Monday, December 7, 2009

Salt bath furnaces

Modern Salt bath furnaces are used for a number of heat treatment applications such as: Preheating • Austenitizing • Martempering • Neutral Hardening • High-Speed Tool Hardening • Tempering Nitriding • Carburizing • Solution Heat Treating • Dip Brazing

Modern systems offer quick ramp-up and high heating uniformity with temperatures maintained to within 5 degrees throughout the bath providing high and uniform processing results

Today's furnaces are heated by electricity, oil or gas. In procuring Salt Bath Furnaces special attention should be given so operator safety standards and local environmental requirements are met and that waste treatment technologies involved meet government regulations and provide for a comprehensive and cost-effective waste management such as effective sludge removal systems.

Modern systems included modular stages in order to accomodate pre- and post-treatments that are combined with fully programmable controls that offer precise in-line processing capabilities.

Typical wire and wire rod applications include: Annealing •Nitriding •Melting •Tempering •Hardening •Brazing •Galvanizing •Aluminizing as well as Surface treatment of various metals & alloys

Typical processing tempratures for wire applications are 500°C to 1100°C (900°F to 2000+°F)

The electrically heated furnaces are equipped with either resistance or silicon carbide heating elements, mounted externally to either a square or round metal pot holding molten media.

At elevated temperatures silicon carbide heating elements are prefered. The elements are made in one piece from high-density, high-purity, self-bonded silicon carbide.

For a round pot furnace helically-wound wire heaters are a common configuration.

Schematic view of Furnace with over - the - top mounted electrodes
Features include: 6" thick tile walls make the pot last for years. The removable electrodes can be changed without emptying furnace.
The use of large cross section electrodes assure long heater life and provides electrodynamic circulation for uniform temperature.

Furnace with submerged electrodes Operating temprature 850&degC to 1260&degC (1550°F to 2300°F) for Austenitizing

Submerged electrodes provides long life & trouble free service with periodic maintenance. Electrodes, embedded in the wall, below the salt line,are sealed against corrosive attack at the air-salt interface. The submerged electrode design also lends itself to economical 3 phase operation in deeper furnaces when 2 or more tiers of electrodes are utilized.

Isothermal Quench Furnaces
For Austempering & Martempering

Molten salt quenching for austempering or martempering requires a furnace capable of several functions, and the resulting designs are much different from salt bath furnaces employed in heating or austenitizing work.

Four primary factors should be considered for all Quench Furnaces:

1.Bath Temperature Uniformity is essential for consistent metallurgical properties. Internal heating of the bath can be accomplished electrically by using either immersion heaters or electrodes. Gas-fired immersion tubes can also be employed. It is important to automatically dissipate heat from work being quenched by either internal or external cooling means. For light duty, forced air or air/water can be directed to the exterior of the pot. For heavy duty, controlled cooling is provided by internal coils, using water or air/water injection.

2.Proper Agitation of the bath is necessary to remove heat quickly and efficently from the work.mConstant or variable speed propeller agitation provides a uniform flow through the workload being quenched.

3. To achieve optimum hardness and other physical properties, foreign bath contaminants must be removed. In most furnaces this is accomplished by a continuous filtering and automatic dumping. A common system like that supplied by AJAX Electric Company comprises a scoop-shaped filter basket connected to a driving cylinder mounted adjacent to the furnace wall, the basket is automatically raised up and out of the bath - automating chloride removal. Readily installed on new furnaces, the mechanism can also be retrofitted on existing salt baths.

4. For isothermal quenching of heavier sections, water increases quenching power. Small amounts of water added to a nitrate-nitrite salt drives vapor from the bath as hot work enters and gives a faster cooling rate. Both manual and automatic systems are available for adjusting the frequency and duration of water additions.


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