Heat treatment processing method for refining grains of large steel forgings

Abstract

The invention discloses a heat treatment processing method for grain refinement of a large steel forge piece. The heat treatment processing method comprises the following steps: S1, selecting steel suitable for grain refinement; s2, the forge piece is heated to be lower than 700 DEG C at a certain heating speed; s3, the forge piece continues to be heated to the austenitizing temperature; s4, normalizing treatment is conducted on the forge piece; s5, the normalized forge piece is subjected to multiple times of heat treatment cycle operation; s6, the forge piece is heated to the temperature higher than Ac3; s7, tempering treatment is conducted according to requirements; and S8, after heat treatment is completed, ultrasonic flaw detection and other necessary quality detection are conducted on the forge piece. The strength and toughness of the forge piece are remarkably improved, so that the overall mechanical performance of the forge piece is improved, fine crystal grains can more effectively hinder dislocation movement, the material is more difficult to deform when bearing external force, thick and uneven structures and widmannstatten structures generated in the forging process can be eliminated through heat treatment, and the mechanical performance of the forge piece is improved. The microstructure of the whole forge piece is more uniform, and the comprehensive performance of the material is improved.

Description

Technical Field

[0001] The invention relates to the technical field of steel forging processing, in particular to a heat treatment processing method for refining grains of large steel forgings. Background Art

[0002] Large steel forgings refer to steel parts with large size and complex shape produced by forging process, which are usually used in important equipment and engineering structures. Due to its important role and special manufacturing requirements. According to the definition of China Forging Association, large forgings refer to free forgings produced by hydraulic presses of more than 1,000 tons or free forging hammers of more than 5 tons, and die forgings produced by hot die forging equipment of more than 6,000 tons or die forging hammers of more than 10 tons. Large forgings can be divided into four categories: large die forgings, large free forgings, rings and large-diameter thick-walled seamless pipes. The manufacture of large steel forgings usually includes the following steps: steelmaking, forging, heat treatment, machining and physical and chemical testing. Forging is the core process, which involves heating, plastic deformation and cooling processes, with the aim of obtaining the desired shape and microstructure. Heat treatment of large steel forgings to refine grains is a process aimed at improving the microstructure of forgings and improving their performance.

[0003] At present, in the process of producing and forging large steel forgings, the internal structure of the steel forgings after processing has poor mechanical properties, poor rigidity and toughness, and is prone to brittle cracking. Therefore, it is necessary to develop a heat treatment processing method for refining the grains of large steel forgings to solve the above-mentioned technical problems. Summary of the invention

[0004] The purpose of the present invention is to provide a heat treatment processing method for refining the grains of large steel forgings, so as to solve the problem raised in the above-mentioned background technology that during the current production and forging process of large steel forgings, the internal structure of the steel forgings after processing and forming has poor mechanical properties, poor rigidity and toughness, and is prone to brittle cracking.

[0005] To achieve the above-mentioned purpose, the present invention provides the following technical scheme: A heat treatment method for refining grains of large steel forgings, comprising the following method steps: S1, selecting steel suitable for refining grains, heating the steel ingot to an appropriate forging temperature, and forging to form a desired shape and size; S2, heating the forging to below 700°C at a certain heating rate to reduce the temperature difference and thermal stress inside the forging; S3, continuing to heat the forging to an austenitizing temperature, which is higher than the phase transformation temperature Ac3 of the material, and keeping it at this temperature for a period of time to ensure that the temperature inside the forging is uniform and complete the austenite transformation; S4, The forgings are normalized, and after being taken out of the heating furnace, they are cooled to room temperature with air, and the cooling rate is controlled to avoid excessive cooling that causes grain growth; S5. The forgings after normalization are subjected to multiple heat treatment cycles; S6. The forgings are heated to a temperature higher than Ac3, quickly cooled to room temperature, and this process is repeated several times; S7. After the heat treatment process for grain refinement, tempering treatment is required as required to adjust the hardness and toughness of the forgings; S8. After the heat treatment is completed, the forgings are subjected to ultrasonic flaw detection and other necessary quality inspections to ensure that there are no defects such as white spots, and to verify that the grain size meets the requirements.

