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Vacuum Heat Treatment

Vacuum Technology is the Basis for Process Innovation in Heat Treatment.

Heat Treatment is the process in which metallic/steel parts are exposed completely or partially to time-temperature sequences in order to change the mechanical and/or corrosion properties. There are numerous application areas, e.g.:

• Annealing
• Hardening
• Tempering
• Aging
• Case hardening

to achieve a higher strength of the material, better wear resistance or to improve the corrosion behaviour of the components.

All of these processes need a temperature up to 1.000 °C and higher as well as especially developed furnaces to achieve such ranges. From the past there are well-known technologies for the above processes, e.g.:

• Technology using molten salt
• Furnace for protective and/or activated atmospheres

 

Specific tools and dies                                                                 

 

 

 

1. Gears and shafts, 2. Vacuum heat treatment, 3. Atmospheric heat treatment

Oxidation occurs on the part's surface when exposed to the atmosphere (air). This results in costly and time-consuming post treatments. Therefore, heat treatment is preferably conducted in an oxygen-free atmosphere. In addition to the use of high-purity protective gases, vacuum allows the best protection against oxidation, thus being the most cost-efficient atmosphere.

Such furnaces are also used for high temperature brazing, a well established joining process.

Hardened steel structure Martensite                                      

Annealing
Annealing is one type of heat treatment comprising heating up to a specific temperature, holding and cooling down slowly. Such processes are generally used to obtain a softer structure of the part and to optimise material structure for subsequent working steps (machining, forming). Parameters depend on the material and the desired structure.

Hardening and Tempering
Hardening is a typical heat treatment process combining heating to specific temperatures (mostly above 900 °C) and direct fast cooling or quenching of the part. The requirements are selected to change the materials’ structure partially or completely into martensite. The part undergoes tempering treatment after hardening in order to obtain high ductility and toughness.

Case Hardening
One of the important processes is the case hardening or carburizing process. Parts are heated up to 900 °C - 1.000 °C and by adding specific gases (hydrocarbons) into the atmosphere of the furnace the part’s surface is enriched by absorbing carbon. Following this treatment the part is quenched in order to achieve the required properties. This results in higher resistance to stresses and friction on the component’s surface. The core of the part remains somewhat softer and more ductile which allows the part to carry high stresses through its entire life. For example, all gear parts for transmissions are treated this way.

Brazing
Brazing is a process for joining components, whereby a filler melts under temperature and joins the components together after solidification. In this process, the solidus temperature of the parts to be joined is not reached. In high temperature brazing (above 900 °C) which ideally happens in vacuum, the atmosphere (vacuum) takes on the duties of the fluxing agent.

 

 

1. Piezo-Common-Rail diesel injection system, 2. Modular vacuum heat treatment furnace (Company Wegener)

Advantages of Vacuum Heat Treatment

Vacuum as "Protective Atmosphere"
• No toxic protective gases containing CO
• No health hazards in the work shop
• No danger of explosion or open flames
• No furnace conditioning
• Use of inert gases (nitrogen or helium)
• No CO2 emission

Vacuum Carburizing
• Use of various processes
• Use of different gases
• Shorter carburizing cycles than in conventional technology
• Higher carburizing temperatures offer potential to further reduce process time
• Small gas consumption l instead of m3

Gas Quenching Instead of Oil Quenching
• Clean, dry parts after hardening
• No washing machine - no disposal of washing water
• No maintenance of washing equipment
• No complicated washing water chemistry
• Saves space
• Cost benefits
• Quenching intensity is controlled via gas pressure or gas velocity
• No vapor blankets during quenching
• Homogeneous quenching
• Reduced distortion

Surface Influences
• Free of surface oxidation
• No surface decarburization
• Bright, metallic, shiny parts 

Plant Operation / Installation / Maintenance
• No idling over the weekend
• No continuous gas consumption
• Short heat-up times
• Fast access to installed modules
• No fire detection or sprinkler system
• No open flames
• No flammable gas mixtures
• Cold-Wall Technology
• No gas emission
• Minimum energy loss
• No heat radiation to atmosphere

Auxiliary Equipment of Protective Gas Plants are no longer required, like:
• Fire safety equipment
• Sprinkler system
• Exhausts
• CO2 extinguisher for the oil bath
• Measuring CO concentration in the shop
• Smoke exhaust in the roof (automatic opening and closing)
• Oil-proof floor or tank
• Methanol storage

 

Typical hard metal parts