Centrifugal casting launder for nodular cast iron pipe and preparation method of centrifugal casting launder

Abstract

The invention provides a nodular cast iron pipe centrifugal casting launder and a preparation method. The launder comprises the following chemical components in percentage by mass: 0.17 to 0.21 percent of C, 0.35 to 0.50 percent of Si, 0.30 to 0.45 percent of Mn, less than or equal to 0.025 percent of P, less than or equal to 0.025 percent of S, 2.45 to 2.65 percent of Cr, 1.10 to 1.30 percent of Mo, 0.05 to 0.09 percent of Nb, 0.08 to 0.11 percent of V, 0.02 to 0.04 percent of Y, 0.002 to 0.004 percent of B and the balance of Fe and other inevitable impurities. The high-temperature creep resistance of the casting launder under continuous thermal shock of 1050 DEG C is improved by more than two times, the problem of runner deformation caused by thermal stress concentration of an existing product is effectively solved, and the high-temperature service life is remarkably prolonged by 2-9 times.

Description

Technical Field

[0001] This invention belongs to the field of casting and is applicable to casting channels used in high-temperature environments. Specifically, it relates to a centrifugal casting channel for ductile iron pipes and its preparation method. Background Technology

[0002] In the centrifugal casting of ductile iron pipes, the casting trough, as the core process equipment of the centrifuge, plays a crucial role in accurately guiding high-temperature molten iron into a high-speed rotating mold. This component must withstand extreme thermal shock during its service life: its working end is continuously in contact with high-temperature molten metal, while its tail is exposed to the air, creating an axial temperature gradient exceeding 1200°C. More specifically, the centrifugal casting trough employs a slender cantilever structure design up to 6 meters long, which must withstand combined stresses during high-speed centrifugal operation, including thermal stress, mechanical vibration loads, and the impact of molten metal flow.

[0003] Currently, the industry commonly uses alloy structural steel (such as 30CrMo and 21CrMo) to manufacture casting channels. While these materials offer good resistance to melting and loss, significant technical defects have been exposed in practical applications. Material testing data shows that structural steel with conventional compositions experiences a 45% decrease in elastic modulus at 800℃, and its coefficient of thermal expansion reaches as high as 16×10⁻⁶. -6 The temperature drops to a certain level, causing irreversible bending deformation of 10-15mm to occur in the casting trough after 20-30 consecutive cycles. This deformation not only disrupts the trajectory accuracy of molten iron pouring, resulting in quality defects such as uneven wall thickness and surface cold shuts in the cast pipe, but also leads to a decrease in equipment utilization and production line capacity loss due to frequent trough replacements.

[0004] Current technology suffers from two cognitive biases: first, it overemphasizes the material's resistance to melting while neglecting its high-temperature structural stability; second, it fails to consider the unique stress characteristics of slender cantilever components, and traditional alloy designs do not specifically optimize creep resistance, leading to stress relaxation under long-term thermo-mechanical coupling. Industry statistics show that the scrap rate of cast pipes due to flow channel deformation is as high as 3.8%, causing direct economic losses exceeding ten million yuan annually, and has become a bottleneck restricting the upgrading of centrifugal casting technology. Summary of the Invention

[0005] To address the problems of thermal deformation and insufficient service life of traditional centrifugal casting channels for ductile iron pipes used in steel structures under high-temperature conditions, this invention innovatively proposes a novel casting channel technology based on scientifically optimized chemical composition.

[0006] The technical solution adopted in this invention is as follows: a centrifugal casting channel for ductile iron pipes, wherein the chemical composition of the channel, by mass percentage, is: C: 0.17~0.21, Si: 0.35~0.50, Mn: 0.30~0.45, P: ≤0.025, S: ≤0.025, Cr: 2.45~2.65, Mo: 1.10~1.30, Nb: 0.05~0.09, V: 0.08~0.11, Y: 0.02~0.04, B: 0.002~0.004, with the balance being Fe and other unavoidable impurities.

[0007] Preferably, the chemical composition of the flow channel, by mass percentage, is: C: 0.18, Si: 0.38, Mn: 0.33, P: ≤0.012, S: ≤0.003, Cr: 2.55, Mo: 1.19, Nb: 0.06, V: 0.09, Y: 0.02, B: 0.003, with the balance being Fe and other unavoidable impurities.

[0008] A method for preparing a centrifugal casting trough for ductile iron pipes includes the following steps:

[0009] The ingots are smelted and cast using the EAF-LF-VD process, with the following parameters: electric furnace final temperature 1580-1600℃, carbon content controlled at 0.25-0.30%; refining furnace is stirred with argon gas at a flow rate of 50-70 L / min, and the Y content of the fed rare earth cored wire is accurate to ±0.005%; VD furnace vacuum degree ≤67 Pa, smelting time 15 min, hydrogen content controlled at ≤1.5 ppm, and nitrogen content slightly below 0.025%.

