Tensile Strength Question Continued

Alan Baker wrote:
In article ,
"Dick" wrote:

okay, got pounds per Square Inch and elongation differences and the failure
sequence in my noodle.

For simplicity, let's use 120,000 and 80,000 psi numbers.

So if the 120,000 # piece is .667 square inches
and the 80,000# piece is 1.0 square inch,,,are the elongations the same???

Thanks, Dick

Ummm...

Think about it for a moment. To be different strengths, the two steels
must differ in chemical composition, right?

Surprisingly enough, that is not true. A treatable steel can be
hardened and annealed over a broad range of tensile strengths.
The bulk composition of the alloy does not change, but the
crystaline structure and the compositions of those crystals
changes.

Crudely stated, metals consist of microscopic crystals that butt
up against each other. The simplest heat treatable alloy,
carbon steel, is just iron and carbon. When fully annealed
(softest) all of the carbon is collected into tiny crstals
of graphite and the bulk of the material is pure iron crystals.
In that form, it really isn't steel at all, it is just wrought iron.
Heat treating will cause some of the carbon to combine chemically
with the iron and form carbides. Then you have steel and there
will be some pure iron crystals, some pure graphite crystals,
and some carbide crystals but the bulk proportion of
carbon atoms to iron atoms will remain the same.

The different hardnesses depend partly on how much carbide is
formed and partly on the geometry of the atoms in those crystals.
The geometry within the crystals can be manipulated by heating
to a temperature at which the geometry (packing) changes and then
cooling it so fast that they 'freeze' in that state.

All this happens without any change in the bulk composition of the
alloy. Which, I grant you is quite counter-intuitive.

Do you think that that
wouldn't also lead to different elongation under the same load?


The relationship between elongation and applied load depends on
a property called Young's modulus. It depends almost entirely on
the bulk composition of the material and is independent of the
strength of the material. Heat treating an alloy to a different
strength
does not affect its modulus.

Iron has the highest modulus of any of the natural elements which is
why really big stars become supernovae. More relevent to the
present discussion, small differences in the compositon of a
steel have almost no effect on their moduli. Thus all steel alloys
that are 99% iron have esentially the same modulus as pure
iron.

Even the 18-8 stainless steels that are less than 65% iron have
moduli that about 90% that of pure iron.

Neat, eh?

--

FF