Explanation of heat treatment stress of steel

Heat treatment furnace is a general designation of industrial furnaces for various metal heat treatment of metal workpieces. The temperature is generally lower than that of the heating furnace. Heat treatment furnace can adopt various types of heating furnace, but it is required to strictly control furnace temperature and furnace atmosphere. Metal heat treatment is a process that put metal workpiece in a certain medium for heating, heat preservation and cooling, and control its performance by changing the surface or internal structure of metal materials.

Explanation of heat treatment stress of steel:

In the process of heating and cooling, due to the difference of cooling speed and time between the surface and the center, temperature difference will be formed, which will lead to the uneven volume expansion and contraction and generate stress, that is, thermal stress. Under the action of thermal stress, the surface temperature is lower than the center, and the shrinkage is also greater than the center, which makes the center under tension. When the cooling is over, the surface is compressed and the center is under tension because the final cooling volume shrinkage of the center cannot be carried out freely. That is to say, under the action of thermal stress, the surface layer of the workpiece is compressed and the center part is pulled. This phenomenon is affected by cooling rate, material composition and heat treatment process. The higher the cooling rate, the higher the carbon content and the alloy composition, the larger the non-uniform plastic deformation under the action of thermal stress in the cooling process, and the larger the residual stress finally formed. On the other hand, in the process of heat treatment, due to the change of microstructure, i.e. the transformation from austenite to martensite, the increase of specific volume will be accompanied by the expansion of the volume of the workpiece, and the transformation of various parts of the workpiece will lead to the inconsistency of the volume growth and produce the structural stress. The final result of the change of tissue stress is the tensile stress on the surface and the compressive stress on the center, which is just opposite to the thermal stress. The size of the structure stress is related to the cooling rate, shape and chemical composition of the workpiece in the martensitic transformation zone.

It has been proved that thermal stress and tissue stress will occur in any workpiece during Nantong heat treatment as long as there is phase transformation. But the thermal stress has been produced before the organization transformation, and the organization stress is produced in the organization transformation process. In the whole cooling process, the comprehensive effect of thermal stress and organization stress is the actual stress in the workpiece. The results of these two kinds of stress are very complex, which are affected by many factors, such as composition, shape, heat treatment process and so on. In terms of its development process, there are only two types, i.e. thermal stress and tissue stress. When the direction of action is opposite, they cancel out, and when the direction of action is the same, they overlap each other. Whether they are mutually offset or superposed, the two stresses should have a dominant factor. When the thermal stress is dominant, the result is that the center of the workpiece is under tension and the surface is under pressure. When the tissue stress is dominant, the result is the tension on the compression surface of the workpiece center.

Main application fields of metal heat treatment:

1. Welding: cutting tool, drilling tool, cutting tool, mirror frame, compressor, pressure gauge, contact point of relay, welding of copper wire of transformer winding, liquid storage (welding of air filling nozzle, welding of stainless steel meal and kitchen utensils).

2. Heat treatment: quenching of mechanical metal parts (surface, inner hole, part and whole) such as gear, machine guide rail, hardware tools, pneumatic tools, electric tools, hydraulic parts, nodular cast iron, auto parts, etc.

3. Diathermy forming: standard parts, small hardware parts, heating and annealing of metal materials for drawing, modeling, embossing, bending, smashing, steel wire (iron wire) heating and nail making, stainless steel products annealing, drawing, expanding, etc.