Steel Tempering: Factors, Techniques, and Treatments
Factors Affecting Steel Tempering
Several factors influence the achievement of optimal steel tempering, including:
- Steel type and structural state
- Thermal conductivity
- Part size
- Cooling medium
The initial structure’s grain size affects the critical cooling speed; larger grains slow down austenite transformation.
Hardenability
Hardenability refers to steel’s ability to achieve depth of hardening, not its toughness. Methods for measuring hardenability include:
- Impact fracture observation
- Acid etching to reveal structures
- Hardness variation studies (most accurate)
Jominy Test
Standardized Hardenability Assessment
The Jominy test (adopted by AISI and SAE) determines crucial steel characteristics, especially for setting property boundaries in standard steels. This test provides information on:
- Minimum and maximum achievable hardness
- Hardenability
- Cooling media effects
Jominy discovered a direct correlation between tensile mechanical properties and steel hardness.
Steel Tempering Techniques
Cooling Methods
Common cooling media include:
- Water: Its low boiling point can reduce cooling speed.
- Oil: Mineral-based oils with viscosities between 5 and 9 degrees are typical.
- Mercury: Used for special cases requiring extreme hardness, despite its high cost.
The cooling process involves three steps:
- Heat loss by conduction and radiation through the gaseous layer.
- Vapor transport cooling, influenced by bath viscosity and cracking potential.
- Cooling by conduction and convection in the liquid.
Patenting
Isothermal Transformation for High-Tensile Wire
Patenting is used before or during high-tensile wire drawing. It involves heating the wire to full austenitization, then cooling it in a molten lead bath (350-600°C). This isothermal annealing process requires careful temperature control.
Austenmpering
Tempering for Tools and Small Parts
Austenmpering is effective for tools and small steel parts, minimizing fissures and tensions associated with martensitic structures. It is less economical for large pieces.
Martenpering
Stress Reduction in Hardening
Martenpering aims to achieve martensitic hardening while eliminating stress cracks. This is done by cooling the austenitized metal in a molten salt bath (200-300°C). It’s used for manufacturing bearings, gears, etc.
Post-Tempering Treatments
Tempering
Complementary Annealing
Tempering is a complementary annealing process following quenching. Quenching to achieve a martensitic structure can induce internal tensions. Tempering improves mechanical properties by adjusting annealing temperature and duration.
Surface Treatments
Enhancing Toughness and Wear Resistance
Surface treatments with varying compositions are used for parts requiring both toughness and high hardness/wear resistance. This is achieved by combining low-carbon steels (for toughness) with high-alloy steels (for hardness). Common methods include:
- Cementation
- Carbonitriding
- Sulfanization