vacuum manipulation of final stage sintering

Open Access
Author:
Li, Li
Graduate Program:
Engineering Science
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 28, 2011
Committee Members:
  • Donald Francis Heaney Jr., Thesis Advisor
Keywords:
  • Evaporation/Condensation
  • Pore Mobility
  • Density
  • Vacuum
  • Final Stage Sintering
Abstract:
Achieving full-density in iron-based P/M materials has long been a topic of interest. Density is a predominant factor in the performance of materials. The effect of density on a wide variety of properties of powder metallurgy has been thoroughly documented. Generally, as density is increased almost all properties, including strength and magnetic performance, are improved. 440C stainless steel is a martensitic stainless steel with high corrosion resistance, good resistance to the atmosphere and mild acids. It is used in high quality knife blades and rolling element bearings. 440C stainless steel was densified to 98-99% density by sintering in 4-10 Torr dynamic partial pressure conditions while the same alloy stops to densify at 92-95% density at 760 Torr due to the mechanisms that occur during the final stage of sintering. Case studies were carried out to determine how vacuum affects final stage sintering. This was done by comparing the samples sintered in 4-10 Torr dynamic partial pressure and 760 Torr. The density, hardness and carbon level of samples were also examined. The microstructure was then observed after grinding and polishing. Density increased when partial vacuum of 4 Torr was applied. Hardness also increased as density increased. In addition, the pores are more likely to be on the grain boundaries in partial vacuum than in typical atmosphere. The dominate mechanism for this behavior is believed to be evaporation/condensation which is promoted by partial vacuum and resulted in keeping the pores on the grain boundaries, which was verified by microstructural examinations. Final-stage densification is critically dependent on minimized grain growth and attachment of the pores to the grain boundaries. When a vacuum is applied, the ability for metals to evaporate and transport across the pore is greater, thus the higher sintered density can be achieved.