Butterfly Valve Disc by Investment Casting

1. Pengenalan

In butterfly valves, the disc serves as the primary flow-control element, directly affecting pressure drop, sealing integrity, and actuation torque.

Akibatnya, disc design and manufacturing determine valve performance far more than peripheral components.

Pelaburan Pelaburan has emerged as the preferred method to produce complex, high-precision discs that meet stringent service requirements.

Dalam artikel ini, we explore every stage—from design and material selection to casting, penamat, and validation—providing professional, data-driven insights and emphasizing best practices.

2. Investment Casting Overview

Pelaburan Pelaburan, also known as lost-wax casting, is a time-tested method for creating intricate metal components.

The process begins with a wax pattern, which is coated with a ceramic shell to form a mold.

After dewaxing and high-temperature firing, molten metal is poured into the cavity, and the final part is finished through shot-blasting and machining.

Compared to sand casting or machining, investment casting offers near-net shape geometry with tight tolerances (± 0.1 mm) and surface finishes as smooth as Ra ≤ 1.6 μm.

This precision is vital for butterfly valve discs, where even minor deviations can compromise sealing integrity.

Typical disc dimensions range from 50 mm ke 1,500 mm in diameter, with weights spanning 0.5 kg ke 50 kg, bergantung pada permohonan.

3. Materials Selection for Butterfly Valve Discs

Choosing the right alloy for an investment-cast butterfly valve disc demands balancing Rintangan kakisan, kekuatan mekanikal, temperature capability, dan kos.

Di bawah, we explore four material families—each with its advantages—and highlight quantitative property targets to guide specification.

Keluli tahan karat Austenitic (CF8 / CF8m / CF3 / CF3m)

Why Choose Them? Austenitic grades offer excellent general-service corrosion resistance in water, mild acids, and steam up to 200 ° C..

Thanks to their face-centered cubic (FCC) struktur, they maintain toughness down to –50 °C.

Transitional Note:

For valves exposed to chlorides or weak acids, upgrading from CF8 to CF8M (316) doubles the Pitting Resistance Equivalent Number (Kayu) from ~18 to ~25, markedly extending service life in seawater or brine.

Dupleks & Super-Duplex Stainless Steels (Mis., SAF 2205, 2507)

Why Choose Them? Duplex grades combine austenite and ferrite phases to deliver higher yield strength (~ 800 MPA) and superior chloride-stress-corrosion-cracking (SCC) rintangan.

Wawasan data:

In full-strength seawater (3.5 % NaCl), 2205 discs resist pitting at up to 80 ° C., versus only ~ 60 ° C untuk 316L, making them the go-to for subsea valves.

Nickel-Base Alloys (Inconel 625, Monel 400)

Why Choose Them? Nickel-based superalloys withstand temperatures above 550 °C and resist oxidation, sulfidation, and chlorination—ideal for suhu tinggi dan sour-gas aplikasi.

Application Example:

A steam-injection valve in a gas-turbine system specified an investment-cast Inconel 625 cakera,

which operated leak-free at 575 ° C dan 40 bar for over 18 months.

4. Butterfly Valve Disc Design Considerations

Designing a butterfly valve disc involves a delicate balance between hydraulic performance, Integriti struktur, dan kebolehan.

Akibatnya, engineers must evaluate geometry, pressure loading, Dinamik Aliran, material distribution,

and gating strategy—each factor contributing to reliable operation over millions of cycles.

Disc Profile: Cambered vs. Flat

Pertama dan terpenting, The disc profile dictates flow resistance and torque.

A cambered or “waisted” disc—curved on both faces—reduces flow separation by up to 20% compared with a flat disc and lowers actuation torque by approximately 25% in typical 150 mm, PN16 valves.

Di samping itu, camber creates a self-centering hydrodynamic force, which enhances mid-stroke stability and extends seal life.

Sebaliknya, flat discs remain popular in low-pressure (≤ 10 bar) and simple on/off applications, as they simplify tooling and machining.

Ketebalan dinding & Structural Rigidity

Moving on, Ketebalan dinding determines both rigidity and cast quality.

For investment-cast discs, a nominal thickness of 4–8 mm supports pressure ratings up to 40 bar while avoiding shrinkage porosity.

