Heat treatment process method of martensitic stainless steel casting

Abstract

The invention relates to the technical field of casting, in particular to a heat treatment process method of a martensitic stainless steel casting, which comprises the following steps: S1, heating the casting to 450-700 DEG C, preserving heat, cooling to below 400 DEG C along with a furnace, and then discharging and air-cooling; s2, the casting treated in the step S1 is heated to 1020-1100 DEG C, heat preservation is conducted, then air cooling is conducted, and a fine and uniform martensite structure is obtained; s3, the casting treated in the step S2 is heated to be between Ac1 and Ac3, and air cooling is conducted after heat preservation; and S4, the casting treated in the step S3 is heated to 690-730 DEG C, heat preservation is conducted, and then air cooling is conducted. By introducing two-phase region normalizing, the plasticity and the impact toughness of the martensitic stainless steel casting are remarkably improved on the premise that the strength is not sacrificed, and the fundamental problem that the strength and the toughness of the GX8CrNi12 casting are difficult to consider at the same time is solved.

Description

Technical Field

[0001] This application relates to the field of casting technology, and in particular to a heat treatment process for martensitic stainless steel castings. Background Technology

[0002] GX8CrNi12 is a typical low-carbon martensitic stainless steel. Due to its excellent corrosion resistance, high strength, and weldability, it is widely used in high-end equipment fields such as nuclear power and petrochemicals, especially as key pressure-bearing castings such as the main steam valve shell and high-pressure inner cylinder of nuclear power turbines. Under the requirement of a 60-year design life of nuclear power units, these components must not only possess stable mechanical properties, but also simultaneously meet comprehensive indicators such as high strength, high plasticity, and excellent impact toughness. This places extremely high demands on the material's microstructure uniformity, residual stress control, and the matching of strength and toughness.

[0003] Currently, the conventional heat treatment process for large GX8CrNi12 castings mostly adopts a two-stage process of "high-temperature normalizing + high-temperature tempering". Although this method can eliminate casting segregation to a certain extent and obtain tempered martensite structure, it is difficult to balance strength and toughness: if the normalizing temperature is increased to improve the uniformity of the structure, the resulting martensite is hard and brittle, and even with subsequent high-temperature tempering, it is difficult to fully restore plasticity and toughness; if the tempering temperature is decreased to retain strength, the impact performance is significantly insufficient. In addition, the coarse carbides, Laves phase, and continuous δ-ferrite network in the as-cast structure are difficult to effectively break down or spheroidize in traditional processes, further restricting the improvement of overall performance.

[0004] Therefore, there is an urgent need for a new heat treatment approach that can significantly improve the plasticity and toughness of materials without sacrificing strength. Summary of the Invention

[0005] This application solves the problem that existing GX8CrNi12 martensitic stainless steel large castings, under the existing heat treatment process, are difficult to effectively control the microstructure, resulting in excessively high strength and insufficient plasticity and toughness (or vice versa), and improves the comprehensive mechanical properties of strength and good plasticity and toughness.

[0006] This application provides a heat treatment process for martensitic stainless steel castings, comprising the following steps: S1: After heating the GX8CrNi12 casting to 450~700℃ and holding it at that temperature, cool it in the furnace to below 400℃ before taking it out of the furnace and air cooling it. S2: The casting treated in step S1 is heated to 1020~1100℃ and held at that temperature before being air-cooled to obtain a fine and uniform martensitic structure. S3: Heat the casting processed in step S2 to A. c1 With A c3 In between, heat preservation followed by air cooling; S4: Heat the casting after step S3 to 690~730℃, hold for a period of time, and then air cool.

[0007] Preferably, the heat preservation time in step S3 is 2 to 6 hours.

[0008] Preferably, the heat preservation time in step S2 is 3 to 8 hours.

[0009] Preferably, the heat preservation time in step S1 is 4 to 10 hours.

[0010] Preferably, the heat preservation time in step S4 is 4 to 8 hours.

[0011] Preferably, the A c1 The temperature is 725~740℃, and the A c3 The temperature ranges from 790℃ to 810℃.

[0012] This application achieves structural restructuring and performance decoupling by introducing "two-phase normalizing"; it achieves a significant improvement in plasticity and impact toughness without sacrificing strength. Attached Figure Description

[0013] Figure 1 This is a schematic flowchart of a heat treatment process for martensitic stainless steel castings provided in an embodiment of this application. Detailed Implementation

[0014] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0015] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0016] Please refer to Figure 1 , Figure 1 This is a schematic flow chart of a heat treatment process for martensitic stainless steel castings provided in an embodiment of this application, including: S1: After heating the GX8CrNi12 casting to 450~700℃ and holding it at that temperature, cool it down to below 400℃ in the furnace before taking it out and air cooling it.

