Steels_ Metallurgy and Applications, Third Edition

Engineering steels 203

Heat treatment aspects

Isothermal transforma~on dial,ares

Isothermal transformation diagrams were first published by Bain and Davenport l

in the United States in 1930 and paved the way to the detailed understanding

of the effects of alloying elements on the heat treatment response in steels.

A steel is first heated to a temperature in the austenitic range, typically 20"C

above At3, and then cooled rapidly in a bath to a lower temperature, allowing

isothermal transformation to proceed. The progress of transformation can be

followed by dilatometry, the degree of transformation depending upon the

holding time at temperature. As illustrated schematically in Figure 3.1, the

start and finish of transformation to ferrite, pearlite, bainite and martensite are

then shown on a diagram as a function of temperature and time. Isothermal

transformation diagrams, also known as TTr (time-temperature-transformation)

diagrams, are simple in concept but are not representative of the majority

of commercial heat treatments which involve a continuous-cooling operation.

However, martempering and austempering are examples of heat treatment that

employ isothermal sequences.

A schematic illustration of martempering is given in Figure 3.2. Following a

conventional austenitizing treatment, a component is cooled rapidly to a temper-

ature just above the Ms temperature and held at this temperature long enough to

ensure that it attains a uniform temperature from surface to centre. The material is

then air cooled through the Ms-Mf temperature range to form martensite and is

subsequently tempered. Martempering is therefore not a tempering operation but

a treatment that leads to low levels of residual stresses and minimizes distortion

and cracking.

--Ae 3 ..........

A + F+C

---M,,

----Mr

,,1 ,, ,, ,

Log time

Figure 3.1 Isothermal transformation diagram