High-toughness martensitic stainless steel for petroleum sucker rod and heat treatment method thereof

Abstract

The present application belongs to the technical field of petroleum drilling and production equipment materials, and particularly relates to a high-strength and high-toughness martensitic stainless steel for petroleum sucker rod and a heat treatment method thereof. In view of the problems that the existing petroleum sucker rod material is insufficient in strength and toughness, and corrosion resistance and fatigue resistance are difficult to be coordinated under sulfur-containing corrosive medium and alternating load, the present application firstly reduces the cost through specific alloy design without or with low cobalt, and cooperates with the twice heat treatment process of "specific temperature quenching + precise medium-temperature tempering", so as to effectively stimulate the secondary hardening effect of vanadium and niobium micro-alloy elements in the steel at the tempering temperature of 500-600 DEG C, so that the rod part has high toughness while obtaining ultrahigh strength, and has good corrosion resistance and fatigue resistance, and can be used for the key positions of downhole operation.

Description

Technical Field

[0001] This invention belongs to the field of materials technology for oil drilling and production equipment, specifically relating to a high-strength, tough, and corrosion-resistant martensitic stainless steel for oil sucker rods and its heat treatment method. Background Technology

[0002] The information disclosed in this background section is intended only to enhance understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] Martensitic stainless steel rods are key components in oil drilling, engineering machinery, and hydraulic transmission systems. They are primarily used to withstand complex tensile, compressive, torsional, and impact loads, as well as harsh corrosive environments. Therefore, these rods not only need to possess extremely high strength and hardness, but also excellent toughness, fatigue resistance, and good corrosion resistance to ensure long-term safe and reliable operation under extreme conditions.

[0004] High-strength and high-toughness martensitic stainless steels typically achieve their corrosion resistance by adding higher proportions of alloying elements such as chromium (Cr), nickel (Ni), and molybdenum (Mo), and supplementing them with microalloying elements such as vanadium (V) and niobium (Nb) to enhance strength and toughness. For example, chromium is the core element providing corrosion resistance to stainless steel, nickel and molybdenum can stabilize the microstructure and enhance resistance to pitting corrosion, while the addition of vanadium and niobium can produce a precipitation strengthening effect by forming carbonitrides, refining the grains, and thus improving the overall mechanical properties of the steel.

[0005] Currently, traditional heat treatment methods for martensitic stainless steel rods are mainly divided into two categories: "quenching + low-temperature tempering (usually referring to tempering processes between 150-250℃)" and "quenching + high-temperature tempering (usually referring to tempering processes above 650℃)". However, both methods have significant performance shortcomings. While the "quenching + low-temperature tempering" process can achieve high surface hardness and strength, it results in severely insufficient material toughness and low impact energy, making the rods susceptible to brittle fracture under impact loads. Furthermore, controlling carbide precipitation during low-temperature tempering is difficult, easily impairing the material's corrosion resistance. On the other hand, while the "quenching + high-temperature tempering" process can achieve good toughness and plasticity, it inevitably causes a significant loss of strength and hardness, making it difficult to meet the ultra-high strength requirements of modern high-load rods. In addition, this process cannot effectively utilize the precipitation strengthening potential of microalloying elements such as vanadium and niobium, resulting in a waste of alloy resources.

[0006] Existing heat treatment methods generally suffer from drawbacks such as a single process route and insufficient performance control, resulting in a trade-off between the strength and toughness of the rods, making it impossible to achieve a synergistic improvement in both, and making it difficult to guarantee corrosion resistance. Some traditional processes cannot precisely coordinate the austenitizing temperature and tempering temperature parameters, thus failing to accurately control the dissolution and precipitation behavior and microstructure of carbides, ultimately affecting the stability and consistency of the rod's performance. Summary of the Invention

[0007] To address the problems of existing technologies, this invention proposes a novel chemical composition method and heat treatment method for high-strength, tough, and corrosion-resistant martensitic stainless steel oil rods. Unlike existing technologies that rely on adding expensive cobalt (Co) and employing complex thermomechanical processing to achieve ultimate strength, this invention utilizes a special alloying treatment of the steel parts, optimizes the heat treatment process, and precisely controls key process parameters. This aims to solve problems such as strength-toughness imbalance, unsatisfactory corrosion resistance, and large performance fluctuations in existing technologies. Through the improvements of this invention, high-performance martensitic stainless steel oil rods with ultra-high strength, excellent toughness, and reliable corrosion resistance can be stably produced while controlling production costs.

