In the field of fluid transportation, multi-stage pumps are widely used in key industrial scenarios such as petrochemical, water conservancy, power, and mining due to their core advantages of high head and large flow rate. As the core pressure-bearing component and fluid passage carrier of multi-stage pumps, the structural integrity, dimensional accuracy, and material performance of the pump body directly determine the operational efficiency, reliability, and service life of the pump set. The casting process, as the mainstream technical route for pump body manufacturing, requires precise control over the entire process, including material selection, mold design, melting and casting, heat treatment, and subsequent inspection, in order to meet the strict requirements of multi-stage pumps under complex working conditions.
I. Material Selection for Casting the Pump Body of Multistage Pumps: The Core Premise of Meeting Working Conditions Requirements
The working environment of the pump body of a multistage pump is often accompanied by high pressure, high-speed fluid erosion, medium corrosion (such as acid and alkali solutions, fluid containing solid particles), and periodic temperature changes. Therefore, the material selection must take into account three core indicators: mechanical properties, corrosion resistance, and process adaptability, to avoid faults such as cracking, wear, or leakage of the pump body due to improper material selection.
From the perspective of industrial application practice, gray cast iron is the most widely used in multi-stage pumps for conveying clean water and low-temperature, low-pressure media due to its excellent casting performance, shock absorption, and cost advantages. Among them, HT250 and HT300, with tensile strengths of up to 250MPa and 300MPa respectively, can meet the demands of most civilian and light industrial scenarios. However, for industrial-grade multi-stage pumps conveying high-temperature (above 200℃) and high-pressure (above 10MPa) media such as steam condensate and hot oil, ductile iron becomes a better choice. QT450-10 and QT500-7 not only have strengths close to those of steel, but their spherical graphite structure also significantly enhances the material's toughness and fatigue resistance, effectively resisting the periodic loads caused by fluid pulsation.
In corrosive working conditions, the application of special alloy materials becomes crucial. When transporting strong corrosive media containing chloride ions, sulfides, etc., 304 and 316L stainless steels can achieve excellent corrosion resistance due to the passive film formed by chromium and nickel elements. Among them, 316L, with the addition of molybdenum, has significantly better resistance to pitting and crevice corrosion than 304, and is suitable for applications in the chemical industry, seawater desalination, etc. In high-concentration acid and alkali environments, duplex stainless steel (such as 2205) with a dual-phase structure of ferrite and austenite, combines high strength and corrosion resistance, and can meet the long-term stable operation requirements of pump bodies under extreme conditions.
II. Casting Mold Design: The Fundamental Step to Ensure the Structural Accuracy of the Pump Body
The structure of a multistage pump is complex, with multiple series-connected flow channels, impeller cavities and sealing surfaces inside. The flow channels of different stages need to maintain coaxiality and perpendicularity; otherwise, it will cause vortices in the pump body, increase hydraulic losses and even lead to pump body vibration. Therefore, the design of the casting mold should aim at "precisely replicating the structure and optimizing the filling process", and focus on breaking through the following technical difficulties.
In terms of mold structure design, the first step is to plan the parting surface based on the three-dimensional model of the pump body, ensuring that the parting surface avoids critical precision areas such as the sealing surface and flange joint surface, thereby minimizing the impact of flash removal on dimensional accuracy. For the complex internal flow channels of the pump body, a sand core combination process should be adopted. The overall flow channel is divided into multiple separately manufacturable sand cores (such as the first-stage flow channel sand core and the second-stage flow channel sand core), and positioning pins and positioning slots are set on the sand cores to ensure that the coaxiality error of the flow channel after assembly is controlled within 0.1mm/m. At the same time, the mold should be reasonably designed with a gating and riser system: the gate location should avoid the stress concentration areas of the pump body (such as the flange root), and bottom gating or stepped gating should be used to ensure that the molten metal fills the mold smoothly, avoiding the impact on the sand mold that could cause sand inclusion and sand holes defects. The risers should be placed at the thickest part of the pump body wall (such as the pump body flange and the intersection of flow channels) to eliminate shrinkage cavities and porosity inside the casting through feeding, ensuring the density of critical parts of the pump body.
In terms of mold material selection and processing accuracy control, the mold body (such as the sand box and the mold base plate) is usually fabricated by welding Q235 steel plates, and its flatness must be precisely controlled within 0.05mm/m through milling machine processing. For the production of sand cores, the appropriate process should be selected based on batch requirements. For small-batch production, resin sand manual core making can be adopted, while for large-batch production, hot core box and cold core box core making processes are preferred. The size tolerance of the sand cores should be kept within ±0.1mm by using automated equipment. Additionally, exhaust channels must be set up in the mold to promptly expel gases from the cavity during the metal filling process, preventing gas entrapment and the formation of porosity defects. Generally, one exhaust hole with a diameter of 2-3mm should be provided for every 100cm² of the sand mold surface, and the exhaust holes should extend to the surface of the sand mold to ensure smooth gas evacuation.
III. Melting and Pouring Process: The Crucial Steps Determining the Intrinsic Quality of the Pump Body
The quality of molten metal during smelting directly affects the chemical composition, purity and mechanical properties of the casting, while the pouring process determines whether the molten metal can completely fill the mold cavity. Together, they form the "internal quality defense line" of the casting multi-stage pump body.
