then air-cooled to 300°C, so that the dendritic crystal segregation grains are basically eliminated and the grains are initially refined.

[0052] Step 5: Second normalizing: When the workpiece temperature is measured to be ≤300°C, the workpiece is put back into the furnace and heated to 860-870°C at a rate of 70-90°C / h, and then kept warm. The second target holding time is calculated according to (2.1-2.4) min / mm*workpiece thickness (mm). After reaching the second target holding time, heating is stopped, and the workpiece is taken out of the furnace along with the trolley to the air cooling area. After the blower is turned on for forced air cooling for 40-45 minutes, it is air-cooled to ≤300°C. An infrared temperature measuring gun is used to detect the surface temperature of the workpiece. Specifically, the second target holding time T2=t2*h, where t2 is the holding time coefficient, the range of t2 is 2.1-2.4, and the unit of t2 is min / mm; h is the workpiece thickness, and the unit of h is mm.

[0053] Specifically, although the grains are obviously refined after the first normalizing, the grain size is refined to level 5 to 6 according to metallographic detection, and the dendritic crystal segregation is basically eliminated, but the grain size has not yet reached the requirement, so a second normalizing is required; in the second normalizing, the multi-alloy workpiece is heated at a heating rate of 70 to 90°C, which provides a gentle heating method. This slow and stable heating process helps to prevent the increase of thermal stress caused by too fast a heating rate, effectively avoids the tendency of the workpiece to cause severe deformation or cracking, and ensures the quality and integrity of the final product; the normalizing temperature of 860 to 870°C can eliminate the cast segregation structure, adjust the structure to bainite + a small amount of ferrite, and refine the grains. In this embodiment, the workpiece is heated to 860°C at a rate of 70°C / h, and its second target holding time is 44min (i.e., the workpiece thickness is 20mm, and the second target holding time = 2.2min / mm*20mm); after 44min, the heating is stopped, and the workpiece is taken out of the furnace along with the trolley to the air cooling area, and the blower is turned on for forced air cooling for 40min, and then air-cooled to 300°C. The second normalizing temperature is slightly higher than the critical temperature AC3 (i.e., the austenitizing temperature), but lower than the first normalizing temperature. When the second normalizing is performed at 860°C, it can ensure that the material can promote a more ideal organizational transformation during the austenitizing process again, avoiding the problem of grain coarsening that may be caused by excessively high temperatures, thereby further refining the grains and improving the comprehensive mechanical properties of the material. After two normalizings, the workpiece has a grain size of 6.5, the cast dendritic crystal segregation structure is basically eliminated, and the structure is bainite + a small amount of ferrite, which meets the technical requirements of the product.

[0054] Step 6: Tempering: When the workpiece temperature is measured to be ≤300℃, the workpiece is put back into the furnace and heated to 660~680℃ at a rate of 80~105℃ / h, and kept warm for 7~8h. After the end of the heat preservation, the workpiece is pulled out of the furnace again and exposed to the air for natural cooling. In particular, in this step, when the parts are exposed to still air for cooling, it is strictly forbidden to use forced air cooling by fans. This process is the final heat treatment. When comprehensively balancing production efficiency, when using natural air cooling, the heat treatment stress is relatively small, which is conducive to reducing the deformation of the workpiece.

[0055] Step 7: Send for metallographic and hardness inspection: After the workpiece is treated with "secondary normalizing", the metallographic inspection shows that its grain size reaches level 6.5, and its grains are obviously refined and uniform. After "twice normalizing + tempering", the metallographic inspection shows that its grain size reaches level 7, and the average hardness of the product is 227HB, which meets the requirement of ≤260HB and meets the technical requirements of the product.

[0056] In this scheme, when the heat treatment method of "950℃ first normalizing + 860℃ second normalizing + 670℃ tempering" is used to batch process workpieces, the heat treatment cycle is 60.5h and the natural gas consumption is 6059.6m3 The degree of decarburization on the workpiece surface is slight, and the grain size can reach level 6. The average hardness test value of the product is 227HB, which meets the technical requirements of the product. At the same time, the qualified rate of the workpiece is 100%, which can greatly improve the production efficiency and quality of the product.

[0057] In this scheme, the selection of the temperature for the two normalizing processes is the result of multiple tests. The details are as follows:

[0058] (1) Select several workpieces as kiel specimens for normalizing temperature selection;

[0059] (2) According to the AC3 point temperature (i.e., austenitizing temperature) of the GS32CrNiMo6V material, the Keel specimens were numbered 1, 2, 3, 4, and 5, and the following temperature tests were performed respectively.

