Isothermal Transformation Diagram
TTT (Time,Temperature,Transformation) Curve
This diagram charts the transformations in microstructure that a given steel
will undergo at various rates of cooling through a range of temperatures. For a
heat-treater that is particular about his results it can be indispensable. The
Most prominent feature of the chart is the "pearlite nose" that comes from the
right toward the left hand side, creating a narrow area around 1100oF
through which the successful quench must pass in the given amount of time. In
shallow hardening steels this area is much narrower. In deeper hardening steels
the area widens to allow slower quench mediums like oils and air. By noting the
time in seconds that you have to get your chosen steel below the "nose" you can
better choose a quench that is suited for your needs.
The next feature to notice is the Ms point (indicated here by the green line at 450oF. This is the point at which steel actually starts
the hardening process as martensite begins to form. Before this line the internal
structure of the steel still consists of austenite that remains, since pearlite
was not formed. This unstable austenite will quickly transform into a more
stable structure as soon as a change in conditions allows for nucleation of
such structures. Below 500oF stresses caused by the shearing effect
of tilting at atomic interfaces known as habit planes trigger the crystaline
formation of martensite. Halfway down from the Ms point 50% martensite
will have precipitated from the austenite and at the bottom, hopefully room temperature
the transformation will be complete.
If care is not taken within this range the possibility exists that the austenite
could stabilize resulting in retained austenite. Steels having larger amounts of
substitutional alloy elements become much more prone to this phenomenon due
to interference with the shearing action by strengthening the austenite at the
habit plane interface. This is why many stainless alloys can benefit from sub
zero quenching. The severe drop in temperature overcomes the stabilizing effects
and transforms the remaining austenite.
A quench that is insufficient to miss the pearlite nose will result in varying
amounts of fine pearlite formation and will keep the steel from reaching maximum hardness.
Back to Info Index