[0008] Based on the above-mentioned technical achievements, the present invention provides the following technical solution: In a first aspect, a high-strength and high-toughness martensitic stainless steel is provided, having the following composition by mass percentage: The composition consists of 0.015-0.08% carbon (C), 0.30-0.35% silicon (Si), 0.20-0.25% manganese (Mn), 13.30-13.40% chromium (Cr), 1.80-2.0% nickel (Ni), 0.15-0.25% molybdenum (Mo), 0.30-0.35% copper (Cu), 0.05-0.10% vanadium (V), 0.01-0.05% niobium (Nb), and 0.02-0.025% nitrogen (N); the balance is Fe and unavoidable impurities, of which phosphorus (P) is less than or equal to 0.02% and sulfur (S) is less than or equal to 0.005%.

[0009] In the above composition, chromium is the main strengthening element of martensitic stainless steel, vanadium and niobium are microalloying elements, and nitrogen, in conjunction with carbon, refines the grains. This invention discovers that vanadium (V) and niobium (Nb) can effectively stimulate the "secondary hardening" effect of stainless steel parts during the tempering stage, effectively improving their strength properties. Holding the tempering process at 500℃-600℃ facilitates the dispersion and precipitation of alloying elements such as vanadium and niobium in the form of nanoscale carbonitrides.

[0010] Therefore, in a second aspect, the present invention provides a heat treatment method for the high-strength and high-toughness martensitic stainless steel described in the first aspect, comprising the following steps: (1) Quenching: Using steel billets that meet the above composition requirements as raw materials, heat to 1000℃-1100℃ and hold for 30-90 minutes to fully dissolve and austenitize the steel billet raw materials, and then rapidly cool to obtain supersaturated lath martensite structure; (2) Tempering: Reheat the steel parts after quenching in step (1) to 500℃-600℃ and hold for 120-240 minutes to allow the carbonitrides of vanadium and niobium to precipitate out. After cooling to room temperature, the product is obtained.

[0011] In step (1) above: The preferred heating temperature is 1020℃-1060℃, the suitable heating rate is 10-15℃ / min, and the holding time is 50-70 minutes.

[0012] The cooling rate is ≥20℃ / s, thereby ensuring the complete transformation of austenite into martensite and eliminating non-martensitic structures such as pearlite and bainite. The cooling rate can be achieved by methods such as oil quenching or forced air cooling.

[0013] In step (2) above: The heating rate is 5℃-10℃ / min, the preferred heating temperature is 540℃-580℃, and the holding time is 160~200 minutes. The above temperature range and heating time are conducive to the precise control of the precipitation size and distribution of vanadium and niobium carbides, thereby achieving the best ratio of strength and toughness.

[0014] The tempering process described in step (2) is preferably carried out in an air resistance furnace.

[0015] The cooling method is to allow it to cool naturally in the air.

[0016] The beneficial effects of this invention are as follows: By rationally controlling the content of multiple elements and simultaneously employing a two-stage heat treatment process of "quenching at a specific temperature + precise medium-temperature tempering", this invention breaks through the bottleneck of the mutual constraint between strength and toughness in traditional martensitic stainless steel rods.

[0017] Specifically, this invention employs a two-stage heat treatment process of "specific temperature quenching + precise medium-temperature tempering," effectively activating the secondary hardening effect of vanadium and niobium microalloying elements in the steel at a tempering temperature of 500-600℃ (optimal 540-580℃). This results in rods achieving both ultra-high strength (yield strength ≥950 MPa, tensile strength ≥1050 MPa) and high toughness (room temperature impact energy ≥70 J). Furthermore, corrosion resistance and fatigue resistance tests were conducted on the embodiments under specific environmental conditions. This process, through a higher tempering temperature, inhibits the precipitation of harmful carbides, ensuring the chromium content of the matrix, enabling the product to maintain high strength while possessing excellent corrosion resistance. Simulated acidic brine containing saturated H2S (pH approximately 3.5) was tested using NACE. According to TM0177-2016 standard method A test, the steel rod of Example 1, under a stress of 900 MPa (approximately 90% of its yield strength), did not fracture for 720 hours, exhibiting excellent resistance to sulfide stress corrosion cracking (SSCC). The nanoscale carbonitride microstructure dispersed on the resulting tempered martensitic matrix also significantly improved fatigue resistance. Axial tensile-tensile fatigue tests (stress ratio R=0.1) conducted according to ASTM E466 standard showed that the fatigue limit (10... 7 With a strength of up to 485 MPa (approximately 48% of its yield strength), it exhibits excellent resistance to high-cycle fatigue, sufficient to cope with complex alternating loads downhole, making it particularly suitable for key components such as oil drill pipes and hydraulic piston rods. In addition, the well-defined process window ensures uniform and stable performance during mass production, resulting in high yield and significant economic benefits. Attached Figure Description

[0018] 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.