[0060] The first normalizing temperature selection: the temperature of the No. 1 Keel sample is 870℃; the temperature of the No. 2 Keel sample is 950℃; the temperature of the No. 3 Keel sample is 980℃; the No. 1, No. 2 and No. 3 Keel samples are placed in a small electric furnace in the laboratory for simulation tests. The only difference between the No. 1, 2 and 3 Keel samples is the temperature, while other conditions are the same.

[0061] The experimental results show that the normalizing treatment of the Keel specimen is as follows:

[0062] Keel sample number Normalizing Treatment Effect No. 1 870℃ first normalizing treatment The grain size is 0 to 2, and the dendritic crystal segregation has not been eliminated No. 2 950℃ first normalizing treatment The grain size is 5-6, and the dendritic crystal segregation is basically eliminated. No.3 980℃ first normalizing treatment The grain size is level 5, and dendritic crystal segregation is basically eliminated

[0063] According to the above results, the temperature of 950°C is closer to the technical requirements of the product, so this scheme will select 950°C as the temperature of the first normalizing treatment.

[0064] Second normalizing temperature selection: The first normalizing temperature of the No. 2 Keel sample is 950℃, and the second normalizing temperature is 860℃; the first normalizing temperature of the No. 4 Keel sample is 950℃, and the second normalizing temperature is 840℃; the first normalizing temperature of the No. 5 Keel sample is 950℃, and the second normalizing temperature is 880℃; the No. 2, No. 4 and No. 5 Keel samples are placed in a small electric furnace in the laboratory for simulation tests. The only difference between the No. 2, No. 4 and No. 5 Keel samples is that the second normalizing temperature is different, while other conditions are the same.

[0065] The experimental results of the two normalizing treatments of the Kiel specimen are as follows:

[0066]

[0067] According to the above results, and taking into account batch processing and production efficiency, this plan selects the heat treatment method of "the first normalizing temperature is 950℃, the second normalizing temperature is 860℃".

[0068] In this scheme, (1) under the condition of the first normalizing (950℃) treatment of GS32CrNiMo6V casting, it is beneficial to cut off the connection between the original (cast) dendritic segregation structure and the newly formed grains, and the austenite composition is homogenized, so that the cast segregation structure is recrystallized, the dendritic crystal segregation grains are basically eliminated, and the grains are initially refined; under the condition of the second normalizing (860℃) treatment, the cast segregation structure is eliminated, and its structure is adjusted to bainite + a small amount of ferrite, and the grains are refined; under the condition of the second stress relief annealing, the grains are further refined;

[0069] (2) It breaks the technical prejudice in existing technologies that “production efficiency and production quality” cannot be achieved at the same time.

[0070] (3) The problem in the prior art that the high temperature of 1100-1250°C is difficult to reach in general furnaces, and the workpiece surface is severely oxidized and decarburized, the surface is burned, and the workpiece surface quality is poor is solved.

[0071] (4) The insulation time coefficient of the workpiece is selected as (1.8~2.1) min / mm or (2.1~2.4) min / mm. The insulation time is organically linked to the effective cross-sectional thickness of the product, expanding the application range of the material. It is suitable for both test specimens, small cross-sectional products and products with thicker casting cross-sections.

[0072] (5) The holding time coefficient of the first normalizing is (1.8~2.1) min / mm, and the holding time of the second normalizing is (2.1~2.4) min / mm. The austenitizing temperature set for the first normalizing is relatively high, so the normalizing holding coefficient is selected to be lower than the holding coefficient of the second normalizing to avoid overheating of the workpiece due to prolonged heating at a high austenitizing temperature. The austenitizing temperature of the second normalizing is lower, and the normalizing holding coefficient is selected to be larger, which can ensure sufficient austenitization, reduce the thermal stress of the workpiece during heating, and continue to optimize the grain.

[0073] (6) After insulation, cooling adopts the method of "air cooling + air cooling", so that the workpiece can be cooled to the process design temperature quickly and the stress of the workpiece can be reduced.

[0074] (7) The "double normalizing + tempering" in this scheme can achieve a tensile strength of more than 900Mpa, and has good plasticity and impact toughness, which can meet the requirements of large load capacity and durability of chain plate castings.