Tambahan pula, transitional fillet radii of 3-5 mm at the hub–disc junction prevent stress concentration and promote uniform solidification.

Finite-element analysis (FEA) routinely confirms that such sections deflect less than 0.2 mm under a 16 bar differential, thereby maintaining seal integrity.

Pressure Balancing & Reinforcement

Selain itu, designers often incorporate pressure-balancing holes atau relief grooves in larger butterfly valve discs (≥ 300 mm) to equalize inlet and outlet pressures.

By reducing net unbalanced force by up to 60%, these features shrink actuator sizing by one class.

Di samping itu, localized ribbing on the downstream face—typically 4–6 ribs dari 5 mm thickness—further stiffens the disc without appreciable weight gain.

Hydrodynamics & Torque Reduction

Equally important, hydrodynamic contours ensure smooth flow transitions.

Computational fluid dynamics (Cfd) analyses highlight that rounded leading edges with a curvature radius of 0.1× disc diameter delay flow separation,

improving the discharge coefficient (Cd) from ~0.65 to ~0.75 at 50% opening.

Akibatnya, actuation torque drops by 15-20%, directly translating into lower operational energy costs.

Gating, Riser Placement & Kebolehan

Akhirnya, gating and riser design adapt the disc geometry for defect-free casting.

Engineers place the main gate at the disc hub, where metal pools promote directional solidification toward a single peripheral riser.

This layout ensures feeding into the last solidifying zones, reducing shrinkage defects to under 0.5% of castings.

In tandem, a shell thickness of 6 mm and controlled cooling rates (≤ 5 ° C/min) avoid thermal shock and microcracking.

5. Butterfly Valve Disc by Investment Casting Process Details

Investment casting—often called lost wax—transforms a precision wax pattern into a metal butterfly valve disc via a ceramic mold.

Among various shell systems, silica‐sol binders have emerged as the industry standard for high‐integrity, dimensionally accurate castings.

Wax Tooling & Pattern Production

• High‐Precision Dies: CNC-machined die cavities produce wax patterns within ±0.05 % of nominal dimensions.
• Pattern Assembly: Engineers attach sprues and gating systems—designed for hub‐first metal flow—to each pattern, assembling them on wax trees that hold 20–50 discs per pour.

Ceramic Shell Building (Silica Sol Coating):

The wax assembly is dipped into a silica sol slurry (a colloidal solution of colloidal silica and fine refractory particles) dan disalut dengan stuko (zircon or fused silica sand).

This process is repeated 8–12 times, with each layer dried at 70–100°C to build a shell thickness of 5–7 mm.

Silica sol shells offer superior thermal stability and surface finish compared to water glass or ethyl silicate systems.

Dewaxing dan menembak:

The shell is heated to 850–950°C in a controlled furnace to melt out the wax (dewaxing) and sinter the ceramic shell.

This step eliminates residual hydrocarbons and strengthens the shell to withstand molten metal.

The firing temperature is carefully calibrated to avoid cracking while ensuring the shell’s refractoriness matches the alloy being cast (Mis., 1,500–1,600°C for stainless steels).

Lebur logam & Pouring Practices

• Crucible & Furnace: Gunakan vacuum induction furnaces (VIM) to melt alloys—stainless, dupleks, or nickel‐base—maintaining O₂ < 50 ppm and H₂ < 5 ppm for clean castings.
• Menuangkan suhu: Maintain 1 480-1 520 ° C. for CF8/CF8M; 1 550-1 600 ° C. for Inconel 625.
• Inert Shrouding & Pressure Pour: Employ argon or nitrogen shrouds over the mold and apply slight positive pressure (0.1–0.3 bar) to drive metal into thin sections, reducing gas porosity to < 0.2 %.

Penyingkiran dan penamat shell:

Selepas pemejalan, the ceramic shell is removed via shot-blasting (using aluminum oxide grit) to reveal the near-net shape disc.

Final finishing includes trimming gates/risers and polishing to achieve surface roughness (Ra) ≤ 1.6 μm,

critical for minimizing flow turbulence in the valve.

