A heat treatment method for improving the uniformity of the structure of nitrogen-containing 4Cr13 plastic mold steel

Abstract

This invention discloses a heat treatment method for improving the uniformity of the microstructure of nitrogen-containing 4Cr13 plastic mold steel. The method involves smelting and casting plastic mold steel ingots or billets according to the following chemical composition by mass percentage: C: 0.3~0.4%, Si: 0.3~0.5%, Mn: 0.3~0.5%, Cr: 13~14%, N: 0.1~0.2%, C+N: 0.4~0.45%, with the balance being Fe and unavoidable impurities. The heat treatment includes the following sequential steps: (1) high-temperature homogenization treatment; (2) forging; (3) post-forging homogenization treatment; (4) spheroidizing annealing treatment; and (5) quenching and tempering treatment. Its beneficial effects are: based on the elimination of coarse carbides and the provision of dispersed spheroidizing nuclei through high-temperature homogenization and post-forging homogenization, the spheroidizing isothermal temperature is raised to near Ac1, utilizing the high diffusion rate in the high-temperature region to achieve full spheroidization of the carbides. Chain carbides are basically eliminated, and carbides with a diameter of ≤0.2μm account for more than 30% of the total carbides. The annealing hardness is 185-195HB, and the microstructure uniformity and impact toughness are significantly better than those of traditional processes.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for metallic materials, and specifically relates to a heat treatment method for improving the uniformity of the microstructure of nitrogen-containing 4Cr13 plastic mold steel. Background Technology

[0002] The service performance of plastic mold steel is highly dependent on the uniformity of its microstructure. 4Cr13 martensitic stainless steel is widely used in high-mirror-finish plastic molds due to its excellent corrosion resistance and polishing properties. This type of steel has a high alloy element content, making it prone to dendritic segregation and large-sized liquid carbides during solidification. These liquid carbides have high thermal stability and are difficult to eliminate, severely damaging mold quality and lifespan. Currently, a process route of "high-temperature diffusion + forging + spheroidizing annealing" is commonly used to obtain a uniform and fine spheroidized microstructure.

[0003] To further improve the corrosion resistance and mechanical properties of 4Cr13 steel, adding nitrogen (i.e., forming nitrogen-containing 4Cr13 steel) has become an effective alloying method. The addition of nitrogen can strengthen the matrix through solid solution, refine the grains, and promote the formation of more stable carbonitrides. However, the introduction of nitrogen also brings new challenges to the control of microstructure homogenization: on the one hand, nitrogen can alter the precipitation behavior of carbides, promoting the formation of chromium-rich M... 23 C6 type carbides tend towards more stable M 23 (C,N)6 type carbonitride transformation: these precipitates are more difficult to dissolve and diffuse at high temperatures. On the other hand, during solidification and hot working, nitrogen exacerbates the microscopic segregation tendency, making it easier for coarse, angular liquid carbonitrides to form between dendrites. These hard and brittle phases are difficult to eliminate in subsequent heat treatment, severely disrupting the matrix continuity and causing even greater damage to impact toughness and polishing performance than ordinary carbides. High-temperature homogenization can effectively reduce segregation and refine liquid carbides. Studies have shown that a significant liquid carbide band still exists after diffusion at 1180℃, and it can only be basically eliminated after holding at 1210℃ for 12 hours. This process has an excessively long holding time, high energy consumption, and increases the risk of grain coarsening and surface oxidation.

[0004] Spheroidizing annealing is a key process for obtaining granular pearlite and improving machinability. The measured Ac1 of the 4Cr13 steel used in this study was approximately 860℃, and the Ac3 was approximately 920℃. Existing spheroidizing annealing processes are divided into ordinary annealing (holding at 850-880℃ followed by slow cooling to 600℃) and isothermal spheroidizing annealing (heating at 850-880℃ followed by furnace cooling to 730-770℃ for 3-4 hours, then slow cooling to 500℃). Both processes involve prolonged slow cooling. The existing process has the following bottlenecks: First, the annealing hardness is too high and the microstructure is not uniform enough. After ordinary annealing at 850℃, the hardness is as high as 210-260 HB, and although it is reduced to 190-210 HB after annealing at 870℃, there are still liquid carbide bands. Second, the ability to control the morphology of carbides is insufficient. The isothermal time of processes such as patent CN107058704A is as long as 3-8 hours, and chain-like carbides still exist after treatment. Third, the carbides are prone to coarsening. 23 C6 type carbides are prone to forming chain-like aggregates during slow cooling, which significantly reduces toughness.

