Centrifugal Casting: Spinning Molten Metal Into Seamless Pipe

How It Works: An Inertial Mold With No Core

In true centrifugal casting the mold is a hollow cylinder — a permanent water-cooled metal die (the de Lavaud process) or a resin/refractory-lined sand tube — rotated about its own axis while a metered charge of molten metal is poured in through a launder. Because the mold spins, every element of liquid feels a centripetal requirement that must be supplied by a pressure gradient in the melt. The metal therefore climbs the wall and self-forms into an annulus whose outer diameter is the die bore and whose inner diameter is a free liquid surface defined purely by the volume of metal poured and the rotational speed.

• Horizontal axis: the die spins on rollers about a horizontal centerline; used for long pipe (up to ~6 m and beyond) and cylinder liners.
• Vertical axis: used for short, large-diameter rings; gravity is no longer negligible relative to length, so the bore takes a slight parabolic profile (thicker at the bottom).

Two related processes share the physics: semi-centrifugal casting spins a mold containing a central sprue and can use a core for wheels and pulleys, and centrifuge (pressure) casting arranges small mold cavities around a rotating axis so inertia acts as a feeding head. Only true centrifugal casting eliminates the core entirely.

The Governing Physics: Pressure From ρω²r

Treat the spinning melt as a rigid body in solid-body rotation. In the rotating frame the radial momentum balance gives dP/dr = ρω²r, where ρ is the melt density (≈ 7,000 kg/m³ for iron), ω is angular velocity in rad/s, and r is radius. Integrating from the free (bore) surface at rᵢ out to the wall at r₀:

P(r₀) = ½ρω²(r₀² − rᵢ²)

For a 300 mm-OD pipe (r₀ = 0.15 m, rᵢ = 0.13 m) spinning at 1,000 rpm (ω ≈ 105 rad/s), the wall pressure is ½·7000·105²·(0.15²−0.13²) ≈ 0.22 MPa — a real feeding head no gravity riser could match on a horizontal part. The dimensionless measure of the field is the G-factor:

G = ω²r / g = (2πN/60)² r / g

Practical spun pipe runs G ≈ 60–90; rings and rolls run 100–150. Foundries often target the empirical Constant of Casting C = N²·D (with N in rpm, D in m) between roughly 1×10⁵ and 3×10⁵ so the metal is pinned firmly (no raining or slipping) without spraying. The same field is a density separator: buoyant force on an inclusion of density ρᵢ scales as (ρ−ρᵢ)ω²r, so slag, sulfides, and gas migrate to the bore while the dense metal stays at the wall — the origin of centrifugal casting's clean load-bearing zone.

Controlling Variables and the Core Trade-Offs

Wall quality is a tug-of-war between too little and too much rotation, set against pour temperature and mold heat extraction.

• Speed (ω): too low and gravity wins on the upswing — metal rains off the top of a horizontal die, giving lap defects and an eccentric wall; too high and hoop stress from ρω²r cracks the solidifying skin (longitudinal hot tears) and can spray metal. The window widens with diameter, so large rolls tolerate lower rpm than small liners.
• Pour temperature / superheat: typically 80–150 °C above liquidus. High superheat delays solidification and improves feed but coarsens grain and worsens die life; low superheat risks cold laps and misruns.
• Mold temperature & coating: a metal die is pre-heated (150–250 °C) and sprayed with a refractory wash (silica, zircon, or graphite) whose thickness tunes the cooling rate — the wash both protects the die and slows the chill to control the white-iron depth in gray/ductile pipe.
• Pour rate & traversing: long pipe is poured through a traversing launder that lays a helical film of metal along the die, controlling wall uniformity and avoiding cold shuts at the far end.

The core trade-off: centrifugal casting buys you a core-free, high-yield, directionally sound part, but only for axisymmetric geometry, and the bore surface is always the dirtiest metal — it must be bored out (typically 3–6 mm of stock removed), so the process is uneconomic unless the machining allowance is small relative to the wall.

