A heat treatment method for eliminating banded structure of low alloy steel

Abstract

This invention discloses a heat treatment method for eliminating banded structures in low-alloy steel, comprising the following steps: heating a low-alloy steel billet with banded structures to A... C3 The billet is heated to 145-155℃ above the first quenching temperature and held for 50-70 minutes, then water-cooled to complete the first quenching. The billet is then heated to 15-25℃ below the first quenching temperature and held for 20-40 minutes, then water-cooled to complete the second quenching. The billet is then heated to 15-25℃ below the second quenching temperature and held for 20-40 minutes, then water-cooled to complete the third quenching. After the third quenching, the billet is heated to A... C3 Heat the billet to 65-75℃ above the required temperature and hold for 20-40 minutes. After holding, water cool it. Finally, temper the billet to 550-650℃ and hold for 30-50 minutes. Then air cool it to 15-40℃.

Description

Technical Field

[0001] This invention belongs to the field of metal heat treatment technology, specifically relating to a heat treatment method for eliminating banded structures in low alloy steel (AISI 4330). Background Technology

[0002] The statements herein provide only background information in relation to this invention and do not necessarily constitute prior art.

[0003] AISI 4330 steel is a low-alloy high-strength steel used in fracturing pumps. With the rapid development of my country's oil extraction industry, the quality requirements for steel used in fracturing pumps are becoming increasingly stringent. However, due to the addition of alloying elements, segregation during the smelting process, and improper handling during the solidification process of the cast billet, a banded structure with alternating ferrite and pearlite distribution is easily formed, severely affecting the mechanical properties of the steel. The inhomogeneity of the banded structure causes anisotropy in the steel, resulting in uneven hardness distribution and a significant reduction in transverse plasticity and toughness. Under external forces, the poor-performing banded structures become weak areas, generating crack initiation points and shortening the service life of the steel. The conventional method for eliminating banded structures is diffusion annealing at a high temperature (usually around 1200℃) for a long time (more than ten hours) to achieve uniform composition and structure. However, this method is time-consuming, labor-intensive, and costly. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the purpose of this invention is to provide a heat treatment method for eliminating banded structures in low-alloy steel.

[0005] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0006] A heat treatment method for eliminating banded microstructure in low-alloy steel includes the following steps:

[0007] The low-alloy steel billet with banded structure is heated to A. C3 Set the temperature to 145-155℃ and hold for 50-70 minutes. After holding, cool with water to complete the first quenching.

[0008] Heat the steel billet to 15-25°C below the first quenching temperature, hold for 20-40 minutes, and then cool it with water to complete the second quenching.

[0009] Heat the steel billet to 15-25°C below the second quenching temperature, hold for 20-40 minutes, and then cool it with water to complete the third quenching.

[0010] After the third quenching, the steel billet is heated to A. C3 Keep the temperature at 65-75℃ above the required temperature for 20-40 minutes, then cool with water after the heat preservation period.

[0011] Finally, temper the steel billet to 550-650℃, hold for 30-50 minutes, and then air cool to 15-40℃.

[0012] The purpose of holding the first quench for a longer period is to allow for more complete diffusion of carbon (C) in the steel. The second and third quenches are to refine the grains through multiple phase transformations. The phase interfaces generated during the phase transformation can absorb and consume unstable grain boundary energy, thereby promoting further grain refinement. In addition, the shorter holding time during the second and third quenches prevents grain growth.

[0013] The purpose of quenching and tempering is to give steel good comprehensive mechanical properties.

[0014] In some embodiments, the alloying element content in the low alloy steel is less than 5%.

[0015] Preferably, the low-alloy steel has the following composition: C: 0.27–0.35%, Si: 0.15–0.35%, Mn: 0.3–0.6%, Cr: 0.7–1.0%, Mo: 0.35–0.5%, Ni: 1.65–2.0%, P ≤ 0.015%, and S ≤ 0.008%.

[0016] In some embodiments, prior to the first quenching, the process further includes observing the banded structure and grain size of the low-alloy steel billet and determining carbon segregation. Grain size is determined by EBSD. Carbon segregation is determined using electron probe microanalysis (EPMA).

[0017] The preferred method for observing the banded structure of low alloy steel billets is as follows: cut a sample block from the low alloy steel billet, grind and mirror polish the sample block, then etch it with 4% nitric acid alcohol solution for 15-20 seconds, and observe the banded structure under an optical microscope.

[0018] In some embodiments, the A of low alloy steel billets C3 The temperature was determined by measuring the expansion curve.

[0019] In some embodiments, the heating rate during the quenching process is 5-20℃ / min.

[0020] Preferably, the heating rate during the quenching process is 8-12℃ / min.

[0021] In some embodiments, the heat preservation coefficient during the quenching process is 2 mm / min.

[0022] Preferably, the heat preservation coefficient during the tempering and heat preservation process is 2 mm / min.

[0023] The beneficial effects achieved by one or more embodiments of the present invention described above are as follows:

[0024] The heat treatment method for eliminating banded structures in low-alloy steel (AISI 4330) of this invention employs a three-stage stepped quenching and holding experimental scheme, avoiding conventional high-temperature (around 1200℃) and long-term (more than ten hours) diffusion annealing. This reduces costs while achieving the goal of eliminating banded structures in the steel. Carbon is fully diffused, resulting in significant grain refinement. After tempering, the overall mechanical properties of the steel are improved, with substantial increases in Brinell hardness and -40℃ impact toughness. Attached Figure Description

[0025] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0026] Figure 1 This is a process curve diagram of the heat treatment method for eliminating banded structures in low-alloy steel (AISI 4330) according to Embodiment 1 of the present invention.

