GTBL042-11 GTBL042-Callister-v3 October 4, 2007 11:59
2nd Revised Pages11.5 Isothermal Transformation Diagrams • 423Time (s)Temperature (°C)Temperature (°F)10 –1 110102 103 104 10520040080060010001200A Eutectoid temperature 1400AAM + AM(start)
M(50%)
M(90%)50%PB1000200300400500600700800AA++PBFigure 11.23 The
complete isothermal
transformation
diagram for an iron–
carbon alloy of
eutectoid composition:
A, austenite; B,
bainite; M, martensite;
P, pearlite.because carbon diffusion must be virtually nonexistent.^4 The horizontal and linear
character of these lines indicates that the martensitic transformation is independent
of time; it is a function only of the temperature to which the alloy is quenched or
athermal rapidly cooled. A transformation of this type is termed anathermal transformation.
transformation Consider an alloy of eutectoid composition that is very rapidly cooled from
a temperature above 727◦C (1341◦F) to, say, 165◦C (330◦F). From the isothermal
transformation diagram (Figure 11.23) it may be noted that 50% of the austenite
will immediately transform to martensite; as long as this temperature is maintained,
there will be no further transformation.
The presence of alloying elements other than carbon (e.g., Cr, Ni, Mo, and W)
may cause significant changes in the positions and shapes of the curves in the isother-
mal transformation diagrams. These include (1) shifting to longer times the nose of
the austenite-to-pearlite transformation (and also a proeutectoid phase nose, if such
exists), and (2) the formation of a separate bainite nose. These alterations may be
observed by comparing Figures 11.23 and 11.24, which are isothermal transformation
diagrams for carbon and alloy steels, respectively.
plain carbon steel Steels in which carbon is the prime alloying element are termedplain carbon
alloy steel steels,whereasalloy steelscontain appreciable concentrations of other elements,
including those cited in the preceding paragraph. Chapter 13 tells more about the
classification and properties of ferrous alloys.(^4) The alloy that is the subject of Figure 11.22 is not an iron-carbon alloy of eutectoid
composition; furthermore, its 100% martensite transformation temperature lies below the
ambient. Since the photomicrograph was taken at room temperature, some austenite (i.e.,
the retained austenite) is present, having not transformed to martensite.