This lab explores the effects of heat treatment on both the mechanical properties of steel,
and also on the microstructure. To better understand the effects, a TTT (time-temperature-
transformation) curve is analyzed in ferrous metallurgy. In this experiment, a cold-worked, steel
alloy with an inducted temperature gradient was used. It was found that
smaller grains create a
harder substance, thus making martensite the hardest phase, then pearlite, and then ferrite. These
phases were obtained by cooling austenite differently, holding it at different temperatures.
Steel, an alloy of iron and carbon, is widely used in engineering for many applications,
from transportation, to household materials. This versatility is partially due to the many variation
of its properties, which are in a large part due to the thermomechanical processes which are
imposed on steel.
A very important necessity, then, is to understand how to use this heat
treatment to optimize performance in ferrous alloys.
Carbon is soluble in iron because the carbon atoms are able to fit into the interstitial sites
of the iron atom without distorting it too much. In the FCC phase of iron, austenite,
approximately 2% carbon can dissolve, whereas only around .02% of carbon can dissolve in the
BCC phase (ferrite).
By cooling austenite below the eutectoid temperature, it becomes unstable, precipitating
the Fe as ferrite, and distributing the carbon atoms into Fe
C (cementite). The nucleation rate of
ferrite and cementite is low, and transformation occurs when the specimen is held at a
temperature for a long time. As the temperature becomes increasing lower, the nucleation rate
increases, until below 540
C where it becomes slower again, as the carbon atoms lose mobility
in austenite, and must diffuse to Fe