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Thermal processes

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1.Thermal processes:


     The processes are particularly applicable to improvements and enhancements of  performance and quality of raw materials or workpieces which are heated under vacuum environment or protective gas atmosphere by resistance heating or induction heating to reach appropriate temperature and hold this temperature a period of time and then cooled in different cooling rates through different cooling medium to change their surface or internal microstructure to eliminate or control defects.   



     The processes are divided into the following: 


-Sintering:Powders or powder compacts are heated under vacuum environment or protective gas atmosphere in temperature, which is lower then basic melting point temperature of their main compositions to force the powder particles solidifying into aggregates to obtain compacted sintered products.


-Brazing:The brazing is a process to join workpieces by heating under vacuum environment. During the processing, liquid solders  with lower melting point temperature than the workpieces diffuse into tiny gaps between the workpieces in the effect of capillary force to until the gap filled to form firm combination of the workpieces. 


-Quenching:Steel or alloy workpieces are heated under vacuum environment or protective atmosphere to appropriate temperature and hold it a period of time, after that the workpieces are cooled down rapidly through cooling medium to lead their structure changed from austenite into martensite to make hardness improvement. Based on different cooling medium, the quenching is divided into:gas quenching, oil quenching and water quenching.


-Tempering:Quenched workpieces are heated under vacuum environment in different temperature to obtain tempered martensite, troostite or sorbite with a certain hardness and malleability after holding the temperature a period time and than cooling down.Quenching and tempering are inseparable,  quenching with high temperature tempering is called hardening & tempering(tempered sorbite).


-Annealing:The annealing is a process to force internal structure of steel workpieces to be or close to equilibrium state to improve their malleability and plasticity which are processed by heating, certain temperature holding and cooling inside furnace under vacuum environment or protective atmosphere to relieve inner stress caused by work hardening. 


-Solution:Workpieces made of stainless steel or alloy are processed under vacuum environment or protective atmosphere by heating, certain temperature holding and cooling inside furnace to make their internal alloy phase sufficient solubility leading to internal structure changing to form solid solutions with relieved inner stress and improved plasticity, toughness and corrosion resistance so that the workpieces are able to be post processed or formed.  


-Aging:Solution processed workpieces are heated to certain temperature under vacuum environment and then hold this temperature a period of time to prompt the workpieces property, shape and dimension changing with time so as to attain inner stress relief, structure and dimension stabilization and mechanical performance improvement.  


-Normalizing:Steel workpieces are heated above austenitizing temperature, hold this temperature a period of time and then cool down  them slowly to change their internal matrix structure. 


-Degassing:Workpieces are processed under vacuum environment with heating and certain temperature holding to force inner gaseous impurities releasing from the workpieces so as to attain inhibition of aging embrittlement and mechanical property improvement.  


-Magnetic annealing:This annealing process is applicable to inner stress relief of soft magnets caused by  rolled machining or stamped machining.


-Dehydroxylation:This process is applied to quality improvement and service life extension of quartz workpieces roasting and exhausting under high vacuum and temperature.



2.Thermal processes comparison


-Vacuum heat treatment:Clean processing environment, non-oxidation and non-decarburization, dewaxing and degassing, inhibition of internal metal elements volatilization from workpieces,  surface brightening, less deformation of quenched workpieces,  improvement of abrasion resistance, fatigue strength and machining property, process stability and repeatability, energy save, environment protection, less cycle of cost recovery.


-Controlled atmosphere heat treatment: Processed workpieces surface roughness and non-brightness, lack metal elements on surface, erosion heat elements,  overuse of gas,  difficult reducing gas recovery.


-Atmospheric heat treatment:Oxide film on workpieces, harsh processing environment, less application.



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