[0075] (8) This scheme adopts a slower and more uniform heating rate (i.e., 70-90℃ / h or 80-105℃ / h), which can make the temperature inside and outside the workpiece rise evenly, avoiding thermal stress concentration caused by excessive temperature difference. Slow heating helps the internal structure of the workpiece to transform evenly, avoiding local structural unevenness or incomplete phase change caused by rapid heating. In addition, it can also avoid energy waste caused by excessively rapid heating, and optimize energy utilization efficiency while ensuring quality.

[0076] In summary, the overall process in this scheme overcomes the shortcomings of the GS32CrNiMo6V casting material itself, and reaches the high standard requirements of "normalizing + tempering" for delivery, grain size ≥ 6 levels, and surface hardness ≤ 260HB. In addition, this scheme breaks the industry convention. On the basis of using existing processing equipment, this scheme cleverly adds the normalizing step, and finely controls the heating and cooling operations during the two normalizing and tempering processes, which can eliminate the inheritance of the cast dendritic crystal segregation organization, adjust the organization to obtain bainite + a small amount of ferrite; homogenize the organization, refine the grains, avoid the equipment limitations and surface oxidation and decarburization problems caused by high-temperature treatment, shorten the production cycle, and significantly improve production efficiency and workpiece quality.

[0077] Comparative Example 1:

[0078] The workpiece with the same specifications as in the first embodiment is heat treated by diffusion annealing at 1100-1250°C + normalizing + tempering. Specifically, as follows:

[0079] (1) Diffusion annealing: The workpiece temperature is ≤350℃, and the temperature is raised to 1200℃ at a rate of 70-90℃ / h, and kept at 1200℃ for 12h. After the insulation, the workpiece is cooled to below 400℃ at a cooling rate of 50-60℃ / h, and then taken out of the furnace for air cooling.

[0080] (2) Normalizing: The normalizing temperature is 900℃. After the insulation is completed, the workpiece is taken out of the furnace and air-cooled for 40 to 45 minutes, and then air-cooled to ≤300℃.

[0081] (3) Tempering: The tempering temperature is 670℃. After the insulation is completed, the workpiece is taken out of the furnace and air-cooled.

[0082] Through the heat treatment method in comparative example 1, the metallographic inspection diagram is as follows Figure 3 As shown, the heat treatment cycle is 79.5h, the workpiece surface is severely decarburized, the surface decarburization is 5 to 6mm, and there is a certain amount of burn loss, forming local small pits, and the hardness is 215HB. The local pits formed need to be polished and rounded. Specifically, about 30% of the pits of the workpiece need to be repaired by welding, then polished and partially tempered, that is, the qualified rate is 70%. In comparison, the qualified rate of this solution is higher, and the heat treatment cycle can be shortened by 23.8%, and natural gas can save 3086.2m3 / furnace, thereby reducing production costs.

[0083] Comparative Example 2:

[0084] The workpiece with the same specifications as in the first embodiment is used, and the heat treatment method is diffusion annealing + conventional annealing. Specifically, as follows:

[0085] (1) Diffusion annealing: The applicable temperature range of diffusion annealing is 1100-1250° C. In the prior art, the temperature of 1180-1200° C. is usually used for diffusion annealing.

[0086] (2) Conventional annealing: The applicable temperature range of conventional annealing is 800-900°C. In the prior art, a temperature of 860-900°C is usually used for complete annealing.

[0087] By the heat treatment method in comparative example 2, the heat treatment cycle is 74h, the surface decarburization of the workpiece is serious, the surface decarburization is 6-7mm, and there is a certain amount of burn loss, forming local small pits, and the hardness is 200HB. The local pits formed need to be polished and rounded. About 35% of the pits of the workpiece need to be repaired by welding, then polished and locally tempered, that is, the qualified rate is 65%. In comparison, the qualified rate of this scheme is higher, and its heat treatment cycle can be shortened by 18.2%, and natural gas can be saved by 2115.4m3 / furnace, thereby reducing production costs.

[0088] According to the different heat treatment methods used in Example 1, Comparative Example 1 and Comparative Example 2, the effects are shown in the following table:

[0089]

[0090] In summary, this solution optimizes the heat treatment process, which not only shortens the heat treatment cycle, significantly improves product quality and product qualification rate, but also saves a lot of energy costs (such as saving natural gas costs), reduces energy consumption and emissions, and is in line with the development trend of green manufacturing.

[0091] Comparative Example 3:

[0092] The difference between this comparative example and the first embodiment is that the insulation time coefficient of the first target insulation time is different.