Final Heat Treatment

• Penyelesaian Penyepuh: Heat discs to 1 050 ° C. (CF8/CF3M) atau 1 100 ° C. (aloi nikel) untuk 30 min,
  then water‐quench to dissolve segregated phases and optimize corrosion resistance.
• Melegakan tekanan (Pilihan): A 650 ° C., 1-hour hold can mitigate residual stresses from finishing operations.

Advantages of Silica Sol for Butterfly Valve Discs

• Kemasan permukaan: Silica sol shells produce smoother surfaces than traditional methods, reducing the need for post-casting machining.
  This is vital for discs operating in high-purity environments like pharmaceutical or potable water systems.
• Ketepatan dimensi: The rigid shell structure maintains tight tolerances (± 0.1 mm), ensuring concentricity and flatness critical for disc-seat alignment.
• Kestabilan terma: Silica sol’s high refractoriness (up to 1,600°C) prevents shell distortion during pouring, preserving intricate pressure-balancing features on the disc.
• Keserasian bahan: Ideal for casting austenitic steels, duplex alloys, dan superalloy berasaskan nikel, which are common in butterfly valve applications.

6. Surface Integrity & Rintangan kakisan

As‐Cast Surface Finish and Post‐Cast Polishing

Even with high‐precision silica‐sol shells, as‐cast discs typically emerge with Ra 2.5–3.5 µm.

Walau bagaimanapun, investment casting’s fine ceramic grains limit surface peaks to under 10 μm in height. To meet valve‐industry standards—which often require Ra ≤ 1.6 μm—manufacturers apply:

• Vibratory Tumbling: Ceramic media and light abrasives reduce Ra by 30–40% in 2–4 hours.
• Precision Polishing: CNC‐guided polishing with diamond paste (3 µm grit) achieves Ra ≤ 0.8 µm on sealing faces, ensuring leak-free performance.

These steps eliminate surface micro‐notches that could initiate corrosion pits or damage elastomeric seats.

Acar & Passivation Cycles

To build a uniform passive film and remove embedded inclusions, butterfly valve discs undergo:

• Acar: Immersion in a 10 % HNO₃–2 % HF solution at 50 °C for 20–30 min dissolves surface oxides and scale.
• Rinse & Neutralization: Subsequent rinsing in deionized water and a sodium bicarbonate bath neutralizes residual acids.
• Passivation: A second dip in 20 % Hno₃ pada 60 ° C untuk 30 min promotes formation of a 2-5 nm Cr₂O₃ film,
  verified via ASTM A967 citrate testing.

Surface analytical studies show a 30 % increase in Cr content at the outermost 50 nm,

translating into a passive‐film breakdown potential rise of +50 mV in potentiodynamic tests.

Corrosion Performance in Representative Media

Persekitaran	Disc Material	Corrosion Rate	Test Standard
Air laut (3.5% NaCl di 25 ° C.)	CF8m / 316	0.05 mm/tahun	ASTM B117 salt spray
Ferric Chloride (pitting test)	CF8m / 316	No pitting < 24 h	ASTM G48 Method A
10% H₂SO₄ at room temperature	CF3m / 316L.	0.10 mm/tahun	ASTM G31 immersion
Superheated Steam @ 550 ° C.	Inconel 625	0.02 mm/tahun	Ni‐alloy oxidation test

High‐Temperature Oxidation and Stress‐Corrosion Cracking

For applications above ambient:

• Rintangan pengoksidaan: Inconel 625 discs exhibit < 0.02 mm/year oxide scale growth in air at 550 ° C..
• Rintangan SCC: Duplex‐cast SAF 2205 discs show no chloride SCC when tested per ASTM G36 pada 80 ° C dan 1000 psi for 720 h, outperforming 316L by 40 %.

7. Butterfly Valve Disc Casting Tolerance

Maintaining tight dimensional tolerances on the cast disc ensures proper fit, reliable sealing, and minimal post-cast machining.

Investment casting delivers finer tolerances than sand casting, but designers must still specify realistic expectations to balance cost and performance.

