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.)
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 of torque)."
"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 measure.
Cool questions but I gotta go to town, my axle parts are in will-call. ;)
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 :-)
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 alignment.
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 same tempereture.
I think that is the critical part, which graph actually shows the behavior in tool use.
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