[0005] Traditional isothermal spheroidizing annealing processes select isothermal temperatures far below Ac1, primarily to avoid the decomposition of austenite into lamellar pearlite. However, the deeper reason lies in the fact that insufficiently homogenized 4Cr13 steel contains a large number of coarse, stable liquid carbides and severe elemental segregation. During spheroidizing annealing, there is a severe shortage of dispersed nuclei for carbide attachment, necessitating forced nucleation in the low-temperature region with significant supercooling. Therefore, directly using high-temperature isothermal annealing near Ac1 results in a lack of nucleation sites, leading to uneven austenite decomposition and the formation of lamellar pearlite instead of a uniform spheroidized structure, making it difficult to guarantee the spheroidizing effect. Summary of the Invention

[0006] To address the aforementioned problems, this invention aims to provide a heat treatment method for improving the uniformity of the microstructure of nitrogen-containing 4Cr13 plastic mold steel. It employs a synergistic process of "high-temperature homogenization + post-forging homogenization + high-temperature isothermal spheroidizing annealing." The preceding steps eliminate coarse carbides and provide dispersed spheroidizing nuclei, creating conditions for raising the isothermal temperature to near Ac1. This results in improved microstructure uniformity while achieving superior spheroidizing effects, thus solving the problems of severe segregation, poor carbide morphology, and difficulty in eliminating chain-like carbides in existing technologies.

[0007] This invention adds a post-forging homogenization treatment after forging. Through recrystallization and element diffusion in the austenite region, it further eliminates the microstructure inhomogeneity caused by forging deformation, refines the carbide distribution, and provides dispersed undissolved carbide nuclei for subsequent high-temperature isothermal spheroidization, thereby reducing the dependence of the spheroidization process on low-temperature nucleation.

[0008] The core innovation of this invention lies in the following: First, high-temperature homogenization (1250-1280℃) effectively dissolves coarse liquid carbides and reduces elemental segregation. Then, post-forging homogenization (1040±10℃) further eliminates the microstructure inhomogeneity caused by forging deformation, refines the carbide distribution, and provides dispersed undissolved carbide nuclei for subsequent high-temperature isothermal spheroidization. On this basis, the isothermal temperature in spheroidizing annealing is significantly increased to around Ac1 (840-860℃). Taking advantage of the high atomic diffusion rate in the high-temperature region around Ac1, the carbides are rapidly spheroidized on sufficient spheroidization nuclei, thereby obtaining a uniform and fine spheroidized microstructure and effectively eliminating chain-like carbides.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: A heat treatment method for improving the microstructure uniformity of nitrogen-containing 4Cr13 plastic mold steel, comprising smelting and casting plastic mold steel ingots or billets according to the following chemical composition by mass percentage: C: 0.3~0.4%, Si: 0.3~0.5%, Mn: 0.3~0.5%, Cr: 13~14%, N: 0.1~0.2%, C+N: 0.4~0.45%, with the balance being Fe and unavoidable impurities; the heat treatment method includes the following steps performed sequentially: (1) High temperature homogenization treatment: Heat the plastic mold steel ingot or billet to 1250-1280℃, keep it at that temperature for 3-6 hours, and then air cool it; (2) Forging: Slowly heat to 800℃ at a heating rate of less than 80℃ / h, then rapidly heat to 1160-1200℃ at a heating rate of more than 80℃ / h and hold for 1-2 hours, forging and transforming into a slab, with a final forging temperature ≥850℃; (3) Homogenization treatment after forging: Heat the slab to 1040±10℃, hold for 2-3 hours, and then air cool; (4) Spheroidizing annealing treatment: The steel treated in step (3) is treated according to the following procedure: heated to 880-900℃ and held for 1-2 hours; furnace cooled to 840-860℃ at a cooling rate of 15-25℃ / h and held for 1-3 hours; then furnace cooled to 600-620℃ at a cooling rate of 5-15℃ / h, and then air cooled after being removed from the furnace. (5) Quenching and tempering treatment: After holding at 1020-1040℃, oil quenching is performed; then tempering is carried out at 500℃ for 2 hours.

