Heat Treatment of Castings

Heat treatment transforms raw castings—often brittle and non-uniform—into high-performance components with tailored mechanical and physical properties.

By precisely controlling temperature profiles, soak times, and cooling rates, foundries manipulate an alloy’s microstructure to achieve predictable outcomes.

In this comprehensive article, we delve into the purposes, metallurgical underpinnings, key objectives, primary processes, alloy-specific considerations, process control, and real-world applications of casting heat treatments.

1. Introduction

In casting production, uncontrolled solidification yields large grains, segregation, and residual stress levels exceeding 200 MPa.

Consequently, heat treatment serves three critical roles:

1. Microstructure Modification: It converts as-cast dendrites and segregation zones into refined grains or precipitates, directly influencing hardness (up to 65 HRC in steels) and toughness.
2. Stress Relief: By reducing internal stresses by up to 80%, it prevents distortion during machining and eliminates cracking in service.
3. Property Optimization: It balances hardness, ductility, strength, and fatigue life—often a trade-off requiring careful cycle design.

Moreover, ferrous alloys (carbon steels, alloy steels, ductile and gray iron) leverage phase transformations, such as austenite to martensite, to achieve high wear resistance.

In contrast, non-ferrous alloys (aluminum, copper, nickel) typically utilize solid-solution and precipitation hardening to reach tensile strengths of 300–800 MPa.

Understanding these differences forms the foundation for effective heat-treatment strategies.

2. Metallurgical Fundamentals

Phase Transformations in Steels

Steels exhibit numerous phase changes:

• Austenite (γ-Fe): Stable above 720 °C, face-centered cubic.
• Ferrite (α-Fe): Stable below 720 °C, body-centered cubic.
• Pearlite: Alternating layers of ferrite and cementite form during slow cooling.
• Martensite: Hard, body-centered tetragonal phase achieved by quenching at cooling rates >100 °C/s.

TTT and CCT Concepts

• Time-Temperature-Transformation (TTT) Diagrams show isothermal holds that yield 100% pearlite at 600 °C after ~10 s.

  

• Continuous Cooling Transformation (CCT) Curves predict phase fractions during actual cooling ramps (e.g., quench in oil at 20–50 °C/s yields ~90% martensite).

3. Primary Heat-Treatment Processes

LangHe Foundry relies on a core suite of heat-treatment techniques to tailor casting properties.

Each process targets specific microstructural changes—whether softening for machinability or hardening for wear resistance.

Below, we examine the seven principal methods, their typical parameters, and the mechanical benefits they deliver.

Annealing

Purpose: Soften the casting, relieve stress, and improve ductility.

• Process: Heat to a temperature just above the alloy’s recrystallization point (steels: 650–700 °C; aluminum alloys: 300–400 °C), hold for 1–4 hours, then furnace-cool at 20–50 °C/h.
• Outcome: Hardness drops by 30–40 HRC in quenched steels, while elongation rises by 15–25%. Residual stresses fall by up to 80%, reducing the risk of distortion during machining.

Normalizing

Purpose: Refine grain structure and homogenize the microstructure for predictable strength.

• Process: Heat carbon steels to 900–950 °C (above Ac₃), soak 30–60 minutes, then air-cool.
• Outcome: Grain size typically refines by one ASTM grade; tensile strength variance narrows to ±5%, and surface hardness stabilizes within ±10 HB.

Quenching

Purpose: Produce a hard martensitic or bainitic matrix in ferrous alloys.

• Process: Heat above the upper critical temperature (950–1050 °C), then quench in water (cooling rate > 100 °C/s), oil (20–50 °C/s), or polymer solutions.
• Outcome: Martensite content reaches ≥ 90%, yielding hardness of 55–65 HRC and ultimate tensile strengths up to 1200 MPa. Note: Aluminum, copper, and nickel alloys typically soften to a solutionized condition for subsequent ageing.

Tempering

Purpose: Reduce brittleness of quenched steels, trade some hardness for toughness.

• Process: Reheat martensitic castings to 200–650 °C, soak 1–2 hours, then air-cool.
• Outcome: Hardness adjusts from 60 HRC down to 30–50 HRC, while Charpy impact energy increases by 40–60%, dramatically improving resistance to dynamic loads.

Precipitation Hardening (Ageing)

Purpose: Strengthen non-ferrous alloys via fine precipitate formation.

• Process:

• • Aluminum (6xxx series): Solution-treat at 530 °C, quench, then age at 160 °C for 6–12 hours.
  • Nickel alloys: Age at 700–800 °C for 4–8 hours.
• Outcome: Yield strength climbs by 30–50% (e.g., 6061-T6 yields ~240 MPa vs. 150 MPa in T4), while retaining elongation ≥ 10–12%.

