Spinning of spherical heads
Spherical heads are key components of pressure vessels, storage tanks, pipelines, and other equipment. They feature uniform force distribution and excellent pressure resistance, making them widely used in the petrochemical, nuclear power, aerospace, and other fields. Spinning is an advanced process for manufacturing spherical heads. It uses a rotating mold to drive the blank, while a rotating wheel applies radial pressure to the blank, gradually deforming it into a spherical shape. Compared to traditional stamping, it offers advantages such as high material utilization (up to 90% or more), high forming precision, small equipment tonnage, and suitability for the manufacture of large and special-shaped heads. The spinning process for spherical heads involves multiple technologies, including metal plastic deformation, mechanical property evolution, and mold design. Its quality directly impacts the safety and service life of the pressure vessel, necessitating strict control of forming process parameters and quality inspection procedures.
The preparation of the blank and the design of the mold for spinning spherical heads are fundamental to the success of the process. The blank is typically made of a material with good plasticity, such as low-carbon steel (such as Q245R), stainless steel (such as 304), or alloy steel (such as 16MnDR). The blank is a circular flat plate, and its diameter is calculated based on the nominal diameter, radius of curvature, and wall thickness of the spherical head. Generally, the blank diameter is 10%-15% larger than the head diameter to ensure sufficient material for forming and trimming. The blank requires pretreatment: surface scale and oil stains are removed, and surface quality is improved through pickling or sandblasting. Thick plate blanks (thickness > 10mm) are preheated (200-300°C) to reduce the material’s yield strength and improve plasticity. The mold consists of a spindle-driven core mold (forming mold) and a movable roller (tool mold). The core mold’s spherical radius must be 0.5%-1% larger than the head’s designed radius to compensate for post-forming elastic rebound. The roller’s working surface is an arc, with a radius typically 5-10 times the head’s wall thickness to avoid surface damage. By accurately calculating the blank’s dimensions and optimizing the mold’s corner radius, a pressure vessel manufacturer has increased the material utilization rate of spherical heads from 75% to 92%, reducing material waste.
Controlling the spinning process parameters significantly impacts head quality. Key parameters include spindle speed, wheel feed rate, wheel pressure, and pass deformation. These parameters must be tailored to the blank material, thickness, and head size. Spindle speed typically ranges from 50-200 rpm. Too high a speed can cause the blank to yaw due to centrifugal force, while too low a speed can reduce production efficiency. For heads with a diameter of 2-3 m, a speed of 80-120 rpm is ideal. The wheel feed rate (axial feed) ranges from 50-200 mm/min. Too high a speed can cause uneven deformation, resulting in wrinkles or cracks, while too low a speed increases processing time and mold wear. Generally, the feed rate is adjusted based on the pass deformation, with a low value (50-100 mm/min) for the first pass and a high value (100-200 mm/min) for subsequent passes. The roller pressure should be gradually increased. Initially, the pressure should be sufficient to prevent the blank from slipping. This pressure should be gradually increased as the forming process progresses. The maximum pressure should be calculated based on the material’s yield strength to ensure deformation of 10%-20% per pass (e.g., for a 10mm thick blank, the thickness decreases by 1-2mm per pass). In one case study, the “low speed + gradual pressure” parameter combination increased the forming qualification rate for spherical heads from 82% to 98%.
The metal deformation patterns and quality control during the spinning process are central to the process. The spinning deformation of spherical heads is a localized, continuous plastic deformation. Under the action of the spinning wheel, the blank undergoes three stages: elastic deformation, plastic deformation, and finalization. The deformation zone is primarily concentrated in the contact area between the spinning wheel and the blank (approximately 3-5 times the blank thickness). The metal in this area is subjected to radial compression and axial tension, resulting in plastic flow and conformation to the core mold. The following quality issues must be controlled during the spinning process: wrinkles (caused by excessive feed speed or insufficient pressure) can be eliminated by reducing the feed speed or increasing the pressure; cracks (caused by excessive deformation or insufficient material plasticity) require reducing the deformation per pass or performing intermediate annealing (for stainless steel heads, the annealing temperature is 1050-1100°C); and uneven wall thickness (caused by inaccurate spinning wheel positioning) requires precise control of the spinning wheel trajectory through a CNC system to keep the wall thickness variation within ±10%. The formed head needs to be inspected for size: diameter error ≤±1%, sphericity error ≤1mm/m, surface roughness Ra ≤12.5μm, and ultrasonic testing is used to check whether there are cracks or delamination inside.
The selection of spinning equipment and automation technology are key to improving efficiency. Based on head size and production batch size, spinning equipment is divided into horizontal and vertical spinning machines. Horizontal spinning machines are suitable for small and medium-sized heads with a diameter of less than 2m, offering ease of operation and low equipment costs. Vertical spinning machines are suitable for large heads with a diameter of more than 2m, as they avoid deformation caused by the weight of the blank. The degree of CNC control of the equipment directly affects forming accuracy. Modern CNC spinning machines utilize multi-axis linkage control systems (e.g., 3-5 axes) that precisely control the feed trajectory and pressure of the spinning wheel, achieving repeatable positioning accuracy of ±0.05mm, ensuring consistency in batch production. For large-scale production, automatic loading and unloading systems and online inspection devices can be installed to automate the entire process from blank to finished product, increasing production efficiency by 3-5 times compared to manual operation. After a nuclear power equipment plant introduced a 5-axis CNC vertical spinning machine, the production cycle of large spherical heads with a diameter of 5m was shortened from 72 hours to 24 hours, and the dimensional accuracy was controlled within 0.5mm, meeting the stringent requirements of nuclear power equipment.
Heat treatment and surface treatment after spinning spherical heads are crucial for ensuring performance. Work hardening during the spinning process increases the material’s hardness and decreases its toughness, requiring heat treatment to eliminate these effects. Mild steel heads undergo normalizing (900-950°C for 1-2 hours, air cooling) to refine the grain size and restore plasticity. Stainless steel heads undergo solution treatment (1050-1100°C for 0.5-1 hour, water cooling) to dissolve carbides and improve corrosion resistance. After heat treatment, heads require surface treatment to remove the oxide scale (pickling or mechanical polishing). For heads requiring painting, a primer and topcoat (such as epoxy zinc-rich primer) are sprayed to a dry film thickness of ≥120μm to enhance corrosion resistance. Stainless steel heads undergo passivation (immersion in nitric acid solution) to form an oxide film and enhance corrosion resistance. After treatment, the heads undergo a hydraulic pressure test (test pressure is 1.25 times the design pressure), maintaining pressure for 30 minutes to ensure no leakage. Surface defects are then inspected using magnetic particle or penetrant testing. Through rigorous heat treatment and testing procedures, a chemical equipment plant has extended the service life of spherical heads to over 20 years, far exceeding the industry average.
