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A D V A N C E D
M A T E R I A L S
&
P R O C E S S E S | J A N U A R Y
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metallography were open to interpreta-
tion and each successive improvement
in lens quality increased available mag-
nifications, which further clarifiedmicro-
structural details.
Dippenaar related these devel-
opments to current work using laser-
scanning
confocal
microscopy
to
observe high-temperature microstruc-
ture development in-situ. Using this
technique, important changes in micro-
structure such as solidification, and
diffusional and displacive solid-solid
phase transformations in real time, may
be observed. Laser-scanning confocal
microscope development now allows
unprecedented observation of these
phenomena as they happen, and inter-
pretation of these results requires many
years of study moving forward. The abil-
ity to characterize materials at higher
resolutions and in real processing envi-
ronments is a critical area where techno-
logical developments promise to unveil
previously misunderstood fundamen-
tals of metallurgy.
An area where improved char-
acterization will continue to enhance
understanding and result in vastly im-
proved performance is steel processing.
Prof. Emeritus George Krauss, FASM,
Colorado School of Mines, gave an ex-
ample by discussing the phenomena of
steel embrittlement. Several conditions
exist where low ductility is observed in
steels, including quench cracking, tem-
per embrittlement, tempered marten-
site embrittlement, hydrogen embrit-
tlement, and quench embrittlement.
There was a focus on quench embrit-
tlement, which results in intergran-
ular fracture on prior austenite grain
boundaries.
Krauss summarized the efforts
over the past 40 years to understand
low toughness that occurs in quenched
and tempered steels with carbon levels
of approximately 0.5 wt% and above.
Through high quality characterization
by various methods, it has been shown
that cementite formation on prior aus-
tenite grain boundaries is associated
with this reduction in ductility, and that
alloying elements such as phosphorous
further aggravate the problem. Over
time, a map of tempering temperature
vs. carbon content has been developed,
showing that quench embrittlement
occurs only in high carbon steels tem-
pered at low temperatures. Understand-
ing the mechanisms behind phenomena
such as quench embrittlement allow the
use of high strength steels with micro-
structures designed to avoid conditions
where brittle failure would occur.
The
fifth
speaker,
Robert
Glodowski
of
RJG
Metallurgical
LLC, highlighted the importance of
Observation of formation of Wid-
manstätten ferrite (W), from ferrite
allotriomorphs (A) during cooling from
austenite (
γ
) in an Fe-C alloy.
understanding microstructural devel-
opment as a function of processing in
his presentation, “The Evolution of Fer-
rous Grain Size Control: Standards and
Practice.” In the late 1950s, it was often
necessary to specify use of alloying for
austenite grain size control to ensure
that hot rolled steels were heat treat-
able, and that the quality of the result-
ing products was satisfactory. However,
as steelmaking and rolling processes
steadily improved, the need for grain
size control via alloying and prescribed
processing procedures for as-rolled
steels disappeared.
Improved processing technolo-
gies enabled controlled thermal and
mechanical rolling techniques, which
eliminated the need to separately roll
for shape and heat-treat for proper-
ties. Because the fine-grained austenite
quality requirement is often still cited
in standards, the perception that they
are still needed remains, even though
they do not benefit modern as-rolled
steels. Grain-refining alloy additions
and associated testing increases the
steelmaking process cost, and may in
fact cause other quality problems and
an unwarranted sense of security to the
user. This is another example where a
fundamental understanding of micro-
structural development as a function
of processing is imperative to enable
manufacturing with the highest possible
quality at the lowest possible cost.
Cross-section of meteorite shows 3Dmicrostructure formed over millions of years.