Odlievanie piesku remains a cornerstone of the metal‐forming industry, leveraging reusable or expendable molds packed with sand to shape complex geometries.
After pouring molten metal into these sand cavities and allowing it to solidify, manufacturers often apply targeted heat‐treatment cycles.
These thermal processes refine hardness, mikroštruktúra, and mechanical performance to meet rigorous customer specifications.
V tomto článku, we’ll explore:
- Why heat treat sand castings?
- The three fundamental stages of heat treatment
- Common heat‐treatment methods (žíhanie, normalizácia, otužovanie, temperovanie)
- Quantifiable benefits—with data—of each approach
1. Why Heat Treat Sand Castings?
Sand‐cast components—ranging from heavy-duty engine blocks (weighing up to 200 kg) to precision gearbox housings—often require enhanced pevnosť v ťahu, únava, alebo machináovateľnosť.
Uncontrolled cooling in the mold can create uneven microstructures, leaving internal stresses or coarse grain sizes that impair performance.
Integráciou controlled heating and cooling cycles, zlievarne môžu:
- Refine grain size to <50 µm for uniform mechanical properties
- Relieve up to 80% of residual stresses from solidification
- Tailor hardness from 150 HBW (žíhané) až 600 HBW (stvrdnuté)
Následne, heat treatment transforms as-cast parts into reliable, high-performance components suitable for automotive, letectvo, and industrial power systems.
2. The Three Fundamental Stages of Heat Treatment
Každý heat‐treatment protocol for sand castings follows three core stages.
Although temperatures, hold times, and cooling media vary by alloy and desired outcome, the sequence remains consistent:
| Etapa | Účel | Kľúčové úvahy |
|---|---|---|
| 1. Kúrenie | Bring the entire casting to target temperature without distortion | Ramp rates typically 50–100 °C/hour; use uniform furnace atmosphere to prevent decarburization |
| 2. Soaking | Maintain temperature long enough for full microstructural transformation | 1–4 hours depending on section thickness; ensure uniform temperature ±5 °C |
| 3. Chladenie | Achieve desired final structure by controlled quenching or slow cooling | Air cool, olej/uhasiť, or salt bath; cooling rate 1–50 °C/sec |
Failure to control any stage can introduce cracks, deformácia, or non-uniform properties—undermining the casting’s integrity.
3. Common Sand Casting Heat Treatment Methods
While all methods share the three‐stage framework, differences in temperature ranges, soak durations, and cooling rates yield distinct outcomes:
Žíhanie
- Spracovanie: Ramp to ~50 °C above the alloy’s upper critical temperature (Napr., 900 °C for low-alloy steel), hold 2–3 hours, then furnace‐cool at ≤20 °C/hr.
- Vyplývať: Zmäkčuje materiál (down to ~200 HBW), relieves nearly 90% of residual stress, and produces a fully spheroidized mikroštruktúra.
- Prípady použitia: Zlepšuje machináovateľnosť for complex CNC work; ideal when subsequent forming or machining demands ductile, stress-free metal.
Normalizácia
- Spracovanie: Heat to 30–50 °C above the annealing range (Napr., 950 °C for carbon steels), hold 1–2 hours, potom air‐cool (≈25 °C/min).
- Vyplývať: Refines grains to 20–40 µm, boosts hardness by ~20% (Napr., od 200 HBW to 250 HBW), and yields a more uniform ferrite-pearlite structure.
- Prípady použitia: Vylepšuje tvrdosť a machináovateľnosť in parts subject to moderate loads, such as pump housings and structural brackets.
Otužovanie (Zhasnutie)
- Spracovanie: Austenitize at 800–900 °C (v závislosti od zliatiny), hold 30 minutes per 25 mm section thickness, potom quench rapidly vo vode, soľankou, alebo olej.
- Vyplývať: Forms a martenzitické alebo bainitic structure that raises hardness to 450–600 HBW.
- Prípady použitia: Critical for wear-resistant components, such as gear teeth, shear blades, and high-stress connecting rods.