Solution Treatment & Aging (Non-Ferrous)

Purpose: Dissolve alloying elements, then re-precipitate them for optimal hardness and corrosion resistance.

• Process: Heat to the solvus temperature (e.g., 520 °C for 17-4 PH stainless), hold 30 minutes, water-quench, and age (e.g., 480 °C for 4 hours).
• Outcome: Achieves controlled hardness (Rockwell C 38–44 in PH stainless) and uniform mechanical properties throughout the casting.

Case Hardening (Carburizing, Carbonitriding, Nitriding)

Purpose: Impart a wear-resistant surface shell over a tough core.

• Process Options:

• • Carburizing: 900–950 °C in a carbon-rich atmosphere for 2–8 hours; quench to form a 0.5–2 mm case at 60–65 HRC.
  • Carbonitriding: Similar to carburizing but with added ammonia, creating a mixed carbon-nitrogen case for enhanced fatigue life.
  • Gas Nitriding: 520–580 °C in ammonia for 10–20 hours, yielding surface hardness up to 900 HV without quenching.

• Outcome: Surface wear rates drop by 70–90%, while core toughness remains high—ideal for gears, camshafts, and bearing surfaces.

4. Cast Alloy-Specific Considerations

While general principles of heat treatment apply across many materials, each alloy system responds uniquely to thermal processing.

Differences in chemical composition, phase stability, and thermal conductivity require specialized strategies to maximize performance.

In this section, we will examine important alloy-specific considerations for cast steels, irons, aluminum, copper, and nickel-based systems.

Carbon Steels & Alloy Steels

Key Factors:

• Hardenability: Directly influenced by carbon content and alloying elements like Cr, Mo, and Ni. For instance, 0.4% carbon steels reach ~55 HRC after oil quenching, while low-carbon steels (<0.2% C) may barely harden without additional alloying.
• Critical Cooling Rates: Must quench fast enough to form martensite but avoid cracking or distortion.
  Steels with higher alloy content (e.g., 4140, 4340) allow slower quenching media like oil or polymer solutions, reducing thermal shock.

Special Notes:

• Tempering post-quenching is crucial to balance hardness and toughness.
• Normalization can help improve isotropy and prepare for hardening operations.

Ductile (SG) & Gray Cast Irons

Key Factors:

• Matrix Control: Heat treatment (e.g., austempering) transforms pearlitic or ferritic matrices to bainitic structures in ductile iron, boosting tensile strength to ~1200 MPa with 10–20% elongation.
• Graphite Shape Preservation: Must prevent graphite nodules (in SG Iron) or flakes (in gray iron) from degrading, as this severely impacts mechanical performance.

Special Notes:

• Stress relief annealing (~550–650 °C) is common to reduce internal stresses without significantly altering graphite morphology.
• Normalizing can enhance strength, but it must be carefully controlled to avoid excessive hardness.

Aluminum Alloys

Key Factors:

• Precipitation Hardening: Dominates strength development in 2xxx, 6xxx, and 7xxx series alloys.
  T6 treatments (solution heat treatment + artificial aging) can double yield strength compared to as-cast conditions.
• Distortion Sensitivity: Aluminum’s high thermal conductivity and low melting point (~660 °C) make careful ramp rates and quench controls essential to minimize warping.

Special Notes:

• Typical T6 treatment for A356 castings:

• • Solution heat treat at 540 °C for 8–12 hours
  • Quench in water at 60 °C
  • Age at 155 °C for 4–6 hours

Results in yield strengths up to 250 MPa, with elongations of ~5–8%.

Copper & Copper-Based Alloys

Key Factors:

• Solid Solution vs. Precipitation Hardening: Brasses (Cu-Zn) mainly benefit from cold working and annealing, while bronzes (Cu-Sn) and aluminum bronzes (Cu-Al) respond well to age-hardening treatments.
• Overaging Risk: Excessive aging can coarsen precipitates, dramatically reducing strength and corrosion resistance.

Special Notes:

• Aluminum bronze castings (e.g., C95400):

• • Solution treat at 900–950 °C
  • Water quench
  • Age at 300–400 °C to achieve tensile strengths up to 700 MPa.

Nickel-Based Alloys

Key Factors:

• Precipitation-Hardening Alloys (e.g., Inconel, Incoloy, Hastelloy): Require precise control over ageing temperatures and times to maximize yield strength without sacrificing ductility.
• Resistance to Overaging: These alloys offer excellent thermal stability, but incorrect heat treatment can still cause embrittlement.

Special Notes:

• Typical treatment for Inconel 718 castings:

• • Solution treated at 980 °C
  • Age at 720 °C for 8 hours, then furnace cool to 620 °C and hold 8 more hours.

• Outcome: Tensile strengths exceed 1200 MPa, with excellent creep and fatigue resistance at elevated temperatures.