[0017] Large GX8CrNi12 castings, after complete solidification and cooling to room temperature, contain enormous residual casting stress, structural stress, and thermal stress. At this point, the material's plasticity and toughness are extremely poor. Directly flame-cutting (such as gas cutting) the riser or patching can easily cause cold cracks or even render the casting unusable. Therefore, stress-relieving annealing is necessary for the casting.

[0018] Stress-relieving annealing temperature range: 450~700℃, lower than material A c1 Phase transition point (A in the text) c1 The lower critical temperature of GX8CrNi12 steel is approximately 730℃, therefore no austenitic phase transformation occurs. This step aims to eliminate residual casting stress caused by uneven shrinkage during solidification and cooling. Specifically, the casting is heated to the target temperature in the furnace, held for several hours (usually 4-10 hours), and then cooled. During cooling, it is first slowly cooled in the furnace to below 400℃ before being removed and air-cooled to avoid generating new thermal stress during cooling. In this process, the matrix remains as-cast martensite + δ-ferrite + carbides, but the internal micro-stress is significantly reduced.

[0019] S2: The casting treated in step S1 is heated to 1020~1100℃ and held at that temperature before being air-cooled to obtain a fine and uniform martensitic structure.

[0020] The as-cast microstructure of the casting treated in step S1 exhibits severe segregation and brittle phases, which cannot be eliminated by direct tempering. This step, a high-temperature normalizing process heating the casting to 1020~1100℃, is the first step in "resetting" the microstructure, providing a clean and uniform initial matrix for subsequent performance control. If the temperature is too low, the precipitated phases will not dissolve sufficiently; if it is too high, it will lead to coarsening of the austenite grains, impairing toughness.

[0021] This step fully dissolves the coarse, unevenly distributed carbides and intermetallic phases in the as-cast microstructure; achieves homogenization of alloying elements (Cr, Ni, etc.); and obtains fine, uniform austenitic grains.

[0022] Afterwards, air cooling (not water quenching) is performed. Since GX8CrNi12 is a low-carbon martensitic steel, air cooling can produce fine lath martensite.

[0023] Furthermore, holding the temperature at 1020~1100℃ for 3~8 hours allows the sparingly soluble Laves phase in the as-cast state to fully dissolve, achieving compositional homogenization. Simultaneously, large castings have thick cross-sections and slow heat transfer, requiring sufficient holding time to ensure phase transformation also occurs in the core; sufficient dissolution and precipitation of the phase prevents abnormal precipitation or brittle phase residue during subsequent heat treatment; providing a clean and uniform initial martensitic matrix for subsequent two-phase normalizing is a prerequisite for achieving high performance.

[0024] S3: Heat the casting processed in step S2 to A.c1 With A c3 Between these, the insulation is followed by air cooling.

[0025] This step is two-phase normalizing, heating the casting to A... c1 (Approximately 730℃) and A c3 (A) c3 The upper critical temperature is between approximately 810℃, which is the region where the α (ferrite) and γ (austenite) phases coexist.

[0026] During this process, the original martensite partially undergoes reverse transformation at this temperature to form carbon-rich austenite islands. Simultaneously, carbides undergo spheroidization, aggregation, and even partial dissolution. After the holding period, air cooling occurs, and the newly formed austenite transforms again into new martensite with a significantly reduced dislocation density.

[0027] This step is not a simple tempering process, but a reconstruction cycle of "martensite → austenite → neomartensite". The original martensite, which is "high strength and hardness but brittle", is transformed into neomartensite and spherical carbides, which are "moderate in strength and excellent in toughness".

[0028] This step involves setting the normalizing temperature in the two-phase region at A. c1 With A c3 The temperature is maintained for 2-6 hours to ensure sufficient time for martensite reversal, carbide spheroidization, and component diffusion. If the time is too short (<2 hours), incomplete martensite reversal may occur, resulting in poor microstructure reconstruction; if the time is too long (>6 hours), grain coarsening or increased energy consumption may result. In this embodiment, 2-6 hours is the optimal window for balancing microstructure control efficiency and production cost, ensuring the quality of new martensite formation and optimized carbide morphology.

[0029] Traditional processes rely solely on tempering to soften martensite, but this makes it difficult to maintain strength. This step actively "resets" the martensite structure through a controllable reverse phase transformation mechanism, reducing the strengthening effect without sacrificing the advantages of fine grains, thus achieving a balance between high strength and high elongation.

