|
197. Metallographic Analysis of
PM Fracture Surfaces
The fracture surfaces produced by breaking PM materials
appear substantially different from those generated on parts made by other
metalworking techniques. Although the characteristics of the fractured
regions are the same, ductile dimples, cleavage, etc., the interior pore
surfaces add complications to a fractographic analysis. The smooth surfaces
of the inherent porosity are not only common with PM materials, they are
expected. Part density, the effectiveness of sintering, and the history of
the part in the green state have large effects on the microstructure and
subsequently, on the appearance of the broken surfaces. The fraction of pore
edges, which appear as smooth free particle surfaces, varies with density,
where low-density regions display large amounts of these free surfaces.
Sintering causes the particles forced into intimate contact during
compaction to form metallurgical bonds. Better sintering results in neck
growth and a more homogeneous distribution of alloying elements.
Inappropriate handling of green parts or compaction/ejection problems can
create internal cracks within the parts prior to sintering.
Techniques will be presented both to
prepare surfaces for analysis and to quantify the appearance of fracture
surfaces. Examples of PM materials broken using different types of loading
will be used to demonstrate the use of these methods. In addition, an
investigation using an FC-0205 material was conducted to investigate the
effects of the three variables mentioned above. Test pieces were compacted
to several densities and sintered using various temperatures to produce
density and sinter quality variations. In addition, cracks were introduced
into green test pieces and examined after sintering. SEM analysis of the
fractures was used in concert with light microscopy on prepared
cross-sections to evaluate and quantify the resulting fracture surfaces.
|
|
196. Sintering of Powder
Premixes - A Brief Overview Advances in
the understanding of the sintering of powder premixes have contributed
significantly to the growth of the ferrous powder metallurgy industry. This
includes sintering both in the solid state and in the presence of a liquid
phase. In this article, the sintering of iron powder premixes containing: 1)
graphite; 2) nickel and graphite; 3) copper and graphite; 4) Phosphorus as
ferrophosphorus; and, 5) boron as ferroboron are discussed. The evolution of
microstructure and mechanical properties are discussed as well. |
|
157. Quantifying the Degree-of-Sinter in Ferrous P/M Materials
Improvements in the physical and mechanical properties of pressed and
sintered ferrous materials are made during the sintering process. Particle
bonding and alloying by diffusion occur during sintering with property
enhancements resulting as the sintering time is increased. The effects of
sintering are visible as changes in microstructural features, such as
particle boundaries and pore edges. Some of the improvements in sintering
appear as a loss in particle boundaries, smoother pore edges, and a
lessening in the number of angular features between particles. The
appearance of these features and characteristics, in conjunction with their
frequency of occurrence, is often referred to as degree-of-sinter.
Quantification of the degree-of-sinter can be performed on properly prepared
metallographic specimens using well-understood stereological practices.
Three test methods will be discussed as techniques for quantifying and
separating materials sintered to varying degrees. Additionally, images of an
iron-copper-carbon premix, sintered at varying times, will be used to
illustrate these microstructural changes. |
|