[0010] Furthermore, in step (1), the high-temperature homogenization treatment temperature is 1260±5℃, and the holding time is 4-5 hours.

[0011] Furthermore, in step (3), the homogenization treatment temperature after forging is 1040℃ and the holding time is 3 hours.

[0012] Further, in step (4), the spheroidizing annealing process is specifically as follows: heating to 890°C and holding for 1 hour; furnace cooling at 20°C / h to 850°C and holding for 2 hours; furnace cooling at 10°C / h to 600-620°C, followed by air cooling after removal from the furnace.

[0013] The plastic mold steel prepared according to the above heat treatment method has the following chemical composition by mass percentage: C: 0.3~0.4%, Si: 0.3~0.5%, Mn: 0.3~0.5%, Cr: 13~14%, N: 0.1~0.2%, C+N: 0.4~0.45%, with the balance being Fe and unavoidable impurities. Its microstructure is uniform, with chain carbides largely eliminated. Carbides ≤0.2μm account for more than 30% of the total carbides area, and the average grain size is less than 312μm. The solubility of liquid-precipitated carbides is greater than 97.8%, the carbide area fraction is less than 0.4%, the annealed hardness is 185-195HB, and the Charpy U-notch transverse impact energy (A) of the heat-treated material is [not specified]. ku The impact energy of the Charpy U-notch is above 4J, and the longitudinal impact energy is above 6J. The uniformity of the structure and the impact toughness are significantly better than those of the traditional process.

[0014] Compared with the prior art, the present invention has the following beneficial effects: (1) High-temperature homogenization effect This invention raises the high-temperature homogenization temperature to 1250-1280℃, utilizing the high diffusion coefficient of alloying elements at high temperatures to achieve element homogenization. After treatment, element segregation is significantly improved. Simultaneously, the grain size does not increase significantly, with an average grain size of 312 μm; liquid carbide precipitation is significantly reduced, with a solubility of 97.8%, and the remaining carbide area fraction is only 0.4%. These improvements provide dispersed carbide nuclei for subsequent spheroidizing annealing.

[0015] (2) Homogenization effect after forging This invention incorporates a post-forging homogenization treatment at 1040±10℃ for 2-3 hours after forging. This step, through recrystallization and element diffusion in the austenite region, further eliminates the microstructure inhomogeneity caused by forging deformation, refines the carbide distribution, and makes the initial microstructure before spheroidizing annealing more uniform and dispersed. This treatment provides sufficient undissolved carbide nuclei for subsequent spheroidizing annealing, reduces the dependence of the spheroidizing process on low-temperature nucleation, and thus creates the necessary microstructure conditions for raising the isothermal spheroidizing temperature to near Ac1.

[0016] (3) Effect of high-temperature isothermal spheroidizing annealing Traditional spheroidizing annealing processes follow the design principle of "isothermal temperature much lower than Ac1." For 4Cr13 steel with Ac1 of 860℃, the isothermal temperature is typically between 730-770℃. This invention breaks through this traditional concept, significantly increasing the isothermal spheroidizing temperature to near Ac1 (840-860℃). The theoretical basis for this design is that in the high-temperature region near Ac1, the atomic diffusion rate is significantly increased, and the carbide spheroidizing process changes from being controlled by interfacial reaction to diffusion control, resulting in a significant improvement in spheroidizing effect. Simultaneously, because the isothermal temperature is not higher than Ac1, an appropriate amount of undissolved carbides is retained in the microstructure as spheroidizing nuclei, avoiding the formation of lamellar pearlite after austenite homogenization. This invention employs a "high-temperature isothermal + two-stage slow cooling" spheroidizing process, achieving excellent spheroidizing results at 850℃ for 2 hours.

