Vacuum Hardening

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Process Features

Bright, clean work - no scale or oxidation
Low distortion
Power used only during processing
No dangerous gas atmosphere
No expensive salt disposal
No exposure to hot or live items
Excellent reproducible results
Fully automatic cycles
Environmentally clean

Problem

Salt baths and furnaces with air or protective atmospheres all produce hardened work with some degree of surface scaling or oxidation.

Minimisation of distortion in these types of furnace is also difficult to control and occurs through a number of causes: thermal shock due to plunging components into molten salt baths, plastic deformation occurring when parts are transferred to a quench tank, distortion caused by general handling during the hardening process.

Solution

Components heat treated through vacuum furnaces are carefully loaded at room temperature into the chamber. They are supported on ceramic, graphitic or heat resisting steel grids, plates, baskets or specially designed fixtures depending on the shape and complexity of the part.

The furnace is evacuated with vacuum pumps to operating pressure. This is commonly low vacuum (10-2 mbar range) for tool and high speed steels, higher vacuum levels (10-4mbar range) are used for materials more difficult to keep clean and bright, titanium for example. When the correct vacuum level has been achieved the charge is heated at a controlled rate to the preheat temperature.

With heavy sectioned parts and complicated shapes a number of intermediate preheats are used before the austenitising temperature is reached. Because the heat transfer is less efficient in vacuum furnaces, lower temperatures and longer times are often employed. This reduces the risk of insipient melting and excessive grain growth, both problems encountered when heat treating high speed steels in salt baths.

After the correct soak time the load is rapidly cooled (quenched) using nitrogen gas to transfer the stored heat in the components to a water cooled heat exchanger. 
The pressure of the cooling gas can be controlled up to 6 bar. Speed of quench increases with greater gas pressures.

The bright, clean, shiny finish obtained on machined parts after vacuum hardening means that the surface suffers no decarburisation, oxidation or discoloration. Critical items may be machined and ground to almost final dimensional tolerances before heat treatment. The resulting components are cheaper to manufacture, have less inherent distortion and commonly exhibit superior metallurgical properties over parts processed through other types of furnace.

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