Data point: Proper quenching can increase tensile strength from 350 MPA (ako odliatok) do konca 1,200 MPA.
Temperovanie
- Spracovanie: Reheat hardened castings to 150–650 °C (below the lower critical point), soak for 1–2 hours, potom air‐cool.
- Vyplývať: Relieves brittleness, balancing hardness (down to 350–500 HBW) with improved nárazová húževnatosť (až 40 J v Charpyho testoch).
- Prípady použitia: Final step after hardening for parts like crankshafts, where a compromise between strength and toughness ensures durability.
4. Benefits of Sand Casting Heat Treatment
Applying controlled heat‐treatment cycles to sand‐cast components unlocks a range of performance and manufacturing advantages.
Below are the key benefits—each backed by quantitative data where available—that drive quality, konzistencia, and cost‐effectiveness:
Optimized Hardness and Strength
- Quantifiable Gain: Hardness rises from ~200 HBW (ako odliatok) do konca 500 HBW after quench-and-tempering, a >150 % zvýšiť.
- Dopad: Improved wear resistance extends tool life and minimizes maintenance downtime in abrasive service environments.
Stress Relief and Dimensional Stability
- Zníženie stresu: Annealing can alleviate up to 90 % of residual stresses accumulated during solidification.
- Prínos: Reduced distortion and cracking during subsequent machining, zváranie, or service loading—resulting in tighter tolerances (±0.1 mm vs. ±0.5 mm as-cast).
Refined Microstructure and Toughness
- Grain Size Control: Normalizing refines grain diameter from 60 µm down to 30 µm, boosting impact toughness by up to 25 %.
- Výsledok: Enhanced resistance to shock and cyclic loading, critical for gearbox housings and high‐horsepower engine components.
Improved Machinability
- Surface Hardness Adjustment: Annealed castings (180–220 HBW) machine 20–30 % faster than as-cast parts.
- Vyplývať: Lower tool wear and shorter cycle times in CNC milling and turning—reducing perpart machining cost by up to 15 %.
Mechanické vlastnosti na mieru
- Všestrannosť: By varying soak times and quench media, foundries can dial in tensile strengths from 350 MPa až nad 1,200 MPA.
- Výhoda: Enables one alloy to serve multiple roles—from ductile pump housings to high‐strength drive shafts—without changing raw material.
Enhanced Fatigue Life
- Dátový bod: Components that undergo stress‐relief and tempering exhibit a 30–50 % increase in fatigue life during accelerated testing.
- Aplikácia: Extends service intervals for parts in repetitive load scenarios such as agricultural equipment and construction machinery.
Controlled Magnetic and Electrical Properties
- Prispôsobiteľnosť: Heat treatment can adjust electrical conductivity by ±10 % and magnetic permeability in steel castings for specialized electromagnetic applications.
- Relevantnosť: Ideal for motor housings, držiaky snímačov, and EMI‐sensitive enclosures.
| Prínos | Žíhanie | Normalizácia | Otužovanie + Temperovanie |
|---|---|---|---|
| Tvrdosť (HBW) | 180–220 | 230–270 | 350–600 |
| Veľkosť zrna (µm) | 40–60 | 20–40 | 10–20 |
| Úľava od zvyškového stresu (%) | 90–95 | 70–80 | 50–60 |
| Tensile Strength Increase (%) | — | +20 | +250 |
| Charpyho húževnatosť (J) | 80–100 | 60–80 | 20–40 |
5. Záver
Selecting the appropriate sand casting heat treatment path depends on the zliatinová chémia, casting geometry, a intended service conditions.
By controlling heating rates, soak times, and cooling profiles, manufacturers transform raw sand-cast parts into components
with predictable, high-performance characteristics—ready for CNC machining, kovanie, or direct installation in critical assemblies.
To learn more about optimizing heat treatment for your sand-cast components, contact our team of metallurgical experts.
Leveraging data-driven process controls, we ensure every casting achieves its full potential in strength, trvanlivosť, a spoľahlivosť.
