Ever since Alvin posted a link to a graph showing tempering temperature
and its effects on hardness with curves for torsion and charpy results
for 1095 I've been thinking. (anybody scared yet:) ) The curve shows
a local maximum for torsion with a temper to 325 F. The charpy curve
shows a classic gradual curve with a higher force with higher temper
temp. So I've been wondering and just can't seem to find anything to
satisify my curisoty (sp), is there any reason to choose torsional
toughness over charpy?
And while were there could someone outline the testing procedures for
each. (I think charpy is to secure the sample and strike it to
determine the force required to break the piece.)
ps I hope the question makes sense.
In the older books (Bain's, Grossman's etc) the Charpy and Izod
tests show a mild-hump at 325F and drop off and then climb again.
ASM's "Tool Steels" by Roberts and Cary, 4th Edition...
"Usually, brittleness is considered to be the opposite of toughness.
This is not necessarily true; a brittle material may be either very
weak or very strong. Brittleness implies that a material shows
little or no plasticity to fracture, with the magnitude of the
elastic strength of the material not clearly implied."
"Toughness can be defined as a combination of two factors:
1. Ductility: the ability to deform before breaking, which
represents the amount of plastic plus elastic deformation
up to the point of failure.
2. Strength: the ability to resist permanent deformation or the
elastic strength of the material.
If only one component is to be used to describe toughness, the
second (strength) appears more practical for tool steels, since
large amounts of flow or deformation are rarely permissible with
fine tools. Strength is largely determined by hardness. Ductility,
although somewhat affected by hardness, is largely a function of
the state of stress."
"Tension testing of hardened tool steels (above Rockwell C 55) is
not a common procedure. Above this hardness level, nonaxiality of
specimen alignment causes erractic results. As yet, no convenient
method has been devised for achieving the degree of alignment
necessary to permit reliable measurement of tensile properties of
fully hardened tool steels. Thus, tension testing of tool steels is
normally confined to steels in the annealed or overtempered
condition and to the hot work and shock-resisting types of tool
steel that are employed at hardness levels below Rockwell C 55."
Ok, so it goes through compression testing, notched and unnotched
impact testing, bend tests and explain why none of them work worth
anything for fully hardened tool steels.
"A torsion test can be used to measure both the strength and
ductility of tool steels more accrately than can be done with a
tension test, particularly since these steels are characterized by
high strength and low dutility. However due to the low ductility, a
high ratio of length to diameter of the specimen is necessary (16
to 1 instead of the usual 5.78 to 1) so that sufficiently large
strain values (usually recorded in degrees or angles of twist) can
be obtained for a given stress (expressed generally as inch-pounds
"The torsional test does indicate various peaks in the "toughness"
of tool steels that are not often shown by other tests."
That's important since back in the late 20's and early 30's they
were actually looking for a test that would reliably show those,
since -they knew from experience- there was toughness peaks of some
sort from heat treating industrial tooling. They had experienced
(for example) a tool drawn at 325F wouldn't deform or break just
keep going :) and one drawn at 200F would break and one drawn at
500F would also break and one drawn at 650F would deform.
Charpy and Izod (notched or unnotched) measure how high a hammer
will swing when it's free to swing after release. The difference
in swing height after it goes by and breaks the sample, is what they
Cool questions but I gotta go to town, my axle parts are in
Alvin in AZ
Seems to me (if I understand all this) that the best example of torsional
strength at high hardness would be a drill or tap. I was doing some serious
cussing recently at taps that didn't have the strength to do the job I
wanted them too... Funny how the tap stood up much better when I increased
the drill size a bit :-)
On Fri, 24 Feb 2006 snipped-for-privacy@XX.com wrote:
show the same thing. Concerning the graphs that don't, I wonder if points
were actually taken at those co-ordinates (people often do just extrapolate
smooth lines through non-existant data), or could it be due simply to grain
The temper embrittlement is usually argued to be due to weakening at the
grain boundries due to precipitates and thus the alignment of the piece
being tested in charpy/izod would dramatically effect the results depending
on if the load went across or along the grain boundries.
Crucible is now testing along multiple alignments, but doesn't give graphs,
just some points. However when they give a decent amount of points, such as
for A1, they again show the same kind of peaked behavior, though often
shifted. For example the charpy-c notch values from Crucible show a maximum
at 500F which is a minimum on the torsional graph when air cooled from the
I think that is the critical part, which graph actually shows the behavior
in tool use.
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