Salt Bath Quenching

Salt bath quenching is a metal heat treatment process in which a workpiece, after being austenitized by heating, is immersed into molten salt liquid at a specific temperature for rapid cooling (quenching) to achieve the required microstructure and properties.

1. Core Principle

• Medium: Molten salt (such as chloride, nitrate mixture) is used as heating/cooling medium.

• Process:

1. The workpiece is heated quickly and uniformly to quenching temperature in salt bath (isolated from air to prevent oxidation and decarburization).

2. Quickly transferred to low-temperature salt bath (for marquenching/isothermal annealing) or cooled directly in salt bath.

3. The controllable cooling rate of salt bath (between water and oil) is used to reduce deformation and cracking.

2. Main Types

• Marquenching (Grade Quenching): Insulated in salt bath above Ms point (martensite start temperature), cooled uniformly then air-cooled.

◦ Application: Complex and thin-walled parts (tools, dies), minimal deformation.

• Isothermal Quenching (Bainite Quenching): Long-term insulation in salt bath near Ms point, austenite directly transformed into bainite.

◦ Features: High hardness + high toughness, excellent comprehensive performance.

3. Main Advantages

• Small deformation, low cracking risk: uniform cooling, low thermal/structural stress.

• Good surface quality: no oxidation, no decarburization, smooth surface.

• Controllable performance: obtain martensite, bainite accurately by adjusting salt temperature/composition.

• Fast heating: thermal conductivity much higher than air furnace, high efficiency.

4. Disadvantages

• Pollution/corrosion: toxic salt fumes, workpieces need thorough cleaning to prevent corrosion.

• High cost: high salt consumption and environmental treatment cost.

• Poor working conditions: high temperature, fumes, strong ventilation required.

5. Typical Applications

• Tool steel: taps, drills, dies (small deformation, high hardness required).

• Bearing steel: GCr15 bearings (bainite structure, long fatigue life).

• Precision parts: gears, special-shaped parts, thin-walled parts.

• Die steel: hot-work dies, cold-work dies.