If a martensitic steel like W1 is quenched to 350F and then held there to
temper, the 50% retained austenite transforms to bainite, see Krauss.
However what happens to the austenite if it is tempered hot, ~ 950F. I have
read it won't transform the austenite but conditions it so during the
cooling martensite forms. The basic reason is that during the temper the MS
point of the steel is raised. What I don't understand is why it doesn't
transform to pearlite and instead is "conditioned".
Regarding toughness testing, if you examine torsional graphs they show a
huge peak at low temperatures, usually ~325 F and then a sudden fall off
after that usually attributed to cementite precipitation. However the charpy
and izod tests don't show this at all. I was wondering if this isn't just an
issue of grain alignment of the test piece because if the grain was aligned
with the break as opposed to against it it would make a huge difference if
the grain boundries were reduced in toughness due to cementite formation.
Is this the case. Does anyone know of charpy tests over a wide tempering
range which include the grain orientation both opposing the with the break
and do they have the same peaked nature of the torsional graph. Crucible has
the tests for some steels, but only at select points they don't have the
full range of values over a wide tempering range, 300-1000F.
In regards to compression strength, Crucible notes that CPM versions of the
same steels are 10-15% stronger under compression at the exact same HRC
hardness. Now a HRC value is a test of compression to the point that plastic
deformation is seen over an extent far larger than the grain size of the
steel so I would think it would hold to compression in general. What is the
theory behind the difference in behavior between compression say a mm sized
hole (HRC) to a cm point, hitting a knife edge into a nail and having it
deform (this is one of the promoted advantages).
- posted 16 years ago