Internal Stress in Forging Parts During Heating Process
Forging is one of the most common and important processes of metalforming technology. Forging is generally used to produce components with a complex shape with a large number of passes and a difficult heat treatment. It is a complex process involving both mechanical and thermal actions. The heating process, especially the correct control of the heating temperature, is significant for the production of quality components. With the heating of the work piece during the forging process, the temperature of the piece rises and the volume of the material expands, however, due to the restrictive effect of the die and the restraint of the parts inside the die a certain amount of internal stress is generated. It is noticeable that the effects of heat treatment modify the microstructure of steels, as well as other metals, affecting the final mechanical properties of the material. The ability of the metal to resist deformation under a given load, as well as other mechanical properties, is closely linked with the magnitude and distribution of these stresses. An important part of the success of the forging process, is the control of the internal stresses generated during the heat treatment.
It is well known that metals expand with the increase of temperature, and the expansion is non-uniform in the dimensions of the parts due to the inner restrain of the die. As a result of the non-uniform expansion, internal stress/strain is generated in the metal as internal forces due to the restrictive effect of the die. The internal strain generated along with the heating of the work piece during the forging process is the most significant cause of cracking, and significantly reduce the fatigues strength and toughness of the product, which limits its use under dynamic loading conditions. Tocolatin, D.L. et al. (1982) studied the heating process of metal pieces for aerospace applications and the relation between the internal stress generated during the heating and part distortion, and increased the understanding of the effect of internal stresses on work piece geometry. It was found that internal stresses were generated during the heating process and changed the geometry of the part and the distribution of internal stresses. The magnitude of the stress and strain generated during heating is largely related to time and temperature, which are the two most important parameters that affect the physical properties of the material.
In the literature, a number of methods to reduce the magnitude of the stress generated in the work piece during heating are proposed directly or indirectly. Some of the proposed solutions are direct ones such as “quenching and tempering” and “preheating”. Other solutions are more indirect and they rely on reducing thermal contraction during cooling, with techniques such as “annealing” or “cooldown” after the heating. “Part illumination” is one of the newer solutions that is used for large pieces and involves the use of an infrared lamp, light bulbs or a crane mounted lamp which are used to irradiate the parts and reduce the cooling effect. As well as these, some other solutions such as “vacuum heat-treatment” and “finite element simulations” have been suggested for more specific requirements. In general, the solutions described above can be effective in reducing the stresses generated by the heating process and hence increasing the quality of the produced components.
In conclusion, it is clear that the heating process during the forging process plays an important role in determining the internal stress generated in the work piece. An adequate control of the thermal process is essential to reduce the internal stress generated by the heat treatment.
