The quenching process of lower bainite

I. Basic Principle of Lower Bainite Quenching

II. Process Steps and Parameters of Lower Bainite Quenching

1. Austenitization Stage (Heating and Holding)

• Purpose: To fully dissolve carbides in the steel and obtain a uniform, fine austenitic structure, laying the foundation for subsequent transformation.
• Process Parameters:
  • Austenitizing Temperature: Determined by the steel composition, generally 30~50°C above Ac3 (for hypoeutectoid steels) or between Ac1 and Ac3 (for hypereutectoid steels, to avoid network carbides). For example:
    • Medium carbon steels (e.g., 45 steel): 820~860°C;
    • Medium carbon low-alloy steels (e.g., 40Cr): 840~880°C;
    • High carbon steels (e.g., T8 steel): 780~820°C (to prevent coarse grains from overheating).
  • Holding Time: Determined by the workpiece thickness and loading capacity to ensure austenite homogenization. Typically 1~3 hours (shorter for small workpieces, up to 30 minutes; longer for large workpieces), avoiding underheating (incomplete austenitization) or overheating (coarse grains, leading to reduced performance).

2. Rapid Cooling Stage (Avoiding Pearlite/Upper Bainite Zones)

• Purpose: To rapidly cool the austenitized workpiece to the lower bainite transformation temperature range (200~350°C), preventing premature transformation in the pearlite (500~600°C) or upper bainite (350~500°C) zones, ensuring "directional" transformation of austenite into lower bainite.
• Process Parameters:
  • Cooling Medium: Must provide sufficient cooling rate (exceeding the steel's critical cooling rate). Common media include:
    • Molten salts (e.g., nitrate-nitrite baths, with low melting points and easy temperature control);
    • Mineral oils (suitable for small workpieces, with slightly slower cooling than molten salts);
    • Polymer solutions (e.g., polyvinyl alcohol solutions, with adjustable cooling capacity).
  • Cooling Endpoint: Rapidly cool the workpiece to 200~350°C (specific temperature determined by the steel's TTT curve; e.g., 250~300°C for low-alloy steels).

3. Isothermal Transformation Stage (Core Step)

• Purpose: Hold at the set low temperature to fully transform austenite into lower bainite (minimizing retained austenite).
• Process Parameters:
  • Isothermal Temperature: Usually 200~350°C (characteristic range for lower bainite). Lower temperatures produce finer lower bainite laths, increasing strength and hardness but slowing transformation (requiring longer holding); higher temperatures (near 350°C) may form partial upper bainite, reducing toughness.
  • Isothermal Time: Determined by the steel's TTT curve to ensure complete austenite transformation. Examples:
    • 40Cr steel at 280°C: ~2~4 hours;
    • 60Si2Mn spring steel at 250°C: ~3~5 hours.
  • Judgment Criteria: Microstructural inspection (lower bainite appears as dark needle-like/lamellar structures without pearlite or martensite) or empirical formulas (e.g., extend time by 0.5~1 hour per 10mm thickness).

4. Post-Isothermal Cooling

• After isothermal holding, the workpiece can be air-cooled to room temperature (no need for continued rapid cooling). Austenite has already fully transformed into lower bainite, so air cooling avoids martensite formation or other adverse structures and prevents stress cracking from excessive cooling rates.

III. Suitable Steel Grades

• Medium carbon steels: 45, 50, etc.;
• Medium carbon low-alloy steels: 40Cr, 42CrMo, 35CrMo, etc.;
• Spring steels: 60Si2Mn, 50CrVA, etc.;
• Bearing steels: GCr15 (control austenitizing temperature to avoid coarse grains).

IV. Performance Characteristics and Applications

• Performance Advantages: Lower bainite has a hardness of HRC45~55 (close to martensite) but 2~3 times higher impact toughness (αk) than martensite (e.g., 40Cr after lower bainite quenching has αk≥80J/cm² vs. 30~50J/cm² for martensitic quenching), with excellent fatigue strength and wear resistance.
• Applications: Parts requiring "strength-toughness balance," such as gears, drive shafts, connecting rods, springs, and bearing rings.

V. Key Process Points and Precautions

1. Austenitization Control: Excessively high temperatures cause coarse austenite grains and roughened lower bainite, reducing toughness; low temperatures lead to incomplete austenitization, with residual carbides impairing transformation uniformity.
2. Cooling Rate: Must rapidly pass through the pearlite zone (500~600°C); otherwise, pearlite forms, drastically reducing strength (select media with appropriate cooling capacity, e.g., molten salt baths, to ensure rates exceed critical values).
3. Isothermal Parameters: Strictly follow TTT curves for temperature and time—deviations cause excessive retained austenite (reduced hardness) or partial upper bainite (reduced toughness).
4. Workpiece Geometry: For large or complex parts, control heating/cooling rates to avoid stress cracking (use stepped heating or pre-cooling to higher temperatures before isothermal holding).