Abstract: Owing to their excellent mechanical properties and corrosion resistance, ductile iron pipes are widely used in municipal water supply, gas transmission, and industrial fluid transportation. Centrifugal casting is the primary manufacturing process for ductile iron pipes. However, there is still considerable scope for improving product quality consistency, production efficiency, and energy efficiency. This paper examines the centrifugal casting process, analyzes its existing challenges, and proposes equipment improvements in six key areas: casting system optimization, temperature field control, rotational speed regulation, cooling process optimization, molten metal purification, and automation system upgrades. The effectiveness of the improved process is comprehensively evaluated through inspections of pipe inner and outer wall quality, mechanical property testing, production efficiency analysis, and energy consumption assessment, providing a reference for the optimization and upgrading of ductile iron pipe manufacturing processes.
With the rapid development of infrastructure construction, the demand for ductile iron pipes and the requirements for their quality continue to increase, as these pipes serve as critical materials for fluid transportation. Centrifugal casting has become the mainstream technology for ductile iron pipe production due to its ability to produce pipes with uniform wall thickness, dense structures, and excellent mechanical properties. Its technological level directly determines the stability of pipe quality and the economic efficiency of production. Currently, the traditional centrifugal casting process for ductile iron pipes faces many challenges. The push for higher quality, greater efficiency, and lower energy consumption in ductile iron pipes has made systematic refinement of the centrifugal casting process not just desirable, but imperative for the industry. The growing industrial demand for ductile iron pipes with superior quality, higher production efficiency, and lower energy consumption is driving a systematic overhaul of the centrifugal casting process—an evolution that has become imperative for the sector. This paper addresses critical process bottlenecks by proposing targeted, practice-based improvements to key equipment and procedures, and validates their feasibility and effectiveness through systematic performance evaluation. The aim is to provide technical support for ductile iron pipe manufacturers to enhance their product competitiveness and achieve greener, more efficient production.
Centrifugal casting for ductile iron pipes is a specialized manufacturing process in which centrifugal force is used to distribute molten metal uniformly along the inner surface of a rotating mold, enabling rapid solidification and the formation of high-quality pipes. The core principle of centrifugal casting is to inject qualified molten ductile iron into a high-speed rotating horizontal mold through a gating system. Under the action of centrifugal force, the molten metal is forced against the inner wall of the mold and rapidly solidifies due to the cooling effect of the mold, ultimately forming a ductile iron pipe with uniform wall thickness and a dense microstructure. As the core manufacturing process for ductile iron pipes, centrifugal casting plays a decisive role in determining pipe quality consistency, mechanical performance, and production efficiency. Compared with conventional processes such as sand casting, centrifugal casting has become the mainstream technology for ductile iron pipe production worldwide, owing to its advantages of uniform metal distribution, high casting density, superior mechanical properties, and high automation potential. It is widely used in infrastructure fields such as municipal water supply, drainage, gas transmission, and industrial fluid transportation, playing an essential role in ensuring the long-term safety and durability of ductile iron pipes during operation. The current process faces several key challenges. First, improper gating system design causes non-uniform molten metal flow, leading to excessive variations in cast pipe wall thickness and the formation of defects such as shrinkage cavities and slag inclusions in certain regions. Second, insufficient control of the temperature field leads to microstructural non-uniformity in the cast pipe, which adversely affects its mechanical properties and service performance. Third, the absence of a scientific basis for determining rotational speed parameters limits the adaptability of the process to the production of ductile iron pipes with varying specifications. Fourth, inadequate cooling performance restricts productive capacity; and fifth, insufficient automation renders the process vulnerable to human operational errors. Furthermore, the traditional casting process, with its fixed temperature and speed parameters, proves ill-suited for large-diameter pipe production—especially for pipes longer than 6 m, where thermal gradients in the molten metal lead to significant property inhomogeneity between the two ends.
Targeted improvements have been made to the casting trough, distributor, and guide device to resolve the inefficiencies of the conventional system. The trough features a new high-alumina refractory lining whose thermal conductivity is 30% lower than that of the original lining material, effectively curtailing heat loss. In addition, the trough cross-section has been converted from a rectangular to a trapezoidal geometry, featuring side walls inclined at 15° and a bottom width widened from 120 mm to 150 mm—modifications that effectively suppress molten metal splashing while simultaneously minimizing heat loss. The distributor has been reconfigured into a Y-shaped, double-layer architecture, with the upper layer incorporating an adjustable baffle. The baffle utilizes a sliding mechanism that affords high-precision positioning. The baffle adjustment accuracy reaches ±2 mm, enabling precise control of the molten metal flow distribution ratio according to different pipe specifications. The guide device adopts an adjustable-angle arc-shaped guide plate with a specially treated surface, maintaining a surface roughness of Ra 3.2. This design ensures that the molten metal enters the mold at an optimal angle, thereby improving the filling process. Furthermore, an intelligent monitoring system is integrated into key areas of the gating system, incorporating eight high-precision platinum-rhodium thermocouples and two infrared cameras to achieve real-time monitoring of temperature field variations during the casting process.