[0017] (4) Synergistic effect The nitrogen-containing 4Cr13 steel treated by this invention exhibits a near-complete elimination of chain carbides, with carbides ≤0.2μm accounting for over 30% of the total carbides (33%-37% in the examples). The annealed hardness reaches 185-195HB, meeting machining requirements. Transverse impact toughness is improved by over 33% compared to conventional processes, and longitudinal impact toughness by over 62%. This performance improvement stems from the synergistic effect of the three-step process: high-temperature homogenization, post-forging homogenization, and high-temperature isothermal spheroidization. The preceding steps eliminate coarse, liquid-precipitated carbides, reduce elemental segregation, refine the microstructure, and provide dispersed spheroidization nuclei, creating the necessary microstructural conditions for subsequent high-temperature isothermal spheroidization. High-temperature isothermal spheroidization, utilizing the high diffusion rate near Ac1, achieves full spheroidization and microstructural homogenization of carbides while retaining undissolved nuclei. Attached Figure Description

[0018] Figure 1 The spheroidized annealed microstructure of nitrogen-containing 4Cr13 plastic mold steel in Example 1 is shown.

[0019] Figure 2 This is the spheroidized annealed microstructure of nitrogen-containing 4Cr13 plastic mold steel in Example 2.

[0020] Figure 3 This is the spheroidized annealed microstructure of nitrogen-containing 4Cr13 plastic mold steel in Example 3.

[0021] Figure 4 This is a comparative example of the spheroidized annealed microstructure of nitrogen-containing 4Cr13 plastic mold steel. Detailed Implementation

[0022] The present invention will now be described in detail with reference to the embodiments and accompanying drawings. It should be noted that the described embodiments are only intended to facilitate the understanding of the present invention and do not constitute any limitation thereof. Example 1

[0023] This embodiment provides a heat treatment method for improving the uniformity of the microstructure of nitrogen-containing 4Cr13 plastic mold steel. The plastic mold steel ingot or billet is obtained by smelting and casting according to the following chemical composition by mass percentage: C: 0.3%, Si: 0.3%, Mn: 0.3%, Cr: 13%, N: 0.12%, C+N: 0.42%, with the balance being Fe and unavoidable impurities. The heat treatment method includes the following steps performed sequentially: (1) High temperature homogenization treatment: The nitrogen-containing 4Cr13 plastic mold steel ingot is heated to 1260℃, kept at the temperature for 4 hours, and then air-cooled to room temperature.

[0024] (2) Forging: The ingot treated in step (1) is slowly heated to 800°C, then rapidly heated to 1180°C, held for 1.5 hours and then forged. The final forging temperature is 880°C.

[0025] (3) Homogenization treatment after forging: Heat the forged steel to 1040℃, hold for 3 hours, and then air cool.

[0026] (4) Spheroidizing annealing treatment: The steel treated in step (3) is spheroidized annealed according to the following procedure: heated to 890°C and held for 1 hour; furnace cooled to 850°C at a cooling rate of 20°C / h and held for 2 hours; furnace cooled to 600°C at a cooling rate of 10°C / h and then air cooled to room temperature.

[0027] (5) Quenching and tempering treatment: After spheroidizing annealing, the steel is held at 1030℃ and then oil quenched; then tempered at 500℃ for 2 hours.

[0028] The nitrogen-containing 4Cr13 plastic mold steel treated in this embodiment exhibits a uniform microstructure, with chain carbides largely eliminated. Carbides ≤0.2μm account for 35.3% of the total carbides, achieving an annealed hardness of 192HB. The final heat-treated material has a transverse impact energy of 4.5J and a longitudinal impact energy of 6.5J. The average grain size is less than 312μm; the solubility of liquid-precipitated carbides is greater than 97.8%, and the carbide area fraction is less than 0.4%. Compared to the comparative example, the segregation ratios of Mn, Cr, Si, V, and Ni decreased from 1.69, 1.72, 1.22, 1.96, and 2.21 before treatment to 1.39, 1.10, 1.05, 1.28, and 1.69, respectively, indicating a significant reduction in segregation. Example 2

[0029] This embodiment provides a heat treatment method for improving the uniformity of the microstructure of nitrogen-containing 4Cr13 plastic mold steel. The plastic mold steel ingot or billet is obtained by smelting and casting according to the following chemical composition by mass percentage: C: 0.35%, Si: 0.4%, Mn: 0.4%, Cr: 13.5%, N: 0.1%, C+N: 0.45%, with the balance being Fe and unavoidable impurities. The heat treatment method includes the following steps performed sequentially: (1) High temperature homogenization treatment: Heat the ingot to 1250℃, keep it at that temperature for 5 hours, and then air cool it to room temperature.

