Single-Chamber Furnaces, Type MonoTherm

The process of heat treatment has been used for centuries in order to specifically change the properties of components. In the course of development, the processes have fundamentally changed. Up to the early 20th century the processes were conducted in a normal atmospheric environment. The use of protective gases has further improved the quality of the components.

Heat treatment development experienced a significant boost with the introduction of vacuum technology. At first, this technology was only used for special materials in aviation technology but soon widely spread to harden high alloyed tool steels.

The following reasons are essential for the change to vacuum technology:

• Reduced distortion
• Clean and dry parts, no oxidation
• Simple and reproducible treatment of parts
• Fully automated processes
• Hardening and tempering in one system

For the heat treatment, parts are loaded in batches into the vacuum furnace. The vacuum furnace is a pressure vessel, equipped with insulation as well as a heating system. After loading the furnace, the vessel is evacuated, thus the air and at the same time any potential for oxidation is removed from the furnace. The parts can be heated either in vacuum or under convection. When using convective heating, an inert gas, usually nitrogen up to 2 bar, is filled into the furnace after evacuation. By circulating the gas during heating, excellent temperature uniformity is reached, which has a positive effect on reducing distortion. Convective heating up to 900 °C also shortens the cycle time. Subsequently, the load is heated in vacuum to the required austenitizing temperature. After an adequate soaking time at austenitizing temperature
the parts are cooled using gas quenching.

The type of gas used and the necessary pressure depends on the part (material, geometry) as well as on the required treatment results. Most parts of cold and hot work tool steels as well as high-speed steels can be hardened with nitrogen at a quenching pressure up to 10 bar.

Mid and some low alloyed tool steels, such as ball bearing steel 100 Cr 6, require quenching gases with better thermal properties such as helium, or higher quenching pressures up to 20 bar. Most of the low alloyed steels like case hardening steels are requiring increased quenching speeds which may not be achievable in singlechamber vacuum heat treatment furnaces. These parts have to be quenched in a "cold quenching chamber".

ALD has developed such furnace types:
• Double-chamber vacuum furnace, Type DualTherm®
• Linked multi-chamber furnace, Type ModulTherm®
• Special furnaces