5. Process Parameters & Control

In the heat treatment of castings, precise control over process parameters is essential to achieve the desired material properties consistently.

Variations in temperature, time, atmosphere, and cooling conditions can dramatically affect the microstructure and, consequently, the mechanical performance of the casting.

This section explores the major parameters and best practices for controlling them.

Furnace Types and Atmosphere Control

Furnace Selection:

• Air Furnaces: Suitable for general heat treatment of steels where slight oxidation is acceptable.
• Protective Atmosphere Furnaces: Use inert gases (e.g., nitrogen, argon) or reducing gases (e.g., hydrogen) to prevent oxidation and decarburization.
• Vacuum Furnaces: Ideal for high-value alloys (e.g., nickel-based superalloys, titanium) requiring ultra-clean surfaces and minimal contamination.

Data Point:
In vacuum heat treatment, residual oxygen levels are typically kept below 10⁻⁶ atm to prevent oxide formation.

Best Practice:
Use atmosphere monitoring sensors and automated flow control systems to maintain consistent gas composition during processing.

Heating Parameters

Soak Temperature and Time:

• Temperature Accuracy: Must stay within ±5 °C of the target temperature for critical applications.
• Soak Time: Depends on casting thickness and alloy type; a common rule of thumb is 1 hour per inch (25 mm) of section thickness.
• Ramp Rates: Controlled heating rates (e.g., 50–150 °C/hour) prevent thermal shock and minimize distortion, especially for aluminum and complex steel castings.

Monitoring:
Multi-zone furnaces with independent controls ensure temperature uniformity across large or complex castings.

Cooling and Quenching Control

Cooling Media:

• Water Quench: Extremely rapid, suitable for steels but risks distortion and cracking.
• Oil Quench: Slower cooling, often used for alloy steels to reduce thermal stresses.
• Polymer Quench: Adjustable cooling rates by modifying polymer concentration; combines the benefits of oil and water.
• Air or Gas Cooling: Used where minimal quenching stress is required (e.g., some aluminum alloys).

Key Cooling Parameters:

• Agitation: Improves heat extraction and prevents vapor blanket formation around the part.
• Temperature Control: Cooling media should be kept within specific temperature ranges; for example, oil quenches are often maintained between 60–80 °C to ensure uniform cooling.

Example:
For 4340 steel, oil quenching from 845 °C typically achieves martensitic structures with minimal cracking compared to water quenching.

Process Monitoring and Data Logging

Instrumentation:

• Thermocouples: Attached directly to representative parts to monitor real-time temperatures.
• Furnace Control Systems: Modern setups use PLCs (Programmable Logic Controllers) for automatic recipe management.
• Data Loggers: Record temperature profiles, soak times, and cooling curves for full traceability and quality audits.

Best Practice:
Utilize redundant thermocouple systems (load thermocouples and survey thermocouples) to cross-validate furnace conditions.

6. Industrial Applications & Case Studies

Automotive Brake Rotors

• Process: Normalize at 900 °C, quench in oil, temper at 450 °C for 2 h.
• Outcome: Achieve 45 HRC, minimal warping <0.05 mm under thermal cycling.

Oil & Gas Pump Impellers

• Alloy: 718 Ni-base.
• Cycle: Solution treat at 980 °C, quench, age at 718 °C for 8 h, then 621 °C for 8 h.
• Result: UTS 1200 MPa and SCC resistance in sour service.

Aerospace Turbine Cases

• Material: 17-4 PH stainless.
• Treatment: H900 (490 °C × 4 h) yields 1050 MPa UTS and excellent fatigue strength.

Heavy Equipment Gearboxes

• Steel: 4340 alloy.
• Process: Carburize at 930 °C for 6 h, quench, temper at 160 °C.
• Benefit: Surface 62 HRC, core 35 HRC, enduring heavy-load cycles.

7. Conclusion

Heat treatment remains indispensable in casting production, offering a versatile toolkit to modify microstructures and engineer precise mechanical properties.

By mastering metallurgical fundamentals—phase transformations, TTT/CCT principles, and hardening mechanisms—and by exercising strict control over furnace atmospheres, soak times, and cooling rates,

foundries deliver castings with optimized hardness, strength, ductility, and fatigue life.

Through rigorous testing and alloy-specific adjustments, heat treatment elevates cast components from raw form to mission-ready parts across automotive, oil & gas, aerospace, and heavy-equipment industries.

Moving forward, innovations in induction heating, digital process controls, and integrated additive manufacturing promise even greater efficiency, consistency, and performance in casting heat treatments.

At LangHe, we are happy to discuss your project at an early stage in the design process to ensure that whatever the alloy selected or post-casting treatment applied, the end result will meet your mechanical and performance specifications.

To discuss your requirements, email [email protected].