[0006] Preferably, in step S1, low alloy steel with good heat treatment response is selected as the forging material, and the forging temperature is controlled at 500-600°C.

[0007] Preferably, in step S3, the forging is heated to a temperature above 700° C. and the heating rate is increased to form a uniform austenite structure.

[0008] Preferably, the heating temperature of each normalizing treatment in step S5 may be slightly lower than that of the previous normalizing treatment to promote grain refinement.

[0009] Preferably, in step S7, grain refinement is achieved by controlling the heating and cooling processes, and a special cooling method is adopted, including spray cooling or controlled atmosphere cooling method.

[0010] Preferably, in step S8, the forging is subjected to quality inspection after the heat treatment, including hardness test, metallographic inspection and performance test.

[0011] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention significantly improves the strength and toughness of the forging, thereby improving its overall mechanical properties. The fine grains can more effectively hinder the movement of dislocations, making the material more difficult to deform when subjected to external forces. Heat treatment can eliminate the coarse uneven structure and Widmanstatten structure generated during the forging process, making the microstructure of the entire forging more uniform, which is beneficial to improving the comprehensive performance of the material. (2) The grain refinement process of the present invention can reduce the initiation and propagation of fatigue cracks, thereby improving the fatigue life of forgings. This is particularly important for structural parts that are subjected to cyclic loads. Fine grains can improve the impact toughness of the material, making the forgings less likely to break when subjected to impact loads. Heat treatment can eliminate or reduce the residual stress generated during the forging process, reducing the risk of cracking of the forgings due to stress concentration during subsequent processing or use. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 The figure is a flowchart of the method steps of the present invention. DETAILED DESCRIPTION

[0013] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0014] See also Figure 1 The present invention provides an embodiment: a heat treatment method for refining grains of large steel forgings, comprising the following method steps: S1, selecting steel suitable for grain refinement, heating the steel ingot to an appropriate forging temperature, and forging to form a desired shape and size; S2, heating the forging to below 700°C at a certain heating rate to reduce the temperature difference and thermal stress inside the forging; S3, continuing to heat the forging to an austenitizing temperature, which is higher than the phase transition temperature Ac3 of the material, and keeping it at this temperature for a period of time to ensure that the temperature inside the forging is uniform and complete the austenite transformation; S4, After normalizing treatment, the forging is taken out of the heating furnace and cooled to room temperature by air, and the cooling rate is controlled to avoid excessive cooling that causes grain growth; S5, the forgings after normalizing are subjected to multiple heat treatment cycles; S6, the forgings are heated to a temperature higher than Ac3, quickly cooled to room temperature, and this process is repeated several times; S7, after the heat treatment process for grain refinement, tempering treatment is performed as required to adjust the hardness and toughness of the forgings; S8, after the heat treatment is completed, the forgings are subjected to ultrasonic flaw detection and other necessary quality inspections to ensure that there are no defects such as white spots, and to verify that the grain size meets the requirements.

[0015] Furthermore, in step S1, low alloy steel with good heat treatment response is selected as the forging material, and the forging temperature is controlled at 500-600°C. Before reaching 700°C, a slower heating rate is usually used to reduce the thermal stress inside the forging. Once the temperature reaches above 700°C, the heating rate can be significantly increased. At this time, the phase transformation in the forging is accelerated, and ferrite and pearlite are rapidly transformed into austenite.

[0016] Furthermore, in step S3, the heating temperature of the forging is above 700°C, and the heating rate is increased to form a uniform austenite structure. The heating above 700°C is to transform the ferrite and pearlite structures inside the forging into austenite. Austenite is an iron-carbon alloy with a face-centered cubic crystal structure, which has good plasticity and is conducive to subsequent forging and forming. In the austenite state, the forging is also easier to achieve the desired mechanical properties through heat treatment.