[0027] Figure 2 The images show metallographic images of the samples before and after heat treatment in Example 1 of the present invention, and the C element distribution diagram characterized by EPMA. (a) Before heat treatment; (b) After heat treatment. Detailed Implementation

[0028] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0029] The present invention will be further described below with reference to the embodiments.

[0030] Example 1

[0031] Step 1: Determine the original banded microstructure characteristics of the low-alloy steel (AISI 4330). Optical microscopy, EPMA, and EBSD analyses were performed on the banded microstructure, confirming it to be ferrite + pearlite, with significant carbon segregation. Figure 2 As shown.

[0032] Take a 15mm thick steel billet. The chemical composition of this low alloy steel (AISI 4330) by mass percentage is: C 0.35%, Si 0.19%, Mn 0.55%, P 0.0081%, S 0.0056%, Cr 0.91%, Ni 2.10%, Mo 0.44%, Cu 0.0812%, V 0.0729%, with the remainder being Fe and unavoidable impurities.

[0033] Step Two: Three-stage stepped quenching: An expansion test was performed on the sample steel to determine its A... C3 The temperature was approximately 800℃. The sample was placed in an electric resistance furnace, with a heating rate of 10℃ / min. In the first stage, the billet was heated to 950±5℃ and held for 1 hour to allow for full diffusion of carbon, followed by water cooling. In the second stage, the billet was heated to 930±5℃ and held for 30 minutes, followed by water cooling. In the third stage, the billet was heated to 910±5℃ and held for 30 minutes, followed by water cooling.

[0034] Step 3: Tempering treatment: Quenching temperature is 870±5℃, hold for 30 minutes and then water cool. According to the service performance indicators, tempering temperature is selected as 600℃, hold for 40 minutes, and then air cool to room temperature after tempering.

[0035] Step 4: Cut a sample block from the heat-treated steel billet, grind and mirror-polish the sample block, then etch it with a 4% nitric acid alcohol solution for 15-20 seconds. Observe the banded structure under an optical microscope, and analyze the diffusion of carbon element using EPMA. The elimination of banded structure and the distribution of carbon element are shown below. Figure 2 As shown in the figure. In addition, EBSD characterization of the grain size before and after heat treatment showed a significant grain refinement effect.

[0036] Comparative Example 1

[0037] The difference from Example 1 is that the step of “heating the billet to 950±5℃ and holding it for 1 hour in the first stage to allow C element to diffuse fully, and then water cooling after the holding period” is omitted. All other steps are the same as in Example 1.

[0038] Comparative Example 2

[0039] The difference from Example 1 is that the step of “heating the billet to 930±5℃ and holding it for 30 minutes in the second stage, followed by water cooling after holding” is omitted. All other steps are the same as in Example 1.

[0040] Comparative Example 3

[0041] The difference from Example 1 is that the step of "heating the billet to 910±5℃ and holding it for 30 minutes in the third stage, followed by water cooling after holding" is omitted. All other steps are the same as in Example 1.

[0042] The Brinell hardness (the Brinell hardness value is the average of 4 dots) and -40℃ impact test (the -40℃ impact value is the average of 5 tests) were performed on the samples of Example 1 and Comparative Examples 1-3 before and after heat treatment. The test results are shown in Table 1.

[0043] Table 1

[0044]

[0045] In summary, it can be seen that after three-stage step quenching and tempering treatment, the banded structure of low alloy steel (AISI 4330) is eliminated, the diffusion of carbon element is obvious, and the grains are refined. The mechanical properties of the steel are greatly improved, the hardness is increased, and the impact toughness at -40℃ is significantly improved.

[0046] Comparative Example 1 failed to achieve a significant improvement in impact toughness at -40°C. The results obtained by Comparative Examples 2 and 3 were similar, but the improvement effects on hardness and impact toughness at -40°C were not as good as those of Example 1.

[0047] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method of eliminating banded microstructure in low alloy steel comprising the steps of: It comprises the following steps: ​ The low-alloy steel blank with banded structure is heated to A C3 above 145-155℃, and held for 50-70 min, and then water-cooled after the holding to complete the first quenching; The steel billet is heated to 15-25℃ lower than the first quenching temperature, and kept for 20-40 minutes, and then water-cooled after the keeping to complete the second quenching; The steel billet is heated to 15-25℃ lower than the second quenching temperature, and kept for 20-40 minutes, and then water-cooled after the keeping to complete the third quenching; After the third quenching, the billet is heated to A C3 65-75°C above the temperature, and the temperature is kept for 20-40 min. After the temperature keeping, the billet is water-cooled. Finally, the steel billet is tempered to 550-650℃, kept for 30-50 minutes, and then air-cooled to 15-40℃, and then it is finished; The content of the alloying elements in the low-alloy steel is less than 5%. Before the first quenching, it further comprises the steps of observing the banded structure and grain size of the low-alloy steel blank and determining the C element segregation; the A C3 The temperature is measured by the expansion curve, so as to determine the quenching temperature.

2. The method of claim 1, wherein the method is characterized by: The method for observing the banded structure of the low-alloy steel billet is that: a sample block is cut from the low-alloy steel billet, the sample block is polished and mirror-polished, and then corroded with 4% nitric acid alcohol solution for 15-20 seconds, and then the banded structure is observed under an optical microscope.

3. The method of claim 1, wherein the method is characterized by: The heating speed in the quenching process is 5-20℃ / min.

4. The method of claim 3, wherein the method is characterized by: The heating speed in the quenching process is 8-12℃ / min.

5. The method of claim 1, wherein the method further comprises: The keeping coefficient in the quenching keeping process is 2mm / min.

6. The method of claim 5, wherein the method further comprises: The keeping coefficient in the tempering keeping process is 2mm / min.

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