heat treatment furnace Changes in structural stress

The stress of heat treatment of steel workpiece in the process of heating and cooling, due to the surface and center of the cooling rate and time inconsistency, formation temperature, will lead to uneven volume expansion and contraction stress, thermal stress. Under the action of thermal stress in heat treatment furnace, due to the surface temperature is lower than the core, and make the heart of contraction is greater than the core department in tension, when at the end of the cooling, due to the heart of the final cooling volume shrinkage can't free and make the surface tension of compression of the core. That is, under the action of thermal stress, the surface of the workpiece is finally compressed and the core is pulled. This phenomenon is affected by cooling rate, material composition and heat treatment process.
The higher the cooling speed, the higher the carbon content and alloy composition, the greater the uneven plastic deformation under thermal stress during the cooling process, and the greater the residual stress. On the other hand, when steel is transformed from austenite to martensite in the heat treatment process, the increase of specific volume will accompany the expansion of workpiece. The phase change of each part of the workpiece leads to the inconsistency of volume growth and tissue stress.

The ultimate result of the change of tissue stress is the tensile stress on the surface and the compressive stress on the industrial furnace, just opposite to the thermal stress. The magnitude of tissue stress is related to the cooling rate, shape and chemical composition of the workpiece in the martensite phase transition zone. It is proved by practice that thermal stress and tissue stress will occur in any workpiece during heat treatment as long as there is phase change.