[0030] (2) Forging: The ingot treated in step (1) is slowly heated to 800°C, then rapidly heated to 1160°C, and held for 2 hours before forging deformation. The final forging temperature is 850°C.

[0031] (3) Homogenization treatment after forging: Heat the forged steel to 1030℃, hold for 3 hours, and then air cool.

[0032] (4) Spheroidizing annealing treatment: The steel treated in step (3) is spheroidized annealed according to the following procedure: heated to 880°C and held for 2 hours; furnace cooled to 840°C at a cooling rate of 15°C / h and held for 3 hours; furnace cooled to 600°C at a cooling rate of 5°C / h and then air cooled to room temperature.

[0033] (5) Quenching and tempering treatment: After spheroidizing annealing, the steel is held at 1030℃ and then oil quenched; then tempered at 500℃ for 2 hours.

[0034] The nitrogen-containing 4Cr13 plastic mold steel treated in this embodiment exhibits a uniform microstructure, with chain carbides largely eliminated. Carbides ≤0.2μm account for 37% of the total carbides, and the annealed hardness is 190HB. The final heat-treated material has a transverse impact energy of 5J and a longitudinal impact energy of 7J. The average grain size is less than 312μm; the solubility of liquid-precipitated carbides is greater than 97.8%, and the carbide area fraction is less than 0.4%. Example 3

[0035] This embodiment provides a heat treatment method for improving the uniformity of the microstructure of nitrogen-containing 4Cr13 plastic mold steel. The plastic mold steel ingot or billet is obtained by smelting and casting according to the following chemical composition by mass percentage: C: 0.32%, Si: 0.5%, Mn: 0.5%, Cr: 14%, N: 0.13%, C+N: 0.45%, with the balance being Fe and unavoidable impurities. The heat treatment method includes the following steps performed sequentially: (1) High temperature homogenization treatment: The nitrogen-containing 4Cr13 plastic mold steel ingot is heated to 1260℃, held for 3 hours, and then air-cooled to room temperature.

[0036] (2) Forging: The ingot treated in step (1) is slowly heated to 800°C at a heating rate of 60°C / h, and then rapidly heated to 1200°C at a heating rate of 100°C / h. After holding at the temperature for 1 hour, it is forged and deformed, and the final forging temperature is 900°C.

[0037] (3) Homogenization treatment after forging: Heat the forged steel to 1050℃, hold for 2 hours, and then air cool.

[0038] (4) Spheroidizing annealing treatment: The steel treated in step (3) is spheroidized annealed according to the following procedure: heated to 900°C and held for 1 hour; furnace cooled to 860°C at a cooling rate of 25°C / h and held for 1 hour; furnace cooled to 600°C at a cooling rate of 15°C / h and then air cooled to room temperature.

[0039] (5) Quenching and tempering treatment: After spheroidizing annealing, the steel is held at 1030℃ and then oil quenched; then tempered at 500℃ for 2 hours.

[0040] The nitrogen-containing 4Cr13 plastic mold steel treated in this embodiment has a uniform microstructure, with chain carbides basically eliminated, and carbides ≤0.2μm accounting for 33% of the total carbides. Figure 3 As shown in the figure, the annealed hardness is 195HB. The final heat-treated material has a transverse impact energy of 4J and a longitudinal impact energy of 6.5J. The average grain size is less than 312μm; the solubility of liquid-precipitated carbides is greater than 97.8%, and the carbide area fraction is less than 0.4%. Comparative Example

[0041] The chemical composition of this comparative example is C: 0.32%, Si: 0.5%, Mn: 0.5%, Cr: 14%, N: 0.13%, C+N: 0.45%, with the balance being Fe and unavoidable impurities; a typical existing technology process is used for comparison with the embodiments of the present invention.

[0042] (1) High temperature homogenization treatment: The nitrogen-containing 4Cr13 plastic mold steel ingot is heated to 1210℃, held for 12 hours, and then air-cooled.

[0043] (2) Forging: The ingot treated in step (1) is heated to 1150°C for forging, and the final forging temperature is 850°C.

