Powder Metallurgical Components: Improvement of Surface Integrity by Deep Rolling and Case Hardening

In many cases, powder metallurgical components are heat treated in order to improve the component behavior. An established heat treatment technique for wrought materials is case-hardening. But up to now, this procedure is not applied for powder metallurgical components, since the existing porous microstructure avoids adjusting a defined carbon depth profile. As a consequence a carbonization of the core and a full hardening can occur, leading to a disadvantageous phase and residual stress distribution. However, a new approach for case hardening of powder metallurgical components is to perform a surface densification prior to the heat treatment. In this study this approach is realized by using a deep rolling process. In order to verify the functionality of the process chain, the surface integrity was analyzed after each processing step using X-ray diffraction measurements and materialographic methods. There is clear evidence that the open porous structure could be transferred into a surface densified graded porous layer. As a consequence the carburization is dominated by solid state diffusion enabling to adjust a defined carbon depth profile leading to a martensitic surface layer and compressive residual stresses in the near surface zone. Subsequent fatigue strength experiments for sintered samples as well as for additional densified or densified plus case-hardened specimens were performed in order to demonstrate the enhanced mechanical properties. A significant increase in fatigue strength can be measured.