An intelligent temperature field control system is introduced, combining multi-point temperature measurement with zoned heating to achieve precise control of the temperature field during the casting process. A high-precision infrared thermometer is installed at the inlet of the gating system, enabling real-time temperature measurement and data transmission to the control system for precise process regulation. The heating power is automatically regulated based on temperature variations across different regions. Independently controlled induction heating units are installed on both sides of the casting tank, where a PID-based control strategy enables dynamic temperature adjustment to maintain the molten metal temperature within the optimal casting range of 1380°C–1420°C. In addition, multiple temperature sensors are installed on the mold surface to establish a temperature field distribution model. A segmented heating strategy is adopted to achieve a uniform preheating temperature gradient and ensure optimal mold thermal conditions. The improved temperature field control technology controls molten metal temperature fluctuations within ±15°C, thereby improving the uniformity of graphite morphology in the cast pipe and mitigating defects resulting from non-uniform temperature distribution.
To accommodate the production requirements of ductile iron pipes with different specifications, an adaptive speed control system based on fuzzy control is developed. The system enables intelligent rotational speed regulation by establishing a correlation model between mold speed and critical pipe parameters, including wall thickness and length. The system adopts variable-frequency speed control and is equipped with a high-precision encoder with a resolution of 0.01 r/min for real-time speed feedback, enabling closed-loop speed regulation. The control system employs a Siemens S7-200SMART series programmable logic controller (PLC) as the core controller and is equipped with a Schneider 22 kW variable frequency drive, achieving a speed control accuracy of ±0.1 r/min. During the initial pouring stage, the system automatically calculates the optimal starting speed based on the cast pipe specifications and then gradually adjusts the speed to a stable level according to the molten metal filling conditions. For large-diameter cast pipes with DN600 and above, a three-stage speed control strategy consisting of low-speed start-up, rapid acceleration, and steady-speed operation is adopted to prevent molten metal stratification. Furthermore, the newly developed speed control system incorporates a self-learning function that continuously optimizes control parameters based on historical production data. The system is equipped with overspeed alarm and automatic protection functions. When speed fluctuations exceed the preset limit, the system automatically switches to protection mode.
A new zoned precision cooling control system is adopted to enhance the cooling process. The outer surface of the mold is divided into three independently controlled zones, each equipped with a dedicated spray device and flow control valve. By adjusting the cooling water flow rate, the cooling intensity in each zone can be independently controlled. The cooling water system adopts a closed-loop circulation design, with the water temperature maintained between 18°C and 22°C and the water pressure controlled within the range of 0.3–0.4 MPa. An air curtain isolation device is installed at the mold end to prevent cooling water overflow from interfering with the pouring process. The specially designed system uses high-pressure nitrogen to create a stable isolation barrier, effectively preventing cooling water from entering the pouring zone. An intelligent cooling control strategy is developed to accommodate the cooling characteristics of ductile iron pipes with different specifications. Twenty-four high-precision infrared sensors are installed on the mold surface to enable real-time temperature monitoring. A real-time temperature field monitoring network is established, with a data acquisition interval of 0.5 seconds. Based on the temperature field distribution characteristics, the control system automatically calculates the optimal cooling parameters for each zone, enabling dynamic adjustment of cooling intensity. For large-diameter ductile iron pipes, a progressive cooling strategy is adopted, in which cooling begins at the middle section of the mold and gradually extends toward both ends to prevent thermal stress concentration caused by uneven cooling. Simultaneously, electromagnetic flow meters and pressure sensors are installed in the cooling water pipelines to ensure stable and controllable cooling water flow rates in each zone.
To improve molten metal cleanliness, a novel composite purification process is developed. A new slag-forming system is introduced during the medium-frequency induction furnace smelting process. Adjusting the slag composition improves the deoxidation and desulfurization efficiency of the melt. A ceramic filter with a 2 mm mesh size is installed at the tapping outlet to effectively capture inclusions and impurities in the molten metal. A cyclone slag remover is installed at the inlet of the casting system to further separate inclusions and impurities through centrifugal force. During casting, argon gas is introduced for stirring and purification, with the gas flow rate controlled at 4–6 L/min and the treatment time maintained at 3–5 min, effectively reducing inclusion defects in the cast pipes.
To meet the automation requirements of the production process, a PLC-based centralized control system was developed to enable fully automated control throughout the entire production process, from raw material batching to casting. The system adopts a distributed control architecture, with a Siemens S7-1500 series PLC as the main controller and a 17-inch touchscreen serving as the human-machine interface (HMI). Multiple control loops are established for key processes, including temperature control, speed regulation, and cooling control. Data exchange among these control loops is achieved through an industrial Ethernet network. The system incorporates functions including real-time data acquisition, automatic process parameter optimization, and fault diagnosis. Based on production requirements, it can automatically generate optimized process parameters, enhancing the consistency and stability of product quality.
