Figure 1. Typical tool steel load for vacuum hardening (courtesy of Nevada Heat Treating)
Question
Can you provide a candid and independent assessment of tool steel hardening using vacuum technology?
Figure 2. Typical vacuum tool steel hardening furnace (courtesy of VAC AERO International)
Answer
Again, I can sure try, having been actively involved in the development of vacuum technology for this group of materials. Vacuum technology has process capability over the full range of tool steel grades. It is capable of achieving temperature uniformity at austenitizing temperature in the range of ±5°F (±3°C). When using vacuum furnaces, the critical issues are proper design and careful control of:1. Part loading and racking
2. Ramp rates
3. Preheating temperatures and times
4. Austenitizing temperature and time at temperature
5. Cooling rates of parts
Vacuum Furnace Advantages
1. Variable heating rates - convection heating in lower-temperature (<1600°F or 870°C) ranges
2. Clean parts (extreme surface finishes), particularly if the parts are gas quenched
3. Ability to handle physically large part sizes, complex shapes and heavy load weights
4. Flexibility for intermediate heating and cooling
5. Superior programming capability and cycle flexibility
6. Load documentation (time/temperature cycles)
7. Environmental, energy consumption and safety
Vacuum Furnace Disadvantages
1. Initial capital equipment cost
2. Slight loss of surface hardness (0.5–1.0 HRC) for certain tool steel grades (Note: this can be overcome but special features/options are required.)