[0030] S4: Heat the casting after step S3 to 690~730℃, hold for a period of time, and then air cool.

[0031] The casting treated in step S3 is heated to 690~730℃, slightly lower than or close to A. c1 This falls under the category of high-temperature tempering; it allows the new martensite formed in step S3 to be fully tempered, precipitating dispersed carbides, eliminating micro-stress in the new martensite, promoting the decomposition of residual austenite, and stabilizing the final microstructure and properties.

[0032] Air cooling was used to avoid introducing new stresses through rapid cooling, ultimately resulting in tempered sorbite (fine carbides dispersed in a ferrite matrix) and a small amount of stable δ-ferrite. Although the two-phase normalizing in step S3 significantly improved toughness, the new martensite still exhibited some brittleness. This tempering step is the performance "finalization" stage, ensuring the stability of the material microstructure and improving reliability.

[0033] This embodiment breaks through the traditional "normalizing + tempering" mode and achieves microstructure reconstruction and performance decoupling by introducing "two-phase normalizing". It significantly improves plasticity and impact toughness without sacrificing strength, solving the fundamental problem of the difficulty in balancing strength and toughness in GX8CrNi12 castings. The four stages work together to balance stress relief, microstructure homogenization, strengthening control and toughness optimization, thereby improving reliability.

[0034] The heat treatment process for martensitic stainless steel castings provided in this application is described below with reference to a specific embodiment: The material involved in this embodiment is GX8CrNi12, whose material composition is C: 0.01~0.10, Si: ≤0.50, Mn: 0.40~0.80, P: ≤0.025, S: ≤0.015, Ni: 0.90~1.50, Cr: 11.50~13.00, Mo: ≤0.50, and the remainder is iron. The process method in this embodiment includes annealing, high-temperature normalizing, two-phase normalizing, and tempering.

[0035] (1) Annealing: After the casting has completely solidified and cooled to room temperature, it contains enormous residual casting stress, structural stress, and thermal stress. At this point, the material's plasticity and toughness are extremely poor. Directly flame-cutting (such as gas cutting) the riser or patching can easily cause cold cracks or even render the casting unusable. Stress-relieving annealing is necessary for the casting.

[0036] After solidification, the casting cools rapidly, resulting in a mixed microstructure of martensite, δ-ferrite, and retained austenite, with severe dendritic segregation. High temperatures increase atomic mobility, causing the high-density dislocations in the as-cast martensite to move, rearrange, and partially annihilate. Additionally, carbon in the supersaturated as-cast martensite begins to precipitate as carbides. Although these precipitates are coarse, the process itself reduces lattice distortion and helps relax stress. Since the casting will subsequently undergo quality heat treatment, only partial stress relief is needed to ensure no cracking during cutting; the stress-relieving annealing temperature can be set between 450℃ and 700℃.

[0037] (2) High-temperature normalizing: The purpose of high-temperature normalizing heat treatment is to fully dissolve the precipitates in the as-cast part, forming smaller-grained austenite after austenitization, and then forming a martensitic structure with uniform grain size after cooling. Because GX8CrNi12 material contains high levels of Cr, Ni, and Mo alloying elements and their compounds, as well as precipitates such as Z-phase and Laves phase, coupled with dendrites formed during the solidification of the molten steel, the microstructure of the casting is inhomogeneous. Therefore, in order to dissolve these compound structures and precipitates while simultaneously homogenizing the material microstructure, the normalizing temperature needs to be as high as possible, much higher than the A-phase. c3 At the same time, the normalizing temperature should not be too high, as this would lead to coarse grains and deteriorate the material's toughness. Cr is the most important element in heat-resistant steel for its anti-oxidation and anti-hot corrosion properties. Generally, the lowest dissolution temperature of Cr compounds is 1020℃, so the normalizing holding temperature is between 1020℃ and 1100℃.

[0038] The CCT curves of the simulated material show that GX8CrNi12 has a relatively low critical cooling rate for martensite formation, making it a typical air-hardening steel. If cooled from the austenitic temperature to room temperature over a relatively wide cooling rate range, the austenitic structure can be completely transformed into martensite. After the casting is austenitized, the cooling rate has little effect on performance fluctuations. Therefore, air cooling is sufficient.

[0039] The normalizing austenitization process of this type of material is relatively fast, so a low coefficient can be used for the holding time. Considering the actual furnace holding effect and temperature conditions, as well as the specific casting wall thickness and the large number of alloying elements in the material, the holding coefficient can be 2 min / mm based on the maximum wall thickness of the casting, but at least 6 hours is required.