[0093] Specifically, in this comparative example, the holding time coefficient of the first target holding time is 1 min / mm, that is, the holding time coefficient of the first normalizing (temperature of 950° C.) is 1 min / mm, and the rest is the same as in the embodiment.

[0094] Comparative Example 4:

[0095] The difference between this comparative example and the first embodiment is that the holding time coefficient of the first target holding time is different, that is, the holding time coefficient of the first normalizing (temperature of 950° C.) is different.

[0096] Specifically, in this comparative example, the holding time coefficient of the first target holding time is 3.5 min / mm, that is, the holding time coefficient of the first normalizing (950° C. temperature) is 3.5 min / mm, and the rest is the same as in the embodiment.

[0097] According to the different heat treatment methods used in Example 1, Comparative Example 3 and Comparative Example 4, the effects are shown in the following table:

[0098]

[0099] Experimental data show that the holding time significantly affects product performance. Specifically, if the holding time is too short (such as in Comparative Example 3) or too long (such as in Comparative Example 4), not only will the product hardness be reduced due to severe decarburization of the product, but the product grain size qualification rate and surface qualification rate will also be reduced.

[0100] The above is only an embodiment of the present invention, and the common knowledge such as the known specific technical solutions and / or characteristics in the solution is not described in detail here. It should be pointed out that for those skilled in the art, without departing from the technical solution of the present invention, several modifications and improvements can be made, which should also be regarded as the protection scope of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A heat treatment process for refining the grains of GS32CrNiMo6V castings, characterized in that: include: First normalizing: heat the workpiece to 950-960℃ at a rate of 70-90℃ / h and keep it warm. The first target holding time is calculated based on 1.8-2.1 minutes per millimeter * workpiece thickness; After reaching the first target holding time, stop heating, take the workpiece out of the furnace and place it in the air cooling area for forced air cooling for 50 to 55 minutes, and then air cool it to ≤300°C; Second normalizing: When the workpiece temperature is ≤300℃, put it back into the furnace, heat it to 860~870℃ at a speed of 70~90℃ / h, and keep it warm. The second target holding time is calculated according to 2.1~2.4 minutes per millimeter * workpiece thickness. After reaching the second target holding time, stop heating, take the workpiece out of the furnace to the air cooling area, force air cooling for 40~45 minutes, and then air cool to ≤300℃; Tempering: When the workpiece temperature is ≤300℃, put it back into the furnace and heat it to 660~680℃ at a rate of 80~105℃ / h. Keep it warm for 7~8h. After the insulation is completed, take the workpiece out of the furnace and expose it to the air for natural cooling. The workpiece is a chain plate steel casting. After processing, the grain size of the chain plate steel casting is ≥ grade 6 and the surface hardness is ≤ 260HB.

2. A heat treatment process for refining the grains of GS32CrNiMo6V castings according to claim 1, characterized in that: Before the first normalizing, stress relief annealing is required. The content of the stress relief annealing is as follows: the temperature of the workpiece entering the furnace is ≤300°C, the workpiece is heated to 620±20°C at a speed of 80-90°C / h, kept warm for 6-8h, cooled in the furnace to ≤300°C, and then air-cooled out of the furnace.

3. A heat treatment process for refining the grains of GS32CrNiMo6V castings according to claim 2, characterized in that: After the stress relief annealing, a painting treatment is required, and the content of the painting treatment is as follows: an anti-oxidation coating is applied to the surface of the workpiece.

4. A heat treatment process for refining the grains of GS32CrNiMo6V castings according to claim 3, characterized in that: After the painting process, the furnace loading process is required, and the content of the furnace loading process is as follows: a number of workpieces are placed in the effective heating area of ​​the furnace temperature field in the trolley furnace.

5. A heat treatment process for refining the grains of GS32CrNiMo6V castings according to claim 4, characterized in that: The distance between adjacent workpieces is ≥100mm.

6. A heat treatment process for refining the grains of GS32CrNiMo6V castings according to claim 4, characterized in that: Place metal shims on the bottom of the trolley to isolate the workpiece from the bottom of the trolley.

7. A heat treatment process for refining the grains of GS32CrNiMo6V castings according to claim 6, characterized in that: The height of the shim is ≥300mm.

8. A heat treatment process for refining the grains of GS32CrNiMo6V castings according to claim 1, characterized in that: A blower is used for forced air cooling.

9. A heat treatment process for refining the grains of GS32CrNiMo6V castings according to claim 1, characterized in that: Use an infrared temperature measuring gun to detect the surface temperature of the workpiece.

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