Figure 1. Salt bath hardening of a 96-inch-long tool steel broach (photograph courtesy of Upton Industries)

Question:
Can you provide a candid and independent assessment of tool-steel hardening using salt bath equipment?

Answer:
I can sure try, having been fortunate enough in my career to do salt bath heat treating. First, it is important to understand that salt bath technology has process capability over the full range of tool-steel grades. In addition, it is capable of achieving temperature uniformity at austenitizing temperature in the range of ±5°F (±3°C).

When using salt bath equipment, the critical issues are:
Control of operator-induced variability
Properly maintaining bath chemistry and integrity

Figure 2. Typical salt bath hardening line (photograph courtesy of Upton Industries)

Salt Bath Advantages

1. The heat-transfer rate between the liquid salt and the steel is in the order of 3-4 times faster than between a radiant or convection (gas) environment and steel.
2. Work immersed in salt is “automatically preheated” as a “cocoon” of frozen salt is formed on immersion, which melts fairly rapidly.
3. Uniformity of heat transfer (conduction) is more efficient for short process cycle times.

Other noteworthy features:

1. Component distortion is minimized by the buoyancy effect of the salt (e.g., a component that weighs 10 pounds in air weighs only 3.3 lbs in liquid salt).
2. Finer (prior) austenitic grain sizes due in large part to shorter times at preheat temperature.
3. Use of convection or electrochemical stirring can achieve temperature variations in the salt bath of ±3.5°F (±2°C).

Salt Bath Disadvantages

1. Limited protection of the tool-steel parts from oxidation and decarburization either in the bath or during bath transfers
2. Numerous environmental issues (air pollution, water pollution, waste disposal, permits, etc.)
3. The operating range of the bath must be controlled by adjusting salt composition.
4. Control systems (i.e. programming, computer controls, etc.) lack sophistication
5. High operator dependency for accuracy and repeatability
5. High heat discharge to the environment
6. Labor intensive
7. Maintenance intensive – chemistry & equipment (type and complexity)