[0010] Preferably, the high-temperature creep resistance of the flow channel is: steady-state creep rate ≤ 4.0 × 10⁻⁶ under conditions of 1050℃ and 50MPa. -8 s -1 The fracture time is ≥200h.

[0011] The beneficial effects obtained by the present invention are as follows: the high-temperature creep resistance of the casting channel of the present invention under continuous thermal shock at 1050℃ is improved by more than 2 times, effectively solving the problem of channel deformation caused by thermal stress concentration in existing products, and significantly extending the high-temperature service life by 2-9 times.

[0012] The flow channels of this invention contain Nb (0.05–0.09% by mass), which suppresses high-temperature embrittlement caused by the σ phase, forms stable NbC, pins grain boundaries, hinders dislocation climb, and reduces the creep rate. The flow channels also contain V (0.08–0.11% by mass), which is stable at high temperatures (V₄C₃) and suppresses high-temperature dislocation movement. Furthermore, the flow channels contain B (0.002–0.004% by mass), which strengthens grain boundaries and improves high-temperature creep strength. Finally, the flow channels contain Y (0.02–0.04% by mass), which purifies grain boundaries and, synergistically with B, improves grain boundary strength at high temperatures. Detailed Implementation

[0013] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0014] The technical solution adopted in this invention is as follows: a centrifugal casting channel for ductile iron pipes, wherein the chemical composition of the channel, by mass percentage, is: C: 0.17~0.21, Si: 0.35~0.50, Mn: 0.30~0.45, P: ≤0.025, S: ≤0.025, Cr: 2.45~2.65, Mo: 1.10~1.30, Nb: 0.05~0.09, V: 0.08~0.11, Y: 0.02~0.04, B: 0.002~0.004, with the balance being Fe and other unavoidable impurities.

[0015] A method for preparing a centrifugal casting trough for ductile iron pipes includes the following steps:

[0016] The ingots are smelted and cast using the EAF-LF-VD process, with the following parameters: electric furnace final temperature 1580-1600℃, carbon content controlled at 0.25-0.30%; refining furnace is stirred with argon gas at a flow rate of 50-70 L / min, and the Y content of the fed rare earth cored wire is accurate to ±0.005%; VD furnace vacuum degree ≤67 Pa, smelting time 15 min, hydrogen content controlled at ≤1.5 ppm, and nitrogen content slightly below 0.025%.

[0017] In practice:

[0018] The present invention adopts the following technical solution for the production and manufacturing of flow channels.

[0019] Manufacturing of round blanks for flow channels: High-purity EAF-LF-VD technology is used to produce round blanks for flow channels.

[0020] Casting channel manufacturing: The manufacturing of the centrifugal casting casting channel for the above-mentioned ductile iron pipe includes the following steps:

[0021] (1) Hot pressing: The flow channel has a complex shape and needs to be heated to 950-1100℃ (austenitizing temperature) before hot pressing to avoid material cracking;

[0022] (2) Submerged arc welding: Preheat to 250-300℃, interpass temperature ≤350℃;

[0023] (3) Turning: Low-speed turning to ensure the inner surface finish Ra≤3.2μm and reduce the flow resistance of molten metal;

[0024] (4) Tempering treatment: After heating to 880-920℃ and holding, oil quench to obtain martensitic structure. After holding at 560-650℃ for 2-4 hours, air cool to adjust hardness and toughness;

[0025] (5) Stress relief annealing: If the weld deformation of the flow channel is large, it needs to be kept at 600-650℃ and then slowly cooled to eliminate residual stress; (6) High temperature oxidation resistant coating.

[0026] For the mechanical property verification of the centrifugally cast flow channel of the new ductile iron pipe, this scheme implements a standardized testing system: room temperature mechanical property characterization, and tensile testing is carried out on a WDW-300 microcomputer-controlled electronic universal testing machine according to GB / T 228.1-2021 standard, with a strain rate set at 0.005s. -1 Stress-strain curves were acquired synchronously. Key performance indicators were required: tensile strength (Rm): ≥860MPa; specified plastic elongation (Rp0.2): ≥720MPa; elongation after fracture (A): ≥12%; reduction of area (Z): ≥40%. V-notch Charpy impact tests were conducted using a JBN-300 impact testing machine according to GB / T 229-2020, with a test temperature gradient of 25℃±2℃, requiring an impact energy absorption (KV2) ≥50J. Surface hardness testing of the surface reinforcement layer was performed using an HR-150A Rockwell hardness tester according to GB / T 230.1-2018 standard, with a load of 1471N (150kgf) and a diamond cone indenter, requiring a hardness value ≥24HRC.

