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194. Effect of Post Sintering
Thermal Treatments on Dimensional Precision and Mechanical Properties in
Sinter-Hardening PM Steels
Dimensional precision is a critical parameter in net shape processing of
ferrous PM components. PM parts producers continue to pursue larger parts,
but absolute tolerances dictated by the end user generally do not scale with
part size. Therefore, in larger parts, the variation in percentage change in
size, or dimensional change, must be reduced. Beyond the dimensional changes
associated with pressing and sintering of typical low alloy PM steels,
sinter-hardenable alloys present some unique challenges and opportunities
for PM part manufacturing. The ability to harden a part in the sintering
furnace eliminates the need for a secondary quenching operation. The
resulting microstructure of untempered martensite is, however, not ideal for
dimensional stability and mechanical properties. Tempering hardened steels
results in improved mechanical properties and dimensional shrinkage, as the
martensite converts to a more stable ferrite and carbide microstructure of
higher density. In addition, many sinter-hardening grades contain high Cu
and C contents that result in relatively high amounts of retained austenite.
Retained austenite can improve impact and ductility properties, but
contributes to dimensional instability as it can transform to lower density
bainite and/or martensite with thermal fluctuations. Proper thermal
treatments of sinter-hardened steels are necessary to obtain the best
combination of mechanical properties and dimensional control. This paper
reviews the effects of different post-sintering thermal treatments on the
dimensional, microstructural and mechanical property changes of
sinter-hardened PM steels. |
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186. Dimensional Precision
in Sinter-Hardening PM Steels
Dimensional precision is a critical parameter in net shape processing of
ferrous PM components. Beyond the dimensional changes associated with
pressing and sintering of typical low alloy PM steels, sinter-hardening
alloys undergo a transformation from austenite to martensite. The formation
of martensite results in a large expansion during cooling, as martensite is
the lowest density phase in steels. Tempering hardened steels results in
shrinkage, as the martensite converts to a ferrite and carbide
microstructure of higher density. Both of these transformations have a large
impact on the dimensional change. In addition, martensitic regions with high
Cu and C contents may contain large amounts of retained austenite. As
austenite is the highest density phase, retained austenite results in less
growth of the compact. The presence of martensite and retained austenite, in
addition to the tempering step, all play a role in the final dimensions of a
component. This paper reviews two sinter-hardening grades and investigates
the dimensional and microstructural changes of those grades through
different post-sintering thermal treatments. |
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184. Dimensional Control in
Cu-Ni Containing Ferrous PM Alloys Elemental additives, such as
copper and nickel, modify the dimensional change in sintered parts.
Typically, Cu causes growth and Ni causes shrinkage. Interactions between
Cu, Ni and C complicate these simple trends leading to more complex
behavior. With the use of prealloyed Mo base materials, these alloy systems
can be used for sinter-hardening applications. This paper investigates the
dimensional and microstructural changes of Cu-Ni containing PM alloys during
the sintering process. |
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