Below are typical toleransi guidelines for investment-cast butterfly valve discs, based on ISO 8062-3 (CT8) and industry practice:

Ciri	Nominal Size Range	Toleransi	Nota
Overall Diameter	Hingga 200 mm	± 0.10 mm	Ensures concentricity with valve body; critical for full-bore applications
	200–400 mm	± 0.15 mm
	> 400 mm	± 0.20 mm
Ketebalan dinding	3–8 mm	± 10 % of nominal	Designers maintain 4–8 mm sections to avoid shrink porosity
Hub Bore Diameter	Hingga 50 mm	− 0 / + 0.05 mm	Slip fit onto shaft; may require reaming to H7 for precision actuators
	50–100 mm	− 0 / + 0.10 mm
Bolt Circle & Lubang	PCD Ø up to 300 mm	± 0.10 mm	Matches pipe flange standards (Mis., Ansi, Dari)
	PCD Ø > 300 mm	± 0.15 mm
Out-of-Roundness	Any circular feature	≤ 0.05 % of diameter	Ensures seal compression uniformity
Kebosanan (Seating Face)	Across disc face	≤ 0.05 mm	Critical for valve shut-off; often ground to final dimension
Edge Profile Radii	Fillet / chamfers	± 0.5 mm	Designers specify 3–5 mm radii to balance flow and stress concentration

Implikasi praktikal

1. Seal Engagement: Tolerances on seating faces and out-of-roundness directly impact packing and O-ring compression, affecting leak tightness.
2. Actuation Alignment: Hub-bore accuracy ensures concentric disc rotation, reducing eccentric loading on bearings and actuators.
3. Elaun pemesinan: While many butterfly valve discs meet finish tolerances as-cast, critical sealing surfaces often receive a light grind (0.2–0.5 mm stock) to guarantee flatness and surface finish.
4. Inspection Strategy: Coordinate-measuring machine (Cmm) audits of 100 % of discs validate compliance; statistical process control (SPC) flags trends before they exceed CT8 limits.

8. Ini Supply Value-Added Services

Beyond production of the investment-cast disc itself, Ini now bundles a suite of value-added services that accelerate time-to-market, reduce in-house workload:

Pemesinan ketepatan

• CNC beralih & Penggilingan: Suppliers often deliver discs with finished hub bores, keyways,
  and bolt-hole patterns to H7/H8 tolerances (± 0.02 mm), eliminating secondary machining.
• Balancing & Penggerudian: Static or dynamic balancing to G6.3 grade limits (< 2.5 µm unbalance per mm) for discs ≥ 300 mm diameter, plus optional bleed or balance-hole drilling.

Rawatan haba

• Solution Penyepuhlindapan: Vacuum or salt-bath anneals at 1 050-1 100 °C followed
  by rapid quenching restore duplex and austenitic microstructures, ensuring full corrosion resistance.
• Melegakan tekanan: Sub-critical holds at 600–650 °C for 1–2 hours reduce residual stresses
  from machining or welding by up to 60%, preventing distortion in final assembly.

Rawatan permukaan

• Menggilap & Lapping: Final finishes down to Ra ≤ 0.4 µm on sealing faces ensure leak-free performance; typical turnaround: 1–3 days per batch of 20–50 discs.
• Salutan & Linings: Epoksi, Ptfe, or ceramic coatings add chemical resistance in aggressive media; thickness control to ±10 µm meets OEM specifications.

Custom Packaging & Logistics

• Protective Crating: ISO-compliant wooden crates with anti-corrosion VCI inserts, shock-monitoring sensors, and humidity indicators safeguard discs during transit.
• Fast-Track Shipping: Expedited air-freight or “milk-run” consolidation cuts lead times to 2–3 weeks from order to door, compared with standard sea-freight of 6–8 weeks.

9. Kesimpulan

Investment casting provides a one-step route to high-performance butterfly valve discs, delivering complex geometries, toleransi yang ketat (± 0.1 mm), and superior surface finishes (Ra ≤ 1.6 μm).

By selecting appropriate alloys—ranging from CF8M stainless to Inconel 625—and applying rigorous process controls and inspections,

manufacturers achieve discs that meet mechanical targets (tensile ≥ 550 MPA; pemanjangan ≥ 25 %), exhibit outstanding corrosion resistance,

and sustain demanding service conditions across water treatment, minyak & gas, dan sektor penjanaan kuasa.