Blue brittleness

I don't have much detailed information about blue brittleness other
than ASM Metals Handbook. I read in an old metallurgy text that
quick cooling from 400-600F might avoid the problem, but not so sure
that it isn't talking about Cr-Ni steels only and their "temper
brittleness" instead.
ASM about blue brittleness-
"Killed steels are virtually free of this kind of brittleness."
So the dip in a torsional toughness test graph at 400F and the steel
doesn't re-gain its strength until 600F is not because of being in
the "blue brittle stage"? Because the steels tested are usually the
higher quality tool steels, surely killed and many vacuum remelted
even.
Being self taught on the subject I've got holes in my understanding
of high carbon and tool steel metallurgy. I'll appreciate and help
filling them in. ;)
Alvin in AZ (hobby knifemaker)
ps- gave up my seat in the fall metallurgy class to someone going
for a degree and it isn't offered again until next fall :/
Reply to
alvinj
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Blue-brittleness is relevant in nitrogen-containing BCC steels, were nitrogen is not bound as nitrides or carbo-nitrides.
In the above mentioned temperature-range, nitrogen diffusion is fast such that it can diffuse to dislocations. The dislocations' mobility is thus strongly reduced and brittleness results.
HTH
Michael Dahms
BTW: Please use SI-units. This is a world-wide newsgroup.
Reply to
Michael Dahms
Some answers to some of the questions you are not asking. Not guaranteed to be accurate for any unspecified steel.
You are unclear about whether you are talking about embrittlement of the material when it is held at temperature or at room temperature after being held at temperature.
If you are talking about a steel that has been quenched and not tempered then you can get an embrittling effect at room temperature after heating to moderate temperatures due to the transformation of retained austenite to bainite. Some people call this temper brittleness, others do not distinguish between this and the other mechanisms of blue brittleness and related issues.
Nitrogen (and carbon) can cause embrittlement of steels both at elevated temperature and at room temperature depending on the chemistry and thermal history of the steel. Some of these effects are called strain ageing or dynamic strain ageing. Some of these effects occur at lower temperatures than blue brittleness others in the blue brittleness range.
Phosphorous and other elements can cause embrittlement after heating to the blue brittle or higher range.
Secondary hardening elements can cause reductions in room temperature toughness after heating. This is associated with precipitation effects. Often this is at higher temperatures than blue brittleness.
Reply to
David Deuchar

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