[0017] Furthermore, the heating temperature of each normalizing treatment in step S5 may be slightly lower than that of the previous normalizing treatment to promote grain refinement.

[0018] Furthermore, in step S7, grain refinement is achieved by controlling the heating and cooling processes, and by adopting special cooling methods, including spray cooling or controlled atmosphere cooling methods. Spray cooling allows small water droplets to contact the high-temperature surface, quickly absorb heat and evaporate, thereby achieving efficient heat dissipation. Controlled atmosphere cooling allows cooling in a specific atmosphere, which can effectively prevent oxidation and corrosion of the workpiece at high temperatures.

[0019] Furthermore, in step S8, the forgings are subjected to quality inspection after the heat treatment, including hardness test, metallographic test and performance test, which are usually carried out after the heat treatment to check whether the material has reached the specified hardness value. The metallographic test is to evaluate the quality and performance of the material by observing the microstructure inside the material. The performance test includes a tensile test: measuring the tensile strength, yield strength, elongation and cross-sectional shrinkage of the material; an impact test: evaluating the impact toughness of the material, usually carried out in a low temperature environment; and a fatigue test: measuring the fatigue life of the material under repeated stress.

[0020] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0021] The standard parts used in this application document can all be purchased from the market, and the specific connection methods of each part are connected by conventional means such as mature bolts, rivets, welding, etc. in the prior art. In addition, the machinery, parts and equipment all adopt conventional models in the prior art, and the circuit connection adopts the conventional connection method in the prior art, so no specific description is given here.

[0022] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A heat treatment method for refining grains of large steel forgings, characterized in that: The method comprises the following steps: S1. Select steel suitable for grain refinement, heat the steel ingot to the appropriate forging temperature, and forge it to form the desired shape and size; S2. Raise the temperature of the forging to below 700°C at a certain heating rate to reduce the temperature difference and thermal stress inside the forging; S3, continue to heat the forging to the austenitizing temperature, which is higher than the phase transformation temperature Ac3 of the material, and keep it at this temperature for a period of time to ensure that the temperature inside the forging is uniform and complete the austenite transformation; S4. Perform normalizing treatment on the forgings. After taking the forgings out of the heating furnace, use air to cool them to room temperature, and control the cooling speed to avoid excessive cooling that causes grain growth; S5. Perform multiple heat treatment cycles on the forgings after normalizing; S6, heating the forging to a temperature higher than Ac3, rapidly cooling it to room temperature, and repeating this process several times; S7. After the heat treatment process for grain refinement, tempering treatment is required to adjust the hardness and toughness of the forging; S8. After heat treatment, the forgings are subjected to ultrasonic testing and other necessary quality inspections to ensure that there are no defects such as white spots, and to verify that the grain size meets the requirements.

2. A heat treatment method for refining grains of large steel forgings according to claim 1, characterized in that: In the step S1, low alloy steel with good heat treatment response is selected as the forging material, and the forging temperature is controlled at 500-600°C.

3. The heat treatment method for refining grains of large steel forgings according to claim 1, characterized in that: In the step S3, the forging is heated to a temperature above 700° C. and the heating rate is increased to form a uniform austenite structure.

4. The heat treatment method for refining grains of large steel forgings according to claim 1, characterized in that: The heating temperature of each normalizing treatment in step S5 may be slightly lower than that of the previous normalizing treatment to promote grain refinement.

5. The heat treatment method for refining grains of large steel forgings according to claim 1, characterized in that: In step S7, the grain refinement is achieved by controlling the heating and cooling processes, and a special cooling method is adopted, including spray cooling or controlled atmosphere cooling method.

6. The heat treatment method for refining grains of large steel forgings according to claim 1, characterized in that: In the step S8, the forging is subjected to quality inspection after the heat treatment, including hardness test, metallographic inspection and performance test.

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