[0019] Figure 1 The image shows the metallographic structure of the martensitic stainless steel forging billet raw material described in Example 1. Figure 2 The image shows the metallographic structure of the steel part after quenching as described in Example 1; Figure 3 The image shows the metallographic structure of the steel part after tempering as described in Example 1. Figure 4 This is a comparison diagram of the mechanical properties of the embodiments and comparative examples; in, Figure 4 In the middle (a), the yield strength is... Figure 4 (b) represents tensile strength. Figure 4 (c) represents elongation after fracture. Figure 4 The middle (d) is a bar chart comparing impact energy; Figure 5 The corrosion resistance and fatigue resistance of the examples and comparative examples are shown. in, Figure 5 (a) is a bar chart comparing corrosion resistance performance. Figure 5 (b) is a comparative bar chart of fatigue resistance performance. Detailed Implementation

[0020] 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 herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0021] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0022] In the context of this specification, the word "comprising" is considered to mean "especially including". It should not be interpreted as "consisting of only".

[0023] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments and comparative examples.

[0024] Example 1 This embodiment provides a high-strength and high-toughness martensitic stainless steel and its heat treatment method. The high-strength and high-toughness martensitic stainless steel part has the composition shown in Table 1 below: Table 1. Composition (mass fraction %) of stainless steel forging billet raw material in Example 1 The heat treatment method for the above-mentioned high-strength and high-toughness martensitic stainless steel parts specifically includes the following steps: (1) Quenching: Select martensitic stainless steel forging billets that meet the requirements of Table 1 as raw materials, heat the billets in an electric resistance furnace at a heating rate of 10℃ / min to 1040℃, hold for 60min and then oil quench to room temperature.

[0025] (2) Tempering: The steel parts quenched in step (1) are heated to 560°C in a tempering furnace at a heating rate of 10°C / min, held for 180 min, and then air-cooled to obtain the final product.

[0026] The metallographic structure of the above martensitic stainless steel forging billet is shown in the figure. Figure 1 As shown, the metallographic structure of the steel part obtained after quenching in step (2) is as follows. Figure 2 As shown, the metallographic structure of the steel part obtained after tempering in step (3) above is as follows. Figure 3 As shown; according to Figure 3 The results show that nanoscale carbonitrides are dispersed on the martensitic matrix obtained after tempering.

[0027] Mechanical property tests were conducted on samples of the tempered steel component. The test results showed that the steel component had a yield strength of 1005 MPa, a tensile strength of 1120 MPa, an elongation after fracture of 16%, and a room temperature impact energy of 75 J. Furthermore, the steel component exhibited excellent comprehensive service performance. Tested according to NACE TM0177 standards, under an applied stress of 900 MPa (approximately 90% of its yield strength), no sulfide stress corrosion cracking occurred for 720 hours, and its axial tensile fatigue limit reached 485 MPa.

[0028] Example 2 This embodiment also provides a high-strength and high-toughness martensitic stainless steel and its heat treatment method. The difference from Embodiment 1 is that the martensitic stainless steel forging billet raw material used has the composition shown in Table 2 below: Table 2. Composition (mass fraction %) of stainless steel forging billet raw material in Example 2 Its heat treatment method specifically includes the following steps: (1) Quenching: Select martensitic stainless steel forging billets that meet the requirements of Table 2 as raw materials, heat the billets in an electric resistance furnace at a heating rate of 15℃ / min to 1100℃, hold for 90min and then oil quench to room temperature.

[0029] (2) Tempering: The steel parts quenched in step (1) are heated to 580°C in a tempering furnace at a heating rate of 5°C / min, held for 240 min, and then air-cooled to obtain the final product.

[0030] Mechanical property tests were conducted on samples of the tempered steel parts from this embodiment. The test results showed that the yield strength of the rod reached 995 MPa, the tensile strength was 1110 MPa, the elongation after fracture was 16.5%, and the room temperature impact energy was 78 J. This embodiment also exhibits excellent corrosion resistance and fatigue resistance. NACE TM0177 standard test results showed that no fracture occurred after 720 hours of testing under an applied stress of 895 MPa (approximately 90% of its yield strength). Its axial tensile-tensile fatigue limit was 480 MPa, with performance comparable to Example 1, demonstrating the stability and reproducibility of the process of this invention.

[0031] The above embodiments demonstrate that the heat treatment method of the present invention has good adaptability and stability to raw materials from different batches, good product performance consistency, and is suitable for mass production.

[0032] Comparative Example 1 The difference between this comparative example and Example 1 is that the chemical composition of the martensitic stainless steel forging billet raw material was adjusted, and the key microalloying elements V and Nb were removed. The specific composition is shown in Table 3 below: Table 3. Composition (mass fraction %) of stainless steel forging raw material for Comparative Example 1 The heat treatment method is the same as in Example 1.