[0040] (3) Two-phase zone normalization: High-temperature normalizing dissolves a large amount of carbides, and alloying elements (Cr, Mo, V, etc.) dissolve in austenite. During tempering, a large number of fine, coherent / semi-coherent alloying carbides precipitate from the supersaturated martensite, producing a strong precipitation strengthening effect. Simultaneously, the matrix retains a high level of dissolved carbon and alloying elements, resulting in strong solid solution strengthening. At this stage, the material exhibits relatively high strength and hardness, but poor ductility and toughness. In two-phase normalizing, the martensite obtained from the first normalizing undergoes a high-temperature tempering process. This process involves a dramatic recovery phenomenon, with dislocation rearrangement and annihilation occurring in the high-dislocation-density martensite, significantly reducing the dislocation density. Although subsequent cooling forms new secondary martensite, this new martensite forms on a "tempered and softened" matrix, and its overall dislocation density and distortion energy are still far lower than the microstructure obtained from only one normalizing. Meanwhile, the carbides precipitated during heating continuously mature and spheroidize, significantly reducing their ability to pin dislocations, meaning the precipitation strengthening effect is greatly weakened. The coarsening and growth of carbides consumes carbon and alloying elements in the matrix, leading to a simultaneous decrease in the solid solution strengthening effect of the matrix. Therefore, two-phase normalizing can better match the material's strength, hardness, and ductility.

[0041] A of GX8CrNi12 material c1 The temperature is 725~740℃, A c3 The temperature range is 790℃ to 810℃. The normalizing temperature in the two-phase region is selected at point A. c1 and A c3 In this embodiment, the normalizing temperature in the two-phase region can be controlled between 730℃ and 800℃.

[0042] (4) Tempering: M of GX8CrNi12 material f When the temperature is above 100℃, ensure that the casting cools naturally in air to M before tempering. f The following points are considered: If the tempering temperature is too low, the tempering effect will be insignificant, resulting in excessively high material strength but poor ductility and toughness. If the tempering temperature is too high, austenite will reform and re-harden during subsequent cooling, which is detrimental to the control of mechanical properties. Based on the commonly used performance requirements of this material, the tempering temperature is determined to be between 690 and 730℃. Because the tempering temperature is relatively low, the driving force for microstructure transformation is slightly lower; therefore, the tempering time should be 3 hours longer than normalizing to ensure complete martensitic transformation.

[0043] Table 1 shows examples of test specimens after implementing the process method parameters provided in this application: Table 1:

[0044]

[0045] Table 2 shows examples of performance test results after implementing the process method provided in this application: Table 2:

[0046] m / MPa <![CDATA[ R p 0.2 / MPa]]> （%） （%） Impact KV2 / J, room temperature Hardness / HB Test piece 1 617 463 28 73 138 192 Test piece 2 620 455 28 74 152 191 Test piece 3 648 490 26 72 162 204 Test piece 4 655 501 23 71 108 207 The example results show that after treatment with this process, the castings exhibit tensile strength of 610–660 MPa, elongation ≥23%, reduction of area ≥70%, impact energy ≥100 J, and hardness of 190–210 HB. This exceeds the levels of existing processes (traditional processes typically have elongation <20% and impact energy <80 J); it also meets the comprehensive requirements for strength, plasticity, toughness, and hardness of nuclear-grade standards such as RCC-M.

[0047] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0048] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A heat treatment process for martensitic stainless steel castings, characterized in that, Includes the following steps: S1: After heating the GX8CrNi12 casting to 450~700℃ and holding it at that temperature, cool it in the furnace to below 400℃ before taking it out of the furnace and air cooling it. S2: The casting treated in step S1 is heated to 1020~1100℃ and held at that temperature before being air-cooled to obtain a fine and uniform martensitic structure. S3: Heat the casting processed in step S2 to A. c1 With A c3 In between, heat preservation followed by air cooling; S4: Heat the casting after step S3 to 690~730℃, hold for a period of time, and then air cool.

2. The heat treatment process according to claim 1, characterized in that, The heat preservation time in step S3 is 2 to 6 hours.

3. The heat treatment process according to claim 1, characterized in that, The heat preservation time in step S2 is 3 to 8 hours.

4. The heat treatment process according to claim 1, characterized in that, The heat preservation time in step S1 is 4 to 10 hours.

5. The heat treatment process according to claim 1, characterized in that, The heat preservation time in step S4 is 4 to 8 hours.

6. The heat treatment process according to claim 1, characterized in that, The A c1 The temperature is 725~740℃, and the A c3 The temperature ranges from 790℃ to 810℃.

Images