Approximately 20 years ago, salt bath heat treating was deemed "messy" and the costs associated with the related drag-out were relatively high. With the advent of new salt bath technology, equipment manufacturers and salt suppliers are providing equipment that is compact in size, optimizing production output, and offering the capability of salt reclamation thereby contributing to a cleaner environment.

Fig. 1. Heating rate comparison for four heating methods


New salt bath technology is growing in its acceptance as a cost-effective, environmentally acceptable process for treating a variety of products from austempered ductile iron (ADI) to high-speed tool steels and non-metals such as glass, rubber, plastic, etc. Characteristics such as controllability, versatility, minimal floor space, heat transfer efficiency and economy are comparable to or exceed other processes. The new age of salt bath technology features modernized equipment, automated parts, handling lines and safe chemicals. With the proper salt bath system design - carefully matching the chemicals, equipment and application - capital investment costs can be lower than alternative methods. Energy and labor costs are also reduced and post-treatment operations are minimized.

Fig. 2. Over the top, knee-type electrode system. (Photo courtesy of Industrial Heat Treat, Beamsville, Ontario)

Heat Transfer Efficiency/Speed of Heating

Figure 1 compares four heating methods by utilizing a 2" diameter SAE 1040 steel cylinder with type K thermocouple located in the center of the test piece. The advantage of salt as the heating media over other types of furnace equipment can be easily seen.

Fig. 3. Partially immersed knee type electrode system

Furnace Selection - Electric or Gas - Temperature Uniformity

Over the Top, Knee-Type Electrode System

Fuel: Electric. Extremely easy to remove and replace electrodes.

Uniformity, ± 15°F (approx.)

Partially Immersed Knee Type Electrode System

Fuel: Electric. Back partial wall must be removable to install new electrodes and wall must be re-bricked.

Uniformity, ± 15°F (approx.)

Totally Submerged Electrode System

Fuel: Electric. Furnace must be re-bricked to install new electrodes.

Uniformity, ± 10°F (approx.)

Gas Fired Tangentially Heated System

Fuel: Gas. Preventive maintenance includes sludge removal and pot revolving to avoid burner impingement.

Uniformity, ± 15-20°F (approx.)

Fig. 4. Totally submerged electrode system (Photo courtesy of Advanced Cast Produccts Inc., Meadville, PA)

Benefits of Modern Salt Bath Technology

Uniform Heat Transfer

Salt bath heat-up time is three to four times faster than that of other heating methods. The uniform, rapid heating results in less cracking and distortion thereby reducing scrap. The molten salt freezes on the parts at immersion, insulating the components until their temperature exceeds the melting point of the salt, minimizing thermal shock.

Safety

Modern salt baths are non-flammable, have very low volatility and are capable of being non-polluting.

Automation

Salt bath heat-treating systems can be automated to eliminate all manual handling operations (except loading and unloading). Controlled robotic parts handling equipment can transfer parts efficiently in high production applications improving productivity rates and lowering costs.

Fig. 5. Gas fired tangentially heated system (Photo courtesy of A&M Heat Treating, Mississauga, Ontario)
Accurate Controls

Salt bath temperatures are accurately maintained with a broad operating range designed to match the bath with the metal characteristics.

Versatile

Parts can be partially immersed for selective treating. Variable time cycles can be utilized and the work can be immersed or withdrawn from the bath at any time.

Surface Protection

Salt baths protect metal surfaces from oxidation or scaling by excluding air during immersion. After immersion, a thin salt "cocoon" remains to shield the metal from oxidization.

Conclusion

Capital costs, high production rates, minimal distortion or breakage, and small space requirements combine to make salt bath heating efficient, economical, and very versatile. The salt products themselves are not expensive and the equipment compares favorably with other types of furnaces. Additionally, highly skilled personnel are not required to operate salt bath furnaces.

For more information: Brian Reed is President of Park Thermal International (1996) Corporation, 62 Todd Rd., Georgetown, ON L7G 4R7 Canada. Ph: 877-834-4303; Fax: 905-877-6205; e-mail: info@parkthermal.com; web: www.parkthermal.com


Additional related information may be found by searching for these (and other) key words/terms via BNP Media LINX at www.industrialheating.com: salt bath, oxidation, heat transfer, thermal shock