Heat
treatment is a metal thermal processing technology which can realize the
material that in solid state to obtain the expected structure and performance
through the means of heating, heat preservation and cooling, that is, to improve the internal quality of
the work-piece.
Heat treatment generally does not change the shape and
the overall chemical composition of the work-piece. It will endow or improve
the performance of the work-piece by changing the internal micro-structure of
it or the chemical composition of its surface.
Integral
heat treatment is widely used in the treatment of valves and their accessories.
Its mainly process is heating, heat preservation and cooling.The quality of
this process determines the final heat treatment effect.
The integral
heat treatment roughly has annealing, normalizing, quenching and tempering four
basic types.
1. Quenching
Quenching has the shortest cooling time, usually
cooling with cold water or oil.
2. Normalizing
Its cooling rate is between quenching and annealing,
generally by air cooling. Normalizing can refine the crystal grain of steel
during the slightly faster cooling, which can not only obtain satisfactory
strength, but also significantly improve the toughness and reduce the cracking
tendency of the work-piece.
3. Annealing
Its cooling speed is the slowest, generally through
furnace cooling. The purpose is to
improve machinability, stabilize size and reduce hardness ,residual stress and deformation
and crack tendency. It helps to refine grain, adjust micro-structure and
eliminate micro-structure defects.
4 Tempering
Tempering is to reheat the quenched work-piece at the
appropriate temperature which is lower than the critical one. After a period of
heat preservation , the work-piece will be cooled by air, water, oil and other
media. Under some circumstance, there will be two or several tempering.
After
heat treatment , metal can realize the improvement of mechanical properties,
the elimination of residual stress and the improvement of metal
machinability.