In the smelting stage, it is necessary to formulate differentiated smelting process parameters based on the type of material. For cast steel materials, medium-frequency induction furnaces are typically used for smelting, and the smelting temperature should be controlled within 1600-1660℃. Meanwhile, alloying elements such as ferrosilicon and ferromanganese are added to adjust the chemical composition to prevent an increase in the brittleness or a decrease in the strength of the castings due to fluctuations in composition. During the smelting process, slag removal and degassing treatments are also required. By adding slagging agents, inclusions in the molten metal can be adsorbed.
The core of the casting process lies in controlling the casting temperature and speed to ensure a smooth filling of the molten metal. Due to the high melting point of stainless steel, the casting temperature needs to be raised to 1550-1600℃. The casting speed should be dynamically adjusted according to the wall thickness of the pump body. For thin-walled areas with a thickness of 5-10mm, a faster casting speed (15-20kg/s) should be adopted to prevent premature solidification of the molten metal during the filling process. For thick-walled areas with a thickness of over 30mm, the speed should be appropriately reduced (5-10kg/s) to minimize gas entrapment. Additionally, during the casting process, the liquid level of the molten metal should be kept steadily rising to avoid flow interruption, ensuring that all parts of the mold cavity are fully filled.
IV. Heat Treatment Process: A Necessary Means to Optimize the Mechanical Performance of Pumps
After casting, the pump body of a multistage pump often has problems such as internal stress concentration and uneven structure. If no heat treatment is carried out, it will not only affect the mechanical properties of the pump body, but also may cause deformation or cracking due to stress release during subsequent processing or use. Therefore, a scientific heat treatment process should be formulated based on the material type and performance requirements to achieve the goal of "eliminating internal stress, optimizing microstructure, and improving mechanical properties".
The heat treatment of stainless steel pump bodies should focus on the balance between corrosion resistance and mechanical properties. For austenitic stainless steels such as 304 and 316L, solution treatment is the core process - heating the castings to 1050-1100℃, holding for 1-2 hours, and then rapidly quenching in water can ensure that carbon is fully dissolved in the austenitic matrix, preventing carbide precipitation at the grain boundaries and thus maintaining the material's corrosion resistance. For 2205 duplex stainless steel, a "solution + aging" process is required. Solution treatment can obtain a uniform duplex structure, and aging treatment (holding at 450-550℃ for 2-3 hours) can further enhance strength by precipitating intermetallic compounds, meeting the requirements of high-pressure working conditions.
V. Quality Inspection and Defect Repair: The Last Line of Defense for Ensuring the Pump Body Meets Standards Before Leaving the Factory
As a pressure-bearing component, the pump body of a multistage pump may lead to medium leakage during operation and even cause safety accidents due to quality defects such as cracks, pores and shrinkage cavities. Therefore, a comprehensive quality inspection system should be established to conduct a thorough screening of the appearance, dimensions and internal quality of the pump body, and to carry out standardized repairs for the detected qualified defects.
Appearance and dimensional inspection are fundamental steps in quality control. For appearance inspection, visual inspection combined with penetrant testing (PT) should be adopted, focusing on checking whether there are cracks, sand holes, slag inclusions and other defects on the surface of the pump body. Penetrant testing can detect surface opening defects, with a sensitivity of up to 0.1mm. For dimensional inspection, a three-coordinate measuring instrument should be used to measure key dimensions such as the flange diameter, flow channel coaxiality and sealing surface flatness of the pump body, ensuring that the dimensional tolerances meet the design requirements.
Internal quality inspection is the core to ensuring the long-term stable operation of the pump body. Ultrasonic testing (UT) can be used to detect volume defects such as shrinkage cavities and porosity inside the pump body, capable of identifying internal defects with a depth of ≥ 2mm, and the detection range can cover the entire thickness direction of the pump body. For critical areas (such as the root of the flange and the intersection of flow channels), radiographic testing (RT) is also required. By penetrating the casting with radiation and forming an image, it can accurately identify linear defects such as internal cracks and inclusions, ensuring that the internal density of the pump body meets the standard requirements.
For minor defects found during inspection (such as pores with a diameter of ≤ 2mm and micro-cracks with a length of ≤ 5mm), the repair process of spot welding can be adopted, but the repair process must be strictly controlled: before spot welding, the defective area should be ground and cleaned to expose the original metal color; the welding material should be of the same composition as the pump body material (for example, stainless steel pump bodies should use stainless steel electrodes of the same material); after spot welding, local heat treatment should be carried out to eliminate the stress of spot welding, and re-inspection should be conducted to ensure that the quality of the repaired area meets the standards.
VI. Conclusion
The manufacturing of multi-stage pump bodies through casting is a systematic project that integrates materials science, mold engineering, thermal processing techniques, and quality inspection. The precision control of each link directly affects the operational performance and safety reliability of the pump set. With the continuous increase in the demand for multi-stage pumps with "high head, high efficiency, and long service life" in the industrial field, casting technology also needs to develop towards greater accuracy, higher efficiency, and more environmental friendliness - for instance, by optimizing the melting and casting parameters through numerical simulation technology to reduce trial-and-error costs; using 3D printing technology to manufacture complex sand cores to enhance the accuracy of the flow channels; and promoting low-energy consumption heat treatment processes to reduce energy consumption during the production process. Only by continuously promoting technological innovation and process upgrading can the quality level of cast multi-stage pump bodies be continuously improved, providing a solid guarantee for the stable operation in the field of fluid transportation.