[0033] Mechanical property tests were conducted on samples of the rod. The test results showed that the rod had a yield strength of 820 MPa, a tensile strength of 950 MPa, an elongation after fracture of 15%, and an impact energy of 50 J at room temperature. Due to the lack of V and Nb microalloying elements in its composition, its corrosion resistance and fatigue resistance were severely deteriorated. According to the NACE TM0177 standard test, under the applied stress of 740 MPa (about 90% of its yield strength), sulfide stress corrosion cracking occurred in only 300 hours, and the critical stress was significantly reduced; its axial tensile fatigue limit was only 400 MPa.

[0034] This comparative example shows that without V and Nb microalloying elements, even with the optimal heat treatment process of this invention, the "secondary hardening" effect cannot be activated, resulting in the performance of the produced rods being significantly lower than that of this invention.

[0035] Comparative Example 2 The difference between this comparative example and Example 1 is that the heat treatment process parameters of the martensitic stainless steel forging billet were adjusted, and the traditional high-temperature tempering process parameters were adopted, specifically: In step (2), the quenched workpiece is heated to 680°C in a tempering furnace at a heating rate of 10°C / min, held for 180 min, and then air-cooled.

[0036] All other conditions were identical to those in Example 1, and the composition of the martensitic stainless steel forging used was the same as in Table 1. Mechanical property tests were conducted on samples of the rod. The results showed that the rod had a yield strength of 720 MPa, a tensile strength of 880 MPa, an elongation after fracture of 20%, and a room temperature impact energy of 105 J. Although high toughness was achieved through high-temperature tempering, strength, corrosion resistance, and fatigue resistance were significantly sacrificed. NACE TM0177 standard testing showed that while it could pass a 720-hour test under relatively low applied stress (650 MPa, approximately 90% of its yield strength), the risk of failure was very high under more complex and variable working conditions, making it unsuitable for practical applications. Its axial tensile-tensile fatigue limit was only 380 MPa, which would also be insufficient to meet the long-life requirements of high-load components in practical applications.

[0037] This comparison shows that if the traditional high-temperature tempering process is used, the resulting rods will have high toughness, but the strength loss will be huge, and they will not meet the performance requirements of high-strength rods.

[0038] 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 high-strength and high-toughness martensitic stainless steel for oil sucker rods, characterized in that... The composition comprises the following components by mass percentage: 0.05-0.10% carbon, 0.30-0.35% silicon, 0.20-0.25% manganese, 13.30-13.40% chromium, 1.80-1.90% nickel, 0.15-0.25% molybdenum, 0.30-0.35% copper, 0.05-0.10% vanadium, 0.01-0.05% niobium, and 0.02-0.025% nitrogen; the balance being Fe and unavoidable impurities, of which phosphorus is less than or equal to 0.02% and sulfur is less than or equal to 0.005%.

2. The heat treatment method for high-strength and high-toughness martensitic stainless steel for oil sucker rods as described in claim 1, characterized in that, Includes the following steps: (1) Quenching: Using steel billets that meet the above composition requirements as raw materials, heat to 1000℃-1100℃ and hold for 30-90 minutes to fully dissolve and austenitize the steel billet raw materials, and then rapidly cool to obtain supersaturated lath martensite structure; (2) Tempering: Reheat the steel parts after quenching in step (1) to 500℃-600℃ and hold for 120-240 minutes to allow the carbonitrides of vanadium and niobium to precipitate out. After cooling to room temperature, the product is obtained.

3. The heat treatment method for high-strength and high-toughness martensitic stainless steel for oil drill pipe as described in claim 2, characterized in that, In step (1): the heating temperature is 1020℃-1060℃, the heating rate is 10-15℃ / min, and the holding time is 50-70 minutes.

4. The heat treatment method for high-strength and high-toughness martensitic stainless steel for oil drill pipe as described in claim 2, characterized in that, In step (1): the cooling rate is ≥20℃ / s.

5. The heat treatment method for high-strength and high-toughness martensitic stainless steel for oil sucker rods as described in claim 4, characterized in that... The cooling method is oil quenching or forced air cooling.

6. The heat treatment method for high-strength and high-toughness martensitic stainless steel for oil drill pipe as described in claim 2, characterized in that, In step (2): the heating rate is 5℃-10℃ / min, the heating temperature is 540℃-580℃, and the holding time is 160~200 minutes.

7. The heat treatment method for high-strength and high-toughness martensitic stainless steel for oil sucker rods as described in claim 2, characterized in that... The tempering process described in step (2) is carried out in an air resistance furnace.

8. The heat treatment method for high-strength and high-toughness martensitic stainless steel for oil sucker rods as described in claim 2, characterized in that, The cooling method described in step (2) is to allow the object to cool naturally in the air.

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