[0044] (3) Post-forging heat treatment: air cooling after forging.

[0045] (4) Spheroidizing annealing treatment: The traditional isothermal spheroidizing annealing process is adopted: heating to 870℃ and holding for 3 hours; furnace cooling to 750℃ at a cooling rate of 30℃ / h and holding for 4 hours; furnace cooling to 500℃ at a cooling rate of 30℃ / h, and then air cooling after removal from the furnace.

[0046] (5) Quenching and tempering treatment: After spheroidizing annealing, the steel is held at 1030℃ and then oil quenched; then tempered at 500℃ for 2 hours.

[0047] The nitrogen-containing 4Cr13 plastic mold steel treated in this comparative example still exhibits obvious chain carbides in its microstructure, with carbides ≤0.2μm accounting for 18.9% of the total carbides. The annealed hardness is 205HB. The material after final heat treatment has a transverse impact energy of 3J and a longitudinal impact energy of 4J.

[0048] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A heat treatment method for improving the uniformity of microstructure in nitrogen-containing 4Cr13 plastic mold steel, characterized in that, Plastic mold steel ingots or billets are obtained by smelting and casting according to the following chemical composition by mass percentage, wherein C: 0.3~0.4%, Si: 0.3~0.5%, Mn: 0.3~0.5%, Cr: 13~14%, N: 0.1~0.2%, C+N: 0.4~0.45%, balance being Fe and unavoidable impurities; the heat treatment method includes the following steps performed sequentially: (1) High temperature homogenization treatment: Heat the plastic mold steel ingot or billet to 1250-1280℃, keep it at that temperature for 3-6 hours, and then air cool it; (2) Forging: Heat to 800℃ at a heating rate of less than 80℃ / h, then heat to 1160-1200℃ at a heating rate of more than 80℃ / h and hold for 1-2 hours, forging and transforming into a slab, with a final forging temperature ≥850℃; (3) Homogenization treatment after forging: Heat the slab to 1040±10℃, hold for 2-3 hours, and then air cool; (4) Spheroidizing annealing treatment: The steel treated in step (3) is treated according to the following procedure: heated to 880-900℃ and held for 1-2 hours; furnace cooled to 840-860℃ at a cooling rate of 15-25℃ / h and held for 1-3 hours; then furnace cooled to 600-620℃ at a cooling rate of 5-15℃ / h, and then air cooled after being removed from the furnace. (5) Quenching and tempering treatment: oil quenching after holding at 1020-1040℃; It was then tempered at 500°C for 2 hours.

2. The heat treatment method for improving the uniformity of microstructure of nitrogen-containing 4Cr13 plastic mold steel according to claim 1, characterized in that, In step (1), the high-temperature homogenization treatment temperature is 1260±5℃, and the holding time is 4-5 hours.

3. The heat treatment method for improving the uniformity of microstructure of nitrogen-containing 4Cr13 plastic mold steel according to claim 1, characterized in that, In step (3), the homogenization treatment temperature after forging is 1040℃ and the holding time is 3 hours.

4. The heat treatment method for improving the uniformity of microstructure of nitrogen-containing 4Cr13 plastic mold steel according to claim 1, characterized in that, In step (4), the spheroidizing annealing process is as follows: heating to 890℃ and holding for 1 hour; furnace cooling at 20℃ / h to 850℃ and holding for 2 hours; furnace cooling at 10℃ / h to 600-620℃, followed by air cooling after removal from the furnace.

5. A plastic mold steel prepared by the heat treatment method according to any one of claims 1-4, characterized in that, Its chemical composition, by mass percentage, includes: C: 0.3~0.4%, Si: 0.3~0.5%, Mn: 0.3~0.5%, Cr: 13~14%, N: 0.1~0.2%, C+N: 0.4~0.45%, with the balance being Fe and unavoidable impurities; its microstructure is uniform, with carbides ≤0.2μm accounting for more than 30% of the total carbides area, and the average grain size is less than 312μm; the solubility of liquid-precipitated carbides is greater than 97.8%, the carbide area fraction is less than 0.4%, the annealed hardness is 185-195HB, and the transverse impact energy of the Charpy U-notch after heat treatment is greater than 4J, and the longitudinal impact energy of the Charpy U-notch is greater than 6J.

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