In situ Evaluation of Supersolidus Liquid Phase Sintering Phenomena of Stainless Steel 316L: Densification and Distortion

Open Access
Author:
Bollina, Ravi
Graduate Program:
Engineering Science and Mechanics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
April 18, 2005
Committee Members:
  • Randall M German, Committee Chair
  • John Richard Hellmann Jr., Committee Member
  • Ivica Smid, Committee Member
  • Renata S Engel, Committee Member
  • Barbara Shaw, Committee Member
Keywords:
  • Distortion
  • Densification
  • Stainless steel
  • sintering
  • master sintering curve
Abstract:
Supersolidus liquid phase sintering (SLPS) is a variant of liquid phase sintering. In SLPS, prealloyed powders are heated between the solidus and liquidus temperature of the alloy. This thesis focuses on processing of stainless steel 316L via SLPS by adding boron. Various amounts of boron were added to study the effect of boron on densification and distortion. The sintering window for water atomized 316L with 0.2% boron ranges from 1430 to 1435 C and 1225 to 1245C for water atomized 316L with 0.8% boron. The rate of change of liquid content with temperature dV$_{L}$/dt decreases from 1.5%/oC to 0.1%/oC for an increase in boron content from 0 to 0.8%, giving a wider range and better control during sintering. Further, effect of boron on mechanical properties and corrosion properties was researched. It was possible to achieve tensile strength of 476$pm$21 MPa and an yield strength of 250$pm$5 MPa with an elongation of 15$pm$2 \% in water atomized 316L with 0.8\% boron. Fracture analysis indicates the presence of a brittle boride phase along the grain boundary causing intergranular fracture resulting in poor ductility. The crux of this thesis discusses the evolution of apparent viscosity and its relation to the microstructure. Beam bending viscometry was successfully used to evaluate the emph{in situ} apparent viscosity evolution of water atomized 316L with 0.2 and 0.8% boron additions. The apparent viscosity drops from 174 GPa.s at 1200oC to 4 GPa.s at 1275oC with increasing fractional liquid coverage in the water atomized 316L with 0.8% boron. The apparent viscosity calculated from bending beam and was used as an input into a finite element model (FEM) derived from constitutive equations and gives an excellent fit between simulation and experiment. The densification behavior of boron doped stainless steel was modelled using Master Sintering Curve (MSC) (based on work of sintering) for the first time. It is proven that MSC can be used to identify change in densification rate upon liquid formation during SLPS.