[0027] Samples were taken from the centrifugal casting trough of ductile iron pipes, and high-temperature uniaxial tensile creep tests were conducted according to GB / T 2039-2024 "Metallic Materials - Uniaxial Tensile Creep Test Method". Step-type high-temperature uniaxial tensile creep tests were carried out on an RWS-50 electronic creep testing machine. The specimens were machined into circular proportions, and the test results should satisfy the requirement of recording sufficient elongation data to plot the elongation-time curve. Temperature gradients: three groups of comparisons at 850℃ / 950℃ / 1050℃; Stress levels: 180MPa / 100MPa / 50MPa respectively; Duration: 500h for each gradient. The steady-state creep rate at 1050℃ and 50MPa should be ≤4.0×10⁻⁶. -8 s-1 The fracture time is ≥200h, which is significantly better than the performance of conventional materials under the same working conditions.

[0028] The present invention uses EAF-LF-VD smelting technology to smelt three heats of steel for the embodiments of the present invention and three heats of steel for the comparative examples. The composition of each test steel is shown in Table 1. The chemical composition of the steel for the embodiments of the present invention is within the scope of claim 1.

[0029] The high-temperature creep test in the flow channel was conducted by placing a group of specimens under different stresses at a constant uniaxial tensile test temperature. A set of creep curves were obtained, and the relationship between stress and creep rate at a specified time was plotted to determine the creep limit at the specified creep rate. The test results are shown in Table 2.

[0030] Table 1. Chemical composition (%) of the embodiments and comparative examples of the present invention

[0031]

[0032]

[0033] Table 2 Mechanical properties and high-temperature creep test results of the embodiments and comparative examples of the present invention

[0034]

[0035] According to the room temperature mechanical property test results in Table 2, the strength and hardness of the material in the example are significantly higher than those in the comparative example, while the impact toughness is slightly lower, but the decrease is within the controllable range and does not affect the performance of the flow channel in the field. The high temperature constant load creep test results further show that the material in the example exhibits a lower creep rate and a significantly longer fracture time, and has better high temperature durability.

[0036] In summary, the centrifugal casting channel material for ductile iron pipes of the present invention has a reasonable composition. The centrifugal casting channel using the present invention exhibits excellent comprehensive mechanical properties and significantly superior high-temperature creep resistance compared to ordinary structural steel. The centrifugal casting channel of the present invention can meet the design, manufacturing, and usage requirements of centrifugal casting channels for ductile iron pipes and can be promoted and applied to the field of casting structures.

[0037] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A centrifugal casting trough for ductile iron pipes, characterized in that: The chemical composition of the flow channel, by mass percentage, is as follows: C: 0.17–0.21, Si: 0.35–0.50, Mn: 0.30–0.45, P: ≤0.025, S: ≤0.025, Cr: 2.45–2.65, Mo: 1.10–1.30, Nb: 0.05–0.09, V: 0.08–0.11, Y: 0.02–0.04, B: 0.002–0.004, with the balance being Fe and other unavoidable impurities.

2. The centrifugal casting flow channel for ductile iron pipes according to claim 1, characterized in that: The chemical composition of the flow channel, by mass percentage, is: C: 0.18, Si: 0.38, Mn: 0.33, P: ≤0.012, S: ≤0.003, Cr: 2.55, Mo: 1.19, Nb: 0.06, V: 0.09, Y: 0.02, B: 0.003, with the balance being Fe and other unavoidable impurities.

3. The method for preparing a centrifugal casting channel for ductile iron pipes as described in claim 1 or 2, characterized in that: Includes the following steps: The ingots are smelted and cast using the EAF-LF-VD process, with the following parameters: electric furnace final temperature 1580-1600℃, carbon content controlled at 0.25-0.30%; refining furnace is stirred with argon gas at a flow rate of 50-70 L / min, and the Y content of the fed rare earth cored wire is accurate to ±0.005%; VD furnace vacuum degree ≤67 Pa, smelting time 15 min, hydrogen content controlled at ≤1.5 ppm, and nitrogen content slightly below 0.025%.

4. The method for preparing a centrifugal casting channel for ductile iron pipes according to claim 3, characterized in that: The high-temperature creep resistance of the flow channel is as follows: steady-state creep rate ≤ 4.0 × 10⁻⁶ under conditions of 1050℃ and 50MPa. -8 s -1 The fracture time is ≥200h.

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