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Fig. 6. 22 FEM modeling of heat treatment process

Heat treatment process sequences
The heat treatment process can be divided into three sequences, Fig. 6. 23. The first sequence, called
the heating sequence, the material is heated according to a predetermined temperature sequence.
The allowable heating curve is constrained by a minimum rate due to requirements for change in
microstructure and maximum rate due to problem with large thermal gradients. For example, complex
geometry may result in uneven heating due to geometry; this may generate plastic deformation or
cracks. The second sequence is the holding sequence where the wanted effects usually take place, for
example ageing or stress relieving. This temperature may change in for example precipitation
hardening, where a higher temperature first are used to get one type of precipitates, and a lower
temperature where other precipitates are formed. The last sequence is the cooling sequence.
Dependent on the heat treatment carried out different requirements made on this sequence. In
quenching, a certain cooling rate is needed to obtain the desired microstructure. However, in ageing
the cooling rate is not an equally important parameter. Common requirement for all cooling sequences
is that it should not generate cracks or plastic deformation.

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