Systematic testing and analysis of 500 batches of ductile iron pipes manufactured with the improved process revealed significant enhancements in inner and outer wall quality. Portable roughness tester measurements showed that the surface roughness Ra value of the inner wall of the cast pipe was stably controlled within 45µm, a 30% reduction compared to before the improvement. Measurements using a portable roughness tester indicate that the inner wall surface roughness (Ra) was consistently maintained below 45 µm, representing a 30% reduction from the pre-improvement level. The surface roughness Ra value of the outer wall was controlled within 60 μm, meeting the specified design requirements. An ultrasonic thickness gauge was used to measure the wall thickness of the cast pipes through sampling inspection. The measurements showed that the wall thickness deviation was controlled within ±1.2 mm, outperforming the national standard requirement of ±2 mm. Comprehensive endoscopic examination of the inner wall surface revealed no discernible shrinkage cavities, slag inclusions, or other macroscopic defects. X-ray flaw detection results revealed a significant improvement in the internal integrity of the cast pipes, with porosity reduced to below 0.8%. The surface finish and overall appearance quality were also significantly enhanced, increasing the first-pass yield rate from 92% to 98%.
Universal testing machine tests revealed that the improved-process cast pipes exhibit tensile strength exceeding 420 MPa, yield strength above 300 MPa, and elongation over 10%—all of which meet or exceed the standard requirements. Brinell hardness testing demonstrated a uniform hardness distribution throughout the cast pipes. The hardness fluctuation along the axial direction was limited to within ±15 HB, while the radial hardness gradient remained below 20 HB. In bending tests, the cast pipes withstood the maximum applied load without cracking, demonstrating excellent toughness. Flattening tests also showed no cracks or fractures after 30% deformation. Metallographic examination revealed that the ductile iron matrix consists predominantly of pearlite, with a graphite nodularity exceeding 90% (Grade 2) and a nodule count of over 150 spheres/mm², indicating a well-developed nodular microstructure.
Statistical analysis of production data collected during six months of continuous operation after implementing the improved process demonstrated a significant increase in production efficiency. By optimizing the gating system structure and implementing an automated control system, the daily production capacity of a single production line increased from 85 to 110 pipes, corresponding to a 29% increase. Product changeover time was reduced from 45 minutes to 25 minutes, equipment commissioning time was shortened by 40%, and the implementation of the automated control system reduced the number of operators from 8 to 5, significantly enhancing labor productivity. Through precise temperature field control and improved cooling processes, the cooling time was shortened. The production cycle time was reduced from 35 minutes to 28 minutes, the product scrap rate decreased from 8% to 2%, and the rework rate was reduced by 65%, significantly minimizing material waste and improving raw material utilization.
A comparison of energy consumption data before and after the improvement over a six-month period revealed significant energy-saving and consumption-reduction benefits achieved by the new process. During the smelting process, the optimized heating control strategy reduced the energy consumption of the medium-frequency furnace from 680 kW·h/t to 580 kW·h/t, resulting in a 15% energy saving. The improved cooling system adopts a closed-loop design, increasing the cooling water reuse rate to 95% and reducing daily water consumption by 40%. By improving the thermal insulation performance of the casting system, heat loss was reduced by 25%, and the overall molten metal yield increased by 8 percentage points. The implementation of the new dust removal system increased dust removal efficiency to 98% while reducing equipment operating energy consumption. The overall energy consumption per unit product decreased by 22% compared to before the improvement, equivalent to a saving of 85 kg/t of standard coal, resulting in significant annual cost savings and demonstrating good economic benefits.
This paper focuses on the improvement and application of the centrifugal casting process for ductile iron pipes. Through an analysis of the current process status, the limitations of traditional methods in quality control, production efficiency, and energy consumption optimization were identified. A comprehensive equipment improvement strategy was proposed, covering casting system optimization, temperature control, speed regulation, cooling process enhancement, molten metal purification, and automation system upgrades. The practical value of the improved process was verified through a multi-dimensional evaluation of its application performance. The results demonstrate that the improved centrifugal casting process effectively addresses many limitations of the traditional process, highlighting the necessity and practicality of the proposed improvements. The optimization of the centrifugal casting process for ductile iron pipes remains a continuous and iterative endeavor. Future developments may involve deeper integration of intelligent technologies and the Industrial Internet to achieve real-time production monitoring, dynamic process control, and precise optimization of process parameters. With ongoing technological innovation and practical advancement, the centrifugal casting process for ductile iron pipes will continue to be optimized, providing higher-quality pipe products and stronger technical support for infrastructure development.
