What types of salts are used in a salt bath curing line

What are their typical operating temperature ranges?

Salt bath curing lines generally employ two categories of salt formulations, depending on the intended heat treatment stage:

Low-temperature salts (for tempering and aging): Typically composed of nitrates and nitrites, such as a 50/50 mixture of sodium nitrate (NaNO₃) and potassium nitrate (KNO₃). Their operating range is usually between 160°C and 550°C. These salts offer good fluidity and thermal stability below 600°C.

High-temperature salts (for hardening and austenitizing): Usually based on chlorides, such as barium chloride (BaCl₂), sodium chloride (NaCl), or calcium chloride (CaCl₂). A common mixture is 50% BaCl₂ and 50% NaCl, operating between 600°C and 950°C, with some formulations reaching up to 1300°C for high-speed steel processing.

The selection of salt depends on the workpiece material and the required transformation temperature. Chloride-based salts are more aggressive and require more frequent maintenance.

What are the main advantages of a salt bath curing line over conventional atmospheric furnaces?

Temperature uniformity: Molten salt provides direct contact heat transfer, keeping workpiece temperature variation within ±5°C, compared to ±15°C or more in many forced-air furnaces.

Reduced oxidation and decarburization: The molten salt forms a protective coating on the metal surface, limiting oxygen exposure. For neutral salt baths, decarburization depths are often less than 0.05 mm.

Faster heating rates: Liquid-to-solid heat transfer coefficients are higher than gas-to-solid, reducing soaking time by 30–50% for similar section thicknesses.

Lower distortion: Even heating minimizes thermal gradients, which is beneficial for complex shapes or thin-walled components.

What safety and environmental concerns are associated with salt bath curing lines?

Salt bath lines present specific operational risks and waste management requirements:

Burn hazards: Molten salts at 500–900°C can cause severe thermal injury. Splashing can occur if damp workpieces are introduced, as water vaporizes rapidly. Standard practice requires preheating workpieces to 150–200°C before immersion or ensuring they are completely dry.

Toxic fumes: Chloride-based salts, especially those containing barium chloride, may generate small amounts of chlorine or hydrogen chloride gas when reacting with atmospheric moisture at high temperatures. Adequate exhaust ventilation (typically 20–30 air changes per hour) is required.

Waste disposal: Spent salts may contain heavy metals (e.g., barium or cyanide from older carburizing salts). Disposal is regulated as hazardous waste in most industrial regions. Neutralization and precipitation treatments are needed before landfill discharge.

Skin and eye protection: Operators must wear face shields, aluminized suits, and insulated gloves. Standard cotton or leather gloves can retain absorbed salt and cause contact burns.

How is the salt bath maintained to ensure consistent curing quality?

Routine maintenance of a salt bath curing line focuses on chemical composition and physical cleanliness:

Rectification (regeneration): Over time, high-temperature chlorides react with oxygen and moisture, forming oxides and hydroxides that raise viscosity and reduce heat transfer. Weekly rectification involves adding rectifiers (e.g., ammonium chloride or silicon tetrachloride) to convert oxides back to chlorides.

Sludge removal: Oxides, carbon residues, and foreign particles settle at the bottom of the bath. Sludge volume is checked monthly by dipping a steel rod; when sludge exceeds 10–15% of bath depth, the bath is drained, and sludge is mechanically removed.

Electrode cleaning: For electrically heated baths, carbon build-up on graphite or metal electrodes increases resistance. Electrodes are mechanically cleaned every 4–6 weeks or when the current draw rises by 15% above baseline.