**2.3. Determining the influence of cooling rate on the microstructure and properties**

30 Heat Treatment – Conventional and Novel Applications

**Figure 10.** Interrelation between the heterogeneity factor (ν) of former austenite grain size in the cast

steel and the austenitizing parameters for: a) mean diameter; b) mean grain area

**Figure 11.** Distributions of frequency of cumulated grains

In order to determine the influence of cooling rates, allowing proper selection of parameters for the regenerative heat treatment, the TTT curves were plotted for L21HMF cast steel. On the basis of results achieved by means of dilatometric tests, a graph was drawn up, as shown in Fig 12. It illustrates the influence of cooling rate in the temperature range of 800 ÷ 500 oC on the structure and hardness of the investigated cast steel.

**Figure 12.** Influence of the cooling rate on structure and hardness of the cast steel

Analysis of the curves presented in Fig. 10 allows to state that in the case of L21HMF cast steel, whose chemical composition is given in Table 1, austenite cooled at 0.004 K/s ≤ v8-5 ≤ 0.017 K/s gets transformed into ferrite and pearlite. The rate of cooling for austenite: 0.023 < v8-5 ≤ 0.869 K/s makes it possible to obtain ferritic – pearlitic – bainitic structures. Whereas after cooling of the cast steel at the range of 0.869 K/s < v8-5 ≤ 14.630 K/s bainitic – ferritic structures were obtained, with an increasing bainite volume fraction as the cooling rate increased. Bainitic structure with around 6% volume fraction was received for the cooling rate of v8-5 14.630 K/s.
