Here is the professional English translation of the complete production process for malleable iron pipe fittings:
I. Modeling process
1. Design: 3D modeling based on fitting specifications (elbows, tees, couplings) with determination of parting lines and shrinkage allowance (typically 1.5-2%).
2. Material Selection: High-strength alloy steel (e.g., H13) to withstand repeated molten iron impact.
3. Machining: CNC precision machining of mold cavities, surface polished to Ra≤0.8μm.
4. Trial Validation: Test casting to verify dimensional accuracy, optimizing gating/riser systems.
II. Sand Core & Mold Box Preparation
1. Sand Treatment:
– Resin sand: Furan resin + catalyst (1.2-1.8% of sand weight)
– Green sand: Bentonite (8-10%), coal dust (3-5%)
2. Core Making:
– Hot-box process: Core shooter injects resin sand into 200-250°C molds (curing: 30-60 sec)
– Cold-box process: Triethylamine hardening
3. Mold Assembly: Assemble cope/drag boxes per drawings, position cores (clearance ≤0.5mm).
III. Melting & Pouring
1. Raw Material Ratio:
– Pig iron (Q10-Q12) 40% + Scrap steel 50% + Returns 10%
– Carbon equivalent CE=3.8-4.2% (C:2.5-2.7%, Si:1.2-1.6%)
2. Melting:
– Medium-frequency furnace (1-5 ton capacity) heats to 1520-1550°C
– Modularization: Fe, Si, Mg added via ladle injection (Mg:6-8%, dosage:1.2-1.5%)
3. Pouring:
– Rapid pouring (≤15 sec/fitting) to avoid cold shuts
– Temperature control: 1380-1420°C (higher for thin-wall fittings)
IV. Annealing Treatment (Core Process)
1. Stage 1 Graphitization:
– Heat to 920-960°C, hold 15-20 hrs to decompose cementite into tempered carbon.
2. Rapid Cooling:
– Furnace cool to 720-750°C (rate: 40-50°C/h).
3. Stage 2 Graphitization:
– Hold at 700-720°C for 8-12 hrs to eliminate eutectoid cementite.
4. Cooling:
– Furnace cool to 600°C then air cool, achieving ferritic matrix (hardness:120-150HB).
V. Blasting, Cleaning & Finishing
1. Shot Blasting:
– Steel grit Ø0.8-1.2mm, pressure 0.5-0.7MPa, removing >90% sand residue.
2. Flash Grinding:
– Grind parting-line flash (allowance ≤0.3mm).
3. Gate Removal:
– Plasma-cut residual risers, leave 2-3mm machining allowance.
VI. Machining Operations
1. Straightening:
– Hydraulic presses correct distortion (straightness tolerance: ≤0.5mm/m).
2. Socket Edge Deburring:
– Special scrapers remove burrs (Ra≤6.3μm).
3. Chamfering:
– CNC lathes cut 30°×1.5mm guide chamfers for assembly.
VII. Surface Treatment
1. Hot-Dip Galvanizing:
– Pickling (15-20% HCl, 40°C) → Fluxing (ZnCl₂+NH₄Cl) → Zinc bath (445-465°C, 3-5min)
– Coating thickness: ≥55μm (ISO 1461).
2. Passivation:
– Chromate passivation (Cr⁶⁺ 0.5g/L) enhances corrosion resistance.
VIII. Threading Process
1. Thread Cutting:
– CNC pipe thread lathes produce NPT/BSPT threads (1:16 taper).
2. Precision Control:
– GO/NO-GO gauge inspection (pitch diameter tolerance: ±0.05mm).
3. Antirust Oil Impregnation:
– Thread immersion in ISO VG 32 oil, forming a 10-15μm protective film.
IX. Quality Inspection
1. Pressure Testing:
– Hydrostatic test at 2.5× working pressure (30-sec hold, no leakage).
2. NDT:
– Magnetic particle inspection (ASTM E709) for surface cracks.
3. Coating Verification:
– Magnetic thickness gauge checks zinc layer (≥65μm).
X. Packaging & Storage
1. Anticorrosion Packaging:
– Cartons + plastic bag sealing.
2. Carton Specifications:
– corrugated boxes labeled with specs/batch/standards + Pallets (e.g., GB/T 3287).
3. Warehousing:
– Humidity ≤60%, isolated from acidic/alkaline environments.
This process ensures fittings meet: Tensile strength ≥350MPa, elongation ≥10% (per GB/T 9440), suitable for high-pressure fluid systems. Production follows ISO 9001/TS 16949 for quality management.