Sizing and Scale: From rpm to Wall Thickness

Design starts from the finished part and works outward through solidification and machining stock. A clean sequence:

• Set wall thickness t from the service requirement, then add bore machining stock (3–6 mm) and OD stock, giving cast r₀ and rᵢ.
• Charge volume: for length L, V = πL(r₀² − rᵢ²); mass m = ρV. A DN300 × 6 m ductile pipe has V ≈ π·6·(0.16²−0.15²) ≈ 0.058 m³, so m ≈ 410 kg of iron per pour.
• Rotational speed from the target G-factor: N = (60/2π)√(G·g/r₀). For G = 75 at r₀ = 0.16 m, N ≈ 645 rpm. Cross-check against C = N²D ≈ 1.3×10⁵ — comfortably in the pinning range.
• Solidification time follows Chvorinov's rule, t_s = B(V/A)ⁿ with n ≈ 2; because the die is a strong chill (metal-mold heat flux ~1–3 MW/m²), a 12 mm iron wall freezes in a few seconds to tens of seconds, producing a fine columnar chill zone at the OD transitioning to equiaxed grains toward the hotter bore.
• Power: the drive supplies the kinetic energy ½Iω² of die + melt (I = ½mr² for the annulus) plus bearing/windage losses; a mid-size horizontal machine runs a 15–75 kW variable-speed drive to reach and hold speed through the pour.

The scale range is enormous: cylinder liners of 50–150 mm OD spun at 2,000–3,000 rpm, water main up to DN1200, and steel rolls or nuclear-grade rings exceeding 2 m OD and many tonnes spun at only a few hundred rpm.

Real Applications and Hardware

Centrifugal casting dominates wherever a dense, seamless, axisymmetric part is needed at volume:

• Ductile-iron pressure pipe: the flagship product, produced to AWWA C151 / ISO 2531 / EN 545. De Lavaud water-cooled metal dies spin DN80–DN1200 pipe; the as-cast pipe is annealed to convert brittle carbides, then cement-mortar lined. Tensile strengths of ~420 MPa (with ~300 MPa 0.2% proof yield) and elongation >10% come from graphite-nodule ductile iron (ASTM A536-equivalent 65-45-12).
• Engine cylinder liners: gray-iron and Ni-resist liners are spun to a fine pearlitic matrix with dispersed graphite for wear and scuffing resistance.
• Bimetallic mill rolls & rolls for paper/steel: a hard, wear-resistant alloy shell (high-Cr iron, ~800 HV) is cast centrifugally, then a tough nodular-iron core is cast into the still-hot shell — a two-alloy metallurgical bond impossible in a static mold.
• Alloy-steel and superalloy rings: valve bodies, gun tubes, and jet-engine seal rings in stainless (ASTM A426/A447), Inconel, and even titanium (in inert/vacuum machines) exploit the oxide-flushing action for aerospace-grade cleanliness.

The hardware is deceptively simple: a rigid die on driven hydrodynamic or roller bearings, a VFD spindle drive, a tilting/traversing launder and ladle, water cooling galleries or spray, and an extractor that pulls the shrunk casting once it has clamped-then-released the die on cooling.

Failure Modes, Limits, and Best Practice

The characteristic defects of centrifugal casting all trace back to the same ρω²r field being wrong for the moment in solidification:

• Banding / segregation: unsteady pour or too-low G lets successive metal layers freeze as visible bands with entrapped oxide film between them; fixed by steady traverse and correct speed.
• Longitudinal hot tears: the tensile hoop stress σ_θ = ρω²r² in the fragile solidifying skin, plus mold restraint against thermal contraction, splits the wall lengthwise if speed is too high or the die too cold.
• Raining / laps: too-low speed on a horizontal die — metal detaches at top-of-rotation and drops, giving folds and eccentric walls.
• Density segregation gone wrong: useful when it flushes slag to the bore, harmful when alloy carbides or heavy phases (e.g., in high-Cr iron or leaded bronze) macro-segregate radially, leaving a soft or hard band the design didn't intend.
• Bore contamination: the last-to-freeze inner surface concentrates inclusions and shrinkage; it must be machined away, so an inadequate bore allowance leaves defects in the finished part.

Best practice: pick N from a validated C = N²D window, control superheat to 80–150 °C, coat and preheat the die, pour at a steady traverse rate, and set bore/OD machining stock so all suspect metal is removed. Verify with ultrasonic and radiographic inspection per the governing ASTM/ASME spec, and heat-treat (anneal or normalize) to homogenize the directional chill structure. The immutable limit remains geometry — if the part is not a body of revolution, no amount of spin will feed it, and you return to sand or investment casting.

True centrifugal casting versus conventional static (sand) casting for hollow parts

Attribute	Centrifugal casting	Static / sand casting
Bore formation	Free surface set by ω (no core)	Sand or metal core required
Feeding pressure	Inertial, ρω²r (up to ~1 MPa at wall)	Gravity head + risers only
Metal yield	85–95% (little gating, machined bore)	45–70% (gates, risers, sprue)
Wall soundness	Directional, oxide/slag driven to bore	Shrinkage porosity, floating inclusions
Grain structure	Fine chill columnar → equiaxed at bore	Coarser, less directional
Best geometry	Axisymmetric: pipe, ring, liner, roll	Arbitrary 3-D shapes